JPS6049653B2 - Manufacturing method for synthetic resin molded products - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a synthetic resin molded article mainly made of crystalline synthetic resin, and particularly to a method for manufacturing a coated molded article having the same dimensions as an unpainted molded article. .

Vehicle parts such as bumpers are molded from synthetic resins, mainly crystalline synthetic resins such as polypropylene, and some are used unpainted, while others are used with various baked-on coatings. There is.

The conventional manufacturing method for painted molded products is to wash with an organic solvent such as trichloroethane after molding, apply a primer coat, bake at 80°C for 30 minutes, and then apply a topcoat and bake at 90°C. The image is being baked for a minute. However, in such conventional manufacturing methods for synthetic resin molded products, the resin that shrinks during molding recrystallizes and heat shrinks during the baking process, resulting in smaller products than unpainted molded products. To prevent this, it had to be molded using a separate large mold.

Further, since the baking temperature is 80 to 90°C, there is a problem that the heat shrinkage rate varies and the dimensions of the finished product after coating are not constant. This invention was made by focusing on these conventional problems, and involves molding using a synthetic resin having a predetermined molding shrinkage rate, and then molding the resin at a temperature lower than the baking temperature prior to the paint baking process. heat treated,
The purpose is to solve the above-mentioned problems by making the overall shrinkage rate due to molding and heat treatment match the molding shrinkage rate of a molded product that is not coated.

This invention relates to a method for manufacturing synthetic resin molded products in which a synthetic resin mainly composed of crystalline synthetic resin is molded and then a paint baking process is performed. A method for producing a synthetic resin molded article, characterized in that it is first heat treated at a temperature higher than the baking temperature, so that the overall shrinkage rate due to molding and heat treatment matches the molding shrinkage rate of a molded article that is not coated. It is.

As a result of examining the causes of differences in heat shrinkage rates in conventional methods, it was found that when 30 to 4 powder baking was performed at a temperature of 80 to 90°C, the shrinkage rate varied; It was found that when the material was reheated at 90°C after the test, the shrinkage progressed further and the variation became larger.

The drawing shows a vehicle pumper (molding shrinkage rate 1511000 and 14
11000) for the mold (longitudinal width 170
? ) is a graph showing the results of measuring the change in shrinkage rate over time for each heating temperature, and shows that the above phenomenon occurs. On the other hand, when heat-treated at 100'C or higher, there is no variation in the shrinkage rate, and the shrinkage rate becomes constant at about 30 to 6 gin, indicating that the shrinkage rate is stable. It was also found that once the material is heat-treated at a high temperature, especially when the shrinkage rate is stabilized by the heat treatment at a high temperature, no shrinkage occurs even if the material is further heat-treated at a lower temperature. The present invention makes use of this phenomenon. When heat treatment is performed at a temperature higher than the baking temperature, at which the shrinkage rate of the resin stabilizes early, the recrystallization of the resin ends and the shrinkage rate remains constant. Therefore, even if baking is performed at a temperature lower than the above heat treatment temperature, no thermal shrinkage occurs.

In addition, the molding shrinkage rate (shrinkage rate during molding) of crystalline synthetic resins can be adjusted by selecting additives such as fillers, so the overall shrinkage rate due to molding and heat treatment can be adjusted by controlling the overall shrinkage rate during molding and heat treatment. If you select the molding shrinkage rate of the resin to match the molding shrinkage rate of the unpainted molded part, the size of the painted molded part and the unpainted molded part can be reduced using the same mold. It is possible to match the dimensions of the coated molded product, and to eliminate variations in the dimensions of the coated molded product. In other words, if a molded product is molded with a resin that has a lower mold shrinkage than an unpainted molded product, a larger molded product will be produced, but by heat-treating this, it is possible to mold with a resin that has a higher mold shrinkage. , the dimensions are the same as the unpainted molding, and even if the heat-treated molded product is then painted and baked, no heat shrinkage will occur, so the final product can be either a painted molded product or an unpainted molded product. The dimensions will be the same.

The heat treatment temperature of the molded product is higher than the baking temperature, which is the temperature at which the shrinkage rate stabilizes early. In the case of polypropylene, recrystallization is completed when heat treatment is performed at 90 to 110°C for 30 to 60 minutes. The shrinkage rate becomes stable.

The subsequent painting process is similar to the conventional method; for example, after applying a primer, baking is performed at 80°C, and after applying a topcoat, baking is performed at 90°C. The resin used for molding is mainly a crystalline synthetic resin, and examples of the crystalline synthetic resin include polyolefin resins such as polypropylene.

To adjust the molding shrinkage rate of the resin, the degree of polymerization of the resin itself,
This can be done by changing the regularity etc., but it can be easily done by mixing additives.
Examples of additives include fillers such as talc and other additives such as EPDM, and the molding shrinkage rate can be adjusted by adjusting the type and amount of additives. The heat treatment temperature and time for the molded article can be appropriately selected in accordance with the resin to be treated and the subsequent baking temperature within a range in which the shrinkage rate is stabilized.

Further, the present invention is applicable to the production of vehicle parts such as bumpers and radiator grilles, as well as molded products for uses other than vehicles. As explained above, according to the present invention, molding is performed using a synthetic resin that is mainly composed of crystalline synthetic resin and has a predetermined molding shrinkage rate, and is heated to a temperature higher than the baking temperature prior to the paint baking process. The overall shrinkage rate due to molding and heat treatment is made to match the molding shrinkage rate of the unpainted molded product, so the dimensions of the coated molded product and the unpainted molded product are the same. In this case, it is possible to eliminate variations in the dimensions of the coated molded product, and the same mold can be used for molding.

Next, examples of the present invention will be described.

Example: A synthetic resin mainly composed of polypropylene is mixed with talc as a filler to adjust the molding shrinkage rate. This resin is used to mold an automobile bumper. After cooling, the molded product is degreased and washed with trichloroethane, and then heated at 1000C for 1 After heat treatment for several hours, a primer coating was applied and baked at 80'C for 3 minutes, and a top coat was applied and baked at 90'C for 4 minutes.

Table 1 shows the shrinkage rate for the mold in each step. Also, Comparative Example 1 is a case where no heat treatment is performed, Comparative Example 2 is a case where no filler is mixed and molded and no painting is performed.
As Comparative Example 3, the results obtained when molding and painting were performed without mixing filler are also shown. From the above results, in the case of the example, the shrinkage rate becomes stable due to heat treatment, does not shrink due to subsequent baking, and the molded product without painting (
It can be seen that the shrinkage rate is the same as that of Comparative Example 2).

[Brief explanation of the drawing]

The drawing is a graph showing the results of measuring the change in shrinkage rate over time of polypropylene by changing the heating temperature, and the numbers appended to the curves indicate the heating temperature.

Claims (1)

[Scope of Claims] 1. In a method for manufacturing a synthetic resin molded article, which involves molding a synthetic resin mainly composed of crystalline synthetic resin and then performing a paint baking process, Synthetic resin molding characterized by heat treatment at a temperature higher than the baking temperature prior to the paint baking process so that the overall shrinkage rate due to molding and heat treatment matches the molding shrinkage rate of a molded product that is not painted. method of manufacturing the product. 2. The method for producing a synthetic resin molded article according to claim 1, wherein the molding shrinkage of the synthetic resin is controlled by incorporating additives. 3. The method for producing a synthetic resin molded article according to claim 2, wherein the additive is a filler. 4. The method for producing a synthetic resin molded article according to any one of claims 1 to 3, wherein the synthetic resin is polypropylene. 5. The method for manufacturing a synthetic resin molded article according to any one of claims 1 to 4, wherein the heating temperature is 90 to 110°C.