Dimensional Springback in Interior Panels: Reading Shrinkage After Demould

Interior panels keep moving after they leave the tool; set a conditioning time before measurement so the springback is captured rather than argued over.

What the failure looks like on the line

Interior transport panels see temperature swings and long fixtures, so a shape that is acceptable off the tool can move after installation.

Panels measured straight off the tool pass, then fail the same check a day later as edges pull in and the fixing line shifts.

The formed sheet leaves the tool above ambient temperature and continues to contract as it cools, while frozen-in orientation relaxes more slowly. Measurement taken before that contraction finishes records a part that no longer exists on the vehicle or assembly.

The useful question is not whether the part conforms, but which condition was never written into the specification. On a buyer's specification this distinction matters: a part can measure inside tolerance and still fail in use, because the drawing records geometry while the complaint records behaviour. Conditioning time before measurement is usually the first value both sides compare, and it is also the one most often quoted without the conditions attached to it.

The process window that decides it

Four variables move this result more than the rest. The ranges below are illustrative engineering guidance for a first discussion; confirm them on your own tool, material grade and part geometry before they become contract terms.

Table 1. Starting window for a joint process trial. Values are engineering guidance, not a measured survey.

Conditioning time before measurement is typically held at 24 h at 20-23 C. Measuring earlier understates the final shrinkage Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Demould part temperature is typically held at Below 50-60 C. Hot release increases the amount of post-demould movement When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Mould shrinkage allowance is typically held at 0.4-0.8 % for amorphous sheet. A wrong allowance shifts every fixing point off datum A change here is cheap during a trial and expensive after tooling release, so treat it as a controlled variable with a documented re-approval.

Cooling fixture hold is typically held at Until uniformly below 50 C. Releasing to free cooling lets the panel distort unconstrained Put it on the change-control list, otherwise an unannounced material or setting substitution will be mistaken for ordinary process noise.

Reading the response curve

The left panel shows cumulative dimensional change (mm/metre) as conditioning time after demould (h) increases: the response rises from 4 to 24 on the illustrative 0–100 scale. The dashed line at 20 marks a proposed acceptance limit, not a universal standard. Move that line only after the test method and the sample plan have been agreed, otherwise two suppliers can both be compliant and still deliver different parts.

Read the shape rather than a single point. A curve that changes quickly near the limit is more sensitive to ordinary process noise than one that approaches it gradually, and that difference should be reflected in how tightly the setting is controlled and how often it is checked.

Where the causes actually sit

The right panel of Figure 1 splits audited failures of this type into five groups. The percentages are illustrative engineering guidance for prioritising an investigation, not a measured survey and not a prediction for your line.

Table 2. Cause split and the corresponding first check. Shares are indicative and sum to 100.

Measured before conditioning accounts for roughly 32% of the illustrative split. Record the demould temperature and the time each part leaves the cooling fixture. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Released too hot accounts for roughly 25% of the illustrative split. Measure the same datums immediately after release and again after the agreed conditioning period. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Shrinkage allowance wrong accounts for roughly 19% of the illustrative split. Log the ambient temperature at both measurement events. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Checks before sign-off

Work through these in order; measuring a later variable before an earlier one hides the cause rather than finding it.

1. Record the demould temperature and the time each part leaves the cooling fixture.
2. Measure the same datums immediately after release and again after the agreed conditioning period.
3. Log the ambient temperature at both measurement events.
4. Compare the measured movement against the tool's shrinkage allowance.
5. Freeze the measurement timing in the incoming-inspection method before sign-off.

Keep a retained reference sample beside every result. When a batch is disputed, the comparison that settles it is almost always between the retained part and the current one under the same method, not between either part and a number on a drawing. Record the instrument, the operator, the conditioning time and the position on the part or sheet for each reading.

Writing it into the specification and the evidence pack

Critical dimensions shall be measured after the agreed conditioning period at a stated ambient temperature, with the demould-to-measurement interval recorded for each lot.

Write the requirement as a behaviour at a stated condition, not as a brand of material or a machine setting. State the test method, the conditioning, the sample plan and the rule for deciding a pass. Where a document is needed, ask the supplier for documentation that covers the actual construction and check its scope against your own market before approval; a document for the base material alone does not describe the finished part you are buying.

At first-article approval, freeze the drawing revision, the material and colour identification, the process window, the tool and cavity identification, and the inspection method. Any later change to grade, site, tool, or window should trigger a new first article rather than a paper waiver. Keep retained samples from the approved lot and from each production lot so that a future comparison is possible.

Table 3. Minimum evidence pack for a first-article decision and a later batch dispute.

All ranges in this article are illustrative engineering guidance rather than measured data. Treat them as a starting agenda for a process trial, not as a certificate for a batch that has not been run.