JPS6049652B2 - 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 a crystalline synthetic resin, and particularly to a method for manufacturing a synthetic resin molded article having a baked coating film.

BACKGROUND OF THE INVENTION Vehicle parts such as bumpers are made of synthetic resin, mainly made of crystalline synthetic resin such as polypropylene, and then painted. The conventional manufacturing method for this type of synthetic resin molded product is to wash it with an organic solvent such as trichloroethane after molding, apply a primer coat, bake it at 80°C for 30 minutes, and then apply a topcoat to give it a temperature of 90°C.
Baking was performed at °C for 4 minutes. However, in such conventional manufacturing methods for synthetic resin molded products, the molded product that has shrunk during molding shrinks again in the baking process due to recrystallization of the resin, and since the baking temperature is 80 to 900C, the shrinkage rate is low. There was a problem that the dimensions of the final product were not constant.

This invention has been made by focusing on these conventional problems, and solves the above problems by heat-treating the molded product at a temperature higher than the baking temperature prior to the paint baking process. The purpose is to.

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, in which the molded product is heated at a temperature higher than the baking temperature prior to the paint baking process. This is a method for manufacturing a synthetic resin molded product, which is characterized by a process.

As a result of examining the causes of differences in shrinkage rates in conventional methods, it was found that when baking was performed for 30 minutes at a temperature of 80 to 900C, there was variation in shrinkage rate, and that after baking at 80℃, 90℃ It was found that when baking was performed again, the shrinkage progressed further and the variation became larger.

The drawing shows a vehicle pumper (molding shrinkage rate 1511000 and 14
11,000), the heating temperature, especially the mold (
Longitudinal width 170? ) is a graph showing the results of measuring the change in shrinkage rate over time, and shows that the above phenomenon occurs. On the other hand, when heat treated at 100°C or higher, there is no variation during shrinkage, and the shrinkage is about 30°C
The shrinkage rate became constant after 6 gins, indicating that the shrinkage rate was stable. It was also found that once heat treatment is performed at a high temperature, especially if the shrinkage rate is stabilized by heat treatment at a high temperature, no shrinkage occurs even if the material is subsequently heat treated at a lower temperature. The present invention utilizes such a phenomenon, and after molding a synthetic resin mainly composed of crystalline synthetic resin such as polypropylene, molding is performed at a temperature higher than the baking temperature at which the shrinkage rate of the resin quickly stabilizes, such as polypropylene. In the case of 90~
Heat treatment is performed at 1100C, followed by primer coating, baking, topcoating, and baking in the same manner as in the conventional method.

The time required for the above heat treatment to complete the recrystallization and stabilize the shrinkage rate is, for example,
In the case of heat treatment at 0°C, the temperature is about 30 to 6. By heating for this period of time, the recrystallization of the resin is completed and the shrinkage rate of the molded product becomes constant, so even if it is baked at a low temperature, for example 80°C for primer coating and 90°C for top coating. No shrinkage of the molded product occurs. Therefore, the shrinkage rate of the final product is constant and the dimensions are constant. In addition, although polypropylene was illustrated as an example of crystalline resin in the above description, other resins are also covered by the present invention.

The heat treatment temperature and time can be appropriately selected in accordance with the target resin and the subsequent baking temperature within a range that stabilizes the shrinkage rate. 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, after molding a synthetic resin mainly composed of crystalline synthetic resin, the molded product is heat-treated at a temperature higher than the baking temperature prior to the painting process. After that, the shrinkage rate of the molded product can be kept constant, and since no shrinkage occurs in the subsequent coating process, the dimensions of the final product can be kept constant.

Next, examples of the present invention will be described.

Example A bumper for an automobile was molded from a synthetic resin mainly composed of polypropylene. After cooling, the molded product was degreased and washed with trichloroethane, heat treated at 100°C for 1 hour, and then coated with a primer and heated at 80°C. It was baked for 3 minutes, then top coated and baked for 4 times at 90°C.

Table 1 shows the shrinkage rate for the mold in each step. The results obtained without heat treatment are also shown as comparative examples. From the above results, it can be seen that in the case of the example, the shrinkage rate becomes stable due to the heat treatment and does not shrink due to subsequent baking.

[Brief explanation of drawings]

The drawing is a graph showing the results of measuring changes over time in the shrinkage rate of a mold for each heating temperature for a synthetic resin molded product mainly made of polypropylene, and the numbers appended to the curves indicate the heating temperatures.

Claims (1)

[Scope of Claims] 1. In a method for manufacturing a synthetic resin molded product, which involves molding a synthetic resin mainly composed of crystalline synthetic resin and then performing a paint baking process, the process is performed at a temperature higher than the baking temperature prior to the paint baking process. A method for producing a synthetic resin molded product, which comprises heat-treating the molded product. 2. The method for producing a synthetic resin molded article according to claim 1, wherein the heat treatment is performed until the shrinkage rate of the molded article is stabilized. 3. The method for producing a synthetic resin molded article according to claim 1 or 2, wherein the synthetic resin is mainly polypropylene. 4. The method for manufacturing a synthetic resin molded article according to any one of claims 1 to 3, wherein the heating temperature is 90 to 110°C.