‘Short shot defects’ occasionally appear in injection moulding. In this article, the mechanism by which this defect occurs is explained in a fun way, using the exchange of resin and air as a narrative.
Resin meets air
One day, resin flows into the mould with great force. However, the mould is packed with air. The resin tries to flow while moving the air out of the way.
'It's our path! Get out of the way!’ Resin calls out to the air in a strong voice. Air, however, looks troubled: ‘But... where should we go? and seems to have lost sight of an escape route.
Resin in a hurry and Air unable to escape
Resin, running out of time, is getting more and more frustrated. 'Just retreat as fast as you can, anywhere, whether it's a gap in the PL (parting line) or a nested gap! If it doesn't flow fast, we'll be cooled and hardened in the mould!' He rushes on.
The air rushes to find a way out and tries to escape through the gap, but the resin is moving so fast that it is unlikely that all the air can escape through the narrow gap alone.
Resin rushing in and air approaching its limit
The resin says, ‘Yeah, get out of the way!’ and pushes the air further into the gap. But the air says: ‘I can't do it any more! If it pushes any further, the adiabatic compression will cause it to ignite!’ it cries out.
Operators monitoring the pressure waveform of the molding machine also notice an unusual increase in injection pressure.
Lamentations of the solidified resin
The resin tried desperately to push the air out, but it gradually cooled. The resin finally solidified and stopped flowing, with a frustrated look on the operator's face: ‘Damn... is this it?’
An operator notices a short shot.
An operator who removes a molded product cranes his neck at the incomplete shape left in the product. 'Huh? Is it a short shot? I wonder why?’ and look at the product curiously. In fact, the resin has cooled and hardened in the process because air could not escape, leaving the product unfilled.
Measures to prevent short shots
As this story shows, the cause of short shot defects is ‘insufficient air escape’. If air cannot escape smoothly, the resin flow is obstructed and the product becomes unfilled. The following measures are therefore effective in preventing short shots.
Optimising air vents:
it is important to ensure that air vents are properly positioned in the mold to allow air to escape.
Adjust injection speed:
If the resin injection speed is too fast, air is compressed before it can escape. Adjusting the injection speed appropriately will help air to escape.
Controlling mold temperature:
If the mold is too cold, the resin hardens too quickly, causing short shots. It is also important to carry out molding at the appropriate mold temperature.
Introduction of vacuum degassing equipment: air in the mold can be sucked out in advance using vacuum equipment to minimise disturbance by air.
The world of injection molding is a battle between resin and air. The secret to reducing defects is to create an environment where the two can co-exist in harmony with each other with a little ingenuity!
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