Screw Wrapping

[tim12871]

One of many problems in the injection molding shop I work in is the screws are getting wrapped. This only occurs with the acrylic material on our new machines that have screws designed for acrylic. I have checked to make sure the feed zone is not over heating and the feed throat is cooling properly set at 100 to 110 f...When the press is shut down all the material will be discharged and I will decompress the screw about 2". I have found out that if the screw is not decompressed it wraps almost every time. Also the material feed lines are pulled at the dryer to avoid wet material at start up and increasing the chance to wrap the screw. Does anyone have any suggestions?

[Suhas]

Hi Tim,
Just to understand clearly, you are saying he acrylic material wraps around the screw - correct?
Suhas

[tim12871]

Yes that is correct.

[Suhas]

Since the plastic is not 'moving' it is either that the plastic is too cold or too hot. Can you experiment with a range of temps and see if that helps?
Regards,
Suhas

[brentb]

Lots of people use cracked cast acrylic to REMOVE "wrapping"!
What is helix angle of the "special" screw?
Have you put micrometers on screw and in barrel to check if fit is in tolerance?
If either or both are worn, you could "wrap"

KOM

brent

[rickbatey]

Usually when I see a screw wrapped, it's from a zone or zones that are too hot. You might want to check the barrel zones to see if you have loose or damaged thermocouples causing the over temp.
I use strips of acrylic sheet to break the wrap off. Cut them about 6mm wide and push them into the feed throat as the screw is run. The chunks broken off help push the wrapped stuff off the screw and since its extrusion grade it melts slower and works its way further up the screw.
Rick.

[tim12871]

Thanks everyone for your time suggestions and remedies. I found the rear zone on one of the machines too hot due to a faulty thermal couple, along with improper shut down. I have implemented a shut down procedure specifically for acrylic.
Thanks everyone..