Die flow problems

Die flow problems typically result in appearance problems. These can be related to melt fracture, die lip bulld-up, gels, v- or w-patterns, specks, color variation, lines, and change in optical properties (e.g., transparency, mattness, gloss, haze). 

Melt fracture was discussed in Section 7.5.3.2. It manifests itself as extrudate surface roughness, shark skin, orange peel, and other distortions. Melt fracture can be reduced or eliminated by  

Streamlining the die flow channel is always a good idea but it will increase the cost of a die. For a high-volume product, it generally makes sense to design and manufacture a fully streamlined die. For a small-volume product, it may not make economic sense to design and manufacture a fully streamlined die. 

Reducing the shear stress in the land region can be done by  

Several polymer processing aids (PPA) are available to eliminate or reduce melt fracture. An effective method to eliminate melt fracture in high molecular weight polyolefins is to add a small amount of fluoroelastomer [153], about 500 to 1000 ppm (parts per million). When a fluoroelastomer PPA is added to a polyolefin it usually takes a certain amount of time for a critical coating of fluoropolymer to form on the die. This conditioning time can vary from 5 minutes to more than 1 hour [154]. 

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