A few weeks ago we were asked to print a wind tunnel model for UBC’s Formula Racing team. Since a smooth surface finish was paramount, we printed the model in ABS to permit vapour smoothing.
Presented here are some general techniques for achieving fine surface smoothness, even when the printed item is of mediocre quality.
Printing the Formula team’s nose cone marked the first time using ABS in our printer, so we encountered some calibration errors and the end result was of poor quality and contained noticeable artifacts.
Here’s how the printed nose cone turned out. There are some obvious artifacts in the print, including z-axis ripples, perimeter holes, overheating and overhang issues.
The worst of the problems can be seen more clearly in the next picture.
The general approach to finishing these parts involves repeatedly sanding and vapour smoothing the ABS parts using acetone. I recommend this RepRap blog post for an introduction to ABS vapour smoothing.
The first step is to sand the surface of the print with emery cloth or sandpaper. Start with higher grit (300+) sandpaper for prints that are of high quality, and lower grit for parts with more obvious artifacts.
When sanding, you will start to build up ABS powder on the surface of the printed item. Instead of removing the powder, the trick here is to try to coat the surface of the print with it. This will improve the effectiveness of acetone vapour smoothing.
By first building up a layer of ABS powder on the surface, the vapour will condense onto the print and dissolve the outer layer of powder, which will then fill in small cracks and voids on the surface. The high surface area of the ABS powder means that it dissolves faster than the rest of the print, so surface detail can be retained to a greater extent.
Repeated acetone vapour treatments can be time consuming, so acetone can also be applied directly to the powder with a paintbrush.
After several iterations (or just one with good initial quality), a smooth part is obtained. Here’s how the nose cone turned out.
As you can see, the nose cone is now significantly smoother than before. Most of the artifacts are gone, and the print has adopted a glossy sheen even when dry. The part could be finished even further, but this was sufficient for the Formula team’s needs.
On a semi-related note, after careful calibration the printer no longer encounters issues printing ABS, as demonstrated by one of the most recent prints.