QUICK ANSWER
Wall thickness affects flow, cooling and the ability to deliver packing pressure. Shrinkage is a material-and-process response influenced by geometry, orientation and measurement conditions. Use the exact grade's data as an input, not a universal scale factor, and verify critical dimensions on stable samples in the specified condition before final steel corrections.
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. Map the whole section, including feature intersections
Start with a thickness map and section views of the current CAD revision. Identify the nominal wall, thin flow restrictions and accumulations where ribs, bosses or corner radii meet. A component with a consistent outer silhouette can still contain a large internal mass. Evaluate the full local section instead of measuring only the visible skin or a single isolated rib.
There is no wall thickness that suits every resin grade, flow length and application. The choice must satisfy structural, impact, appearance and manufacturing requirements together. If a change is needed, compare feasible geometries rather than merely reducing every wall to a handbook minimum. Confirm that assembly features and the intended load path remain functional after any coring or transition is introduced.
2. Read shrinkage data with its test conditions
A molding-shrinkage value describes a result under stated material and test conditions. It is not a promise that every dimension of a complex part will contract by the same percentage. Determine whether the data distinguishes directions, reinforcement and specimen geometry. The exact grade, processing history and the constraints imposed by the forming steel can all be important to the dimensional plan.
Also separate the as-molded condition from later changes. Some materials change dimensions as their physical state or moisture content changes. A dimension accepted soon after ejection may not represent the required assembly condition. The product team should state when and how dimensions are measured; a supplier should not have to guess whether a drawing refers to a dry, conditioned, free or assembled part.
| Input | What to establish | Common interpretation to avoid |
|---|---|---|
| Resin data | Exact grade, test basis and direction | Using a family name as a single shrinkage number |
| Part geometry | Thickness, flow path and constraint | Scaling all features identically without review |
| Inspection plan | Time, environment, datums and fixture | Comparing measurements made in different states |
| Trial results | Cavity identity and a stable process | Correcting steel from one untraceable sample |
3. Know what a scalar compensation calculation assumes
For an explicitly illustrative, isotropic case, define linear shrinkage as s = (cavity dimension minus part dimension) divided by cavity dimension. A 100.00 mm target and an assumed s of 1.00 percent give a cavity dimension of 100.00 / (1 - 0.01), approximately 101.01 mm. The assumed percentage is not assigned to any resin in this guide and is not a tooling recommendation.
This calculation explains a convention, not the complete behavior of a molded component. It assumes one uniform factor and ignores directional effects, local packing differences, constrained features and later conditioning. If the technical team uses another definition or correction convention, state it clearly. Keep the calculation linked to the approved material and dimensional strategy rather than embedding an unexplained scale factor in a CAD file.
4. Review filling, packing and cooling as connected constraints
A local thin section may restrict the flow path to a heavier region. Conversely, leaving a large mass to solve a strength concern can create a difficult cooling and dimensional-control problem. These observations are reasons to investigate the geometry and feed strategy together, not proof that a specific process adjustment will correct the part. Use the gate, cooling and process-window reviews as connected inputs.
Simulation can compare alternatives and expose sensitivities, provided its material data and assumptions are appropriate. Include the actual part and feed geometry where the analysis requires them. A successful fill result does not qualify a tolerance, and a predicted shrinkage pattern should not be converted automatically into a steel correction without reviewing the model assumptions and the required sampling evidence.
5. Make steel corrections from controlled evidence
Before tool release, identify dimensions with significant uncertainty and agree how they will be verified or adjusted. Steel-safe planning can be useful where the correction direction and access are genuinely available. Do not label every feature steel-safe without considering whether material would need to be added, how adjacent geometry is affected, and whether the repair process changes the tool's service requirements.
At trial, retain cavity identity, CAD revision, material information, process condition and measurement timing. Establish repeatability before deciding that a nominal size needs to move. Recheck the feature in its required condition after an approved correction and review neighboring dimensions and assembly performance. The deliverable is a traceable dimensional baseline, not simply a tool that makes one sample fit.
PUT IT INTO PRACTICE
Engineering review checklist
Use these prompts for a project review. This page does not record or certify completion.
- A thickness map covers intersections and transitions.
- The exact resin grade and relevant shrinkage data are recorded.
- Directional behavior is considered where applicable.
- Measurement timing and conditioning are defined.
- Critical correction provisions are physically feasible.
- Cavity and process traceability accompany trial dimensions.
Questions engineers ask
Can a resin datasheet determine the final cavity dimensions?
It supplies important input, but not the full answer for a complex part. Geometry, processing, directional behavior and the inspection condition require a project-specific dimensional plan.
Should steel be corrected as soon as a part is out of tolerance?
First verify the measurement method, part state, cavity identity and process stability. Otherwise a steel correction may compensate for a temporary or incorrectly measured condition.
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.
- Protolabs: Injection Molding Wall Thickness Guidelines ↗
Primary manufacturer background on wall consistency and local material concentrations. Its material-family ranges are not used as universal design limits.
- BASF: Ultramid (PA) Product Brochure — Shrinkage and Aftershrinkage ↗
Primary resin-manufacturer explanation of grade, geometry, direction and process influences. PA-specific discussion is not generalized to every polymer.
- BASF: Ultramid Brochure — Conditioning ↗
Primary illustration of why moisture condition and measurement state must be specified for applicable polyamides; no conditioning recipe is prescribed here.