JPH10235695A - Novel molding method for synthetic resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel molding method for providing a molding having excellent external appearance and stress crack resistance. SOLUTION: The method for molding by injecting synthetic resin in a mold cavity comprises the steps of fully filling the cavity with specific gas of three times of the solubility as large as solubility of the air and/or nitrogen in the resin at the time of molding, and injecting the resin in the cavity at least at a low flowing velocity of 200mm/sec or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for molding a synthetic resin.

[0002]

2. Description of the Related Art In injection molding of a synthetic resin, it has always been a challenge to improve mold surface reproducibility while minimizing a decrease in molding productivity. Generally, when the flow velocity of the synthetic resin mold cavity is low, the mold surface reproducibility is poor. Further, in a low-pressure injection molding method having a low injection pressure or a low mold clamping force, the reproducibility of the mold surface tends to deteriorate. Particularly in this low-pressure injection molding, it has been strongly required to improve the mold surface reproducibility.

[0003] Low-pressure injection molding methods with low injection pressure or mold clamping force have been widely introduced and put into practice. Plastics Tec is a low-pressure injection molding method.
hnology, April p. 44 (1994),
Alternatively, there is a gas assist injection molding method introduced in US Pat. This molding method is a molding method in which a synthetic resin is injected into a mold cavity, and then a gas material having a pressure lower than the injection pressure of the synthetic resin is injected. This is a method in which the injection pressure is uniformly transmitted to the parts and low-pressure injection molding is performed.

The oligomer-assisted injection molding method and the liquid-assisted injection molding method shown in EP 059909A1 are methods for similarly performing low-pressure injection molding using an oligomer or a liquid. The synthetic resin is injected in a state where the mold cavity is thickened by slightly retreating the mold clamping plate, and then the synthetic resin is injected at a low pressure by moving the mold clamping plate forward to reduce the mold cavity to a predetermined thickness. Injection compression molding, which fills the entire mold cavity, has recently been widely used.

In the same molding process as injection compression molding, extrusion compression molding in which a synthetic resin is injected into a mold cavity by extrusion molding is also used. The Hettinga low pressure injection molding method, which performs injection at an extremely low speed at an extremely low injection pressure, is disclosed in Synthetic Resins, 37 (9) p. 34 (1992), etc., and are favorably used depending on the shape of the molded product.

[0006] In these low-pressure injection moldings, the filling pressure of the synthetic resin is lower than in general injection molding, and the flow velocity of the synthetic resin in the mold cavity is lower. As described above, when the flow velocity of the synthetic resin in the mold cavity is reduced, the reproducibility of the mold surface of the injection-molded product is poor, the surface is rough, and the stress crack resistance (hereinafter abbreviated as SCR) is also deteriorated. Is required.

In general injection molding, various molding conditions are used to improve mold surface reproducibility and solvent resistance. Among the various molding conditions, the most influential is the mold temperature, and it is effective to increase the mold temperature to around the softening temperature of the synthetic resin. However, when the mold temperature is increased,
The cooling time required for cooling and solidifying the plasticized resin is prolonged, and the molding efficiency is reduced. Therefore, there is a need for a method that improves the reproducibility of the mold surface without increasing the mold temperature and that does not increase the required cooling time even if the mold temperature is increased. Has been introduced.

In WO93 / 06980, etc., a method for coating a heat insulating layer for improving the reproducibility of a mold surface by coating the mold wall surface forming a mold cavity with a material having a low thermal conductivity in general injection molding is introduced. Have been. The method of high-frequency induction heating of the mold surface immediately before molding is disclosed in
No. 0504, USP 4,340,551, USP
4,439,492.

A method of blowing a heated gas body into a mold cavity immediately before molding to heat the mold surface is disclosed in Japanese Patent Publication No. 45-22 / 1972.
No. 020, JP-A-6-170943, JP-A-8-24
No. 4072, each publication. Heating and cooling holes are attached to the mold, and the heating medium is alternately
A method of repeating heating and cooling of a mold by flowing a coolant has also been performed.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a molding method for improving the deterioration of mold surface reproducibility when the flow velocity of a synthetic resin in a mold cavity is low. . Gas assisted injection molding, oligomer assisted injection molding, injection compression molding, extrusion compression molding, Hett
In low pressure injection molding such as inga low pressure injection molding, the flow velocity of a synthetic resin in a mold cavity is generally slow, and more often the resin flow velocity changes abruptly during flow. The slow flow velocity results in poor mold surface reproducibility, and an awkward flow mark generally called a hesitation mark occurs when the flow velocity changes suddenly. Further, the SCR deteriorates due to the slow flow rate of the mold cavity of the synthetic resin. Particularly in gas-assisted injection molding, unsightly flow marks tend to occur on the surface near the gas channel formed in the rib of the molded product. It is required to improve these appearance defects and SCR, and the present invention is intended to answer these.

[0011]

The present inventors have studied the relationship between the low flow rate of resin in the mold cavity, low injection pressure, etc., and the reproducibility of the mold surface of the molded product. Have been studied to arrive at the present invention. That is, the present invention provides: In a molding method in which a synthetic resin is injected into a mold cavity and molded, the mold cavity is filled with a gas having a solubility in the synthetic resin at the time of molding that is three times or more the solubility of air and / or nitrogen in the synthetic resin. And a method of molding a synthetic resin by performing injection including at least low-speed injection of a synthetic resin having a mold cavity flow velocity of 200 mm / sec or less. 2. Mold cavity flow speed of synthetic resin is 100mm
1 above that performs injection including at least low-speed injection of not more than /
Molding method of synthetic resin. 3. The method for molding a synthetic resin according to the above 1 or 2, wherein the molding is performed by filling the mold cavity with a gas body at atmospheric pressure. 4. The gaseous body passes over the mold cavity above atmospheric pressure, 1.1
The method for molding a synthetic resin according to the above 1 or 2, wherein the molding is performed under a pressurized state of less than MPa. 5. The mold cavity is set to 1.1 MPa or more with a gas
The method for molding a synthetic resin according to the above 1 or 2, wherein the molding is performed under a pressurized state of less than 0 MPa. 6. The molding method of the above-mentioned 1, 2, 3, 4 or 5 wherein the molding method of injecting the synthetic resin into the mold cavity and molding is selected from the following low pressure molding methods. (1) Gas assist injection molding. (2) Liquid-assisted injection molding. (3) Oligomer assist injection molding. (4) Injection compression molding. (5) Extrusion compression molding. (6) Hettinga injection molding. 7. The molding method of the above-mentioned 1, 2, 3, 4, 5, or 6, which is used in combination with a molding method selected from the following mold surface reproducibility improving molding methods. (1) A molding method using a mold coated with a heat insulating layer. (2) A molding method in which the surface of a mold is subjected to high-frequency induction heating immediately before molding. (3) A molding method in which a heated gas body is blown into a mold cavity immediately before molding. (4) A molding method in which a heat medium and a refrigerant are alternately flowed into a mold to form the mold. 8. The method of molding a synthetic resin according to the above 1, 2, 3, 4, 5, 6, or 7, wherein the gas is carbon dioxide or a gas mainly containing carbon dioxide.

Hereinafter, the present invention will be described in detail. The synthetic resin used in the present invention is a thermoplastic resin that can be used for general injection molding, such as polyethylene, polyolefin such as polypropylene, polystyrene, styrene-acrylonitrile copolymer, styrene-based resin such as rubber-reinforced polystyrene, polyamide, and polyester. , Polycarbonate, polyoxymethylene, methacrylic resin, vinyl chloride resin and the like. The synthetic resin preferably contains 1 to 60% by weight of a resin reinforcement. The resin-reinforced material includes various fibers such as various rubbers, glass fibers, and carbon fibers, and inorganic powders such as talc, calcium carbonate, and kaolin. Particularly preferred are rubber-reinforced synthetic resins, and among them, rubber-reinforced styrene-based resins are more preferably used. Rubber reinforced polystyrene, ABS resin,
AAS resin, MBS resin and the like can be most preferably used.

The molded articles formed by the method of the present invention include various optical parts, housings for light electric appliances and electronic appliances, various daily necessities,
It is a commonly used synthetic resin injection molded product such as various industrial parts. The mold described in the present invention is a mold generally used for molding a synthetic resin such as iron or a steel material containing iron as a main component, aluminum or an alloy containing aluminum as a main component, a zinc alloy, and a beryllium-copper alloy. Is included. Particularly, a mold made of a steel material can be used favorably.

As the molding method of the present invention, a molding method in which a synthetic resin is injected into a mold cavity and molded can be widely used, and general injection molding can be favorably used. A particularly good use is in low pressure injection molding. Low-pressure injection molding includes gas-assisted injection molding, liquid-assisted injection molding, oligomer-assisted injection molding, injection compression molding, extrusion compression molding, and Hetti.
nga low pressure injection molding or the like.

In the molding of the present invention, the flow velocity of the synthetic resin in the mold cavity is 200 mm / sec or less, preferably 100 m / sec.
Molded by injection molding including at least low-speed injection of m / sec or less. This includes various cases such as a case where the injection speed of the synthetic resin temporarily becomes low, a case where the flow stops momentarily, and a case where the injection speed of the synthetic resin is entirely low. The flow velocity of the synthetic resin in the mold cavity can be calculated by measuring the flow distance of the resin in the mold cavity and the injection time of the synthetic resin based on the advancement time of the injection screw and the like. When plastic resin is injected by gas-assisted injection molding, and then a gas body is injected to fill the mold cavity, the synthetic resin is generally injected at a high pressure, and then the gas body is generally injected at a pressure less than half of the synthetic resin. But
The resin flow rate after switching to a low pressure gas is significantly reduced. Where a high-pressure synthetic resin is switched to a low-pressure gas, an unsightly flow mark is generated, which is generally called a hesitation mark. The resin flows at a low speed in a portion ahead of the hesitation mark, and the mold surface reproducibility generally deteriorates.

The gas filling the mold cavity of the present invention is:
It is a gas body having a solubility of the gas body in the synthetic resin of 3 times or more at the time of molding, preferably 4 times or more, more preferably 5 times or more, and the larger the solubility, the more preferable the gas body.
The solubility during molding is around the solidification temperature of the synthetic resin,
And the solubility under the molding pressure of the synthetic resin. Further, the gas body is selected from the constraints of not deteriorating the resin, being inexpensive, and having little environmental destruction.
The solubility of the gas in the resin is measured by a pressure drop method or the like.

The gaseous substance filling the mold cavity of the present invention is preferably a carbon dioxide gas, a hydrocarbon, a halide or the like, and particularly preferably a carbon dioxide gas. The gas body can be used even if it is a mixture of two or more kinds including those having high solubility. A gas body in which air is mixed with carbon dioxide gas can be used in the present invention as long as its solubility is three times or more. In the case of a mixed gas of carbon dioxide and air, the content of carbon dioxide is preferably 50% by volume or more, more preferably 70% by volume or more, particularly preferably 90% by volume or more, and most preferably 95% by volume or more.
It is preferable that the content is not less than 100% by volume and as close to 100% as possible.

Carbon dioxide gas dissolves well in the synthetic resin, becomes a good plasticizer, improves the fluidity of the synthetic resin, and can be used particularly favorably. Next, the amount of carbon dioxide dissolved in each synthetic resin,
Glass transition temperature of synthetic resin by dissolution of carbon dioxide (hereinafter T
Abbreviated as g. ) Will be described with reference to the drawings. 2 to 11 show reports described in various documents. In other words, FIGS.
p. 279 (1989), FIGS. 4, 5, 6, and 7
And FIG. Appl. Polym. Sci. , V
ol. 30 p. 8 and FIG. 11 are J. 4019 (1985). Polym. Sci. , Vol. 30 p.
FIG. 9 shows the results of J. 2633 (1985). Membran
e Sci. , Vol. 5 p. 63 (1979). C (cm ³ CO ₂ (STP) / cm ³ p) indicating the amount of carbon dioxide dissolved in each resin in the figure.
polymer) is 35 ° C. of carbon dioxide dissolved in the resin,
The capacity at 20 atm.

FIGS. 2 and 3 show the dissolution amounts of carbon dioxide and nitrogen gas in polystyrene (PS). Carbon dioxide has a dissolution amount of about 10 times that of nitrogen gas and has a large plasticizing effect. . 4 and 5 show the amount of carbon dioxide dissolved in polystyrene, and FIG. 6 shows the amount of decrease in Tg due to the dissolution of carbon dioxide. Polystyrene can easily reduce Tg by dissolving carbon dioxide gas.

FIGS. 7 and 8 are graphs showing the amount of carbon dioxide dissolved in polymethyl methacrylate (PMMA) and polyvinylidene fluoride (PVF ₂ ) polymer alloys and the decrease in Tg due to the dissolution of carbon dioxide. Tg can be reduced by gas dissolution. FIGS. 9 and 10 show the amounts of dissolved carbon dioxide in polycarbonate (PC) and polysulfone.

When polystyrene is used as the resin and carbon dioxide is used as the gas, the carbon dioxide gas is dissolved at a solidification temperature of 100 ° C. at a pressure of about 1.5 MPa and 1% by weight. on the other hand,
In the combination of polystyrene and nitrogen, only 0.1% by weight or less of nitrogen dissolves under the same conditions, and the presence of nitrogen at the interface between the mold and the resin lowers the transfer state of the mold surface state to the molded product. Become. If nitrogen remains on the weld line, the weld line becomes more conspicuous, and if it remains on the general part as an air trap, a trace of pox remains.

FIG. 11 is a diagram collectively showing the amount of decrease in Tg due to dissolution of carbon dioxide in each synthetic resin. The decrease in Tg due to the dissolved amount of carbon dioxide is almost the same except for polycarbonate. Polycarbonate is Tg by dissolving carbon dioxide
Is particularly large. The gas filling the mold cavity is used at various pressures, but generally from atmospheric pressure to 10 MPa
Is preferably selected in the range. The specific gas pressure described in the present invention is indicated by an absolute pressure. That is, 0.1 MPa substantially corresponds to the atmospheric pressure. The pressure of the specific gas body is selected and used depending on the type of the molding method, the type of the specific gas, and the like. In order to minimize the amount of gas used in one step and to simplify the structure of the mold seal and the gas supply device, the pressure is preferably as low as possible and generally from atmospheric pressure to 5 MPa. If the pressure is increased, the force for opening the mold cannot be ignored, the sealing of the mold becomes difficult, and problems such as safety tend to occur.

The molten synthetic resin injected into the mold cavity flows into the resin cavity at the mold cavity (flow front).
After that, it flows toward the mold wall surface, contacts the cooled mold wall surface, is cooled and solidified, and forms a solidified layer on the mold wall surface. Immediately applying a high pressure to the synthetic resin in contact with the mold wall improves the mold surface reproducibility, but if a slow pressure is applied to the synthetic resin in contact with the mold wall, the synthetic resin will cool down before the mold wall And the mold surface reproducibility deteriorates. Therefore, when the high-speed injection is performed, the mold surface reproducibility is generally good, and when the low-speed injection is performed, the mold surface reproducibility is deteriorated.

According to the present invention, the mold cavity is filled with a gas having a solubility in the synthetic resin of at least three times the solubility of air and / or nitrogen in the synthetic resin at the time of molding carbon dioxide or the like, and injection molding is performed. This is a method in which the synthetic resin is molded while absorbing the gas. That is, the synthetic resin that has reached the flow front comes into contact with the gas in the mold cavity, and a part of the gas is absorbed by the flow front of the synthetic resin. The softening temperature of the synthetic resin having absorbed the gas body is lowered, and the softening temperature is pressed against the mold wall surface to improve mold surface reproducibility. The slower the injection speed, the longer the contact time between the flow front synthetic resin and the gas in the mold cavity,
As a result of the longer gas absorption time, the amount of gas absorbed by the synthetic resin that has reached the flow front is increased. From this, when the mold cavity is filled with a gas having a solubility in the synthetic resin at the time of molding that is three times or more the solubility of air and / or nitrogen in the synthetic resin, the mold of the synthetic resin is molded. The lower the cavity flow rate, the greater the effect of filling the mold cavity with the gas, and the greater the effect of improving mold surface reproducibility.

This will be described with reference to FIG. FIG.
2 is a graph showing the relationship between the flow rate of the mold cavity of the synthetic resin and the glossiness of the molded product (which has the same meaning as the mold surface reproducibility when the mold surface is a mirror surface). Take the curve shown in When the mold cavity of the present invention is filled with a gas having a solubility in synthetic resin of 3 or more times greater than the solubility of air and / or nitrogen in synthetic resin at the time of molding, and injection molding is performed, a curve X is obtained. . The gloss difference ΔA at the time of low-speed injection is larger than the gloss difference ΔB at the time of high-speed injection. That is, the effect of filling the mold cavity with the gas body at the time of low-speed injection is increased.

The higher the gas pressure of the gas filling the mold cavity, the greater the amount of gas absorbed by the synthetic resin, and the greater the effect of improving mold surface reproducibility. The effect appears accordingly. The gas pressure is selected as required, and is atmospheric pressure, over atmospheric pressure and 1.1M
A pressure of less than Pa, a pressure of more than 1.1 MPa and less than 10 MPa, or the like is used. When the pressure of the gas body is low,
It is preferable to use it in combination with other various molding methods for improving other mold surface reproducibility.

Further, the present invention will be described with reference to the drawings. FIG.
FIG. 3 is a diagram illustrating gas-assisted injection molding, oligomer-assisted injection molding, and liquid-assisted injection molding of low-pressure injection molding in which the present invention is implemented. FIG. 14 shows a cross section of a molded article near a gas channel formed in a gas assist injection molded article.

FIG. 15 shows the relationship between the temperature and the viscosity of various fluids. FIG. 16 shows another example of molding such as gas-assisted injection molding. FIG. 17 is a diagram illustrating injection compression molding of low pressure injection molding in which the present invention is implemented. In FIG. 13, a synthetic resin 3 is placed in a mold cavity 2 formed by a mold 1.
And then inject a gas body 5 to mold cavity 2
Is satisfied, and then the mold 1 is opened and the molded product 6 is taken out.
Although the synthetic resin 3 is injected at a high injection pressure, the injection pressure of the gas body 5 is significantly lower than the injection pressure of the synthetic resin 3 and generally less than half the injection pressure of the synthetic resin, resulting in low-pressure injection molding. Therefore, the flow velocity of the injected synthetic resin in the mold cavity changes in conjunction with the switching of the injection from the synthetic resin to the gas body. Where the flow velocity changes, an unsightly flow mark generally called a hesitation mark remains on the surface of the molded article. Further, the mold surface reproducibility is poor between the hesitation mark and the flow end.
In addition, if the filling is performed at a lower flow rate, the SCR of the molded product also deteriorates.

The present invention is a molding method which solves these problems. That is, injection molding is performed in a state where the mold cavity is filled with a specific gas body having a solubility in synthetic resin of 3 times or more that of air and / or nitrogen in synthetic resin at the time of molding carbon dioxide or the like. As the flow velocity of the resin in the mold cavity decreases, the amount of the specific gas absorbed by the synthetic resin increases, thereby improving the mold surface reproducibility. In gas-assisted injection molding, a method of injecting an amount of synthetic resin that fills a mold cavity and then press-fitting an amount of gas that shrinks by cooling the synthetic resin in the mold is also widely used. The gas body selectively travels through the thick part of the mold cavity,
Form a gas channel. The present invention is also effective in improving various flow marks and the like generated on the surface of the molded article near the gas channel. In FIG. 14, in the gas assist injection molding, a gas channel 9 is formed in a thick portion at the base of the rib 8 of the molded product 7. Surface 1 of this gas channel 9
The flow mark generated at 0 can be improved by the method of the present invention.

Similar results can be obtained by changing the gas body of the gas-assisted injection molding to another fluid. FIG. 15 is a graph showing the relationship between the temperature and the viscosity of the various fluids shown in Table 1, and shows that at the temperature at the time of injection, 1/100 of the viscosity of the synthetic resin injected first.
A fluid having a viscosity of 50 or less can be similarly used. The case where a liquid is used as a fluid is liquid-assisted injection molding, and the case where a low molecular weight polymer is used is oligomer-assisted injection molding. In the liquid-assisted injection molding and the oligomer-assisted injection molding, the same result as that described with reference to FIGS. 13 and 14 is obtained.

[0031]

[Table 1]

FIG. 16 shows another molded article formed by gas-assisted injection molding. In FIG. 16, a thick part 14 is formed on a molded product 11 from a gate 12 to a resin flowing end 13. When the synthetic resin is injected from the gate 12 and then the gas is injected, the gas selectively travels through the thick portion, and the gas channel 15 is formed. This gas channel 1
The injection pressure is uniformly transmitted to the resin flowing end 13 by the gas pressure of 5. FIG. 16B shows an AA ′ cross section of the molded product in FIG. Also in this molded product, the same problem of the mold surface reproducibility and SCR as described with reference to FIG. 13 occurs, and a good molded product is obtained by using the molding method of the present invention.

FIG. 17 shows injection compression molding which is one of the low pressure injection moldings described in the present invention. In FIG. 17, the synthetic resin 3 is injected while the mold cavity 2 is made thick by retreating the mold 1 by slightly retracting the mold clamping plate of the injection molding machine, and then injecting the mold clamping plate. The synthetic resin is filled into the mold cavity at a low pressure by advancing the mold 1 to advance the moving side of the mold 1 to make the mold cavity a predetermined thickness. At the stage where the synthetic resin is injected and then the mold cavity thickness begins to decrease, the flow velocity of the synthetic resin in the mold cavity generally changes, producing unsightly flow marks at the points of change. In addition, when filling the mold cavity with a low pressure by reducing the thickness of the mold cavity, the flow velocity of the synthetic resin in the mold cavity is slow, so that the reproducibility of the mold surface of the molded product in that part is deteriorated, and the glossiness is reduced. Deteriorate. Further, since the flow velocity of the synthetic resin in the mold cavity is reduced, the SC
R also gets worse. By using the method of the present invention, these poor appearance and SCR are improved. In FIG. 17, instead of injecting the synthetic resin from the injection molding machine, the same effect can be obtained in the extrusion compression molding in which the resin is injected from the extrusion molding machine and compressed.

FIG. 18 shows a flow pattern of the synthetic resin in the mold cavity, and FIG. 19 shows that shear heat is generated by the flow of the resin. In FIG. 18, the cooled mold 2
When the synthetic resin that has been heat-plasticized is injected into the mold cavity constituted by 2, the injected synthetic resin comes into contact with the cooled mold wall surface and immediately forms the solidified layer 23. Then, the subsequently injected synthetic resin proceeds in the solidified layer 23, reaches the flow front 16 and then flows 17 toward the mold wall surface, so-called fountain flow. The flow rate 18 of the synthetic resin is the highest at the center 19 of the mold cavity as shown by the velocity distribution curve 20, and becomes slower as it approaches the solidified layer 23. According to the present invention, the mold cavity is filled with a specific gas having a solubility in synthetic resin, such as carbon dioxide, which is three times or more greater than the solubility of air and / or nitrogen in the synthetic resin, and injection molding is performed. This is a method in which the synthetic resin is molded while absorbing the specific gas body. That is, the synthetic resin that has reached the flow front contacts the specific gas body, and a part of the specific gas body is absorbed by the synthetic resin at the flow front. The softening temperature of the synthetic resin that has absorbed the specific gas body is lowered, and the mold surface reproducibility is improved. The lower the injection speed, the longer the specific gas body absorption time and the larger the amount of the specific gas body absorbed by the synthetic resin in the flow front portion. From this, when the mold cavity is filled with the specific gas body and molded, the effect of using the specific gas body increases as the flow velocity of the synthetic resin in the mold cavity decreases, and the effect of improving the mold surface reproducibility is reduced. growing.

A shear rate distribution curve 2 is shown for the flowing synthetic resin.
The shear force shown in FIG. 1 is generated, and heat is generated in proportion to the shear force. This heat value is extremely large when the injection speed is high, and is generated at a position in contact with the solidified layer, and functions to prevent the solidified layer from increasing. FIG. 19 shows how much heat is generated depending on the injection speed. FIG. 19 shows the temperature distribution of the synthetic resin immediately after filling the mold by changing the flow rate of the synthetic resin and calculating the calorific value under the following conditions. Mold: Steel Mold cavity size: 60 × 290 × 2 mm (width × length × thickness) Synthetic resin: Asahi Kasei Polystyrene 492 Synthetic resin temperature: 240 ° C. Mold temperature: 50 ° C. Injection filling time: 0.1 second, 0 .4 seconds, 1.0 seconds Software used for calculation: MOLD FLOW PTY
Ltd. 25mm (A), 145mm from the MOLDFLOW / FLOW gate
(B) and three points of 265 mm (C) show the temperature distribution in the cross-section of the resin immediately after filling the resin. When the flow rate of the synthetic resin is high, the amount of heat generation is extremely large, and the heat is generated along the solidified layer, so that the growth of the solidified layer is suppressed, and the solidified layer is filled in a thin state. That is, in general injection molding of a synthetic resin, the solidification layer is suppressed from increasing due to self-heating, and molding is performed while securing a flow path. On the other hand, the low-speed injection generates less heat, and the low-speed flow causes the solidified layer to be thickened during the flow, and the solidified layer is filled in a thick state. In this sense, low-speed injection molding means that injection molding is performed under extremely severe conditions for flow. It is presumed that the thickened solidified layer deteriorates the SCR of the molded product. According to the present invention, the mold cavity is filled with a specific gas having a solubility in synthetic resin, such as carbon dioxide, which is three times or more greater than the solubility of air and / or nitrogen in the synthetic resin, and injection molding is performed. This is a method in which the synthetic resin is molded while absorbing the specific gas body. The synthetic resin having a reduced softening temperature by absorbing the specific gas body forms a solidified layer, and the solidified layer can be thinned accordingly. The slower the injection speed, the longer the absorption time of the specific gas body, the greater the amount of the specific gas body absorbed by the synthetic resin that has reached the flow front, and the thinner the solidified layer can be.

The present invention also has the effect of improving the SCR of the molded article. Next, the improvement of the SCR will be described with reference to FIG. In FIG. 20, when synthetic resin is injection-molded in a cooled metal mold, the injected molten resin comes into contact with the cooled mold wall surface, and a solidified layer 24 is immediately formed on the surface layer. The inner core 25 is gradually cooled and contracts as a heat insulating layer. In the sufficiently cooled molded product, compressive stress remains in the solidified layer 24 as the surface layer due to shrinkage of the inner core 25, and the residual compressive stress is determined by the thickness C of the inner core 25 and the solidified layer 2 as the surface layer.
4 is proportional to the thickness S ratio, C / S (20- (a)
I). The solidified layer 24 formed immediately after injection when injection molding is performed using a mold cavity filled with a specific gas body,
As compared with the case of injection molding with a general mold not filled with a specific gas body, the molded product becomes thinner, and the residual compressive stress of a sufficiently cooled molded product increases in proportion to C '/ S' (20-
(B) I), its residual compressive stress is larger than that of a case where it is molded by a general mold that does not fill the specific gas body. When bending stress is applied to the injection molded article, the compressive stress on the concave side 26 of the molded article is further increased, and the compressive stress is released on the convex side 27 of the molded article and tensile stress is applied (20- (a) II). ,
(20- (b) II). When the residual compressive stress of the surface layer is large (b), the surface tensile stress applied when bending is small. Since the compressive strength of the synthetic resin is much higher than the tensile strength, when a bending stress is applied to the synthetic resin, the synthetic resin is generally broken from the convex side of the molded product.
Therefore, when the tensile stress of the surface layer of the molded product is reduced (b), the molded product is less likely to be broken. As described above, the molded article molded by the mold filled with the specific gas body has a high fracture resistance when a stress is applied, which is presumed to be the reason that the SCR becomes strong.

The low flow rate of the synthetic resin in the mold cavity in low pressure injection molding will be described with reference to FIG. FIG.
Shows the change over time of the resin pressure applied to the mold wall during injection molding.
The difference between general injection molding and gas assist injection molding is shown as a model. In FIG. 21, the pressure curves near the gate are indicated by A and a, and the pressure curves near the flow end are indicated by C and A.
The pressure curve in the middle of the flow is indicated by B and b. In the case of general injection molding, the pressure applied to the vicinity of the gate rises in the curve of A and A 'and becomes the maximum in the full shot, and the pressure applied in the middle of the resin flow rises in the curve B and becomes the maximum in the full shot. The pressure applied to the vicinity of the end increases in the curve of C, becomes maximum at the full shot, and decreases with the dwell time. The maximum value of each of A, B, and C increases as the position is closer to the gate, and the final applied pressure greatly differs depending on the position of the molded product.

On the other hand, in the case of gas-assisted injection molding, the resin is first injected at a high pressure, the injection of the resin is stopped during the injection, and a gas having a pressure significantly lower than the resin injection pressure is injected. Fill the cavity. In this gas-assisted injection molding, the pressure applied to the vicinity of the gate first rises in a curve A, and the resin is switched to a gas during injection. At 28, the resin pressure becomes substantially the same as that of the gas. It changes at a constant pressure. The pressure curve at the point where the gas body is injected during the flow of the resin rises as indicated by the curve b, becomes almost the same as the gas body pressure at the time of a full shot, and is maintained at the time of holding pressure. The pressure curve near the flow end is indicated by c, which is almost the same as the gas pressure at the time of a full shot, and that pressure is almost maintained at the time of dwell. Gas assist injection molding is characterized in that the pressure near the gate, during the flow, and at the end of the flow are almost the same during a full shot.
As a result, the gas-assisted injection molding has the advantage that the stress remaining on the molded product becomes uniform and the dimensional accuracy of the molded product is improved. In the gas assist injection molding, as shown by the curves b and c, the rise of the resin pressure is slow, the flow speed in the mold cavity in this portion is slow, and the appearance of the molded product and the SCR in this portion are deteriorated. The present invention can particularly improve this part.

The present invention can be favorably used in combination with existing molding methods for improving the reproducibility of various mold surfaces. When the specific gas body pressure in the mold cavity is low, particularly when the specific gas body pressure is less than 1.1 MPa, it can be used favorably. When the specific gas pressure is 1.1 MPa or more,
Strictly regulated by the High Pressure Gas Control Law in terms of safety,
It is preferable to use at less than 1.1 MPa. (1) A molding method using a mold coated with a heat insulating layer. (2) A molding method in which the surface of a mold is subjected to high-frequency induction heating immediately before molding. (3) A molding method in which a heated gas body is blown into a mold cavity immediately before molding. (4) A molding method in which a heat medium and a refrigerant are alternately flowed into a mold to form the mold.

A method of high-frequency induction heating of a mold surface immediately before molding is disclosed in Japanese Patent Publication No. 58-40504, US Pat.
0551, US Pat. No. 4,439,492 and the like. A method of blowing a heated gas body into a mold cavity immediately before molding to heat the mold surface is disclosed in Japanese Patent Publication No. 45-22020.
JP-A-6-170943, JP-A-8-24407
This is the method described in each publication of No. 2.

It is also possible to use a method in which heating and cooling holes are attached to a mold, and a heating medium and a coolant are alternately flowed to repeatedly heat and cool the mold. In this case, steam is particularly suitable for the heat medium, and water is suitable for the refrigerant. The heat-insulating layer coating mold method is a molding method introduced in WO93 / 06980 or the like, and is a molding method using a mold in which a surface forming a mold cavity of a metal mold is covered with a heat-insulating layer. A heat-insulating layer-coated mold covered with a heat-insulating layer made of a heat-resistant polymer is preferable,
Furthermore, a heat-insulating-layer-coated mold having a thin metal layer on the heat-insulating layer surface can also be used. Next, the heat-insulating-layer-coated mold will be described in detail. The heat-resistant polymer favorably used for the heat-insulating layer of the heat-insulating layer-coated mold is a polymer having a softening temperature near or above the molding temperature of the synthetic resin to be molded, and is preferably a glass transition temperature. Is 140 ° C. or higher, preferably 160 ° C. or higher, and / or has a melting point of 200 ° C. or higher,
More preferably, it is a heat-resistant polymer having a temperature of 250 ° C. or higher. The heat conductivity of the heat-resistant polymer is generally 0.0001 to 0.00
2 cal / cm · sec · ° C., which is much smaller than metal. Further, the elongation at break of the heat-resistant polymer is preferably at least 3.5%, more preferably a polymer having a toughness of at least 5%. The elongation at break is measured according to ASTM D638, and the tensile speed at the time of measurement is 5 mm / min. The polymer that can be used favorably as the heat insulating layer is a heat-resistant polymer having an aromatic ring in the main chain. For example, various amorphous heat-resistant polymers and various polyimides that are soluble in an organic solvent can be used. Examples of the non-crystalline heat-resistant polymer include polysulfone, polyethersulfone, and polyetherimide. These amorphous heat-resistant polymers can be used as the heat-insulating layer of the present invention by lowering the coefficient of thermal expansion by adding a filler such as carbon fiber.
There are various types of polyimide, linear high molecular weight polyimide,
Polyamideimide and partially crosslinked polyimide can be used favorably. In general, straight-chain high molecular weight polyimides have large breaking elongation, are tough, have excellent durability, and can be used particularly favorably. Furthermore, selected thermosetting resins can also be used as a heat insulating layer. That is, a cured thermosetting resin having a glass transition temperature of 140 ° C. or more and a breaking elongation of 3.5% or more can be used favorably. Further, a thermosetting resin cured product having a small coefficient of thermal expansion, that is, an epoxy resin containing various fillers in an appropriate amount can be used. Epoxy resins generally have a large coefficient of thermal expansion, and the difference in coefficient of thermal expansion from a metal mold is large. However, an appropriate amount of powder such as glass, silica, talc, clay, zirconium silicate, lithium silicate, calcium carbonate, alumina, and mica having a small coefficient of thermal expansion, glass fiber, whisker, carbon fiber, etc. The epoxy resin mixed with a filler in which the difference in the coefficient of thermal expansion from the metal mold is reduced can be favorably used as a heat insulating layer. To these filler-compounded epoxy resins, various compounds which impart toughness such as polyethersulfone, polyetherimide, nylon, rubber, etc. are added to lower the thermal expansion coefficient, and the compounded epoxy resin which gives toughness is Can be used well.

A mold in which a thin metal layer is further coated on the heat insulating layer can be used. The metal layer can be coated in various ways,
Good coverage by plating. The plating described here is chemical plating (electroless plating) and electrolytic plating. Generally, plating is performed through some of the following steps. That is, first, chemical plating is performed in contact with the heat insulating layer. Pretreatment → Chemical corrosion (chemical etching with acid or alkali: making the surface moderately uneven) → Neutralization → Sensitization treatment (Adsorption of a reducing metal salt on the surface of the synthetic resin to activate it) → Activation treatment (a precious metal such as palladium having a catalytic action is applied to the resin surface) → Chemical plating (chemical nickel plating, chemical copper plating, etc.) → electrolytic plating (electrolytic nickel plating, electrolytic copper plating, electrolytic chromium plating, etc.).

In order to increase the adhesion between the heat-insulating layer and the plating layer, the heat-insulating material forming the outermost surface of the heat-insulating layer may be made of an inorganic material such as calcium carbonate, silicon oxide, titanium oxide, barium carbonate, barium sulfate, or various polymers. It is possible to mix the fine powder of an organic substance and elute the powder by chemical corrosion to make the surface have appropriate unevenness. FIG. 22 shows an example of a mold for carrying out the molding method of the present invention, in which a specific gas having a mold cavity whose solubility in a synthetic resin at the time of molding is at least three times the solubility of air and / or nitrogen in a synthetic resin. 1 shows an example of a mold structure for sealing a body. In FIG. 22, a gap 29 through which a gas body can easily pass is provided on the entire parting surface of a mold 39 constituting a mold cavity 38, and a groove 30 for the gas body is provided around the gap 29. An O-ring groove 31 is provided around the periphery to seal the pressurized gas body in the mold cavity, and an O-ring 32 is provided. Gas body groove 30
Is connected to the passage 33, the specific gas body source 34 and the safety valve 3
It is connected to 5. The gas of the specific gas source 34 is supplied to the mold cavity 38 by opening and closing the on-off valve 36. A specific gas is blown into the mold cavity immediately before molding, and the sprue 3
7, the mold cavity 38 is filled with a specific gas. The nozzle of the injection molding machine is brought into contact with the sprue 37 while the mold cavity 38 is filled with a specific gas body at atmospheric pressure,
After pressurized as required, synthetic resin is injected.
Although the pressure of the specific gas in the mold cavity increases in response to the injection of the synthetic resin, the pressure of the gas in the mold cavity is adjusted by the safety valve 35.

Although the present invention has been described mainly with respect to injection molding, other molding methods for injecting a synthetic resin into a mold cavity can be used as well.

[0045]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following molds, substances, measuring methods and the like are used. Mold: made of steel (S55C), mold cavity is 2
mm (thickness) x 100 mm x 100 mm, and has a side gate. The mold surface is mirror-like and chrome-plated. Heat insulation layer coating mold: A polyimide coating mold is used as the heat insulation layer coating mold. (1) Polyimide precursor and cured polyimide: straight-chain high molecular weight polyimide precursor solution (Trenice # 3000, trade name, manufactured by Toray Industries, Inc.). The performance of the cured polyimide was as follows: Tg: 300 ° C., thermal conductivity: 0.0005 ca
l / cm · sec · ° C. (2) Polyimide coated mold: A polyimide precursor solution is applied to each main mold, heated to 160 ° C. to partially imidize, and then the coating and heating at 160 ° C. are repeated 2 to 4 times. The mold surface is heated to 100 ° C., imidized to 100%, and the surface of the mold is coated with polyimide, and the surface is polished to a mirror surface to form a polyimide-coated mold having a predetermined polyimide thickness. Synthetic resin to be injection molded: rubber reinforced polystyrene (Stylon 495 manufactured by Asahi Kasei Corporation) (HIPS4
95. )) Gloss measurement method: JIS K7105, reflection angle 60
Every time

[0046]

EXAMPLE 1 HIPS 495 is injection-molded using a mold without a heat insulating layer and a polyimide-coated mold coated with a polyimide having a thickness of 0.025 mm and a thickness of 0.05 mm, respectively.
Molding conditions are: cylinder temperature 220 ° C, mold temperature 35 ° C
It is. By adjusting the hydraulic control valve of the injection molding machine, the forward speed of the injection cylinder is kept constant,
Injection molding is performed at a constant flow rate of the synthetic resin in the mold cavity. The molded article was molded at each flow rate of the synthetic resin, and the gloss of the molded article was measured. The results are shown in FIG. Further, the mold cavity is filled with a carbon dioxide gas of 1.0 MPa, and molded using a 0.025 mm polyimide layer-coated mold. The gloss of the molded product differs significantly depending on the flow velocity of the mold cavity of the synthetic resin, and the gloss of the molded product increases as the flow speed increases. The effect of filling the mold cavity with carbon dioxide gas is significant when the flow velocity of the synthetic resin in the mold is 200 mm / sec or less.
When it is less than 00 mm / sec, it becomes even larger.

[0047]

According to the method of the present invention, the appearance of molded articles and SCR
An injection-molded article excellent in quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the flow rate of a synthetic resin mold cavity and the glossiness of a molded product in Example 1.

FIG. 2 is a graph showing the amount of carbon dioxide dissolved in polystyrene.

FIG. 3 is a graph showing the amount of dissolved nitrogen gas in polystyrene.

FIG. 4 is a graph showing the amount of carbon dioxide dissolved in polystyrene.

FIG. 5 is a graph showing the amount of carbon dioxide dissolved in polystyrene.

FIG. 6 shows Tg by dissolving carbon dioxide gas in polystyrene.
It is a graph which shows the fall amount of.

FIG. 7 is a graph showing the amount of carbon dioxide dissolved in a PMMA / PVF ₂ polymer alloy.

FIG. 8 is a graph showing the amount of decrease in Tg due to dissolution of carbon dioxide in a PMMA / PVF ₂ polymer alloy.

FIG. 9 is a graph showing the amount of carbon dioxide dissolved in polycarbonate.

FIG. 10 is a graph showing the amount of carbon dioxide dissolved in polysulfone.

FIG. 11 is a graph showing a decrease in Tg due to dissolution of carbon dioxide in each synthetic resin.

FIG. 12 is a graph showing the relationship between the flow rate of a synthetic resin and the glossiness of a molded product.

FIG. 13 is a diagram illustrating gas-assisted injection molding, oligomer-assisted injection molding, and liquid-assisted injection molding of low-pressure injection molding in which the present invention is implemented.

FIG. 14 is a partial cross-sectional view showing the vicinity of a gas channel of a gas-assisted injection molded product.

FIG. 15 is a graph showing the relationship between temperature and viscosity of various fluids.

FIG. 16 is a (a) top view of an example of a molded article formed by gas-assisted injection molding. (B) It is sectional drawing in the AA 'line.

FIG. 17 is a diagram illustrating injection compression molding of low pressure injection molding in which the present invention is implemented.

FIG. 18 is an explanatory view showing a mold cavity flow pattern of a synthetic resin.

FIG. 19 is a diagram showing calculated values of shear heat generation during flow of a synthetic resin in a mold cavity.

FIG. 20 shows an SCR in low-speed injection molding by using a mold whose cavity is filled with a specific gas body whose solubility in synthetic resin at the time of molding is at least three times the solubility of air and / or nitrogen in synthetic resin. It is explanatory drawing which shows that is improved. (A) a case using a general mold not filled with the specific gas body, and (b) a case using a mold filled with the specific gas body.

FIG. 21 is a graph showing a change over time of a resin pressure applied to a mold wall surface during injection molding of a synthetic resin.

FIG. 22 is a sectional view showing a mold structure embodying the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Mold cavity 3 Synthetic resin 4 Where flow velocity changes 5 Gas body 6 Molded product 7 Molded product 8 Rib 9 Gas channel 10 Surface part 11 Molded product 12 Gate 13 Resin flow end part 14 Thick part 15 Gas channel 16 Flow Tip 17 Flow toward Mold Wall 18 Flow Rate of Synthetic Resin 19 Center of Mold Cavity 20 Velocity Distribution Curve 21 Shear Velocity Distribution Curve 22 Mold 23 Solidified Layer 24 Solidified Layer 25 Inner Core 26 Concave Side of Molded Article 27 Molded Article Convex side 28 Where resin is switched to gas 29 Gap 30 Gas groove 31 O-ring groove 32 O-ring 33 Passage 34 Gas source 35 Safety valve 36 Open / close valve 37 Sprue 38 Mold cavity 39 Mold

Claims (8)

[Claims] In a molding method in which a synthetic resin is injected into a mold cavity and molded, a gas body whose solubility in the synthetic resin at the time of molding is at least three times the solubility of air and / or nitrogen in the synthetic resin. A method of molding a synthetic resin that fills a mold cavity and performs injection including at least low-speed injection at a flow rate of the synthetic resin of 200 mm / sec or less. 2. The mold cavity flowing velocity of the synthetic resin is 1
2. The method of molding a synthetic resin according to claim 1, wherein injection including at least low-speed injection of not more than 00 mm / sec. 3. The method for molding a synthetic resin according to claim 1, wherein the molding is performed by filling the mold cavity with a gaseous substance to atmospheric pressure. 4. The method for molding a synthetic resin according to claim 1, wherein the molding is performed by filling the mold cavity with a gas body at a pressure exceeding atmospheric pressure and less than 1.1 MPa. 5. The mold cavity is set to 1.1 MPa with a gas body.
The method of molding a synthetic resin according to claim 1 or 2, wherein the molding is performed under a pressurized state of less than 10 MPa. 6. The molding method for a synthetic resin according to claim 1, wherein the molding method for injecting the synthetic resin into the mold cavity is a molding method selected from the following low pressure molding methods. . (1) Gas assist injection molding (2) Liquid assist injection molding (3) Oligomer assist injection molding (4) Injection compression molding (5) Extrusion compression molding (6) Hettinga low pressure injection molding 7. The method for molding a synthetic resin according to claim 1, which is used in combination with a molding method selected from the following mold surface reproducibility improving molding methods. (1) A molding method using a mold coated with a heat insulating layer. (2) A molding method in which the surface of a mold is subjected to high-frequency induction heating immediately before molding. (3) A molding method in which a heated gas body is blown into a mold cavity immediately before molding. (4) A molding method in which a heat medium and a refrigerant are alternately flowed into a mold to form the mold. 8. The gas body according to claim 1, wherein the gas body is carbon dioxide gas or a gas body mainly composed of carbon dioxide gas.
Or the molding method of the synthetic resin of 7.