IL43707A - Resilient plastic foam bodies and method of making the same - Google Patents

Description

QWVy*? Π0»\?1 *QO O *)SPB 0*Φ*Μ 0»D1* RESILIENT PLASTIC POAH BODIES AHD HETHOD OP HAKHJO THB SAME The present invention relates generally to articles made from an exradable, expandable synthetic plastic composition, and more particularly to the production of resilien plastic foam bodies having a nesting or inter* locking configuration for use as a free-flowing, loose-fill packing material.

Packing materials comprising a multiplicity of plastic foam bodies made from an extradable, expandable thermoplastic material, such as a polystyrene composition, are well-known. As disclosed, for example, in U.S. Patent Re. 27,243, dated December 14, 1971 and assigned to the assignee of the present invention, such expanded plastic packing materials have many advantages over conventional materials such as excelsior or shredded newspaper. Among others, the expanded plastic materials are resistan to fungus growth, do not readily absorb moisture, and have good heat and electrical insulation properties. In addition, the plastic bodies may be configured to pour, freely and low around an object placed in a shipping container, then to Interlock and prevent settling of the packing material when compressed* With packing materials In general, and expanded plastic loose-fill tsaterlale in particular, it is Important that the product have a low bulk density so that the gross weight of a filled package, and consequently its shipping cost, is minimized. Eeducing bulk density also reduces manufacturing costs, since less raw materials Is needed for a given volume of product. It is, therefore, a principal bulk density. More specifically. It 1s an object of the Invention to provide φ a packing material comprising a multiplicity of expanded plastic bodies con- Ilk figured to have a low bulk density.

Another mportant object of the Invention 1s to provide such expanded plastic bodies configured to Interlock or nest with other similar bodies when massed together, 1n order to prevent settling caused by relative movement of adjacent bodies.

A more specific object of the Invention 1s to provide such bodies having highly frictlonal surfaces so that non-shifting contact between them 1s ehhanced.

U.S. Patent 3,400,037 describes an arrangement 1n which foam 1s extruded nto a rod which Is subsequently cut nto disks. The cut disks are subjected to steam to cause warping. The ultimate shapes may Include those of a saddle or concave-convex configuration which differ from the present Invention 1n view of the uniform thickness throughout. This U.S. patent does not disclose or suggest a process similar to that 1n the Invention 1n which the extruded mass has a concave surface when cut and which process Involves the production of a temperature gradient which combines with the other properties of the severed mass during expansion to provide a plurality of severed masses as a part of the expansion process rather than requi ing a separate step to produce warpage.

Methods for producing extruded, expanded synthetic plastic packing materials are known. Such methods are disclosed, for example, 1n U.S. Patent 3,188,264 and above-cited U.S. Patent Re. 27,243. Generally, previous methods comprise extruding an expandable synthetic plastic composition, such as polystyrene, through a die orifice, and allowing 1t to expand and form an elongated rod or strip. The expanded rod 1s fed Into conveying apparatus, commonly called a "puller" , which supports the flexible rod for a distance forward of the extruder and feeds 1t to a cutter which slices the rod Into segments of a desired length. Several drawbacks attend such prior art methods, however. First, t 1s necessary to provide a puller to support the extrudate (I .e. , the rod or strip) as 1t expands and cools prior to cutting. Second, the puller speed must be carefully controlled to match that of both the extruder and the cutter. presents the usual maintenance problems.

Other drawbacks of known prior art methods relate to cutting the expanded plastic rod. Cutting an already expanded foam ruptures cells on either side of the cut, leading vto an open-celled outer surface jancL ^ ooscin^ eeak ^fbam oodyT The broken cells also limit the expansion of the body in a subsequent steaming process, resulting in a final product having a higher bulk density. Rupturing of foam cells by the cutter is particularly significant when thin sections or chips are produced, since the ruptures may extend through the entire thickness of such a chip· In addition to the product quality problems, cutting an already expanded foam rod requires considerable mechanical power, and, particularly with polystyrene foams, is hard on blades. Wear and breakage of the cutter blades force frequent stops for blade changes, increasing production costs.

Accordingly, a further important object of the present invention is the provision of an improved method for producing the expanded plastic article of manufacture of this invention.

More specifically, it is an object of the invention to provide an improved method which eliminates both the need for an intermediate conveying or pulling apparatus, and the necessity for frequent blade changes in the cutting apparatus.

A further more specific object is to provide such a method whereby a packing material comprising a ability to interlock and prevent settling, can be produced at a lower cost.

These and other objects of the instant invention will become more apparent as the description which follows is read in conjunction with the accompanying drawings, wherein: Fig* 1 is a foreshortened schematic view in side elevation of apparatus for producing the article of manufacture of the present invention; Fig. 2 is a side view, partially broken away, of an initially obtained hollow, substantially hemispherical foam body before subsequent post-expansion processing; Figs. 3 and 4 are side views, partially broken away, of initially obtained foam bodies after subsequent post-expansion process steps; Fig. 5 is a side view, partially broken away, showing the hollow, substantially hemispherical articles of the invention In nested relationship; Fig. 6 is a foreshortened plan view, partially broken away to disclose details of Internal construction, of the extruder and cutter components of the apparatus shown In Fig. 1; Fig. 7 is a sectional view taken along line 7-7 of Fig. 6; Fig. 8 is a partial plan view in enlarged scale of the cutter apparatus shown in Fig. 6; and Fig. 9 Is an elevation view, partially broken away, taken on view line 9-9 of Fig. 8. - having a hollow, substantiall hemispherical configuration, and subsequently expanding them to obtain a packing material having a desirably lower bulk density. Apparatus useful for practicing the invention is schematically Illustrated in Fig. , and includes an extrusion device, designated generally at 10, including a hopper 12 containing a supply of an extrudable, expandable thermoplastic composition, preferably in the form of granules or beads. Such compositions are well known In the art and include compositions comprising polystyrene or other suitable polymers as disclosed, for example. In U.S. patent numbers 2,983,692, 2,941,964, 2,941,965, 3,066,382 and 3,251,728.

A mixture comprising 98+ per cent by weight extrusion grade expandable polystyrene beads, sold by oppers Co., Inc. under the trademark Dylite, together with suitable expanding agents, has been employed with good success.

The constricted, bottom discharge end 14 of hopper 12 communicates with the infeed end of extruder portion 16, Portion 16, which will be described in greater detail below, Includes a variable root diameter extruder screw (not shown) mounted for rotation inside extruder barrel 18 and driven by an electric motor 20 through a gear reduction unit 22. As expandable thermoplastic beads flo from the hopper Into extruder portion 16, rotation of the screw moves them from the infeed end of barrel 18, adjacent discharge end 14, toward an extruder die 24. The solid thermoplastic composition is subjected to increasing it is moved alon b the screw due ; o the in* mixture is heated to a higher temperature until it melts into a viscous liquid mixture and is thereafter agitated or fractionated such as with the aid of pins (not shown) mounted on the screw to an extent necessary to produce foaming and a consequent small and uniform open-cell structure* The molten material is then forced out through an orifice in extruder die 2h$ and as a mass of the molten plastic composition emerges from the ori ice* it is contin-nuously divided Into a multiplicity of thin sections by a revolving cutting mechanism 26 which cuts the emerging composition at or very near the die opening* It will he appreciated that the molten thermoplastic composition within die 2k is under substantial pressure* So as the molten plastic leaves the die, exposure to the lower pre8sure of the surrounding atmosphere allows the expand* ing agents in the composition to release gas and expand the composition into a cellular structure many times the diameter of the orifice* It is an important feature of the present invention that the expandable thermoplastic composition is agitated to the point of foaming to cause repeated cell breakdown and is divided or cut Into thin sections as it emerges from the orifice In a molten state and before appreciable expansion has occurred* By so doing* the extruded composition or mixture is oomprlBed of small* closed* uniform cells which are not ruptured during cutting resulting in smooth, closed-celled surfaces where the cuts are made* Thus* bodies may be formed from very thin sec ions without the drawbacks previously mentioned* In greatly reduces wear of the cutting blades and nearly eliminates blade breakage* As the thin sections are cut from the extrudate* they are propelled by cutting meohanism 26 toward a collection hopper 26· During their travel through the airy the cut sections rapidly expand into foam bodies* This expansion is accompanied by simultaneous cooling, so that the bodies solidify a short distance from the die and before reaching collection hopper 28. he thus formed bodies have a thickness in direotions normal to the planes of the cuts. wHich is nonuniform and which decreases from a central portion of the body r: ■ to the outer portion- of the /body. Fig. 2 illustrates such an initially expanded foam body 30 which, as shown, includes a convex surface 30a and a concave surface 30b with the intersection of these surfaces forming an edge 30c which delineates the perimeter of the concave surface.

The initially expanded foam bodies collected by hopper 28 are conveyed by a blower fan 32 upwardly through a duct 34 into a storage container 36. The articles thus produced may be used for various purposes, such as a packing material. However, in their initially expanded form the foam bodies have a bulk density which is undesirably high for use as an economical loose-fill packing material. For this reason, the initially expanded bodies are typically re-expanded in a steam atmosphere to provide a significantly larger foam body having a reduced bulk density.

Referring again to Fig. 1, initially expanded bodies 30 are fed from storage container 36 into a post-expansion u«-t 42 wherein the bodies are deposited onto a moving conveyor in an atmosphere of steam. As the bodies progress through the post-expansion unit to a point of discharge, the environment of elevated temperature and steam causes the bodies to substantially expand in dimension while retaining the same configuration. Fig. 3 shows a re-expanded body 38 resulting from subjecting an initially expanded body 30 to the action of steam at an elevated Upon leaving post-expansion unit 42, the re-expanded bodies are conveyed to storage container 44. This container preferably is made of screen or other open mean material to permit free circulation of air and thus facilitate drying of the re-expanded bodies.

It should be noted that initially expanded bodies SO may be re-expanded immediately after their formation.

However, a greater degree of expansion, and consequently a lower bulk density, results from storing the initially formed bodies for a few hours prior to re-expanding them.

Valve means 46 at the outlet end of storage container 36 allows a quantity of bodies 30 to be accumulated and stored p ior to the re-expansion process.

Again, the re-expanded bodies 38 may be used as is. But preferably, they are stored for several hours and again subjected to an atmosphere of steam at elevated temperature to again provide a substantial Increase in dimen-eicm and consequent reduction in bulk density. Fig. 4a. shows a wice re-expanded body 48 resulting from re-expanding body 38 (Fig. 3) in this manner.

Referring now to Fig. 6 of the drawings, extruder portion 16 includes an lnfeed housing 50 secured at rearward flanged end 52 to the output end of gear reduction unit 22. lnfeed housing 50 has a central bore 54 with the lnfeed end of an extruder screw 56 removably received therein. At Its rear end, extruder screw 56 is connected for rotation to gear reduction uni 22. Discharge end 1 of hopper 12 communicates with bore 54 and projects upward therefrom surrounding bore 54 end communicating with infeed and drain conduits 60 and 62, respectively. Housing 50 thus is maintained at a temperature sufficiently low to prevent the thermoplastic composition from melting in the infeed housing or in the discharge end of the hopper.

Forward flanged end 64 of housing 50 is secured to the rearward flanged end 66 of extruder barrel 18.

Barrel 18 has an axial bore 68 coaxial with and of the same diameter as bore 54, and receives the forward portion of extruder screw 56 therein. As illustrated, extruder screw 56 has a helical rib 70 of constant pitch and diameter, and a root diameter which increases uniformly from the Infeed end to the outfeed end of the screw.

Forward flanged end 72 of barrel 18 is secured to rearward flanged end 74 of extruder die 24. Die 24 is provided with a central bore 76 coaxial with bore 68 of the extruder barrel. Die bore 76 tapers from a diameter substantially the same as that of bore 68 to a smaller diameter at outlet orifice 77, which predetermines the diameter of the extruded composition prior to expansion.

To provide for controlling the temperature of the thermoplastic composition as it progresses forward, i.e. * to the right in Fig. 6, through bore 68, a cooling conduit 78 is coiled helically about barrel 18, retained in a helical groove provided in the outer surface of the barrel. The rearward end of conduit 78 is connected through a control valve 80 to a supply conduit 60. The forward end of conduit 78 terminates rearward of the conduit 62.

A second cooling conduit 86 s colled helically above the forward end of the extruder barrel. One end of this conduit Is connected through a control valve 88 to supply conduit 60, and the opposite end through T-coupllng 82 to outlet conduit 84.

Removably enclosing cooling co l 78 and extruder barrel 18 Is a pair of electrical heater units 90 and 92. Referring to Fig. 7 along with Fig. 6, each heater unit Includes a pair of elongate, semi-circular members 93 of electrically non-conducting material, each containing an electrical heating element. Members 93 are joined together along one of their adjacent edges by hinge means 94, and along their opposite adjacent edges by clamp means 96.

Thus, the two halves of a heater unit easily may be removed and replaced if necessary. A third heater unit 98 of similar construction removably encircles extruder die 24 forward of mounting flange 74.

Each of the three heater units has Its own adjust* able control so that each associated zoae of the extruder stay be varied in temperature independently of the others.

Referring now to Figs. 8 and 9 in conjunction with Fig. 6, cutting mechanism 26 includes a cutter wheel 100: suitably mounted on output sha t 102 of an electric motor 104. This motor preferably Is of the variable speed type, so that the rotational speed of the shaft, and hence the cutter wheel, ma be controlled. As best shown In Figs, 8 and 9, wheel 100 includes a pair of spaced opposing cir 100 is a cu tec blade 110, which may suitably be an injector-type razor blade.

As shown in the drawings, the end portions of blade 110 are received within a pair of opposing slot-like apertures 112 in flange portions 106, securely retained by a pair of set screws 114. Blade 110 is disposed so that, with rotation of wheel 100, cutting edge 115 intercepts molten plastic mass 79 (Fig. 8) as it emerges from outlet orifice 77 of die 24, and divides the composition into thin sections before appreciable expansion of the composition has occurred. It will be evident that the thickness of these sections is dependent upon the linear speed of the molten extrudate (controlled by extruder screw 56) and the rotational speed of cutter wheel 100. Although either may be varied, ordinarily, the thickness of the cut sections is changed by adjusting the speed of cutter motor 104.

As will be appreciated, the mass of viscous, molten plastic flowing through bore 76 flows most slowly In the region adjacent the walls of the bore, with the flow rate increasing toward the center of the bore. Thus, after a mass of extrudate has been intercepted and cut off by blade 110, the emerging plastic will bulge out from orifice 77 so that the portion of mass 79 first extruded will have a convex , partially spherical surface. Continued extrusion of the plastic composition would, of course, produce a rod-like mass. The speed of cutter wheel 100 is adjusted, however, so that the extrudate is cut into thin sections, each in part bounded by a convex, partially spherical surface, and bodies having the desired hollow, oubotantlolly homlophorl' eelr configuration previously described.

Referring again to Fig* 1, post-expansion unit 42 comprises a hollow, closed tank 120 provided with means for Introducing water Into the bottom of the tank, such as valved Inlet 122. An electrical heater 124 Is provided within the tank for heating the water to boiling and thus generating an atmosphere of steam within the tank. A generally circular perforate plate 126 Is supported for rotation within the tank by a vertical shaft 128 which extends down through the bottom of the tank. The lower end of shaft 128 Is connected through gear means 130 to an electric drive motor 132.

An elongate deflector blade 136 Is mounted within tank 120 closely overlying perforate plate 126. Blade 136 extends Inwardly from one end of the tank to Intercept Initially expanded foam bodies previously deposited upon the plate behind blade 136 through duct 138* After the bodies have been subjected to the atmosphere of steam within tank 120 while being carried on plate 126 for a revolution thereof, they are Intercepted by blade 136 and deflected outwardly through an opening 140 In the front wall of the tank. From tank 120, they are conveyed to storage container 44 by a blower 142 and Its associated duct 144. As mentioned previously, the re-expanded bodies preferably are stored In container 44 for several hours, then again re-expanded by a second exposure to a steam atmosphere in apparatus similar to expansion unit 42, To Illustrate the operation of the a paratus sole* by Koppers Co. , Inc. under the trademark Dylite and designated KFP 533 is mixed wth 0.4% by weight ammonium Blue ZNK (a blue pigment) and poured into hoppe 12 of extrusion device 10, The extruder screw o device 10 is approximately 20 inches long and one inch in diameter, and motor 20 is adjusted to provide a screw speed of about 100 ram. Outle orifice 77 of die 24 is 11/64" in diameter.

The flow of cooling liquid through chamber 58 is adjuste to maintain the temperature at the infeed end of the screw at about 100*F. Heater unit fO and control valve 80 are adjusted to provide a temperature of about 220*FV in the aone of barrel 18 adjacent heater 90, and the temperature in the aone adjacent heater 92 is similarly adjusted to about 230*F. Heater unit 98 is set to provide a die temperature of about 230*F. Cutter wheel 100, which has a diameter of about 41/2" and a cutting radius of about 2*% la adjusted to a speed of about 3600 rpm.

Under these conditions, foam bodies 30 are pro* duced having a hollow, substantially hemispherical configuration as shown in Fig. 2 with a diameter of about 1/2 Inch. The initially expanded foam bodies have a bulk density Of about 2 pounda per cubic foot, measured by weighing a tared container of known volume after it has been filled with the foam bodies, compacted b moderate vibration, and refilled and re*compacted until no further settling occurs. the thus produced initially expanded foam bodies 1 3/4 minutes* This exposure expands bodies 30» result- j ing in re-expanded bodies 38 having the configuration shown in Fig* 3 ith an ewerall diameter of about 13/16 inch and a bulk dnsity of about 0*l|5 pounds per cubic foot* A group of similar bodies 30 re-expanded immediately after forming has a bulk density of about 0*65 pounds per cubic foot* Re-expanded bodies 38 are again stored or about 4 1/2 hours, then exposed to steam at atmospheric pressure for about 1 1/4 minutes* The resulting re-expanded bodies 1(8 have a configuration similar to that shown in Fig* 1 with a diameter of about 1 inch and a bulk density of about 0*3 pounds per cubic foot* In the illustrated operation of the apparatus just described, various factors contribute beneficially or ad* vereely to the characteristics of the resultant articles* As the molten mass of thermoplastic material leaves the die with a cell structure that is small* closed and uniform due to the agitation and foaming just prior to extrusion* reduced pressure allows the composition to expand into a cellular structure* To provide the configuraion desired, it is important that the molten extrudate be cut as closely as possible to outlet orifice 77 of die 24 so that the plastic material is divided before any appreciable expansion has occurred* Thus* again referring to Figs* 8 and 9» face 81 of die 24 is configured to complement the circular path (dot-dash line 1*6) described by cutting edge 1 6 of blade 110* Although in Fig* 9 path 146 is shown passing slightly in front of face 81 for clarity of illustration* die and cutter wheel 100 preferably are disposed so that edge 116 lightly "brushes" the face of the die* In addition* It has been found desirable to mount blade 110 In wheel 100 so that it is inclined at an angle "A" of about 10 to 15* to a tangent o£ path 146 taken at edge 116.

As previously mentioned, the thickness of the section cut from the molten extrudate depends upon the realtionshlp between the linear movement of the molten thermoplastic out of orifice 77 and the rotational speed of the cutter blade. Either or both of these factors Is adjusted to produce an initially expanded foam body having the desired hollow, substantially hemicphcgic*¾ configuration. When the section cut from the extrudate is too thin, the resulting bodies are smaller in diameter and have a shallow concave surface 30c (Fig. 2), resulting in increased bulk density. An overly thick section of extrudate results in a body having a predominantly lenticular configuration* Another actor contributing to the ultimate characteristics of the finished article is the degree of heating of the thermoplastic composition in the extruder. For example, it has been found that when the temperature of the molten composition Is too high, the cut sections expand rapidly but produce a foam having internally ruptured cells and decreased strength. When the temper* ature of the molten composition is too low, the Initial expansion is not su ficient to produce a body having a substantially hemispherical configuration.

Additionally, as will be evident, the degree of expansion of the foam bodies in the post-expansion unit varies with the amount of time the bodies are exposured to may produce collapsed bodies, rendering the product unsuitable as a loose-fill packing material.

Also, It is known that storage of the Initially expanded bodies for at least 11/2 but preferably about four to eight hours before the post-expansion steam exposure result9 in re-expanded bodies having a greater size, and consequently a lower bulk density than those re-expanded immediately after formation. This is thought to result from the diffusion of blowing agent from the solidified plastic into the Initially formed cells of the foam which, just after the initial expansion, contain a partial vacuum. In the same manner, storage of the re-expanded foam bodies, preferably for about fou to eight hours, followed by a second exposure in a steam atmosphere produces re-expanded bodies having a still lower bulk density.

A preferred product of the invention is a Qf cellular foam having a substantially continuous outer skin Integral with and covering the foam. As previously mentioned, and as shown in Fig. 5, articles having such configuration flow freely, as from a storage container, to completely fill a shipping container and surround an object contained therein. Further, the articles of the invention, when massed together, tend to nest or interlock with each other. Thus, in a mass of packing material comprising a multiplicity of such bodies wherein a plurality are of graded or substantially similar size, the convex portions of at least some of the bodies seat against the perimeters of the con- Fig. 5, wherein the convex portion of a substantially hemispherical body 150 is seated against the perimeter(s) of the concave portion of similar body 152. Since the radius of curvature of the convex portion of such a body is greater than the radius of curvature of the concave portion, the portions do not fully mate, and thus void spaces exist between them, such as that indicated at 154. As will be appreciated, such void spaces result in a reduction in the bulk density of a mass of such material.

In addition to the non-shifting and non-settling characteristics of the packing material of the invention, the known high friction surface characteristics of the expanded material enhances non-shifting contact betwee abutting bodies when a mass of such a material is compressed.

As used herein, "substantially hemispherical" foam bodies includes foam bodies having a true hemispherical configuration such as shown in Fig. 4a. In addition it Includes bodies which are somewhat more than and somewhat less than a true hemisphere, such as shown in Figs. 4b and 4c, respective-ly. Also, It will be appreciated that in a process such as described above, the expanded plastic articles formed may be as previous-Ly notedj somewhat ovate or otherwise Irregular in configurations Such Irregularly shaped bodies are also considered to be "substantially hemispherical" as the term is used herein.

There is thus provided by the instant invention a loose-fill packing material of improved characteristics and a method by which this material may be readily and economically produced. possible without; departing from the spirit of the invention. For example, other expandable synthetic plastic compositions well known in the art may be employed for the purpose of the present invention, it being recognized that in such event the temperature conditions above illustrated may require modifications in accordance with the physical properties of the particular composition used.

Claims (18)

43707/2 WHAT IS CLAIMED IS:

1. A loose fill element comprising a plastic foam body having a generally closed-celled outer surface and a generally open-celled interior. loose fill element^

2. The invention/ defined in claim 1, wherein the generally open-celled interior has a relatively saall and uniform call structure, loose fill element-'

3. · The inventiondefined in claia 1, wherein the body is formed by being cut from a mess of plastic foam which is in a molten and highly fractionated state. loose fill elements

4. Tha inventionde ined in claim 3, wherein the body has a substantially non-uniform thickness in directions normal to the plana of the cut.

5. The invention ,¾efined in claim 1, wherein the body has a generally convex surface and a generally concave surface intersecting the generally convex eurfaca. loose fill element

6. Tha Invention.- de ined in claim 5, wherein the generally convex surface has greater curvature than the generally concave surface and intersects the generally concave eurface along a closed loop defining the perimeter* loose fill element,

7. · The Invention de ined in claim 1, wherein the plastic foam is an expanded synthetic thermoplastic composition. loose fill elemen j

8. The Inven ionde ined in claim lt wherein the element is of hollow, generally hemispherical shape. loose fill element .j

9. The lnve tlor* de ined in claim 1, wherein the element has a density of less than 0.5 lbs./ft.J. loose fill elecent-

10. The invention defined in claim I, wherein 43707/2 -defining a concave portion of the element whereby the convex portions of at least some of the elements are seated against ^ the perimeters of the concave portions of other elements without being fully received within the concave portions when a plurality of the elements are massed together. element

11. The loose fill/ defined in claim 10, wherein the outer surfaces of the elements have relatively high friction to provide non-shifting contact between abutting elements when a plurality of the elements are massed together.

12. The method of making loose fill packing material from an extrudable, expandable plastic foam composition comprising the steps of: continuously extruding a mass of said composition while 1n a molten state through a die orifice at a temperature and at a rate to produce a temperature gradient and a velocity gradient across the cross-section of the orifice, with the extrusion producing at the orifice a protruding mass of unsoll-dlfled material bounded by a convex surface; and repetitively cutting the protruding mass as it emerges from the orifice and while 1t is still in the molten state to provide a plurality of severed masses.

13. The method defined In claim 12, wherein the mass is extruded at a pressure exceeding atmospheric pressure through the orifice Into a region of atmospheric pressure, and further Including the steps of partially solidifying the mass after cutting such that the skin of the mass recomblnes Into a closed cell characteristic and exposing the severed mass to atmospheric pressure to cause substantial expansion of internal cells of the severed mass and warpage due to the temperature gradient while maintaining the closed cell characteristic of the skin to form a foam lastic bod havin a hollow substantiall hemis herical 43707/2

14. The method defined 1n claim 12, wherein the cutting occurs along a path which 1s substantially linear as compared to said convex surface and at a rate sufficiently high relative to the average extrusion velocity to produce a severed mass having a thickness substantially less than the width thereof, said mass being bounded by said convex surface and an edge extending in a plane subtending said convex surface.

15. The method . defined 1n claim 12, further including the stpp of subjecting the severed mass to an atmosphere of steam for a time sufficient to enlarge its size substantially.

16. The jmethod defined In claim 12, further including the steps of storing the severed mass at atmospheric temperature and pressure for a minimum of about 1 1/2 hours, and subjecting the severed mass, following such storage, to an atmosphere of steam for a time sufficient to re-enlarge its size substantially*

17. The method defined 1n claim 12, wherein said plastic foam composition comprises a polystyrene composition.

18. The method defined in claim 15, wherein the severed mass 1s subjected to the steam atmosphere for a time sufficient to enlarge its size to provide, In a packing material consisting of a multiplicity of such severed masses, a bulk density of about 0.65 to about 0.45 pounds per cubic foot. 19, The ; method , defined 1n claim 18 wherein the severed mass 1s subjected to the steam atmosphere for a time sufficient to enlarge Its size to provide, In a packing material consisting of a multiplicity of such re-enlarged masses, a bulk density of less than about 0.40 pounds per cubic foot.