color design materials

Color and Appearance in Plastics

The appearance of a molded plastic part is often an important design element. While this involves color, it also involves gloss and texture.

Color is an important aspect of appearance, but it also involves gloss and texture. Furthermore, cosmetic defects (flow lines, knit lines, parting lines, gate vestige, gate flush, dark specs, etc.) can weak havoc on the appearance. Selecting a thermoplastic material for optimal appearance should include considerations for all of these issues.

Color

Involves visible light (including the wavelengths of the light source, how they are reflected by the object that is being viewed, and how they are perceived by the viewer), but also by other physical, physiological, and psychological factors. Color is a complex subject, and a thorough discussion is well beyond the scope of this article. However, at a basic level, we need to understand that the appearance of an object involves not only the color of the object being viewed, but also the gloss level and texture of the surface that is reflecting the light. In the world of plastics, gloss and texture are poorly understood phenomena, yet they both play an important role in the appearance of a molded plastic part.

colored Lego blocks

Gloss

Used to describe the manner in which a surface reflects light. The surface can reflect light in a pure mirror-like fashion (known as a specular reflection), it can scatter the light in a number of different directions (known as diffuse reflection), or it can absorb the light. There can also be a combination of all three. Many people confuse the term gloss with the concept of smoothness. However, gloss is not a description of surface smoothness (or roughness), it is a property of the material itself. As an example, a marble surface, even when polished perfectly smooth, will not provide a pure specular reflection like a common mirror, which is usually made by taking a piece of polished glass and coating it with a thin layer of aluminum or silver. Both aluminum and silver are high gloss materials, whereas marble is a lower gloss material.

The concept of gloss can be explained by the sheen (the shiny-ness) that is offered in various paint finishes, which can range from very dull (also known as flat or matte), and then getting progressively more shiny as the gloss level increases, from flat through eggshell, satin, silk, semi-gloss, and high gloss. Note: these terms are not standardized, and while the color or surface smoothness of the paint does not change, the final appearance most definitely does.

Unfortunately, gloss levels of thermoplastic materials are rarely reported. Some materials, like ABS and polycarbonate, have an intrinsically high gloss level, and they come out of the mold with very high gloss, looking shiny and bright. Other materials, like polypropylene, have a lower gloss level, more like a semi-gloss. As a result, they come out of the mold looking a bit duller. And with some materials, like nylon and polyester, the gloss level can be affected by the processing conditions. So selecting a material based on gloss level can be a bit of a challenge. Quite often, this kind of information can be considered privileged and proprietary information, or as a matter of tribal knowledge.

A color chip of molded PET polyester. Note the change in gloss at different thicknesses.

Texture

Texture also plays a critical role in appearance. Anyone who has ever held a textured plastic color chip can tell you that the appearance changes as the texture changes over the surface of the chip. In plastic parts, the term texture refers to surface texture, and it is used to describe the physical condition of the part surface.

When we describe the surface texture of a molded part, we typically think of the textures (plural) that were on the surface of the mold, which have been transferred to the surface of the part. We often forget that the textured surfaces in the molded part are NOT a perfect replica of the textured surfaces that were in the mold. The molded part will have surface imperfections, minor deviations here and there, etc. Even if the mold surface was highly polished, the molded plastic part will NOT have the same level of smoothness as the tool. Furthermore, I would postulate that every thermoplastic material will have its own unique signature in how it replicates the mold surface. I think of this as the material texture.

In addition, unlike metal fabrication, molded plastic parts rarely undergo any secondary finishing processes (buffing, polishing, etc.). So the surface texture on a plastic part is typically an “as fabricated” texture, and will be dependent on the specific plastic material as much as the mold texture (and also the processing conditions). Unfortunately, there is no published data on material texture for molded plastics, as far as I know.

Natural Color

Finally, there is is the natural color of the material itself. Every thermoplastic material has its own innate color. Some are clear, some are a milky white, others are brown, some are even black. (A thermoplastic material in its innate color state is referred to as natural, regardless of the exact color.) Furthermore, every thermoplastic material responds to pigments in a different way. These pigments—whether they be dyes, solids, powders, or flakes—interact with the base resin to create the final color of the molded part. However, how the material responds to the pigment is a critical aspect of the overall appearance. Is the final color rich and vibrant? Or is it dull and muted? Does the pigment affect the gloss level of the material? Does it affect the surface texture?

Appearance Specifications

Most designers and engineers understand the concept of color specifications. Design teams often go to great lengths to establish color palettes for the project, with specific color targets. The color specifications often involve descriptions of lighting and viewing conditions, approved materials and color codes, and sometimes even acceptable color tolerances. Part texture is typically described by general notes on the part drawings. However, gloss level is rarely addressed. I would encourage you to consider all of these factors in developing specifications for appearance.

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