Castool Community

1. There is plenty of strong evidence from metal flow studies (even the old plasticine experiments and then the work of Valberg and others in the 90’s) that clearly illustrates the frictional effect of the container wall during direct extrusion.  Reducing the container temperature with reference to the billet temperature will only enhance that effect.
2. The frictional effect helps collect billet skin and enriched surface layers (plus contaminants) and allow this material to accumulate into the rear of the billet and be discarded in the butt.  Admittedly some of this accumulated garbage will flow into the extrusion interior – but for many commodity 6063 type extrusions where the emphasis is more on surface finish, this is quite acceptable.
3. Recent studies show that not all the billet skin flows backward with typical container/billet temperature differences of around 70F, but some actually seeps forward and cause some surface finish issues on the extrusion.  Maintaining this delta T or higher minimizes the risk of forward flow of billet skin and helps maintain an acceptable surface finish. (I very much doubt forward flow can be totally controlled and avoided – there are too many other secondary factors at work also – edge distance of die aperture to container, press alignment, butt shear, dummy block performance etc.).
4. An additional benefit using a colder container liner compared to the incoming billet temperature, relates to partial chilling of the billet, offsetting some of the extrusion deformation temperature gain towards the end of the extrude cycle.  This effectively compliments the benefits of a taper heated billet (colder back end) and thus helps achieve isothermal extrusion conditions and maximize extrusion speed.
5. With indirect extrusion the situation is quite different.  First there is no container/billet friction and billet surface has a much higher tendency to flow forward.  Heat loss into the closure block at the back of the container, typically means the container temperature has to be controlled – at least at the back end – ideally higher than the incoming billet temperature, or at least as close to as possible.  This allows the extruder to optimize indirect extrusion productivity by maintaining maximum extrusion speed throughout the entire live cycle.

Therefore, with direct extrusion the container should be designed to run around 70F below the billet temperature. Less delta T for longer live cycle times (e.g. hard alloy extrusion).  Using a QR container with the front zones closer to the incoming billet temperature (say only 20F below) and the back zones approx 70-100F colder than the front is clearly advantageous.

For indirect extrusion, target the container temperature to be close to the billet temperature, with the back end zones hotter by around 70F if at all possible (QR container technology can do this)..