A dimple on a cosmetic cover and an internal cavity in a load-bearing section can both be consequences of local shrinkage, but they do not present the same acceptance risk or demand the same remedy. When injection molding wall thickness changes at a rib, boss, corner or junction, the thicker mass cools differently from the adjacent wall. The practical buyer question is not simply whether a mark is visible; it is whether the observation is cosmetic, functional, repeatable, limited to a flow-path location, or tied to a change in resin, colour, cavity or process window.
Before approving a tool modification, ask for evidence that distinguishes a geometry issue from a packing, gate-freeze, machine or cooling issue. BASF describes sink marks as surface indentations that occur mainly where a wall section increases, while its troubleshooting guide describes voids as hollow spaces that normally occur in the molten centre and in thick sections.[1] Those descriptions give a useful starting hypothesis, not a substitute for reviewing the actual part, material grade and molded samples.
Sink marks and voids can share a shrinkage mechanism; whether the outer skin yields or has already solidified changes the visible outcome.[1][2]
01
Classify the evidence before naming the cause
Start by locating the condition on the drawing and on actual parts. Record cavity, resin lot, colour, setting revision and image orientation. For a sink, compare depth, gloss and position against nearby ribs, bosses and section transitions. For a suspected void, agree an inspection route appropriate to the part: sectioning can reveal an internal cavity, while CT or another validated method may suit parts that cannot be cut. Do not treat every bubble-like indication as a shrinkage void; gas-related phenomena need their own evidence trail.[1]
A short-run comparison is more useful than a broad request to “fix the sink.” Keep resin, colourant, tool, sample condition and inspection method constant where possible, then compare defined settings. Mark whether the condition occurs in every cavity or only one, near or far from the gate, and before or after ejection. This stops a cosmetic observation becoming an unsupported conclusion about the tool.
02
Read the part as a local shrinkage problem
Thickness is rarely a single drawing number. A nominal wall may look consistent until a rib root, boss base, locating feature or intersection concentrates mass. Covestro notes that material characteristics, temperatures and pressures, part stiffness and wall thickness all influence shrinkage.[2] Ask for a section map showing nominal walls, local maximum sections, radii, cored regions and flow direction; a CAD view may hide the mass that matters.
For injection molding ribs, review root thickness, root radius, intersections, spacing, the opposite show surface and whether coring or a gusset can deliver stiffness with less mass. Celanese’s Celcon POM guide recommends a rib no thicker than half the adjacent wall to reduce sinks, voids and distortion, but it is a resin-family guide, not a universal release criterion.[3] Ask what functional purpose the extra material serves before accepting a thicker rib or steel change.
03
Check packing and gate freeze before cutting steel
Packing can compensate shrinkage only while material can enter through the gate. BASF recommends incrementally increasing holding time and observing when part weight or cavity pressure stops changing; it links inadequate compensation during cooling to both sink marks and voids.[1] Request the switchover position, hold-pressure profile, hold time, cushion, part weight and, where available, cavity-pressure trace. Judge the entire response: a setting that improves one sink may create flash, ejection difficulty or stress elsewhere.
Injection molding gate design is a pressure-transfer review, not an automatic mandate to relocate the gate. Gate size, gate land, runner restriction, flow length, local wall section and gate freeze can all affect packing. BASF advises gating into the greatest wall section where possible when thin-wall gating causes sink marks.[1] Application depends on appearance zones, weld lines, fibre orientation, warpage, ejection and tool layout. Check simulation options against representative samples and the agreed acceptance method.
04
Use trials to decide whether the remedy is process or geometry
A buyer need not prescribe settings, but should require a trial that answers a defined question. Establish a baseline with the nominated grade and stable process; then change one controlled variable or planned design feature at a time. If a repeatable packing or hold-time study changes the condition without adverse effects, a permanent tool change may be premature. If it persists across a validated process window and is concentrated at a thick mass, a geometry review is more credible. Resin behaviour and functional requirements remain part-specific.[1][2]
When geometry is the likely driver, compare options that preserve function: coring a boss, thinning and radiusing a rib root, adding a gusset, smoothing a transition, revising a pad or changing a gate concept. Review effects on strength, assembly, sealing, texture, tolerance, mouldability and maintenance. Release the selected option under normal drawing and sample-approval control; a guideline or one optimized trial does not prove identical behaviour in every cavity.
| Observed pattern | Working hypothesis to test | Evidence a buyer can request | Tool-change decision |
|---|---|---|---|
| Shallow depression opposite a rib or boss | Local mass is shrinking after the surface skin forms. | Section map, images under repeatable lighting, baseline and packing-study samples. | Review rib or boss geometry only after the controlled process comparison. |
| Internal cavity in a thick area | Local shrinkage was not adequately compensated, or the indication is not a shrinkage void. | Agreed inspection result, section location, material and process traceability. | Confirm the mechanism before altering steel or gate layout. |
| Condition worsens far from the gate | Pressure transfer or premature gate freeze may be limiting local packing. | Part-weight or cavity-pressure study, gate/runner drawing and sample map. | Compare a validated process adjustment and gate concept before committing. |
| One cavity behaves differently | A cavity-specific thermal, venting, gate or mechanical condition may be present. | Cavity-coded samples, maintenance history and cooling/setting records. | Investigate the cavity condition before changing the common part design. |
05
Turn the review into an approval record
The decision record should state what is acceptable and why. Define the face or internal region, inspection method, sample condition, lighting or section location, and decision owner. A controlled appearance limit may suit a cosmetic sink. A suspected void in a structural, pressure-retaining or sealing region may need separate functional evaluation. Do not make an internal indication a generic pass/fail rule without its location, size, service load and product requirements.
Link the sample set to the CAD and tool revision, material grade and colour, process study, inspection results and disposition. If the team elects a tool change, record the mechanism it is intended to address and the post-change comparison. If it keeps the tool unchanged, record the validated process window and acceptance basis. This turns a defect discussion into a controlled purchasing decision.
Practical takeaway
Before you move
to the next review.
- Map each mark or suspected internal cavity to a drawing location, cavity identifier, sample condition and visible-face requirement.
- Request a local wall-section and flow-path review that includes ribs, bosses, radii, transitions and cored areas.
- Review a controlled packing and hold-time study with part weight or cavity-pressure evidence where that measurement is available.
- Agree the inspection method and acceptance basis before interpreting an internal indication as a functional defect.
- Approve a steel, gate or geometry change only with its documented trade-offs, revision control and post-change verification plan.
FAQ
Common questions
Are sink marks and voids the same defect?
No. They can arise from related local shrinkage and inadequate packing, but a sink is a surface depression while a void is an internal hollow space. The location, inspection method and acceptance consequence can therefore differ.[1][2]
Should a buyer always ask for a gate change when a sink is far from the gate?
No. Distance from the gate is a reason to investigate pressure transfer and gate-freeze behaviour, not proof that a gate move is the best answer. Review the controlled process study, local thickness, appearance constraints and effects on the rest of the part before selecting a remedy.[1]
Can a rib-thickness rule be used as an automatic approval criterion?
No. Material-maker design guides can provide useful starting points, but the appropriate proportion depends on resin, local geometry, function, appearance requirement and process. Use the guide alongside a part-specific review and representative molded samples.[3]
References
Source material
- [1] BASF, “Injection Molding Problems in Engineering Thermoplastics: Causes and Solutions.”
- [2] Covestro, “The Fundamentals of Shrinkage in Thermoplastics.”
- [3] Celanese, “Celcon® Acetal Copolymer Design Guide.”
This guide is general information for early procurement and engineering discussion. It does not replace product-specific DFM, qualification, validation, regulatory assessment or legal advice.