Views: 0 Author: Site Editor Publish Time: 2026-04-09 Origin: Site
On the injection molding shop floor, few things are more frustrating than pulling parts out of the mold only to find they're out of spec, have sink marks on the surface, or are warped like a potato chip. Behind these problems, more often than not, stands one culprit — uneven shrinkage.
Today, we're going to break this down completely: why it happens, and how you can solve it step by step.
All plastics shrink as they cool — that's just physics. But when different areas of the same part shrink by different amounts, trouble follows:
Sink marks (localized depressions): Unattractive appearance, may affect assembly
Warpage (distortion): Parts won't lie flat — often scrap
Dimensional non-conformance: Batch scrap, costs skyrocket
Making things worse, uneven shrinkage often doesn't reveal itself until after the mold opens. By the time you see it, you may have already produced hundreds of bad parts.
This is the structural root cause. Thick sections cool slowly and shrink more; thin sections cool quickly and shrink less. The two fight each other — at best you get sink marks, at worst warpage.
Typical case: A flat plate with reinforcing ribs. The rib base is thicker than the plate, and visible sink marks appear on the surface directly above the ribs.
Pack & hold pressure keeps feeding material into the cavity to compensate for shrinkage. If it's inadequate:
Thick sections and areas far from the gate will be "starved" and shrink excessively
Result: Sink marks or internal voids
The gate determines the melt flow path. If you place the gate in a thin section, the melt has to travel a long distance to fill a thick section — by the time it gets there, the far end has already cooled, pressure doesn't transfer, and shrinkage is high.
The layout, flow rate, and condition (scale, blockages) of your cooling circuits directly affect mold surface temperature distribution. A temperature difference of more than 10°C will cause significant variation in shrinkage rates.
An easily overlooked point: Warpage caused by non-uniform cooling often only becomes apparent after ejection, making it easy to mistake for ejection deformation.
Semi-crystalline materials (PP, PE, PA): Crystallinity is highly dependent on cooling rate. These materials have a wide shrinkage range and are very process-sensitive.
Amorphous materials (ABS, PC, PS): Relatively more stable, but still affected by temperature and pressure.
Melt temperature too high → Overall shrinkage increases, amplifying local differences
Injection speed too fast → Severe molecular orientation leads to anisotropic shrinkage (different shrinkage rates in flow direction vs. cross direction)
Mold temperature too high or too low → Affects crystallization rate and cooling consistency
Defect Appearance | Most Likely Cause |
|---|---|
Sink mark at thick section | Insufficient pack/hold |
Excessive shrinkage far from gate | Poor gate location or insufficient pack/hold |
Overall warpage (saddle shape) | Non-uniform cooling |
Bending along flow direction | Severe molecular orientation |
Use a thermal imager or contact pyrometer to measure the mold surface temperature immediately after opening. Key criterion: Temperature difference across different areas of the same part should be controlled within ±5–10°C. If the difference exceeds this, fix cooling first.
Priority from highest to lowest (easiest/most cost-effective first):
Increase pack/hold pressure and time → Most direct and effective
Reduce melt temperature → Reduces overall shrinkage
Adjust mold temperature → Balances cooling rate
Adjust injection speed → Improves filling pattern
Modify the mold (last resort) → Wall thickness, gate, cooling channels
Problem: A PA66 gear pump housing with an 8mm thick flange and 4mm side walls. Obvious sink marks appeared at the center of the flange, and the part was warped by 0.35mm (requirement ≤0.15mm).
Diagnosis process:
Observation: Sink marks were located in the last area to fill
Mold temperature measurement: Flange area 65°C, side wall area 45°C — a 20°C difference
Solutions:
Added a dedicated cooling circuit for the flange area (original design had only a series circuit)
Increased pack pressure from 60 bar to 85 bar, and pack time from 3s to 5s
Results: Sink marks disappeared, warpage reduced to 0.12mm, and yield increased from 68% to 94%.
If you're still in the product development phase, these points will save you enormous trouble later:
Keep wall thickness as uniform as possible. Where thickness changes are unavoidable, use gradual transitions or radii.
Place gates in thick sections — let the melt fill thick areas first, then flow to thin sections.
Design cooling circuits intelligently. Hot zones (thick sections, near the gate) need dedicated, aggressive cooling.
For semi-crystalline materials, allow extra margin for mold shrinkage adjustment.
When facing an uneven shrinkage problem, check in this order:
Is wall thickness significantly non-uniform?
Is pack/hold pressure or time insufficient?
Is gate location appropriate?
Is mold cooling uniform (temperature difference ≤10°C)?
Is melt temperature too high?
Is the material unusually process-sensitive (e.g., unfilled PP or PA)?
Uneven shrinkage is a systemic problem — it's rarely caused by a single factor. The good news is that in most cases, prioritizing pack/hold and cooling adjustments will solve 80% of the problems.
If process adjustments have limited effect, don't stubbornly keep trying. It may be a mold design issue, and modifying the mold sooner is more cost-effective than producing bad parts indefinitely.
I hope this article helps you avoid some unnecessary trial and error on the shop floor. If you have a specific part or material in mind, feel free to leave a comment below.
