How Acetone Vapor Smoothing Evens Out 3D-Printed Plastics

Acetone vapor smoothing is a process used to reduce the surface roughness of certain 3D-printed polymers, primarily for aesthetic reasons. Due to the nature of fused deposition modeling (FDM), which lays down discrete layers, the surface finish of parts is often very bumpy. This can be refined by using a smaller layer height, but this increases the print time and only goes so far. Acetone vapor smoothing dissolves the outermost layer of the print to smooth the surface, leaving behind a glossy finish. The acetone vapor smoothing method increases the aesthetic appeal of 3D printed parts and it’s faster than other surface smoothing processes.

Acetone vapor smoothing works on any material that can be dissolved by acetone. Some filament types that don’t respond the same way to acetone may be damaged by the process or may remain completely unaffected. The five most common 3D printed materials which benefit from acetone vapor smoothing are:

1. ABS (acrylonitrile butadiene styrene)
2. ASA (acrylonitrile styrene acrylate)
3. PMMA (polymethyl methacrylate)
4. HIPS (high-impact polystyrene)
5. PC (polycarbonate) 
6. Examples of materials that do not benefit from this post-processing include: PLA, PETG, Nylon, and TPU. 

For more information, see our guide to 3D printing.

How It Works

There are six key steps to the acetone vapor smoothing process:

1. Preparation

During preparation, sand the 3D printed part to remove any particularly coarse surfaces, including those with 3D printed supports. Use a clean file to ensure no foreign particles get lodged in the material. This surface preparation does not take long but is required to remove larger deformities. 

Evaluate how to suspend the part so the acetone vapor contacts all surfaces that need smoothing. This usually means only the sides and top faces will be smoothed during a single session. Also, ensure that the part will fit inside the container with the lid closed. The part will need a base to rest on that will not dissolve in acetone. Metal foil is usually a good option. 

2. Acetone Vapor Generation

There are several ways to generate acetone vapors. The easiest way is to add some acetone to a container and cover (but do not seal) it. This method takes advantage of the fact that acetone quickly evaporates in the air. Make sure that the container is not also made of acetone-soluble material. Glass is usually best as it does not react to the vapor and it is transparent so you can see the part while the process runs. To improve the quality of this method, you can soak a paper towel with acetone and suspend it around (but not touching) the part. 

3. Object Suspension

Next, place the part into the container. The part must be suspended in the container such that it is not touching either the liquid acetone or the acetone-soaked paper. If it does touch, the part will dissolve quicker at the contact point.

4. Vapor Exposure

Leave the part in the container and monitor it continuously. There is no set timeframe or method of calculating how long the part might need to be in the container. This will depend on several variables including the rate of acetone evaporation, the size of the container, the size of the part, and the initial surface roughness.

5. Smoothing Process

Once the liquid acetone has vaporized and started to dissolve rough surface features and layer lines, keep a close eye on the object to ensure ‘over smoothing’ doesn’t occur. This is why optically transparent containers are best. Proper observation will ensure a glossy finished product. 

6. Ventilation and Drying

The final stage of the acetone vapor smoothing process is ventilation and drying. Remove the container carefully to prevent contact between the paper towels and the 3D-printed part. Place it in an isolated, well-ventilated area so that any excess acetone can evaporate into the air. It may take anywhere from a few hours to a couple of days to dry out and harden completely. This stage is important to ensure the finished product will have a smooth, rigid finish that is ready to be handled and used. 

Advantages of Acetone Vapor Smoothing

1. Time and Cost-Efficiency

Compared to other post-processing methods that use sandpaper, heat guns, or filler material, acetone vapor smoothing is quicker and cheaper. Those other methods concentrate effects on only one area at a time, so they’re more labor-intensive in general. The acetone vapor, by contrast, affects all areas of the model equally. Thus, acetone vapor smoothing can actually boost your productivity.

2. Ability to Smooth Complex Geometries

Acetone vapor smoothing has advantages over other methods because it can reduce the surface roughness of complex geometries evenly. Other methods have trouble adding or removing an even layer of material, especially over complex geometry. 

3. Strength and Integrity

Prior to acetone vapor smoothing, FDM prints are very anisotropic. This means that their mechanical properties differ when loaded in different directions, and it’s considered a major problem with FDM 3D printing. The vapor smoothing, however, helps to even that directionality out, putting the parts closer to isotropy. In short, the acetone vapor smoothing creates more bonds in the z-axis (perpendicular to the print bed) but decreases strength in the x-axis and y-axis (parallel to the print bed). Such can be advantageous if your item is likely to see z-axis loads. 

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