Sunday, December 2, 2012

Improving part surface finish and accuracy for casting

3D printing is great, but RepRap style printers generally don't produce great surface quality, while also having problems reproducing exact dimensions.  Most people address the second issue by filing their parts to fit, but I wanted to see if I could simply print test parts and adjust the dimensions a bit.

This seems to work pretty well, the part shown above had the post mounting holes pattern negatives (the pegs in the above photo) off by about 1.2 mm.  So I just subtracted that from the initial diameter and regenerated the part with my Python CSG library and, to my surprise, the diameters came out correct to within 2 thou.  The same goes for the horizontally oriented cylinder, one simple iteration of correction produced the correct diameters within a very small tolerance.  I've also found that printing external perimeters at 20 mm/s while printing everything else at the maximum reliable 80 mm/s feedrate for my RepRap gives the best surface finish and fastest printing time of all the options I've tried.

To improve the surface finish of the parts I tried filling the surface with Bondo body-filler to fill in the filament marks.  I wasn't sure if the Bondo would dissolve the PLA, but it seems to be fine.  After a light surface sanding the final part had a very clean surface finish, with little to no filament marks and consistent, accurate diameters.

The pink in the above photo is the Bondo fairing compound.  You can get an idea of how thin the coating is since you can still see the dimension adjustments I'd marked on the part through the fairing.  This coating just fills in the small gaps between the filament and allows the surface to be quickly sanded smooth.  This should dramatically improve the surface finish of cast parts.

Spending a bit more time on the patterns, embedding positives of the cores to be used in casting (the vertical pegs and horizontal cylinder above) and moving to an open-topped casting method should improve the quality of my cast parts considerably.  By using an open mold, I'm hoping that the air-bubbles that ruined the previous part can be popped at the exposed resin surface easily.  I've also remove sharp concave corners in the pattern, by filleting the relevant edges, in the hopes of improving these features.  Finally the time spent finishing the pattern should pay off in spades, since it will reduce cleanup of each cast part by the same amount.

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