Dimensional and Weight Deviations
Dimensions/weight varying from shot to shot are the sign of an unstable process. The melt cushion, non-return valve wear and effective holding time are checked.
Dimensional and weight deviations are when parts coming out of the same mold differ from one another. It is not as important as a single part being defective; the real problem is that the process is not repeatable. Its source is often not in the machine but in the surrounding conditions that feed the machine: raw material moisture, dosing ratio, mold temperature and the condition of the non-return valve.
How to Recognize It
- Dimensions/weight fluctuate from shot to shot or between shifts.
- If the melt cushion fluctuates, mechanical suspicion (non-return valve/barrel wear) comes first.
Physical Causes
- The non-return valve closes at a different speed/amount from shot to shot (wear, low first injection speed, insufficient decompression, short strokes, low viscosity).
- The holding time ends before the gate freezes — the melt flows back in an undefined way.
- The mold wall temperature or cooling time has changed; a batch/color change has not been compensated for.
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- WEAR TEST: set a very long constant holding phase and watch the screw position. If the screw does not stop, the non-return valve/barrel is worn; shift the metering zone and repeat — if it still does not stop, the valve is broken.
- Determine the effective holding time with the weighing method (weight plateau) and set it to that value.
- Keep the cushion in the 2-5 mm band; adapt the metering stroke.
- Increase the first injection speed stage (so the valve closes quickly); set sufficient decompression.
- When parameters change (temperature, time, batch), redetermine the freeze point and the holding level.
Mold, Design and Material Solutions
- Check the shot weight/screw diameter ratio (very short strokes affect the valve adversely).
- Consider a non-return valve with a different operating principle.
- Review the runner dimensioning and mold temperature control.
Important Notes
- ⚠️ Increasing the cooling time raises the processing shrinkage and IMPROVES dimensional accuracy — contrary to intuition.
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Flash | Flash increases the weight but leaves an excess visible only at the parting surface. With a dimensional deviation the part looks clean; only its dimensions do not hold. |
| Warpage | Warpage is a distortion of shape; the weight does not change. With a dimensional deviation the weight fluctuates too. |
Frequently asked questions
Why does the shot weight fluctuate?
The most frequent reason is a leaking non-return valve; as the screw advances, part of the melt slips back and a different amount passes in every shot. A worn screw and barrel create the same picture. The cushion value changing from shot to shot confirms this diagnosis.
How much of the dimensional drift is down to the raw material?
More than expected. Fluctuations in moisture content, regrind ratio and colorant dosing change the viscosity; with the same parameters you get different filling. Gravimetric dosing and proper drying are the invisible foundation of dimensional stability.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice