back end defect - coring

Hello, I'm learning about the problem which called "back end defect" (or "coring" in some places). This problem reduce the yield of the
extrusion process in Aluminium, and I asked myself what is the way to prevent this phenomenon during the extrusion process. I'm sure that other peoples met this problem before me, and I'll be thankful to learn from their experience. Thanks Yaver
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Indirect extrusion can eliminate a lot of this defect; but the Butt size needs to be controlled.
TC

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yaver wrote:

Coring is because of impurities, isn't it? (as I remember it)...
Med venlig hilsen / Best regards Martin Jrgensen
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No coring from extrusion process is not related to impurities. Extrusion core can occur is very pure metals. This is typically called extrusion core and occurs at the end of the extrusion , sometimes as much as 30 % of the length of the extrusion. The question was related to extrusion not casting
Coring as related to casting is usually realted to shrinkage porosity
Tom.

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There is only one way to eliminate "back end defect" in direct extrusion. butt length should get longer.
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