JPH11245256A - Method for injection molding of non-crystalline thermoplastic resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a molded article with excellent appearance by transferring highly precisely a mold surface condition to the molded article, in injection molding of a non-crystalline thermoplastic resin. SOLUTION: In a method for injection molding wherein carbon dioxide gas is fed in advance into the cavity of a mold and then, a molted resin is filled by injection, when surface temp. of the cavity of the mold is Tm( deg.C) and glass transition temp. of a non-crystalline thermoplastic resin is Tg( deg.C) and pressure of carbon dioxide gas is P (MPa), for a part where transferring properties of the surface of the mold are required, Tm is set so as to satisfy Tg-5>=Tm>=Tg-(5×P)-35 and for a part where the transferring properties of the surface of the mold are not required, Tm is set so as to satisfy Tm< Tg-(5×P)-35.

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 transferring a mold surface state to the surface of a molded article at a high level in molding an amorphous thermoplastic resin.

[0002]

2. Description of the Related Art In molding a thermoplastic resin, the temperature of a mold is usually kept sufficiently lower than the temperature at which the molding resin solidifies. This is necessary for cooling a resin material having extremely low thermal conductivity from a molten state to a temperature at which it can be taken out as a molded product in a short time. Further, in order to transfer a mold surface state to a molded article to a high degree, it is necessary to press a resin having a low viscosity into a mold with a high pressure. However, if the mold temperature is lower than the solidification temperature of the resin, the filling and solidification of the resin will proceed simultaneously, and the resin in contact with the mold near the flow front will be rapidly cooled and the viscosity will increase, Since it is solidified while being pressed against the surface of the mold with a low pressure, it is difficult to transfer the surface state of the mold to the molded product at a high level. For this reason, ordinary injection molding tends to cause poor appearance such as uneven gloss, weld lines, flow marks, and jetting, and fine pit transfer failures in precision molded products such as optical disks, and short shots occur in thin parts. Sometimes.

In order to improve the transferability of the mold surface, it is necessary to prevent or minimize the solidification of the resin during the resin filling step.

In injection molding of a thermoplastic resin, there has always been a demand for economical enhancement of mold surface transferability without lengthening the molding cycle time. Various methods have been proposed as means for improving the mold surface transferability. For example, there are the following methods.

(1) A method of repeating heating and cooling of a mold surface by alternately flowing a heat medium and a coolant through the mold (Plastic)
Technology, Vol. 34 (June), 1
50 (1988)). (2) A method of selectively heating the mold surface by high-frequency induction heating immediately before molding (US Pat. No. 4,439,492). (3) A method in which an insulating layer and a conductive layer are provided on the surface of a mold, and the conductive layer is energized and heated (Plym. Eng. Sci., Vo)
l. 34 (11), 894 (1994), etc.). (4) Method of radiantly heating the mold surface (synthetic resin, Vo
l. 42 (1), 48 (1996), etc.). (5) A heat insulating layer coating method (USP5) in which a mold surface is coated with a heat insulating layer, and the mold surface is heated while being heated by the molding resin itself.
362226, WO97 / 04938, etc.).

[0006] All of these are methods of molding while heating the mold surface during injection molding. By heating the mold surface above the solidification temperature of the resin when the injected molten resin is pressed against the mold wall surface. This is a molding method for improving the mold surface transferability.

In addition, in injection molding of a foaming resin containing a foaming agent or water, a pressurized gaseous body is introduced into a mold cavity prior to filling with a resin in order to eliminate surface defects such as swirl marks on the surface of a molded product due to foaming gas. , And pressurized to form a molded article.
This method applies a gas pressure to the mold cavity in advance to prevent bubbles generated by foaming gas or vaporized moisture from exploding at the flow front of the molten resin flowing through the mold cavity and causing a surface defect. In this case, it is sufficient that the gas body does not oxidize and degrade the resin. In general, air is used, and all inert gas bodies can be used in this molding method.

This counterpressure method is a method used for injection molding of a resin containing a foaming agent or a resin which is insufficiently dried. When used for molding a general non-foamable resin, the gas pressure existing in the cavity is reduced. However, when the air enters the gap between the molten resin and the mold and hinders transfer, or when the gaseous body is air, the part where the air is compressed by the resin in the cavity becomes a state of high oxygen concentration at high temperature, resulting in oxidative deterioration of the resin. It can be said that there is no effect of increasing the mold surface transferability only because of problems such as causing mold failure. For this reason, in order to transfer the mold surface state to the molded product to a high degree, there are also methods such as opening the mold slightly during resin filling to release air in the cavity, and reducing the pressure inside the mold by a vacuum pump. in use.

Japanese Unexamined Patent Publication (Kokai) No. 62-231715 discloses a method for molding a water-containing polymer alloy by using a counter pressure method for injection molding. The gas body for pre-pressurizing a mold cavity is air or nitrogen. And an inert gas such as carbon dioxide. Furthermore, Japanese Patent Laid-Open No. 61
JP-A-213111 discloses that two kinds of monomers are mixed,
With respect to reaction injection molding for injection, a method is disclosed in which a mold cavity is replaced with carbon dioxide at atmospheric pressure and then molded to reduce voids generated when air is entrained in the resin during resin filling. However, none of the publications discloses a method for improving the mold surface transferability defect due to the solidification of the resin during the resin filling step.

Also, J. J. Appl. Polym. Sc
i. , Vol. 30,2633 (1985), when carbon dioxide is absorbed into a resin, it acts as a plasticizer for the resin, and the glass transition temperature (T
_g ), but is not widely applied to resin molding. As a few application examples, German Patent DE 4314869 discloses a method of dissolving carbon dioxide and hydrocarbons in a supercritical state in a bioabsorbable polyester in a high-pressure container to lower the glass transition temperature. A method for molding a resin is disclosed. However, this method lowers the glass transition temperature of the entire resin, so it is necessary to use a mold temperature lower than usual for molding because of the decrease in the glass transition temperature, and transfer failure due to solidification during the resin filling step is caused. There is no effect to prevent.

[0011]

SUMMARY OF THE INVENTION The present invention suppresses solidification and viscosity increase of a resin during a resin filling step in molding an amorphous thermoplastic resin, and transfers a mold surface state to a molded article to a high degree. It is to provide a method economically.

[0012]

As a result of investigations by the present inventors to solve the above-mentioned problems, there has been found no conventional technique for improving the mold surface transferability by heating the mold surface. The present inventors have found that the surface state of a mold can be highly transferred to a molded product by a different method, and have completed the present invention.

That is, the present invention relates to an injection molding method in which a carbon dioxide gas is injected into a mold cavity in advance, and then the molten amorphous thermoplastic resin is injected and filled. The cooling of the resin is performed at a mold cavity surface temperature equal to or higher than the solidification temperature of the resin surface reduced by the dissolution of the carbon dioxide gas.

In the present invention, preferably, the mold cavity surface temperature is T _m (° C.), the glass transition temperature of the amorphous thermoplastic resin is T _g (° C.), and the carbon dioxide gas pressure is P (M
When Pa) is set, good mold surface transferability can be obtained by setting _Tm so as to satisfy the following expression.

T _g -5 ≧ T _m ≧ T _g − (5 × P) −35
(However, P ≧ 0.01) Moreover, in order to obtain a better mold surface _{transferability, T g -5 ≧ T m ≧} T g - (5 × P) may be -20 and configuration.

Further, under the above conditions, such as when the target molded article has a thick-walled portion such as a rib or a boss, the mold cavity surface temperature on the side where high mold surface transferability is not required is adjusted to the above _Tm . By setting it below the lower limit,
It is possible to prevent sinks occurring in the thick-walled portion.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention focuses on a gas body in a mold cavity, which has conventionally been considered to inhibit the transfer of a mold surface. The mechanism by which the effect is exhibited is as follows. Can be considered.

In the injection molding, the resin always flows in a laminar flow in the mold cavity, and when it contacts the cooled mold wall surface, a solidified layer is formed at the interface, and the resin to be filled later is inside the solidified layer. Flows forward and reaches a flow front, and then flows toward a mold wall surface, called a fountain flow. If the mold cavity is filled with resin after filling the mold cavity with a specific gaseous substance such as carbon dioxide at an appropriate gas pressure, the gaseous body is absorbed at the flow front of the flowing resin or enters the interface between the mold and the resin and enters the resin surface. Dissolve in layer. The gas dissolved in the resin acts as a plasticizer to selectively lower the solidification temperature only on the resin surface or to lower the melt viscosity of the resin. The solidification temperature drops only in the thin resin surface layer,
When the solidification temperature is equal to or lower than the mold surface temperature, no solidification occurs during the resin filling step, and the mold surface transferability of the molded product can be significantly improved. The gas dissolved in the resin surface layer diffuses into the resin with time, and the solidification temperature of the resin surface layer rises. Therefore, the surface layer solidifies within the usual resin cooling time and can be taken out as a product.

As a result, during the resin filling step, molding is performed while lowering the solidification temperature of the resin surface in contact with the mold, and the mold surface is appropriately set in accordance with the solidification temperature of the resin which is lowered due to the plasticizer effect of the gas body. As a result, the present invention has been achieved which achieves both high mold surface transferability and high productivity.

The resin used in the present invention is an amorphous thermoplastic resin which can be generally used for injection molding. Specifically, the amorphous thermoplastic resin, a blend of plural kinds thereof, or an amorphous thermoplastic resin can be used. A resin in which a crystalline thermoplastic resin is partially mixed with a thermoplastic resin, specifically, polystyrene, styrene-acrylonitrile copolymer, rubber-reinforced polystyrene, styrene-methyl methacrylate copolymer, ABS resin, styrene-methyl methacrylate- Styrene resins such as butadiene copolymers, polymethyl methacrylate, methacrylic resins such as methyl methacrylate-styrene copolymers, and modified polyphenylene ethers, polysulfones, polyethersulfones, and polyethers containing polyvinyl acetate, polycarbonate, polyphenylene ether, or polystyrene. Ruimido, polyarylate, polyamideimide, polyvinyl chloride, vinyl chloride - ethylene copolymer, vinyl chloride - vinyl chloride resin such as vinyl acetate copolymer can be particularly preferably used. Further, a resin in which various fillers such as an inorganic substance and an organic substance are mixed with these resins can be used.

The solidification temperature of the resin described in the present invention is a temperature at which a molten thermoplastic resin solidifies in a mold, and is T _g for an amorphous resin. For non-compatible system polymer alloy, a T _g of the amorphous resin constituting the sea island structure.

The carbon dioxide gas used in the present invention has a high solubility in a thermoplastic resin and has an effect of plasticizing the resin. Therefore, by filling the mold cavity in advance, the resin is absorbed by the resin surface during the resin filling step, and the solidification temperature of the resin surface in contact with the mold is lowered. In the case of a gaseous substance having a solubility in a resin of about air or nitrogen, as is conventionally known, it merely inhibits the transfer of the surface of the mold in the cavity and cannot expect an effect of lowering the solidification temperature. Carbon dioxide is a gas body suitable for the present invention in terms of safety, price, ease of handling, and the like.

The dissolved amount of each resin of the carbon dioxide used as the gas material to the present invention will be described with reference to the drawings such as reduction of T _g of the resin with carbon dioxide dissolved.

1 to 5 show reports described in various documents. That is, FIG. 1 shows the molding process '96 (J
SPP '96 Tech, Papers), 279,
(1966), FIG. Appl. Poly
m. Sci. , Vol. 30, 4019 (1985), FIG. Appl. Polym. Sci. , V
ol. 30, 2633 (1985), FIG.
Membrane Sci. , Vol. 5, 63 (19
79) respectively. FIG. 5 shows Poly.
m. J. , Vol. 22 is a diagram created based on data cited from 22 (1), 77 (1990).

FIG. 1 is a diagram showing the amount of carbon dioxide dissolved in polystyrene.

[0026] Figure 2 shows the decrease T _g of with carbon dioxide dissolved. Polystyrene can easily lower T _g by dissolving carbon dioxide.

FIG. 3 is a graph showing the amount of carbon dioxide dissolved into the polymethylmethacrylate and polyvinylidene fluoride polymer alloy, the decrease T _g of by dissolving carbon dioxide, to lower a T _g by dissolving carbon dioxide it can.

FIG. 4 shows the amount of carbon dioxide dissolved in polycarbonate.

FIG. 5 is a diagram collectively showing the carbon dioxide pressure of each resin and the amount of decrease in _Tg due to dissolution of carbon dioxide.

The gas pressure sealed in the mold cavity is such that as the pressure becomes higher, a larger amount of gas is dissolved in the resin.
The solidification temperature is lower, and solidification during the resin filling step can be prevented even at a low mold temperature. Practically, the required gas pressure is determined from the required mold surface transferability, the type of resin, the mold temperature, etc. If the mold temperature is set high, sufficient transferability can be obtained at a low gas pressure. Can also. At this time, since the gas pressure, the amount of gas dissolved in the resin in the equilibrium state, and the amount of solidification temperature decrease are almost proportional to each other, the gas pressure in the mold cavity and the amount of solidification temperature decrease of the resin surface layer are also proportional. .

Therefore, assuming that the gas pressure filled in the mold cavity is P (MPa), the solidification temperature drop of the resin surface layer in the equilibrium state is about 15 × P (° C.) from FIG. However, in actual molding, the gas absorption time is short, and the gas absorbed by the resin diffuses into the resin, so that the resin surface does not absorb the gas to an equilibrium state and the solidification temperature of the resin surface decreases. The amount varies depending on the type of the resin, but is generally 5 × P (° C.). Therefore, the mold cavity surface temperature T _m (° C.) is set to [T _g − (5 × P)].
When the temperature is set to (° C.) or more, solidification during the resin filling step can be suppressed or prevented, and the mold surface transferability of the molded product can be improved. Practically, even if the lower limit of _Tm is lower than the solidification temperature of the resin by about 35 ° C., the mold surface transferability is improved, so that [T _g − (5 × P) −35]
(° C.) or more, and the upper limit of T _m can be said to be (T _g −5) (° C.) from the necessity of heat transfer from the resin to the mold.

The solidification temperature of the resin surface rises during the cooling step because the gas diffuses toward the center of the thickness of the molded product,
It approaches T _g and becomes higher than the mold temperature.

Further, in order to obtain better mold surface transferability than the above-mentioned molding, [T _g- (5 × P) -20] (° C.)
As described above, the temperature is preferably set to (T _g −5) (° C.) or less.

Further, the molding process, Vol. 2 (6), 50
5 (1990), when one of the mold surface temperatures is set to _Tg or more and the other is set to _Tg or less, sink on the molded product surface due to resin shrinkage occurs on the side where the mold surface temperature is low. It is known to This phenomenon is the same in the present invention, when high mold surface transferability is not required for one surface or a part of the surface such as the back surface of a molded product, and when a thick-walled portion such as a rib or a boss exists. High transferability is not required, and only the mold surface temperature on the side having the thick-walled portion is the lower limit of _{Tm according to} the present invention [ _Tg- (5 * P) -3.
5] When the temperature is lower than (° C.), it is difficult to cause sink marks on the surface on the opposite side where high mold surface transferability is required, which is preferable.
In addition, the surface temperature of the mold cavity on the side where the surface transfer property is required is set to [T _g- (5 × P) -2
0] (° C.) or higher, the mold cavity surface temperature at which surface transferability is not required is [T _g − (5 ×
P) -20] (° C.).

The lower limit of the gas pressure in the mold cavity varies depending on the degree to which the gas is compressed by the resin in the resin filling step, but is practically preferably 0.01 MPa or more, more preferably 0.1 MPa or more. That is all. Even at a pressure lower than this, since carbon dioxide has high solubility in a resin and a plasticizer effect, it is possible to obtain an effect of improving transferability equal to or higher than that when the cavity is depressurized by a vacuum pump. When used at low pressure, it is preferable to replace the cavity with carbon dioxide as much as possible.

The upper limit of the pressure is not particularly limited.
If the pressure is too high, problems such as the force to open the mold cannot be ignored and the mold is difficult to seal are likely to occur. Therefore, 15 MPa or less is practical. Gas pressure minimizes the amount of gas used in one process,
In order to simplify the structure of the mold seal and the gas supply device, it is preferable that the temperature is low as long as the required effect can be obtained.

The air remaining in the mold at the time of closing the mold is preferably a gas used during or after completion of the mold clamping, and is preferably replaced. However, when the gas pressure used exceeds 1 MPa,
The effects of air are often negligible.

After filling the resin, the gas pushed out of the cavity is released to atmospheric pressure. The release of the gas is performed after the cavity is filled with the molten resin. After the resin is filled, it is desirable to apply a sufficient pressure to the resin in the cavity until the surface of the molded product is solidified in order to transfer the surface state of the mold to the molded product. In particular, when transferring a point-like dent shape on the mold surface, it is necessary to press the resin against the mold against the gas pressure inside the dent, and in such a case, the resin is higher than normal molding. It is desirable to mold with resin pressure.

The gas dissolved in the resin gradually diffuses into the air if the molded article is left in the air after the resin is molded. No bubbles are generated in the molded article due to diffusion, and the mechanical performance of the molded article after diffusion is not different from that produced by a normal molding method.

It is preferable that a device for supplying and discharging a gas body to and from the cavity, a gas pipe, and a mold take measures to prevent liquefaction of the gas body. This means that at a temperature where liquefaction of gas occurs, not only can a high gas pressure not be obtained,
When the liquefied gas comes into contact with the resin in the cavity, a large amount of the gas dissolves into the resin, and after the gas pressure is released, the surface of the molded product foams, resulting in poor appearance. As a measure to prevent liquefaction, the gas body was heated by a heater, and the temperature of the gas body flow path and the mold were kept above the critical temperature of the gas body, and the gas body was pushed out of the cavity during resin filling. In order to prevent a significant pressure increase due to the above, a pressure release valve capable of maintaining the gas pressure in the cavity and the pipe in an arbitrary range, and a gas reservoir in which a gas body can flow backward from the cavity are provided. However, it is not preferable to excessively increase the temperature of the gas in order to prevent liquefaction of the gas, because the gas in the cavity decreases due to expansion of the gas.

Normally, in order to make the mold hermetically sealed by molding by a counter pressure method or the like, the parting surface and each plate are sealed with an O-ring, and movable pins such as protruding pins communicating with the mold cavity are also O-shaped. A method such as sealing with a ring or covering the whole protruding plate portion to which the protruding pin is fixed to make it airtight is adopted.

When an O-ring is used for sealing the protruding pin, it is necessary to pass the protruding pin after inserting the O-ring between the two plates. At this time, if the O-ring is damaged by the edge of the tip of the protruding pin, or if the pin insertion resistance is large, the O-ring is twisted, and reliable sealing performance cannot often be secured. On the other hand, when sealing is performed with rubber packing having a U-shaped cross section in the radial direction (hereinafter referred to as “U packing”), the insertion resistance is small when the protruding pin is inserted, and the pin tip is damaged or twisted at the edge of the tip. The mold can be easily assembled without the need, and a highly reliable sealing property can be obtained.

In the case where the movable pin is sealed with packing, the pressurized gas body that has entered the gap around the pin between the mold cavity and the packing is confined in the gap by filling with resin, and the surface of the molded product is cooled and the mold surface is cooled. Away from the mold cavity, it may flow into the mold cavity, denting the surface of the molded product that is not sufficiently solidified, or expanding and deforming the molded product when the mold is opened. When such a problem occurs, the mold is provided with a groove or a hole that allows the pressurized gas body entering the gap around the pin to be discharged out of the mold from a path other than the mold cavity. It is desirable to exhaust simultaneously with the discharge of the gas body pushed out of the cavity. FIG. 6 shows an example of the structure of a mold capable of discharging a pressurized gas body out of the mold from a path other than the mold cavity. In FIG. 6, 8 is a gap, 9 is a gas flow channel, 1
0 is a hole communicating from the gas flow channel 9 to the outside of the mold, 11 is an O-ring, 12 is a protruding pin, 13 is a cavity block,
14 is a backup plate.

The gas body can be injected into the mold cavity by using a mold structure generally used for venting the mold cavity. A slit provided on the parting surface on the outer periphery of the mold cavity, a mold insert, and the like. And a gap between protruding pins, a venting pin, and a nest made of a porous sintered body can be used. When the mold cavity is replaced with a gas substance near the atmospheric pressure, the air in the mold cavity is reduced to 100% with as little gas as possible in as short a time as possible.
An economical method of near replacement is required, and a method of blowing a gas from a mold sprue is suitable. In this method, before filling the mold cavity with the resin, a gas body is injected from near the mold sprue and molded, whereby the gas body is pushed by the resin and the air remaining in the cavity by the gas body is removed by the mold. It will be molded while being discharged out of the mold. That is, if the vicinity of the sprue, runner, and gate of the mold is sufficiently replaced with a gas, the gas that comes into contact with the resin is always the injected gas.

[0045]

EXAMPLES The resin used in the examples of the present invention was rubber-reinforced polystyrene (HIPS, Asahi Kasei Kogyo Co., Ltd., "Stylon 4").
00 "), styrene-butadiene block polymer (S
Bblock, Asaflex 81 manufactured by Asahi Kasei Corporation
0 ”), acrylonitrile-butadiene-styrene copolymer (ABS,“ Styrac ABS1 ”manufactured by Asahi Kasei Corporation)
80 "), a methacrylic resin (PMMA," Delpet 80NH "manufactured by Asahi Kasei Kogyo Co., Ltd.), which is colored black so that defects on the surface of the molded product are easily seen.

The T _{g of} each resin was measured using DSC (“PYRIS 1” manufactured by PerkinElmer), and the inflection point of the specific heat with respect to the temperature change when the temperature was raised at 10 ° C./min was defined as T _g . The T _g of each resin was 99 ° C. for HIPS and SBblock.
k is 104 ° C, ABS is 109 ° C, PMMA is 114 ° C
Met.

As the gas, carbon dioxide having a purity of 99% or more was used.

The injection molding machine "SG5" manufactured by Sumitomo Heavy Industries, Ltd.
0 "was used.

The molded product is 120 mm × 60 m with a thickness of 2 mm.
m rectangular plate with a 10mm diameter round hole in the center,
3mm height, 3mm width, 20 length near resin filling end
mm ribs are provided. FIG. 6 shows the structure of the mold, and FIG. 7 shows the structure of the gas supply device. The mold surface was matte with _a surface roughness R _{a of} 5 μm using electric discharge machining on the fixed side. The gate had a width of 3 mm, a thickness of 2 mm, and a land length of 3 mm. The cross section of the runner is almost square with an average width of 4 mm and a depth of 4 mm, the runner length is 140 mm, and the sprue is an average diameter of 4 m.
m, length 55mm, 3.5mm diameter of nozzle touch part
And In the mold cavity, a gap 8 for gas supply and release and a gas passage groove 9 are provided on the outer periphery of the cavity, and the hole 10 is connected to a gas supply device outside the mold.
An O-ring was provided on the outer periphery of the sprue for gas sealing, and the mold cavity had an airtight structure.

In the gas supply device, the cylinder 16 filled with liquefied carbon dioxide was kept at 40 ° C. and used as a gas supply source. The gas body passes through a heater 17 from a cylinder 16 and is adjusted to a predetermined pressure by a pressure reducing valve 18, and then stored in a gas reservoir 19 having an internal capacity of 100 cm ³ kept at about 40 ° C. The gas supply to the mold cavity is performed by opening the supply solenoid valve 20 downstream of the gas reservoir 19 and closing the release solenoid valve 21 at the same time. linked. Almost simultaneously with the completion of the resin filling, the supply electromagnetic valve 20 is closed and the release electromagnetic valve 21 is opened to release the gas body out of the mold.

(Embodiment 1) The cylinder set temperature is HIP
S, SBblock is 220 ° C, ABS and PMMA are 240 ° C, and the mold surface temperature is changed for all resins in the range of 40 to 90 ° C in steps of 10 ° C. At each temperature, the carbon dioxide in the mold cavity is changed. Pressure is 0-8MPa
The resin was filled in the range of 1 MPa at intervals of 1 MPa, and the resin was filled with a filling time of 0.6 seconds.
The pressure was maintained at 40 MPa for 5 seconds, and after cooling for 20 seconds, the molded product was taken out. The carbon dioxide filled in the mold was released to the atmosphere at the same time as the resin filling was completed.

[0052] T _g of the resin surface layer portion to decrease by absorbing carbon dioxide during resin filling directly for measurement by no minimum appearance mold surface state in the molding of the above are transferred almost completely molded article defects is eliminated the difference between the mold temperature T _m and the DSC according to the resin T _g was used as an index a T _g lowering effect of carbon dioxide.

At each mold surface temperature, a weld line formed on the satin portion of the molded product and uneven gloss around the mold were evaluated. The weld line was not visually observed, and the width was about 0.5 μm with a 500 × laser microscope. The minimum carbon dioxide pressure P ₁ at which the weld line is confirmed and there is almost no gloss unevenness, and the minimum carbon dioxide pressure P ₂ at which the weld line is not visible even with a laser microscope and there is no gloss unevenness at all. Was. Here, due to the roughness of the carbon dioxide pressure condition, P ₁ and P ₂ each include an error of ± 0.5 MPa at the maximum.

The results are shown in Tables 1 and 2 below and FIGS. 8 and 9.

[0055]

[Table 1]

[0056]

[Table 2]

From Table 1 and FIG. 8, T _g −T _m ≦ (5 × P)
+35, i.e., T _m ≧ T _g - if (5 × P) -35, it can be seen that substantially the mold surface transferability of good molded articles for various resins can be obtained. Also, from Table 2 and FIG.
T _g −T _m ≦ (5 × P) +20, that is, T _m ≧ T _g −
If (5 × P) −20, it can be seen that a molded product having a better surface can be obtained.

(Example 2) Using HIPS, the cylinder set temperature was set to 200 ° C and 240 ° C, and the mold surface temperature was set to 6 ° C.
At 0 ° C., the carbon dioxide pressure in the mold cavity
After filling the resin in the range of 8 MPa in steps of 1 MPa, filling the resin with a filling time of 0.6 seconds, the resin pressure in the cylinder 1
The pressure was maintained at 20 MPa for 5 seconds, and after cooling for 20 seconds, the molded product was taken out. The carbon dioxide filled in the mold was released to the atmosphere at the same time as the resin filling was completed.

In the same manner as in Example 1, the weld line formed in the satin portion of the molded product and the surrounding gloss unevenness were evaluated.
P ₁ and P ₂ were determined. The results are shown in Table 3 below.

[0060]

[Table 3]

As shown in Table 3, it can be said that P ₁ and P ₂ are almost constant irrespective of the temperature of the molten resin. Incidentally, the high P ₁ slightly at the cylinder setting temperature of 200 ° C., to a temperature of the resin has increased the melt viscosity becomes low, the same holding pressure reduces the resin pressure in the mold, the mold surface state This is probably because the transfer to the resin became difficult.

(Example 3) Using HIPS, the fixed-side mold surface temperature was set to 70 ° C, the moving side was set to 35 ° C, the carbon dioxide pressure in the mold cavity was set to 5 MPa, and the resin was filled in 0.6 seconds. Then, the pressure was maintained for 5 seconds at a resin pressure of 120 MPa in the cylinder, and after cooling for 20 seconds, the molded product was taken out. The carbon dioxide filled in the mold was released to the atmosphere at the same time as the resin filling was completed.

When the sink was observed on the opposite side of the rib portion of the molded product, no sink was observed.

Comparative Example 1 The surface temperature of the fixed mold was 35 ° C.
A molded article was obtained in the same manner as in Example 3 except that the above conditions were satisfied. Observation of sink on the opposite side of the rib part of the molded product revealed obvious sink.

Comparative Example 2 A molded product was obtained in the same manner as in Example 3 except that the inside of the mold cavity was set to the atmospheric pressure. Observation of sink on the opposite side of the rib part of the molded product revealed obvious sink.

[0066]

As described above, according to the present invention,
It is possible to economically transfer the mold surface state to a molded product at a high level. Furthermore, in the present invention, by setting the mold cavity surface temperature at a portion where surface transferability is not required to be low, it is possible to prevent the occurrence of sink marks even when ribs or bosses are present. For this reason, post-processing such as painting, which has been unavoidably performed when the appearance of the molded product is poor, becomes unnecessary, and the cost of the molded product can be significantly reduced. In addition, because it is not possible to transfer the fine mold surface state uniformly to the molded product, the productivity of flat lenses molded by press molding, which is less productive than injection molding, is significantly increased, and a new It can be expected to have the effect of creating application fields for injection molding.

Examples of the molded article satisfactorily molded by the molding method of the present invention include resin injection molded articles such as optical equipment parts, light electric equipment, electronic equipment, office equipment housings, various automobile parts, various daily necessities, and the like. Can be Injection molding with multi-point gate,
As a result, the present invention is suitable for improving the appearance of housings of electronic equipment, electric equipment, office equipment, etc., in which a large number of weld lines are generated, matte molded products, and patterned grain molded products. In addition, lenses such as lenticular lenses and Fresnel lenses molded using a transparent synthetic resin, recording disks such as optical disks,
It is also suitable for injection molded products of various optical components such as a light guide plate and a diffusion plate which are liquid crystal display components. These molded products formed by the present invention have improved mold surface reproducibility, improved gloss, reduced appearance defects due to weld lines, improved reproducibility of sharp edges on the mold surface, and improved fine mold surface irregularities. Not only has the effect of improving reproducibility, but also reduces internal strain near the molded product surface generated during the resin filling process, reduces birefringence, improves chemical resistance, and improves plating performance by reducing the orientation of the compounded rubber There are other effects as well. By filling high-pressure gas in the cavity, generation of gas from the melt front generated during the resin filling step is suppressed, so that mold contamination is reduced and the mold release force of the molded product is reduced. Such effects are expected.

[Brief description of the drawings]

FIG. 1 is a diagram showing the amount of carbon dioxide dissolved in polystyrene.

FIG. 2 is a graph showing the amount of decrease in T _g due to dissolution of carbon dioxide in polystyrene.

FIG. 3 is a graph showing the amount of carbon dioxide dissolved in a PMMA / PVF ₂ polymer alloy.

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

FIG. 5 is a graph showing the amount of decrease in T _g due to dissolution of carbon dioxide in various resins.

FIG. 6 is a sectional view of an example of a mold used in the present invention.

FIG. 7 is a diagram showing a structure of an example of a gas supply device used in the present invention.

FIG. 8 is a diagram showing the results of Example 1 of the present invention.

FIG. 9 is a diagram showing the results of Example 1 of the present invention.

[Explanation of symbols]

 Reference Signs List 8 gap 9 gas passage groove 10 hole from gas passage groove to outside of mold 11 O-ring 12 protrusion pin 13 cavity block 14 backup plate 16 cylinder 17 heater 18 pressure reducing valve 19 gas reservoir 20 supply electromagnetic valve 21 release Solenoid valve 22 Pressure release valve

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI // B29K 101: 12

Claims (5)

[Claims] 1. An injection molding method in which a carbon dioxide gas is injected into a mold cavity in advance, and then a molten amorphous thermoplastic resin is injected and filled, wherein cooling of the resin filled in the mold cavity is performed. A mold cavity surface temperature equal to or higher than the solidification temperature of the resin surface reduced by the dissolution of the carbon dioxide gas. 2. The mold cavity surface temperature is T _m (° C.),
The glass transition temperature of the amorphous thermoplastic resin T _g (℃), when the pressure of carbon dioxide gas was P (MPa), amorphous of claim 1, wherein setting the T _m so as to satisfy the following formula Injection molding method for conductive thermoplastic resin. _{T g -5 ≧ T m ≧ T} g - (5 × P) -35 ( where,
P ≧ 0.01) 3. The injection molding method for an amorphous thermoplastic resin according to claim 2, wherein the mold cavity surface temperature is partially set to _Tm which satisfies the following equation. T _m <T _g − (5 × P) −35 (However, P ≧ 0.0
1) 4. The mold cavity surface temperature is T _m (° C.),
The glass transition temperature of the amorphous thermoplastic resin T _g (℃), when the pressure of carbon dioxide gas was P (MPa), amorphous of claim 1, wherein setting the T _m so as to satisfy the following formula Injection molding method for conductive thermoplastic resin. _{T g -5 ≧ T m ≧ T} g - (5 × P) -20 ( where,
P ≧ 0.01) 5. The injection molding method for an amorphous thermoplastic resin according to claim 4, wherein the mold cavity surface temperature is partially set to _Tm which satisfies the following expression. T _m <T _g − (5 × P) −20 (where P ≧ 0.0
1)