Crystalline material needs time as more as possible to process its micro-structural crystallization. For injection molding part with crystalline material, the skin layer is at the status more like amorphous because material wherein is directly contact injection mold and cooled by injection mold temperature so fast that time is not enough to process crystallization; while core layer can be more at crystalline status because material wherein is cooled slower and time is comparatively longer for processing crystallization. Injection mold temperature is the key factor to influence the allowable time for material's crystallization process. Increasing injection mold temperature could reduce cooling rate, providing more time, to make amorphous (skin) layer thinner and crystalline (core) layer thicker.
Not only is it possible to avoid an amorphous layer, but you can make the skin almost totally crystalline under the right circumstances. I have done so and it results in PP with very interesting properties. Descriptions of how to do it have been in the literature (books and paper) for decades although no-one seems to have paid attention. Higher injection mold temperature gives the crystals time to form instead of locking in an amorphous structure. Nevertheless, it is possible to make PP with a crystalline surface under normal injection molding conditions with some tricks.
The article comes from China injection mold manufacturer - Mold Best Assurance Company Limited, website is www.mbamoldanddesign.com
