Appliance Fascias: A Visible Colour Step Between Two Cartons

Two shipments of an appliance fascia look matched under the booth light but show a clear step when fitted side by side on the assembly line.

What the failure looks like on the line

High-volume household parts are audited on cosmetics and fit across long runs, so a slow process drift is more expensive than a single bad shot.

Assembled units show a visible shade difference between fascias from consecutive deliveries, although each batch passed its own colour check. The step is most obvious across a large flat area near the control panel.

Small changes in masterbatch let-down ratio, resin lot tint or residence time shift the colour of a moulded part, and a large untextured flat amplifies the difference by specular reflection. Metamerism means two parts can read as equal under one illuminant yet differ under another. A dry-colour blend or a poorly purged changeover adds a further within-lot drift.

Start with the failure as it is reported, not with the parameter that is easiest to measure. 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. Masterbatch let-down ratio 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.

Masterbatch let-down ratio is typically held at 1.5-3.0 %. Below the window the tint cannot mask resin-lot variation Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Melt temperature is typically held at 210-240 C. Excess residence heat shifts the pigment and yellows the shade When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Back pressure is typically held at 5-12 MPa. Low back pressure leaves the colourant poorly dispersed 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.

Changeover purge shots is typically held at 8-15 shots. Too few purge shots carry the previous tint into the new lot 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 colour drift (relative index) as shots since colour changeover increases: the response rises from 10 to 90 on the illustrative 0–100 scale. The dashed line at 60 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.

Masterbatch let-down drift accounts for roughly 30% of the illustrative split. Measure delta-E against the signed master panel under two agreed illuminants. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Resin lot tint change accounts for roughly 24% of the illustrative split. Compare a part from each carton fitted side by side on the assembly, not separately. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Metameric check under one illuminant accounts for roughly 20% of the illustrative split. Log the masterbatch lot and let-down ratio for both deliveries. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Checks before sign-off

In practice the sequence below is cheaper to run than a root-cause debate after a rejected delivery.

1. Measure delta-E against the signed master panel under two agreed illuminants.
2. Compare a part from each carton fitted side by side on the assembly, not separately.
3. Log the masterbatch lot and let-down ratio for both deliveries.
4. Check the purge record between the two colour campaigns.
5. Confirm the colour-measurement aperture and geometry match the approved method.

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

Each delivery shall be measured against the signed master panel under two agreed illuminants and reported as delta-E; a batch exceeding the agreed delta-E limit is rejectable, and the disposition shall be made on parts fitted adjacent to one another.

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.