Why Screws Break — and How to Prevent It

1. Screw material and temperature

Temperature plays a small role; most screw failures come from screws with poor material toughness or screws that have simply worn out and need replacing.

Control temperatures carefully. At start-up, always run screw decompression (suck-back) before injecting. Without decompression, the screw tip is still wrapped in material; purging or injecting directly can snap the screw tip or crack the check ring.

2. Abrasive materials

Glass-filled (GF) materials are hard on screws and machines. They flow poorly and need high injection speed. If not handled well, they can crack the screw tip and check ring, or even break the screw into several pieces.

3. Back pressure

Excessive back pressure increases the load on the screw during recovery and can also cause it to break.

Prevention

• When running GF materials, if the machine is stopped for more than 3 minutes, purge the barrel before restarting. PP, which is relatively tacky, works well for cleaning the screw and preventing degraded material from coating the screw and tip.
• Purge at reduced screw speed so the material melts fully as it rotates.
• Whatever the material, when the machine is shut down, purge all remaining material from the barrel so it is ready for the next run.