3D printing has made it incredibly easy to produce small runs of plastic parts, but getting rid of the 3D printed look can be tricky and time-consuming. When you need a smooth and polished finish, or you want to make exact copies of an existing injection molded part, casting resin parts in silicone molds is an excellent option. [Eric Strebel] has plenty of experience with the process, and demonstrates it in detail while creating copies of violin chin rests that are no longer in production. It’s an interesting application, where 3D-printed layer lines are not just an aesthetic issue, but something that would irritate the user’s skin if present.
Creating silicone molds requires a bit of forethought about the mold design. You want to select the split line to make it as easy as possible to remove the finished parts, while also placing the resin pouring sprue and vents to prevent air bubbles from getting trapped in the mold. In [Erics] case, it’s impossible to use a simple planar split line, so he mounts the master part on a block of wood and uses cardboard and modeling clay to create a volume where the second side of the mold will protrude in the first side. It’s important to note that sulfur-free clay must be used, otherwise the silicone might not cure.
One side of the silicon mold is cast first, and after curing it is placed back in the mold box with the master part to allow casting the other side of the mold. At this point [Eric] super glues the sprue-former and vent rods to the master parts before molding the second side. A release agent consisting of petroleum jelly and naphtha is added wherever the two sides of the mold will touch, to prevent them from sticking together.
Bubbles are your enemy while resin casting, so ideally you need a vacuum chamber to degas the silicone and resin before casting, and a pressure chamber to allow the resin part to cure. While pouring the silicone for the molds, the mold box is placed on a vibration table to allow any bubbles to rise to the surface. While the entire mold-making and molding process is time-consuming, the copied parts are almost indistinguishable from the original.
[Eric] has also shown us how to make much larger silicone molds in the past. If you find yourself making lots of different-sized mold boxes, it might be worth building an adjustable mold box.
This is what 3D printing was intended.
Maybe the resin ones could do a sufficient job. But this hops, skips and jumps over CAD, faff and finishing while presenting a totally separate set of challenges. Good stuff!
Not that I’m good at playing a violin, but I do have a face and assume that if the face was not covered in beard, a nice smooth textured surface would be preferable for a chin rest.
It would be great if one day 3D scanners and 3D printers would work as well as their 2D equivalents. Nowadays 40 EUR buys an multifunction inkjet that makes passable copies of most documents and photos. Imagine if one could just scan to blender, sculpt a little if needed and print back out.
This is cool, but if layer texture is a problem with 3-D prints of these, why not smooth the prints with the acetone or isopropanol methods?