CA1114126A - Method for manufacturing thin-walled articles from crystalline thermoplastic material by thermoforming - Google Patents

Method for manufacturing thin-walled articles from crystalline thermoplastic material by thermoforming

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Publication number
CA1114126A
CA1114126A CA308,019A CA308019A CA1114126A CA 1114126 A CA1114126 A CA 1114126A CA 308019 A CA308019 A CA 308019A CA 1114126 A CA1114126 A CA 1114126A
Authority
CA
Canada
Prior art keywords
web
temperature
sheet
thermoforming
cooling
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
Application number
CA308,019A
Other languages
French (fr)
Inventor
Alfons W. Thiel
Barbara Geppert
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Bellaplast GmbH
Original Assignee
Bellaplast GmbH
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Bellaplast GmbH filed Critical Bellaplast GmbH
Application granted granted Critical
Publication of CA1114126A publication Critical patent/CA1114126A/en
Expired legal-status Critical Current

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Classifications

    • BPERFORMING OPERATIONS; TRANSPORTING
    • B29WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29CSHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C51/00Shaping by thermoforming, i.e. shaping sheets or sheet like preforms after heating, e.g. shaping sheets in matched moulds or by deep-drawing; Apparatus therefor
    • B29C51/26Component parts, details or accessories; Auxiliary operations
    • B29C51/42Heating or cooling
    • B29C51/421Heating or cooling of preforms, specially adapted for thermoforming
    • B29C51/422Heating or cooling of preforms, specially adapted for thermoforming to produce a temperature differential
    • B29C51/423Heating or cooling of preforms, specially adapted for thermoforming to produce a temperature differential through the thickness of the preform
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B29WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29CSHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C48/00Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
    • B29C48/03Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor characterised by the shape of the extruded material at extrusion
    • B29C48/07Flat, e.g. panels
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B29WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29CSHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C48/00Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
    • B29C48/03Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor characterised by the shape of the extruded material at extrusion
    • B29C48/07Flat, e.g. panels
    • B29C48/08Flat, e.g. panels flexible, e.g. films
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B29WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29CSHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C48/00Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
    • B29C48/25Component parts, details or accessories; Auxiliary operations
    • B29C48/88Thermal treatment of the stream of extruded material, e.g. cooling
    • B29C48/911Cooling
    • B29C48/9135Cooling of flat articles, e.g. using specially adapted supporting means
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B29WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29CSHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C48/00Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
    • B29C48/25Component parts, details or accessories; Auxiliary operations
    • B29C48/88Thermal treatment of the stream of extruded material, e.g. cooling
    • B29C48/911Cooling
    • B29C48/9135Cooling of flat articles, e.g. using specially adapted supporting means
    • B29C48/914Cooling of flat articles, e.g. using specially adapted supporting means cooling drums

Landscapes

  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Blow-Moulding Or Thermoforming Of Plastics Or The Like (AREA)
  • Extrusion Moulding Of Plastics Or The Like (AREA)
  • Laminated Bodies (AREA)
  • Casting Or Compression Moulding Of Plastics Or The Like (AREA)

Abstract

Abstract of the Disclosure The present invention relates to an improved method for manu-facturing thin-walled articles from crystalline thermoplastic material by thermoforming a web of said material. The method includes providing a web of said material wherein the inner core of the web is at a temperature higher than the upper limit of the critical crystalline melting temperature range while the outer surface regions of the web are at a temperature lower than the lower limit of the critical crystalline melting temperature range. The web is then subjected to thermoforming with or without heating of the outer surface regions.

Description

This invention relates to the manufacture of thin-walled articles such as cups, plates and the like containers, of synthetic crystalline thermoplastic material and particularly to a method taking into account the crystalline character of the thermoplastic material used.
In manufacturing thin-walled articles by thermo forming sheets or webs of crystalline thermoplastic material it is shown in US~PS 3,709,976 to heat the cold web or sheet to a temperature of 6 to 30 C, preferably 6 to 17 C below the crystalline melting range. This would be for instance for polypropylene a temperature of about 150 to 165C. Such known method has the principal defect that the sheets or webs of crystalline thermoplastic material must be heated from outside so that the surface regions of the sheets or webs are much more heated than the inner core portions, according to this known method it is necessary to heat the outer surface regions of the webs or sheets very much above the crystalline melting point, or if it is desired to avoid this the outer surfaces are heated to above the crystal-line melting temperature range while the inner core material is retained in a relatively cool condition in which it is only elastically deformed. The articles shaped by such method therefore do not have satisfactory dimensional stability under heat.
In a similar method shown by US-PS 3,157,719 polypropylene is ex-truded and separated into sheet-form and cooled down practically to room temperature to thereafter be sub~ected for a short period to a preliminary heat treatment at a temperature of about 130 to 140C. In order to shape articles such polypropylene sheets are transported sheet by sheet directly adjacent to the shaping tool and each one of the sheets is subjected to the combined action of a second heating up to the softening temperature of the polypropylene and simultaneously applying vacuum to the tool in order to -~
draw the said softened sheet in contact with the tool surface. However, it ~; '`` :' ~' ',':
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is impossible to control crystal gro~th in the polypropylene during such kind of thermal forming, because softening the material necessitates that it be heated up to a temperature within or higher than the critical c~stalline melting temperature range of the material. On the other hand shaping by drawing said sheet in contact with a tool surface does not in-volve cooling down the material through the said critical crystalline melt- -ing temperature range by a sufficiently high cooling rate in order to con-trol or avoid growth of crystals in the material.
Accordingly, it would be advantageous to have a method of thermo forming taking into account the special character of crystalline thermo-plastic material to be thermoformed in order to control growth of crystals in the said material during the entire process. Such control may take the form of substantially avoiding crystal growth or may take the form of allow-ing crystal growth to a desired extent.
Accordingly, the present invention provides a method of manufactur-ing thin-walled articles from crystalline thermoplastic material including thermoforming a web or sheet of said thermop].astic material while reducing the web or sheet thickness and cutting out shaped articles from said web or sheet, wherein said thermoplastic material has a critical crystalline melt-ing temperature range and said method further includes a temperature pre-conditioning of said web or sheet to suitable temperature conditions for thermoforming and is characterized in that said temperature preconditioning involves providing at the inner core of said web or sheet a temperature higher than the upper limit of said critical crystalline melting temperature range and providing at the outer surface regions of said web or sheet a temperature lower than the lower limit of said critical crystalline melting temperature range, and in that during said thermoforming step while reducing the thickness of said web or sheet and immediately thereafter the thermo-
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plastic material in said core of said web or sheet is rapidly cooled down r~
through said critical crystalline melting temperature range ~h~e controlling crystal growth in said core material.
More particularly, according to the present invention temperature preconditioning of the crystalline thermoplastic material involves providing a special temperature profile in the web or sheet to be thermoformed. Such special temperature profile provides that the inner core material of the web or sheet is brought to a temperature for thermoforming which is higher than the upper limit of the critical crystalline melting temperature range of the respective material, while the outer surface regions of the web or sheet are brought to a temperature which is lower than the said critical crystalline melting temperature range. By providing such temperature profile it is pos- ;
sible to control or substantially avoid growth of crystals in the outer sur-face regions of the web or sheet while the inner core material is in a con-dition practically free of crystals. By the combined action of substantial reducing in thickness with a rapid cooling it is possible to positively control crystal growth during the thermoforming step. Accordingly, it is possible to obtain outer surface layers of an article wall having a more or less fine crystalline structure and an inner core region of the article wall having a predetermined controlled crystalline structure. If the outer sur-face layers are not reheated before the web or sheet enters the thermo-forming step a substantial stretching effect is obtained within the said outer surface regions during the thermoforming step. Such stretching in the outer surface regions of an article wall for many cases may have special advantages, e.g. the material brittleness is very much reduced in the outer layers of the article wall and so the mechanical behavior of the article is very much improved.
As indicated the hot core material is cooled down during the
- 3 -- : - , thermoforming step in combination with a substantial reducing of wall thickness. Following such procedure it is possible to obtain relatively rapid cooling down of the core material through the critical crystalline melting temperature range even though the said crystalline thermoplastic material has a very low thermal conductivity. The combined action of reduc-ing the wall thickness during thermoforming and also cooling the thermally formed wall surfaces results in improved control of cooling conditions through the critical crystalline melting temperature range and therefore also in improved control of crystal growth in the inner core region of an article wall.
By such means the mechanical behavior of the core region of the article wall becomes more or less controllable. On the other hand a rela~
tively high shaping temperature of the core material can be used in order to get articles having high dimensional stability under heat. Fur~her it is a special advantage of such articles, the outer surface layers of which are in a stretched condition, that the core region of the article wall has special sti~fness and stability in shape under heat while the stretched outer sur-face regions of the article are tough and therefore of improved mechanical properties. The articles manufactured in accordance with the present inven-tion have wall properties in which an increased elastic modulus and an in-creased impact strength in the outer surface regions of the article wall are combined with an increased stiffness and thermal stability in the core region of the article wall. Further the invention has special advantages for the method itself due to the fact that the rapid temperature transfer through the critical crystalline melting temperature range is obtained in combination with reducing the wall thickness during the thermal forming step. Therefore it is possible to work with relatively thick webs or sheets in the present method, for instance such webs or sheets having thickness
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more than 3 mm.
In the present novel method ~he temperature provided for the corematerial of said web or sheet may be close to but higher than the upper limit of the critical crystalline melting temperature range of the crystal-line thermoplastic material used. When using this feature cooling down the core material through the critical crystalline melting temperature range may be substantially improved.
Further in the present novel method the temperature provided for ths outer surface regions of said web or sheet may be close to but lower than the lower limit of the critical crystalline melting temperature range of said crystalline thermoplostic material.
The present novel method for manufacturing thin-walled articles from crystalline thermoplastic material by thermoforming may be performed according to different kinds of embodiments. One embodiment provides that said temperature preconditioning prior to thermoforming includes heating a web or sheet of said thermoplastic material throughout its thickness to a temperature higher than the upper limit of sa,id critical crystalline melting te~perature range and cooling down under predetermined cooling rate con-ditions the outer surface regions of said web or sheet to a temperature lower than the lower limit of said critical melting temperature range, said cool-ing down rate conditions being adapted to controll crystal growth of the material of said outer surface regions of the web or sheet.
In the above described embodiment two different steps for cooling ~, down through the critical christalline melting temperature range can be used, namely a first precooling step for cooling down the outer surface regions of ~;, the web or sheet and a second cooling step for cooling down the inner core material during the thermoforming step. By these two separate cooling steps ';~
an improved and more precise temperature control is possible. With respect
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to the fact ~hat the first cooling step involves a pre-cooling of the web at its surface regions and the second cooling step includes cooling the core material after reducing the wall thickness to obtain the thin article wall, both these provided cooling steps are highly effective The preferred embodiment of the present novel method is character-ized by an uninterrupted sequence of steps comprising a) extruding a continuous hot web of said crystalline thermo-plastic material at a conventional extrusion temperature above the crystal-line melting temperature range of said material, b) substantially immediately rapidly precooling the opposite surfaces of said web to form along said surfaces thin supportive layers having a temperature in the range wherein said material is not further plastically deformable and wherein further growth of crystals is essentially avoided, provided that the hotter core material between said layers is cooled to a temperature ad~acent to but above the crystalline melting temperature range and is held in a substantially crystal free condition, c) transporting the so pre-cooled web to a thermoforming station within a short time period so as to essentia].ly maintain the above mentioned temperature profile produced by precooling over the web thickness and d) then thermoforming said web to an article of desired shape under rapid cooling rate conditions while essentially controlling crystal growth in said material while cooling through said crystalline melting tem-perature range under said cooling rate conditions.
Such preferred embodiment is able to combine the technical advan-tages of a high effective in-line method of production with the special ~
measures and precautions to avoid or to control growth of crystals during the method steps when the said crystalline thermoplastic material is cooled down through the critical temperature range of the material used. So-called ,, ' ,: " .

in-line methods are known by IJS-PS 1~,039,609. The principals of such known inline processes comprise plasticizing thermoplastic material, preferably of granular kind, by heating and compressing in an extruderpress, casting the -said material by a coat hanger nozzle to form a web and precooling said web for stabilizing to make such web able to be transported to a thermal forming station and thereafter thermoforming said web to form the desired articles.
Such known inline processes are highly effective for manufacturing articles but they involve the necessity to cool down the thermoplastic material from its extrusion temperature to the normal room temperature during one or more stages of the process. In the known in-line methods such cooling down does not take into account any control of the crystal growth in the material if crystalline thermoplastic material will be used in such a process. There-fore it is a special advantage of the said preferred embodiment of the invention to introduce special cooling conditions at the pre-cooling and stabilizing stage and also at the thermoforming stage of the known in-line process such that the high effectiveness of the in-line process is fully exploited while all necessary measures and precautions to control the growth of crystals in the material become fully effective in such in-line process.
One preferred embodiment of the present invention provides for suostantially maintaining the temperature profile of the web as produced by :
pre-cooling until said web is subjected to the thermoforming step. But for ~-other purposes it is possible to reheat the outer surface regions of the web so as to get a temperature which is adjacent to but lower than the lower limit temperature of said crystalline melting temperature range. This latter - -possibility is advantageous for shaping articles which have somewhat dif-ficult surface shape. Thus the surface material gets somewhat more duct,il-ity for thermal forming, but the crystalline conditions of the surface mate-rial is not changed by such reheating. Such reheating may be applied to one ~:
. ~ , -i' ''" ' ;" ~' "' ' or both of the outer surfaces. A further possibility is to reheat one or both of the outer web surfaces to a temperature within or above the crystal-line melting range. But when thus reheating, the crystalline conditions of the surface material regions are more or less changed and the surface regions must also be rapidly cooled down through the critical crystalline melting temperature range during thethermoforming step. These variations using sur-face reheating may for example be advantageous for the forming of parts with very sharp corners, which would tend to be rounded by toughly elastic skins.
Also, in the case of reheating surface layers above the critical crystalline melting temperature range of the material, sag problems have to be kept in mind. However the advantages of avoiding long heating times and more precise and uniform temperature control for many cases is more important than using precautions with respect to said sag problems.
Cooling means in the pre-cooling step as well as in the thermo-forming step may be used as is optimum for each special material and case.
Thus it is possible for pre-cooling to use contact of the web surface with cooled surface means having good and high heat conductivity. During the thermal forming step additional cooling means can be provided for the back-side surface of the article if only one cooled shaping tool surface is pro-vided for one surface of the article wall. In such case for instancepowdered dry ice may be blown onto the said backside of the web during the thermal forming step.
~ ome embodiments of the present invention are described in more detail in connection with the enclosed drawings. In particular in drawings which illustrate embodiments of the invention:
Figure 1 is a diagrammatic view showing an embodiment of the present invention for shaping thin-walled articles from crystalline thermo-plastic material in which the transferr time period between the stabilizing . . - -: .

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and thermoforming steps of the web is minimized;
Figure 2A and Figure 2B are graphic views illustrating temperature relationships in the crystalline thermoplastic material according to the embodiment illustrated in figure 1;
Figure 3 is an enlarged fragmentary view partly in secion illus-trating internal web conditions;
Figure 4A and Figure 4~ are enlarged fragmentary views in section illustrating web shaping operations in connection with the embodiment accord-ing to figure l; ~~
Figure 5 is a still more enlarged fra gentary view in section illustrating the crystalline conditions in the wall of an article shaped -according to the embodiment shown in figure l;
Figure 6 is a diagrammatic view showing a second embodiment of the present invention in which a reheating s-tep iB provided before the web enters the thermal forming station;
Figure 7 illustrates graphic views a, b and c relating to reheat-ing the outer surface layers of the web and Figure 8 illustrates enlarged fragmentary views in section similar to figure 5 and showing the article wall obtained under conditions according - :
to figure 7a, b and c. ~
Figure 1 illustrates the operational sequence of a preferred ~ `
embodiment for the manu~acture of thin-walled articles from crystalline thermoplastic material. This embodiment comprises an extrusion device 1 .
suitable for receiving granular crystalline thermoplastic material and com-pressing and heating it continuously until it is liquified and reaches a temperature TE above the crystalline melting temperature range ~ o~ the material. The liquified thermoplastic material, processed by the extrusion press 1 is conveyed into a coat hanger nozzle 2 having a broad outlet slot, _ g - ' ?
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the upper and lower wall of which is additionally provided with temperature regulators.
The hot liquified thermoplastic material continuously flowing out from said coat hanger nozzle 2 forms a hot plastic web I, which is immediate-ly fed to a stabilizing station 3, which may be constructed as a sequence of cooled metallic rollers providing an effective contact between the cooled roller surfaces and the outer surfaces of the plastic web I. The web is stabilized in such stabilizing station 3 by pre-cooling its opposite surfaces in such manner that thin solidified supportive layers of said thermoplastic material are produced at those surfaces whereby the web becomes self support-ing. However, in view of the low heat conductivity of the thermoplastic material used the inner core material of the web remains relatively hot and in any case at a te~perature which is above the crystalline melting temper-ature range ~ of the material.
Pre-cooling is done very rapidly, so that the crystal nuclei con- `
tained in the material do not have time to grow substantially. Therefore the material of said solidified outer supportive layers of the web obtains a very fine crystalline structure during the pre-cooling step. As the inner core material of the web remains at a temperature above the crystalline melting temperature range there will be some transition regions between said inner core and said outer layers in which the temperature conditions are such that some growth of crystals may occur, by pre-cooling very rapidly such transition regions become very thin and have practically no influence.
If desired for any reason the pre-cooling in this first embodiment may be done less rapidly. When pre-cooling in such manner some crystal growth will occur in the outer supportive layer material. It is thus pos-sible to control crystal growth in said outer layers according to the rate ~ ;
of pre-cooling used.

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From said stabilizinæ station 3 the continuously advancing web is fed to a motion control feed conversion device ~ for converting the con-tinuous advance of the web into an intermittent advance. Such feed con-version device 4 may be constructed as known from US-PS 4,039,609. ~ ~ ~
The web coming from such motion control feed conversion device 4 is immediately introduced to the thermo~orming station 6, containing thermal forming means which may be constructed as known form US-PS 4,039,609 and fragmentary and schematically shown in figuresl~a and 4b. ~uring such thermo-forming operation the precooled outer supportive layers are shaped substan-tailly be stretching deformation whereas the almost plastic material of theweb core will be plastically deformed and distributed between the stretch-formed outer layers. During and after such mechanical shaping substantially by stretching of the outer supportive layers and plastically deformin~ and distributing the plastlc core material betwe~en the outer layers, the shaped web or article wall respectively is rapdily cooled in order that crystal growth in the cooling core material may be controlled or minimized. If desired such cooling rate may be somewhat slower to control the growth of the crystal in the core material in any desi.red manner.
; In connection with said first embodiment of the present invention it is important to minimize the time period which any part o~ the web requires in order to be transported from the stabilizing step to the forming step. By minimizing the transport time period between the stabilizing and the thermal forming steps the temperature conditions (or the temperature profile) as produced by the stabilizing step are practically maintained until cooling in the said thermal forming step begins. It is thus possible to control the growth of the crystals in the web material during said trans-port time period if a temperature profile of the web is produced in the pre-cooling step such that the core material temperature is above the crystalline A

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melting temperature range of the material and is in a plastic state in which no growth of crystals occurs, whereas the temperature of the outer layers is such that it is below the lower limit of the critical crystalline melting temperature range and so further growth of crystals in the outer layer mate-rial does not occur.
Figures 2A and 2B are graphics to illustrate the temperature relationship and the special temperature conditions which should preferably be used in connection with the embodiment as generally described above in connection with figure 1. hs shown infigures 2A and 2B the interesting temperature ranges may be considered as follows: ~`
There is a lower temperature limit TC at which melting o~ crystals will begin when heating the crystalline thermoplastic material in question.
Below such critical temperature limit TC there is a temperature range ~
having a lower temperature limit TA. In this temperature range ~ the crys-talline thermoplastic material is thermoformable but such thermo forming is almost a stretching action so that an article shaped under tempera~ure con-.
ditions in the temperature range d~is more or less elastically deformed and stretched and has poor dimensional stability under heat. Thermoforming under temperature conditions below TA is practically impossible.
,~ 20 Above the critical temperature TC there is the critical crystal-line melting temperature range having an upper limit temperature TM. Within this critical temperature range ~ growth of crystals occurs; e.g. when cool-ing down crystalline thermoplastic material through this temperature range . Above the upper limit temperature TM f the critical crystalline melting temperature range ~ for most crystalline thermoplastic materials is a tem-perature range ~ which in connection with the present invention has been i -found to be especially suitable for thermoforming. This is true especially for the lower part Yl Of this temperature range y whereas the upper part Y2 .~

: :: : :: - :: . :
. - :::~: ::
-: :
: ' - :-:: , : :: . : , also is a suitable temperature range for thermoforming. The upper limit temperature TB f the said temperature range ~ is followed by an upper temperature range ~ which is especially useful for in~ection and extrusion processes. So figure 2B shows an extrusion temperature TE which is within the said upper temperature range ~.
From figures2A and 2B may be seen the special problem that must be faced when using an in-line process for producing articles of crystalline thermoplastic material; namely, that during such in-line process, cooling down must be provided from the extursion temperature TE to a temperature, for instance of temperature TA, at which the material of the article solidifies and which requires cooling the material through the critical temperature range ~ within which growth of crystals occurs.
As may be seen from figure 2A within the above mentioned temper-ature range ~, the so-called crystalline melting temperature range, sub-stantial crystalline changements in the material occur. Such changements include melting of crystals as well as growth of crystals. The lower limit of this critical temperature range ~ may be called the critical temperature C and the upper limit may be the crystalline melting point TM. When heat-ing the material above TC to a temperature within the said crystalline melt-ing temperature range ~ the small crystals which are already contained inthe cooler material begin to grow, but on the other hand the said crystals begin to melt if the material is further heated. By such reason the dotted line curve in figure 2A which is relative to heating the material was found to be substantially flatter than the full line curves relating to cooling the material. When cooling the crystalline thermoplastic material from a temperature above the crystalline melting point TM through the critical tem-perature range ~ to a temperature lower than the critical temperature TC at ` ~irst the material is in an amorphous condition. When the material reaches ;
, :.' : . , , . ~ ~: ~ , . ' , '' .' ' . ~ ~ ~.', `'': . '.

temperatures within the said critical temperature range ~ crystals begin to develop and to grow. The growth of crystals and the final size which the crystals will reach by such cooling depends on the length of time during which the temperature of the material is within said critical temperature range ~. This may be seen by the three different curves in figure 2A; S for cooling slowly, M for a medium cooling rate and R for cooling rapidly.
Therefore it is possible to control the growth of crystals in a crystalline thermoplastic material by using a predetermined cooling rate within the critical temperature range ~. If cooling slowly a relatively 10 coarse crystalline structure of the material will be obtained, whereas if cooling rapidly a relatively fine crystalline structure of the material is obtainable.
Having in mind the method described above in connection with fig-ure 1 it is clear that when cooling down the material from the extrusion temperature TE to the normal room temperature of the article at any time it is necessary to cool the material through the critical temperature range ~.
It is therefore advantageous to cool said material through the critical tem-perature range ~ so as to control the growth of crystals in the material in any desired manner.
Figure 2B shows a preferred possibility for such cooling in which growth of crystals in the outer layers of the web material is intended to be avoided as much as possible. Therefore the thermoplastic material is heated for extrusion such that it flows out from the coa~-hanger nozzle 2 at an extrusion temperature TE substantially above the crystalline melting point TM. This is shown by curve E in figure 2B. During the stabilizing step which is illustrated by the curves G the outer layers of the web are cooled down rapidly such that their temperature is very much lower than the criti-cal Temperature TC whereas the core material is cooled to a temperature t - 14 -! ~

. . . , ' ' . .
' ' ' ' ' ' ~ .
~ ', ' ' 1.

which is in any case above the crystalline melting point TM. As shown in figure 2B by the several curves different pre-cooling conditions may be used so that the temperature profile of the core material may vary within some limits. But in any case the core material temperature has to be retained above the crystalline melting point TM. By such processing only some tran-sition regions between the cooled outer layers and the core material will have a temperature between TC and TM. Normally the thickness of such tran-sition regions is very small, but as shown by the several curves G there is some possibility to make such regions more or less thicker by varying the pre-cooling conditions used and the pre-cooling rate used at the stabilizing step or by providing an outer surface temperature of the web which is nearer to the critical temperature TC than it is shown in figure 2B.
As shown by curve H in figure 2B the thermoforming step should contain a very rapid cooling so that the core material distributed between the outer surface regions of the web will become rapdily cooled down through the critical temperature range ~ and to a temperature lower than the critical temperature Tc. During thermoforming rapid cooling is enhanced when the thickness of the web is much reduced to get the final wall thickness of the article and so the cooling effect is obtained in the inner core region when-ever the thermoplastic material has relatively low heat conductibility. Itwas found in connection with the present invention that the cooling down rate in the core material can be so controlled that the growth of the crys- ;
tals can be practically minimized during cooling in the thermoforming step.
On the other hand if it is intended for any reason to obtain some coarser crystalline structure in the core of the article wall the cooling rate may be controlled to be slower so that the crystalline material in the core has time enough for some crystal growth. In connection with the present inven-tion for any given material a predetermined rate of cooling in the thermal '' ~,, - .

forming step may be provided in view of the desired control of crystal growth in the core material.
Figure 3 shows a web having outer solidified supportive layers l and plastic core material C having a temperature above the crystalline melt-ing point TM. As shown in figure 4A and figure llB the web thickness is sub-stantially reduced during the thermoforming step. Thus the hotter inner core material when distributed between the outer layers or regions 1 becomes a relatively thin inner layer, so that it is possible to cool down rapidly such thin inner layer material during the thermoforming step. Thereby it is possible to provide a cooling down rate through the critical temperature range ~ (figure 2B) such that crystal growth can be sufficiently controlled.
In order to improve cooling at the surface which is not in contact with the cooled tool surface liquid or gaseous cooling medium or powdered dry ice may be introduced into the shaping tool at K.
After the articles have been shaped the web may be fed into a trimming station 7 to cut out the shaped articles from the web or they may be trimmed out while in the forming tool. The residual web is broughtto a receiving station 8 provided with a suitable device 81 for reducing the web material into granular form, so that this residual material may be fed back and recycled through a metering device 82 to mix with fresh material at the extrusion device 1 in a predetermined ratio.
In this first embodiment as illustrated in figures lto 5 crystal-line thermoplastic material of different kind may be used.
In an especially preferred embodiment an isotactic polypropylene is used having a crystallinety in the range of 60 to 70% and a crystalline melting temperature range of 160 to 170 C. The web will be extruded at tem-peratures in the range between 220 C and 250 C. After stabilizing the web will have a temperature at its outer surfaces of about 120 C and a temper-, . ~ ., , , :

~ , . .
: : :

.
.

, ature in the core material in the range of 170 to 200 C. Immediately beforeentering the thermal forming step the te~perature at the outer surfaces of the web will be about 130 to 160 C and the core material will be at about 170 to 200C.
If polypropylene material is used which has a crystalline melting point lower than 160 to 170 C, the extrusion temperature and the temperature of the core material and the temperature at the outer surfaces of the web can be adequately provided at lower levels. Normally the crystalline melt-ing point of each material to be used is known and listed by the producer of the material but in any case it is possible to find the crystalline melting ~ ?
point of a material to be used by some adequate checking which can be easily made.
Figure 5 shows, in a highly enlarged scale, a section through the wall of an article made under the conditions of the preferred manner of execution of the present invention. Due to the fact that the material in the outer layers of the web is pre-cooled to a temperature lower than the lower limit of the crystalline melting temperature range by a high cooling rate only relatively small crystals have the possibility to develop in said outer surface region material. Further by maintaining the said outer sur-;; 20 face region material at such low temperature during the thermal forming step said regions are shaped substantially by stretching. Thus the outer regions ; of the article wall contain a fine structure of small crystals 21 which are oriented by stretching during the thermal forming step. In the core material at first no growth of crystals occurs because such material is held at a temperature above the upper limit TM of the said crystalline melting temper-` ature range ~ before entering the thermal forming step. But during said : thermal forming step cooling occurs in the inner core material at a somewhat ~, slower cooling rate than is done during pre-cooling at the outer surface . ' ' .

, .: ., , . , . . - . , " ~ . . , ,.:' . ' ~:
:: ' : . . . ' . . . .

regions. So in the core region of the article wall middlesized crystals 22 develop. As such crystal development occurs during and after the distribu-tion of the plastic core material within the outer surface regions no stretching effect occurs with respect to these crystals 22. By such pre-cooling and thermal forming steps the article wall obtains a core material region having medium crystallinety and containing medium size crystals; a maximum of stiffness of this inner core of the wall article is obtained. On the other hand the outer layers or surface regions of the article wall con-tain a fine crystalline structure being stretched during thermal forming, so that the outer layers or outer surface regions of the wall have an increased ductility and an increased impact strength.
The above described preferred embodiment of the method may be varied by reheating the outer surface layers by external means immediately before thermal forming. Figure 7a and figure 8a show an embodiment wherein the outer surface layers of the web are reheated by external means immedi-ately before entering the thermal forming step such that the temperature at these outer surface layers is ad~acent to but lower than the lower limit TC
oP the crystalline melting temperature range ~. Such reheating may be desirable in some cases for instance if especially fine surface structures have to be formed at the article surface.
Such reheating may be done in an apparatus as diagrammatically sho~n in figure 6. Such apparatus is basically the same as that one of fig ure 1, except for a reheating means 9 which is provided at the entry of the thermal forming station 6.
All the other parts of the apparatus may be the same as described above in connection with figure 1. Therefore in figure 6 the same reference numbers for the same parts are used.
As may be seen from figure 7a the surface temperature of the web .

' '' :' ~ ' ' .:~' .' ,, ': ~ ', is increased such that it is ad~acent to but lower than the lower limit TC
of the critical crystalline melting temperature range ~. All the other tem- ;
perature conditions may be the same ones as shown in connection with figure 2B.
As shown in figure 8a such reheating has some influence with re-spect to the structure of the finished article wall, since the hotter core material of the web is able to rewarm the above mentioned transition regions between the said inner core and the said outer layers. Thereby some con-trolled growth of crystals can occur in these transition regions and such 10 transition regions will become somewhat thicker. These transition regions ~
therefore will have some equalizing function between the core and the surface `
layers of the article wall. Figure ôa shows that in the inner core region crystals 22 of medium size are produced and at surfaces of the article wall stretched crystals 21 of small size are present in the thermoplastic mate-rial. Between these a third type of crystalF; 23 is present which are some-what larger in size than the crystals 21 of l,he surface regions but sub-stantially smaller in size than the crystals 22 in the core material. These crystals 23 of the third type are essentially unstretched and unoriented because they are produced mainly during the said thermal forming step.
A further modification of the method is shown in connection with figure 7b and figure 8b. To produce articles under this modification the ~ _ same apparatus may be used as described above in connection with figure 6. `
In this case the one surface of the web which is provided to be brought in contact with the shaping tool in the thermal forming step is reheated much more so that its temperature will assume a value above the upper limit TM f the critical crystalline melting temperature range. As shown in figure 7b the outer temperature of this one web surface is near to the upper limit of the upper preferred thermal forming temperature range Yl- Therefore the -:: '-thermoplastic material in this surface region comes into plastic state and must be cooled down through the critical crystalline melting temperature ~ -range ~ during the thermal forming step. Such a second cooling is not dif-ficult because this thin surface layer of the web comes into contact with the cooled tool surface and will be very rapidly cooled down. As shown in figure 8b by such processing, the one surface of the article wall contains small crystals 21 but in unstretched condition and crystals 23 of the third type but in unstretched condition also. The second surface of the article wall has been shaped under practically the same conditions as described 10 above in connection with figure 8a, and so this second surface region con-tains small crystals 21 in stretched condition and crystals 23 of the said third type but in somewhat stretched condition. The inner core of the article wall is structured in the same manner as that one of figure 8a, and contains medium size crystals 22 in an unstretched condition.
A third possible example for modification is shown in figures7c and 8c. The process means to be used are practically the same as shown in figure 6, but reheating by external means is made such that both of the web surfaces are reheated such as described above in connectîon with the one web surface to a temperature above the crystalline melting point TM.
When processing in this third manner, the same conditions are pro-vided at the wall surface formed at the cooled tool surface as in the example of figure 7b and figure 8b. So this contact shaped article wall surface -contains small crystals 21 in unstretched conditions and third type crystals 23 in unstretched condition also. In contrast to the embodiment in figure 7b and 8b the second surface of the article wall only contains crystals 23 of the third type which crystallize during the final cooling in the thermal forming step. The inner core of the article wall in this modification con-tains medium size crystals practically the same as in the examples of figures ~,.. ~.. ~. . .... - . . . . .
. . .

~ ' ' ' ' ,' .' ~ ' ~ ' 5 and 8a and 8b.
If it is desired to avoid any crystal growth in the transition regions during the thermal forming step some modification may be provided in the thermal forming such as shown by a dotted line arrow K in figure 4B.
This dotted line arrow K means that a fluid or particulate cooling medium may be introduced into the closed thermal forming tool and onto the one sur-face of the shaped article wall which is not in contact with the cooled shaping tool surface. For instance powdered dry ice may be introduced at K
and blown onto the said free surface of the shaped article wall. When doing 10 so the over all cooling of the article wall is faster. By such higher cool-ing rate the growth of crystals at all parts within the wall can be held reduced, so that the crystals in the core material will have substantially smaller size than as shown at 22 in figures8a, 8b and 8c.
In connection with the invention crystalline thermoplastic mate-rials of different kind may be used. Preferably crystalline olefine mate-rials may be used in the present process for manufacturing articles.
Special materials sùita~le to be used in this connection may be:
Polyethylene (middle pressure production), having a density in the range between 0,924 and 0,945 (g/cm3), a crystalline melting temperature 20 range between 115 and 127 C and a crystallinity of 65 to 76%.
Polyethylene (low pressure production), having a density between 0,945 and 0,965 (g/cm3), a crystalline melting temperature range of 127 to 137C and a crystallinity of 75 to 95%.
Isotactic polypropylene having a density in the range between o,go8 and 0,905 (g/cm3), a crystalline melting temperature range between 140 and 170 C and a crystallinity of 60 to 70%.
Random copolymerisation products of ethylene and propylene. Block copolymerisation products of ethylene and propylene.

. ., '' ' '

Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of manufacturing thin-walled articles from crystalline thermoplastic material including thermoforming a web or sheet of said thermo-plastic material, while reducing the web or sheet thickness and cutting out shaped articles from said web or sheet wherein said thermoplastic material has a critical crystalline melting temperature range and said method further includes a temperature preconditioning of said web or sheet to suitable tem-perature conditions for thermoforming and is characterized in that said tem-perature preconditioning involves providing at the inner core of said web or sheet a temperature higher than the upper limit of said critical crystalline melting temperature range and providing at the outer surface regions of said web or sheet a temperature lower than the lower limit of said critical crystalline melting temperature range, and in that during said thermoforming step while reducing the thickness of said web or sheet and immediately there-after the thermoplastic material in said core of said web or sheet is rapidly cooled down through said critical crystalline melting temperature range thereby crystal growth in said core material.
2. The method defined in claim 1 wherein the temperature provided at the core material of said temperature preconditioned web or sheet is close to but higher than the upper limit of the critical crystalline melting tem-perature range of said crystalline thermoplastic material.
3. The method defined in claim 1 wherein the temperature provided at the outer surface regions of said temperature preconditioned web or sheet is close to but lower than the lower limit of the critical crystalline melt-ing temperature range of the said crystalline thermoplastic material.
4. The method defined in claim 1 wherein said temperature pre-conditioning for thermoforming includes heating a web or sheet of said thermoplastic material throughout to a temperature higher than the upper limit of said critical crystalline melting temperature range and cooling down under predetermined cooling rate conditions the said outer surface regions of said web or sheet to a temperature lower than the lower limit of said critical melting temperature range, said cooling rate conditions being adapted for controlling growth of crystals in the material of the said outer surface regions of the web or sheet.
5. The method defined in claim 1, characterized by an uninterrupted sequence of steps comprising a) extruding a continuous hot web of said crystalline thermo-plastic material at a conventional extrusion temperature above the crystal-line melting temperature range of said material b) substantially immediately rapidly precooling the opposite surfaces of said web to form along said surfaces thin supportive layers hav-ine a temperature in the range wherein said material is not further plas-tically deformable and wherein further growth of crystals is essentially avoided, provided that the hotter core material between said layers is cooled to a temperature adjacent to but above the crystalline melting temperature range and is held in a substantially crystal free condition, c) transporting to a thermoforming station the so precooled web within a short time period so as to essentially maintain over the web thick-ness the above mentioned temperature profile produced by precooling and d) then thermoforming said web to an article of desired shape under rapid cooling rate conditions, while essentially controlling crystal growth in said material while cooling through said crystalline melting tem-perature range under said cooling rate conditions.
6. The method defined in claim 5 wherein the said precooling of the opposite surfaces of said web is such that crystal growth in the material of said thin supportive layer is essentially avoided during said precooling, provided that the hotter core material between said layer is kept at a tem-perature above the crystalline melting temperature range and in a crystal free condition.
7. The method defined in claim 5 wherein rapidly cooling said web during said thermoforming to an article of desired shape is such that growth of crystals is essentailly avoided or controlled and that only relatively small crystals are developed during said thermoforming.
8. The method defined in claim 5 wherein the web is subjected to a forming tool operation for thermoforming in which the said thin outer sup-portive layers are shaped practically by elastic deformation which is stabilized by final cooling of the article, whereas the warmer core is deformed in a plastic state between the said outer layers to produce an inner wall layer free of oriented crystals, the maximum average size of said crystals being controlled by said cooling rate during the said thermoforming step.
9. The method defined in claim 1 wherein the thermoplastic material is isotactic polypropylene, the extrusion temperature being within the range of 220°C to 250°C, and the temperature preconditioned web has a core temper-ature in the range of 170 to 200°C and the outer layer temperature in the range of 130 to 160°C.
10. The method as defined in claim 1 wherein the thermoplastic mate-rial is a low pressure produced polyethylene having a density of 0,945 to 0.965 g/cm and a crystallinity of 75 to 95%.
11. The method as defined in claim 1 wherein the thermoplastic material is a random co-polymerisation product of ethylene and propylene.
12. The method as defined in claim 1 wherein the thermoplastic material is a block co-polymerisation product of ethylene and propylene.
13. The method as defined in claim 1 wherein pre-cooling of the web is obtained by contact of the web surfaces with the cooled surfaces of heat conducting cooling means.
14. The method defined in claim 1 wherein during said thermoforming one surface of said web or sheet is cooled by contact with a cooled surface of a thermoforming tool while cooling of the opposite surface of said web or sheet not in contact with the cooled tool surface is obtained by contact with a gaseous, liquid or powdered cooling medium.
15. The method as defined in claim 14 wherein cooling of the said web surface not in contact with the cooled tool surface is obtained by con-tact with a powdered dry ice blown thereon.
16. The method as defined in claim 1 wherein immediately before entering the said thermoforming step the said precooled web or sheet of crystalline thermoplastic material at one or both of its surfaces is reheated by external means to a temperature adjacent to but below the lower limit TC
of said crystalline melting temperature range.
17. The method defined in claim 1 wherein immediately before enter-ing the said thermoforming step the precooled web or sheet of crystalline thermoplastic material at one or both of its surfaces is reheated by external means to a temperature within the said crystalline melting temperature range.
18. The method defined in claim 1 wherein immediately before enter-ing the said thermoforming step the precooled web sheet of crystalline thermoplastic material at one or both of its surfaces is reheated by external means to a temperature above the upper limit TM of said crystalline melting temperature range.
CA308,019A 1977-07-25 1978-07-24 Method for manufacturing thin-walled articles from crystalline thermoplastic material by thermoforming Expired CA1114126A (en)

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FR2398592B1 (en) 1984-04-20
GB2001576B (en) 1982-03-03
ZA784183B (en) 1979-07-25
FR2398593B1 (en) 1984-04-13
CA1114125A (en) 1981-12-15
IT1097332B (en) 1985-08-31
ATA508778A (en) 1982-09-15
IT7826034A0 (en) 1978-07-24
FR2398593A1 (en) 1979-02-23
BE869217A (en) 1979-01-24
NL7807832A (en) 1979-01-29
JPS5440870A (en) 1979-03-31
ES471997A1 (en) 1979-02-16
DE2830740C2 (en) 1985-11-07
ZA784184B (en) 1979-07-25
AT370672B (en) 1983-04-25
AR215724A1 (en) 1979-10-31
SE7807756L (en) 1979-01-26
GB2001576A (en) 1979-02-07
GB2001577B (en) 1982-01-13
IT1097331B (en) 1985-08-31
DE2830740A1 (en) 1979-02-15
FR2398592A1 (en) 1979-02-23
GB2001577A (en) 1979-02-07
IT7826035A0 (en) 1978-07-24
SE423878B (en) 1982-06-14
AT370671B (en) 1983-04-25
NL7807831A (en) 1979-01-29
ATA508878A (en) 1982-09-15
DE2830788A1 (en) 1979-02-15

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