Nearly any attempt at PLA smoothing requires some amount of sanding. The best technique is to use multiple grits of sandpaper, starting from course, like 200 grit, and continuing to smooth, like 4000 grit, if available. Sand in a circular motion against the grain of the layer lines.
Resist the urge to use a power tool because the high vibrations created by electric tools will easily melt the plastic, deforming the surface. Even when sanding manually, understand that friction generates heat, which can damage your print by melting the plastic.
The best sandpaper is waterproof or “wet/dry” sandpaper because you can wet it in order to cool the plastic. You should also rinse your print often to dislodge any particles created by the sanding process. An additional benefit is that water stops small particles of plastic from becoming airborne and inhaled. Wet/dry sandpaper can be distinguished because the grit is black rather than tan or beige.
When PLA smoothing, sanding by itself will never entirely eliminate layer lines. One method that works well is applying polishing compound to fill in any remaining layer lines. Polishing compounds are often sold in bars, each with different grades, similar to sandpaper. These compounds are generally designed for use with metals, but work well enough with plastic. Simply wipe some compound onto a buffer wheel or similar attachment for a rotary tool, then apply it to the plastic for a smooth and shiny finish. As with sanding, remember that buffing generates friction and therefore heat. When PLA smoothing with a buffer wheel, a slower speed is always better.
Another method when PLA smoothing is to use a sandable high-fill primer spray paint, such as the one sold by Rustoleum in most hardware stores. Thin, even coats dry quickly, at which point you can sand the primer back down to the plastic so that it remains only in the depressions of the layer lines. Repeat the process until all the imperfections of the print have been filled in.
When performing these two procedures, always use primer and paint from the same manufacturer and never mix acrylics and enamels. Doing so runs the risk of causing the paint to crack, easily destroying all the hard work you’ve done to smooth out your print.
Another product worth mentioning is XTC3D from Smooth-On. It uses two liquids, one catalyzing the other, to create a resin-like material that can be brushed onto a 3D print. The coating self-levels uniformly without leaving brush strokes. While Smooth-On states that XTC3D will not melt the plastic, the resin definitely heats up as the mixture begins to set. The end result is a smooth and hard exterior that can be painted over or used as the final coating.
A less popular option for PLA smoothing is to use a heat gun along the surface. This technique requires a certain amount of practice because the tool obviously outputs a great deal of heat, which can destroy your 3D print. Pre-sanding your model actually helps expose the plastic, making the final effect much more effective. The key is to keep the gun moving along the surface of the model, never allowing the gun to linger on one area which can easily melt the plastic.
While PLA generally does not emit noxious odors when extruded, applying heat will certainly produce fumes as the plastic softens. No matter which PLA smoothing technique you choose, always try to work in a well-ventilated space because heated plastic, resins and spray paints all emit potentially harmful fumes.
A well-known technique for smooth ABS 3D prints is to place the print in a container filled with heated acetone mist. The acetone interacts with the ABS and melts the plastic in a slow and controlled way to create a super smooth model. Unfortunately, you can’t perform the same procedure on PLA because it doesn’t interact the same way with acetone. Certain other chemicals will do the same but they are difficult to acquire and are very dangerous and toxic to use.
Polymaker, a company best known for its filaments, unveiled their own smoothing solution called Polysher. Using their own formulation of PLA-like filament, a 3D print is lowered into a sealed chamber where isopropyl alchohol is atomized, creating a mist around the object to be smoothed. The 3D print rotates in the alchohol mist, which reacts with the plastic in order to melt it. The longer the process, the more the print will smooth. The surface will be tacky when it comes out of the Polysher so it’s best to let it sit for about a half hour before handing. Doing so also improves the final appearance, which might not seem as smooth as you would expect at first.
As with the other procedures outlined here, a bit of sanding helps the process but it’s not necessary. A tip is to expose the 3D print to shorter sessions in the Polysher and turning the model between sessions. Amazingly, this technique will produce nearly transparent prints when using Polymaker’s clear PolySmooth filament.
Creating smooth 3D prints will always require some work. Sanding will either prepare your print for other processes or else create a near-flawless surface by itself. No matter which procedure you choose, there are always some caveats to any PLA smoothing process.
All processes remove material from your print. Sharp edges will be dulled and it is very difficult to work around fine details. By designing your model into sub-assemblies, each piece can be smoothed using the most appropriate process and assembled into a final, smooth and professional-looking model. No matter the process, a little work will produce results that will bely the fact that your 3D print was produced on a low-cost 3D printer.
License: The text of "PLA Smoothing – A Beginner’s Guide" by All3DP is licensed under a Creative Commons Attribution 4.0 International License.
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