Quick answer: Before tooling, treat shrinkage as a range of directional, process- and geometry-dependent behavior. Build a risk map from the exact grade, flow path, wall distribution, packing access, cooling, constraint and measurement state; then preserve steel-safe correction for dimensions that cannot be predicted confidently.
Release Decision Matrix
| Risk driver | Pre-tooling question | Planned evidence |
|---|---|---|
| Material | Exact grade, filler, moisture, crystallinity and data condition? | Supplier data plus molded-part correlation |
| Geometry | Which sections shrink or constrain differently? | Wall/section map and directional analysis |
| Process | Can gates pack CTQs before freeze, and can cooling be balanced? | Simulation or T1 weight, pressure and thermal study |
| Tolerance | How is the dimension datumed and conditioned? | Measurement plan and steel-safe map |
Approval Checklist
- Use exact grade, reinforcement, color and conditioned material data.
- Map wall, mass, ribs, bosses, asymmetry and rigid constraints.
- Review flow direction, weld lines, gate freeze and packing access.
- Assess circuit balance, mold temperature and ejection state.
- Classify CTQs by datum, tool feature and process sensitivity.
- Define measurement time, environment, fixture and cavity coverage.
- Preserve realistic steel-safe or replaceable-insert options.
- Plan T1 evidence before committing dimensional correction.
Start With Exact-Grade Evidence
Record resin manufacturer, grade, reinforcement, additives, color and expected conditioning. Review shrinkage values by flow direction, specimen and process when available. A broad polymer-family range is useful for screening but too weak to promise a tight product dimension.
Consider moisture, crystallinity, recycled content and supplier variation. State whether the project has relevant historical molds and where their geometry or process differs from the new part.
Map Geometry and Constraint
Create a wall and thermal-mass map. Mark ribs, bosses, thick pads, long spans, asymmetric sections and rigid assembly interfaces. These zones cool, orient and contract differently. A nominal global scale factor cannot represent every feature.
Identify free shrinkage versus constrained shrinkage. Corners, inserts, actions and opposing walls can store stress or redirect movement into bow and twist. Review the shape the product can tolerate, not only individual coordinate dimensions.
Include Gate and Packing Access
Gate location sets flow direction and how holding pressure reaches the cavity. Regions isolated after a thin section or early gate freeze can show more variable shrinkage. Reinforcement orientation can make dimensions respond differently along and across the fill path.
Compare gate options around CTQs, weld lines and appearance. Plan a gate-freeze and part-weight study at T1 so packing assumptions can be verified before steel changes are requested.
Review Cooling and Ejection State
Uneven mold temperature produces different shrinkage histories on opposite surfaces or cavities. Deep cores, inserts, slides and thick intersections need specific cooling review. Define intended mold temperature and circuit capability before estimating dimensional behavior.
The part may continue moving after ejection. Plan ejection temperature, handling, fixtures and measurement timing. Assembly or packaging should not become an uncontrolled cooling fixture.
Build a Dimensional and Steel-Safe Map
Classify dimensions as tool-controlled, process-sensitive, flexible or assembly-derived. Mark datums, measurement method and conditioning. Review which tool steel controls each dimension and whether correction requires removal, welding, a replaceable insert or product change.
Use steel-safe conditions for high-risk fits where geometry permits. Keep stock and access realistic; steel-safe is a correction strategy, not a support that every dimensional error can be moved in either direction.
Define the T1 Learning Plan
Specify resin, process conditions, cavities, sample timing, free-state and fixture measurements, part weight, pressure and temperature evidence. Measure at a center condition before exploring the practical window. Separate mold-size error from process and conditioning effects.
Assign correction authority and update the shrinkage assumption after evidence. Retain the rationale, actual steel change and final process so future projects can use relevant history without pretending it is universal.
Illustrative Shrinkage Risk Map
Illustrative engineering example—not a claimed CKMOLD customer result: A glass-filled nylon frame has a long datum span parallel to flow and a short latch dimension across flow. The team assigns different directional risk, controls conditioning time, places the latch in a replaceable insert and plans cavity measurements after dry and conditioned states. One scalar shrinkage value is not used for both CTQs.
Supporting Review Resources
Questions Before Approval
Can one published shrinkage value size an entire mold?
It is only a starting input. Direction, geometry, packing, cooling, constraint and conditioning can require different corrections.
Why does glass-filled plastic shrink differently by direction?
Fibers align with flow and constrain contraction differently along and across their orientation.
What is a steel-safe dimension?
It begins with a tool condition that can be adjusted in the expected direction by removing steel after measurement.
When should shrinkage be finalized?
Use pre-tooling evidence for the initial design, then update the working model from controlled T1 and production validation.
Apply the Injection Molding Shrinkage Risk Review Before Tooling review to your released design. Share the function, exact resin, annual volume, CTQs, cosmetic limits and launch timing through the CKMOLD project form. If CAD is relevant, send it directly to jerry@ckmold.com; the form does not require an upload.
Detailed Shrinkage Decisions
Use these focused guides when the project reaches a narrower engineering decision: