Process for producing thermoplastic resin hollow molded article

Details Process for producing thermoplastic resin hollow molded article Process for producing thermoplastic resin hollow molded article

1998-06-10

2001-02-13

Silbaugh, Jan H. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of fluid pressure differential to...

Producing multilayer work or article

C425S572000, C425S577000, C425S519000, C249S082000

Reexamination Certificate

active

06187251

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a molded article having a void inside, which molded article is made of a thermoplastic resin. The thermoplastic resin is, hereinafter, referred to as a “resin”. The molded article having the void inside and made of the resin is referred to as a “resin hollow molded article” or a “hollow molded article”.
2. Description of Related Art
Japanese unexamined patent publications Nos. hei 4-212822 and hei 7-144336 disclose a process for producing a hollow molded article utilizing a core retreating method. In this process, a molten resin is injected with an extremely high pressure into a cavity which is completely closed with a predetermined clearance. After filling the cavity with the resin, a movable core is retreated to expand the cavity while injecting the high pressure gas into the molten resin. Consequently, a void is formed in a resin substrate. However, in such a hollow molding process utilizing an injection molding method, an extremely high clamping force must be applied to a mold because the molten resin is injected with a high pressure in order to fill the cavity. Therefore, a clamping machine inevitably becomes large and expensive. Additionally, a molded article to be obtained suffers from deformation and warp due to residual stress. Moreover, a pressure to be applied to the movable core is very high because the molten resin is injected into the cavity with a high pressure. Hence a machine for holding the movable core and retreating the same is obliged to become large in order to hold the movable core at a predetermined position with opposing to the high pressure of the molten resin and in order to retreat it smoothly.
SUMMARY OF THE INVENTION
An object of the present invention is to dissolve aforementioned problems of the conventional process. In other words, the object of the present invention is to provide a process for producing a hollow molded article of high quality which does not suffer from either deformation or warp using a simple and cheap machine.
According to an aspect of the present invention, there is provided a process for producing a hollow molded article, comprising the following steps of:
Step (A) providing a molding machine comprising a mold including the first mold half and the second mold half, wherein the first mold half has the first mold surface, the second mold half has the second mold surface, the mold surfaces together define a cavity, the mold halves can relatively move so that the mold surfaces approach and go away from each other, at least one of the mold halves comprises a body and a slidable block, the mold surface of the mold half comprising the body and the slidable block comprises a surface of the body and a surface of the slidable block, the slidable block can move from the first position to the second position along the same direction of movement that the first and second mold halves move between their closed and unclosed states, the movement of the slidable block from the first position to the second position can expand the cavity, and the molding machine has means for supplying compressed gas into the cavity;
Step (B) supplying a thermoplastic resin, which is in a molten state, to between the first mold surface and the second mold surface while the mold being in an unclosed state;
Step (C) relatively bringing the mold halves close to each other to press the thermoplastic resin and spread the same so that the cavity is filled with the thermoplastic resin;
Step (D) injecting the compressed gas into the thermoplastic resin which is located in the cavity by the operation of the means for supplying the compressed gas;
Step (E) moving the slidable block from the first position to the second position; and
Step (F) moving at least one of the mold halves relative to the other mold half to open the mold and to take out the thermoplastic resin hollow molded article.
In an embodiment of the present invention, the above steps are performed in the order, Step (A), Step (B), Step (C), Step (D), Step (E), Step (F). In another embodiment of the present invention, the above steps are performed in the order, Step (A), Step (B), Step (C), Step (E), Step (D), Step (F). In still another embodiment of the present invention, at least a part of Step (B) is performed with at least a part of Step (C). In yet another embodiment of the present invention, at least a part of Step (C) is performed with at least a part of Step (D). In an embodiment of the present invention, at least a part of Step (D) is performed with at least a part of Step (E). In another embodiment of the present invention, Step (B), Step (C), Step (D) and Step (E) are performed under a condition that temperature of a mold surface of the slidable block is lower than that of a mold surface of the body. In still another embodiment of the present invention, Step (B), Step (C), Step (D) and Step (E) are performed under a condition that escape of heat from the thermoplastic resin to a part of the body which is located in the vicinity of the slidable block can be prevented. In yet another embodiment of the present invention, supply of the thermoplastic resin is started when the slidable block exists at the first position and the mold surface of the slidable block existing at the first position is depressed below the surface of the body which is adjacent to the slidable block, in Step (B). In an embodiment of the present invention, pressure of the compressed gas in Step (D) is not lower than about 1 kgf/cm
2
and lower than about 10 kgf/cm
2
. In still another embodiment of the present invention, a further step of supplying a skin material to between the mold surfaces of the mold halves is performed prior to Step (B). In yet another embodiment of the present invention, the compressed gas is injected into the thermoplastic resin which is located over the body in Step (D). In an embodiment of the present invention, the compressed gas is injected into both the thermoplastic resin which is located over the slidable block and the thermoplastic resin which is located over the body in Step (D). In another embodiment of the present invention, a further step of ejecting gas from a void which was formed in the thermoplastic resin is performed after the beginning of Step (D). In still another embodiment of the present invention, a further step of changing the pressure which is applied to the thermoplastic resin is performed after the completion of filling the cavity with the thermoplastic resin. In yet another embodiment of the present invention, a further step of reducing the pressure which is applied to the thermoplastic resin is performed after the completion of filling the cavity with the thermoplastic resin. In an embodiment of the present invention, the step of injecting the compressed gas is performed after the reduction of the pressure applied to the thermoplastic resin. In another embodiment of the present invention, the step of moving the slidable block is performed after the reduction of the pressure applied to the thermoplastic resin.
The above and other aspects, embodiments, features and advantages of the present invention will become clear from the following description taken in conjunction with the accompanying drawings.


REFERENCES:
patent: 5277865 (1994-01-01), Hara et al.
patent: 5534216 (1996-07-01), Kamiyama
patent: 5584470 (1996-12-01), Klotz
patent: 5656234 (1997-08-01), Kaneshi et al.
patent: 5885518 (1999-03-01), Hendry
patent: 0 624 447 (1994-11-01), None
patent: 0 679 491 (1995-11-01), None
patent: 1-168425 (1989-07-01), None
patent: 4-212822 (1992-08-01), None
patent: 5-278076 (1993-10-01), None
patent: 5-301257 (1993-11-01), None
patent: 6-315970 (1994-11-01), None
patent: 7-144336 (1995-06-01), None
patent: 7-156184 (1995-06-01), None
patent: 7-186178 (1995-07-01), None
Patent Abstracts of Japan, vol. 96, No. 3, Mar. 29, 1996, & JP 07 290486, Nov. 7, 1995.
Patent Abstracts of Japan, vol. 96, No. 12, Dec. 26, 1996, & JP 08 216178, Aug. 27, 1996.

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