JPS59101340A - Method of foaming and injection molding - Google Patents

Abstract

PURPOSE:To obtain an injection molded item whose surface is smooth and which is not affected adversely with an expanding agent, by employing water as expanding agent in a molding method wherein until a cavity is filled with an expandable resin, the expandable resin is prevented by a pressure gas from expanding. CONSTITUTION:After water as expanding agent is added to an expandable resin by for example a method wherein generally a solid substance (e.g. silica gel, etc.) is added and mixed that can liberate water of crystallization or absorbed water at the plasticising temperature of said expanding resin, the resin is injected while the resin is prevented by the action of a gas (e.g. preferably air at 10kg/cm<2>) at a pressure capable of suppressing the expansion of the resin from expanding until the mold cavity is filled with the resin. Subsequently, the pressure gas is released to allow the resin to expand to obtain the intended molded item. EFFECT:The obtained item is free from defects such as molding sink, warping, etc. and the molding can be effected economically.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a foam injection molded article with a smooth surface.

Conventionally, synthetic resin foam injection molded products have good dimensional accuracy and can be manufactured as thick injection molded products, so they are widely used for applications that require strength and precision, such as housings for electronic application equipment. has been done. However, due to foaming during injection, general foam injection molded products exhibit a unique spiral-like rough surface on their surfaces, and when applied to the above-mentioned applications where commercial value is important, the surface must be finished with a coating. The current situation is that it requires a great deal of effort and expense. Also, since the rough surface requires finishing,
It became impossible to create a free design by transferring the mold pattern.
Many design constraints are imposed. As a countermeasure to this problem, the so-called counterbrella sure method is known as a technique for obtaining a foam injection molded product with a smooth surface. In this method, sufficient pressure gas is applied to suppress foaming of the resin during injection until the mold cavity is injected and filled with foamable thermoplastic resin, and then the pressure gas is released. This is a method for manufacturing a foam injection molded article that allows foaming of resin within a mold. In this case, as the base resin is injected and filled into the mold cavity, its volume decreases according to its coefficient of thermal expansion as it cools, but the foaming pressurized gas that is dissolved and dispersed inside the base resin is vaporized. The expansion compensates for the volume corresponding to the thermal contraction of the base resin, and as a whole remains constant at the mold volume throughout, resulting in cooling assimilation. At this time, the entire surface layer of the injected resin forms an assimilated skin as soon as it comes into contact with the cooling surface of the mold cavity and can no longer foam, resulting in a foamed product with a smooth surface. Therefore, in this method, that is, in the counterbrella sure method, a foamed injection product having a very low expansion ratio within the heat shrinkage volume of the base resin can be obtained. When a more highly foamed product is required, the method of Japanese Patent Publication No. 39-22213 is used, in which, in the injection process mentioned above, the Koken cavity is enlarged to enable further foaming after releasing the pressurized gas. Under the application of the above pressure gas, the pressure gas is encapsulated and injected into the foamable resin, and after filling the mold cavity, the side pressure gas is released to allow foaming in the hollow space occupied by the encapsulated gas. The method of Japanese Patent Publication No. 53-25352 can be applied.

Hereinafter, all foam injection molding methods that involve a process in which pressurized gas is not applied to the mold cavity will be referred to as the counterbrella sure method. Although foamed injection molded articles can now be produced, the following difficult problems still remain. In other words, the first problem is that commonly used organic chemical blowing agents that generate gas through chemical thermal decomposition at the plasticizing temperature of the resin may color the base resin or cause deterioration. The decomposition products cause contamination of molded products, molds, etc.
There are many negative effects such as corrosion of the molding machine. In addition, the decomposition temperature of substances with slightly less negative effects may be extremely high, or
Prohibitively expensive. In addition, the decomposition residue of inorganic blowing agents such as hydrogen carbonate (IJum) has deliquescent properties, and when it absorbs moisture in the molded product, the resulting solution may sweat, and its evaporated dry product may form white powder. There are many problems that arise. As a second problem, generally the pressure gas used is 10 to 9/cIl (
Economically, it is most preferable to use air at the following cage pressure. Gases with pressures higher than 9/7 in 10 are high-pressure gases, and there are many problems that are extremely difficult in terms of handling and equipment. Therefore, when attempting to use a gas of 10 powder/- or less in this counter breather method, there are significant restrictions on the amount of blowing agent used as a condition to suppress foaming of the resin during injection. It is a matter of receiving. This usage limit varies slightly depending on the blowing agent, type of resin, and injection temperature, but the amount of decomposed gas generated per gram of general organic chemical blowing agents is approximately ≧200CC (under standard conditions, the same applies below). Since the usage limit according to the above-mentioned restrictions is about 0.2%, this amount of generated gas corresponds to about 240% of the base resin volume. In general, the number of air bubbles in foam injection molded products used for the housing of electronic equipment, etc., is less than 20 cm compared to the base resin. It is also somewhat insufficient to provide pressure to expand the bubbles against the viscosity of the filled base resin, which contracts as it cools. In general, Japanese Patent Publication No. 39-22213, and Japanese Patent Publication No. 53-253
It is clear that the application of Publication No. 52 is completely insufficient. Therefore, in this case, defects such as sink marks and warpage occur, which are seen in ordinary thick injection molded products, and the product cannot be used for applications requiring the above-mentioned precision.

The present invention solves the problem of obtaining a foamed molded article with a smooth surface as described above. That is, the present invention provides the following advantages: ``Until the foamable synthetic resin is injected and filled into the mold cavity,
A foaming injection molding method comprising using water as the main foaming agent in foaming injection molding, which includes a step of applying gas at a pressure capable of suppressing foaming of the injected resin into the mold cavity. The main objective is to provide the following.

Conventionally, it has not been possible to use a blowing agent mainly consisting of d1 water in foam injection molding. The reason for this is presumed to be that water vapor released due to foaming during injection condenses on the cooling surface of the mold cavity and forms water droplets, making it impossible to continue molding. However, in the present invention, the foaming of the resin during injection into the mold cavity is suppressed, and as soon as the resin fills the mold cavity, the entire surface of the resin is cooled and hardened to form a smooth and hard skin layer. However, escape of generated gas due to foaming of the internal resin can be suppressed.

As mentioned above, there are various methods for supplying water as a blowing agent in the present invention. When the required amount of the blowing agent is within the amount that the base resin can serve as a carrier, it is mixed with the base resin and then injected. Although it can be fed to a molding machine, in general, a solid substance that releases crystal water, absorbed water, etc. at the temperature of plasticization of the base resin is used as R[
Mix in a quantitative manner and supply to the injection molding machine. Materials used as these water carriers include calcium sulfate, activated clay, silica gel, and talcum powder. In addition, after melt-kneading a large amount of dehydrated products of these substances with the base material, add and mix the amount of water that can be hydrated to retain the water, and mix the required amount with the base resin in a so-called concentrate type. It can also be supplied to an injection molding machine.

The amount of water added in this manner varies depending on conditions such as the type of base resin, desired expansion ratio, injection temperature, etc., but M is approximately 3 times the volume of the base resin in terms of water vapor volume. It was confirmed that a foamed injection molded article that can be sufficiently used for the above purpose could be produced in the following manner. It was also confirmed that with this amount of water used, the gas pressure exerted on the mold cavity was approximately ≧sufficient at 10 Kf/-. Generally, the synthetic resin that forms the base material used for the above purpose is high-impact polystyrene, ABS
resin, styrene-modified polyphenylene ether resin, etc. under pressure. If the solubility of water is within the above range at the time of injection and the pressure is sufficiently lower than the equilibrium vapor pressure of ordinary water, foaming at the time of injection can be suppressed.

In addition, the amount of foaming agent used and the gas pressure applied to the mold cavity will of course vary depending on the conditions, so when determining the amount, first conduct a trial to determine the desired foaming ratio. A sufficient amount of gas is determined so as not to cause sink marks on the product, and then a sufficient pressure of the gas to eliminate so-called silver streaks is also determined by trial.

In addition, the heat distortion temperature of the base resin (ASTM, D648
) is low and water cannot exist as a gas at this temperature and normal pressure, so in this case, a chemical blowing agent of Sunset is used as an auxiliary. The amount to be used is also determined by trial, but the indicator for its use is the amount in which the volume of gas generated by the chemical foaming agent corresponds to the foaming expansion volume of the base resin. In this case, the water may exist as a gas at a partial pressure at that temperature and may contribute to maintaining the shape of the foam molded article.

In this case, although the adverse effects caused by the use of the chemical blowing agent described above remain, the amount thereof is so small that dilution can be alleviated to a great extent.

Next, an example will be explained. Each of the following examples uses a general-purpose 20 oz injection molding machine, an average thickness of 7 y, external dimensions of 260 w, JIX of 350 m, and internal dimensions of 160 ma x 2.
It was molded using a 50I frame-shaped mold.

91 Example 1 Grafted polyphenylene ether resin (trade name Cylon 201V, manufactured by Asahi Kasei Corporation) 10 as the base resin
0 parts by weight was moistened with 0.2 parts of mineral oil, and 1.2 parts of calcium sulfate powder (Ca SO42H2O) was added and mixed thereto.

The injection temperature was 260°C, which did not reach the mold cavity, and the air pressure was 8 Kq/lri, and the mold cavity was filled with injection. The foamed injection molded product taken out after cooling had a smooth surface, no sink marks were observed, the expansion ratio was 1.06 times, and no discoloration was observed.

Example 2 The injection resin amount was 80% that of Example 1, and the same raw material composition, the same injection temperature, and the same air pressure were used. Also, special public service 1973-
Injection molding was carried out using the method of No. 25352, with 150 Ky/i of nitrogen gas incorporated.

The appearance of the obtained foam was similar to that of all the examples, and its cut surface showed uniform foamed cells.

Example 3 ABS resin (trade name, Stylac 1) was used as the base resin.
91, manufactured by Mukasei Co., Ltd.) moistened with 100 parts by weight and 0.2 parts by weight of mineral oil, containing 0.2 parts by weight of water in 1 part by weight of talc powder, and 0.02 parts of azodicarbonate. Amide was added and mixed to obtain a raw material.

The mold cavity was filled with injection at an injection temperature of 220 DEG C. and an air pressure of 9 Kf/- on the mold cavity.

The foamed injection molded product taken out after cooling had a smooth surface and no pale yellow sink marks were observed.

Comparative Example A comparative example was carried out under the same conditions as in Example 3 except for the raw material composition. The raw material ABS was the same as in Example 3, and it was moistened with 0.2% mineral oil and mixed with 0.2% azosialbonamide to obtain a raw material for injection molding. Although the obtained foamed molded product had a smooth surface, some sink marks were observed on the inside and inner corners, and it had a severe yellow discoloration.

As described above, in the present invention, water, which is the main blowing agent, and its carrier are generally inert to the basic resin, with some exceptions, and do not exhibit any adverse effects such as color sticking. In addition, for the manufacture of foam molded products that can be applied to electronic application equipment, etc., 10
The economic efficiency of producing smooth-surfaced foam injection molded bodies with pressures in the mold cavity of less than KIi/Cd is significant.

Patent applicant: Asahi Kasei Industries, Ltd.

Claims (1)

[Scope of Claims] L. Foaming injection comprising the step of applying gas at a pressure capable of suppressing foaming of the injected resin into the mold cavity until the mold cavity is injected and filled with foamable synthetic resin. Foaming injection molding method 2 characterized in that water is used as the main blowing agent in molding In the method according to claim 1, water as a blowing agent is used to thermoplasticize the base resin. A method for foam injection molding, characterized in that a solid that releases water upon heating is fed to an injection molding machine in the form of a solid that releases water upon heating. , calcium sulfate, activated clay, silica gel, and Merck powder. In the method according to claim 1, the pressurized gas does not reach the mold cavity. The pressure is less than the gauge pressure toKg/-J, and the volume of water used as the foaming agent is less than three times the volume of the base resin as the volume of water vapor converted to the standard state. Molding method & Foaming injection molding in the method according to claim 1, characterized in that a compound generated by thermal decomposition is added in an amount of gas that does not exceed the foaming expansion volume of the intended molded product. Method