Understanding Jetting…

I was asked this question the other day when discussing molding defects…

What actually causes Jetting?

My Response
This defect occurs when the polymer shoots into the mold and does not create a smooth laminar, or fountain, flow as it first enters the mold cavity. A laminar flow occurs when the polymer enters the mold cavity from the center, spreads out to the edges of the mold surface to allow more material to enter through the center. In the case of jetting, the polymer streams into the mold cavity and shoots forward like a cylinder of polymer with the first polymer to enter the mold continuing towards the opposite wall rather than spread to the nearest wall. 
Additional Thoughts
Jetting, non-laminar flow, can be caused by one of three major factors:
Low Material Temperature
High 1st Stage Injection
Poor Mold Design


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