posted June 01, 2004 11:23 AM               

Desired output rate.
Available HP.
Gear ratio.
L/D
Vented?
Head pressure characteristics.
Cooled barrel?
Cooled screw?

These are just some of the considerations which go into a proper screw design.

Tom C

posted June 05, 2004 10:11 AM            

HIPS is a very simple material. It will run on just about any screw, so if you have a machine already, try it, and the results will indicate screw changes better than any theory.

Venting is very common for HIPS. Sometimes people manage without it, but they may then need a conventional (not dehumidifying) dryer. In a humid climate you'll need one and mabe even both.

If I had to pick a design for a 120mm, 32:1 vented line, I'd put the vent at flight 23, put a parallel Maddock section just before the vent and another (helical) one near the end, use 14mm feed depth (a little deep to allow direct scrap reuse) and 4 mm first metering depth, return to 14mm in the 4-turn vent zone, and end up with 7 mm in the 2-turn last zone (after the second Maddock, which is helical to get some pumping action). I once saw a screw with a toothed blister (like a gear) at the end of the first stage which allowed a deeper first-stage metering zone (less heat development) and better venting (more surface exposed) but I don't know anyone who uses them, which means either that they don't work, or they work so well that their users don't want others to know.

If you already have a conventional single-stage extruder, say a 4.5" 24:1 and plan to dry in the hopper, you might use 15mm in and 5mm out.