High injection pressure is a symptom, not a diagnosis. The required pressure reflects resistance through the machine nozzle, runner, gate and cavity plus the pressure needed to pack the part. When pressure rises or reaches the machine limit, the correct response is to locate where resistance or demand changed before altering multiple settings.
This article presents a diagnostic method. It is not a report of a named or anonymous customer project.
Separate Fill Pressure From Pack Pressure
First determine whether the pressure problem occurs before or after velocity-to-pressure transfer. A rise during filling points toward material condition, temperature, nozzle restriction, runner or gate resistance, cavity geometry, venting or machine response. A rise during packing may be connected to an early-freezing gate, excessive pack demand, overfilled regions or a transfer problem.
Confirm the Evidence Before Changing the Process
- Verify the resin manufacturer, grade, lot, colorant and permitted regrind.
- Check drying or conditioning records when the material requires them.
- Record actual melt and mold temperatures with appropriate methods.
- Review fill time, transfer position, cushion, peak pressure and part weight.
- Inspect the nozzle, sprue, runner, gate and vents for restriction or contamination.
- Compare the machine’s available injection pressure, flow and clamp capacity with the mold demand.
Use a Short-Shot Sequence to Locate Resistance
A controlled series of short shots can show how the flow front moves, whether one cavity or region leads, where hesitation occurs and where air may be trapped. Keep material preparation and relevant conditions consistent. Do not interpret the last unfilled area automatically as the cause; it may simply be the end of an upstream imbalance.
| Observed pattern | Possible cause to test | Next evidence |
|---|---|---|
| Pressure rises before the gate fills | Nozzle, sprue or runner restriction | Inspect dimensions, temperature and obstruction |
| One cavity consistently lags | Runner or gate imbalance | Compare fill pattern, part weight and cavity pressure if available |
| Burning near end of fill | Trapped or compressed gas | Map air trap, inspect vents and review fill speed |
| Part fills but pressure spikes in pack | Late transfer or excessive pack demand | Check transfer basis, gate seal and part weight response |
Check Mold and Part Geometry
Thin regions, abrupt thickness transitions, long flow lengths, undersized gates and runners, sharp turns, deep ribs and multiple flow splits can increase resistance. A process change may create a temporary improvement without resolving the geometry. Use DFM and flow analysis when the pressure pattern is geometry-dependent.
Do Not Trade One Defect for Another
Higher melt temperature or faster filling may reduce pressure in some conditions, but can also affect degradation, shear, flash, burning, orientation and cycle time. More pack pressure may improve one sink area while increasing stress, flash or ejection load elsewhere. Stay within resin, machine and tool limits and change one justified factor at a time.
Illustrative Diagnostic Example
This example is hypothetical. A multi-cavity part begins reaching the pressure limit after a material-color change. The team freezes the known-good mold and machine setup, verifies dryer and material records, weighs each cavity, and runs a short-shot sequence. The imbalance appears before the cavities rather than at the end of fill. Inspection finds a partial restriction in one runner branch. The lesson is not that every pressure problem is a blocked runner; it is that evidence can locate the stage of the system that changed.
Release Criteria After a Correction
Confirm that the part meets dimensional, appearance and functional requirements across cavities and that the selected settings have operating margin. Record the root cause, corrective action, approved setup and maintenance implication. If steel or gate geometry changed, repeat the measurements and tests affected by that change.
Questions About High-Pressure Molding
Does a high-pressure alarm mean the machine is too small?
Not necessarily. Machine capability is one possibility, but restrictions, temperature, material, transfer and mold conditions should be checked before that conclusion.
Can increasing melt temperature solve the problem?
It can reduce viscosity within the grade’s processing limits, but it may not address a blocked flow path or poor geometry and can introduce degradation or cycle-time risks.
When should the gate be enlarged?
Only after evidence shows the gate is the limiting restriction and the effect on vestige, packing, freeze time, shear and appearance has been reviewed.
If a mold is pressure-limited or unstable, provide fill patterns, defect photographs, resin information, process records and the mold concept through CKMOLD’s engineering contact. For a new tool, review the mold design process before steel cutting.