JP3472637B2 - Hollow injection molding method and mold - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an injection molding method for molding a hollow molded article by pressurizing a molten gas injected into a mold cavity of a mold with a pressurized gas, and a mold used therefor. More specifically, it relates to improvement of the surface condition of the obtained hollow molded article.

[0002]

2. Description of the Related Art Hollow injection molding is disclosed in Japanese Examined Patent Publication Sho 57-14968.
It is a technique known from Japanese Patent Publication No. JP, etc., and in recent years, it is rapidly becoming popular in the molding of various interior parts of automobiles and housings of household electric appliances.

The above conventional hollow injection molding is generally performed through the following steps.

First, a predetermined amount of molten resin is injected into a mold cavity of a mold having a pressurized gas press-in mechanism (the pressurized gas press-in mechanism may be built in the injection nozzle in some cases), and then pressurized. Pressurized gas is injected into the molten resin for a predetermined time from the gas injection mechanism. The pressurized gas that has been press-fitted enters the inside of the molten resin and flows to form a hollow portion. Following the pressurization of pressurized gas for this predetermined time,
Hold this gas pressure for a predetermined time so that it is maintained in the molded product, and after the resin in the mold cavity is cooled,
After releasing the pressurized gas inside the molded product, the mold is opened and the hollow molded product is taken out.

When the above-mentioned hollow injection molding is applied to a molded article having a partially thick portion and a pressurized gas is injected into the thick portion, volumetric shrinkage due to cooling of the resin occurs in the thick portion. There is an advantage that easy sinkage can be prevented.

[0006]

However, the hollow molded article obtained by the above-mentioned conventional hollow injection molding often has its own poor appearance. That is, it is uneven gloss that occurs on the surface of the portion where the hollow portion is formed.

The gloss unevenness appears as a line or a band having a strong gloss, which is generated at the boundary between the surface region corresponding to the hollow portion and the other surface region. Especially when the hollow part is formed in the thick part protruding to the back surface, a strong linear or strip-like gloss portion along the contour of the thick part that is the back surface is generated, and between other areas. Uneven gloss, that is, uneven gloss. This gloss unevenness is an important quality issue especially for products that are not subjected to post-processing such as painting and coating, for example, interior and exterior parts of automobiles and housings of household electric appliances, and the application of the hollow injection molding method to them. Can be difficult.

It is an object of the present invention to prevent uneven luster that occurs along the formed hollow portion when molding a hollow molded article by hollow injection molding.

[0009]

For this purpose, in the invention of claim 1, a pressurized gas is pressed into the molten resin injected into the mold cavity of the mold to form a partial thick portion. Existence
Then, in the hollow injection molding method of molding a molded body having a hollow portion in the thick portion, after injecting a molten resin into a mold cavity, pressurizing gas is injected to remove excess molten resin in the mold cavity. Extruded out of the mold cavity to the thick part
This is a hollow injection molding method in which a hollow portion is formed , then the pressure in the hollow portion is momentarily lowered, and then cooled under the lowered pressure .

Further, in the invention of claim 7 , the mold used in the above hollow injection molding method has a volume sufficiently larger than the amount of the surplus molten resin extruded out of the mold cavity and the atmosphere. The resin reservoir which is open or connected to the recovery tank is a mold which is connected to the mold cavity through an outflow passage having an opening / closing valve.

The present invention will be further described with reference to the drawings.

FIGS. 1 and 2 show the procedure for carrying out the hollow injection molding method of the present invention using a mold 2 as shown in FIGS. 1 (a) and 3.

First, the mold 2 will be described with reference to FIG.
As shown in (a) and FIG. 3, the mold has a mold cavity 1 in which a rib-shaped thick-walled molding portion 3 is provided across a central portion of a substantially rectangular plate-shaped portion. . The mold cavity 1 is filled with molten resin from the injection nozzle 4 via the runner 5, and pressurized gas is injected from a pressurized gas injection mechanism 6 connected to a gas supply device (not shown). It is what is done.

The gas supply device supplies the gas pressurized by, for example, a compressor to the pressurized gas injection mechanism 6, and is usually electrically connected to the injector to set the timing of starting the injection of the pressurized gas. It can be controlled according to the operation of the injection machine. In addition, it is possible to control the time during which the pressurized gas is being injected, the auxiliary holding pressure described later, and the like.

The filling of the molten resin and the pressurization of the pressurized gas are performed from the left end portion of the thick-walled molding portion 3 in the drawing, and the right end portion of the thick-walled molding portion 3 flows out to the outside of the mold 2. A path 7 is provided. An on-off valve 8 that opens and closes the outflow passage 7 is interposed in the outflow passage 7.

As the opening / closing valve 8, for example, a valve that moves forward / backward by hydraulic pressure to close / open the outflow passage 7 can be used. The opening / closing operation by the opening / closing valve 8 can be automatically controlled by electrically connecting to the gas supply device or connecting a timer.

Next, the hollow injection method of the present invention will be described with reference to FIGS. 1 and 2.

First, as shown in FIG. 1A, the mold is clamped, and as shown in FIG. 1B, the shutoff valve 9 of the injection nozzle 4 is opened with the opening / closing valve 8 closed. The molten resin is injected into the mold cavity 1.

The injection amount of the molten resin is an amount sufficient to fill the mold cavity 1 or a little insufficient amount to fill the mold cavity 1. Specifically, it is preferable to inject the molten resin in an amount of 90% by volume or more of the mold cavity volume. In particular, the mold reproducibility of the obtained hollow molded product is improved, and a weld line like a weld line which tends to be generated when the molten resin is pushed into the unfilled and left mold cavity 1 portion by pressurization of a pressurized gas described later. In order to prevent the occurrence of surface unevenness, it is preferable to inject an excessive amount of molten resin that exceeds 100% by volume of the mold volume.

As the resin, a thermoplastic resin used in injection molding or extrusion molding can be used. Also,
A thermosetting resin can also be used if necessary. Various commonly used additives, fillers and other fillers can be added to these resins.

Next, as shown in FIG. 1 (c), the shutoff valve 9 of the injection nozzle 4 is provided in order to prevent the backflow of the pressurized gas.
Is closed and pressurized gas is injected into the molten resin in the mold cavity 1 for a predetermined time through the pressurized gas injection mechanism 6, and the state is maintained after completion of the injection. Further, the on-off valve 8 is opened before the pressurization of the pressurized gas is started, at the same time as the start of the pressurization, or after the start of the pressurization and before the completion of the press-fitting.

By the way, the present inventors believe that the above-mentioned cause of the uneven glossiness is caused by the excessive pressure of the pressurized gas acting along the press-fitting path when the pressurized gas is press-fitted. If the pressure of the pressurized gas is lowered, the action of excessive pressure can be prevented, but this will not allow sufficient pressurization of the pressurized gas. Therefore, in the present invention, the pressurizing gas having a sufficient pressure is injected and the opening / closing valve 8 is opened as described above.

That is, when the on-off valve 8 is opened, the excess molten resin due to the pressurization of the pressurized gas causes the excess resin to flow through the outflow passage 7 and the mold cavity, as shown in FIG. 1 will be pushed out. Therefore, the pressurized gas can penetrate deeply into the deep portion in a short time without great resistance, and the excessive pressure of the pressurized gas is prevented from acting for a long time along the press-fitting path, so that uneven gloss is generated. It will be prevented. Further, the excess molten resin being pushed out means that the amount of resin in the portion where the hollow portion 11 is formed is reduced, and the decrease in the amount of resin also suppresses the occurrence of sink marks.

The time during which the pressurization of the pressurized gas is continued is the same as the press-in time in ordinary hollow injection molding. Open / close valve 8
As described above, the timing of opening the pressurizing gas may be before starting the press-fitting of the pressurized gas, at the same time as the start of the press-fitting, or after the start of the press-fitting until the completion of the press-fitting. It is preferable that the press-in is started and the press-in is completed after a predetermined short time has elapsed. The predetermined short time means the size of the hollow molded product to be molded and the hollow portion 11 formed therein.
The time is about 1/2 to 1/10 of the pressurization time of the pressurized gas, although it varies depending on the length of the gas. If the opening / closing valve 8 opens too early, sink marks may occur depending on the size and shape of the molded product. Conversely, if the opening / closing valve 8 opens later than the completion of pressurization of the pressurized gas, the high pressure of the pressurized gas The action time becomes longer and uneven gloss is likely to occur.

The flow direction of the pressurized gas injected under pressure has a strong tendency toward the outflow direction of the surplus molten resin, that is, the outflow path 7. Therefore, it is preferable that the position where the outflow passage 7 is provided is provided at the end portion of the hollow portion 11 to be formed by the press-fitting of the pressurized gas when viewed from the press-fitting position of the pressurized gas. By doing so, it becomes easy to form the hollow portion 11 along a predetermined route. In the case of the mold 2 shown in the drawing, the hollow portion 11 can be formed along the thick-walled molded portion 3 by guiding the pressurized gas that has been press-fitted from the pressurized gas press-fitting mechanism 6 to the outflow passage 7.

In the illustrated mold 2, the outflow passage 7 is provided only at one location, but the outflow passage 7 is not limited to one location, and a plurality of locations can be provided for one die cavity 1. . For example, when the thick-walled molding portion 3 exists in a cross shape, the pressurized gas press-fitting mechanism 6 is provided to one end of the thick-walled molding portion 3 and the remaining three ends of the thick-wall molding portion 3 are provided. It is possible to provide an outflow passage 7 in which an on-off valve 8 is interposed in each part. Further, when a plurality of outflow passages 7 are provided, by shifting the opening / closing timing of the on-off valve 8 in each outflow passage 7,
It is also possible to control the flow direction of the pressurized gas.

The pressurized gas press-in mechanism 6 shown in the figure has an opening in the mold cavity 1, and is usually constituted by a nozzle-like member having a narrow opening through which pressurized gas can pass but molten resin can be prevented from entering. To be done. In addition, when it is composed of a nozzle-shaped member having an opening / closing mechanism that is closed when the molten resin is injected and is opened when the pressurized gas is injected, or as in the illustrated pressurized gas injection mechanism 6, a direct mold cavity is used. 1
Rather than installing for sprue 10 and runner 5
It may be provided for. Furthermore, the pressurized gas injection mechanism 6 may be built in the injection nozzle 4 to inject the pressurized gas into the mold cavity 1 along the same path as the molten resin.

As the pressurizing gas to be injected, one that does not react with the resin used at the temperature and pressure during injection molding is used. For example, air, carbon dioxide gas, nitrogen or the like is used, but an inert gas such as nitrogen is preferable.

When the opening / closing valve 8 is opened, the hollow portion 11 formed at the time of pressurizing the pressurized gas further grows and extends into the outflow passage 7, and finally, as shown in FIG. 2 (a). , The resin layer at the tip of the outflow passage 7 is broken. That is, a part of the resin wall surrounding the hollow portion 11 is broken and the pressurized gas inside the hollow portion 11 is released to the outside, so that the pressure inside the hollow portion 11 instantaneously drops. And, by this, the hollow portion 11
It is possible to prevent the occurrence of uneven gloss due to the action of a high pressure gas after forming the film.

The momentary pressure drop in the hollow portion 11 may be performed by lowering the pressure in the hollow portion 11 to the atmospheric pressure. However, when the pressure in the hollow portion 11 is lowered to the atmospheric pressure and cooling is performed thereafter. Depending on the shape of the hollow molded product, some sink marks may occur. Therefore, in order to prevent this, it is preferable to leave the auxiliary holding pressure exceeding the atmospheric pressure in the hollow portion 11.

The auxiliary pressure holding pressure is preferably 1% to 50% of the pressure of the pressurized gas that is press-fitted to form the hollow portion 11. If the auxiliary holding pressure is too low, it is difficult to sufficiently prevent the sink mark, and if it is too high, it may cause uneven gloss.

The auxiliary holding pressure can be obtained by closing the on-off valve 8 before the pressure in the hollow portion 11 drops to atmospheric pressure, as shown in FIG. 2 (b). Further, the holding pressure can be obtained by replenishing the pressurized gas having a low pressure through the pressurized gas injection mechanism 6 after closing the opening / closing valve 8.

The instantaneous drop in the pressure in the hollow portion 11 is achieved by connecting the pressurized gas injection mechanism 6 to a recovery system having a recovery tank (not shown) and adding the pressurized gas in the hollow portion 11 to it. It can also be performed by causing a rapid reverse flow through the pressurized gas injection mechanism 6. Alternatively, a part of the molded product may be cut by using a shearing mechanism (not shown), or a hollow needle pin may be penetrated toward the hollow portion 11 or the like. However, since it is difficult to precisely adjust the timing of activating these to reduce the pressure, it is preferable to rupture the hollow molded article as described above.

As described above, after the pressure inside the hollow portion 11 is set to the atmospheric pressure or the auxiliary holding pressure, the hollow molded article is taken out after a necessary cooling time. When the pressure in the hollow portion 11 is reduced to atmospheric pressure, the mold 2 is used as it is.
Although the hollow molded product can be taken out by opening, when the auxiliary holding pressure is applied to the hollow portion 11, for example, after the inside of the hollow portion 11 is opened to the atmosphere via the pressurized gas injection mechanism 6. It is preferable to open the mold 1.

FIG. 4 shows another mold 2 used in the hollow injection molding method of the present invention, which is substantially the same as the mold 2 described in FIGS. 1 to 3, except that the outflow passage 7 has a resin reservoir. 12 is different from the mold 2 described in FIGS. 1 to 3 in that the exhaust passage 13 extends from the resin reservoir 12 to the outside of the mold 2. The resin reservoir 12 has a sufficiently large volume as compared with the amount of resin pushed out by pressurizing the pressurized gas. The state of the mold 2 shown in FIG.
This corresponds to the state of the mold 2 in (a).

The hollow injection molding method using this mold 2 is shown in FIG.
2 is the same as that of FIG. 2, but the resin wall surrounding the hollow portion 11 is broken in the resin reservoir 12, and the pressurized gas in the hollow portion 11 is released from the resin reservoir 12 to the atmosphere via the exhaust passage 13. It is different in that it is released to.

That is, by the pressurization of the pressurized gas and the opening of the on-off valve 8, the excess molten resin is pushed out into the resin reservoir 12 through the outflow passage 7, and the pressurized gas also passes through the resin reservoir 1.
2 into the molten resin, the hollow portion 11 grows excessively in the resin reservoir 12, and finally a part of the resin wall is broken, so that the pressurized gas in the hollow portion 11 is discharged from the exhaust passage 13 to the atmosphere. Will be released to.

When the mold 2 shown in FIG. 4 is used,
Excess molten resin remains in the resin reservoir 12 and is taken out from the mold 2 when taking out the hollow molded product.
The collection becomes easy. Further, in FIG. 4, the resin reservoir 12 is open to the atmosphere, but a recovery tank (not shown)
It is also possible to collect the pressurized gas released from the hollow portion 11 by connecting it to the recovery system having the above. Furthermore, 1
It is also possible to provide a plurality of resin reservoirs 12 for one mold cavity 1 by connecting to different mold cavity 1 positions.

FIG. 5 shows still another example of the mold 2 used in the hollow injection molding method of the present invention, which has two resin reservoirs 12 which are connected to each other in series. One of the resin reservoirs 12 has an outflow passage 7 with an opening / closing valve 8 interposed.
Is connected to the mold cavity 1, and the other resin reservoir 12 is connected to an exhaust passage 13 communicating with the outside of the mold 2. One resin reservoir 12 may be provided as in the mold of FIG. 4, but if two resin reservoirs 12 are provided in series as described above, it is possible to cope with the formation of hollow portions 11 of various sizes, and the resin inside the hollow portion 11 can be formed. It becomes easy to prevent the resin from jumping out of the mold 2 when the pressurized gas is released.

A pipe 17, in which a filter 14, a pressure sensor 15, and a gas release valve 16 are sequentially interposed in the exhaust passage 13,
Are connected. 18 is a timer, pressure sensor 1
The gas release valve 16 is closed when the pressure detected by 5 reaches the above-mentioned holding pressure, and the gas release valve 16 is opened again after the set pressure holding time has elapsed.

The state of the mold 2 shown in FIG. 5 is as shown in FIG.
This corresponds to the state of the mold 2 in (a). Further, 19 is a sealing material for preventing leakage of the pressurized gas released from the hollow portion 11.

In the mold 2 of FIG. 5, the gas release valve 16 is opened at the same time as the opening / closing valve 8, and excess melting is caused by pressurization of the pressurized gas and opening of the opening / closing valve 8 and the gas release valve 16. The resin first stays in the resin reservoir 12 on the outflow passage 7 side, and with the growth of the hollow portion 11, a part of the resin wall is broken in the resin reservoir 12a or in another resin reservoir 12b, and the inside of the hollow portion 11 is broken. Pressurized gas flows out. Resin reservoir 12a, 12b
This outflowing pressurized gas is guided to the pipe 17 and is released to the atmosphere through the filter 14 and the gas release valve 16.

The filter 14 is for removing the resin pieces accompanying the flow of the pressurized gas flowing out. When the pressure sensor 15 detects that the pressure inside the pipe 17 has reached the preset holding pressure, the gas release valve 16 is closed for the time set by the timer 18 to hold the pressure. After the pressure holding time has elapsed, the gas release valve 16 is opened again to release the pressurized gas remaining in the hollow portion 11 and the pipe 17,
The mold 2 is opened and the hollow molded product is taken out.

In FIG. 5, the tip of the pipe 17 is open to the atmosphere, but it is connected to a recovery system having a recovery tank (not shown), and the pressurized gas released from the inside of the hollow portion 11 is connected. Can also be collected and recycled. It is also possible to provide a plurality of sets of resin reservoirs 12 connected in series and connect them to different mold cavity 1 positions.

[0045]

【Example】

Example 1 Hollow injection molding was performed using a mold 2 as shown in FIG.

First, with respect to the mold cavity 1, a sufficient amount (1 volume of the mold cavity 1 to mold a product shape) is formed.
A polypropylene resin (containing 20% by weight of talc, black) of 100% by volume) was injected from the injection nozzle 4. At this time, the open / close valve 8 and the gas release valve 16 were in a closed state, and the mold cavity 1 was filled with the molten resin.

Next, after the injection of the molten resin is completed, the shutoff valve 9 of the injection nozzle 4 is closed, and at the same time, 200 kg / c of the pressurized gas injection mechanism 6 into the molten resin in the thick-walled molding portion 3.
Nitrogen gas having a pressure of m ² was injected for 4 seconds. As a result, the nitrogen gas enters the molten resin in the thick-walled molding portion 3 to form the hollow portion 11.

The open / close valve 8 was opened 3 seconds after the start of the nitrogen gas press-in (1 second before the end of the nitrogen gas press-in), and at the same time the gas release valve 16 was opened. When the outflow passage 7 was opened, the nitrogen gas pushed the surplus molten resin into the resin reservoir 12, broke through the resin wall in the resin reservoir 12, and was released into the atmosphere through the pipe 17.

About 2 seconds after the resin wall was ruptured, the pipe 1
The pressure sensor 15 detected that the pressure inside 7 had reached the set value of 20 kg / cm ² , the gas release valve 16 was closed, and the state was kept for 15 seconds preset by the timer 18. After the lapse of 15 seconds, the gas release valve 16 was opened again to open the inside of the hollow portion 11 to the atmosphere, and then the mold 2 was opened to take out the hollow molded article.

A time chart in this embodiment is shown in FIG.

On the surface of the obtained hollow molded article, neither the unevenness of gloss, which was a problem in hollow injection molding, nor the occurrence of sink marks was observed.

Example 2 The pressure of the nitrogen gas to be press-fitted was 150 kg / cm ² , the press-in time was 5 seconds, and the opening / closing valve 8 and the gas release valve 16 were opened 4 seconds after the start of the pressurization of nitrogen gas (nitrogen gas 1 second before the press-fitting ends), the auxiliary holding pressure is 30 kg / cm
^2. Hollow injection molding was carried out in the same manner as in Example except that the auxiliary pressure holding time was 20 seconds.

On the surface of the obtained hollow molded article, neither the uneven luster, which was a problem in hollow injection molding, nor the occurrence of sink marks was observed.

Example 3 The pressure of nitrogen gas to be press-fitted was 100 kg / cm ² , the press-in time was 5 seconds, and the opening / closing valve 8 and the gas release valve 16 were opened 5 seconds after the start of the pressurization of nitrogen gas (nitrogen gas At the same time when press fitting is completed), the auxiliary holding pressure is 30 kg / c
Hollow injection molding was performed in the same manner as in Example except that m ² and the auxiliary pressure holding time were 20 seconds.

On the surface of the obtained hollow molded article, neither the uneven luster, which was a problem in hollow injection molding, nor the occurrence of sink marks was observed.

Comparative Example 1 The same material as in Example 1 was injected into the mold cavity 1 in the same manner, and then 100 kg / cm ² of nitrogen gas was injected for 5 seconds in the same manner as in Example 1. Subsequent to the completion of the press-fitting of the nitrogen gas, the press-fitting nitrogen gas was sealed as it was for 22 seconds without opening the on-off valve 8 and the gas release valve 16 as in the first embodiment.

On the surface of the obtained hollow molded article, the occurrence of sink marks was not recognized, but uneven gloss, which is a problem in hollow injection molding, was observed.

Comparative Example 2 The same material as in Example 1 was injected into the mold cavity 1 in the same manner, and then 200 kg / cm ² of nitrogen gas was injected for 4 seconds in the same manner as in Example 1. Subsequent to the completion of the pressurization of the nitrogen gas, the pressurized nitrogen gas was sealed and held as it was for 16 seconds without opening the on-off valve 8 and the gas release valve 16 as in Example 1.

Although no sink mark was observed on the surface of the obtained hollow molded article, uneven gloss, which is a problem in hollow injection molding, was observed.

[0060]

EFFECTS OF THE INVENTION The present invention is as described above, and when molding a hollow molded article by hollow injection molding, it is possible to prevent uneven glossiness that occurs along the formed hollow portion, and especially to coat It is intended to facilitate application of hollow injection molding to products that are not subjected to post-processing such as coating and coating, for example, interior and exterior parts of automobiles and housings of household electric appliances.

[Brief description of drawings]

FIG. 1 is a diagram showing a procedure of hollow injection molding according to the present invention.

FIG. 2 is a diagram showing a procedure of hollow injection molding according to the present invention.

FIG. 3 is a view showing an example of a mold used for hollow injection molding according to the present invention.

FIG. 4 is a view showing another example of a mold used for hollow injection molding according to the present invention.

FIG. 5 is a view showing still another example of a mold used for hollow injection molding according to the present invention.

FIG. 6 is a time chart of the first embodiment.

[Explanation of symbols]

1 mold cavity 2 mold 3 Thick part 4 injection nozzles 5 runners 6 Pressurized gas injection mechanism 7 Outflow path 8 on-off valve 9 Shut-off valve 10 sprues 11 Hollow part 12 resin reservoir 13 exhaust path 14 filters 15 Pressure sensor 16 Gas release valve 17 Piping 18 timer

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-4-62118 (JP, A) JP-A-2-215516 (JP, A) JP-A-4-89221 (JP, A) JP-A-5- 309726 (JP, A) JP-A-5-111938 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B29C 45/00-45/84

Claims (7)

(57) [Claims] 1. A pressurizing gas is press-fitted into a molten resin injected into a mold cavity of the mold to have a partial thick portion,
In a hollow injection molding method for molding a molded body having a hollow portion in the thick portion, a molten resin is injected into a mold cavity, and then pressurized gas is injected to remove excess molten resin in the mold cavity. It is pushed out of the cavity and hollow in the thick part
Part, and then the pressure in the hollow part is momentarily dropped.
After that, the hollow injection molding method is characterized by cooling under reduced pressure . 2. The hollow injection molding method according to claim 1, wherein the required auxiliary holding pressure is maintained after the pressure in the hollow portion is instantaneously lowered. 3. The pressure in the hollow portion is instantaneously dropped by releasing the pressurized gas in the hollow portion by breaking a part of a resin wall surrounding the hollow portion due to growth of the hollow portion. The hollow injection molding method according to claim 1 or 2. 4. The hollow injection molding method according to claim 3, wherein the pressurized gas in the hollow portion is released to the recovery system. 5. A mold cavity is provided with an outflow passage having an opening / closing valve at the end of the position where a hollow portion is to be formed, and a molten resin is injected into the mold cavity with the opening / closing valve closed to mold the metal. 3. The hollow injection molding method according to claim 1, wherein the excess molten resin in the mold cavity is extruded from the outflow passage by opening the on-off valve when the pressurized gas is injected after the mold cavity is filled. . 6. A mold cavity is provided with an outflow passage, which is connected to a resin reservoir and has an opening / closing valve, at an end portion of a position where a hollow portion is to be formed in the mold cavity, and the opening / closing valve is closed. After injecting the molten resin into the mold cavity and filling the mold cavity, by opening the open / close valve when the pressurized gas is pressed in, the excess resin in the mold cavity is pushed out into the resin reservoir and the pressurized gas also accumulates in the resin reservoir. The pressure in the hollow portion is momentarily lowered by causing the gas to flow out into the resin reservoir, breaking the resin wall by forming an excessive hollow portion in the resin reservoir, and releasing the pressurized gas in the hollow portion. 1 or 2 hollow injection molding method. 7. After injecting the molten resin into the mold cavity, pressurizing gas is injected to form a hollow portion, and the excess molten resin in the mold cavity is pushed out of the mold cavity and then momentarily. A mold used in a hollow injection molding method for lowering the pressure in the hollow part, which has a sufficiently large volume as compared with the amount of excess molten resin extruded outside the mold cavity and is open to the atmosphere or a recovery tank. The mold is characterized in that the resin reservoir connected to the mold is connected to the mold cavity through an outflow passage having an on-off valve.