June 2010 Design Tip

Texture: When Things Get Rough

Surface finish on a plastic part can serve many functions, from improving grip to hiding fingerprints to facilitating paint adhesion. Protomold offers seven finishes on moulded parts: five polishes and two textures. The five levels of polish are created using manual mould polishing techniques; the two levels of texture are achieved by bead blasting the mould surface after applying a manual base polish.

Protomold’s available textures are light bead blast (T1) and medium bead blast (T2). Many factors can influence your choice of texture. The most obvious is the intended function of the finished part. Either texture can provide an attractive, non-reflective surface. If grip is an issue, that may influence your choice between light and medium texture. Other factors may include secondary processes to be applied to the part and the part’s aesthetic fit with other components of your finished product.

The resin being used can also affect texture choice. For example, olefin resins such as polypropylene can have a waxy feel and, in conditions of high humidity, can become slippery. An appropriate texture can reduce these problems. In such cases, the ability to texture the part’s surface can expand your range of usable resins.

Another factor linking resin choice and surface texture is “sink.” In many cases, a slight sinking of the part surface as the part cools may be no more than a minor cosmetic issue. A textured surface, however, will create shadowing, a visible darkening of areas with sink across a surface. This makes an even slightly sunken area far more apparent, creating a significant cosmetic problem. In such cases there are three possible solutions:

  1. Keep the texture but reduce the thickness of the feature to reduce the likelihood of sink.
  2. Keep the texture but choose a resin that shrinks less to reduce sink.
  3. Reduce or eliminate the surface texture to reduce shadowing.

The last and potentially most serious issue with a textured surface is the possibility of the part sticking in the mould. A textured surface that is perpendicular to or significantly angled away from the direction of mould opening will not present a problem. On a surface that is parallel to the direction of mould opening, however, each little valley in the textured surface acts as a tiny undercut. Forcibly ejecting such a part from the mould can create drag marks on the part surface as the textured mould face scrapes off the high areas of the textured surface. To eliminate drag marks, Protomold requires that surfaces with T1 texture be drafted by at least three degrees and surfaces with T2 texture be drafted at least five degrees. (Of course, surfaces that are created using side actions need not be drafted away from the direction of mould opening; rather, they must be drafted away from the direction of side action cam withdrawal.)

For a variety of reasons, Protomold’s ProtoQuote® design analysis does not look for or identify surface texture. Instead, you will select surface finishes—polish or texture—after you have received your ProtoQuote. Once a ProtoQuote interactive quote is presented (along with mouldability analysis), you can select the surface finish right in the quote and immediately see the resulting effect on cost (see Figure 1).

Figure 1 - Pricing is immediately updated as you make selections on your quote.

Figure 1 - Pricing is immediately updated as you make selections on your quote.

While ProtoQuote does not recognize texture, it does identify surfaces that fall short of the draft required for light or medium texture. Surfaces highlighted in red have less than three degrees of draft. These cannot support light or medium texture (see Figure 2). Any surfaces highlighted in yellow, in a quote, will have at least three but less than five degrees of draft and could support light but not medium texture. In other words, if your ProtoQuote design analysis has areas highlighted in yellow, you can add T1 texture without problems but not T2. If it includes areas highlighted in red, you should either revise your design to increase draft or skip texturing this area entirely.

Figure 2 - Areas highlighted in red have less than three degrees of draft and cannot support light or medium texture.

Figure 2 - Areas highlighted in red have less than three degrees of draft and cannot support light or medium texture.

There are two additional points to keep in mind regarding texture. First, you cannot generally specify texture for just part of a surface (although A- and B-sides of a part can be textured independently). In some cases, however, this partial texturing can be achieved if the area of the surface you do not want textured can be “taped off” during bead blasting. Second, areas that cannot be accessed by the bead-blasting stream cannot be textured. Proto Labs customer service engineers, who can be reached at +44 (0)1952 607 447, can help address either of these issues as well as any other questions you may have regarding surface finish.

Read past Protomold Design Tips.

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