But who checks the check? As in check valve

Published On: April 28th, 2014|Categories: Blog|

As you learn about the injection molding process, you hear the screw amasses molten plastic at the front of the barrel, and then pushes the plastic into the mold under force. How does the screw always move plastic forward when the screw movements include both forward and backward motion? Answer, the Screw tip/check ring assembly. (Sometimes the check ring is called the non-return valve.).

Screw tip/check ring assembly. (Sometimes the check ring is called the non-return valve.).

Screw tip/check ring/thread image

Screwed onto the end of the Screw shaft, this mechanical assembly allows the melted plastic to both amass at the front of the barrel and then get pushed into the mold.

barrel screw

barrell screw: screw tip & non-return vavle, metering zone, compression zone, metering zone, screw root

As the barrel screw turns, the flutes of the screw shuttle molten plastic forward and the plastic pushes the check ring forward. The plastic can flow between the ring and the shaft through the channels of the screw tip into the open barrel at the end. When the “shot” occurs, the screw is pushed forward. The plastic pushes against the check ring and it slides back to seal the path and now acting like a plunger, the plastic is forced forward and into the mold. (See below)

amassing shot, shooting into mold

Amassing shot: check ring foward, screw travel left direction Shooting into mold: check rign back, screw travel right direction

If the screw tip/check ring assembly is not part of the molder’s preventive maintenance activity, variations in molding may be attributed to shot size/volume. This would be due to excessive wear and the check ring not sealing properly. Processing composites that are more abrasive can cause accelerated wear. Also, processing parameters, like not allowing pressures to mitigate (upwards of 5-10,000 psi) prior to rotating the screw can increase wear.

The more you understand the injection molding process, the better a conversation you can have with your molder, or the better you can assess potential molders.


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