The paint performance of PP+EPDM depends to a large extent on the nature of the material itself. Because the polarity of PP itself is very weak, the main paint binding force depends on the distribution of EPDM, and EPDM is required to be evenly distributed on the surface of the product. The paint adhesion on all surfaces is the same.
In the process, It is required to form an appropriate shear force during the flow of the fluid. Through the difference in the flow speed between PP and EPDM, the EPDM is left on the outer surface of the product to achieve the paint requirements. However, due to the requirements in the actual process control due to other aspects The quality of the product, this aspect is relatively easy to be ignored. In the early confirmation of the coating process, the need to verify the impact of injection molded parts on coating performance.
The effect of PP+EPDM on the size of the product is mainly due to the mold shrinkage of the material. As a rubber body, EPDM has a certain shrinkability under the effect of pressure, so under the effect of injection pressure, EPDM has a certain shrinkage. After opening the mold, there is a certain amount of elastic release.
PP as a crystalline material, at different temperatures, the speed of the crystallization will cause a change in the shrinkage rate. In the paint process, the second baking will cause PP recrystallization, so the size of the product, there is More variables.
Melt Index:
The melt index looks at the fluidity of the material, and has a low melt index. Difficulties in material flow will not only cause defects on the surface of the product, but also cause problems such as localized small size. The high melt index and good fluidity will cause Under certain pressure, the amount of melt in the cavity is very sensitive to injection molding and holding pressure, and the stability of the process used to control the size becomes poor.
Material shrinkage:
Deviations in material shrinkage inevitably lead to poor product dimensions. For PP+EPDM, in addition to the properties of the material itself, the shrinkage also depends on the product shape, mold structure, injection speed, pressure and temperature. Materials usually supplied by material suppliers Shrinkage is a range. If it is a certain value, it refers to the mold shrinkage reference value of a larger process window for a certain shape of the product.
EPDM content:
Due to the poor fluidity of EPDM, it will cause surface and size defects. EPDM can be compressed, it will cause local stress concentration, too much EPDM content will cause process control difficulties.
Talc content:
Talc also plays a role in reducing the fluidity, which will mainly affect the surface of the product. At the same time, if the talc powder is not uniformly blended, it will also reduce the paint performance of the product.
Although, in general, the material supplier will not provide the content of the various components of the material, it can be judged from some other properties of the material. The amount of elongation at break depends, in part, on the content of EPDM in the material. The greater the extension rate.
At the same time, the higher the content of talc, the higher the strength. Therefore, under the same material properties, it is also necessary to consider the process performance of the material. Usually the ideal material selection should consider the process window to be as wide as possible.
With the current increase in product size requirements, relatively high shrinkage materials will be used, because the high shrinkage, process changes will affect the size of the product will be smaller.