So today I was making a new ultrasound phantom (a phantom emulates an organ or block of tissue for radiology devices to image - in this case it is a block of soft tissue that matches the ultrasonic properties of muscle with some 3D printed paraffin (Moldilay) blood vessels I will fire out of the block. I 3D printed the outer mold, which took a while (that’s red Proto-Pasta matte HTPLA) and one of my lab folks asked why I didn’t simply cut a mold box with the GF out of acrylic. For those of you planning to use laser cut boxes for molding, It actually does not work well (which is a shame since lasering the box would shave a few hours off the process, and some of my molds are huge (like the pelvis model I have), and I figured for those planning to do this let me save some failures for you.
When you use a liquid cast (such as silicone) the liquid flows to match the shape of the container, and is quite held in via sanction (even if you slather on mold release like I do) so you need to add “draft” to your mold (ice-cube trays are the perfect example they don’t have straight sides for this reason or you’d never get the cubes out). I fillet all corners and add draft which onshape knows how to do via a single click.
I know some folks have done some chamfered edges on the GF with wedges, but for the liquid tight edges you need in a mold, that’s going to be super hard to do with that level of precision:
I do wish GF made a castable compound (like Proofgrade paraffin sheets). One other thing to know if you are investment casting via 3D printed Moldilay, since 3D printed objects normally use infill, the parts float in the silicone, which can cause all sorts of hilarity. I’ve taken to using museum gel to hold it down. and it provides a seal to the outside of the mold wall, which is useful when you go to fire the mold for the wax to drip out.
(note the ragged edge is from some post-processing I did where some dripped on the table)