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Injection molding part with crystalline material

We all know that the skin layer in a molded part with PP is less crystalline than the core in a PP injection molded component. Could you please explain the mechanism behind why it happens? Is it due to the rapid cooling during the cooling cycle during injection molding which prevents the formation of crystalline phases? If there is some literature on the internet which explains the same I would like the links to the same.

Also, I can see some functional benefits from having a crystalline outer skin - e.g. better chemical resistance, better scratch performance in some tests, etc., but is the difference so significant that it is can be justified by the economic cost to achieve the same?

The crystalizing of the material takes time and usually progress better at temperatures a bit higher than the injection mold surface temperature (actually in some cases you might gain cycle time by increasing injection mold temperature when running crystalline materials). Some companies make inductive heaters for the injection mold to increase the surface temperature to get close to the melt temperature to avoid or reduce the frozen layer. Then they apply a rapid cooling system with optimized cooling channel design and in some cases using an expanding gas for cooling.

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.

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