Silicone Design Limitations: Is there a minimum/maximum dimension that is achievable or surface finish that is unattainable?
By: Kevin Franzino
Silicone, especially liquid silicone will conform well to just about any mold shape that can be dreamed up, the difficulty is typically in the mold making process. In the prototyping sector, we see a lot of cutting edge design concepts that push the envelope with features that are very small and precise. Every molder has their own specialty and also their own limitations. At Albright we specialize in quick-turn micro medical device parts molded in liquid silicone rubber. One design limitation that we frequently contend with is the size and geometry of micro features, we like to use "The 3X Rule", which basically means that if you have a microscopic opening in your part design (which would be reflective of a raised boss in the mold) the overall height of that feature shouldn't exceed three times the diameter (or side length) of the feature. That would mean that if you wanted to form a 0.005" diameter hole in your molded part, you wouldn't want to go much more than 0.015" deep into the part, from a tool making standpoint.
The reason for this is due to the radial force imparted by the cutter onto the work. While an endmill is running in a milling machine, it is pushing (slightly) on the part that it is cutting, the whole time the flutes are engaged. This isn't a particularly big deal when you're forming a block with six inch sides, or a even a one inch diameter rod, but when cutting features that are close to the size of a human hair, every bit of length will make it more likely to deflect, or worse snap.
This limitation can frequently be worked around, however. A customer recently came to me with a request for a 0.25mm slot in a molded silicone part reaching approximately 5mm deep, after going back and forth we ultimately settled on a rectangular feature that had a combination of two tapers and a step. In the end we managed to make a 0.25mm opening that was connected to a through hole roughly 5mm deep, and everyone left happy.
In answer to the surface texturing, that's actually an easier one. We sand blast, bead blast, and polish molds all the time. It can help to produce both more aesthetically pleasing parts, as well as parts that mold easier/quicker/more efficiently. If your part design is flexible, it can help a lot on the mold processing end, if one side can be blasted and the other polished, even if they are both light. That can help the parts to more predictably stick to one half of the mold, which can shave seconds off an operator's time. If you are looking for a mold texture that's really fancy, we've had good luck with Mold-Tech in the past.
To sum it up silicone will reproduce features from pretty much any mold that can be created. Avoid excessive undercuts, and if at all possible, respect "The 3X Rule" with micro parts. If your part requires features that are difficult or even "impossible" to machine into a mold, work with your mold designer, odds are there is a compromise somewhere that will keep everyone happy.
If you have further questions on this topic, contact Kevin Franzino at firstname.lastname@example.org.