QUICK ANSWER
A sink is a surface depression; an internal void is a cavity inside the part. They can share a local shrinkage problem, but not every internal bubble is a shrinkage void. Establish the defect type with suitable inspection before changing the process, and verify internal quality rather than judging only the outer surface.
Engineering reference, not a material specification or machine-setting recipe. Validate decisions against the released drawing, resin-grade data and equipment instructions. Equipment work must follow the applicable safety procedures.
1. Establish what the part actually contains
Use controlled lighting and a defined surface reference to document a sink. Map it to nearby ribs, bosses, wall intersections and other concentrations of material. Compare the molded part with the CAD model: an unintended depression is different from a designed relief or a texture effect. Note whether the mark develops immediately or becomes more visible after the part leaves the tool.
For an internal indication, choose an approved inspection method appropriate to the part, such as qualified sectioning or X-ray CT. Preserve the location and orientation. A gas bubble, contamination-related defect and shrinkage void can require different corrective actions. Do not infer internal soundness from an acceptable surface, especially where the region carries load, pressure or another critical function.
2. Connect local mass to the packing path
A thick junction does not cool in the same way as its surrounding thin wall. As the interior changes temperature and volume, the exterior may deform inward or an internal cavity may remain, depending on the local material and process conditions. The useful engineering question is not simply whether the part was packed, but whether the affected region could still receive sufficient material while it needed it.
Trace the route from the gate to that region. A thin intervening section or a gate that seals earlier can limit pressure transmission even when the machine continues its holding stage. Cooling and material behavior influence the result as well. These mechanisms explain why increasing hold time indefinitely is not a universal solution, and why a lower visible sink can coexist with an unresolved internal defect.
| Observation | Investigation direction | Evidence needed before a remedy |
|---|---|---|
| Depression opposite a heavy boss | Local mass and shrinkage compensation | Section geometry and the response to a controlled packing study |
| Internal cavity in a thick region | Shrinkage versus gas-related origin | Internal inspection correlated with preparation and process data |
| One distant region remains defective | Pressure transmission and gate or flow-path behavior | Cavity-specific samples and relevant fill/pack evidence |
3. Test the process without judging by appearance alone
First verify material identity, preparation and actual thermal conditions. Investigate moisture, contamination or air-related causes when the observed internal defect supports them. A recipe adjustment cannot restore a material that has already degraded. Then establish a stable baseline, including repeatable cushion and material delivery, before interpreting a hold-pressure or hold-time comparison.
Relate any packing change to gate-seal evidence, mass, dimensions and internal inspection. Keep pressure definitions and units explicit. A mass increase does not automatically prove that the specific thick section has been corrected. Stop at the approved machine, material and tool limits. A process that removes a cosmetic mark by introducing flash, distortion or excessive stress has not solved the engineering problem.
4. Improve geometry and access to the region
Consider coring unnecessary mass and redesigning rib or boss intersections while preserving the required stiffness and load path. A nominally thin rib can still create a heavy junction when its root, fillet and adjacent features overlap. Review actual cross-sections instead of applying one universal rib-to-wall ratio to every resin, finish and cosmetic requirement.
A gate or runner modification may improve packing access, but it also changes weld locations, vestige, flow orientation and pressure history. Review those consequences before cutting steel. Similarly, a local cooling change should address the measured heat-flow problem rather than assume that colder is always better. Check the ability to manufacture, clean and maintain any proposed circuit or insert.
5. Validate external and internal acceptance together
Set an inspection matrix that covers the actual risk. It can include appearance under specified lighting, section or CT evidence in the suspect region, critical dimensions and the relevant mechanical or leak test. State the conditioning and time-after-molding requirements. If sectioning is destructive, make the sampling and disposition rules clear rather than treating those samples as shippable product.
Compare cavity-identified samples at repeatable conditions. Confirm that the improvement persists within the approved process region. Keep design changes, tool work and material changes traceable. This framework does not prescribe a universal maximum wall thickness or a guaranteed pressure setting for eliminating voids.
PUT IT INTO PRACTICE
Engineering review checklist
Use these prompts for a project review. This page does not record or certify completion.
- Differentiate a surface sink from an internal cavity or gas-related indication.
- Map the defect to actual sections, junctions and the gate-to-feature flow path.
- Verify material preparation and stable process behavior before comparison.
- Evaluate packing changes with internal, dimensional and functional evidence.
- Review the side effects of coring, gate changes and local cooling work.
- Approve the result against both visible and hidden quality requirements.
Questions engineers ask
Can more holding time eliminate every sink mark?
No. Holding time cannot provide additional material through a path that no longer transmits it, and excessive time may add no benefit. Confirm the actual gate and local packing behavior.
Does a smooth surface prove there are no voids?
No. Internal inspection or a suitable functional test may be necessary. The required method depends on the component's risks and acceptance specification.
Sources and application limits
This is independently written educational guidance. The linked manufacturer and engineering references support specific mechanisms or further study; they do not endorse PFMOLD or replace project approval. Supplier recommendations apply to their stated grades, tooling and equipment.
- Toray AMILAN — Common injection-molding defects and their countermeasures ↗
The void and sink sections distinguish internal defects from surface collapse. This is a nylon-specific source, not a universal set of processing limits.
- Protolabs — Addressing Thickness Issues in Moulded Parts ↗
Supports reviewing local geometry and unnecessary mass. The supplier's wall-thickness tables are not reproduced or treated as universal design rules.