The thickness of skin layer is inversely proportional to mold temperature, melt temperature as well as injection speed. In principle, it is possible to obtain minimum thickness of skin layer with elevated mold temperature and melt temperature, at high injection speed. Typically, the skin layer is amorphous because of fast cooling of melt. In addition to that, the shear stress is low. Therefore, both the thermal induced and stress induced crystallization processes are inhibited.

If one likes to crystallize semicrystalline polymer with bare minimum or no skin thickness, shear induced orientation of injection molding could be adopted. The Brunel University and University of Minho use this technique to optimize the crystallization.

There is another feature of skin layer that is not generally addresses and / or understood because of amorphous nature of skin. If one uses fast crystallizing polymer with low Tg, it can be seen that the skin layer crystallizes perpendicular to core layer – our recent findings with fastest crystallizing polymer like polyethylene oxide. The consequence is stress cracking to compensate the crystalline density and poor mechanical properties.

There are a couple of other issues to fight as well. Shear induced segregation will work with diffusion and you will end up with the highest molecular weight material in the core and the lowest at the surface. Russell Composto (UPenn) has done a lot of work on characterizing the gradients of Mw through parts. Certainly functional additives and waxes will make the situation worse. The other factor to consider is shear induced degradation. Both POM and PP go through chain scission as they degrade, which should add to the pool of lower molecular weight materials available to make the gradient even more pronounced.

From a practical perspective, the simplest things you can do to minimize the gradient is to start with a narrow molecular weight material, stabilize it efficiently, minimize waxes, open gates up and slow down injection speeds. If you want to push the envelop, Prof Avraam Isayev (UAkron) holds US Patent 6,713,600 for ultrasonic assisted processing. I’m not sure if this technique would speed up the formation of a gradient or if it would be so disruptive that the gradient never has a chance to form.