JPH07329134A - Injection molding of thermoplastic resin molded product and injection mold apparatus - Google Patents

Abstract

PURPOSE:To obtain a resin product free from a surface flaw such as a sink even in an injection molded product markedly deviated in wall thickness by forcibly generating a void in the thick-walled part of a molded product-by the pressure gas discharged from a mold cavity at time of injection molding. CONSTITUTION:When a resin molded object different in wall thickness is subjected to injection molding, a void insertion pin 6 is arranged at the position corresponding to the thick-walled part of a mold cavity 4. The pressure gas discharged from the mold cavity at the time of injection is allowed to act on the tip of the void insertion pin 6 from an air discharge passage 7 and an air guide passage 8 through the pin 6 to form a void nucleus in the molten resin present in the vicinity of the tip part of the void insertion pin 6. The void necleus grows to the void corresponding to the shrinkage of the resin accompanied by the cooling of the molten resin. Therefore, a resin product free from the surface flaw such as a sink caused by the volumetric shrinkage of an injection molded product markedly deviated in wall thickness is easily produced by existing molding equipment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and a mold apparatus for injection molding thermoplastic resin molded articles having different wall thicknesses without forming defects such as sink marks on the surface due to shrinkage.

[0002]

2. Description of the Related Art When a thermoplastic resin molded product having a thick portion and a thin portion is manufactured by an injection molding method, the thick portion finally solidifies in the process of cooling and solidifying the resin filled in the mold. . Therefore, the volume shrinkage due to the cooling of the resin concentrates on the thick portion, and sink marks or the like tend to occur on the surface of the thick portion. The commercial value of a molded product is significantly reduced by the occurrence of sink marks, deformation, etc. (hereinafter, this is represented by sink marks).
In the conventional injection molding method, in order to compensate for the volume contraction, a pressure holding step is provided immediately after completion of injection and filling of the resin, and a pressure for preventing sink marks is applied to the molten resin in the cavity.
However, when the product shape becomes complicated, the gate seal precedes, sufficient holding pressure cannot be applied, and a product with a satisfactory appearance cannot be obtained.

As means for suppressing the occurrence of sink marks, for example, Japanese Examined Patent Publication No. 61-53208 and Japanese Unexamined Patent Publication No. 63-2686.
In No. 11, etc., high-pressure gas is sent to the cavity through the resin flow path, and holding pressure is applied through the hollow gas flow path.
It is introduced to suppress the generation of surface defects. Since these methods require the use of high-pressure gas, they are expensive and the high-pressure gas itself is not easy to handle. Therefore, as in JP-A-4-45910, a method of recovering and reusing the gas used for hollowing, or in JP-A-5-4253, nitrogen gas is separated from air to form a high-pressure gas source. Although methods of using the same have been proposed, these methods also require a nitrogen source externally and handle high-pressure gas, so the apparatus is expensive and the operation is complicated.

Further, in Japanese Patent Application Laid-Open No. 4-25421,
After injecting the molten resin into the mold cavity with the injection molding machine, with the injection molding machine nozzle once separated from the mold, the screw was retracted without rotation to collect air in the cylinder and a predetermined amount of air was accumulated. A method has been proposed in which after re-contacting the nozzle with the mold at this point, the screw is advanced to inject the accumulated air from the nozzle. However, in this method, the screw reciprocates twice for each shot for the injection operation of both resin and air, and the screw needs to be retracted once without any rotation, so that the material pellets from the hopper must be removed. It is not practical because it is difficult to control supply and it is necessary to take measures to prevent the material pellets remaining in the screw groove from melting.

[0005]

In the case of a resin such as polycarbonate resin or polymethylmethacrylate, which has a high set-up property, the skin developed in the surface layer during injection molding under the condition that the surface layer is rapidly cooled and solidified. Often the strength of the layer exceeds the volumetric shrinkage of the resin. Therefore, the surface layer is not deformed, and a cavity called a void is formed inside the thick portion. In this case, defects such as sink marks are often not generated on the surface of the thick portion.

Therefore, the inventors of the present invention previously proposed an injection molding method, which positively utilizes the above void phenomenon to prevent the occurrence of sink marks, in Japanese Patent Laid-Open No. 5-16177. That is, in this injection molding method, when molding thermoplastic resin molded products having different wall thicknesses, the mold cavity is placed at an arbitrary position (hereinafter referred to as a thick wall portion) corresponding to a thick wall portion where a sink phenomenon occurs. A void insertion pin is provided, and a sufficient amount of molten resin for filling the cavity is injected and filled into the cavity, and a pressurized gas is applied to the pin tip through the void insertion pin,
After inducing voids in the molten resin at the tip of the void insertion pin, the shrinkage force associated with the cooling and solidification of the molten resin causes the voids to grow to a size commensurate with the shrinkage and prevents sinking of the molded product. To do.

Since the pressurized gas used for inducing voids in this method is compressed air at a low pressure of 5 to 15 kgf / cm2, a gas source is secured as compared with the high pressure inert gas used in the conventional hollow injection molding method. It is very easy, but this method also requires an external gas supply facility. In injection molding, when the molten resin is injected and filled in the mold cavity, the air filling the mold cavity is replaced with the resin and discharged to the outside. For this reason, generally, in the injection molding die, a gap is provided around the parting surface, the protruding pin, or the like so that the air is discharged.
However, since the molten resin is injected and filled into the mold cavity at high speed and high pressure, if the exhaust gas is insufficient, the air inside the mold cavity is rapidly adiabatically compressed and the temperature is high enough to cause the resin to burn. It becomes a high pressure.

[0008]

Therefore, if the mold cavity is made airtight and a discharge passage is provided at a specific location near the final filling portion of the molten resin, high pressure air adiabatically compressed by the molten resin is recovered. be able to. The present invention utilizes the recovered compressed air as an air source for generating voids in the above-mentioned Japanese Patent Laid-Open No. 5-16177.

That is, according to the present invention, when a thermoplastic resin molded product having a different thickness is injection-molded, a void insertion pin is provided at an arbitrary position of a mold cavity corresponding to a thick portion where a sink phenomenon easily occurs, A molten resin is injected and filled into the cavity, and a pressurized gas having a pressure that breaks through the resin skin layer generated near the tip of the pin is applied to the pin tip through the void insertion pin to melt near the tip. After inducing voids in the resin, the shrinkage force associated with cooling and solidification of the molten resin causes the voids to grow to a size commensurate with the amount of shrinkage associated with the cooling of the filled resin to form a sink mark of the molded product. In the injection molding method for preventing, by making the entire cavity excluding the molten resin injection port an airtight structure, the air in the cavity compressed with the injection filling of the molten resin,
An injection molding method for a thermoplastic resin molded article, characterized in that the pressurized gas source is guided to the void insertion pin by an air communication path provided from a cavity.

A cavity having a thick portion and a thin portion corresponding to the shape of the thermoplastic resin molded product to be produced, an injection injection gate for injecting a molten resin into the cavity, and a thick portion where a sink phenomenon is likely to occur. Corresponding to the mold cavity portion with one or more void insertion pins fixed in a protruding state, and the mold cavity has an airtight structure except for the molten resin injection port, and the molten resin is filled in the cavity. An injection molding die apparatus, wherein an air outlet near the end and an end of the void insertion pin are connected by an air conducting path. Further, the mold cavity has an airtight structure except for the molten resin injection port, the air discharged from the mold cavity by the injection and filling operation of the molten resin, a device for collecting and storing as compressed air through the air communication path, The injection molding die apparatus is also characterized in that it is equipped with a device for supplying the compressed air from the recovery device to the tip of the void insertion pin.

The present invention will be described in detail below. When molding a thermoplastic resin by an injection molding machine, generally, the average injection pressure of the nozzle part is about 600 to 1500 kgf / cm2,
The average ejection rate is about 150 to 500 cm3 / sec. On the other hand, the average pressure at the tip of the molten resin in the mold cavity decreases due to various pressure loss factors such as viscosity increase due to solidification of the molten resin, sprue, runner, and gate.
It is about 100 to 500 kgf / cm2. The air in the mold cavity is compressed by the filling pressure of the molten resin tip, accumulated as compressed air in the air conducting path, and acts on the resin layer at the tip from the void insertion pin. The pressure of the air at this time is determined by the compression ratio based on the ratio of the volume of the mold cavity and the volume of the air communication path. In general, the volume of a molded product having a thick portion to prevent sink marks in the present invention is significantly larger than the volume of the air communication path, so that it is easy to use 5 to 1
A pressure of about 5 kgf / cm2 can be obtained. In order to efficiently accumulate the air, it is advisable to hermetically seal a part or all of the mold parting surface, the sprue bush, the ejector and the like with an elastic O-ring such as heat resistant synthetic rubber. Also, when the molded product becomes large and the ratio of the volume of the mold cavity to the volume of the air conducting path becomes significantly large, the air compressed in the cavity is temporarily stored in the external pressure storage tank through the air conducting path to induce void induction. The compressed air is controlled to an appropriate air pressure and then supplied to the void insertion pin through the air passage.

The mold according to the present invention is provided with a cavity having a thick portion and a thin portion corresponding to the product shape, and a void insertion pin having a protruding tip is installed at the thick portion of the cavity. An air discharge path or a conduction path from the inside of the cavity is connected to the base of the void insertion pin. A description will be given below with reference to the accompanying drawings. FIG. 1 is a sectional view showing an example of an injection molding die apparatus according to the present invention. In the figure, 1 is an injection molding die, which comprises a fixed die 2 and a movable die 3. A cavity 4 having an inner surface shape corresponding to the shape of the molded product is formed by being sandwiched between the fixed mold 2 and the movable mold 3,
The fixed mold 2 is provided with a sprue 5 for injecting a molten resin. The movable mold 3 has a cavity 4 made of molten resin.
An air discharge passage 7 for collecting the compressed air discharged from the parting surface when being injected into is provided in the vicinity of the molten resin filled end portion of the cavity. Further, a sealing material 9 for preventing air leakage is attached to the fixed mold 2 on the outer periphery thereof so that the cavity has an airtight structure.
The movable mold 3 is provided with a void insertion pin 6 having a sharp pin tip 15 protruding into the cavity at a thick portion 4'of the cavity. The root of the void insertion pin 6 and the air discharge path 7 are connected by an air communication path 8. The void insertion pin 6 may be either a solid pin or a hollow pin as long as it has a sharp tip. In the case of a solid pin, the pressurized gas passes through the peripheral portion, while in the case of a hollow pin, the pressurized gas passes through the inside and is injected into the molten resin from a single hole or a porous nozzle at the tip. What is shown in FIG. 2 is a case of a solid pin, which will be described below.

As shown in FIG. 2, a fitting insertion hole 10 is formed in the movable mold 3 into which the void insertion pin 6 is fitted and inserted. The fitting hole 10 includes a large diameter portion 11 and a small diameter portion 13 located on the cavity 4 side. The large diameter portion 11 has a diameter larger than the cross section of the void insertion pin 6, and an annular space portion 12 is formed between the large diameter portion 11 and the side surface of the void insertion pin 6. The space 12 is connected to the air conducting path 8 at the base of the void insertion pin 6 as a compressed air introducing path. Small diameter part 13
Has a diameter slightly larger than the cross section of the void insertion pin 6, and forms an annular gap portion 14 having a narrow gap with the void insertion pin 6. This narrow gap is set to 0,05 so that the molten resin does not flow in when filling the cavity with the molten resin.
It is set to about mm to 0.1 mm.

In the manufacturing method of the present invention, first, the fixed mold 2 and the movable mold 3 of the mold 1 shown in FIG. 1 are closed, and the sprue 5 is passed through the cavity 4 to fill the cavity with the molten resin. Injection fill. By the above operation, the air existing in the cavity is pushed into the air discharge passage 7 through the slight gap of the parting surface near the filling end portion while being replaced with the molten resin by the injection filling pressure.
Compressed air is accumulated in the air discharge passage 7 and the air communication passage 8 and is accumulated. Next, when the injection filling is completed, the step of cooling the molten resin is started. In this state, the air discharge path 7 and the air communication path 8
The compressed air accumulated in and tries to flow back from the parting surface to the cavity 4, but since the parting surface is sealed by the filling pressure of the injection-filled resin,
Holds compressed.

On the other hand, as shown in FIG. 2, from the air conducting path 8 which has been sealed to the resin skin layer at the tip of the annular void portion 14 through the space portion 12 serving as the compressed air introduction passage and the annular void portion 14. When the compressed pressure of the compressed air decreases due to the volume contraction caused by the progress of cooling of the resin, the filling pressure remaining inside the resin at the thick portion 4 ′ drops below the pressure of the accumulated compressed air. While peeling off the resin surface skin layer 15 ′, it is peeled toward the pin tip of the void insertion pin 6 and the pressure is concentrated on the tip. Then, the thinnest portion of the resin skin layer 15 'generated near the tip of the pin is pierced by the pressure difference between the pressure of the compressed air and the negative pressure state inside the molten resin caused by the volume contraction. The effect that the void insertion pin 6 has a sharp tip shape is that the closer it is to the tip, the more delayed the cooling of the resin skin layer 15 'is to keep the resin skin layer in that portion thin, and the pressure of the compressed air is kept at the tip of the pin. By concentrating it at one point, the compressed air easily penetrates the thin resin skin layer at the tip of the pin to form the through hole 16. Therefore, the void insertion pin 6
The amount of protrusion of the void insertion pin tip 15 from the cavity wall can be reduced to the same extent as its diameter, and the above-mentioned differential pressure can be extremely small. Compressed air is taken into the molten resin from the formed through holes 16 to form void nuclei 17 that serve as nuclei for void growth.

Subsequently, as the cooling process progresses, the void nuclei 17 are sucked by the volume contraction accompanying the cooling and solidification of the surrounding molten resin, and as shown in FIG. 3, the void nuclei 17 are cooled and solidifying around the molten resin. The voids 18 grow to a size proportional to the volume shrinkage involved. At this time, the pressure of the compressed air synergizes with the volumetric shrinkage force of the resin to spread the void nuclei 17 and promote the growth to the voids 18. However, soon the volume of the voids 18 will decrease the pressure. However, instead of the voids that have started to grow, the volumetric shrinkage force of the resin, which increases as the cooling process progresses, acts strongly,
The void 18 continues to grow to a size large enough to prevent sink marks while absorbing the volume contraction force. With this action, the sink effect is exerted from the periphery of the void insertion pin to the entire thick portion, and it is possible to suppress not only the thick portion but also the thin portion in the periphery and the sink of the adjacent thick portion. When the cooling process is completed, the mold is opened and the molded product is taken out.

The obtained molded article has no sink mark on the surface and has a surface property excellent in appearance by completely transferring the shape of the cavity. Further, since the skeleton of the molded product itself can be formed by the freely designed thick portion, it is possible to obtain a molded product having strength and rigidity which cannot be realized by the conventional general injection molded product. The type of resin injection-molded according to the present invention is not particularly limited, and polystyrene, rubber-reinforced polystyrene, styrene-acrylonitrile copolymer, polymethylmethacrylate, polyethylene, polypropylene, AB can be used.
Most thermoplastic resins such as S, polyvinyl chloride, and polycarbonate resins, or resins in which a filler reinforcement is mixed are available.

[0018]

EXAMPLES Examples will be described below with reference to the drawings. Example 1 In this example, as shown in FIG. 4, an injection molded body 40 having four thick ribs 43 on the back surface of a top plate portion 42 of a box-shaped molded product is manufactured. The top plate portion 42 is formed with a sprue 45 for injecting a molten thermoplastic resin. In the product, the sprue 45 is cut off. The rib 43 has a length of 100 mm and a height of 17 mm. As shown in the cross section of the rib portion in FIG. 5, the rib portion has a large thickness of 10 mm at the base 44 as compared with the thickness of 3 mm of the top plate portion 42. ing.

This injection-molded body 40 was molded using the injection-molding die described in FIG. First, the fixed mold 2 and the movable mold 3 are closed, and the sprue 5 is inserted into the cavity 4 so that a sufficient amount of molten rubber-reinforced polystyrene resin to fill the cavity (Estyrene H manufactured by Nippon Steel Chemical Co., Ltd.
-65) was injection-filled with an injection time of 1.5 seconds. After passing through the cooling process for 60 seconds in this state, the movable mold 3 was opened and the injection molded body 40 was taken out from the mold. At this time, a through hole having a diameter of about 0.5 mm is formed in the injection molded body 40 at a position corresponding to the tip of each void insertion pin 6, and injection filling is performed from there through in the longitudinal direction inside the thick rib 43. Voids of a size corresponding to the amount of volumetric shrinkage accompanying cooling and solidification of the molten resin thus grown had grown. Obtained injection molded product 4
When the surface condition of 0 was investigated, the surface part 44 of the top plate 42
No surface defects such as sink marks were detected.

Comparative Example 1 In this comparative example, the air passage 8 of the injection molding die shown in FIG. 1 was sealed and the sealing material 9 was also removed, so that the air inside the cavity from the parting surface was out of the die as usual. And was molded under the same conditions as in Example 1 except for the above. In the injection molded body 40, there is no through hole on the surface of the molded body at a position corresponding to the tip of the void insertion pin 6,
No void was formed in the interior beyond that, and significant sink marks were generated on the side surfaces of the surface portion 44 corresponding to the rib installation portion and the thick rib 43 itself, and the surface quality as a product was poor.

Example 2 In this example, an injection-molded body 40 having the same size as the box-shaped molded product of Example 1 but having a larger size is manufactured. Due to the large size of the molded product, the ratio of the volume of the mold cavity to the volume of the air discharge path is significantly increased, and compressed air is compressed so that the pressure of accumulated compressed air does not increase and sealing becomes difficult. After being temporarily stored in an external pressure storage tank through the discharge passage and being controlled to an appropriate air pressure as the void-inducing compressed air, it was supplied to the void insertion pin through the air passage.

As shown in FIG. 6, the die used is composed of a fixed die 2 and a movable die 3 as in the first embodiment.
And the movable die 3, a cavity 4 having an inner surface shape corresponding to the outer surface shape of the molded product is formed.
Is provided with a sprue 5 for injecting a molten resin. The movable mold 3 is provided with an air discharge passage 7 for collecting compressed air discharged from the parting surface when the molten resin is injected into the cavity 4, and air leakage from the parting surface is provided on the outer periphery thereof. A sealing material 9 is attached to prevent this. From the air discharge path 7 to the air communication path 8
Are provided and are connected to the external air circuit 24.
At least one void insertion pin 6 having a protruding pin tip 15 is installed at each thick portion 4'of the cavity. An external air circuit 24 is connected to the coupler portion 25 at the base of the void insertion pin 6. Outside the mold,
An accumulator tank 2 for storing compressed air collected from the external air circuit 24 and a three-way port solenoid valve 21 for opening and closing the air circuit and the backflow prevention valve 22 from the air discharge passage 7.
0 was installed and combined with the mold body to form a mold device.
The compressed air circuit and accumulator tank were insulated with a heat insulating material.

First, the fixed mold 2 and the movable mold 3 of the mold 1 shown in FIG. 6 are closed, and then the sprue 5 is passed into the cavity 4 to fill the cavity with the rubber-reinforced polystyrene resin in a molten state. (Nippon Steel Chemical's styrene H-65) was injected and filled at an injection time of 1.5 seconds. At this time, the air in the cavity 4 is expelled from the air discharge passage 7 to the air communication passage 8 while being compressed, and is accumulated in the accumulator tank 20 through the check valve 22 of the connected external air circuit. The pressure of the compressed air in the accumulator tank 20 is adjusted to 5 kgf / c by the pressure adjusting valve 23. One second after the completion of injection of the molten resin, the solenoid valve 21 is opened, and the compressed air accumulated in the accumulator tank (5 kgf / c)
Was applied to the tip of the void insertion pin 6. After 8 seconds, the solenoid valve 21 was closed to stop the supply of compressed air, and the air circuit from the void insertion pin was opened to the atmosphere. After passing through the cooling step for 60 seconds, the injection molded body 40 was taken out of the mold 1. At a position corresponding to the tip of the void insertion pin 6 of the obtained injection molded body 40, a diameter of 0.5 mm is formed on the surface of the molded body.
Front and rear through-holes were opened, and voids were formed inside the holes. When the surface condition of the obtained injection-molded article 40 was investigated, no surface defects such as sink marks were detected.

Comparative Example 2 In this Comparative Example, the mold apparatus of Example 2 was used, and similarly to Example 2, first, the fixed mold 2 and the movable mold 3 of the mold 1 shown in FIG. 6 were closed, and then, A rubber-reinforced polystyrene resin in a molten state sufficient to fill the cavity through the sprue 5 (Esstyrene H manufactured by Nippon Steel Chemical Co., Ltd.
-65) was injection-filled with an injection time of 1.5 seconds. However, the injection molded body 40 was taken out of the mold 1 through a cooling process for 60 seconds while the solenoid valve 21 was closed.
The obtained injection-molded body 40 has no through hole on the surface of the molded body at a position corresponding to the tip of the void insertion pin 6, and no void is formed inside the tip, and the surface portion 44 corresponding to the rib installation portion is formed. In addition, a significant sink mark was generated on the side surface of the thick rib 43 itself, and the surface quality as a product was extremely poor.

[0025]

As described above, in the present invention, when molding thermoplastic resin moldings having different wall thicknesses, a void insertion pin is provided at a thick wall portion of a mold cavity where a sink phenomenon occurs. Injecting and filling the cavity with a sufficient amount of molten resin to fill the cavity, and at that time, pressurizing gas that is replaced with the molten resin and discharged from the cavity is passed through the void insertion pin from the air passage. To induce a void in the molten resin at the tip of the void insertion pin, and then grow the void to a size commensurate with the shrinkage due to the shrinkage force accompanying cooling and solidification of the molten resin. This is to prevent sinking of the molded product.

As a result, even if the obtained injection-molded article has a complicated shape having an extremely thick portion and a thin portion,
It has no surface defects such as sink marks, and has high surface value and high commercial value. Moreover, this molding method can omit the pressure-holding step compared to the conventional injection molding method, in which the filling and holding pressure of the molten resin are kept high to prevent sink marks. It has the advantages that the machine can be used and molding strain can be kept low. In addition, this molding method does not involve an unreasonable flow of resin, such as flowing high pressure gas inside the thick part to hollow it out, as compared with the conventional injection molding method that uses high pressure gas as an auxiliary. It is an injection molding method that is extremely advantageous in practical use because it is extremely easy and does not require special high pressure gas that is difficult to handle. In addition, compared to the injection molding method that we previously proposed in Japanese Unexamined Patent Publication No. 5-16177, in which the void phenomenon is positively used to prevent sink marks, external gas supply equipment is not required, and existing molding It can be easily implemented with equipment.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of an injection molding die device according to the present invention.

FIG. 2 is a cross-sectional view showing a state in which void nuclei are induced at the tip of a void insertion pin with pressurized air according to the present invention.

FIG. 3 is a cross-sectional view showing a growth state of voids according to the progress of resin cooling according to the present invention.

FIG. 4 is a partially cutaway perspective view showing a box-shaped molded product injection-molded in Example 1 of the present invention.

5 is a cross-sectional view of a part of the rib portion of FIG.

FIG. 6 is a partial cross-sectional view illustrating compressed air recovery in the mold cavity used in Example 2 of the present invention and supply of compressed air to the tip of the void insertion pin.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Injection molding mold 2 Fixed mold 3 Movable mold 4 Cavity 5 Sprue 6 Void insertion pin 7 Air discharge passage 8 Air conduction passage 9 Sealant 4'Thick wall portion 10 Fitting insertion hole 11 Large diameter portion 12 Annular space portion 13 Small diameter part 14 Annular void part 15 Pin tip 15 'Resin surface skin layer 16 Through hole 17 Void core 18 Void 20 Accumulation tank 21 Three-way solenoid valve 22 Backflow prevention valve 24 External air circuit 25 Coupler part 40 Injection molded body 42 Top plate Part 43 Thick rib 44 Root of rib 45 Sprue

Claims (3)

[Claims] 1. When a thermoplastic resin molded product having different wall thickness is injection molded, a void insertion pin is provided at an arbitrary position of a mold cavity corresponding to a thick portion where a sink phenomenon easily occurs, and the cavity is provided in the cavity. A molten resin is injected and filled, and a pressurized gas having a pressure to break through the resin skin layer generated near the tip of the pin is applied to the pin tip through the void insertion pin, and the molten resin near the tip is injected into the molten resin. After inducing voids, the shrinkage force caused by cooling and solidification of the molten resin causes the voids to grow to a size commensurate with the amount of shrinkage caused by cooling of the filled resin, preventing injection of moldings. In the molding method, by making the entire cavity excluding the molten resin injection port into an airtight structure, the air in the cavity that is compressed as the molten resin is injected and filled is provided with an air communication path from the cavity. The method of injection molding a thermoplastic resin molded article, characterized in that a pressurized gas source that leads to the void insertion pin and breaks through the resin skin layer is provided. 2. A cavity having a thick portion and a thin portion corresponding to the shape of a thermoplastic resin molded product to be manufactured, an injection injection gate for injecting a molten resin into the cavity, and a thick wall where a sink phenomenon easily occurs. A mold cavity portion corresponding to a portion, and a single or a plurality of void insertion pins fixed in a protruding state, and the mold cavity is an airtight structure except the molten resin injection port, the molten resin of the cavity An injection molding die apparatus, wherein an air discharge port near a filling end portion and a tip of the void insertion pin are connected by an air communication path. 3. The mold cavity has an airtight structure except for the molten resin injection port, and the air discharged from the mold cavity by the injection and filling operation of the molten resin is recovered and stored as compressed air via an air passage. 3. The injection molding die device according to claim 2, further comprising: a device for supplying the compressed air to the tip of the void insertion pin from the recovery device.