Billet Extrusion: How to Prevent Coring at the Back End?

[yaver]

Hello,
I'm learning about the phenomenon of coring (back end defect) during the process of aluminium extrusion. I try to find a solution to this problem.
Is there anyone who know how can I prevent or even reduce this problem from the extrusion process
thanks
Yaver

 

[PerennialII]

Do you mean a some sort of a limit forming problem ?

 

[Astronuc]

Coring (or coning or piping) is usually a front end problem in the extrusion of round barstock from round billets, and it is usually a problem with softer metals. I am somewhat familiar with this problem because it resulted in some defective barstock which led to product failures in the field.

The problems arises because there is a huge gradient in the hydrostatic and shear stresses at the front face of billet that is extruded through a die, and perhaps increases as the billet diameter increases in comparison to the die hole diameter. Perhaps a similar problem exists at the back end.

Also die lubrication might be an issue.

Adding a disk of softer material (Al) between the billet and mandrel might help.

But then one needs to cut off the Al portion at the back end.