Screw Slippage: Why Your Vertical Machine Runs Inconsistently

One common reason for unstable production on a vertical machine is screw slippage.

The screw tip assembly

The screw tip assembly has three parts: the screw tip, the check ring and the check ring seat. Together they seal, mix and plasticize the material. Many part quality problems are closely linked to these components, and small issues here can sharply raise scrap rates or even force a shutdown for repair.

What slippage looks like

During recovery, the screw rotates and retracts to convey material for the next shot. Slippage occurs when the screw keeps rotating but stops moving axially. Material may build up at the feed throat instead of being conveyed forward. Slippage tends to degrade the material before injection, reduces part quality (for example short shots) and lengthens the cycle.

Quick checks

• Purge with a small amount of material from the rear of the barrel and check melt temperature. With short residence time, melt temperature will be lower than the barrel setting.
• Inspect molded parts: marbling, black specks or faint streaks show the material is not being properly mixed in the barrel.

Fixes

• Gradually raise the feed-zone temperature until screw rotation and retraction are synchronized. Sometimes the barrel must be set above the recommended temperature to reach this point.
• Check back pressure. If it is set too high, the screw cannot generate enough forward melt pressure to overcome it and will rotate in place without retracting — adding work and heat to the melt and hurting part quality and cycle time. Back pressure is adjusted with the control valve on the injection unit.