Summary: Quantify your ABS shrinkage rates and scale your model to compensate...maybe.
ABS is known to contract or shrink as it cools. The amount of shrinkage depends on the quality of your filament as well as your printer settings and capabilities. Heated build platforms and enclosed print chambers can minimize these shrinkage problems that can lead to warping in ABS parts. I never noticed much of a problem with the final size of my ABS parts until I started printing small holes. I was designing a bunch of 3 mm holes into my parts and each needed to be drilled out after printing to get the hardware to fit.
So I decided to print a bunch of ABS pieces and compare the CAD dimensions to the final printed dimensions. I used MatterHackers Red Pro Series ABS filament for these prints. All prints were with 0.2 mm layers at 230°C with 90°C bed temperature, 50% fan, and 107% flow. I also used the brim option for adhesion. I started with cubes 5, 10, 15, and 20 mm square.
When the prints were finished, I carefully measured the length, width and height of each cube with a set of precision calipers. Ironically, all dimensions actually increased except for two measurements. There doesn't appear to be any correlation between the size of the print and the amount of error. Note: I tried carefully to measure at points on the cube that were clean, avoiding any print fragments or over-extrusions.
|shrinkage of cubes|
Next I wanted to test hole sizes. I designed a 3 mm thick card with 5, 10, 15, and 20 mm diameter holes in it. It was printed using the same settings as above and measured with the calipers. In all cases, the diameter of the holes were smaller than designed. And this time, there was a clear decrease in print error as the size of the hole increased.
|shrinkage of holes|
As it turns out, it looks like shrinkage isn't as much of a problem as I thought it was. In fact the cubes were slightly oversized. Perhaps it is the quality of the filament. Or maybe the files I printed are accidentally designed in a way that minimizes shrinkage. Or does the extra flow at the nozzle make up for any shrinkage? I don't know, but so far the dimensions of ABS parts from the UM2 are almost perfect. But, if you do find you are having problems with final dimensions, you can always repeat this test and scale your final prints. As for the holes, you may want to oversize your small holes accordingly or simply drill them out to their final size.
|for perspective, this is the average error in the cubes|