With most materials, molding gives you the highest degree of control over the finish of your parts. Direct machining, on the other hand can leave tool marks, and 3D printing processes like SLA or FDM will almost certainly leave signs of their production as well. These imperfections can be minimized or eliminated completely with secondary processes, and there are plenty of service providers who can perform those secondary processes quickly and efficiently. In a previous position I specialized in last-minute finishing and damage control for prototypers facing deadlines and needing help finishing up (occasionally while the client waited in the boardroom). In this environment I learned a surprising variety of techniques for refinishing machined and 3D-printed parts. For obvious reasons, these are best suited to one-offs or extremely small numbers of parts when you are squeezed for time or money or both.
The most obvious way of removing roughness or providing a uniform surface is abrasion. Sandpaper can be used to smooth both plastic and machined metal. It will leave uniformly scratched or matte finish and requires careful application to get into tight spaces and avoid swirls, deep scratches, and other obvious marks. Hand polishing with a rag and rubbing compound is an option if you would like to take that finish further. The final result depends on the grit of the compound; for a very smooth or polished finish on a rough piece you’ll want to work in steps using increasingly fine compound or rouge. The results of this process can be quite impressive with final product being as shiny and polished as its injection molded counterpart. It can take a long time but it is a fairly straightforward method if you have patience—lots of patience. (You might want to throw on that audio book you’ve been meaning to get to or put on your favorite radio station).
Flame smoothing or polishing, as the name implies, is a simple matter of applying heat to a rough plastic surface to soften it, then allowing it to self-level and cool. Depending on the size of the piece, you can use a heat gun, propane torch, or even a butane lighter or match. (Indirect application of a cleaner burning flame will produce a better finish, while matches and lighters can leave a smoke residue that needs to be cleaned off.) This technique works particularly well on clear thermoplastics, allowing you to regain internal clarity in just a few seconds. For obvious reasons be careful of how much heat you apply; the last thing you want is your hard work going up in flames, literally.
Among many solvent options, acetone dissolves many plastics including ABS and polycarbonate and can be used to soften and level rough surfaces. Dipping is the most aggressive way of applying acetone and should only be used on larger parts with larger features that are not likely to creep or droop after immersion in the solvent. Brushing allows more targeted application; however, it can be messy and is likely leave brush strokes. An acetone spray or Vapor bath can produce excellent results and, if carefully monitored, is the most controllable application method. The vapor bath method involves heating a small amount of acetone in an open, heatproof vessel in which the part is either hung or raised above the liquid. A gentle heat source—a heating pad, for example; not an open flame—is placed beneath the vessel to vaporize the acetone, which slowly softens and levels the plastic surface. The required time for the process varies and should be carefully monitored. When the desired finish is achieved, the part is removed from the vessel and allowed to harden in its new polished state. Check out this video and detailed description of the vapor bath process.
Note: solvents like acetone are both toxic and flammable and should be used only in well ventilated areas. This is particularly true of heated/vaporized acetone.
Paint, typically sprayed, can change the finish of both plastic and metal. A matte finish is best at hiding blemishes and inconsistencies. In my experience, one of the best spray paints for adhesion to plastic is Krylon Fusion; I have yet to find anything that is as readily available and works as well. Certain floor waxes and linoleum finishes can be applied by rubbing and can fill the marks caused by machining, and lacquer can be sprayed on for a smooth, clear final finish. Both work on plastic and on metal, given the right choice in paint and application.
Rubberized Coating Compound
A variety of rubber-like coatings can be applied by dipping, brushing or spraying to mimic overmolding. You can control placement by masking, and most of the products will air-dry to a shiny and durable surface fit for any board room presentation, though probably not for functional testing.
Adam Poetter is a Proto Labs Customer Service Engineer.