When processing thermoplastics in an injection molding machine, it is important to maintain a solid cushion, no matter what resin you are processing. If you are not maintaining a cushion and it seems to drift high and then low, there are many reasons that can cause this effect. Sometimes, there is not enough back pressure to ensure the same amount of material is in each shot. If this is the case, then simply increase the back pressure to ensure a shot to shot consistency. Or, the screw, barrel and check ring could be worn or damaged.
Here is a safe way to check your machines without having a lot of downtime:
- If you have a tool shop, you can have a small aluminum piece made on a lathe that can fit the radius of the nozzle tip
- Insert this formed piece into the nozzle tip and bring the injection unit forward until it is snugly up against the sprue bushing of the tool
- Charge a shot in manual and then try to shoot the resin
- If the screw tends to drift to 0, then you have an issue with the screw/check ring and or barrel
- If you cannot make an aluminum slug, then you can try to use a piece of wood in the same manner
Good luck and happy processing!
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