CA1151826A - In-line manufacture of printed and die cut polystyrene foam sheets - Google Patents
In-line manufacture of printed and die cut polystyrene foam sheetsInfo
- Publication number
- CA1151826A CA1151826A CA000348554A CA348554A CA1151826A CA 1151826 A CA1151826 A CA 1151826A CA 000348554 A CA000348554 A CA 000348554A CA 348554 A CA348554 A CA 348554A CA 1151826 A CA1151826 A CA 1151826A
- Authority
- CA
- Canada
- Prior art keywords
- ribbon
- passing
- foam
- skin
- thickness
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
- B29C44/34—Auxiliary operations
- B29C44/36—Feeding the material to be shaped
- B29C44/46—Feeding the material to be shaped into an open space or onto moving surfaces, i.e. to make articles of indefinite length
- B29C44/50—Feeding the material to be shaped into an open space or onto moving surfaces, i.e. to make articles of indefinite length using pressure difference, e.g. by extrusion or by spraying
- B29C44/505—Feeding the material to be shaped into an open space or onto moving surfaces, i.e. to make articles of indefinite length using pressure difference, e.g. by extrusion or by spraying extruding the compound through a flat die
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
- B29C44/34—Auxiliary operations
- B29C44/56—After-treatment of articles, e.g. for altering the shape
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29L—INDEXING SCHEME ASSOCIATED WITH SUBCLASS B29C, RELATING TO PARTICULAR ARTICLES
- B29L2007/00—Flat articles, e.g. films or sheets
Abstract
ABSTRACT OF THE DISCLOSURE
This relates to the manufacture of thin printed sheets or boards of expanded polystyrene foam which boards are useful, for example, in the packaging of bacon products.
The invention involves the making of a thin flat sheet of polystyrene foam and thereafter printing and cutting the sheet in a continuous in-line operation while the foam is still in an uncured and somewhat unstable condition.
This relates to the manufacture of thin printed sheets or boards of expanded polystyrene foam which boards are useful, for example, in the packaging of bacon products.
The invention involves the making of a thin flat sheet of polystyrene foam and thereafter printing and cutting the sheet in a continuous in-line operation while the foam is still in an uncured and somewhat unstable condition.
Description
5~2~i IM-LINE MANUFACTURE OF PRINTED AND D~E CUT POJ~STYRENE
~OAM SHEETS
.
Background of the Invention This invention relates generally to the field of packaging and more particuLarly relates to continuous in~line process and apparatus for producing thin printed sheets or boards of expanded polystyrene ~oam which boards are useful, for example, in the packaging of bacon products.
Although the in~ention will be described with particular reference to bacon boards it should be understood that the invention has other applications as well such as in chocolate box liners, cookie bag liners, place mats, bar coasters, and tops for frozen foods and the like.
The board presently used in the packaging of bacon is a printed paperboard which is coated on both sides with a layer of polyethylene plastic. Although this orm of board has met with wide acceptance it does possess a number of disadvantages. For example, the cut edges of the printed board tend to absorb grease and moisture and furthermore the composite paperboard products may tend to harbour bacteria.
Furthermore, the present relatively rigid composite paperboard has a tendency to puncture the overwrapped packaging film both during shipping and also during store handling and it is desirable to provide a softer board to assist in eliminating this problem. It is also considered desirable to provide a packaging material which is light in weight thus ultimately resulting in less waste to incinerate or dispose of.
The present invention thus broadly involves the making of a thin flat sheet of polystyrene foam and thereafter printing and cutting the sheet in a continuous ln-line operation while the foam is still in an uncured and somewhat unstable condition. The process can be carried out very ~15~L826 economically and at a relatively high rate of production.
Summary of the Invention In accordance with the invention in one aspect there is provided a method of continuously manufacturing flexible sheets of cellular polystyrene foam material, including the steps of: .
(a) passing a molten pol~styrene plastic material to whi~h a blowing agent has been added through a die oxifice shaped to al}ow the material to expand into a ribbon of foamed plastics material;
(b) effecting an initial cooling of the opposing major surfaces of the ribbon on emergence from the die orifice to form a thin skin on the surfaces of the ribbon;
(c) biaxially stretching the ribbon of material as it cools;
(d) effecting further cooling and forming of the ribbon by passing it on to a surface of a rotating forming drum;
the method being characterized by the steps of:
(e) compressing the partly cooled ribbon to substantially reduce the thickness thereof by passing the ribbon in pressurized contact with a surface of a roll means;
(f) before or after step (e), passing the ribbon in a path of travel to effect further cooling and stabilization of same;
(g) passing the partially cooled and stabilized ribbon in a further path of travel through a printing press to apply printing inks to a surface thereof, and (h) subsequently die cutting printed sheets o a selected outline shape ~rom ~he ribbon.
In accordance with the product aspect of the invent~on there is provided a thin flexible sheet of cellular polystyrene foam, the sheet being in a compressed or crushed -- 2 ~
1~LS~L8~
condition in ~he direction of its thickness and biaxially oriented in the plane of the ~heet so that the cells of the foam are in a generally flattened condition and lie with their major dimensions generally parallel to the major surfaces of the~sheet. The foam has a density from about 8 to about 12 pounds per cubic foot and at least one of the major surfaces of the foam sheet has a densified generally continuous skin thereon having.a thickness of at least abou~ .3 mils.
As noted previously, this thin flexible sheet is particularly useful as a backing board for use in the packaging of foodstuff such as bacon although it will be reali~ed that the product finds use in other applications as well. The sheet is provided with an outline shape which is dependent upon its end use and the skin surface will be provided with printing ink or inksthereon of any desired colours and pattern arrangement in register with one another.
In the preferred form of the product both of the major surfaces of the sheet are provided with the continuous skin. Preferably the skin has a thickness not greater than about 1.0 mils with the density preferably being from about 10 to about 11.5 pounds per cubic foot.
The method recited above is carried out by a novel system for continuously manufacturing flexible sheets of foamed thermo-plastics material, which system includes:
(a) an extruder for extruding and expanding a continuous flat ribbon of foamed plastic material;
~b) means adjacent the extruder for rapidly cooling the opposing major surfaces of ~he ribbon to produce a skin on both of such surfaces;
(c) means for biaxially stretching the extruded ribbon of plastics material;
:~, ~5~82~i (d) a rotatable forming drum for engaging the biaxially stretched ribbon and means to rotate the forming drum at a speed sufficient to accommodate the advancing ribbon;
the improvement characterized by:
- (e) the drum having a surface acting to smooth and flatten the ribbon surface as the ribbon cools;
(f) gauge roll means for compressing the partially cooled ribbon to substantially reduce the thickness thereof and to effect a consequent densification of such foam; and (g) printing press means downstream of the forming drum and gauge rol} means to receive the advancing ribbon and to apply printing inks to a major surface of the partially cooled and stabilized ribbon.
In a further feature means are provided to cause the ribbon to pass along an elongated path of travel between the forming drum and the printing press means, with the length of such path of travel being selected to allow a degree of stabilization and cooling of the ribbon to take place prior to the ribbon entering the printing press means.
Preferably, the printing means comprises a plurality of rolls around which the ribbon passes and one of which rolls in-cludes means for applying printing ink in a preselected pattern to a major surface of the ribbon.
In a typical opexation~ a plurality of said printing press means are provided, each of which is arranged to apply a different ink colour to the ribbon surface whereby to build up multi-colour images on the ribbon.
According to a still further feature a die cutter assembly is located after said plurality of printing press means and is adapted to sever the ribbon all around each of the multi-colour images printed thereon; the die .~` j!
~il5~ 6 cutter assembly being arranged such that the remaining ink free waste ribbon portion emerges in a continuous fashion for collection and re-use.
During operation,the printing press units are dri-~en in synchronism thereby to match the speed of advance of the thermoplastic ribbon being fed thereto. Suitable tensioning means are provided to maintain a desired tension in the ribbon as it passes from the forming drum toward the first press unit.
In order to establish the desired degree of tension in the ribbon r and in order to eliminate the slack in the ribbon which occurs at start-up or after the ribbon has been spliced, the apparatus, in accordance with a further aspect of the invention, is provided with a drive system which can be activated to cause the printing units to operate at a speed som~what higher than the speed of the incoming ribbon. After all slack in the ribbon has been taken up and the ribbon is at the desired tension, the overspeed drive is deactivated and the printing units thereafter operate at a speed which is synchronized with the speed of the incoming ribbon.
Brief Description of the Drawings Typical embodiments of the invention will now be described by way of example with reference to the accompanying drawings wherein:
Figs. lA, lB, and lC illustrate a portion of the production line including the extruder die assembly, the biaxial orientor, the forming drum assembly, and the downstre~am printing units and die cutter unit;
Fig~ 2 is a somewhat diagrammatic plan view of the screw extruder, homogenizer, cooler, and extruder die arrangement as in the T.W.Winstead process;
il2~
Fig. 3 is an end view of the extrusion die and mandrel showing an extruded ribbon of foamed plastic passing over the mandrel as in the T.W.Winstead process;
Fig. 4 is a longitudinal section view of the extruder die and mandrel assembly as in the T.W.Winstead process;
Figs. 5 and 6 are side elevation and plan views respectively of the biaxial orientor as in the T.W.Winstead process;
Fig. 7 is a side elevation view of apparatus for maintaining ribbon tension as it passes toward the printing section;
Fig. 8 illustrates a portion of the printing press drive mechanism including the overspeed mechanism;
Fig. 9 is an illustration of the product i.e. a backing board for bacon products.
Fig. 10 is a perspective view showing the foam ribbon cell structure.
Detailed Description Referring first to Fig. 2 of the drawings, there is shown a more or less conventional extruding machine 10 having a hopper 12 and a screw 14 operating in a barrel 16. The screw 14 is rotated at a speed determined by the desired output by a suitable drive means (not shown). The barrel 16 discharges through an outlet screen assembly 18 via heated insulated conduit 20 into the inlet end of a homogenizer 22 which may be of the type well known in the art under the trademark KENICS
THERMGGENIZER. This device mixes or scrambles the polystyrene material after it leaves the extruder 10. It will of course be understood that the inner surface oc the extruder barrel 16 is very hot and the particles of the polystyrene mixture that are against the inner surface are hotter than the ones midway between the inner surface oE the barrel and the outside surface of the screw. The REMICS THERMOGENIZER includes therein a series of static metal ribbon-like parts alterna-tely twisted in the left hand and right hancl directiond and which are held in the center of the melt stream and thus scramble the melt as it flows through the homogenizer~ This serves to mix ithe hotter particles with ~he less hot particles thus giving uni~orm temperature throughout the melt as well as providing for uniform mixing of the various ingredients of the mel.t.
~ fter leaving the homogenizer 22, the melt passes via insulatea heated conduit 24 into a cooling unit 26. Various types of cooling units for hot plastic melts are commercially available. The cooler should be selected to provide uniform cooling of the melt as substantial uniformity of temperature across the width of the ribbon being extruded is required. No more than about a 5F. variation across the width of the ribbon being extruded is permissible as otherwise brittle streaks may appear in the thin ribbon which can cause cracking or breakage therealong in the final product.
The melt, after Ieaving the cooler 26, then passes into the extruder die 28 which will be described in detail hereinafter.
With continued reference to Figure 2 and in a typical process operation, the hopper 12 of the extruder is supplied with any suitable general purpose polystyrene resin, a particularly preferred resin being one produced by Monsanto Chemical Co., under the trademark LUSTREX polystyrene resin and designated as "yellow 500". A suitable blowing agent is "Freon ll''(Reference may be made to Canadian Patent 843,838, June 9, 1970 of T. W. Winstead).
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*Trademark ~ 1 5~
The blowing agent is injected at an intermediate point of the extruder by means o~ a pump 30 and inlet pipe 32.
The injection pressure will be in the order of 3500 pounds per square inch which figure will, of course, vary somewhat depending upon process conditions. The melt pressure at the outlet of the extruder is between about 6000 and 7000 pounds per square inch. The temperature of the melt at this same point is typically about 370F. By the time the melt reaches the extruder die 28 it will have been cooled down to a temperature of about 29~-295F.
Temperature control is of considerable importance, it being noted here that overly high temperatures tend to produce a brittle ribbon.
Reference will now be had to Figures 3 and 4 which show the extruder die and mandrel assembly 28 in detail. For a detailed description of this part of the apparatus, reference may be had to Canadian Patent No. 834,029 issued February 10th, 1970 to Thomas W. Winstead. Basically the extruder embodies a die lip arrangement wherein the discharge slot is arcuate in shape and fed from a feed chamber which diverges from a substantially central inlet point to the arcuate lips. This arrangement provides a uniform distribution of pressure flow at all points along the lip of the die and thereby eliminates strains or distortions in the extruded material and accommodates maximum width-wise expansion of the extruded material thus preventing formation of wrinkles therein. As best seen in Figure 4, the extrusion die assembly includes a main body 36 with a feed passage 38 extending longitudinally through the body. A die head 40 is located at one end of the body. A
first die lip 42 is secured to the die head in radial relation to the passageway 38. An end cap 44 is secured to the die head with its inner face adjacent to but longitudinally spaced s~
from the first die lip 42 to thereby form a complementary second die lip 4~ spaced from the irst die lip. ~n extrusion chamber 48 is bounded by a rear wall which converges from a point adjacent the axis of the plastic feeding passage toward the die lips, and by side walls 50 (see Flg.3) which diverge from said axis toward the die lips. The end cap 44 is provided with a substantially conical recess 51 which is concentric with the feeding passage. An adapter 53 is disposed in the recess and is contoured to define the diverging side walls 50 of the chamber. The extrusion orifice be~ween the spaced die lips 46 and 42 is transversely arcuate with respect to the die head whereby the extruded material from chamber 48 may be extruded radially in a diverging path.
As described in detail in the above noted Canadian patent 834,029, a spreading mandrel 52 is provided adjacent the die lips. The spreading mandrel 52 comprises a vertically disposed transversely arcuate tube mounted on a cross bar 54 which, in turn, is mounted on a threaded adjustment device 56 thereby to permit the elevation of same to be varied relative to the die lips. Mandrel 52 is in the form of a tube through which a cooling fluid is passed to prevent sticking of the extruded material to the tube as it passes over the mandrel.
To further prevent sticking the surface of the mandrel is coated with a suitable non-sticking material such as polytetrafluoro-ethylene. When the mandrel 52 is suitably adjusted with respect to the die head, it will be found that in the space between the arcuate die orifice and the mandrel that the corrugations or wrinkles of the expanding cellular sheet are very efficiently and quickly removed. Optimum efficiency is achieved when the rate of lateral expansion of the extruded material coincides with the rate of flow ~etween the die orifice and the mandrel; when this condition is achieved no wrinkles or corrugations even begin to form. For best results, the mandrel should be maintained at a temperature between 60 and With further reference to Figures 3 and 4 it will be seen that currents of cool air are applied to the opposing surfaces of the ribbon immediately after emergence thereof from the arcuate die orifice. These currents of cool air serve to quickly cool the opposing major surfaces of the ribbon thus inhibiting the development of cells on the foam surfaces, which action, in turn, is responsible for the appearance on the foam surfaces of a substantially continuous skin, the thickness of which is from about 0.3 mils to about 1.0 mils with the skin thickness preferably being kept on the low side of this range for best results. The means for sup~lying the currents of cool air comprise a pair of air lips 55 located on opposing sides of the arcuate die orifice and being co-extensive therewith.
Air lips 55 each include a curved slit-like opening 57 which directs air from a source (not shown) on to the opposing ribbon surfaces just as the ribbon emerges from the die oriflce. ~he curved slit-like openings 56 are parallel to the die openings and direct the cool air generally downwardly toward the die orifice. The air is maintained at a temperature of about 70F.
The operator observes the emerging ribbon and adjusts the air pressure to get the desired skin thickness i.e. the greater the air pressure and flow, the greater the skin thickness. The rapid cooling effec~ solidifj.es the ribbon surface before it expands. The "freezing" of the surfaces can be observed readily i.e. a so-called "frost line" can be observed along the line where the air strikes the ribbon surfaces. The operator can adjust the air flows to obtain a "frost line" on the surface and can, w_~h a little practice, obtain the desired skin thickness. For a further teaching of this aspect of the process reference may be had to U.~. patents 3,461,496; 3,632,266;
3,670,059 and 3,676,537, of T.W.Winstead.
31 ~S~826 After passing over khe mandrel S2 the ribbon Rof extruded material passes toward the bia~ial orienter 60 illustrated in Figs. lA, 5 and 6. ~he biaxial orienter employs a first pair of rolLs 62, a pair of toothed wheels 64 positioned immediately after rolls 62, and a plurality of transfer rolls 66 disposed after the toothed wheels 64. It will be seen that the toothed wheels 64 are of relatively large diameter in comparison to rolls 62, 66 and that the teeth 68 on the periphery thereof are arranged to bite into opposing marginal edges of ribbon R as the latter travels therearound. As best seen in Fig. 6, these toothed wheels 64 are arranged in a diverging relationship relative to one another such that as the ribbon R
travels therearound, it is stretched in the transverse direction.
An adjustment means 70 is provided thereby to permit the degree of transverse stretch of the ribbon to be varied somewhat.
Preferably the amount of transverse stretch is in a two to one ratio. The sets of rolls 62 and 66 are also driven at a speed such that the ribbon R is stretched in the longitudinal direction to provide a stretch ratio of about two to one as well. This stretching takes place primarily between the mandrel 52 and the first roll of the set of rolls 62. The rolls 66 of the transfer roll system are divided into upper and lower groups, each group being mounted to a respective support frame 72, the support frames 72 being adjustable toward and away from one another thereby to allow the angle of wrap of the ribbon around the individual rolls of the system to be varied. The upper frame 72 can also be swung u~ to allow the ribbon R to be threaded through the rolls at start up. By causing the ribbon to travel in an undu-lating path between the sets of rolls, the ribbon is allowed to cool and stabilize somewhat thus preventing shrin~age of theribbon in the lateral direction. It should be noted at this point that the ribbon temperature is about 230F. at the point where longitudinal stretching begins to take place. In order to ~2-~lS~
promote more rapid stabilization of the ribbon, it is highly desirable to cool the first three or four rolls of the transfer roll set 66. This may be accomplished by passing fluid through such rolls in the manner well known generally in the art. The mean temperature of the ribbon as it emerges from the transfer roll system 66 should not be greater than about 180F. If the temperature is significantly greater than thls, a substantial amount of llshrinkback" of the web may occur ln the transverse direct.ion.
If shrinkback problems are encountered, the frames 72 can be moved closer toward one another thus increasing the degree of surface contact between the ribbon and the rolls 66 thus increasing the amount of lateral friction between the rolls and the ribbon to eliminate lateral shrinkback.
The ribbon then passes toward a rorming drum 80~ The forming drum comprises an aluminum drum having a smooth cylindrical surface with which the ribbon comes into contact. As seen in Fig. lA, the forming drum 80 is of relatively large diameter, for example, about 5 feet in diameter, such drum being mounted ;~
for rotation about a horizontal axis 82 and being driven at a constant rate of rotation via a suitable motor and gear reduction drive (not shown) at a speed commensurate with the speed of ~advance of the ribbon as it passes from the biaxial orienter and transfer roll system. The forming drum 80 is preferably mounted in a partial enclosure 84 within which are disposed fans 86 and 88 which project currents of air toward the ribbon which is being carried around the periphery of the drum thereby to effect further cooling and stabiliza~ion of the ribbon. By .
adjusting these air currents, the forming drum surface can be maintained at a temperature of about 110-120F.
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With further reference to Fig. lA it will be seen that the ribbon R moves toward and onto the surface of the forming drum 80 by way of guide rolls 90 with -the ribbon then passing through a nip 92 defined between the forming drum surface and a chill roll 94O This roll 94 is maintained at a surface temperature o about 65F. and its primary purpose is to effect further cooling of the ribbon while at the same time effecting a partial calendaring of such ribbon. By way-of example, it i5 noted here that the ribbon thickness out of the forming die assembly 28 is approximately 0.125 inch in thickness. After the biaxial orientation, ribbon thickness is about .08 inch i.e.
80 mils. After passing through nip 92 between forming drum 80 and chill roll 94, the ribbon thickness is approximately 0.050 inch.
The ribbon subsequently enters a further nip 96 defined between forming drum 80 and a gauging roll 98. This gauging roll 98 is positioned relative to the forming drum surface such that the ribbon can be brought toward its final thickness of approximately 0.020 inch. The final thickness is achieved later on at the first press unit at the nip between rolls 150, 152. Those skilled in the art will of course appreciate that these figures are given by way of example only and the amount of calendering or transverse compression can be varied somewhat thereby to provide the desired degree of ribbon thickness.
With particular reference to Figs. lA, lB and lC, it will be seen that the ribbon R moves away from the forming drum 80 in an elongated path of travel defined by guide rolls 100, 102, 104 over a splicing table 106 and thence via infeed tension unit 108 and draw roll system 150, 152 into the printing assembly 110. Th~ printing assembly 110 comprises a plurality of serially arranged printing units of which only two, i.e. 112 and 114 are illustrateZ. The number of printing units is, of course, dependent upon the number of colours to be applied to the ribbon surface.
It .s noted here that in a typical operation a , .
. ., --- 1~5~826 ribbon speed of about 150 feet per minute is common. By way of example the distance between the forming drum 80 and the first press unit 112 may be about 30 feet. The travel time of ribbon R from forming drum 80 to the first press unit 112 provides additional time which allows the uncured ribbon to further stabilize and cool. It will of course be kept in mind that ribbon R has relatively low thermal conductivity and thus this additional cooling time is desirable thereby to allow the interior portions of the ribbon to become more stable. If 1~ insufficient time is allowed between the forming drum 80 and the first press unit 112, the ribbon may be overly soft and the printing unit may tend to emboss the ribbon surface in an undesirable manner and the ribbon may elongate thus causing registration problems as the ribbon passes through the several press units.
The space provided also has another practical purpose in that it provides the necessary room for an elongated spllcing ;~
table 106 which facilitates manual splicing of the continually extruded foam ribbon to the stationary leader which has been pre-threaded along the ribbon path through the printing and die sections At this point it is desirable to refer to Figs. 7and 8 which illustrate the ribbon tensioning device and the printing section overspeed drive assemblies respectively. The infeed tensioning assembly includes a spaced apart pair of rolls 116, 118 mounted between opposing ends of a pair o~ arms 120, ~only one arm being shown in Fig. 7). Arms 120, in turn, are rotatably mounted at their mid-point for rotation on a support member 122 which is affixed to the frame of the first printing unit 112. The rolls 116 and 118 are back pressured by an air cylinder ~24 and lin~age assembly 126 in a clockwise direction to resist the counter-clockwise torque caused by tightenlng ribbon tension. A pressure regulating valve (not shown) can preset the air pressure to regulate the ~L~5~8Z6 tension in the ribbon. By observing the position and direction of rot~tion OI arms 120~ the operator can determine if the ribbon is tightening or loosening or is remaining at the predetermined ribbon tension.
Reference may now be had to Fig. 8 which shows a portion of the press assembly drive train. There is shown a main drive unit 130 which is of any suitable conventional construction and having mounted on its output shaft a pair of axially spaced drive sprockets 132 and 134. The drive sprocket 134 is of somewhat larger diameter than drive sprocket 132;
sprocket 134 may have 48 teeth,for example, while sprocket 132 may have, for example,40 teeth. The sprocket 132 is coupled to the drive shaft via a one-way overrunning clutch 136 of any desired standard construction. The sprocket 134 is coupled to drive shaft 134 via an air cylinder operated clutch 138, such air actuated clutch also being of any standard commerclally available variety. The sprocket 134 is coupled to sprocket 140 while sprocket 132 is coupled to sprocket 142. Sprockets 140 and 142 are of the same diameter and they are both affixed to line shaft 144, such line shaft 144 extending alongside the several printing units and the die cutting unit 170 and being coupled thereto in a conventional manner (not shown). During ordinary operation of the system, the main drive 130 drives the printing units via sprocket 132 which in turn drives sprocket 1~2 to rotate line shaft 144 at a predetermined speed thereby driving the various press units at a speed which is synchronized with the speed of travel of the incoming ribbon R. During this mode of operation the air operated clutch 138 is deactivated so that sprocket 134 simply free-wheels on the main drive shaft. Howevex, during start-up operations, as will be described more ~ully hereinaEter, it is - ~6 --~15~3%~
desired to operate the printing press and ~ie cutting sections at a higher than normal rate of speed and in this instance, the air operated clutch is activated thereby to engage sprocket 134 with the main drive shaft. Since sprocket 134 is larger than sprocket 132 it obviously drives sprocke-t 140 at a higher rate of speed thus resulting in a higher speed of operation of the printing and die cutting u~its. For example, when the ribbon splice is made with the continuously moving ribbon to the stationary ribbon leader in the press section, a press start-up button is manually operated and the press section runs at a speed synchronized with the speed of the inccming ribbon i.e. the a~ove described sprocket 132 drives sprocket 142. At this point, there is usually a considerable degree of slackness in the ribbon before the draw rolls 150 and 152 and scm.e slackness in the ribbon going through the press section. At the start-up point this can be observed by the position of the arms 120 of the infeed tension assembly 108. At this point, the operator presses a press speed-up button which serves to activate the air operated clutch 138 thus automatically shifting the press and die cutting sections into the overspeed mode of operation. m is overspeed may be about 10% higher than the speed of the incoming ribbon. The operator holds the speed-up button down until the slackness in the ribkon disappears and the arm 120 revolves to its medial position. m e operator then releases the speed-up button and the press section returns to synchronous speed, matching the speed of the incc~ing extruded rib~onO
~ ith further reference to Figs. lB and lC, it is noted here that the several prlnting units 112 are sc~ewhat modified flexigraphic printing units utilizing rubber printing plates. The modifications basically concern anti-backlash gears in the plate cylinder gear train to cc~pensate for the thickness of -the ribbon being printed ard to maintain inter-colour register, and the use of large diameter ribbon carrier rolls to reduce curl effects in the uncured rib~on. The carrier rolls are designed for relatively lcw inertia, are well balanced, and use 1GW friction bearings to reduce drag on the ribbon and avoid stretching of ~he ribbon. Since other aspects of this flexographic printing process are well knawn in the art of printing a detailed description is considered unnecessary. Other printing processes, such as rotogravure, etc. can be used.
It will be seen from ~15182~
the drawings that the incoming ribbon R passes between a draw roll 150 and a chilled roll 152. ~s noted previously, these rolls may be used to further compress the ribbon to control the web thickness. The ribbon then passes downwardly around a series of rolls 15~ and thence upwardly through a nip defined between a plate cylinder 156 and an impression roll 158. An engraved inker roll 160 rotates in contact with the plate cylinder 156, the inker roll 160 being i~mersed in a bath of ink contained in trough 162, with a suitable doctor blade 164 being provided to remove excess ink from the inker roll. The printed ribbon then passes upwardly around guide roll 166 and through a conventional dryer arrangement 168 which serves to dry the applied ink following which the ribbon passes on to the next printing uni~. The printing units are all o~ essentially the same construction as that described above and the number of such printing units is, of course, dependent on the number of colours to be applied. A simple two colour system would require the use of only two printing units but for most purposes four or five printing units will be required.
Insofar as the printing inks are concerned, various conventional printing inks based on nitrocellulose or acrylic resins were found to be reasonably satisfactory. The best combination of properties insofar as flexibility, adhesion, slip, abrasion resistance and odour are concerned was obtained using the "Flexo Styrene" series of inks produced by General Printing Ink Company, Toronto,Canada. This series of inks is based on a combination of nitrocellulose and cellulose acetate propionate. The solvents used with the ink are denatured anhydrous ethyl alcohol, normal propyl acetate and normal propyl alcohol. The addition of one percent micronized polyethylene to the ink provides additional slip~
After the ribbon e~erges from the printing section ~L~S~l~3Z6 110 it enters into a die cutter and delivery unit 170. ~his unit is illustrated diagra~matically in Figure lC. Again, since units of this nature are basically well known in the art a detailed description of same is considered unnecessary. The ribbon first passes around a cooled roll 172, around guide roll 174 and thence between the rolls of a scoring unit 176. At this point it is advisable to refer to Fig. 9 which illustrates a segment of the ribbon R and illustrates the outline shape of the die cut bacon board blanks thereon. It will be seen that each bacon board includes a flap portion F and a body portion B. Between the flap portion F and body portion B there is a continuous score line S which has a series of spaced apart fine slits P
therealong. These slits facilitate folding of the flap portion relative to the bcdy portion. Th~ score line S is provided by scoring unit 176. Followlng the scoring unit, the ribbon passes dcwnwardly into the nip b~tween cutter roll 180 and anvil roll 182.- The cutter roll is provided with a raised sharp edged lip which, r~hen it comes into contact with the ribbon R, penetrates therethrough and severs such ribbon all around the outline contour of the bacon board which has been printed on the ribbon surface. At the same time the roll 180 also cuts the above noted slits P along the score line S. After this operation, the ribbon passes dcwnwardly around delivery roll 184 and be-20 tween guide rolls 186 following which the ribkon waste is drawn vertically up-wardly at a sharp angle with such action causing the cut bacon koards to pass outwardly onto a slowly moving surface 188 of a delivery table r,~th the result being that the bacon boards are deposited on such takle in an over-lapping shingle-like fashion. The remaining ladder-like waste portion of the ribbon is then forwarded to a waste cutter with the waste material being ultimately collected and ultimately fed back into the extruder hopper 12 with the result being that there is virtually no wastage of materials.
This is possible since the printed design is spaced about 1/8 of an inch inwardly of the die cut edge all around the die cut edge thus avoiding ink contamination of the waste ribbon material, which contamination would otherwise ultimately adversely dis-colour the extruded ribbon webs.
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Reference may now be had to Table 1 which illustrates the physical characteristics of a number of foam products. Samples No. 2-5 were made generally in accordance with the principles of the present invention. Samples No. 3 and 4 were considered to have the best properties insofar as the purposes of the present invention are concerned. Sample No.
6 which was included here for purposes of comparison is a relatively low density foam meat tray; this foam was not bi-axially oriented after extrusion, nor was it provided with skin surfaces nor was the foam compressed or calendared thereby to control the thickness thereof as in the other samples. Table 1 should be reviewed in conjunction with ~igure 10 which illustrates the cellular structure of the foam as well as the manner in which cell size is measured and determined.
It will be seen from a review of Table l and Fig.
10 that the cells of the bacon board samples have a relatively squashed or flattened appearance with diameters in the machine direction ranging from about .23 to .28 mm. and diameters in the cross machine direction varying from about .2~ to about .30
~OAM SHEETS
.
Background of the Invention This invention relates generally to the field of packaging and more particuLarly relates to continuous in~line process and apparatus for producing thin printed sheets or boards of expanded polystyrene ~oam which boards are useful, for example, in the packaging of bacon products.
Although the in~ention will be described with particular reference to bacon boards it should be understood that the invention has other applications as well such as in chocolate box liners, cookie bag liners, place mats, bar coasters, and tops for frozen foods and the like.
The board presently used in the packaging of bacon is a printed paperboard which is coated on both sides with a layer of polyethylene plastic. Although this orm of board has met with wide acceptance it does possess a number of disadvantages. For example, the cut edges of the printed board tend to absorb grease and moisture and furthermore the composite paperboard products may tend to harbour bacteria.
Furthermore, the present relatively rigid composite paperboard has a tendency to puncture the overwrapped packaging film both during shipping and also during store handling and it is desirable to provide a softer board to assist in eliminating this problem. It is also considered desirable to provide a packaging material which is light in weight thus ultimately resulting in less waste to incinerate or dispose of.
The present invention thus broadly involves the making of a thin flat sheet of polystyrene foam and thereafter printing and cutting the sheet in a continuous ln-line operation while the foam is still in an uncured and somewhat unstable condition. The process can be carried out very ~15~L826 economically and at a relatively high rate of production.
Summary of the Invention In accordance with the invention in one aspect there is provided a method of continuously manufacturing flexible sheets of cellular polystyrene foam material, including the steps of: .
(a) passing a molten pol~styrene plastic material to whi~h a blowing agent has been added through a die oxifice shaped to al}ow the material to expand into a ribbon of foamed plastics material;
(b) effecting an initial cooling of the opposing major surfaces of the ribbon on emergence from the die orifice to form a thin skin on the surfaces of the ribbon;
(c) biaxially stretching the ribbon of material as it cools;
(d) effecting further cooling and forming of the ribbon by passing it on to a surface of a rotating forming drum;
the method being characterized by the steps of:
(e) compressing the partly cooled ribbon to substantially reduce the thickness thereof by passing the ribbon in pressurized contact with a surface of a roll means;
(f) before or after step (e), passing the ribbon in a path of travel to effect further cooling and stabilization of same;
(g) passing the partially cooled and stabilized ribbon in a further path of travel through a printing press to apply printing inks to a surface thereof, and (h) subsequently die cutting printed sheets o a selected outline shape ~rom ~he ribbon.
In accordance with the product aspect of the invent~on there is provided a thin flexible sheet of cellular polystyrene foam, the sheet being in a compressed or crushed -- 2 ~
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condition in ~he direction of its thickness and biaxially oriented in the plane of the ~heet so that the cells of the foam are in a generally flattened condition and lie with their major dimensions generally parallel to the major surfaces of the~sheet. The foam has a density from about 8 to about 12 pounds per cubic foot and at least one of the major surfaces of the foam sheet has a densified generally continuous skin thereon having.a thickness of at least abou~ .3 mils.
As noted previously, this thin flexible sheet is particularly useful as a backing board for use in the packaging of foodstuff such as bacon although it will be reali~ed that the product finds use in other applications as well. The sheet is provided with an outline shape which is dependent upon its end use and the skin surface will be provided with printing ink or inksthereon of any desired colours and pattern arrangement in register with one another.
In the preferred form of the product both of the major surfaces of the sheet are provided with the continuous skin. Preferably the skin has a thickness not greater than about 1.0 mils with the density preferably being from about 10 to about 11.5 pounds per cubic foot.
The method recited above is carried out by a novel system for continuously manufacturing flexible sheets of foamed thermo-plastics material, which system includes:
(a) an extruder for extruding and expanding a continuous flat ribbon of foamed plastic material;
~b) means adjacent the extruder for rapidly cooling the opposing major surfaces of ~he ribbon to produce a skin on both of such surfaces;
(c) means for biaxially stretching the extruded ribbon of plastics material;
:~, ~5~82~i (d) a rotatable forming drum for engaging the biaxially stretched ribbon and means to rotate the forming drum at a speed sufficient to accommodate the advancing ribbon;
the improvement characterized by:
- (e) the drum having a surface acting to smooth and flatten the ribbon surface as the ribbon cools;
(f) gauge roll means for compressing the partially cooled ribbon to substantially reduce the thickness thereof and to effect a consequent densification of such foam; and (g) printing press means downstream of the forming drum and gauge rol} means to receive the advancing ribbon and to apply printing inks to a major surface of the partially cooled and stabilized ribbon.
In a further feature means are provided to cause the ribbon to pass along an elongated path of travel between the forming drum and the printing press means, with the length of such path of travel being selected to allow a degree of stabilization and cooling of the ribbon to take place prior to the ribbon entering the printing press means.
Preferably, the printing means comprises a plurality of rolls around which the ribbon passes and one of which rolls in-cludes means for applying printing ink in a preselected pattern to a major surface of the ribbon.
In a typical opexation~ a plurality of said printing press means are provided, each of which is arranged to apply a different ink colour to the ribbon surface whereby to build up multi-colour images on the ribbon.
According to a still further feature a die cutter assembly is located after said plurality of printing press means and is adapted to sever the ribbon all around each of the multi-colour images printed thereon; the die .~` j!
~il5~ 6 cutter assembly being arranged such that the remaining ink free waste ribbon portion emerges in a continuous fashion for collection and re-use.
During operation,the printing press units are dri-~en in synchronism thereby to match the speed of advance of the thermoplastic ribbon being fed thereto. Suitable tensioning means are provided to maintain a desired tension in the ribbon as it passes from the forming drum toward the first press unit.
In order to establish the desired degree of tension in the ribbon r and in order to eliminate the slack in the ribbon which occurs at start-up or after the ribbon has been spliced, the apparatus, in accordance with a further aspect of the invention, is provided with a drive system which can be activated to cause the printing units to operate at a speed som~what higher than the speed of the incoming ribbon. After all slack in the ribbon has been taken up and the ribbon is at the desired tension, the overspeed drive is deactivated and the printing units thereafter operate at a speed which is synchronized with the speed of the incoming ribbon.
Brief Description of the Drawings Typical embodiments of the invention will now be described by way of example with reference to the accompanying drawings wherein:
Figs. lA, lB, and lC illustrate a portion of the production line including the extruder die assembly, the biaxial orientor, the forming drum assembly, and the downstre~am printing units and die cutter unit;
Fig~ 2 is a somewhat diagrammatic plan view of the screw extruder, homogenizer, cooler, and extruder die arrangement as in the T.W.Winstead process;
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Fig. 3 is an end view of the extrusion die and mandrel showing an extruded ribbon of foamed plastic passing over the mandrel as in the T.W.Winstead process;
Fig. 4 is a longitudinal section view of the extruder die and mandrel assembly as in the T.W.Winstead process;
Figs. 5 and 6 are side elevation and plan views respectively of the biaxial orientor as in the T.W.Winstead process;
Fig. 7 is a side elevation view of apparatus for maintaining ribbon tension as it passes toward the printing section;
Fig. 8 illustrates a portion of the printing press drive mechanism including the overspeed mechanism;
Fig. 9 is an illustration of the product i.e. a backing board for bacon products.
Fig. 10 is a perspective view showing the foam ribbon cell structure.
Detailed Description Referring first to Fig. 2 of the drawings, there is shown a more or less conventional extruding machine 10 having a hopper 12 and a screw 14 operating in a barrel 16. The screw 14 is rotated at a speed determined by the desired output by a suitable drive means (not shown). The barrel 16 discharges through an outlet screen assembly 18 via heated insulated conduit 20 into the inlet end of a homogenizer 22 which may be of the type well known in the art under the trademark KENICS
THERMGGENIZER. This device mixes or scrambles the polystyrene material after it leaves the extruder 10. It will of course be understood that the inner surface oc the extruder barrel 16 is very hot and the particles of the polystyrene mixture that are against the inner surface are hotter than the ones midway between the inner surface oE the barrel and the outside surface of the screw. The REMICS THERMOGENIZER includes therein a series of static metal ribbon-like parts alterna-tely twisted in the left hand and right hancl directiond and which are held in the center of the melt stream and thus scramble the melt as it flows through the homogenizer~ This serves to mix ithe hotter particles with ~he less hot particles thus giving uni~orm temperature throughout the melt as well as providing for uniform mixing of the various ingredients of the mel.t.
~ fter leaving the homogenizer 22, the melt passes via insulatea heated conduit 24 into a cooling unit 26. Various types of cooling units for hot plastic melts are commercially available. The cooler should be selected to provide uniform cooling of the melt as substantial uniformity of temperature across the width of the ribbon being extruded is required. No more than about a 5F. variation across the width of the ribbon being extruded is permissible as otherwise brittle streaks may appear in the thin ribbon which can cause cracking or breakage therealong in the final product.
The melt, after Ieaving the cooler 26, then passes into the extruder die 28 which will be described in detail hereinafter.
With continued reference to Figure 2 and in a typical process operation, the hopper 12 of the extruder is supplied with any suitable general purpose polystyrene resin, a particularly preferred resin being one produced by Monsanto Chemical Co., under the trademark LUSTREX polystyrene resin and designated as "yellow 500". A suitable blowing agent is "Freon ll''(Reference may be made to Canadian Patent 843,838, June 9, 1970 of T. W. Winstead).
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The blowing agent is injected at an intermediate point of the extruder by means o~ a pump 30 and inlet pipe 32.
The injection pressure will be in the order of 3500 pounds per square inch which figure will, of course, vary somewhat depending upon process conditions. The melt pressure at the outlet of the extruder is between about 6000 and 7000 pounds per square inch. The temperature of the melt at this same point is typically about 370F. By the time the melt reaches the extruder die 28 it will have been cooled down to a temperature of about 29~-295F.
Temperature control is of considerable importance, it being noted here that overly high temperatures tend to produce a brittle ribbon.
Reference will now be had to Figures 3 and 4 which show the extruder die and mandrel assembly 28 in detail. For a detailed description of this part of the apparatus, reference may be had to Canadian Patent No. 834,029 issued February 10th, 1970 to Thomas W. Winstead. Basically the extruder embodies a die lip arrangement wherein the discharge slot is arcuate in shape and fed from a feed chamber which diverges from a substantially central inlet point to the arcuate lips. This arrangement provides a uniform distribution of pressure flow at all points along the lip of the die and thereby eliminates strains or distortions in the extruded material and accommodates maximum width-wise expansion of the extruded material thus preventing formation of wrinkles therein. As best seen in Figure 4, the extrusion die assembly includes a main body 36 with a feed passage 38 extending longitudinally through the body. A die head 40 is located at one end of the body. A
first die lip 42 is secured to the die head in radial relation to the passageway 38. An end cap 44 is secured to the die head with its inner face adjacent to but longitudinally spaced s~
from the first die lip 42 to thereby form a complementary second die lip 4~ spaced from the irst die lip. ~n extrusion chamber 48 is bounded by a rear wall which converges from a point adjacent the axis of the plastic feeding passage toward the die lips, and by side walls 50 (see Flg.3) which diverge from said axis toward the die lips. The end cap 44 is provided with a substantially conical recess 51 which is concentric with the feeding passage. An adapter 53 is disposed in the recess and is contoured to define the diverging side walls 50 of the chamber. The extrusion orifice be~ween the spaced die lips 46 and 42 is transversely arcuate with respect to the die head whereby the extruded material from chamber 48 may be extruded radially in a diverging path.
As described in detail in the above noted Canadian patent 834,029, a spreading mandrel 52 is provided adjacent the die lips. The spreading mandrel 52 comprises a vertically disposed transversely arcuate tube mounted on a cross bar 54 which, in turn, is mounted on a threaded adjustment device 56 thereby to permit the elevation of same to be varied relative to the die lips. Mandrel 52 is in the form of a tube through which a cooling fluid is passed to prevent sticking of the extruded material to the tube as it passes over the mandrel.
To further prevent sticking the surface of the mandrel is coated with a suitable non-sticking material such as polytetrafluoro-ethylene. When the mandrel 52 is suitably adjusted with respect to the die head, it will be found that in the space between the arcuate die orifice and the mandrel that the corrugations or wrinkles of the expanding cellular sheet are very efficiently and quickly removed. Optimum efficiency is achieved when the rate of lateral expansion of the extruded material coincides with the rate of flow ~etween the die orifice and the mandrel; when this condition is achieved no wrinkles or corrugations even begin to form. For best results, the mandrel should be maintained at a temperature between 60 and With further reference to Figures 3 and 4 it will be seen that currents of cool air are applied to the opposing surfaces of the ribbon immediately after emergence thereof from the arcuate die orifice. These currents of cool air serve to quickly cool the opposing major surfaces of the ribbon thus inhibiting the development of cells on the foam surfaces, which action, in turn, is responsible for the appearance on the foam surfaces of a substantially continuous skin, the thickness of which is from about 0.3 mils to about 1.0 mils with the skin thickness preferably being kept on the low side of this range for best results. The means for sup~lying the currents of cool air comprise a pair of air lips 55 located on opposing sides of the arcuate die orifice and being co-extensive therewith.
Air lips 55 each include a curved slit-like opening 57 which directs air from a source (not shown) on to the opposing ribbon surfaces just as the ribbon emerges from the die oriflce. ~he curved slit-like openings 56 are parallel to the die openings and direct the cool air generally downwardly toward the die orifice. The air is maintained at a temperature of about 70F.
The operator observes the emerging ribbon and adjusts the air pressure to get the desired skin thickness i.e. the greater the air pressure and flow, the greater the skin thickness. The rapid cooling effec~ solidifj.es the ribbon surface before it expands. The "freezing" of the surfaces can be observed readily i.e. a so-called "frost line" can be observed along the line where the air strikes the ribbon surfaces. The operator can adjust the air flows to obtain a "frost line" on the surface and can, w_~h a little practice, obtain the desired skin thickness. For a further teaching of this aspect of the process reference may be had to U.~. patents 3,461,496; 3,632,266;
3,670,059 and 3,676,537, of T.W.Winstead.
31 ~S~826 After passing over khe mandrel S2 the ribbon Rof extruded material passes toward the bia~ial orienter 60 illustrated in Figs. lA, 5 and 6. ~he biaxial orienter employs a first pair of rolLs 62, a pair of toothed wheels 64 positioned immediately after rolls 62, and a plurality of transfer rolls 66 disposed after the toothed wheels 64. It will be seen that the toothed wheels 64 are of relatively large diameter in comparison to rolls 62, 66 and that the teeth 68 on the periphery thereof are arranged to bite into opposing marginal edges of ribbon R as the latter travels therearound. As best seen in Fig. 6, these toothed wheels 64 are arranged in a diverging relationship relative to one another such that as the ribbon R
travels therearound, it is stretched in the transverse direction.
An adjustment means 70 is provided thereby to permit the degree of transverse stretch of the ribbon to be varied somewhat.
Preferably the amount of transverse stretch is in a two to one ratio. The sets of rolls 62 and 66 are also driven at a speed such that the ribbon R is stretched in the longitudinal direction to provide a stretch ratio of about two to one as well. This stretching takes place primarily between the mandrel 52 and the first roll of the set of rolls 62. The rolls 66 of the transfer roll system are divided into upper and lower groups, each group being mounted to a respective support frame 72, the support frames 72 being adjustable toward and away from one another thereby to allow the angle of wrap of the ribbon around the individual rolls of the system to be varied. The upper frame 72 can also be swung u~ to allow the ribbon R to be threaded through the rolls at start up. By causing the ribbon to travel in an undu-lating path between the sets of rolls, the ribbon is allowed to cool and stabilize somewhat thus preventing shrin~age of theribbon in the lateral direction. It should be noted at this point that the ribbon temperature is about 230F. at the point where longitudinal stretching begins to take place. In order to ~2-~lS~
promote more rapid stabilization of the ribbon, it is highly desirable to cool the first three or four rolls of the transfer roll set 66. This may be accomplished by passing fluid through such rolls in the manner well known generally in the art. The mean temperature of the ribbon as it emerges from the transfer roll system 66 should not be greater than about 180F. If the temperature is significantly greater than thls, a substantial amount of llshrinkback" of the web may occur ln the transverse direct.ion.
If shrinkback problems are encountered, the frames 72 can be moved closer toward one another thus increasing the degree of surface contact between the ribbon and the rolls 66 thus increasing the amount of lateral friction between the rolls and the ribbon to eliminate lateral shrinkback.
The ribbon then passes toward a rorming drum 80~ The forming drum comprises an aluminum drum having a smooth cylindrical surface with which the ribbon comes into contact. As seen in Fig. lA, the forming drum 80 is of relatively large diameter, for example, about 5 feet in diameter, such drum being mounted ;~
for rotation about a horizontal axis 82 and being driven at a constant rate of rotation via a suitable motor and gear reduction drive (not shown) at a speed commensurate with the speed of ~advance of the ribbon as it passes from the biaxial orienter and transfer roll system. The forming drum 80 is preferably mounted in a partial enclosure 84 within which are disposed fans 86 and 88 which project currents of air toward the ribbon which is being carried around the periphery of the drum thereby to effect further cooling and stabiliza~ion of the ribbon. By .
adjusting these air currents, the forming drum surface can be maintained at a temperature of about 110-120F.
32~
With further reference to Fig. lA it will be seen that the ribbon R moves toward and onto the surface of the forming drum 80 by way of guide rolls 90 with -the ribbon then passing through a nip 92 defined between the forming drum surface and a chill roll 94O This roll 94 is maintained at a surface temperature o about 65F. and its primary purpose is to effect further cooling of the ribbon while at the same time effecting a partial calendaring of such ribbon. By way-of example, it i5 noted here that the ribbon thickness out of the forming die assembly 28 is approximately 0.125 inch in thickness. After the biaxial orientation, ribbon thickness is about .08 inch i.e.
80 mils. After passing through nip 92 between forming drum 80 and chill roll 94, the ribbon thickness is approximately 0.050 inch.
The ribbon subsequently enters a further nip 96 defined between forming drum 80 and a gauging roll 98. This gauging roll 98 is positioned relative to the forming drum surface such that the ribbon can be brought toward its final thickness of approximately 0.020 inch. The final thickness is achieved later on at the first press unit at the nip between rolls 150, 152. Those skilled in the art will of course appreciate that these figures are given by way of example only and the amount of calendering or transverse compression can be varied somewhat thereby to provide the desired degree of ribbon thickness.
With particular reference to Figs. lA, lB and lC, it will be seen that the ribbon R moves away from the forming drum 80 in an elongated path of travel defined by guide rolls 100, 102, 104 over a splicing table 106 and thence via infeed tension unit 108 and draw roll system 150, 152 into the printing assembly 110. Th~ printing assembly 110 comprises a plurality of serially arranged printing units of which only two, i.e. 112 and 114 are illustrateZ. The number of printing units is, of course, dependent upon the number of colours to be applied to the ribbon surface.
It .s noted here that in a typical operation a , .
. ., --- 1~5~826 ribbon speed of about 150 feet per minute is common. By way of example the distance between the forming drum 80 and the first press unit 112 may be about 30 feet. The travel time of ribbon R from forming drum 80 to the first press unit 112 provides additional time which allows the uncured ribbon to further stabilize and cool. It will of course be kept in mind that ribbon R has relatively low thermal conductivity and thus this additional cooling time is desirable thereby to allow the interior portions of the ribbon to become more stable. If 1~ insufficient time is allowed between the forming drum 80 and the first press unit 112, the ribbon may be overly soft and the printing unit may tend to emboss the ribbon surface in an undesirable manner and the ribbon may elongate thus causing registration problems as the ribbon passes through the several press units.
The space provided also has another practical purpose in that it provides the necessary room for an elongated spllcing ;~
table 106 which facilitates manual splicing of the continually extruded foam ribbon to the stationary leader which has been pre-threaded along the ribbon path through the printing and die sections At this point it is desirable to refer to Figs. 7and 8 which illustrate the ribbon tensioning device and the printing section overspeed drive assemblies respectively. The infeed tensioning assembly includes a spaced apart pair of rolls 116, 118 mounted between opposing ends of a pair o~ arms 120, ~only one arm being shown in Fig. 7). Arms 120, in turn, are rotatably mounted at their mid-point for rotation on a support member 122 which is affixed to the frame of the first printing unit 112. The rolls 116 and 118 are back pressured by an air cylinder ~24 and lin~age assembly 126 in a clockwise direction to resist the counter-clockwise torque caused by tightenlng ribbon tension. A pressure regulating valve (not shown) can preset the air pressure to regulate the ~L~5~8Z6 tension in the ribbon. By observing the position and direction of rot~tion OI arms 120~ the operator can determine if the ribbon is tightening or loosening or is remaining at the predetermined ribbon tension.
Reference may now be had to Fig. 8 which shows a portion of the press assembly drive train. There is shown a main drive unit 130 which is of any suitable conventional construction and having mounted on its output shaft a pair of axially spaced drive sprockets 132 and 134. The drive sprocket 134 is of somewhat larger diameter than drive sprocket 132;
sprocket 134 may have 48 teeth,for example, while sprocket 132 may have, for example,40 teeth. The sprocket 132 is coupled to the drive shaft via a one-way overrunning clutch 136 of any desired standard construction. The sprocket 134 is coupled to drive shaft 134 via an air cylinder operated clutch 138, such air actuated clutch also being of any standard commerclally available variety. The sprocket 134 is coupled to sprocket 140 while sprocket 132 is coupled to sprocket 142. Sprockets 140 and 142 are of the same diameter and they are both affixed to line shaft 144, such line shaft 144 extending alongside the several printing units and the die cutting unit 170 and being coupled thereto in a conventional manner (not shown). During ordinary operation of the system, the main drive 130 drives the printing units via sprocket 132 which in turn drives sprocket 1~2 to rotate line shaft 144 at a predetermined speed thereby driving the various press units at a speed which is synchronized with the speed of travel of the incoming ribbon R. During this mode of operation the air operated clutch 138 is deactivated so that sprocket 134 simply free-wheels on the main drive shaft. Howevex, during start-up operations, as will be described more ~ully hereinaEter, it is - ~6 --~15~3%~
desired to operate the printing press and ~ie cutting sections at a higher than normal rate of speed and in this instance, the air operated clutch is activated thereby to engage sprocket 134 with the main drive shaft. Since sprocket 134 is larger than sprocket 132 it obviously drives sprocke-t 140 at a higher rate of speed thus resulting in a higher speed of operation of the printing and die cutting u~its. For example, when the ribbon splice is made with the continuously moving ribbon to the stationary ribbon leader in the press section, a press start-up button is manually operated and the press section runs at a speed synchronized with the speed of the inccming ribbon i.e. the a~ove described sprocket 132 drives sprocket 142. At this point, there is usually a considerable degree of slackness in the ribbon before the draw rolls 150 and 152 and scm.e slackness in the ribbon going through the press section. At the start-up point this can be observed by the position of the arms 120 of the infeed tension assembly 108. At this point, the operator presses a press speed-up button which serves to activate the air operated clutch 138 thus automatically shifting the press and die cutting sections into the overspeed mode of operation. m is overspeed may be about 10% higher than the speed of the incoming ribbon. The operator holds the speed-up button down until the slackness in the ribkon disappears and the arm 120 revolves to its medial position. m e operator then releases the speed-up button and the press section returns to synchronous speed, matching the speed of the incc~ing extruded rib~onO
~ ith further reference to Figs. lB and lC, it is noted here that the several prlnting units 112 are sc~ewhat modified flexigraphic printing units utilizing rubber printing plates. The modifications basically concern anti-backlash gears in the plate cylinder gear train to cc~pensate for the thickness of -the ribbon being printed ard to maintain inter-colour register, and the use of large diameter ribbon carrier rolls to reduce curl effects in the uncured rib~on. The carrier rolls are designed for relatively lcw inertia, are well balanced, and use 1GW friction bearings to reduce drag on the ribbon and avoid stretching of ~he ribbon. Since other aspects of this flexographic printing process are well knawn in the art of printing a detailed description is considered unnecessary. Other printing processes, such as rotogravure, etc. can be used.
It will be seen from ~15182~
the drawings that the incoming ribbon R passes between a draw roll 150 and a chilled roll 152. ~s noted previously, these rolls may be used to further compress the ribbon to control the web thickness. The ribbon then passes downwardly around a series of rolls 15~ and thence upwardly through a nip defined between a plate cylinder 156 and an impression roll 158. An engraved inker roll 160 rotates in contact with the plate cylinder 156, the inker roll 160 being i~mersed in a bath of ink contained in trough 162, with a suitable doctor blade 164 being provided to remove excess ink from the inker roll. The printed ribbon then passes upwardly around guide roll 166 and through a conventional dryer arrangement 168 which serves to dry the applied ink following which the ribbon passes on to the next printing uni~. The printing units are all o~ essentially the same construction as that described above and the number of such printing units is, of course, dependent on the number of colours to be applied. A simple two colour system would require the use of only two printing units but for most purposes four or five printing units will be required.
Insofar as the printing inks are concerned, various conventional printing inks based on nitrocellulose or acrylic resins were found to be reasonably satisfactory. The best combination of properties insofar as flexibility, adhesion, slip, abrasion resistance and odour are concerned was obtained using the "Flexo Styrene" series of inks produced by General Printing Ink Company, Toronto,Canada. This series of inks is based on a combination of nitrocellulose and cellulose acetate propionate. The solvents used with the ink are denatured anhydrous ethyl alcohol, normal propyl acetate and normal propyl alcohol. The addition of one percent micronized polyethylene to the ink provides additional slip~
After the ribbon e~erges from the printing section ~L~S~l~3Z6 110 it enters into a die cutter and delivery unit 170. ~his unit is illustrated diagra~matically in Figure lC. Again, since units of this nature are basically well known in the art a detailed description of same is considered unnecessary. The ribbon first passes around a cooled roll 172, around guide roll 174 and thence between the rolls of a scoring unit 176. At this point it is advisable to refer to Fig. 9 which illustrates a segment of the ribbon R and illustrates the outline shape of the die cut bacon board blanks thereon. It will be seen that each bacon board includes a flap portion F and a body portion B. Between the flap portion F and body portion B there is a continuous score line S which has a series of spaced apart fine slits P
therealong. These slits facilitate folding of the flap portion relative to the bcdy portion. Th~ score line S is provided by scoring unit 176. Followlng the scoring unit, the ribbon passes dcwnwardly into the nip b~tween cutter roll 180 and anvil roll 182.- The cutter roll is provided with a raised sharp edged lip which, r~hen it comes into contact with the ribbon R, penetrates therethrough and severs such ribbon all around the outline contour of the bacon board which has been printed on the ribbon surface. At the same time the roll 180 also cuts the above noted slits P along the score line S. After this operation, the ribbon passes dcwnwardly around delivery roll 184 and be-20 tween guide rolls 186 following which the ribkon waste is drawn vertically up-wardly at a sharp angle with such action causing the cut bacon koards to pass outwardly onto a slowly moving surface 188 of a delivery table r,~th the result being that the bacon boards are deposited on such takle in an over-lapping shingle-like fashion. The remaining ladder-like waste portion of the ribbon is then forwarded to a waste cutter with the waste material being ultimately collected and ultimately fed back into the extruder hopper 12 with the result being that there is virtually no wastage of materials.
This is possible since the printed design is spaced about 1/8 of an inch inwardly of the die cut edge all around the die cut edge thus avoiding ink contamination of the waste ribbon material, which contamination would otherwise ultimately adversely dis-colour the extruded ribbon webs.
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Reference may now be had to Table 1 which illustrates the physical characteristics of a number of foam products. Samples No. 2-5 were made generally in accordance with the principles of the present invention. Samples No. 3 and 4 were considered to have the best properties insofar as the purposes of the present invention are concerned. Sample No.
6 which was included here for purposes of comparison is a relatively low density foam meat tray; this foam was not bi-axially oriented after extrusion, nor was it provided with skin surfaces nor was the foam compressed or calendared thereby to control the thickness thereof as in the other samples. Table 1 should be reviewed in conjunction with ~igure 10 which illustrates the cellular structure of the foam as well as the manner in which cell size is measured and determined.
It will be seen from a review of Table l and Fig.
10 that the cells of the bacon board samples have a relatively squashed or flattened appearance with diameters in the machine direction ranging from about .23 to .28 mm. and diameters in the cross machine direction varying from about .2~ to about .30
2~ mm. With reference to skin thickness, all samples with the exception of Sample No. ~ i.e. the meat tray, which had no skin, were found to have about 0.3 to about 1.0 mils of skin on the surface. Two samples, iOe. samples 3 and 4 had skins on both major surfaces, i.e. on the inside and outside surfaces. These two samples were found to have the highest tensile strength in the machine direction. The cross machine direction tensile strength for these two samples was not the highest observed but this may be accounted for by the lower cross machine direction orientations observed. The skinned surfaces give rise to a number of benefits. The skinned surfaces make for a tougher foam 5~326 due to the biaxial orientation of the skin surfaces. The skin also protects the surfaces during the crushing or compressing operation. The biaxial orientation appears to have a beneficial effect on cell structure during the crushing as cell structure is well defined near both surfaces. In the absence of a skin surface on the foam, the cell structures tend to rupture thus producing a poorly defined structure which may be susceptible to premature aging and loss of strength. It might be noted here that care has to be taken to prevent excessive cooling at the die during the skin forming step or the foam will split.
Although there is no need for the skins on opposing sides o~ the foam ribbon to be exactly the same thickness, there should be a reasonable degree of balance between them thereby to mlnimize warping of the final product.
In the preferred form of the invention, the final density is achieved by crushing or compressing a 2.5 lb./ft.3 foam to achieve a final density of the foam ribbon in the range of 10.5 lb./ft.3. In other words, the foam which initially has a thickness of about 80 mils is crushed or compressed back to a recovered value of approximately 20 mils. This represents a four to one crush ratio. These figures,may of course, be varied considerably. At the same time i~ should be kept in mind that overly high crush ratios may cause rupturing of an undue number of the cells of the foam in turn adversely affecting the aging characteristics.
It has previousiy been observed that the orientation levels are relatively high, particularly in the cross-machine or transverse direction. In the example described previously the amount of orientation or stretch was in about a two to one ratio in both the machine direction (longitudinal direction) and in the cross-machine (transverse) direction. This relatively high degree of orientation is considered to be ~ery beneficial and is believed to be responsible for much of the improvement in ~he properties of the foam ribbon observed. With reference to Table 1, in comparing Sample No. 1 with Samples Nos. 3 and 4, the increase in orientation in the cross-machine direction is quite obvious. The dramatic increase in tensile strength, particularly in the cross-machine direction, is attributable in a large measure to the increase in orientation.
With reference to Samples Nos. 2-5 it will be seen that they range in density from a low, in the case of Sample No.2, of 8.1 lb./ft.3 to a high, in the case of Sample No.
Although there is no need for the skins on opposing sides o~ the foam ribbon to be exactly the same thickness, there should be a reasonable degree of balance between them thereby to mlnimize warping of the final product.
In the preferred form of the invention, the final density is achieved by crushing or compressing a 2.5 lb./ft.3 foam to achieve a final density of the foam ribbon in the range of 10.5 lb./ft.3. In other words, the foam which initially has a thickness of about 80 mils is crushed or compressed back to a recovered value of approximately 20 mils. This represents a four to one crush ratio. These figures,may of course, be varied considerably. At the same time i~ should be kept in mind that overly high crush ratios may cause rupturing of an undue number of the cells of the foam in turn adversely affecting the aging characteristics.
It has previousiy been observed that the orientation levels are relatively high, particularly in the cross-machine or transverse direction. In the example described previously the amount of orientation or stretch was in about a two to one ratio in both the machine direction (longitudinal direction) and in the cross-machine (transverse) direction. This relatively high degree of orientation is considered to be ~ery beneficial and is believed to be responsible for much of the improvement in ~he properties of the foam ribbon observed. With reference to Table 1, in comparing Sample No. 1 with Samples Nos. 3 and 4, the increase in orientation in the cross-machine direction is quite obvious. The dramatic increase in tensile strength, particularly in the cross-machine direction, is attributable in a large measure to the increase in orientation.
With reference to Samples Nos. 2-5 it will be seen that they range in density from a low, in the case of Sample No.2, of 8.1 lb./ft.3 to a high, in the case of Sample No.
3, of 11.4 lb./ft.3. Generally speaking, it is recommended that the density of the final product be maintained within the range from about 8 to about 12 lb./ft.3. For best overall results, it is considered desirable to maintain the final density in the order of about 10-11.5 lb./fto3 and this, of course, can be controlled by controlling the degree of crushing or compression being effected by the gauge roll means previously described.
From the foregoing, it is believed that the invention may be readily understood by those skilled in the art without further description 7 it being borne in mind that numerous changes may be made in the details disclosed without departing from the spirit of the invention set forth in the following claims.
From the foregoing, it is believed that the invention may be readily understood by those skilled in the art without further description 7 it being borne in mind that numerous changes may be made in the details disclosed without departing from the spirit of the invention set forth in the following claims.
Claims (11)
1. A method of continuously manufacturing flexible sheets of cellular polystyrene foam material, including the steps of:
(a) passing a molten polystyrene plastic material to which a blowing agent has been added through a die orifice shaped to allow the material to expand into a ribbon of foamed plastics material;
(b) effecting an initial cooling of the opposing major surfaces of the ribbon on emergence from the die orifice to form a thin skin on the surfaces of the ribbon;
(c) biaxially stretching the ribbon of material as it cools;
(d) effecting further cooling and forming of the ribbon by passing it on to a surface of a rotating forming drum;
the method being characterized by the steps of:
(e) compressing the partly cooled ribbon to substantially reduce the thickness thereof by passing the ribbon in pressurized contact with a surface of a roll means;
(f) before or after step (e), passing the ribbon in a path of travel to effect further cooling and stabilization of same;
(g) passing the partially cooled and stabilized ribbon in a further path of travel through a printing press to apply printing inks to a surface thereof, and (h) subsequently die cutting printed sheets of a selected outline shape from the ribbon.
(a) passing a molten polystyrene plastic material to which a blowing agent has been added through a die orifice shaped to allow the material to expand into a ribbon of foamed plastics material;
(b) effecting an initial cooling of the opposing major surfaces of the ribbon on emergence from the die orifice to form a thin skin on the surfaces of the ribbon;
(c) biaxially stretching the ribbon of material as it cools;
(d) effecting further cooling and forming of the ribbon by passing it on to a surface of a rotating forming drum;
the method being characterized by the steps of:
(e) compressing the partly cooled ribbon to substantially reduce the thickness thereof by passing the ribbon in pressurized contact with a surface of a roll means;
(f) before or after step (e), passing the ribbon in a path of travel to effect further cooling and stabilization of same;
(g) passing the partially cooled and stabilized ribbon in a further path of travel through a printing press to apply printing inks to a surface thereof, and (h) subsequently die cutting printed sheets of a selected outline shape from the ribbon.
2. The method of claim 1 further characterized in that compressing step (e) is carried out in two stages, one before step (f) and the other just before step (g).
3. The method of claim 1 further characterized in that the step of compressing the ribbon further flattens the cells of the foam so that they lie with their major axes generally parallel to the major surfaces of the ribbon.
4. The method of claim 3 further characterized in:
that the compressing step or steps reduces the ribbon thickness to about one quarter of its thickness just prior to compression.
that the compressing step or steps reduces the ribbon thickness to about one quarter of its thickness just prior to compression.
5. The method of claim 1; 2 or 3 further characterized in that the step (g) of passing the ribbon through the printing press comprises passing the ribbon in contact with a plurality of roll means, one of which rolls includes means for applying ink in a preselected pattern to a major surface of the ribbon.
6. The method of claim 1, 2 or 3 further characterized in that the step (g) of passing the ribbon through the printing press comprises passing the ribbon in contact with a plurality of roll means, one of which rolls includes means for applying ink in a preselected pattern to a major surface of the ribbon, wherein the printing press includes a plurality of press units, each for applying a different ink colour to the ribbon thereby to build up a series of multi-colour images on the ribbon.
7. The method of claim 1, 2 or 3 further characterized in that the step (g) of passing the ribbon through the printing press comprises passing the ribbon in contact with a plurality of roll means, one of which rolls includes means for applying ink in a preselected pattern to a major surface of the ribbon, wherein the printing press includes a plurality of press units, each for applying a different ink colour to the ribbon units, to build up a series of multi-colour images on the ribbon, and wherein the step of die cutting the printed ribbon is characterized in that the ribbon is severed all around each of the respective images with the remaining waste ribbon portion emerging in a continuous fashion for collection and re-use.
8. A backing board for use in the packaging or foodstuffs, such as bacon, comprising:
a thin flexible sheet of cellular polystyrene foam, said sheet being in a compressed or crushed condition in the direction of its thickness and bi-axially oriented in the plane of the sheet so that the cells of the foam are in a generally flattened condition and lie with their major dimensions generally parallel to the major surfaces of the sheet; the foam having a density from about 8 to about 12 lbs/ft3; at least one of the major surfaces of the foam sheet having a densified generally continuous skin thereon; said skin having a thickness of at least about 0.3 mils, one said skin surface having printing ink or inks thereon.
a thin flexible sheet of cellular polystyrene foam, said sheet being in a compressed or crushed condition in the direction of its thickness and bi-axially oriented in the plane of the sheet so that the cells of the foam are in a generally flattened condition and lie with their major dimensions generally parallel to the major surfaces of the sheet; the foam having a density from about 8 to about 12 lbs/ft3; at least one of the major surfaces of the foam sheet having a densified generally continuous skin thereon; said skin having a thickness of at least about 0.3 mils, one said skin surface having printing ink or inks thereon.
9. The backing board according to claim 8 , wherein said foam has a density from about 10 to 11.5 lb/ft.3.
10. The backing board according to claim 9 wherein said skin has a thickness from about 0.3 to about 1.0 mils.
11. The backing board according to claim 8, 9 or 10 wherein both of said major surfaces are provided with said skin.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000348554A CA1151826A (en) | 1980-03-27 | 1980-03-27 | In-line manufacture of printed and die cut polystyrene foam sheets |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000348554A CA1151826A (en) | 1980-03-27 | 1980-03-27 | In-line manufacture of printed and die cut polystyrene foam sheets |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1151826A true CA1151826A (en) | 1983-08-16 |
Family
ID=4116575
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000348554A Expired CA1151826A (en) | 1980-03-27 | 1980-03-27 | In-line manufacture of printed and die cut polystyrene foam sheets |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1151826A (en) |
-
1980
- 1980-03-27 CA CA000348554A patent/CA1151826A/en not_active Expired
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