US3121255A

US3121255A - Apparatus for extruding plastic compounds

Info

Publication number: US3121255A
Application number: US70323A
Authority: US
Inventors: George E Henning; Melvin W Richter
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1960-11-18
Filing date: 1960-11-18
Publication date: 1964-02-18
Anticipated expiration: 1981-02-18
Also published as: AT243885B; ES272459A1; GB997970A; SE305311B; DE1554979B1

Description

Feb. 18, 1964 as. HENNING ETAL 3,121,255

APPARATUS FOR EXTRUDING PLASTIC COMPOUNDS Filed Nov. 18, 1960 v 4 Sheets-Sheet 1 wx/s/vrons G. E. HENN/NG M W RICHTER Feb. 18, 1964 e. E. HENNING ETAL 3,121,255

APPARATUS FOR EXTRUDING PLASTIC COMPOUNDS Filed Nov. 18. 1960 4 Sheets-Sheet 2 lNl/EN TORS' G. E HE NN/NG BY M m RICHTER A90? 05% A 7' TORNE V Feb. 18, 1964 G. E. HENNING ETAL APPARATUS FOR EXTRUDING PLASTIC COMPOUNDS 4 Sheets-Sheet 3 Filed Nov. 18, 1960 R S mm m O/H M C v wmm w WH fim C W GM A w M B 67 8 a H M 3 80 c 8 56 M C I F H 1954 G. E. HENNING ETAL APPARATUS F OR EXTRUDING PLASTIC COMPOUNDS Filed Nov. 18, 1960 4 Sheets-Sheet 4 FIG. 5

//VVENTOR$' G. E HENN/NG y M W R/CHTER A T TORNE V United States Patent {and 3,121,255 APPARATUS FOR EXTRUDENG PLASTIC CQMPQUNDS George E. Henning, Baltimo e, and Melvin W. Richter,

Perry Hall, Md, assignors to Western Electric Zornpany, Incorporated, a corporation of New York Filed Nov. 18, 195%, Ser. No. 7%,323 7 (Ilaims. ((Jl. 13-43) The present invention relates to extrusion apparatus having a common tool chamber capable of extruding a plurality of coatings of plastic compounds simultaneously on a common core.

In the communications industry, many rural and urban wires, used for transmitting telephone messages from distribution cables to rural and urban subscribers telephones, are formed by stranding twisted pairs of sheathed copper conductors around a central polyethylene-insulated support wire. Each of the copper conductors is covered with a composite sheath which includes an inner layer of solid black, weather-resistant polyethylene and an outer, relatively tough, weather-resistant, colored layer or jacket of polyvinyl chloride.

In the past, the 'mdividual conductors for rural and urban wires were insulated in two separate and distinct extruders; however, when this method of insulating is used, fairly close diameter control of the layer of polyethylene is necessary to minimize problems in subsequent jacketing operations. Further, when the insulation is formed in two separate operations, more scrap is produced, and more maintenance, extra labor, more floor space, and additional equipment is required than when the two coats of insulation are placed on the conductor in one operation. Accordingly, it is highly desirable to insulate the conductors by applying both the inner insulating sheath and outer jacket in one operation. To overcome these disadvantages and to reduce the cost of insulating the multicoat-insulated, copper conductors, extrusion apparatus have been developed to permit both of the insulating coatings to be applied to a conductor simultaneously in a common tool assembly in a single extrusion head.

in the advancing technical art of extrusion, higher and higher rates of extrusion are attainable which magnify the ever-existing problem of providing concentricity of the insulating sheath with respect to the center core. One of the common causes of eccentricity of the plastic insulating sheath with respect to the conductive core, which also causes excessive wearing of the core tube and other extrusion tools, is misalignment of the extruder head with respect to the normal path of travel of the conductive core from a supply thereof to the take-up. If the extruder head is free to move as a result of thermal expansion of the extrusion equipment or is free to turn with respect to a predetermined path of travel of the conductor during the operation of the extruder, the axis of the extruder head will vary, during such operation, with respect to the normal path of travel of the conductive core through the extruder. This variation will affect the centering of the core in the extruded sheath and cause excessive wearing of the core and extrusion tools. Accordingly, it is highly desirable to prevent misalignment of the extruder head with respect to the normal path of travel of the conductive core through the extruder.

Another problem which may be encountered in extrusion processes of this type is a tendency for some particles of particular plastic compounds, such as polyethylene, to adhere to the exit end of the dies from which the particular compound emerges during an extrusion process. The particles so deposited on the die usually build up into relatively large masses which eventually break loose from the end of the die and interfere with the production of uniformly insulated and jacketed coatings applied thereby. This eifect is termed drooling in the extrusion art and the plastic compound which is deposited in this manner is called drool. This drool is undesirable because it diminishes the quality of the extruded covering and is deleterious to the electrical properties of the insulated conductor.

A still further problem to be overcome is the tendency for loose jackets to be produced when certain types of plastic compounds are utilized. Because of the difference in the thermal expansion of polyvinyl chloride compound with respect to the polyethylene compound, precautions must be taken in such an extrusion process to minimize the degree of looseness in the outer jacket of polyvinyl chloride over the inner polyethylene sheath. Also, precautions must be taken to prevent excessive heat from being transmitted from one portion of the apparatus to another.

In order to overcome these and other undesirable features, it is an object of the present invention to provide new and improved apparatus for extruding plastic compounds.

It is another object of the present invention to provide new and improved apparatus for extruding laminated articles of two different plastic compounds simultaneously with two different extruders having a common extrusion head and/ or a common tool extruder.

It is still another object of the present invention to provide new and improved apparatus for extruding simul taneously two coatings of different plastic compounds on a core of an indefinite length.

A further object of the present invention is to provide new and improved means for mounting extrusion apparatus so as to compensate for thermal expansion and contraction of the apparatus during operation.

A still further object of the present invention is to provide new and improved apparatus for preventing detrimental efiects of drool in processes in which two coatings of plastic compounds are extruded simultaneously by minimizing the mass of any particles of drool which may form during an extrusion process, and to minimize the looseness of an outer jacket formed simultaneously with an inner sheath on a conductive core.

Extrusion apparatus embodying certain features of the present invention may include an extrusion cylinder having a common tool chamber secured fixedly to the discharge end thereof. One or more extrusion cylinders, which are mounted movably at the entrance or hopper ends thereof, have the discharge ends thereof secured to the common tool chamber and communicating with an extrusion passage therein. The tool chamber is utilized to place two or more layers of dilierent types of plastic compounds simultaneously on a common core.

The mass of any particles of drool of the plastic compound forming the inner layer of plastic compound on the core may be minimized in accordance with certain principles of the present invention by causing the plastic material, forming the outer layer of plastic compound, to sweep any particles of drool which may tend to form on the end of an extrusion die used to form the inner layer of plastic compound off the end of the die. The velocity of the flow of the plastic compound, which forms the outer layer of compound on the core, adjacent to the end of the die used to form the inner layer may be increased by maintaining the end portion of the die at a temperature, which is higher than the normal temperature of the plastic compound flowing therepast, to cause that portion of the plastic compound to be more fluid than the portion of the compound adjacent to the die utilized for forming the outer jacket.

Other objects and features of the present invention will be more readily understood from the following detailed description of specific embodiments thereof, when read in conjunction with the accompanying drawings in which:

El-G. 1 is a fragmentary plan view of an extrusion apparatus embodying certain features of the present invention with portions thereof broken away for purposes of clarity;

FIG. 2 is an enlarged, fragmentary, perspective view of the extrusion apparatus of FIG. 1, with heat insulating jackets broken away from extrusion cylinders of the apparatus for purposes of clarity;

FIG. 3 is an enlarged, fragmentary, front, elevational view of a common extrusion head of the apparatus of F168. 1 and 2 and illustrating an alternate embodiment of a pedestal for supporting the extruder head with portions thereof broken away for purposes of clarity;

FIG. 4 is an enlarged, sectional view of the extruder head of FIG. 3, taken substantially along line 44 thereof;

FIG. 5 is an enlarged, fragmentary, sectional view of a portion of the extruder head of FIG. 1, taken along line 5-5 thereof, and

FIG. 6 is an enlarged, fragmentary, sectional view of a portion of the extruder head of FIG. 5, taken along line 66 thereof.

Referring now to the drawings, there is shown a crosshead-type, multicoat extruder, designated generally by the numeral 10. The extruder is designed specifically for extruding successive layers of two ditlerent plastic materials on a 19 gauge conductive core or strand 11 to form an insulated and jacketed conductor for urban or rural wire, designated generally by the numeral 12, having a QGZO-inch polyethylene insulating sheath 14 and a (MiG-inch polyvinyl chloride jacket 15. The extruder it) includes a common extruder head, designated generally by the numeral 13, into which two extrusion cylinders, designated generally by the numerals 16 and 17 are secured threadcdly. The longitudinal axes of the cylinders 5 and 17 are contained within a common horizontal plane and converge toward the discharge ends 18 and 19 thereof.

The entrance ends 22 and 23 of each of the extrusion cylinders 16 and 17 are supported in separate, suitably cooled housings, designated generally by the numerals 24 and as, containing associated gear-reduction units (not shown) which are connected operatively, in a conventional manner, to the adjacent end of associated stock screws 3235 (only one of which is illustrated) or" the type disclosed in Garn'urill et al. Patent 2,872,763. Oil in the housings 2d and 2s is maintained substantially at a predetermined temperature by cooling coils in an oil reservoir (not shown). The

bases

36 and 37 of the

housings

24 and 26 are mounted slidably on bearing surfaces 38 33 and 3939, respectively, formed integrally with hearing plates 4tl-4% and secured on a fixed common base 41 by appropriate means (not shown). A key is formed on and projects upwardly from each of the bearing plates idto permit free movement of the housings 2d and only in directions parallel to the axes of the respective cylinders in and 17. Main drive motors 46 and 47 (-FiG. l) are fixed rigidly to the floor and are connected to the gear-reduction units by means of V-belts S'$5t, jack shafts Sit-51 and 52r52 and Para-flex flexible cushion couplings 53-53 obtainable from Dodge Manufacturing Corporation, lviishawaka, Indiana.

To prevent misalignment of the extruder head 13 due to thermal expansion or contraction of the cylinders 1e and '17, the extruder head 13 is supported in a head pedestal, designated generally by the numeral 56. The head pedestal 55 is secured rigidly to the common base 41, which, in turn, is secured fixedly to the floor. The extruder head 13 can be secured rigidly to the head pedestal 55, as illustrated in PEG. 2, by suitable means such as bolts (not shown). However, the head pedestal 5a; is illustrated in FlG. 3 as being provided with an aperture i 57 designed to receive a rectangular projection 58 depending from the extruder head 13. A heat-insulating barr er 6% of a relatively hard and low thermal conductivi;y material, such as asbestos, is interposed between the head pedestal es and the extruder head 13, and between the rectangular projection 5% and the aperture 57 in the pedestal 56. With this arrangement, the extruder head 13 is mounted rigidly to the floor to prevent hori zontal movement thereof. However, the head pedestal 56 is free to expand and contract in a vertical direction under the influence of any heat which may be conducted through the heat-insulating barrier 6% during operation of the extrusion apparatus 19.

When the extrusion cylinders 16 and 17 expand as a result of temperature increases during operation, the rectangular projection 58 will prevent the extruder head 13 from twisting and the entrance ends 22 and 23 of the [-mlers 16 and 17 will be moved away from the extruder on the sli ably mounted housings 2d and 26. "y, when the extrusion cylinders 16 and 17 cool contract, the entrance ends .22 and 23 thereof will move toward the extruder head 13. In all cases, the extrucler head 13 is restrained against any movement, since the head 13 will be maintained in a fixed vertical line by the pedestal 56 and will be maintained in a horizontal plane by the cooled housings 2d and 25 containing the gear-reduction units. in this manner the longitudinal axis or" the extruder head 13 is maintained coincident with the longitudinal axis of the core 11.

Each of the extrusion cylinders 16 and 17 are heated by conventional, thermostatically controlled, electrical band heaters 5959 and have an elongated bore 61 formed in a cylindrical lining 63 which is secured rigidly in the associated cylinder 16 or 17. The gear-reduction units are connected operatively by conventional means (not shown) to the ends of the stock screws 3232 mounted rotatably in the linings 63-63 of the extrusion cylinders 10 and 17, respectively. The stock screws 32-32 are water cooled and are designed to be rotated by the stationary motors do and 47, respectively, through the associated flexible couplings 53-53 and gear-reduction units. Since the couplings 5353' are aligned when the cylinders 1d and 17 of extrusion apparatus 1% are at operating temperatures, which diifer by approximately 1%" B, there is no objectionable vibration of the gear reduction units under full load conditions. When the cylinders 16 and 17 are cold, misalignment of the shafts 51 and 52 associated with the cylinder 17 exceeds inch and the misalignment of the shafts 51 and 52 associated with the cylinder '16 exceeds inch; however, the vibration caused by such misalignment is negligible.

The stock screws 32-32 are rotated for the purpose of advancing suitable plastic compounds, such as polyethylene compound 63 and polyvinyl chloride compound as, from

feed hoppers

72 and 73, respectively, at the entrance ends 22 and 23 of the cylinders 16 and 17, along the extrusion bores 61-161, toward the delivery ends 13 and 19 of the cylinders 16 and 17 and into and through the common head 13. The stock screws 3232 work and knead the plastic compounds as the

respective compounds

63 and 69 are being advanced through the extrusion cylinders 16 and 17 by the screws 3232 to get the plastic compounds 53 and 69 into a suil'iciently fluid state for smooth and uniform extrusion thereof.

Externally threaded ends 7575 (only one of which is shown in FIG. 4) of each of the extrusion cylinders 16 and 17 are screwed into an internally threaded main passage 76 in the fixed common extrusion head 13 which is aligned axially with the associated extrusion cylinder 16 or 17. A perforated and centrally apertured breaker plate 81 is mounted slidably and rotatably in each of the main passages 76'7ti in the head 13 into which the exit ends 18 and 19 of extrusion cylinders 15 and 17 are secured threadedly. The breaker plates 31 are provided with depressions 2S2 and radially inwardly projecting tabs 3383 which permit arched screens 84-84 to be inserted into the breaker plates 8181 during assemblage of the extrusion apparatus 10. The tabs 83- 33 prevent unintended removal of the associated screens 844 when the screens and breaker plates 8l 81 are placed in position in the extrusion head 13.

The breaker plates 8181 are symmetrical about the longitudinal axes of the respective cylinders 16 and 17 and are provided with frustoconically shaped portions 3686 adjacent to the ends thereof opposite the ends of the associated stock screws 32-32. Externally threaded breaker plate retaining nuts 87-87 are secured threadedly within and close the forward end of each of the main passages 7676. The breaker-plate retaining nuts 87 87 are provided with inwardly projecting supports 88 83 having threaded bores 89-89 designed to receive complementarily externally threaded central posts Ell-9t"; projecting from the breaker plates 81Sl. The pitch of the external and internal threads of each of the retaining nuts 37-37 are substantially identical to permit removal of the retaining nuts independently of the associated breaker plates 81-431, in the event that the breaker plates may be hung up or locked in the

respective compounds

63 and 69 for some reason or other.

The projecting support 38 of each of the breaker-plate retaining nuts S7S'7 engages the end of the frustoconical portion 86 of the associated breaker plate 81 and cooperates with a slanting surface 2 of the frustoconical portion 86 to form an annular passage 93 symmetrical about the axis of the associated extrusion cylinder 16 or 17. The annular passage 93 communicates with a main feed passage 9%, in the head 13, leading from the main passage 76 to auxiliary feed passages 96-% and a

pressure gauge

97 or 98 associated with the particular cylinder 16 or 17. This arrangement permits the compound 68 or as to flow through the extruder head 13 regardless of the angular orientation of the retaining nut 87 and breaker plate 81 about the axes of the extrusion cylinder 16 or 17. The retaining nuts 87S7 are also provided with externally threaded blow-out plugs 999 to prevent excessive plastic pressures from being formed in the cylinders 16 and 17.

A common tool chamber is provided in the common extrusion head 13 by an upwardly projecting portion 161 thereof located intermediate thereof equidistantly from the main passages 7676 into which the discharge ends 13 and E of the extrusion cylinders 16 and 17 are secured. The common tool chamber is formed by a generally cylindrical extrusion passage, designated generally by the numeral 152 (FIG. 5), passing through the upwardly project ng portion it'll of the head 13 transversely of the extrusion cylinders 16 and 17. The extrusion passage 1% is provided with a relatively short, threaded, cylindrical bore 193 formed intermediate the ends thereof, a cylindrical counterbore 194 on each side of the threaded bore 1%?) and a threaded counterbore 1% on each side of the two counterbores 104-104.

A hardened, externally threaded retaining insert 107, is screwed into the threaded bore 193 until a circumferential projection 1 38 registers with a tapered shoulder it formed between the bore 103 and the adjacent counterbore 1%. A heat insulating material Hi5 such as silicon carbide or other structurally suitable, relatively low heat conductivity material is pressed into the insert 1437. The heat insulating material 1135 is utilized to reduce the amount of heat conducted from pressure-equalizing chambers containing the relatively hot polyethylene compound 68 to the pressure-equalizing chamber containing the cooler polyvinyl chloride compound 69 to prevent charring of the polyvinyl chloride compound 69 therein.

The hardened retaining insert 167 centers a unitary member which forms a combined polyethylene die and core tube, designated generally by the numeral 111, within the extrusion passage 1G2, and also serves to divide the extrusion passage 162 into two pressure-equalizing chambers in the counterbores 1t94104, one for the polyethylene compound 68 and one for the polyvinyl chloride compound 69. The hardened insert 107 can be readily replaced in the event that plastic-sealing surfaces thereof are damaged. The hardened insert 107 also permits rather etxensive changes in tool design should new plastic compounds or new designs of products require such steps.

Generally, the unitary member which forms the combined polyethylene die and core tube 111 is positioned in an axial be e lid in the retaining insert N7, and has a tapered annular shoulder 117 on the trailing end of the combined die and core tube 111, which registers with the side of the retairu'ng insert 197 and streamlines the flow path of the polyethylene compound 68 so as to eliminate stagnating areas. The entrance portion or the entire unitar y member 111 may be made of a suitable heat insulating and wear resistant material such as Heanium, a trademark of the Heany Company for an artificial sapplnre, to reduce the amount of heat transferred from one of the compounds as or 69 to the other.

An externally and internally threaded core-tube holder, designated generally by the numeral 121, which forms part of a core-tube assembly, is screwed into the threaded connterbore 195 in the trailing end of the extrusion passage lilZ. The core-tube holder 121 has a tapered end portion 122, which has a substantially pyramidal outer periphery with rounded corners (not shown). The flat sides 123123 of the tapered end portion 122 of the coretube holder 121 converge at an included angle of 60.

The core-tube holder 121 is screwed into the threaded counterhore 1536 until the rounded corners of the tapered, pyramidal, end portion 122 of the core-tube holder 121 registers with an internal frustoconical bore 126 forming an approach of the combined polyethylene die and core tube 111, and urges the annular shoulder 117 of the combined die and core tube 111 against the retaining insert 167. The rounded corners of the end portion 122 of the core-tube holder 12?. register with the complementary curved surface of the walls of an approach 126 for the combined polyethylene die and core tube 131 to form four equal passages 127-l27 having circular-segmental cross sections. The passages l27-l27 are utilized to permit the polyethylene compound 68 to pass from the pressure-equalizing chamber, formed in the counterbore lt i between the core-tube holder 121 and the retaining insert 167, into the combined polyethylene die and core tube ill.

An externally threaded, generally cylindrical core'tube tip, designated generally by the numeral 331, having a diamond die E32 mounted in a tapered leading end thereof, is positioned adjustably within a cylindrical bore 133 and threaded counterbore 134 in the core-tube holder 1231. An elongated, cylindrical guide tube 136 is mounted fixedly in a cylindrical bore 137 in the core tube tip 131 so that the elongated cylindrical guide tube serves as a core guide for the conductive core 11 pass'mg therethrough.

An extrusion die, designated generally by the numeral 141, hereinafter referred to as the polyvinyl chloride die, is mounted adjustably in an aperture 42 in a die holder, designated generally by the numeral 143, and is utilized for forming the polyvinyl chloride compound 69 into the jacket 15. The die holder 143 is, in turn, mounted in an aperture in an externally threaded, die retaining nut, designated generally by the numeral 146, which is screwed into the threaded counterbore 1% at the leading end of the extrusion passage 102. The dieretaining nut 14% is turned into the threaded counterbore until the walls of a frustoconical bore 14-7 of the die holder 143 for the polyvinyl chloride extrusion die 141 registers with complementarily curved corners 14"- of a substantially pyramidally shaped portion 149 of the combined die and core tube 111 adjacent to the tip of the combined polyethylene die and core tube 111.

The pyramidal portion 149 of the combined polyethylene extrusion die and core tube ill which regi ters with the frustoconical bore 147 of the die holder 163 is the same general shape as the correspon portion of the core-tube holder 121i, and cooperates with t die holder 1 :3 to form four circular-segmentally shape passages l ll5l through which the polyvinyl chlorid compound 69 passes from a polyvinyl chloride equalizing chamber between the retainer insert 1%? and the dieretaining nut 14% to the die 143. The rounded corners of the pyramidally shaped portions of the combined die and core tube ill and the core-tube holder 121 are oriented preferably so that two of the corners of each of the pyramidally shaped portions are contained a vertical plane containing the axis of the conductive core 11. The flat slanting surfaces of the portion 34) con verge at an included angle 0 34.

The extrusion die 141 has a tapered frustoconical entrance 3152 and a land 159 approximately (MW-inch long forming an exit port for shaping the polyvinyl chloride compound as into the jacket 15 about polyethylene-compound-covered conductive core 11. A land 15%) in excess of (32005 inch results in a loose jacket 15 over the sheath 14. The entrance 152 or" the extrusion die 14-1 converges at an included angle of approximately 16 and communicates with a frusto conical approach 153 of the extrusion die 1 21. The walls of the approach 153 or" the die 14-1 converge on the frustoconical entrance 152 of the die 141 at an included angle of approximately 60.

As mentioned hereinbefore, there is a tendency for some particles of plastic compounds, such as polyethylene compounds, to adhere to the exit end of dies from which the particular compound emerges during an extrusion process. The particles so deposited on the end of the die usually build up into relatively large masses which eventually break loose from the end of the die and ad here to the insulating covering applied thereby. effect is termed drooling in the extrusion art and the plastic compound which is deposited in this manner is called drool. This drool is undesirable because it diminishes the quality of the extruded covering and is deleterious to the electrical properties of the insulated conductor.

If the mass of each particle of drool is minimized, the effect of the drool on the finished extruded article is likewise minimized. Accordingly, the shape of the tip of the combined polyethylene die and core tube ill, the shape of the approach 153 of the polyvinyl chloride die 141, and the relative positions thereof have been desi ned and arranged so that the drool does not build up on the end of the combined polyethylene die and core tube 111, and, therefore, the masses or" the articles of drool are relatively small and the drool has little or no adverse effects on the properties of the insulated and jacketed conductor 12.

The approach 153 of the die 141 is parall l to a frustoconical end 156 of the combined polyethylene die and core tube 111 and is positioned sufficiently close thereto to provide the converging annular constricted passage from about 14 mils thick and approximately 150 mils long through which the polyvinyl chloride compound 69 flows at a relatively high velocity toward the polyethylene sheath 14 on the conductive core ll so as to force any drool tending to form on a sharp edge I57 of the leading end of the combined die and core tube 111 t ward the polyet iylene sheath l4 and minimize the mass of any particles of drool between the polyethylene sheath 14 and the polyvinyl chloride jacket 15.

in order to maintain the desired constricted passage between the die 1 1 and leading end of the combined die and core tube lit, the desired le of the entrance of 0.168 inch and the minimum diameter of 0.096 inch of the orifice of the extrusion die 141 for a l9-gauge conductor 11 having a diameter of 0.936 inch, it may be desirable to to modify the contour of the portion of the extrusion die 341 forming the entrance 152 of the die so that a back relief angle (not shown) or" approximately 30 is formed immediately adjacent to the walls of the approach 153 of the die 141. This arrangement would permit a die 141 to be constructed with the entrance of the die 141 having a larger maximum diameter than otherwise permitted. A second converging frustoconical entrance could be formed adjacent to the back relief angle to form a convex portion in the surface of the orifi e of the extrusion die Lil immediately ahead of the approach 153 thereof having a V-shaped cross section. Alternatively, it may be desirable to place a back relief angle of approximately 5 at the end of the land 35%) or" the die 141.

A low-presure polyethylene extrusion die (i.e., one which ofiers little resistance to compound flow) tends to drool more than dies which have more resistance to compound flow. This is undoubtedly related to the plastic pressure immediately before the compound emerges from the die. Thus, a lov -resist-ance die is a die which will cause relatively little pressure dropping through the die, w ereas a high-resistance die is a die which will cause a comparatively high-pressure drop through the die. The polyethylene compound 63 is formed in a high-resistance die orifice ital of the combined die and core tube 111 having a land, designated by the numeral 162, approximately 0.620-irrch long, and an entrance 163, the walls or" which converge to approximately an 0.077-incl1 diameter at an included angle of approximately 5. With this arrangement, the flow of the polyethylene compound 63 and the drool formed on the tip 157 of the combined die and core tube 111 can be better controlled by the pressure of the polyvinyl chloride compound 69, and hence the tendency for the polyethylene compound 68 to drool can be reduced. The plastic pressures, which range from about 5,500 p.s.i. to about 6,000 psi for the polyethylene compound 63 and from about 2,500 psi. to about 3,000 psi. for the polyvinyl chloride compound 69, as determined by a reading of the gauges 97 and 93, has been found to be the most desirable range of pressures; whereas smaller pressures used on the polyethylene compound 68 cause a greater degree of drooling.

A marked decrease in the accumulation of drool in the exit end or tip 157 of the combined polyethylene die and core tube ill has been produced by providing the tapered, smooth-surfaced tip 156 on the wmbined polyethylene die and core tube 111 of the design illustrated in F G. 6, wherein the conical tip rat has a slope in the ireotion of travel of the conductive core 11 with an included angle of approximteely 60 and the end of the tip 156 is formed in the annular knife edge 157. However, to make a more sturdy structure, it may be desirable to place a 37 /2 slope on the very end of the tip of the combined die and core tube ll, as illustrated in phantorn lines in FIG. 6. The polyvinyl chloride extrusion die 141 should still be constructed and so arranged with respect to the tip 156 of the combined die and core tube 111 that a con-verging annular passage having about 0.014- inch clearance is provided between the conical surface or" the tip 156 of the combined die and'core tube 111 and the approach 153 or" the polyvinyl chloride extrusion die 141. The passage should be kept highly restricted, and can be adjusted by turning a threaded member 166 in the die holder 143.

Since the conical tip 156 of the combined polyethylene die and core tube Bill is ma ntained at a temperature from about 425 F. to about 450 F. by the flow of polyethylene compound ss and the temperature of the polyvinyl chloride extrusion die ldl is maintained at a temperature from about 310 F. to about 325 F. as the polyvinyl chloride compound 69 flows therethrough, the polyvinyl chloride compound 69 will move faster along the surface of the tip 156 of the combined polyethylene die and core tube 111 than along the surface of the relatively cool 9; polyvinyl chloride extrusion die 141. This difierence in temperatures between the tip 156 of the combined die and core tube 111 and the die 141 causes a gradient in the flow of the polyvinyl chloride compound 69 as indicated by the flow vectors illustrated in FIG. 6.

As the polyvinyl chloride compound 69 enters the substantially 16 entrance 152 of the polyvinyl chloride extrusion die 141, the clearance between the tips 156 and the entrance 152, of the die 141, increases with a consequest rise in the static pressure of the polyvinyl chloride compound 69 flowing therethrough. The polyvinyl chloride compound 69 near the tip 156 of the combined polyethylene die and core tube 111 is moving at a velocity close to that of the speed of the conductor 11; and, therefore, the tendency for drool to form and accumulate on the edge 157 of the conical tip 156 will be immediately counteracted by the how of polyvinyl chloride compound 69. Any drool which tends to form at the edge 157 of the tip 156 will be swept off of the edge 157 immediately and will be deposited between the inner face of the polyethylene sheath 14 and polyvinyl chloride jacket 15.

This type of extrusion apparatus can be used satisfactorily to minimize the tendency for particles of drool to adhere to the exit end of the combined die and core tube 111, from which high or low density polyethylene compound emerges during the extrusion process if the extrusion die 141 is designed and arranged with respect to the combined die and core tube 111 in accordance with the following equations:

Ztanfi S: (0.3 to 0.5)D where D=diameter over the PVC jacket 15 and ranges between 0.04 inch and 0.255 inch.

d=diameter over the polyethylene sheath 14.

C=clearance between the approach 153 of the PVC extrusion die 14- 1 and the irustoconical tip 156 on combined polyethylene die and core tube 111.

K=axial length of restricted passage between the approach 153 of the die 141 and the tip 15% of the combined die and core tube 111.

L=axial length of converging angle forming the entrance 152 of the PVC extrusion die 14-1.

F=the axial distance that frustoconical tip 156 of the combined polyethylene die and core tube 111 extends into the entrance 152 of the PVC extrusion die 14-1.

S=space between the entrance end of the extrusion die 141 and the adjacent end of the pyramidally shaped portion of the combined polyethylene die and core tube 111.

oc=il16 included angle of the entrance 152 of the PVC extrusion die 141.

{i=the included angle of the approach 153 of the PVC extrusion die 14 1.

The approach 153 of the extrusion die should be tapered so that the included angle B ranges from between 45 to 90 and should preferably have the same slope as the frustoconical tip 156 of the combined polyethylene die and core tube 111 so that the restricted passage therebetween is formed by substantially parallel surfaces. However, more satisfactory results are obtained when the included angle lies between 60 to 75.

It may be advisable to modify the tip 156 of the combined die and core tube 111 so that the tip has a relatively short taper thereon which has an included angle 10 ranging from 75 to in order to withstand normal wear during operation.

The included angle of the entrance of the extrusion die 14 1 may range between 10 and 16 to suit the geometry of the design.

Desired extrusion characteristics may be achieved by utilizing this type of extrusion apparatus designed in accordance with the above equations for extrud ng an in sulating sheath on a conductive core with high or low density polyethylene compounds being extruded at a pressure ranging from approximately 4,000 psi. to apprommately 7,000 psi. and at a temperature ranging from approximately 400 F. to approximately 475 F., and for jacketing the polyethylene sheathed conductive core with plasticized or semi-rigid polyvinyl chloride compounds being extruded at a pressure ranging from approximately 2,000 psi. to approximately 4,000 p.s.i. and at a temperature ranging from approximately 300 F. to approximately 350 F.

Because of the difference in the thermal expansion of polyvinyl chloride compound 69 with respect to the polyethylene compound 68, precautions must be taken in such an extrusion process to minimize the degree of looseness in the outer polyvinyl chloride jacket 15 over the inner polyethylene sheath 1 The degree of looseness on the polyvinyl chloride jacket 15 can be reduced satisfactorily by increasing the minimum diameter of the die orifice 161 of the combined polyethylene die and core tube 111 used to form the inner polyethylene sheath 14 and of the die orifice of the polyvinyl chloride extrusion die 141 used to form the outer polyvinyl chloride jacket 15 on the inner polyethylene sheath 14 and increasing the speed of the conductor 11 so that a drawing-down effect will occur during the formation of the inner sheath 14 and the outer jacket 15.

Operation During the operation of the extrusion apparatus, the conductor 11 is pulled through the common extrusion head 13 of the extrusion apparatus 10 from a supply stand 1'71 by a capstan 172 driven by suitable means such as a motor 173. The conductor 11 is passed in a predetermined path of travel from the supply stand 171, over a guide sheave 174 aligned with the axis of the extrusion passage 102, through the extruder head 13, into a cooling trough 176 aligned with the axis of the extrusion passage 102, around the capstan 1'72 and is wound subsequently upon a take-up reel 1'77 driven by a suitable means such as a motor 1'78. At the same time, the polyethylene and polyvinyl chloride compounds 68 and 69, respectively, are advanced by the rotating stock screws 32-32, through the extrusion cylinders 16 and 17, screens 8484 and breaker plates 5131.

The plastic compounds 68 and 6? pass from each of the breaker plates 31-31 through the annular passages 9393 located between the breaker plates 81-81 and the breaker plate retaining nuts 87-S7 and into the main feed passages 9-i94. Viewing the extrusion passage 102 from the conductor-exiting side, as shown in FIG. 4, it will be noted that the polyvinyl chloride compound 69 passing through the single feed passage 94, divides into two relatively smaller streams in the auxiliary feed passages %-96 which enter substantially tangentially to the corresponding pressureequalizing chamber in the extru sion passage 102. A similar arrangement also exists for the polyethylene compound 68 on the conductor-entering side of the extrusion head 13.

It will be noted that the extrusion tools are placed equidistantly from the top side or" the two angularly disposed extruder cylinders 15 and 17. The combined polyethylene die and core tube 111 serves as a register member for the other extrusion tool components; namely, the core-tube tip 131 through which the l9-gauge copper conductor 11 enters the extrusion passage 102 and the polyvinyl chloride die 141 which applies the final jacket l l 15 over the polyethylene sheath 14. These details are housed in separate hardened and ground, threaded members 197, 121 and l lo which act to positively seal the plastic compounds 68 and in their respective pressureequalizing chambers. The cooperative arrangement of the extrusion tools also allows the plastic compounds 63 and l 69 to feed into the combined die and core tube 111 and the extrusion 141 through the circular-segmentally shaped passages 127-127 and 151-451 formed by the unique cooperative arrangements which serves to hold the tool members in close concentric alignment. The cross-sectional areas of the passages 127 12 7 and 15l 151 should be at least between and 10 times the crosssectional area of the sheath 14 and jacket 15, respectively.

The polyethylene compound 68 flows from the circularsegmentally shaped passages 127-4127, through an adjustable, converging restricted passage formed between a frustoconical portion 131 of the core-tube tip 151 and the converging frustcconical walls of approach 126 of the combined die and core tube Illl and through the combined polyethylene die and core tube 111 wherein the polyethylene compound 68 is shaped about the conductive core 111 to form the polyethylene sheath 14. The polyvinyl chloride compound as flows from the circular-segmentally shaped passages l5Ii15l through the restricted passage between the extrusion die 141 and the combined die and core tube 111. The polyvinyl chloride compound 69 flows through the constricted passage at a relatively high velocity which is higher adjacent to the combined polyethylene die and core tube 111 than it is adjacent to the polyvinyl chloride extrusion die 141 because of the difference in the temperature of the two elements. The polyvinyl chloride compound 69 flows toward the polyethylene sheath 14 on the conductive core 11 at approximately the same velocity as the conductor 11 is travelling so as to sweep any drool which tends to eminate from the combined polyethylene die and core tube 111 onto the polyethylene sheath 1 2- to minimize the mass of particles of drool between the polyethylene sheath 14 and polyvinyl chloride jacket 15.

Several dififerent arrangements may be used for connecting two or more extruders together to a common tool chamber to be used to place simultaneously two or more layers of plastic compound of different colors or compounds having difierent physical characteristics on a conductive core. A conventional fixed extruder might be converted to a multicoat extruder by adapting a multicoat head to the conventional extruder and securing the discharge end of a movably mounted extruder into the common multicoat extruder head. The movably mounted extruder may be mounted slidably on an axis parallel to the axis of the fixed extruder on the same or opposite side of the common head as the conventional extruder. The axis of the movable extruder could be positioned at a fixed angle with respect to the fixed ex truder and be mounted on a universally, horizontally reciprocable slide. The movable extruder could also be mounted on a pivotable slide so that the axis of the movable extruder could be positioned at a variable angle with respect to the fixed extruder and the movable extruder should then be secured to the common head by a swivel coupling.

It should be noted that the extrusion tools used to shape plastic compounds, which are extruded at relatively low temperatures, and the mounting means for these tools, such as the die 14-1, the die holder 143 and the die-retaining nut 14s, are preferably made of materials having a relatively high coefiic'ent or" thermal conductivity, such as possessed by leaded commercial bronze or other suitable copper alloys so that heat is transmitted therethrough and dissipated to the surrounding atmosphere or an appropriate cooling medium at a relatively rapid rate to reduce the possibility of scorching of heat sensitive plastic compounds being extruded therethrough.

The term xtrusion head and common extrusion head as used in the appended claims are meant to include two separate extrusion cylinders discharging into a common tool chamber. It will be understood that extruder head means secured to the discharge ends of the cylinders may be actually physically separated from each other, except possibly through a common support, and thermally insulated from each other by thermal insulating material or cooling devices and coruiected to a common tool chamber solely by suitable means, such as high-pressure hydraulic conduits and couplings. If such tool chamber were long enough, the inner sheath could be tubed and partially cooled by appropriate means such as a cooling chamber, which is hermetically sealed from the plastic sheath, prior to the application of the outer jacket. Such an arrangement would reduce the looseness of outer jackets made of certain types of plastics.

The term plastic compound, as employed in this sp cification and the following claims, will be understood to include both thermoplastic compounds such as polyethylene and polyvinyl chloride or the like and thermosetting compounds such as neoprene compounds, or other rubbery elastomers, or the like.

it is obvious that the core need not be conductive and that the methods and apparatus described above may be employed for applying two coatings of plastic to a core, for forming laminated strips or form forming pipes or tubes of two plastic materials, by using a stationary core or mandrel instead of a movable core, without departing from the spirit and scope of the invention.

It will be understood that the above-described arrangements are simply illustrative of the application of the principles of the invention. Other arranernents may be devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

What is claimed is:

1. Extrusion apparatus for extruding simultaneously two coats of difierent plastic compounds on a conductive core, which comprises an extrusion head having an extrusion passage through which the conductive core passes, extrusion tools mounted in said extrusion head for forming the plastic compounds into two coatings on'the conductive core, means for supplying the conductive core to the extrusion head, means for taking up the coated conductive core after having passed through the extrusion head, a rigid, stationary, heat-insulated member for maintaining the axis of the extrusion passage of the extrusion 'head coincident with the axis of a portion of the conductive core passing between the supply means and the take-up means, two separate extrusion cyl'mders arranged with their longitudinal axes converging at the exit ends thereof for discharging plastic compounds into the extrusion head, the extrusion cylinders being secured fixedly to the extrusion head, an extrusion screw in each of the extrusion cylinders, a drive unit connected operatively to each of the extrusion screws, the end of each of the extrusion cylinders adjacent to the entrance ends thereof being secured rigidly to the associated drive unit and supported thereby, and bearing means for supporting the drive units slidably for free movement parallel to the axes of the associated extrusion cylinders under the influence or" thermal expansions and contractions encountered during operation.

2. Extrusion apparatus for extruding simultaneously two coats of different plastic compounds on a conductive core, which comprises an extrusion head having an extrusion passage through which the conductive core passes, extrusion tools mounted in said extrusion passage for forming the plastic compounds into two coatings on the conductive core, means for advancing the conductive core in a predetermined path of travel, a rigid, stationary, heatinsulated support for supporting the extrusion head against horizontal movement and maintaining the axis of the extrusion passage of the extrusion head in a vertical plane containing the axis of a portion of the conductive core passing through the predetermined path of travel, two separate extrusion cylinders arranged so that the exit ends thereof discharge the plastic compounds into the extrusion head, the extrusion cylinders being secured fixedly to the extrusion head, an extrusion screw in each cylinder, a drive means connected operatively to each of the extrusion screws, bearing means, and means for movably supporting the entrance ends of the extrusion cylinders and for preventing vertical movement of the extrusion head, the means for supporting the entrance ends of the extrusion cylinders being mounted slidably on said bearing means for free movement under the influence of thermal expansions and contractions encountered during operation.

3. Extrusion apparatus, which comprises an extrusion head having an extrusion passage therein through which plastic compound is forced, an extrusion cylinder secured rigidly to the extrusion head and communicating with the extrusion passage, a perforated breaker plate positioned between the extrusion cylinder and the extrusion passage and having a frustoconical portion adjacent to the end thereof opposite the extrusion cylinder, a screen positioned on the side of the breaker plate adjacent to the extrusion cylinder, substantially radially projecting tabs on the surface of the breaker plate adjacent to the exrusion cylinder for detachably retaining the screen in the breaker plate, the breaker plate having an externally threaded post projecting from the end of the frustoconical portion, and an externally threaded retaining nut received threadedly within and enclosing the forward end of a main passage in the extrusion head extending coaxially with the extrusion cylinder and communicating therewith, the retaning nut being provided with an inwardly projecting support having a threaded bore designed to receive the complementary externally threaded central post projecting from the breaker plate, the projecting support of the retaining nut normally engaging the end of the frustoconioal portion of the breaker plate and cooperating with the slanted surface thereof to form an annular passage symmetricm about the axis of the e"- trusion cylinder through which the plastic material may flow from the extrusion cylinder, through the main passage and to the extrusion passage regardless of the angular orientation of the retaining nut and breaker plate about the axis of the cylinder.

4. Extrusion apparatus for insulating a conductive core by extruding simultaneously an inner sheath of plastic compound and an outer jacket of plastic compound on the conductive core, which comprises an extrusion head having an extrusion passage through which a conductive core passes, a die-retaining nut mounted adjustably in the forward end of the extrusion passage and made of a material having a relatively high coefiicient of thermal conductivity, a retaining insert mounted intermediate of the extrusion passage and made at least in part of heatinsulating material, a core-tube holder mounted adjustably in the extrusion passage, portions of the extrusion passage between the die-retaining nut and the retaining insert and between the retaining insert and the core-tube holder forming relatively large pressure-equalizing chamhers extending coaxially with respect to the extrusion passage, means for maintaining two plastic materials at different temperatures and forcing the two different plastic compounds into the respective pressure-equalizing chambers and through the extrusion passage, the heatinsulating material of the retaining insert reducing the amount of heat being conducted between the two pressure-equalizing chambers as a result of the diilerence in temperatures of the different plastic materials being forced therethrough, a first die mounted in the retaining insert. for formin the plastic sheath on the conductive core and having a frustoconical tip, the entrance end of said first die being made of heat-insulating material, a core-tube tip secured in the core-tube holder and having a frustoconical portion which is designed to cooperate with a first approach of the first die to control the amount of plastic compound passing therebetween, and a second die mounted in the die-retaining nut within the extrusion passage, said second die being made of a material having a relatively high coefiicient of thermal conductivity and having a tapered frustoconical entrance and an exit port for shaping plastic compound flowing therethrough into a jacket about the inner plastic sheath on the conductive core, said second die having a communicating frustoconical approach, the frustoconical portion of the first die projecting into the frustoconical passage formed by the approach of the second die sufficiently to provide a converging annular constricted passage through which plastic compound flows at a relatively high velocity toward the plastic sheath on the conductive core so as to force any drool tending to form on the end of the first die toward the sheath and minimize the mass of any particles of drool between the sheath and the jacket.

5. Extrusion apparatus for extruding simultaneously an inner sheath of polyethylene compound and an outer jacket of polyvinyl chloride compound on a conductive core, which comprises an extrusion head having an extrusion passage through which the conductive core passes, a die-retaining nut mounted adjustably in the forward end of the extrusion passage, a retaining insert mounted intermediate of the extrusion passage, a core-tube holder mounted adjustably in the extrusion passage, portions of the extrusion passage between the die-retaining nut and the retaining insert and between the retaining insert and the core-tube holder forming relatively large pressure-equalizing chambers extending coaxially with respect to the extrusion passage, means for forcing a polyethylene compound and a polyvinyl chloride compound into the pressure-equalizing chambers and through the extrusion passage, a first die mounted in the insert for forming the polyethylene compound sheath on the conductive core and having a substantially pyramidal portion with rounded corners adjacent to the tip end thereof and a frustoconical tip terminating in a sharp annular end, a core-tube tip secured adjustably in the core-tube holder and having a frustoconical portion which is designed to cooperate with the entrance of the first die to control the amount of the polyethylene compound passing thereb-etween, the core-tube tip having a projecting portion which cooperates with an approach of the first die to form circular-segmentaily shaped passages through which the polyethylene compound flows to the first die, the core-tube tip being mounted adjustably in the core-tube holder to regulate the flow of polyethylene between the core-tube tip and the approach of the first die, a die holder having a frustoconical entrance mounted in the die-retaining nut within the passage, and a second die having an extrusion orifice therein mounted axially adjustably within the die holder, said second die having a frustoconical entrance tapered at an included angle of approximately 16 and a land of the order of 0.005 inch long forming an exit port for shaping the polyvinyl chloride compound about the polyethylene-covered conductive core, said second die having a frustoconical approach converging at an included angle of approximately 60, the substantially pyramidal portion of the first die projecting into the frustoconical entrance of the die holder so that the rounded corners of the pyramidal portion of the first die register with the frustoconical entrance of the die holder to form circular-segmentaily shaped passages through which the polyvinyl chloride compound flows into theisecond die, the frustoconical portion of the first die projecting into the approach of the second die suificiently to provide a converging annular constricted passage about 14 mils thick through which the polyvinyl chloride compound flows at a relatively high velocity toward the polyethylene sheath on the conductive core so as to sweep any drool tending to form on the sharp edge of the leading end of the 15 first die oi? the first die and toward the polyethylene sheath and minimize the mass of any particles of drool between the polyethylene sheath and the polyvinyl chloride jacket.

6. In an extrusion apparatus for extruding simultaneously two different layers of piastic compound on a core, an extrusion head means having a common extrusion passage through which the core is passed, extrusion tool-s mounted in said extrusion passage for forming the plastic compounds into two different layers on the core, two separate extrusion cylinders having the respective exit ends thereof secured to the extrusion head, and means for supporting the extrusion cylinders for free axial movement under the influence of thermal expansion and contraction encountered during operation of the extruding head so as to maintain the longitudinal axis of the extruding head coincident with the longitudinal axis of the core.

7. Extrusion apparatus for extruding simultaneously an inner sheath of polyethylene compound and an outer jacket of polyvinyl chloride compound on a conductive core, which comprises an extrusion head; means for forcing a polyethylene compound and a polyvinyl chloride compound into separate sections of the extrusion head; a first die mounted in an extrusion passage of the extrasion head; a first die mounted in an extrusion passage of the extrusion head for forming the polyethylene compound into a sheath on the conductive core and provided with a frustoconical tip terminating in a sharp annular forward end; a core-tube tip secured adjustaoly in the extrusion passage and having a frustoconical portion which is designed to cooperate with an approach of the first die and to regulate the flow of the polyethylene compound between the core-tube tip and the entrance end of the first die; a second die having an extrusion orifice therein mounted adjustably in the extrusion passage in front of the first die, said second die having a frustoconical entrance tapered at an included angle of approximately 16 and a land of the order of 0.005 inch long forming an exit port for shaping the polyvinyl chlorine compound into the jacket about thte polyethylene sheath covering the conductive core, said second die having a communicating frustoconical approach converging upon the frustoconical entrance at an included angle of approximately 60; means for maintaining the frustoconical portion of the first die at a temperature of from about 425 F. to about 450 F. and; means for maintaining the walls of the first-mentioned approach of the second die at a temperature of from about 310 F. to about 325 F., the frustoconical portions of said first die projecting into the approach of the second die sutficiently to provide a converging, annular, constricted passage about 14 mils thick through which the polyvinyl chloride compound flows at a relatively high velocity toward the polyethylene sheath on the conductive core with a velocity gradient which is faster adjacent to the frustoconical portion of the first die than adjacent to the approach of the second die so as to sweep any drool tendin to form on the sharp edge of the leading end of the first die toward the polyethylene sheath and minimize the mass of any particles of drool between the polyethylene sheath and the polyvinyl chloride jacket.

References Cited in the tile of this patent UNITED STATES PATENTS 876,755 Webb Jan. 14, 1908 965,507 Ecaubert July 26, 1910 2,511,986 Martin June 20, 1950 2,790,202 Lorenian Apr. 30, 1957

Claims

6. IN AN EXTRUSION APPARATUS FOR EXTRUDING SIMULTANEOUSLY TWO DIFFERENT LAYERS OF PLASTIC COMPOUND ON A CORE, AN EXTRUSION HEAD MEANS HAVING A COMMON EXTRUSION PASSAGE THROUGH WHICH THE CORE IS PASSED, EXTRUSION TOOLS MOUNTED IN SAID EXTRUSION PASSAGE FOR FORMING THE PLASTIC COMPOUNDS INTO TWO DIFFERENT LAYERS ON THE CORE, TWO SEPERATE EXTRUSION CYLINDERS HAVING THE RESPECTIVE EXIT ENDS THEREOF SECURED TO THE EXTRUSION HEAD, AND MEANS FOR SUPPORTING THE EXTRUSION CYLINDERS FOR FREE AXIAL MOVEMENT UNDER THE INFLUENCE OF THERMAL EXPANSION AND CONTRACTION ENCOUNTERED DURING OPERATION OF THE EXTRUDING HEAD SO AS TO MAINTAIN THE LONGITUDINAL AXIS OF THE EXTRUDING HEAD COINCIDENT WITH THE LONGITUDINAL AXIS OF THE CORE.