JPS6378714A - Method and device for manufacturing injection molding of plastic material - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates generally to injection molding. More particularly, the present invention relates to a method and apparatus for injection molding under high pressure and moldings made by the method and apparatus.

The invention is particularly useful for injecting high pressure gas into a stream of molten plastic as it enters a mold spool and mold space. However, it will be appreciated by those skilled in the art that the present invention has broad application and is suitable for use in many other injection molding applications.

(Prior Art and its Problems) It is widely known that it is effective to force the molten plastic material in an injection mold into contact with the surface of the mold by applying pressure to it. This action helps transfer the exact shape displayed on the surface of the mold to the surface of the plastic material. The use of pressure also aids in the filling action even if the mold space is elongated or narrow and is usually difficult to fill with molten plastic. Such pressure is provided by fluid injected into the plastic material within the mold space. This is effective because the finished molded product uses less plastic material and is lighter than if the molded product were made of solid plastic inside.

Heretofore, conventional injection molding equipment has attempted to co-inject high pressure fluid and molten plastic material into a mold cavity. However, when it is necessary to inject plastic at high pressure, use high-pressure fluid (preferably 280 to 1050 kg).
Difficulties were encountered since the gas (/ad gas) also needed to be injected. Conventional injection molding equipment uses pump devices, such as pistons and cylinders, to increase the pressure of the gas needed to be injected into the flow of molten plastic material. Unfortunately, conventional piston-cylinder designs require approximately two to three seconds to bring the gas to a suitably high pressure. However, normally the injection mold sink operation itself is completed in 2 to 3 seconds, and the gas is pressurized to the extent that the piston cylinder device can enter the mold space (for example, 630 kg).
/cnl), the injection molding of the molten plastic will be completed. At this point, the gas explodes into the mold space and pressurizes the plastic molding, forcing the molten plastic remaining within the mold cavity toward its surface.

Approximately 280-1050 kg/c4 (4,000-15
,000 psi) cannot be obtained with commercially available pressurized cylinders. Although it is true that gas cylinders with a pressure of 420 kg/ci (6,000 psi) are currently available in the industry, gas cylinders pressurized to 175 kg/ci (2,500 psi) are It's just available. Therefore, gas entering from conventional sources must be pressurized before being used in injection mold sink operations.

Unless such pressurization is carried out before the injection mold sink begins, the gas will only be pressurized appropriately to the injection pressure after the molten plastic has already been injected.

As an example, if 90% of the mold space is filled with partially solidified molten plastic, and the product you are molding has a lot of bosses, ridges, and rips, the high-pressure gas may not enter the mold cavity. There will be many bad sink marks on the product. When high pressure gas is injected after injection of the molten plastic, the gas forces the plastic outward and presses the sink mark against the surface of the mold space. However, this puts a shadow mark on the plastic product. This shadow mark is very clear and unacceptable for an A grade finish. Furthermore, gases entering the mold space after the plastic material has substantially ceased to flow will naturally push against the thinner parts of the plastic more than the thicker parts. In other words, the gas moves the plastic somewhat laterally and outwardly. It also creates dents and scratches on the edges of plastic products.

On the other hand, if the product does not have ribs, ridges or bosses, there will be areas where the plastic does not fill the mold space when it initially stops flowing. When gas is introduced into the mold space, the gas presses against the molten plastic in the unfilled areas and spreads the plastic over the entire surface of the mold space. However, a clear demarcation line appears on the surface of the product, said demarcation line proving the position where the plastic first stopped and then the gas pushed it again. This is also unacceptable for a grade A finish. In the two examples mentioned above, post-moulding treatments, such as painting operations, are required, which clearly results in an increase in the price of the molded part.

It has therefore been considered desirable to develop a new injection molding method and apparatus that overcomes the above-mentioned drawbacks while providing better effective results.

(Means for Solving the Problems) A novel method for making injection molded articles is provided based on the present invention.

In particular, according to the invention, a flow of molten plastic is introduced into the mold space at a first pressure.

A quantity of gas at a second pressure at least the same as the first pressure is stored in the storage chamber. Immediately after the molten material passes the gas entry point, gas is introduced into the flow of the molten plastic material to form a gas cavity within the material. The gas exerts pressure on the surrounding plastic material, forcing it towards the surface of the mold space. While the plastic material is being fed into the mold space, gas is continuously injected into the gas cavity. After the molten material completely covers the surface of the mold space, the supply of molten material is stopped. Pressure then continues to be maintained within the gas cavity as the plastic material cools.

According to another aspect of the invention, gas is introduced into the mold sprue.

According to yet another aspect of the invention, the gas is approximately 280-10
50kg/cj (4,000~15,000psi
) is introduced at a pressure of The gas is preferably an inert gas such as nitrogen.

According to yet another aspect of the invention, the method further includes the step of replenishing the reservoir with gas. This replenishment step includes the subsidiary step of introducing low pressure gas into the pump and increasing the pressure of the gas with the pump to equal a second pressure. This gas is then introduced into the storage chamber at a second pressure.

According to another aspect of the invention, the amount of gas introduced into the mold space is not directly measured, but only the gas pressure is controlled.

According to another aspect of the invention, an apparatus for making injection molded articles from plastic materials is shown.

Particularly in accordance with this aspect of the invention, the apparatus includes apparatus for introducing molten plastic material into the mold space at a first pressure. The gas source includes a gas pressurization device for pressurizing the gas to at least a second pressure that is the same as the first pressure. A storage chamber is provided for storing gas at a second pressure for immediate use. Apparatus is provided to begin flowing gas from the storage chamber at a second pressure immediately after the molten plastic passes the gas introduction point. Apparatus is provided for applying pressure to the molten plastic material to hold it against the surface of the mold space until it cools and the mold is ready to connect the shape it displays.

According to another aspect of the invention, the apparatus further includes means for continuously supplying gas to the mold space as long as molten plastic continues to be supplied to the mold space.

According to another aspect of the invention, the second pressure is approximately 280-1
050 kg/cd (4,000~15,000ps
i), this gas is preferably nitrogen.

According to yet another aspect of the invention, the invention includes an injection molded article made by the method described above.

One feature of the present invention is to provide a novel injection molding method that introduces gas into the stream of molten plastic material while it is still flowing.

Another feature of the invention is to provide a method for introducing gas into a stream of molten plastic material immediately after said stream passes through a gas introduction point in order to form gas cavities in the stream as quickly as possible. It is.

Yet another feature of the invention is to continuously inject or feed gas into the mold space as long as the flow of molten plastic material continues to be introduced into the mold space, thus evenly distributing the plastic and gas into the mold space. It is an object of the present invention to provide a method and an apparatus capable of forming a gas injection groove in an injection molded article with a gas injection groove having a substantially constant diameter. This allows the gas to force the molten plastic into contact with all surfaces of the mold space with nearly the same force.

Yet another feature of the invention is that approximately 630-1050 kg/c
Injection molding process and equipment that can be used to mold products from rigid plastics such as acrylics, polycarbonates, rigid PvC that need to be injection molded at pressures of d (9,000 to 15,000 psi). It is to provide.

Yet another feature of the present invention is to provide an apparatus that provides on-site access to high pressure gas for injection molding operations.

Another feature of the invention is to provide an apparatus that does not require measuring the amount of gas entering the gas cavity. In other words, the apparatus and method of the present invention are more forgiving and are not as difficult to operate as conventional methods, but they can be used regardless of whether the amount of gas injected into the mold space is more or less than necessary. This is to avoid any difference.

Still other advantages and features of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention, having physical form in its parts and arrangement of parts, is described in detail herein and illustrated in the accompanying drawings, together with preferred and modified embodiments thereof.

Referring now to the accompanying drawings, which are shown for the purpose of illustrating preferred embodiments and alternative embodiments of the present invention, and are not in any way limiting, FIG. Showing the feeding system. Although the system is primarily designed for injecting gas through sprues in mold spaces and will be described in that regard, the general inventive concept contained herein applies equally to fluids injected at other locations in the mold. Applicable to

In order to carry out the invention, a device A is provided for injecting molten plastic under pressure into the mold space 11 . Place the molten plastic in mold space 1
1. Gas is introduced under high pressure to force (not pack) the Parts molded in this manner have a smooth outer surface with a small amount of sink marks, so that the outer surface is accepted as having a Grade A finish. Injection mold tank press mold space 11
It has a pair of molded parts 9.10 that work together to form the. Moreover, the housing 12 is made of molten plastic 8
A hydraulic screw ram 13 is provided for injecting the liquid into the mold space 11. The screw ram is equipped with a nozzle 14 and a supply chamber 15. The heated and melted plastic material enters the sprue 18 from the nozzle 14 of the supply chamber 15 and flows into the mold space 11 .

FIG. 1 also shows a high pressure gas supply system for injecting gas into the molten plastic at the mouth of mold sprue 18 of mold sprue bushing 19. FIG. The gas supply system includes a gas supply tank 41, a high pressure gas pump assembly 32, and a high pressure gas storage tank 28. The control valve 29 is a high pressure gas directional valve leading to the nozzle 14 and connects the nozzle to the gas storage tank 28 through the gas pipe 22. Tank 28 and gas pump assembly 3
A check valve 25 between 20) prevents backflow of gas into the pump assembly.

An inert gas, such as nitrogen, is stored in the high pressure gas storage tank 28 prior to the start of the Mohr tank cycle. This is the gas pump assembly 320) gas compressor pump 3
This is done by activating 5. A pump 35 supplies gas to a gas supply cylinder 41, that is, a gas supply tank 41.
from there through the gas passage 42 to the suction side of the pump.

A pressure cage 37 and a pressure reducing valve 38 together with a control valve 39 control the gas pressure entering the suction passage 42 of the pump 35.

The gas then enters the tank 28 through the check valve 25 and finally reaches the predetermined gas pressure, thereby activating the pressure setting of the pressure switch 27. This pressure becomes less than 280~1
050kg/an! (4,000~15,000psi
) and is displayed on the pressure gauge 26. When the desired pressure setting is reached, pump 35 switches pressure switch 2.
Stopped by 7. The gas pressure setting value of the pressure switch 27 varies depending on the type of plastic to be injection molded. Soft plastics, such as polyethylene, only require lower pressure settings. However, for example, high viscosity plastics such as acrylic, polycarbonate, rigid PVC, acrylonitrile, etc. require a high pressure setting value for the pressure switch 27. The reason is that these plastics are injected into the mold space 11 under high pressure. The pressure needs to be adjusted for each type of plastic, i.e. a higher setting for acrylonitrile and a lower setting for polyethylene. For example, if the pressure switch is set too high for a product such as polyethylene, this high pressure gas will blow through the plastic without creating gas bubbles within the plastic.

The moment the pump 35 is stopped, the check valve 25 closes to maintain high pressure gas between the three-way gas valve 29 and the check valve 25.

Valve 29 is preferably a two-way pneumatic valve, actuating air cylinder 23 to open and close valve 29.

When the high pressure gas storage tank 28 is sufficiently raised to the required pressure and the solenoid actuated two-way air valves 29°, 30 are in the closed position, the molding cycle is ready to begin.

To begin the molding cycle, a conventional mold press clamping unit (not shown) is closed to clamp the mold parts or mold halves 9.10 under pressure. Next, the nozzle arai 16 is actuated upward by the hydraulic cylinder 17 to open the hole 5 therein (hydraulic ram body 120).
Open in alignment with the hole. In this position, the screw ram 13 is actuated and moves forward to inject the molten plastic material 8 into the mold space 11 .

After a sufficient time has elapsed to allow the advancement of the screw ram 13 to fill the sprue 18 with molten plastic, a valve 29 is opened by the actuating cylinder 23; as soon as the valve 29 is opened, the storage area or high-pressure gas storage tank 28 is drained. High pressure gas enters the gas inlet passage 6 in the center of the nozzle 14 through the gas pipe 22 and check valve 20 . The gas passageway 6 extends to the outlet of the nozzle 14 for supplying gas to the mouth of the sprue 18 and entering the molten plastic stream during a plastic injection molding operation. can.

Also, the plastic flows fairly steadily and the amount of plastic material required for the mold space is typically injected in about 2 to 3 seconds. It is therefore imperative that high pressure fluid, preferably high pressure gas, be available as soon as the molten plastic stream enters the sprue 18. As a related matter,
Once the gas starts flowing, it flows faster than the molten plastic, so the pressure of the gas forces the molten plastic into the mold space 11, creating an internal cavity 21 in the molded part. The flow rate of the gas flowing through the gas inlet passage 6 is not controlled and varies during the molding operation. However, the gas pressure in the containment chamber or high pressure gas storage tank 28 is controlled.

In other words, if the pressure in the containment chamber 28 falls below the pressure setting of the pressure switch 27, which will occur when a sufficient amount of gas flows through the inlet passage 6, the pump 35 will not start. The gas pressure is increased to the required value in the storage tank 28, thus maintaining the pressure of the gas flowing from the gas inlet passage 6 into the molding hollow section 21.

However, the gas pressure in the storage tank 28 does not need to be recharged from the ambient pressure (only the drop in gas pressure in the storage tank 28 has to be restored).
5 is preferably of the type that can generate a small amount of high pressure gas quickly enough to restore the pressure drop in tank 28 and maintain said storage tank 28 at the required pressure during the injection mold gimp cycle. The gas is preferably injected continuously into the mold space. This is effective for the following reasons. That is, both the plastic and the gas are injected evenly into the mold space, and gas grooves with an almost constant diameter are formed in the molded part. This causes the gas to force the molten plastic against all surfaces of the mold with approximately the same amount of force.

In addition, if it is desired to form two or more discontinuous hollow compartments in the molded article B, the flow of gas through the passage 6 may be discontinuous or one or more gas injection points may be provided in the mold space. You can also do it.

The injection pressure of the molten plastic varies somewhat during the mold injection cycle. However, the pressure at which gas exits passageway 6 is relatively constant because pump assembly 32 maintains the gas pressure in chamber 28 approximately at the set point commanded by pressure switch 27.

The method of the present invention is more forgiving and less aggressive than conventional methods because there is no need to meter the amount of gas entering the gas cavity (only the pressure needs to be controlled). It is not important to the invention that more or less gas than is required be injected into the mold space 21. The only thing that needs to be adjusted is the pressure of the gas, which is maintained at approximately a constant level. The amount of gas entering 11 is not directly measured for each mold tank cycle, but may be adjusted for a particular mold cycle by adjusting the gas pressure, plastic injection pressure, and mold design.

The pressure of the gas entering sprue 18 is at least as great as, and preferably greater than, the pressure of the molten plastic entering the sprue. The gas pressure for the types of plastics is 6
30~1050kg/c+j (9,000~15.00
0 psi) and there is no need to store the high pressure gas in the chamber 28 so that the gas is readily available when it is needed to be injected simultaneously with the molten plastic. The end of the gas flow almost coincides with the end of the plastic injection process. Preferably, the gas flow continues until immediately after the screw ram 13 stops its forward movement, and thus until the time when the molten plastic has finished being injected into the mold cavity 11.

When the screw ram 13 completes its forward movement, the gas control valve 29 closes and the gas valve 30 opens, reducing the gas pressure in the tube 22 to the pressure setting of the gas pressure reducing valve 31.

This pressure reducing valve 31 is approximately 70kg/cd (1,000p
si ) to cause the gas pressure in the tube 22 to drop to this value. This pressure is maintained until the molten plastic has cooled sufficiently within the mold 11 to be able to stand on its own. At this time, the gas valve 31 is operated to absorb the remaining high pressure gas in the pipe 22 and reduce the pressure in the pipe 22 to atmospheric pressure. At this time, injection assembly 12 separates from sprue bushing 19, separating mold parts 9, 10 and allowing molded article B to be removed.

It should be noted that the nozzle well 16 is closed by the hydraulic cylinder 17 when the valve 29 is closed. The screw ram 13 can then rotate to store molten plastic 8 for use in the next injection cycle.

Also, during mold removal, the pressure switch 27 activates the gas pump assembly 32 to fill the gas storage tank 28 with gas until the pressure therein reaches the set point indicated by the gauge 26. At this point the pressure switch 27 is shut off. The system is then ready for a repeat cycle with valves 16, 29, 30 in the idle position.

For example, assume that injection molded article B would normally require 160 ounces of plastic to fill mold space 11. If 10% of the weight of the finished product is saved, 0.456 kg (
16 oz) of plastic (i.e., injecting 4.1 kg (146 oz) of plastic). It takes about 32 ounces to move one ounce of plastic.
．． requires 78 cJ (2 cubic inches) and therefore 5
24.48 m (32 cubic inches) of gas is required.

Containment room 28 is approximately 367.8~409.75 cut
(20-25 cubic inches) of high pressure gas. Pump assembly 32 is activated to supply the remaining required gas to chamber 28 and mold space 11. Of course, any suitable number of pre-filled chambers can be provided so that the pump assembly does not have to be activated during all injection mold tank operations.

FIG. 2 shows an alternative high pressure gas source for use with the mold and screw ram apparatus of FIG. The gas source of FIG. 2 replaces the gas source of FIG. 1 at point X. In other words, the first
Gas pipe 43 is removed at point X in the figure, and gas pipe 50 in FIG. 2 is connected to this point.

The operating sequence of the mold and screw ram is the same as previously described. However, high pressure gas is stored for injection in cylinder 53 (FIG. 2) with valve 29 in the closed position. The cylinder or chamber 53 is operated by the pressure reducing valve 56 until the required pressure set point shown on the gauge 55 is reached.
It continues to store gas from the supply tank through. Next, the hydraulic cylinder 51 is actuated to move the gas in the cylinder 53 to the pressure switch 59 by moving the piston 520).
The compressor is compressed to the required high pressure value set at and indicated on the gauge 60. A check valve 57 prevents gas from flowing back into the gas supply tank 54 . The stored high pressure gas is then immediately available for injection into the mold sprue 18 during the mold pump cycle.

In this embodiment, the cylinder 53 is not refilled during the injection molding operation.

The cylinder 53 is refilled from the gas supply tank 54 only in preparation for the next molding cycle and only after the molded part has solidified. It is lowered and gas is admitted into it. Once the cylinder is filled with gas from the supply tank 54, the cylinder 51
The movement of the piston 520, which is pressed by the piston 520, causes the cylinder 53 to fill.

Part moldings for which the method is particularly applicable are moldings having large surface areas supported by thicker areas. Gas is introduced into the molten plastic to cause the plastic to extend along each rib, etc., pressing the molded plastic material against all surfaces of the mold. Typically, the thicker part of molded article B has an internal cavity 21 as shown in FIG.

The invention has been described in terms of preferred embodiments and alternative embodiments. Obvious modifications and changes will occur to others upon a reading and understanding of this specification. However, it is intended that the present invention include all such modifications insofar as they come within the scope of the claims or their equivalents.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a cross section of the mold and screw ram of an injection molding press incorporating the gas pressurizing device of the present invention, and FIG. 2 is another example of the gas pressurizing device of the present invention that can be used in the mold shown in FIG. 1. FIG. 5...hole, 6...gas passage, 8...molten plastic, 9...
・Mold parts, 10...Mold parts, 11...Mold space, 12...
Housing, 13...Screw ram, 1
4... Nozzle, 15... Supply chamber,
16. Old... Nozzle Arai, 18... Molded sprue, 19... Molded sprue bushing, 21.
...inner cavity, 22 ...gas pipe, 25
...Check valve, 26...Pressure gauge, 28...High pressure gas storage tank, 29...
...control valve, 32.old...high pressure gas pump, 38.
...Reducing valve, 41...Gas supply tank. Procedural amendment writing method) Showa year, month, day 1, case description 1988 patent application No. 110437
No. 3, Relationship with the case of the person making the amendment Applicant's name: James D'Abriny 7)' IJ
-4, agent

Claims (24)

[Claims] (1) introducing a flow of molten plastic material into the mold space at a first pressure;
storing the material in a reservoir under pressure to form a gas cavity in the flow of molten plastic material such that the gas exerts pressure on the surrounding plastic material and forces it towards the surface of the mold space; introducing the gas into the material immediately after the material has passed through the gas introduction position; continuing to feed the plastic material into the mold space; and simultaneously continuing to inject gas into the gas cavity; A method of manufacturing an injection molded article comprising: stopping the supply of the molten material when a surface is completely covered with the molten material; and continuing to maintain pressure in the gas cavity until the plastic material cools. 2. The method of claim 1, wherein the gas is introduced from a mold sprue. (3) The method according to claim 1, wherein the gas is nitrogen. (4) The second pressure is approximately 280 to 1050 kg/cm
2 (4,000 to 15,000 psi). (5) replenishing the storage chamber with the gas, the replenishing step including introducing gas at a low pressure into a pump and using the pump to replenish the gas until equal to the second pressure; 2. The method of claim 1, including the steps of increasing the pressure of the gas at the second pressure and introducing the gas at the second pressure into the storage chamber. (6) The method according to claim 5, wherein the amount of gas introduced into the mold space is not measured. (7) a device for introducing molten plastic material into the mold space at a first pressure, a gas supply source, a gas pressurizing device for pressurizing the gas to a second pressure at least equal to said first pressure, the gas being ready for immediate use; a storage chamber for storing gas at the second pressure such that the molten plastic material starts flowing from the storage chamber into the mold space immediately after the molten plastic material passes through a gas introduction point; Apparatus for making injection molded articles from plastic materials, comprising: apparatus for maintaining pressure against the surface of the mold space until the material is able to maintain the shape of the mold space as the material cools. 8. The apparatus of claim 7, further comprising a device for continuously supplying the gas at the second pressure to the mold space while supplying the molten plastic to the mold space. (9) The second pressure is approximately 280 to 1050 kg/cm
8. The apparatus of claim 7, wherein the pressure is between 4,000 and 15,000 psi. (10) The apparatus according to claim 7, wherein the gas is nitrogen. (11) An injection molded product made by the method set forth in claim 1. (12) introducing the molten plastic material into the mold space at a first pressure; storing a certain amount of gas in a storage chamber at a second pressure that is at least the same as the first pressure; the molten plastic material Immediately after the molten material passes through the gas introduction point, the material is injected into the material to form a gas cavity in the material so that the gas exerts pressure on the surrounding plastic material and presses the plastic material against the surface of the mold space. continuing to introduce both gas and molten material into the mold space; stopping the supply of gas; and immediately thereafter stopping the supply of molten material. Method. (13) further comprising the step of maintaining a pressure in the gas cavity that is less than the first and second pressures to keep the molten plastic material pressed against the surface of the mold space as it cools. Claim 1
The method described in Section 2. (14) The method according to claim 12, wherein the amount of gas introduced into the mold space is not measured. (15) The method according to claim 14, wherein the gas pressure introduced into the mold space is maintained at the second pressure. (16) The method according to claim 12, wherein high pressure gas is introduced into the molten plastic after more than half of the molten plastic has filled the mold space. (17) The second pressure is approximately 280 to 1050 kg/c
13. The method of claim 12, wherein m^2 (4,000 to 15,000 psi). (18) maintaining the gas in the reservoir at the second pressure when introducing the gas at the second pressure into the molten plastic material by introducing additional gas at a second pressure into the reservoir; A method according to claim 12. (19) An injection molded article formed by the method according to claim 12. (20) introducing the molten plastic material into the mold space; storing an unmeasured amount of gas in the chamber at high pressure; forming a gas cavity in the molten plastic material; injecting high pressure gas into the plastic material immediately after the plastic material passes through a gas injection point; continuing to inject high pressure gas to expand the gas cavity as the plastic material continues to flow through the mold space; cutting off the supply of said plastic material to the mold space when it has spread throughout the mold space; said gas cavity so that as said plastic material cools, said material is pressed against the surface of the mold and retains the shape assumed by said mold; A method of making injection moldings of plastic materials, which involves the process of holding pressure within. (21) The gas is 280 kg/cm^2 (
21. The method of claim 20, wherein nitrogen is at a pressure greater than 4,000 psi. (22) The method according to claim 20, wherein high pressure gas is injected into the molten plastic material after the mold space is more than half filled with the molten plastic material. (23) A device for injecting molten plastic material in a fluidized state into a mold space, a nitrogen gas storage chamber containing an unmeasured amount of gas, and a device for pressurizing said chamber to a pressure of 280 kg/cm^2 (4,000 psi) or more. an apparatus for injecting said pressurized gas into a stream of molten plastic to form a gas cavity; an apparatus for starting said gas to flow immediately after said molten plastic passes a gas injection location; and an apparatus for injecting said pressurized gas into a stream of molten plastic to form a gas cavity; Apparatus for making injection moldings of plastic materials, including a device for maintaining pressure within the gas cavity to keep the plastic pressed against the mold surface until the surface can sustain the shape it displays. (24) An injection molded product made by the method according to claim 20 or the apparatus according to claim 23.