February 13, 2020 Choosing the Best Surface Finish

A look at how to choose the best surface finish for your part, whether by CNC machining or 3D printing.

One of the big questions when having parts made, whether by CNC machining or 3D printing, is what surface finish to choose. This can make a big difference to the end result and needs to be decided before work starts. In their Protolabs’ Insight video series, experts at Protolabs talk about how to choose and here are some tips from this.

Finish options

Surface finishes are a big question because there are so many different options. The finish can be anything from textured to a smooth finish with a highly cosmetic look to it and many variations in between. So, what do you need to consider when selecting the right one?

There are three main questions Protolabs ask their clients at this point:

1. What development or production stage is the part at – prototype or finished part?
2. What material is being used in the manufacture?
3. What is the end-use application of the part

Material used

While all three are important, let’s start with the material being used as this has perhaps the greatest impact on the finish chosen.

Let’s say you are working with plastics, the most frequently used moulded material. There are hundreds of thermoplastic resins which offer a fine cosmetic finish as well as being flexible and strong. The finish you can achieve does depend on the mould. For example, aluminium moulds are made by having three axes milling and this means it is easy to hand polish or have textured finishes. You can even have a high gloss polish on it.

Other materials may not be quite so flexible. Glass, minerals or materials that are soft or have a high shrink can often have a range of different finishes. But not all work well with a highly cosmetic part because they have unique properties that don’t finish in that way well – streaking is one example of a problem.

Ways to create different finishes

With the developments in both 3D printing and CNC machining, there are often ways to create different finishes that might not have previously been available. An example could be gating, where a liquid resin is added to the mould. This does create a cosmetic defect in the mould by interrupting the surface but may be the best way to complete the finish required.

Another option is to look at liquid silicone rubber for things like injection moulding. It is used in the same way as plastic but mixes two liquids together and then heat cure them to create the final part. Liquid silicone rubber is much easier to work with than plastic and pieces can be easily removed from moulds, so gating isn’t needed or is very minimal.

Get expert design help

The key to all of this is to get the right help at the designs stage. By considering these three questions and some of the practical issues that may be faced and working with an expert in these digital manufacturing techniques, you can get the right finish for your project, regardless of what this is.

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