Warpage in Display Shells: Tracing Residual Stress Back to the Sheet

Flat display shells that bow after forming often start with uneven sheet heating or stored-in roll stress; isolate the source before condemning the tool.

What the failure looks like on the line

Display shells are judged under retail lighting from a metre away, where surface waviness reads more strongly than measured thickness.

The nominally flat face of the display shell bows or twists after trimming, so the part rocks on a flat bench and the printed graphics will not register.

Extruded sheet carries frozen-in orientation from the roll stack, and uneven heating adds a second, thermal gradient through the thickness. When the part is released and trimmed, those unbalanced stresses relax asymmetrically and the flat panel warps.

Buyers and suppliers usually agree on the drawing and disagree on the conditions under which the drawing was checked. 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. Edge-to-centre sheet temperature spread 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.

Edge-to-centre sheet temperature spread is typically held at Within 5-8 C. A larger gradient locks in differential shrinkage and bow Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Sheet conditioning before forming is typically held at Stored flat at 20-23 C for 24 h. Cold or curled sheet carries stored stress into the oven When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Cooling uniformity is typically held at Both faces within 10 C during cooling. One-sided cooling bends the panel toward the cold face 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.

Trim delay after forming is typically held at At least 24 h or until within 5 C of ambient. Trimming warm locks the warped shape into the finished edge 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 flatness deviation (mm/metre) as edge-to-centre temperature spread (c) increases: the response rises from 6 to 70 on the illustrative 0–100 scale. The dashed line at 25 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.

Uneven sheet heating accounts for roughly 31% of the illustrative split. Measure sheet temperature at the centre and at all four corners before the draw. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Stored roll stress in sheet accounts for roughly 26% of the illustrative split. Check incoming sheet for curl and edge wave against the receiving record. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

One-sided cooling accounts for roughly 19% of the illustrative split. Record cooling-air temperature on both faces of the tool. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Checks before sign-off

Run the same sequence on a retained good sample and on the disputed batch so the comparison means something.

1. Measure sheet temperature at the centre and at all four corners before the draw.
2. Check incoming sheet for curl and edge wave against the receiving record.
3. Record cooling-air temperature on both faces of the tool.
4. Compare flatness before and after trimming on the same part.
5. Store a retained flat panel and re-measure it after the agreed conditioning period.

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

Flatness deviation measured on a stated surface plate after conditioning shall stay within the agreed band, with trimming deferred until the part reaches the recorded ambient temperature.

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.

The values quoted here are illustrative engineering guidance, not a measured survey. Agree the test method, the sample size and the acceptance rule before comparing numbers across suppliers.