Why Matte Plus Gloss Looks Cheap: It's the Seam Between Them

A batch of small-appliance housings arrived from the tool shop: one matte top cover and one gloss bottom cover, assembled together. The client photographed the product and questioned why the two halves appeared to be different shades of plastic. The colour chips were confirmed to be correct, and both parts were produced on the same injection moulding machine using the same material batch. This issue has occurred more than once, and the root causes are rarely related to the colour itself.

Typically, there are three primary factors contributing to the perceived colour mismatch: 1) the roughness grade of the matte areas is inconsistent, 2) there is no transition between the gloss and matte boundary, and 3) differential shrinkage between the two materials visually amplifies the step. The order in which these issues are investigated is crucial and should start with the cheapest explanation, moving upwards, before blaming the masterbatch.

First, ensure that both matte surfaces are indeed the same matte. According to the VDI 3400 texture standard, matte finishes range from grades 18 to 45, with Ra values differing by roughly an order of magnitude. A difference of just ten grades can alter reflectance by a factor of two. When two matte surfaces are combined without a transition, the eye perceives the boundary as wider, and any minor imperfections — such as flash, misalignment, or colour drift — become more noticeable.

Second, evaluate the geometry of the boundary between the gloss and matte areas. A hard joint with no radius or step can make the boundary appear wider than it is, magnifying any flaws in the assembly. Two-shot parts are designed for adhesion but not for shrinkage differences. The visual gap left by edge retreat is larger than the nominal design value, leading to a clear visual gap that the human eye detects.

Lastly, consider the shrinkage difference between the two materials. In two-shot parts, the soft material often shrinks more than the rigid material during cooling, creating a visible step at the overmould edge. This is particularly common in ABS and TPE moulding combinations, where ABS has a typical shrinkage of 0.4% to 0.7%, while TPE can shrink significantly more.

Designing the soft material as a wrap-around for the rigid substrate, or seating the soft material into a groove within the rigid part, can help mitigate these shrinkage differences and improve the appearance of the final product.