Barrel and screw for rigid PVC.

[graemeian]

I am setting up a system to injection mold small clear rigid PVC parts. My mold is made of stainless steel. I want to order a new screw and barrel for this job. What screw/barrel material is recommended for the corrosive nature of PVC?

[rickbatey]

Ask your machine supplier because you also should need to reduce the compression ratio to alleviate shear burning. One last question: How much of the barrel capacity are you going to be using? If less than say 60%, you are going to be over-heating the resin and it out gassing (VERY NASTY!). So keep a bucket of water close to the press to drop the purges into.....
Rick.

[graemeian]

I am at 60% because I cannot get two shots. What do you mean by compression ratio?
I am getting a barrel manufactured by a third party. I need to know what materials to use for the barrel and screw.
The machine is a Boy 15s and the company only makes a generic barrel.

[rickbatey]

There is a compression ratio(2.5:1) as well as a length to diameter ratio(L/D) rating for all injection units. From my experience years ago when custom molding PVC, we couldn't run the resin in one press as the compression ratio was 2.5:1. That much compression caused shear burning. I think PVC process sheets call out like 1.5 or 2:1 as the max. Remember that machines around this one will rust as well unless you keep a light oil (we used WD-40) on the plattens and tie bars on all machines adjacent. We used an OEM screw package, GP screw, but all of the machines had wear resistant hard coated barrels and screws. This being a custom molding plant they never knew what resin or mold would come through the door.
BR,
Rick.

[graemeian]

Thank you Rick. I am looking at getting a barrel made with a bimetallic layer containing tungsten carbide. The screw will have a nitride finish.
Do you think the nitride layer is sufficient for both screw and barrel?

[rickbatey]

I think your supplier should be able to tell you that much better than I. PVC is corrosive so I don't know if that coating is the best for this problem. Rick.

[processtech]

Hi Rick
Where does the compression ratio come from, is the based on the barrel volume/volume remaining between screw flights?
processtech

[rickbatey]

It comes from depth of feed zone versus metering zone depth. Like length to diameter ratio is expressed as L/D of 22:1.
Rick.