Saturday, August 14, 2010
Peeling parts off of the raft
I am rather far along in the finger design for the telepresence hand. Now that I have flexible sensors and a need to modify the design to accommodate them, I decided to take the time to reduce the width of the finger design to something approximating my own finger. This entailed reducing its width by 25%.
Doing that was a simple matter of rescaling the STLs in Art of Illusion for the narrower width. Once done, however, the phalanges were so small that using the belt sander to take off the raft was something that was no longer practical.
Heretofore, I have simply printed the raft and part all at 240 C and used the belt sander to take off the raft after printing. With the more delicate narrow finger phalanges, however, I could not hold the parts in my fingers safely and had to use pliers to hold them for the sanding. Unfortunately, getting even removal of the raft by that method was a sometime thing.
After a few goes with the delicate phalanges, I decided that it was time to upgrade the Slice and Dice code to vary temperature by layers in order to make the part separable from the raft.
I use a rather traditional raft with heavy, widely spaced print roads for the first layer to even out any misalignment of the print table and flaws in the print table. I print this at a print speed of F240 and an extrusion rate of S360. For the next and last layer of the raft I quadruple the print speed to F960 whilst keeping the extrusion rate the same. All of this is done at a print temperature of 240 C.
What I decided to do for a removable raft was to keep the first layer as it was and then add a third layer to the raft. The second and third raft layers were to be printed at a lower temperature and then the print itself done at 240 C as before.
I did a series of prints lowering the temperature of raft layers 2 and 3. The sweet spot seems to be at about 215 C. At temperatures higher than that adhesion between raft layer 1 with 2 is too high and adhesion between layers 2 and 3 and the print object is too high as well. I could not get reliable extrusion of ABS at temperatures lower than 215.
The new print protocol produced a raft and print which were reasonably easy to separate with one's fingers as you can see here.
the bits of layers 2 and 3 sticking to the printed part are easily removable with either the belt sander or a piece of conventional sandpaper in just a few moments.