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.
The article comes from China injection mold manufacturer - Mold Best Assurance Company Limited, website is www.mbamoldanddesign.com
