GB1577793A - Method and apparatus for pre-expanding and moulding polymer particles - Google Patents

Description

(54) METHOD AND APPARATUS FOR PRE-EXPANDING AND MOULDING POLYMER PARTICLES (71) We, AMERICAN HOECHST COR PORATION, a Corporation organised and existing under the laws of the State of Delaware, United States of America, of 289 North Main Street, Leominster, Massachussets 01453, United States of America, do hereby declare the invention, for which we pray that patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the production of articles of expanded thermoplastic polymer material. Expanded thermoplastic polymer articles are manufactured from the thermoplastic material which is provided in particulate form generally referred to as beads.

The beads are made expandable during their manufacture by incorporating a so called blowing agent within. Because of their generally good flowing properties such expandable polymer beads can be easily charged into a closed mold and accordingly lend themselves readily to the manufacture of these articles by expansion and fusion in closed molds. Upon the application of heat, the beads undergo a very great expansion, for example, they expand to about 30 to 40 times their original size. If expandable polymer beads were charged directly to the mold for in situ expansion they would occupy very little space and would accumulate on the bottom of the mold. Consequently when heat is applied to the mold, the beads, not being agitated, would not expand uniformly, the hotter particles expanding before the cooler particles. This would result in the premature fusion of the earlier softened and expanded beads. The molded article would be poorly fused and contain large voids and particles of widely varying densities. For these reasons it is usually preferred to partially expand or "pre-expand" the particles in an apparatus known as a preexpander to approximately the desired density of the finished article, fill the mold with the pre-expanded beads and further expand and fuse the pre-expanded beads together to form the finished article by applying heat to the particles in the mold.

The pre-expansion is generally accomplished by heating the beads, which contain a blowing agent, which comprises a gas or a composition which upon which heated to its boiling or decomposition point produces a gas to a temperature at which the bead walls are softened. The softened bead walls expand in response to the pressure exerted by the gaseous blowing agent contained within them.

The process may be carried out by either batch or continuous methods. A typical process for the manufacture of pre-expanded polymer particles is disclosed in U.S. Patent No. 3,023,175, issued on February 27, 1962 to Rodman Jr.

While many of the foamed plastic products commonly in use such as ice chests, floats, etc.

are made of low density plastic foam, there has, especially in recent years, developed a need for high density expandable or foam plastic items such as shoe platforms.

According to one aspect to the present invention there is provided a method of partially expanding expandable thermoplastic polymer particles containing a blowing agent to form particles capable of further expansion comprising: (a) inserting a measured amount of the particles into a vessel; (b) heating the particles in a substantially dry atmosphere within said vessel to a temperature high enough to soften the polymer particles and to volatize the blowing agent, the heat being applied by conduction through the walls of the vessel and radiation therefrom and causing partial expansion of the particles; (c) agitating the particles within said vessel at all times during the heating thereof in a manner so that the particles are repeatedly wiped by an agitator within said vessel against the walls thereof and then returned toward the center of said vessel; (d) subjecting the particles while in their heat softened state to a superatmospheric pressure with a substantially dry gaseous medium after a period of heating in said vessel, the pressure being adequate to substantially prevent further expansion of the particles to obtain a desired generally uniform density thereof; and (e) removing the particles from said vessel with the aid of the gaseous medium under pressure which carries said particles out of the vessel when an exit opening is provided for the particles; the temperature of the particles being continuously maintained above the volatization point of the blowing agent within said particles at all times subsequent to the partial expansion of the particles in said vessel.

According to a further aspect of the invention there is provided apparatus for partially expanding expandable thermoplastic polymer particles containing a blowing agent comprising: (a) a substantially closed vessel; (b) means for inserting a measured amount of said expandable particles within said vessel; (c) means for heating the vessel walls to effect heating of the particles within so as to soften the polymer particles and volatize the blowing agent and thereby cause partial expansion of said particles during their residence in said vessel; (d) agitating means within said vessel for repeatedly wiping the particles within the vessel against the side walls thereof and then returning said particles toward the center thereof before being wiped once more against said side walls; (e) means for supplying a substantially dry pressurized gaseous medium to said vessel to increase the pressure on said particles within while in their heat softened state to a superatmospheric pressure adequate to substantially prevent further expansion of the particles after pre-expansion to a desired uniform density thereof; and (f) means for discharging heated partially expanded particles from an exit opening of the vessel through which the gaseous medium may carry the particles from said vessel.

The inventive method and apparatus disclosed herein is particularly capable of uniformly pre-expanding the commonly available expandable polymer particles or beads to produce high density (of the order of 15 pounds per square inch or greater) preexpanded particles. The pre-expanded particles produced are particularly notable for their uniform density of whatever the desired value and they are dry and flowable and ready to be immediately charged into a mold for molding of the manufactured article.

Thus according to a still further aspect of the present invention there is provided a method of molding expandable thermoplastic polymer particles containing a blowing agent which comprises conveying heated partially expanded thermoplastic polymer particles produced by a method according to the invention to a mold and filling the mold therewith, and heating the particles in the mold to effect further expansion and fusion of the particles to form the desired molded article; the temperature of the particles being continuously maintained above the volatization point of the blowing agent within said particles at all times up to the final expansion and fusion of the particles within the mold.

In another aspect, the invention provides apparatus for molding expandable thermoplastic polymer particles comprising (a) apparatus for partially expanding expandable thermoplastic polymer particles according to the invention, (b) means for conveying the heated partially expanded particles to a mold and for filling the mold therewith while maintaining the temperature of the particles above the volatization point of the blowing agent within, and (c) means for heating the particles in the mold to effect further expansion and fusion of the particles into a desired molded article.

The present invention shows advantages over hitherto known pre-expanders such as disclosed in the aforementioned U.S.

3,023,175 patent which uses steam to heat the particles to pre-expand them in preparation for molding. Such a method not only results in wet particles that must be dried to make them flowable but as described therein this method of pre-expansion results in the requirement for aging the particles for long periods of time in storage bins, or the like, for as long as twenty-four hours prior to their use in molding or irregular foaming will occur in the mold.

The method and apparatus of the present invention eliminates entirely this long aging period and allows one to mold with the preexpanded particles immediately upon their leaving the pre-expander. Thus the substantial storage facilities and materials handling and conveying apparatus required for aging the particles may all be eliminated with a resultant savings benefit.

As a result of the elimination of steam as the heating medium the pre-expanded particles leave the pre-expander of the present invention substantially dry and in a flowable state so that they will not stick or jam and may be used to fill molds of any desired shape. Further since they may be immediately molded they will retain their heat from their exposure in the pre-expander and it is found that relatively little additional heat is required in the mold to cause the particles to further expand therein and fuse together. Since the heating cycle may be thus shortened in the mold the cooling cycle is likewise shortened and thus, for example, an overall molding cycle time saving of up to about 60 percent may be attained. The residence -time of the particles in the pre-expander is likewise very short and thus substantial time and energy savings are made possible by the present invention.

The substantially closed vessel of the apparatus of the invention is provided with means for heating the walls thereof which conveniently is a jacket with a heating coil so that the inside of the vessel is heated by means of conduction and radiation through the walls of the vessel. A measured amount of polymer particles containing a blowing agent is fed into the vessel which also contains an agitator. The agitator is especially designed to repeatedly wipe the particles against the heated walls of the vessel and then return them toward the central portion thereof. Due to the heated condition of the particles the blowing agent within, as it reaches its boiling or decomposition point at volatization temperature, will volatize and apply pressure against the heat softened particle walls to expand the particles in this so called pre-expansion step. After a period of heating, the inside of the vessel is subjected to a superatmospheric pressure with a gaseous medium to substantially prevent further expansion of the particles. Thus due to the substantially uniform heating of all the particles within the pre-expander it is found that at the time of applying the superatmospheric pressure all such particles will have expanded to substantially the same extent and uniform density particles are obtained for molding the final articles. The density of the particles is determined by the temperature to which they are exposed within the vessel and the time period of exposure before superatmospheric pressure is applied thereto. The ability to control the pre-expansion of the particles to enable the production of uniformly high density particles is an advantage of this invention as former pre-expanders have been mainly directcd toward producing lower density pre-cxpanded particles. For example, the pre-expander disclosed in British Patent Specification No. 1,247,950 is directed toward producing pre-expanded particles of ultra-low density of the order of 04--075 pounds per cubic foot and exposes particles to a vacuum during pre-expansion to allow less restrained expansion of the beads.

To make high density pre-expanded beads it is necessary to control the expansion to a great degree which is accomplished by the uniformity of the heating of the particles and the timely application of superatmospheric pressure to the particles as disclosed herein. By control of temperature and length of the heating period it is also possible to produce lower density pre-expanded particles i.e., 1.25--31bs per cubic foot (p.c.f.) in accordance with the invention and the uniformity of the density of the beads can also be maintained. Uniform density can mean savings to the producer since a higher density molded product or portion of a product than desired will result in the use of more of the expensive polymer required to make any article. Uniform density also results in the better appearance of the finally molded product as the visible cell size of the fused particles appears more attractive. Further, weakened portions of the article due to lower density areas are avoided.

When the particles leave the vessel they are dry and flowable and still hot from being heated within the vessel. They may conveniently be fed directly into a mold for molding of the final article or into an insulated storage bin where several batches of the particles from the vessel may be kept until transfer into the mold. Once in the mold further heat is applied to the particles raising their temperature higher than in the pre-expansion thereof. The volatized blowing agent and any gas, such as air, that may have permeated the particle walls will then further expand in volume under the higher temperature applying additional pressure against the softened particle walls to further enlarge the particles forcing them against one another within the mold. The heat softened polymeric particle walls will fuse together under these conditions filling up any space initially between the particles into the mold when initially charged and upon cooling a one-piece cellular coherent article will be formed that may be removed from the mold which is then ready to be filled once more to repeat the molding cycle.

A critical feature of the invention is that the temperature of the expandable polymer beads are continuously maintained above the volatization temperature of the blowing agent at all times subsequent to the preexpansion of the beads up to the final expansion and fusion of the beads in the mold.

By maintaining the expanding agent in its gaseous state there can be no possibility of condensation, for example, of the blowing agent to a liquid, resulting in a reduced pressure inside the particles which may cause them to collapse due to atmospheric pressure thereon distorting any article molded from such particles. While the reduced pressure may be eventually dissipated by allowing air, for example, to permeate the beads in a long aging period as previously described, it is the purpose of this invention to avoid such a reduced pressure by maintaining the blowing agent in its gaseous state at all times subsequent to the pre-expansion of the particles up to the final expansion of the particles within the mold.

Embodiments of the invention will be hereinafter described by way of example with reference to the accompanying drawings in which: Fig. 1 is a partially schematic and partically sectioned side view of apparatus for carrying out the method of the present invention, and Fig. 2 is a partially schematic and partially sectioned front view of the apparatus shown in Fig. 1.

The term "thermoplastic polymer particles" is used generically herein and as such includes all moldable thermoplastic polymer particles regardless of how they are made. Thus, the term includes polymer particles made by aqueous suspension type polymerization, commonly known as beads or pearls; comminuted particles such as those obtained by crushing and grinding slabs of mass polymerized thermoplastic material; and pellets obtained by extruding thermoplastic polymeric materials and cutting the extrudate into small particles generally referred to as pellets.

The class of polymers found useful in the present invention are moldable thermoplastic polymers, particularly vinyl polymers.

Included in this group are, for example, vinyl aromatic polymerizable compounds such as polystyrene and polymers of derivatives of styrene; halogen containing vinyl polymers, including polyvinyl chloride and polyvinylidene chloride; and acrylic polymers, such as polyethyl acrylate and polymethyl methacrylate. Copolymers of the above with each other or with other thermoplastic polymers may also be treated in the method of the invention. The preferred group of polymers for use in the invention are the alkenyl aromatic polymers, particularly polystyrene.

Expanding or blowing agents suitable for use in the present invention are those substances which are gaseous at atmospheric conditions or materials which are liquids or solids at atmospheric conditions but which will, upon being heated to a given temperature, produce a gaseous substance. Thus the term "volatize" is used herein to mean the expansion, evaporation or decomposition to give a gaseous substance of the blowing agent upon heating. The blowing agents are preferably substantially inert to the polymer.

Thus, the blowing agent may be a gas, such as propane or butane, a low boiling inert liquid compound such as, for example, pentane, hexane, trichlorofluoromethane, or a dry chemical compound such as sodium bicarbonate. Combinations of the above blowing agents may also be used in the invention. In general, the blowing agent is present in an amount of about 2 to 15 percent and preferably about 3 to 10 percent based on the total weight of the composition.

In one embodiment of the invention, a predetermined amount of expandable thermoplastic polymer particles, hereinafter referred to as beads in accordance with common usage, containing a blowing agent are inserted into a closed vessel 10 as shown in Figs. 1 and 2. The vessel 10 is preferably cyclindrical in shape and-includes an agitator 12 which is mounted on a shaft 14 extending along-the axis of the cylindrical vessel 10.

The agitator is continuously driven in the direction shown by the arrow by a motor 16 and provides a circulation of the beads in the vessel whereby they are repeatedly wiped against the heated side walls 18 of the vessel and then returned toward the center thereof whereupon they are again urged again out toward the vessel walls to repeat the wiping action and so on.

The beads are heated while inside the vessel 10 by means of their contact with the heat conducting walls of the vessel and from the heat radiating therefrom. The amount of unexpanded beads fed to the vessel should be determined so that the charge when expanded to the desired density gives a volume not more than about 80--85% of the inside vessel volume. The source of heat for the side walls consists preferably of a heating jacket comprising a coil of metal tubing 20 encircling the vessel side walls through which steam under pressure is forced through. The steam conducting coil may be surrounded by an enclosing outer wall 22 to prevent heat loss. Alternatively electrically heated resistant coils may be used to heat the vessel walls or steam may be injected between an inner vessel wall 21 and an outer surrounding wall 22 to form a so called steam jacket. The vessel walls are generally made of any good heat conducting metal such as, for example, aluminium. Provision is made for sealing the vessel to produce a closed vessel which can accomodate the build-up of superatmospheric pressure within. The superatmospheric pressure is conveniently provided by means of a compressed gas (preferably air) source which is connected by conveying line means 24 to the vessel. A valve in the line may be manually and/or electrically operated to open and force the pressurized gas to flow into the vessel raising the gaseous pressure within acting on the beads.

The vessel 10 has inlet means for inserting a predetermined amount of particles within said vessel from a hopper 26 and outlet means including a port or opening through which the particles may exit after the preexpansion of the beads is completed. Any of the variously well known volumetric feeding devices such as shown at 28 may be utilized for charging the measured amount of expandable beads into the vessel. The outlet means may comprise a known simple valve mechanism 30 which may be opened either mechanically or electrically as desired.

The feeding device is preferably also mechanically or electrically operated so that under high speed manufacturing conditions electric timing devices may control the opening and closing of the inlet feeding and outlet means to precisely control the residence time of the beads in the vessel. In practice the inside of the vessel is pressurized to control the bead expansion a normally short time (i.e., 10 seconds) before the outlet means to the vessel is opened so that the pressurized gas within while being relieved through the outlet means will carry the pre-expanded beads out of the vessel.

The heating means described also is preferably provided with temperature controls to provide a means of controlling the amount of heat to which the beads are exposed within the vessel.

As the pre-expanded polymer beads leave the vessel they may be fed directly into molding means through conventional conveying means or may be stored in an insulated storage hopper 32 to await molding, in a manner so that they are maintained in their heated conditions to the extent that the bead temperature is above the volatization temperature of the blowing agent contained within the beads.

The atmosphere inside vessel 10 surrounding the beads should generally be a substantially dry gas, preferably air at substantially atmospheric pressure or slightly above due to the effect of heat thereon. The vessel may be a totally closed vessel, except of course for its inlet and outlet ports when in operation or the vessel may be slightly vented but not enough to preclude its pressurization to the necessary degree when desired.

The temperature to which the expandable polymer beads are exposed to in the vessel will depend upon the type of polymer being processed. As an example, for polystyrene expandable beads, heating generally to a temperature of about 175-180 F will soften the polymer and temperature of up to about 210--220"F may be used provided sufficient agitation is provided. The agitation described above to provide the uniform heating of the beads by the vessel walls further acts to prevent the beads from sticking to each other or agglomerating when the beads are in a heated softened state within the vessel. For final molding of the polystyrene beads the beads may be heated to a slightly higher temperature of about 225"F.

The amount of time the beads remain within the vessel will depend upon a number of variables including the type of polymer they are made of (i.e., the softening temperature range thereof), the amount of blowing agent incorporated within the beads, the temperature within the vessel to which the beads are exposed, the mode of agitation thereof and the desired density of the preexpanded bead to be produced for molding puposes.

The amount of superatmospheric pressure to which the beads are exposed within the vessel to control the density and uniformity of the beads will likewise depend upon the variables mentioned above and will best be determined by gaining experience with the particular polymeric material being used.

Pressures inside the vessel on the order of about 40-50 inches of mercury have been found to be convenient.

As an additional feature of the present method and especially in the production of lower density pre-expanded beads (1.25-3 pcf) the vessel is advantageously pressurized using compressed air for a period immediately after the beads are inserted, the beads being maintained under super-atmospheric pressure during the initial stages of the heating period.

It is believed that the increased pressure on the beads will prevent loss of blowing agent by permeation from the beads during the initial heating stage. The increased pressure is then relieved by venting the vessel after a time period during which the beads are softened and the beads are then allowed to expand under atmospheric pressure. If the vessel is a totally closed one a special selectively operable vent means 34 may be provided to relieve the initially imposed superatmospheric pressure. The retained expanding agent within the beads will then provide further internal pressure from within the beads to expand them to a lower density than would be achieved otherwise. After the beads have expanded the desired amount the vessel 10 is again pressurized as described above to achieve the desired uniform density before the beads are discharged. The maintainance of the beads under superatmospheric pressure during the initial stages of the heating period also prevents premature expanding of certain of the beads which has been found to occur otherwise. The elimination of such premature bead expansion by preventing expansion until all the beads are softened and can expand together when the initially applied superatmospheric pressure is relieved, results in more uniformity of pre-expanded bead density than possible with previously known systems. Thus the application of superatmospheric pressure in the initial stages of the heating period as well as before discharge is found useful to make the more uniform density bead whether in the high density range (i.e., 8-15 pcf) or the aforementioned lower density range (i.e., 125-3 pcf).

The following Example further illustrates the present invention.

EXAMPLE A horizontally mounted cylindrical vessel 24 inches long and 12 inches in diameter of a prototype device was provided with a motor driven axially mounted agitator. A steam carrying coil for steam heating at pressures up to 100 psig is coiled around the vessel and a motor and belt means for continuously driving the agitator is provided. Suitable inlet and outlet ports having electrically operated valves are connected to an electric timing panel are provided in the vessel. A source of compressed air is connected to the vessel and valve means electrically operated in a preset timed sequence is provided to pressurize the vessel at the termination of the heating cycle, the pressurized air discharging the pre-expanded beads when the outlet means are opened. The pre-expanded beads gre carried by a conveying line directly to a standard molding press wherein they are charged into a heated mold and molded in the standard commerical manner to produce finished fused cellular molded articles.

A complete pre-expansion and molding cycle was run to demonstrate the advantages gained in accordance with the objectives of the present invention. The polymer beads used were FOSTAFOAM expandable polystyrene beads (3375) containing about 6% n-pentane by weight as the blowing agent. 106 grams of beads were charged to the vessel having the described steam heating coil in which steam at a pressure of 25 psig passes resulting in a temperature within the interior of the vessel of about 215--225"F. The beads were inside the vessel for a heating period of 1.5 minutes when exposed to a 10 second period of superatmospheric pressure (of the order of about 48 inches of mercury) and then discharged. Such beads were then immediately molded and the molded article having a thickness of 1 inch was sectioned to determine the quality of fusion of the expanded beads in the final product.

The results of tests can be summarized as follows: 1. The pre-expanded beads were of very uniform size and density of 12 pounds per cubic foot (pcf).

2. The molding cycle time saving was of the order of 50% resulting from a dramatic decrease in mold heating time (2 seconds vs.

14 seconds) as compared to molding conventionally pre-expanded high density beads and a very substantial decrease in cooling time (1.5 minutes vs. 5 minutes).

3. The fusion of the beads in the 12 pounds per cubic foot density molded article was considered excellent and the article showed no distortion resulting from so called post expansion.

Similar results were obtained when the prototype device was used to mold articles having densities of 7.5 pcf, 4.5 pcf and 1 5 pcf the only difference being that the heating period required in the pre-expander was greater to produce the lower density articles (i.e., 7.5 pcf required 2 0 minutes, 4.5 required 2 7 minutes and 1-5 pcf required 4.5 minutes).

Among important advantages of the invention, it is estimated that the controlled generally uniform density of the beads leaving the pre-expander results in a savings of about 5 /ó of the polymer material required. The greater density variations of the prior art preexpanders have caused molders to prepare the pre-expanded beads at higher than needed densities so that they have a safety factor and are insured that the pre-expanded bead density will not fall below an acceptable minimum. With the present invention providing substantially more uniform density of the pre-expanded beads, molders are able to reduce the safety factor and expand at lower target densities thus saving expensive polymer materials. Since these polymers are generally derived from petroleum any savin

Claims (18)

**WARNING** start of CLMS field may overlap end of DESC **. driving the agitator is provided. Suitable inlet and outlet ports having electrically operated valves are connected to an electric timing panel are provided in the vessel. A source of compressed air is connected to the vessel and valve means electrically operated in a preset timed sequence is provided to pressurize the vessel at the termination of the heating cycle, the pressurized air discharging the pre-expanded beads when the outlet means are opened. The pre-expanded beads gre carried by a conveying line directly to a standard molding press wherein they are charged into a heated mold and molded in the standard commerical manner to produce finished fused cellular molded articles. A complete pre-expansion and molding cycle was run to demonstrate the advantages gained in accordance with the objectives of the present invention. The polymer beads used were FOSTAFOAM expandable polystyrene beads (3375) containing about 6% n-pentane by weight as the blowing agent. 106 grams of beads were charged to the vessel having the described steam heating coil in which steam at a pressure of 25 psig passes resulting in a temperature within the interior of the vessel of about 215--225"F. The beads were inside the vessel for a heating period of 1.5 minutes when exposed to a 10 second period of superatmospheric pressure (of the order of about 48 inches of mercury) and then discharged. Such beads were then immediately molded and the molded article having a thickness of 1 inch was sectioned to determine the quality of fusion of the expanded beads in the final product. The results of tests can be summarized as follows: 1. The pre-expanded beads were of very uniform size and density of 12 pounds per cubic foot (pcf). 2. The molding cycle time saving was of the order of 50% resulting from a dramatic decrease in mold heating time (2 seconds vs. 14 seconds) as compared to molding conventionally pre-expanded high density beads and a very substantial decrease in cooling time (1.5 minutes vs. 5 minutes). 3. The fusion of the beads in the 12 pounds per cubic foot density molded article was considered excellent and the article showed no distortion resulting from so called post expansion. Similar results were obtained when the prototype device was used to mold articles having densities of 7.5 pcf, 4.5 pcf and 1 5 pcf the only difference being that the heating period required in the pre-expander was greater to produce the lower density articles (i.e., 7.5 pcf required 2 0 minutes, 4.5 required 2 7 minutes and 1-5 pcf required 4.5 minutes). Among important advantages of the invention, it is estimated that the controlled generally uniform density of the beads leaving the pre-expander results in a savings of about 5 /ó of the polymer material required. The greater density variations of the prior art preexpanders have caused molders to prepare the pre-expanded beads at higher than needed densities so that they have a safety factor and are insured that the pre-expanded bead density will not fall below an acceptable minimum. With the present invention providing substantially more uniform density of the pre-expanded beads, molders are able to reduce the safety factor and expand at lower target densities thus saving expensive polymer materials. Since these polymers are generally derived from petroleum any savings in this area is of course important in the present petroleum supply sensitive world climate. The saving in mold cycle time attainable in accordance with the above description of the present invention have further petroleum and other energy source saving ramifications. Heating time period reductions are possible, as explained above, as well as cooling cycle reductions requiring the pumping of cooling fluid, and the eliminating of aging the preexpanded beads and the conveying and reheating requirement attributable thereto, all result in significant energy requirement reductions. It has been estimated that overall utility cost savings due to these economics may amount to as much as 75% over conventional moldings. WHAT WE CLAIM IS:-

1. A method of partially expanding expandable thermoplastic polymer particles containing a blowing agent to form particles capable of further expansion, comprising: (a) inserting a measured amount of the particles into a vessel; (ó) heating the particles in a substantially dry atmosphere within said vessel to a temperature high enough to soften the polymer particles and to volatize the blowing agent, the heating being applied by conduction through the walls of the vessel and radiation therefrom and causing partial expansion of the particles; (c) agitating the particles within said vessel at all times during the heating thereof in a manner so that the particles are repeatedly wiped by an agitator within said vessel against the walls thereof and then returned toward the center of said vessel; (d) subjecting the particles while in their heat softened state to a superatmospheric pressure with a substantially dry gaseous medium after a period of heating in said vessel, the pressure being adequate to substantially prevent further expansion of the particles to obtain a desired generally uniform density thereof; and (e) removing the particles from said vessel with the aid of the gaseous medium under pressure which carries said particles out of the

vessel when an exit opening is provided for the particles; the temperature of the particles being continuously maintained above the volatization point of the blowing agent within said particles at alltimes subsequent to the partial expansion of the particles in said vessel.

2. A method according to Claim 1 wherein the particles are exposed to substantially dry air at about atmospheric pressure during the heating period within the vessel except when the particles are exposed to superatmospheric pressure before the discharge thereof from said vessel.

3. A method according to Claim 1 wherein the particles are exposed to substantially dry air at about atmospheric pressure during the heating period within the vessel except when the particles are exposed to super-atmospheric pressure both for a period soon after insertion into the vessel in order to prevent premature bead expansion and undesirable loss of blowing agent and for a period before discharge of the beads from the vessel in order to control bead expansion to achieve the generally uniform bead density desired.

4. A method according to any one of the preceding Claims wherein the expandable thermoplastic particles are of polystyrene containing about 6% by weight of pentane as the blowing agent.

5. A method according to Claim 1 substantially as herein described.

6. A method of partially expanding expandable thermoplastic polymer particles substantially as herein described in the Example.

7. A method of molding expandable thermoplastic polymer particles containing a blowing agent which comprises conveying heated partially expanded thermoplastic polymer particles produced by a method according to any one of Claims 1 to 4 to a mold and filling the mold therewith, and heating the particles in the molj to effect further expansion and fusion of the particles to form the desired molded article; the temperature of the the particles being continuously maintained above the volatization point of the blowing agent within said particles at all times up to the final expansion and fusion of the particles within the mold.

8. A method as claimed in Claim 7 wherein prior to the final expansion and fusion of the particles within the mold, the temperature of the particles is maintained at a temperature just below that required to effect said final expansion and fusion of the particles within the mold.

9. A method as claimed in Claim 7 substantially as herein described.

10. A method of molding expandable thermoplastic polymer particles substantially as herein described in the Example.

11. Apparatus for partially expanding expandable thermoplastic polymer particles containing a blowing agent comprising: (a) a substantially closed vessel; (b) means for inserting a measured amount of said expandable particles within said vessel; (c) means for heating the vessel walls to effect heating of the particles within so as to soften the polymer particles and volatize the blowing agent and thereby cause partial expansion of said particles during their residence in said vessel; (d) agitating means within said vessel for repeatedly wiping the particles within the vessel against the side walls thereof and then returning said particles toward the center thereof before being wiped once more against said side walls; (e) means for supplying a substantially dry pressurized gaseous medium to said vessel to increase the pressure on said particles within while in their heat softened state to a superatmospheric pressure adequate to substantially prevent further expansion of the particles after pre-expansion to a desired uniform density thereof; and (f) means for discharging heated partially expanded particles from an exit opening of the vessel through which the gaseous medium may carry the particles from said vessel.

12. Apparatus according to Claim 11 substantially as herein described.

13. Apparatus for partially expanding expandable thermoplastic polymer particles substantially as herein described with reference to the accompanying drawings.

14. Apparatus for molding expandable thermoplastic polymer particles comprising (a) apparatus according to any one of Claims 11 to 13, (b) means for conveying the heated partially expanded particles to a mold and for filling the mold therewith while maintaining the temperature of the particles above the volatization point of the blowing agent within, and (c) means for heating the particles in the mold to effect further expansion and fusion of the particles into a desired molded article.

15. Apparatus as claimed in Claim 14 substantially as herein described.

16. Apparatus for molding substantially thermoplastic polymer particles substantially as herein described with reference to the accompanying drawings.

17. Partially expanded expandable thermoplastic particles when ever prepared by a method according to any one of Claims 1 to 6.

18. Molded articles whenever prepared by a method according to any one of Claims 7 to 10.