Orange Peel on Access Doors: When the Sheet Surface Replicates the Tool

A pebbled finish on an access door usually traces back to sheet temperature and tool texture; separate the two before specifying a cosmetic limit.

What the failure looks like on the line

Access doors and covers are assembled repeatedly, and a small forming drift turns into a sealing or latching complaint.

The formed face shows a fine orange-peel ripple with scattered micro-pits that become obvious once the door is painted or lit from an angle.

If the sheet skin is too cool it cannot flow into the tool's micro-texture, so it freezes a coarse replica of its own surface instead of a smooth one; trapped air or moisture at the interface adds small pits. Raising the skin temperature improves replication until the sheet is hot enough to sag or degrade.

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. Sheet surface temperature 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.

Sheet surface temperature is typically held at 160-185 C (ABS/PC-ABS). A cold skin leaves orange peel; an overheated one sags Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Tool surface roughness is typically held at Ra 0.8-1.6 micrometre for painted faces. Coarser texture prints through the formed skin When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Sheet moisture before forming is typically held at Below 0.1 % by mass, or supplier's stated level. Residual moisture boils at the interface and forms micro-pits 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.

Vacuum rise time is typically held at Under 1-2 s to full vacuum. Slow evacuation lets the skin set before it contacts the tool 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 surface replication fidelity (%) as sheet surface temperature (c) increases: the response rises from 35 to 62 on the illustrative 0–100 scale. The dashed line at 75 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.

Sheet skin too cool accounts for roughly 33% of the illustrative split. Measure sheet surface temperature at several points immediately before the draw. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Tool texture too coarse accounts for roughly 24% of the illustrative split. Check the tool face under raking light for texture damage or residue. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Residual sheet moisture accounts for roughly 18% of the illustrative split. Verify the drying record for the sheet lot in use. 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. Measure sheet surface temperature at several points immediately before the draw.
2. Check the tool face under raking light for texture damage or residue.
3. Verify the drying record for the sheet lot in use.
4. Compare the formed face against an agreed gloss and texture limit sample.
5. Inspect an unpainted first off before any coating hides the defect.

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

The formed face shall match the agreed texture and gloss limit sample when assessed under the specified lighting before coating, with the sheet surface temperature recorded for the 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.

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.