Unbalanced Filling and Flow Anomalies
Atypical advance of the flow front, unbalanced filling in multi-cavity molds and core shift. Shear-induced viscosity differences are much larger than generally thought.
Unbalanced filling is when the cavities in a multi-cavity mold fill at different times and to different degrees. Some cavities fill completely while others remain short; in the holding phase the full cavities are then overpacked. The result is parts of different quality coming out of the same mold, and it is not possible to correct them all at once with a single process setting.
How to Recognize It
- In partial fills the flow front advances differently from what is expected; in a multi-cavity mold the cavities fill at different times.
- Secondary weld lines, trapped air or the diesel effect may accompany it.
- In rotationally symmetrical parts the wall thickness becomes thinner on one side because of core shift.
- It can be seen even in naturally balanced (equal-length) runner systems — especially if the main runner and the sub-runner are NOT PERPENDICULAR to each other.
Physical Causes
- CRITICAL CONCEPT: A thermoplastic does NOT have a SINGLE viscosity value at a given temperature. A shear rate profile forms across the flow cross-section; in small cross-sections the viscosity differences can exceed 100 TIMES.
- Low-viscosity layers concentrating in one area of the mold push the flow front partly ahead.
- Insufficient melt temperature, "cold" zones in the hot runner, small flow cross-sections, excessive injection speed and long shear time — all five increase shear.
- Core shift: in parts fed from one side/asymmetrically, partial front advance pushes the core, the wall thickness becomes unbalanced and the effect amplifies itself.
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.
- Reduce the injection speed (lower the shear time and intensity).
- Increase the melt temperature.
- Check the thermal homogeneity of the melt: adapt the AXIAL temperature profile along the metering stroke with the barrel temperature, back pressure and screw speed.
- Measure the temperature homogeneity in the hot runner in the assembled state.
Mold, Design and Material Solutions
- Check the gate and bore cross-sections; make sure they are of sufficient size.
- Use only RIGHT-ANGLED melt deflections; in a cold runner prefer a CIRCULAR (flow-friendly) cross-section and keep the length short.
- Try rotating the torpedo in the hot runner nozzle by 90°.
- For core shift: center and hold the cores in the opposite mold half, design a more stable core, move the injection point.
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 |
|---|
| Short Shot | With a short shot all cavities remain short in the same way; the problem lies in pressure or flowability. With unbalanced filling there is a difference between the cavities. |
| Dimensional and Weight Deviations | Dimensional deviation is variation from shot to shot. With unbalanced filling the difference is within the same shot, between the cavities. |
Frequently asked questions
How can runner imbalance be identified?
A partial fill study is the clearest method: switch off holding, increase the shot volume step by step and observe which cavity fills first. If the order is the same every time, the problem lies in the runner geometry.
How large is the effect of viscosity differences?
Larger than expected. Viscosity can vary markedly between different batches of the same material; moisture content and regrind ratio also affect it. If the filling balance changes from batch to batch, the raw material side should be investigated.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice