JP3371306B2 - Injection molding method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of injecting a gas to injection-mold a molded product having a predetermined portion hollow.

[0002]

2. Description of the Related Art Housings and covers for various molded automobile parts, OA equipment and household electric appliances made of thermoplastic resin are required to have high appearance quality. These molded products have a rib structure in order to give strength to the substrate as needed. Further, it has a boss structure for assembling a molded product. However, on the surface of the substrate on the side opposite to these thick portions, so-called "sink" occurs due to volume shrinkage of the resin due to cooling after injection,
The appearance of the product is significantly deteriorated.

In order to improve the appearance of products such as sink marks in the thick portion, conventionally, in a method of injecting a molten resin into a mold cavity and then injecting a gas to obtain a hollow body, the gas pressure is A method for adjusting a synthetic resin material according to a predetermined profile is known (see Japanese Patent Publication No. 6-6308).

By injecting a gas into the molding space at a relatively low first pressure, a hollow region is formed in a predetermined portion of the resin filled in the molding space, and then a second relatively high pressure is applied. An injection molding method of injecting gas with pressure has been proposed (JP-A-6-23780).

However, as the shape of the molded product becomes more complicated and larger, it becomes necessary to lengthen the gas flow distance in order to stop the sink mark, which requires a high gas pressure. However, in the above method, no specific solution has been proposed, and when high gas pressure is applied,
In the gas channel relatively close to the gas injection position, the phenomenon of gas leaching from the gas channel to the thin wall part occurs, and sometimes the gas channel part swells or cracks, which significantly deteriorates the appearance of the molded product. I had a problem.

[0006]

In view of the above circumstances, the inventors of the present invention have diligently studied the above-mentioned problems, and as a result, have determined the ratio of the gas pressure and the resin pressure at the time of injecting the gas within a specific range. The present invention has been completed by controlling an injection molding method by controlling at the stage of No. 1 to prevent gas leaching, sink marks, and cracks at corners.

That is, the present invention is an injection molding for producing a molded product in which a predetermined portion is hollow by injecting a molten resin into a molding space formed by closing a pair of molding dies and then injecting a gas. In the method, injecting gas in two stages,
Further, the present invention provides an injection molding method characterized in that the ratio of the gas pressure to the resin pressure in the cavity of the mold at the time of injecting gas is controlled as follows. (1) The ratio of the gas pressure to the resin pressure in the first stage is 1 or more and less than 4. (2) The ratio of the gas pressure to the resin pressure in the second stage is 10 or more.

Furthermore, the present invention is an injection molding for producing a molded product in which a predetermined portion is hollow by injecting a molten resin into a molding space formed by closing a pair of molding dies and then injecting a gas. In the method, there is provided an injection molding method characterized by injecting gas in three steps and controlling the ratio of the gas pressure to the resin pressure in the cavity of the mold at the time of injecting the gas as follows. is there. (1) The ratio of the gas pressure to the resin pressure in the first stage is 1 or more and less than 4. (2) The ratio of the gas pressure to the resin pressure in the second stage is 4 or more and less than 10. (3) The ratio of the gas pressure to the resin pressure in the third stage is 10 or more.

Further, according to the present invention, the start time of gas injection based on the injection start time of the molten resin is (6) less than 6 seconds in the first step and (2) more than 10 seconds in the second step. The present invention provides an injection molding method according to item 1. Also,
In the present invention, the start time of gas injection based on the injection start time of the molten resin is (1) less than 6 seconds in the first stage, (2)
3. The injection molding method according to claim 2, wherein the second step is 6 seconds or more and less than 10 seconds, and (3) the third step is 10 seconds or more.

The present invention will be described in detail below. The injection molding method of the present invention is an injection molding method for injecting a molten resin into a molding space formed by closing a pair of molding dies and then injecting a gas to produce a molded product in which a predetermined portion is hollow. This is an improved method of the so-called gas-assisted injection molding method. The gas injection can be started before the injection of the molten resin is completed. The resin pressure in the cavity at the time of gas injection can be measured using a pressure sensor or the like attached to the molding die.

First, the injection of gas in two steps will be described. The ratio of the gas pressure injected in the first step (first gas pressure) to the resin pressure in the cavity at the time of gas injection is 1 or more and less than 4, preferably 1 to 3. For example, using polypropylene, the board thickness is 3m
m, the cross-sectional area of the gas channel part is 100 mm ² , and the outer dimension is 300 mm x 300 mm. When molding a molded product, the resin pressure immediately after filling the mold cavity with a predetermined amount of resin is 10 bar. The injected first gas pressure is greater than or equal to 10 bar and less than 40 bar. The above ratio is 1
If it is less than 4, the hollow part of the molded product is not formed, and if it is 4 or more, gas seepage occurs, which is not preferable. The start time of gas injection based on the injection start time of the molten resin is not particularly limited, the kind of molten resin, the plate thickness of the molded product,
It is determined by the cross-sectional area of the gas channel portion, the gas pressure, etc., and is, for example, less than 6 seconds, preferably 2 to 5 seconds under the conditions of the above-mentioned molded product. Further, the holding time of the first gas pressure may be just before injecting the gas in the second stage.

The ratio of the gas pressure injected in the second stage (second gas pressure) to the resin pressure in the cavity at the time of gas injection is 10 or more, preferably 10-15. For example, under the conditions of the above-mentioned molded product, the resin pressure in the cavity is 8 bar, and the pressure of the second gas injected at this time is 80 bar or more. If the above ratio is less than 10, sink marks are generated at the end of the product or where the thick parts intersect, which is not preferable. Second, based on the injection start time of the molten resin
The start time of the gas pressure to be injected in the stage is not particularly limited, and is determined by the type of molten resin, the plate thickness of the molded product, the cross-sectional area of the gas channel portion, the gas pressure, etc. Under the condition, 10 seconds or more, preferably 1
It is 0 to 20 seconds. The holding time of the second gas pressure is not particularly limited, and an appropriate time until just before the mold is opened may be selected.

Next, the injection of gas in three stages will be described. The ratio of the gas pressure injected in the first stage (first gas pressure) to the resin pressure in the cavity at the time of gas injection is 1 or more and less than 4 as in the case of injecting gas in two stages, It is preferably 1 to 3. For example, in the same molded product as that described when the gas is injected in two steps, the resin pressure in the cavity is 10 bar, and the first gas pressure injected at this time is 10 bar or more and 40 bar or more.
It is less than ar. If the above ratio is less than 1, the hollow part of the molded product is not formed, and if it is 4 or more, gas seepage occurs, which is not preferable. The start time of gas injection based on the injection start time of the molten resin is not particularly limited, it is determined by the type of molten resin, the plate thickness of the molded product, the cross-sectional area of the gas channel portion and the gas pressure, For example, under the conditions of the above-mentioned molded article, it is less than 6 seconds, preferably 2 to 5 seconds. Also,
The holding time of the first gas pressure may be just before the gas is injected in the second stage.

The ratio of the gas pressure injected in the second stage (second gas pressure) to the resin pressure in the cavity at the time of gas injection is 4 or more and less than 10, preferably 5 to 9. . For example, in the above-mentioned molded product, the resin pressure in the cavity is 8 bar, and the pressure of the second gas injected at this time is 32 bar or more and less than 80 bar. If the above ratio is less than 4, the hollow part of the molded product is not formed, and if it is 10 or more, gas seepage occurs, which is not preferable. The start time of the gas pressure injected in the second step based on the injection start time of the molten resin is not particularly limited, and the kind of molten resin, the plate thickness of the molded product, the cross-sectional area of the gas channel portion, and It is determined by the gas pressure and the like, and is, for example, 6 seconds or more and less than 10 seconds, preferably 7 to 9 seconds under the conditions of the above-mentioned molded product. Further, the holding time of the second gas pressure may be just before the gas is injected in the third stage.

The ratio of the gas pressure injected in the third stage (third gas pressure) to the resin pressure in the cavity at the time of gas injection is 10 or more, preferably 10-15. For example, in the above molded product, the resin pressure in the cavity is 1
The pressure of the third gas injected at this time is 100 bar or more. If the above ratio is less than 10, sink marks are generated at the end of the product or where the thick portions intersect, which is not preferable. The start time of the gas pressure injected in the third step based on the injection start time of the molten resin is not particularly limited, and the kind of molten resin, the plate thickness of the molded product, the cross-sectional area of the gas channel portion, and It is determined by the gas pressure and the like. For example, under the conditions of the above-mentioned molded product, 10 seconds or more, preferably 12
More than a second. The holding time of the third gas pressure is not particularly limited, and an appropriate time until just before the mold is opened may be selected.

In the present invention, the gas is injected in two or three steps as described above, but injection molding can be performed in four or more steps depending on the ratio of gas pressure to resin pressure.

The gas used in the present invention is not particularly limited, and examples thereof include air and nitrogen. Of these, nitrogen is preferable.

The molten resin used in the present invention is not particularly limited, and examples thereof include polypropylene, polyethylene, polystyrene, polymethylmethacrylate, polyamide, polycarbonate, polyphenylene ether and the like. Furthermore, these copolymers, for example, an ethylene-propylene copolymer, a styrene-acrylonitrile copolymer, an acrylonitrile-styrene-butadiene copolymer, etc. are also mentioned. The molten resin used in the present invention includes resin compositions based on these homopolymers and copolymers, polymer alloys, composite materials containing fillers, fiber reinforcements and the like. Further, the molten resin used in the present invention may be added with commonly used additives and flame retardants.

The gas injection molding apparatus used in the present invention is not particularly limited, and examples thereof include an air molding apparatus manufactured by Battenfeld.

[0020]

As described above in detail, according to the present invention, by controlling the ratio of the gas pressure to the resin pressure to a specific value, gas exudation, occurrence of "sink", molding It is possible to provide an injection molding method which is free from cracks in the corners of the.

[0021]

EXAMPLES The present invention will be described below based on examples.
The present invention is not limited to these examples. Next, a method for evaluating the appearance of molded products in Examples and Comparative Examples will be described. (1) Sink mark: The surface at the corner where the three thick parts intersect as shown in FIG. 2 is smooth and has no sink mark. X: A sink mark is recognized in the corner portion. (2) Gas leaching ○: No gas leaching from the thick portion to the thin portion is observed. X ... Gas leaching from the thick part to the thin part is recognized.

Example 1 Using polystyrene (registered trademark Sumibrite E183 manufactured by Sumitomo Chemical Co., Ltd., MFR 3.6 g / 10 min), FIG.
As shown in Fig. 6, an injection molding device having a gas injection device and a mold clamping pressure of 240 tons (BM-T24 manufactured by Battenfeld).
00 / 2-630), an injection-molded article having a thick portion as shown in FIGS. 2 and 3 was molded under the conditions shown in Table 1 with gas injection in two stages. In the injection molding apparatus of FIG. 1, molten resin is injected from the injection molding machine 1,
It is poured into the mold 2. Subsequently, gas is injected from the gas injection device 3 into the molten resin. The resin pressure in the cavity at the time of gas injection was measured using a pressure sensor attached to the mold. The molded product of FIG. 2 has an outer dimension of 300 mm × 300 mm, the board thickness 10 of the substrate 5 is 3 mm, and the maximum thickness 8 of the thick portion 8 is 8 mm as shown in the AA ′ sectional view of FIG.
Is 10 mm, the maximum width 9 of the thick portion 4 is 10 mm, the cross-sectional area of the gas channel portion is 100 mm ² , and there is a gas injection port 6 for injecting gas into the thick portion, which is formed by further injecting gas. There is a hollow portion 7. Table 1 shows the evaluation results. Table 1
The symbols in are explained. Each of t1, t2, and t3 is called a delay time, and represents a delay time of each stage from the time when the injection start time of the molten resin is 0 to the start of gas injection. That is, t1 is the gas injection start time of the first step from the injection start time of the molten resin, t2 is the gas injection start time of the second step from the injection start time of the molten resin, and t3 is the third step from the injection start time of the molten resin. Represents the gas injection start time. Further, Pg1, Pg2, and Pg3 represent gas pressures in the first, second, and third stages, respectively, and Pr1,
Pr2 and Pr3 represent the resin pressure in the cavity at the time of each gas injection. In addition, the ratio of the gas pressure in each stage to the resin pressure in the cavity is Pg1 /
Represented as Pr1, Pg2 / Pr2 and Pg3 / Pr3.
In Example 1, t1 was 4 seconds, Pg1 / Pr1 was 3, t2 was 12 seconds, and Pg2 / Pr2 was 13. In addition, the gas pressure holding time is 8 seconds for the first stage and 2 seconds for the second stage.
The stage was 18 seconds.

Example 2 Example 2 was carried out in the same manner as in Example 1 except that the gas injection was performed in three stages and the conditions shown in Table 1 were used. The evaluation results are shown in Table 1.

Comparative Examples 1 to 4 Example 1 was repeated except that the conditions shown in Table 1 were used. The evaluation results are shown in Table 1.

[0025]

[Table 1] ───────────────────────────────────                       Example Comparison Example                     1 2 1 2 3 4 ─────────────────────────────────── First stage   Pg1 bar 30 30 30 100 100 30   Pr1 bar 10 10 10 10 10 10   Pg1 / Pr1 3 3 3 10 10 3   t1 sec 4 4 2 2 4 4 Second stage   Pg2 bar 100 70 − − − 70   Pr2 bar 8 8 − − − 8   Pg2 / Pr2 13 9 − − − 9   t2 sec 12 8 − − − 8 Third stage   Pg3 bar − 100 − − − −   Pr3 bar − 10 − − − −   Pg3 / Pr3 − 10 − − − −   t3 sec − 18 − − − − Evaluation results   Sink ○ ○ × ○ ○ ×   Seepage ○ ○ ○ × ○ ○   Other − − − − Crack − ───────────────────────────────────

[Brief description of drawings]

FIG. 1 is an overall configuration diagram illustrating an outline of an injection molding device according to an embodiment of the present invention.

FIG. 2 is a plan view of a molded product molded by the method of the present invention.

3 is a cross-sectional view taken along the line A-A 'of the molded product shown in FIG.

[Explanation of symbols]

1 ... Injection molding machine 2 ... Mold 3 ... Gas injection device 4 ... Thick part 5 ... Substrate 6 ... Gas inlet 7 ... Hollow part 8 ... Maximum thickness of thick part 9 ... Maximum width of thick part 10 ... Board thickness

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) B29C 45/00-45/84

Claims (4)

(57) [Claims] 1. An injection molding method for producing a molded article in which a predetermined portion is hollow by injecting a molten resin into a molding space formed by closing a pair of molding dies and then injecting a gas into the molding space. An injection molding method characterized in that the gas is injected in stages, and the ratio of the gas pressure to the resin pressure in the cavity of the mold at the time of injecting the gas is controlled as follows. (1) The ratio of the gas pressure to the resin pressure in the first stage is 1 or more and less than 4. (2) The ratio of the gas pressure to the resin pressure in the second stage is 10 or more. 2. An injection molding method for producing a molded article in which a predetermined portion is hollow by injecting a molten resin into a molding space formed by closing a pair of molding dies and then injecting a gas into the molding space. An injection molding method characterized in that the gas is injected in stages, and the ratio of the gas pressure to the resin pressure in the cavity of the mold at the time of injecting the gas is controlled as follows. (1) The ratio of the gas pressure to the resin pressure in the first stage is 1 or more and less than 4. (2) The ratio of the gas pressure to the resin pressure in the second stage is 4 or more and less than 10. (3) The ratio of the gas pressure to the resin pressure in the third stage is 10 or more. 3. The start time of gas injection based on the injection start time of the molten resin is (6) less than 6 seconds in the first step,
(2) The injection molding method according to claim 1, wherein the second step is 10 seconds or more. 4. The start time of gas injection based on the injection start time of the molten resin is (6) less than 6 seconds in the first step,
The injection molding method according to claim 2, wherein (2) the second stage is 6 seconds or more and less than 10 seconds, and (3) the third stage is 10 seconds or more.