Lines in extruded product

Lines can be caused by the die, breaker plate, extruder screw, die lip build-up, and downstream equipment such as calibrators, cooling baths, catapullers, etc. If the line is visible right at the die exit, it must be formed at the die exit (e.g., by die drool), in the die (poor internal surface conditions in the die or build-up in the die), or upstream (breaker plate, screen pack, screw). A single line is often formed in a 

In blown film extrusion, the number of lines in the film frequently corresponds to the number of ports in the die. As a result, these lines are often referred to as port lines. These port lines are obviously related to the die design but they are also very dependent on the melt flow characteristics of the polymer. High molecular weight polymers have very long relaxation times and are more likely to exhibit port lines or other types of die lines. 

Lines can also originate downstream of the die, for instance in the calibrator. Lines can result from an object touching the extrudate or local cold or hot sections. These problems can generally be diagnosed easily by observing at what point in the production line the surface line originates. 

The severity of the weld line problem will be determined by  

The length of time from the point where the melt streams recombined to the exit of the die (residence time) 

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