Gross Melt Fracture in Extrusion
Because there are no universally accepted terms describing the various types of extrudate distortion, each authormust define the terms he intends to use. In this chapter, grossmelt fracture (GMF)will be used to describe a pronounced, chaotic distortion of extrudate that arises from a severe instability of the flow at the entrance to a converging die. A distorted extrudate of this type is shown in Fig. 7.1. This phenomenon aﬀects the entire cross section of the extrudate and is therefore sometimes called volume melt fracture to distinguish it from sharkskin, which aﬀects only the surface of the extrudate and is often called surface melt fracture. However, volume melt fracture also includes the helical extrudate that is caused by a swirling motion of the melt as it enters the converging section of the die. Although this phenomenon does involve the entire cross section of themelt, there is no actual fracture, but it was decided to include it in this chapter anyway.