Shrinkage compensation is the planned difference between the mold cavity and the required molded-part dimension. It is essential, but it is not one universal scale factor. Material grade, flow direction, packing, cooling, geometry, fiber orientation and measurement condition can produce different shrinkage in different areas of the same part.
A robust approach uses supplier data and prior knowledge for the initial tool, preserves steel-safe correction where practical, and updates the tool only after controlled T1 evidence distinguishes a process shift from a cavity condition.
Why Nominal Shrinkage Is Not a Complete Mold Design Rule
A simple screening calculation is:
Cavity dimension ≈ target part dimension ÷ (1 − assumed linear shrinkage).
If a 100.00 mm target dimension uses an illustrative 1.0% assumption, the screening cavity dimension is about 101.01 mm. This example only explains the arithmetic. It is not a recommended value for any material or part.
The formula becomes unreliable when one factor is applied to the entire model. Real parts may shrink differently along and across flow, near thick and thin zones, around ribs or bosses, and between cavities. Reinforced materials can add strong directionality.
Inputs Needed Before the First Tool Design
| Input | What to define | Risk if missing |
|---|---|---|
| Material | Manufacturer, exact grade, reinforcement, color/additives and approved alternatives | Generic polymer data may not represent the production grade |
| Product requirements | Critical dimensions, datums, fit, function, flatness and cosmetic zones | The tool may be optimized around dimensions that do not control product performance |
| Measurement state | Part age, conditioning, temperature, humidity, fixture and method | Different measurement states can produce conflicting correction decisions |
| Process concept | Gate, fill direction, packing, mold temperature, cooling and expected cycle | Shrinkage assumptions become disconnected from the intended process |
| Correction strategy | Steel-safe directions, replaceable inserts and features difficult to change | T1 correction may require welding or major remachining |
How Gate, Packing and Cooling Influence Dimensions
The gate controls the fill path and influences molecular or fiber orientation. Packing can compensate for volumetric contraction while the gate remains effective. Local cooling changes when and how the material solidifies. These effects interact, so a dimension may move because of process conditions even when the cavity is correct.
Before changing steel, verify that the trial used the intended resin, conditioning, machine, mold temperature and a stable process window. Record the actual settings rather than relying on machine “recipes” without context.
Measure T1 Parts for a Correction Decision
- Stabilize the process and document material, lot, drying and machine.
- Identify samples by cavity and molding sequence.
- Condition and measure at the agreed part age and environment.
- Confirm the measurement system is suitable for the tolerance and geometry.
- Compare mean, spread and cavity-to-cavity pattern—not one favorable sample.
- Map dimensional direction to flow, cooling and tool steel conditions.
- Approve a documented correction with affected features and revalidation plan.
Distinguish Process Problems From Steel Problems
| Observed result | Check before cutting steel |
|---|---|
| All cavities move together | Resin, moisture, process center, mold temperature, cooling supply and measurement state |
| One cavity is different | Local cavity dimension, gate, vent, cooling circuit, insert contact and ejection |
| Flow and cross-flow differ | Orientation, reinforced-grade behavior and gate location |
| Mean is near target but spread is wide | Process stability, gate seal, measurement repeatability and cavity variation |
| Dimensions drift with time | Conditioning, moisture uptake, residual stress and the specified measurement time |
Plan Steel-Safe Adjustment
“Steel-safe” means the initial condition permits a likely correction by removing steel rather than adding it. It should be applied selectively because the safe direction depends on the feature and tooling action. An outside dimension, inside dimension, shutoff, texture, draft surface or assembly relationship may require a different strategy.
Document the nominal target, initial cavity condition, adjustment allowance, features affected by the cut and remeasurement plan. For the detailed post-T1 workflow, see Steel-Safe Shrinkage Corrections After T1.
What the Standards Support—and Do Not Support
The public page for ASTM D955-21 describes a test method for measuring thermoplastic shrinkage under specified molding conditions. It states that standard specimens cannot predict absolute shrinkage in actual parts with different flow paths, wall thicknesses, gradients and process conditions.
The official public overview of ISO 20457:2026 describes a plastics-specific framework for dimensional and geometrical tolerances and acceptance conditions. These references support disciplined testing and realistic tolerancing; neither provides a universal shrinkage number for a production part.
Shrinkage Compensation Approval Checklist
- Exact production material and allowed substitutions are controlled.
- Critical dimensions, datums and measurement state are agreed.
- Flow direction, gate, packing and cooling assumptions are reviewed.
- T1 samples are cavity-labelled and measured at the defined time.
- The process is stable before steel correction is approved.
- Directional and local behavior are considered instead of one global factor.
- The correction drawing identifies steel-safe direction and adjacent risks.
- The next trial has a written revalidation plan.
For material-specific planning, review the material selection guide. For a project review, complete the requirements form and send CAD separately to jerry@ckmold.com.
Reference Scope
- ASTM D955-21—Standard Test Method of Measuring Shrinkage from Mold Dimensions of Thermoplastics (public scope/significance).
- ISO 20457:2026—Plastics moulded parts—Tolerances and acceptance conditions (official public overview).
This article does not claim access to paid standard text and does not imply that CKMOLD is certified to these documents.