Extruder Cross Head Die Design, Part I of III

The next three months extrusion technology articles will focus on Extruder Cross Head Design. This
month, we will review the most important aspects of a good cross head design.

'Choke-ring with manifold'

An essential geometric component of a well designed cross head, is the 'choke'. This feature with
preceeding manifold is an excellent device to achieve melt pressure homogenuity, circumferencially. Melt
pressure is free to equalize in the manifold region. Concentricity of the choke is very important, especially
if the choke annulus is narrow, since small differences in annular gap produce large flow variations around
the annular gap. (Flow rate is proportional to the third power of the choke gap.) Another function of the
choke is to provide back pressure, which helps illiminate weld lines, reduces output fluctuatiuon and
allows the screw to adequatly plasticate resin in the barrel.

Diagram: "Pear" Type Flow Diverter for Extruder Cross Head

Flow Diverters
Prior to the choke ring, flow must be diverted from the near side to the far side of the cross-head.
Geometries used to achieve this are generically called flow diverters. They must provide equal pressure
circumferencially and uniform circumferencial heat history

There are numerous types including the "Pear " , "Multi channel", and "Spiral " flow diverter.

In general the more complex flow diverter, the more chance of stagnant regions, the more difficult to
clean, the less easy to duplicate and the more expensive to machine. Simple is better as long as it achieves
circumferencial melt and pressure homogenuity.

Heating System
After the choke and flow diverter geometry, the next most important aspect is the heating system. In
general, a cylinderical cross head is best, fully encased in a cylinderical heating band. A bad heating
system will cause continuously changing extrudate concentricity as the resin flows to the higher
temperature path of least resistance.

Right size for the job
Finally, one should consider Cross head size, or volume. Resins prone to degradation such as P.V.C. and
polyurethane are best suited to a low volume head, since residence time in the head is undesirable.