JP2835965B2 - Method for manufacturing resin molded products - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing a resin molded product used for various household goods and the like.

[Related Art] In recent years, with the development of resins having various characteristics, resins have come to be widely used for household appliances such as home appliances and furniture. In particular, various housings for tableware and home electric appliances utilizing the heat insulating and insulating properties of resin have been developed.

As a method for producing these resin molded articles, generally, a resin which is heated and melted is filled into a pair of molds of a convex mold and a concave mold,
A method of taking out a molded product in a mold after cooling and solidifying is used. However, in this method, a thick molded product is heavy due to its high density and is liable to cause distortion due to resin shrinkage, so-called sink marks and warpage.

As a method of solving this problem, for example, a so-called gas counter pressure method such as US Pat. A so-called sandwich molding method in which the core layer resin to which the foaming agent is added after the layer resin is injected and only the core layer resin is foamed, the inside of the mold is pressurized in advance with air, and the skin layer resin and the core layer resin are foamable. Japanese Patent Application Laid-Open No. Sho 50-17 discloses a method in which a resin is used to fill the resin with an injection molding machine having an optional mechanism, and after the filling is completed, the internal pressure of the mold is released, and the surface of the skin layer is smooth and the skin layer and the core layer are foamed.
No. 476968
A method according to Japanese Patent Application Laid-Open Publication No. H10-21011 has been developed.

Above all, the molded product by the sandwich molding method in which only the core layer resin is foamed has a small distortion to suppress the shrinkage of the skin layer resin from the inside due to the core layer foaming pressure, and the surface of the molded product because the skin layer resin is not foamed. Is smooth, synthetic, and excellent in impact strength.

[Problems to be solved by the invention] However, each method has advantages and disadvantages,
In addition, there is a demand for a lighter molded article, and at present, there is a strong demand for a method of manufacturing a molded article having sufficient strength and light weight and good appearance.

[Means for Solving the Problems] The present invention is a method for producing a resin molded product that solves the above problems.

The present invention is a method for producing a resin molded article having a double structure in which the outside of a core layer resin is wrapped with a skin layer resin,
After filling the molding device with the core layer resin containing the foaming agent and the skin layer resin not containing the foaming agent, the pressure applied to the core layer resin is reduced to foam the core layer resin in the molding device. A method of manufacturing a resin molded product, which comprises injecting a skin layer resin into a molding die and then or simultaneously with the core layer resin.

 Hereinafter, the present invention will be described in detail.

Although the resin molded article according to the production method of the present invention has a double structure of the core layer and the skin layer, the resin of the core layer and the resin of the skin layer may be the same or different, and are selected according to the purpose of the molded article. The resin for the core layer is foamed by adding a foaming agent, but the foaming agent may be foamed by adding a foaming agent to the resin for the skin layer. When foaming the skin layer resin, it is necessary to control the amount of foaming so that the smoothness of the surface of the molded article is not impaired or the strength of the molded article is not reduced.

Also, it may be formed by a mold pressurized with a gas body described in Japanese Patent Application No. 50-17476,
The molding may be carried out by a method described in JP-A-39-22213, that is, by using a mold in which the mold cavity can be enlarged after injection.

The resin forming the skin layer / core layer used in the present invention may be of the same type or different types, and may be of any type such as the same color and different color.

As the usable resin, any resin can be generally used as long as it can be injection molded. For example, polyethylene, polypropylene, polystyrene, styrene-butadiene copolymer, ABS resin, SAN resin, polyvinyl chloride, polymethyl methacrylate, etc. Not only general-purpose resins, but also resins that have chemical and mechanical characteristics such as polyphenylene ether, polycarbonate, polyacetal, and nylon.Use these resins in combination with each other from the viewpoint of economy and aesthetics. It is also possible.

No special agent is required as a foaming agent to be added to the resin, and conventionally commercially available organic and inorganic foaming agents can be used. That is, the critical temperature is high such as pentane and heptane whose boiling point at normal pressure is in the range of 20 to 80 ° C,
Azodichambonamide (ADCA), sodium bicarbonate,
Chemical blowing agents that generate gases such as non-condensable nitrogen and carbon dioxide at the blowing temperature, such as sodium citrate, can also be used.

In the resin molded article produced by the production method of the present invention, the porosity of the core layer is 30 to 80%. If the porosity is less than 30%, the density of the core layer resin is high and the weight of the molded article increases, and if it exceeds 80%, the impact strength of the molded article is undesirably reduced.

Next, a method for producing a resin molded product of the present invention will be described with reference to the drawings.

FIG. 1 is a drawing showing an example of an apparatus for implementing the manufacturing method of the present invention. 1 is a skin layer resin, and 2 is a core layer resin, which is extruded forward (to the left in the drawing) by a skin layer screw 3 and a core layer screw 4, respectively. The injection port is closed by the skin layer needle 5 and the core layer needle 6 except at the time of injection, and the needle retreats due to the pressure of the skin layer resin 1 and the core layer resin 2 pressed by the respective screws, and the resin is injected. The resin is injected into the mold 7 while being pressed by the screw.

The manufacturing method of the present invention is characterized in that foaming, which has conventionally been performed after injection into the mold 7, is performed before injection. Specifically, for example, after filling the core layer resin 2 into the apparatus shown in FIG. 1, the core layer screw 4 is retracted (suck back), the pressure on the core layer resin 2 is reduced, and the core layer resin 2 is removed. Foam. Thereafter, if the core layer screw 4 is advanced again and the core layer resin 2 is injected, a sufficiently foamed core layer is obtained.

[Action] In the resin molded article according to the production method of the present invention, since the core layer is uniformly foamed to the end portion, air bubbles uniformly act on the entire molded article against shrinkage of the resin during the cooling process during molding. There is a function to hold this down. Therefore, sink marks and warpage hardly occur, and a uniform molded product can be obtained as a whole, so that rigidity and impact strength are not uneven. In addition, since the amount of foaming of the core layer is larger than that of a conventional molded article having a double structure, it is extremely light. The foamed portion of the core layer has a cotton-like shape, and has a very small amount of air flow, so that the core layer is also excellent in heat insulation.

Since the manufacturing method of the present invention forms the core layer with the foamed resin, the core layer resin is formed at a portion near the terminal of the resin flow in the mold as in a conventional sandwich molding method using a foamable resin for the core layer. The resin that has been uniformly foamed can be fed to the end of the molded product without protruding. Moreover, the amount of foaming can be easily controlled by changing the type and amount of the foaming agent.

EXAMPLES Hereinafter, specific embodiments of the present invention will be described, but the present invention is not limited thereto.

 Note that all “parts” in the examples are “parts by weight”.

Example 1 A disk-shaped resin molded product having a thickness of 12 mm and a volume of 214 cm ³ was produced using the apparatus shown in FIG.

High impact polystyrene is used for both the core layer and skin layer resin.
Was added. The core layer resin was heated and melted at 220 ° C. together with the skin layer resin while applying a sufficient back pressure to keep the foam in the apparatus without being foamed. After filling, the core layer screw is 2.4
The pressure on the core layer resin was lowered to double the pressure, the core layer resin was foamed in the apparatus, and the resin was injected into the mold at the same time as the skin layer resin. The molded product obtained after cooling was 145 g, and had a foaming ratio of 1.55 times, a porosity to the core layer of 65%, and was free of sink marks and warpage, and was a lightweight material having no protrusion of the core layer.

Example 2 In manufacturing a disc-shaped resin molded product having a thickness of 9 mm,
Add 0.36 parts of ADCA as a foaming agent to 100 parts of the core layer resin, fill the inside of the device while applying back pressure, and then retract the screw so that the volume ratio becomes 1.5 times the volume of the core layer resin after filling Was obtained in the same manner as in Example 1.

The obtained molded article had a weight of 112 g, an expansion ratio of 1.5, and a core layer porosity of 57%, and was lightweight and excellent in appearance.

Example 3 0.18 parts of a foaming agent was added to 100 parts of the skin layer resin, and a molding die previously pressurized to 8 kg / cm ² with a gas was used.
A molded article was obtained in exactly the same manner as in Example 1 except that the resin was filled with both skin and core layers, and then the molding was performed by releasing the gas pressure in the mold.

This molded product was also lightweight as in Example 1, having an expansion ratio of 1.62 times, and was excellent in appearance without sink marks and warpage.

Comparative Example 1 A high-impact polystyrene without any foaming agent was used for both the skin and the core layer resin, and the filling amount of the core layer resin in the apparatus was determined in Example 1. After the core layer screw was retracted, that is, up to the screw retracted position immediately before injection, the resin was heated and melted and the skin layer resin was similarly filled into the apparatus, and then both resins were injected into the mold used in Example 1. . The molded product obtained after cooling was quite heavy, weighing 220 g, and was marked with sink marks.

Comparative Example 2 When filling the core layer resin containing the foaming agent into the apparatus, heating was performed in Example 2 while applying a back pressure sufficient to keep the foaming in the apparatus until the core layer screw was retracted. A molded product was obtained in the same manner as in Example 2 except that the resin was filled with the molten resin.

Although this molded product did not have sink marks and warpage, it weighed 143 g heavier than that of Example 2 and had poor appearance in which the core layer resin had protruded.

Comparative Example 3 A molded product was obtained in exactly the same manner as in Example 2 except that molding was performed without retracting the core layer screw, which was performed in Example 2.

 This molded article was also as heavy as 133 g.

Example 4 Using a soup bowl and a mold having a weight of 142 g made of polypropylene, the skin layer and the core layer were both made of polypropylene, and a molding material obtained by adding 0.5 part of ADCA as a foaming agent to 100 parts of the core layer resin was used. By using the apparatus shown in FIG. 1, the core layer resin was heated and melted while applying a back pressure so as not to foam, and filled in the apparatus. After filling, the core layer screw was retracted so that the volume ratio became 1.7 times the filling amount, and the core layer screw was injected into the mold at the same time as the skin layer resin. The molded product obtained after cooling was 117 g and was lightweight with an expansion ratio of 1.36 times and was excellent in heat retention.

Comparative Example 4 A molded product was obtained in exactly the same manner as in Example 2 except that molding was performed without retracting the core layer screw, which was performed in Example 4.

This molded article had a weight of 133 g and a foaming ratio of 1.2 times and had no sink marks, but was slightly heavier than the molded article of Example 4 and was slightly inferior in heat retention.

FIG. 2 shows the results of comparing the heat insulating properties of the molded products obtained in Example 4 and Comparative Example 4.

 FIG. 3 shows cross sections of the molded products of Example 4 and Comparative Example 4.

[Effects of the Invention] As described above, the production method of the present invention and the resin molded product produced by the production method of the present invention have the following effects.

There is no sink mark or warp and the appearance is good.

Lightweight.

Since it has a double structure, a recycled resin can be used as the core layer resin, which is useful in terms of cost.

Since it can be manufactured using a conventional manufacturing apparatus for a molded product having a double structure, there is no need to change the equipment.

Since the control of the foaming amount is easy, characteristics of the molded article such as strength and weight can be easily changed.

Good heat retention.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing a molding apparatus for carrying out the manufacturing method of the present invention, FIG. It is a photograph which shows the cross section of each molded article of an example and a comparative example. 1: skin layer resin, 2: core layer resin 3: skin layer screw, 4: core layer screw 5: skin layer needle, 6: core layer needle 7: mold

Claims (2)

(57) [Claims] 1. A method for producing a resin molded product having a double structure in which the outside of a core layer resin is wrapped with a skin layer resin,
After filling the molding device with the core layer resin containing the foaming agent and the skin layer resin not containing the foaming agent, the pressure applied to the core layer resin is reduced to foam the core layer resin in the molding device. And a method of injecting the skin layer resin into the molding die and then or simultaneously with the core layer resin. 2. The method according to claim 1, wherein the molding die is pressurized with a gas in advance, and after the skin layer resin and the core layer resin are injected into the die, the gas pressure in the die is released. Item (1).