A batch of small-appliance housings came back from the tool shop: matte top cover, gloss bottom cover, assembled. The client photographed it and asked why the two halves looked like two different colours of plastic. We checked the colour chips — the shade was correct. Same injection moulding machine, same material batch.
We have run into this more than once. When a surface finish combination fails, the cause is rarely the colour itself. In about eight out of ten cases it comes down to three things: the roughness grade of the "matte" areas was never unified, the geometry of the boundary between gloss and matte has no transition, and on two-shot parts the shrinkage difference between the two materials visually amplifies the step.
The order of investigation matters more than the tools. Work from the cheapest explanation upward, and do not start by blaming the masterbatch.
Answer first: three root causes, ranked by frequency
- Two matte surfaces are not the same matte. Under the VDI 3400 texture standard, grades 18 through 45 all count as "matte", but the Ra values differ by roughly an order of magnitude. A 10-grade difference in spark-erosion texture can change reflectance by a factor of two.
- Gloss and matte are butted together with no transition. A hard joint with no radius or step makes the eye read the boundary as wider than it is — and any flash, misalignment or slight colour drift is magnified.
- Two-shot parts are designed for adhesion but not for shrinkage. The visual gap left by edge retreat is larger than the nominal design value.
Investigation order: texture grade → boundary geometry → material shrinkage → colour chips last.
Step 1: confirm both "matte" surfaces are actually the same matte
Matte is not a surface — it is a range. In one project the top cover used the mould shop's off-the-shelf VDI 27 texture, while the newly cut bottom cover was given VDI 33 because that is how the texture operator read "matte". Each part looked fine on its own. Assembled, the top read bright and the bottom read dull; the same colour code looked like two different shades.
What we do now: whenever two matte areas appear on the same product, the drawing states the texture grade and a target gloss value — never the word "matte". Gloss is measured at 60 degrees; matte zones are typically held within 10 GU, and the difference between two zones should not exceed 3 GU. Above that, it is visible under shop downlights.
There is a subtler variant: the same mould and the same texture, but the two cavities were hand-polished to different degrees before etching, so the texture depth differs. The only reliable check is T0 samples compared side by side under the same light angle — never inspected at different times in different rooms.
Judgement call: place the two matte parts side by side in daylight by a window and look at them from about 30 degrees. A clear light/dark boundary means the texture grades differ. If in doubt, use a gloss meter — numbers beat eyes.
Step 2: the width of that boundary is decided by the eye
Gloss next to matte is the highest-contrast combination available. The glossy face collects ambient light into a highlight band, the matte face scatters it, and the human eye automatically reads the boundary between them. Any slight irregularity — flash, misalignment, colour drift — is amplified.
The cheapest fix is a physical divider: a step of about 0.5 mm so the two faces are offset in height, which makes the eye read a step rather than a gap. Second option: a R0.3–R0.5 radius at the joint, so light transitions gradually and the hard edge is broken up.
The worst option is keeping both faces perfectly coplanar and relying on the mould parting line alone. That demands extreme mould precision and process stability; in volume production it will produce edge flash, and once there is flash the part is scrap.
One counter-intuitive lesson: never put the boundary where fingers go. Friction differs sharply between matte and gloss, so the user feels a distinct catch at the boundary and reads it as poor build quality. Keep the grip zone in one finish.
Step 3: on two-shot parts the step is usually shrinkage, not the mould
The drawing says flush; the part has a small step you can feel at the overmould edge. The first instinct is that the mould is wrong. Most of the time it is differential shrinkage. ABS moulding shrinkage is typically 0.4% to 0.7%; TPE is noticeably higher. After overmoulding, the soft material retracts during cooling and its edge pulls back.
You cannot chase this by re-cutting the mould, because shrinkage varies from shot to shot. What works is designing the soft material as a wrap: let the TPE edge overlap the rigid substrate by about 0.2 mm, so after shrinkage it is still covering rather than exposing the hard plastic. Alternatively, seat the TPE into a groove in the rigid part, with the groove slightly deeper than the TPE thickness so the groove wall blocks the retreating edge.
There is also an appearance issue: TPE is naturally matte, so pairing it with a mirror-gloss rigid part creates maximum contrast, and even 0.1 mm of misalignment becomes visible. On one handheld device we ended up pulling the gloss area back 2 mm from the overmould edge and inserting a matte transition band. The step is still physically there; the eye no longer reads it.
Step 4: a colour chip can match and the assembled unit still look off
This sits last not because it matters least, but because it is most often misdiagnosed as the first cause. The same colour code, moulded glossy versus matte, will measure differently in Lab values: matte scatters strongly and makes dark colours read lighter and light colours read duller; gloss concentrates light and makes dark colours read deeper. Comparing a gloss chip against a matte part cannot match.
Our rule: the chip must be produced with the final surface treatment. If the part is matte, etch the texture before shooting the chip; if gloss, polish first. When one product carries both finishes, shoot two chips and evaluate them together, never separately.
Colour tolerance: ΔE below 1.0 is not distinguishable to the eye; 1.0–2.0 is visible to a trained eye and usually ignored by end users; above 2.0 is visible colour drift. Note, however, that ΔE loses reliability when comparing across gloss levels, because the measurement geometry differs — in that case trust the side-by-side visual comparison.
Field checklist
| Symptom | Check first | Root-cause criterion |
|---|---|---|
| Two matte parts differ in depth | Texture grade and measured gloss | Gloss at 60° differs by more than 3 GU |
| Gloss/matte seam looks wider than it is | Whether the boundary has a step or radius | Coplanar parting-line joint, no transition |
| Step felt at the overmould edge | How TPE overlaps the rigid part | TPE fills flush, no overlap or groove |
| Whole unit off-colour but chip passes | Which finish the chip was shot on | Chip and production part differ in gloss condition |
We call this the four-step check internally. Do not reorder it. Skipping step one and going straight to colour usually buys a round of re-shooting chips and re-matching colour, and ends at the same answer.
Article by Hu Yashe (Asher), founder of Hezi Industrial Design — Dongguan, China. 320+ products brought to mass production. Originally published at hezidesign.com.
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