润滑剂可分为外润滑剂和内润滑剂两种。

Lubricants can be divided into two kinds: external lubricants and internal lubricants.

外润滑剂的作用主要是改善聚合物熔体与加工设备的热金属表面的摩擦。它与聚合物相容性较差，容易从熔体内往外迁移，所以能在塑料熔体与金属的交界面形成润滑的薄层。

The role of external lubricants is mainly to improve the friction between polymer melts and hot metal surfaces of processing equipment. It has poor compatibility with polymer and is easy to migrate from the melt, so it can form thin lubricating layer at the interface between plastic melt and metal.

内润滑剂与聚合物有良好的相容性，它在聚合物内部起到降低聚合物分子间内聚力的作用，从而改善塑料熔体的内摩擦生热和熔体的流动性。

The internal lubricant has a good compatibility with the polymer. It plays a role in reducing the cohesion of the polymer molecules within the polymer, thus improving the internal friction heat and the fluidity of the melt.

常用的外润滑剂是硬脂酸及其盐类；内润滑剂是低分子量的聚合物。有的润滑剂还有其他的功用。实际每一种润滑剂都有可以实现某一要求的作用，总是内外润滑共同作用，只是在某一方面更突出一些。同一种润滑剂在不同的聚合物中或不同的加工条件下会表现出不同的润滑作用，如高温、高压下，内润滑剂会被挤压出来而成为外润滑剂。

The commonly used external lubricants are stearic acid and its salts, and the internal lubricants are low molecular weight polymers. Some lubricants have other functions. Actually, every lubricant has the function of achieving certain requirement, which is always combined with internal and external lubrication, but it is more prominent in some aspect. The same lubricant can show different lubricants in different polymers and different processing conditions, such as high temperature and high pressure, and the internal lubricants will be squeezed out and become external lubricants.