Demould Scuffs on Device Housings: When Draft Angle Is Too Small

Vertical walls on device housings drag on release and leave scuffed bands; review draft, tool finish and release practice before the cosmetic limit is set.

What the failure looks like on the line

Equipment housings for clinical environments are judged on cleanability, fit and surface finish, with documentation reviewed separately from the part itself.

Fine vertical scratches appear on the side walls of the housing as the part is lifted off the tool, and rejection concentrates on the visible faces.

A near-vertical wall stays in contact with the tool through most of the release stroke, so the shrinking part slides against the tool surface instead of separating cleanly. Friction at that interface polishes or scores the soft, still-warm polymer before it has fully cooled.

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. Draft angle on external walls 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.

Draft angle on external walls is typically held at 3-5 deg minimum. Below this the part drags along the tool during release 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 finish is typically held at Polished or fine-textured, Ra 0.8-1.6 micrometre. A coarse finish increases friction and visible scoring When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Demould part temperature is typically held at Below 60-70 C before release. Releasing too warm keeps the surface soft enough to mark 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.

Release agent coverage is typically held at 0.5-1.0 g/m2 per cycle. Heavy or patchy deposits transfer to the cosmetic face 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 demould drag force (n) as draft angle (deg) increases: the response falls from 78 to 12 on the illustrative 0–100 scale. The dashed line at 30 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.

Draft angle too small accounts for roughly 38% of the illustrative split. Measure the draft angle on every external wall against the approved drawing. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Tool surface too rough accounts for roughly 22% of the illustrative split. Inspect the tool surface for burrs and polished wear bands under raking light. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Released too warm accounts for roughly 17% of the illustrative split. Record part temperature at the moment release begins. 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 the draft angle on every external wall against the approved drawing.
2. Inspect the tool surface for burrs and polished wear bands under raking light.
3. Record part temperature at the moment release begins.
4. Compare scuff location against the tool's parting and vent features.
5. Agree a cosmetic limit sample that defines the acceptable scuff band.

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

Visible side-wall scuffing shall not exceed the agreed limit sample when inspected under the specified lighting and viewing distance after conditioning.

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.

None of the figures above should be read as a measured survey. They are engineering guidance for framing a joint trial; confirm them on your own tool, grade and geometry before they become contract terms.