Dimensional Stability

Dimensional stability problems occur when a part is inconsistent in part size or in part weig from shot to shor. Parts that are underweight can suffer in structural integrity; overweight parts can also suffer from structural integrity due to molded-in stresses and increased part costs. List causes and solutions for dimensional stability of workpieces

Causes:

1. When injection pressure transfers on time or hydraulic pressure, workpiece weight and size can vary since any change in hydraulic pressure can cause too little material to fill one shot and too much material the next. Transfer due to screw position is found to provide improved object weight and dimensional stability

2. Undersized nozzle or sprue size can restrict the amount of material that enters through the gate and into the cavity

3. Too short a closed mold time will cause the part to underpack and shrink more

4. High mold temperatures can result in too small parts. This can be noticed even more in semicrystalline materials such as polyethylene and polypropylene

Solutions:

1. (1)The screw position can be set to fill the cavity about 95%, and then the remaining 5% can be filled on hold pressure.

2. (2) The pack pressure is increased to fill the cavity with sufficient material, the injection forward time is increased, and the fill time to force more material into the cavity faster is also increased.

3. (3)The cushion is examined visually to verify if it is consistent. If variations can be seen, adjustments to shot size and back pressure can be made to improve cushion consistency.

4. (4)A won check ring can also cause cushion variation. This can be noticed by observing the screw position after moving forward. If the screw shows some rotation after the screw has moved fully forward at the end of its stroke, this indicates a won check ring. Replace the check ring if this condition is seen.

5. (5)Increasing melt temperature within the recommended temperature range will allow the melt to pack out the cavity more than at lower melt temperatures, resulting in increased part size