Designing for Silicon Parts and RPing them.

Currently working on a handheld project and we are considering using silicon for some parts of the housing, in a similar fashion to how Knog lights uses the material.

The engineers I am working with have little experience with “wild” silicon parts, so pushing for the concept, I am trying to gather as much information as I can.

First off from what I understand, you can RP rubber and silicon parts both by casting as well as using objet printers?

When it comes to co-molding, can you build an FDM or SLA part, create a mold, and pour room temperature silicon into it to create a co-mold like prototype?

Also are there any general rules of thumb when it comes to designing silicon parts that act as structure? From what I can tell you can get away with varying material wall thickness and undercuts, but is there anything else to look for?

You can print dual material parts with an Objet printer but most of the samples I’ve played with are nowhere near as durable as an actual cast or molded part would be.

If you’re trying to do overmolded parts, there should be plenty of engineering guidelines in place. If you need, go to a store find as much stuff that is overmolded similar to what you want to do and rip it apart so they can look at it.

When you say “act as structure” - what do you mean? Rubber is useless for anything structural unless you’re using it for something like sealing. You will almost always need a hard plastic substrate behind the rubber for it to bond to. If the rubber is left exposed, you will wind up with a part that can be peeled apart rather easily. You also want to avoid undercuts in my experience. Unless you are using a very soft rubber, having the tool pull against the part is not a good idea and can damage your part or just lower your yield to unacceptable levels.

In my experience the the 3D printed rubber parts are very brittle. If you are going to simulate it for a part that is supposed to flex and bend a lot, like a lid or an iphone case with massive undercuts, I’d say forget it. But - couldn’t hurt to call up your vendor to discuss and ask for samples.

I sometimes use the 3D printed rubber stuff to show overmolded grips, to get a better feeling. In that case it’s almost always 2 halves glued onto an SLS/SLA base. That is fine since it doesn’t move.

If you want to make a part similar to that in your link I would make (order) a silicon mold from an SLA master, and try out a number of Polyurethanes available in various hardnesses. Match it to the silicon properties and you should have a good idea about the real result, and what hardness you need to spec. I’ve seen the process you describe as overmolding (but haven’t done it myself) and it looked like crap. The silicon peeled of and tore way to easily to evaluate any functional properties.

Thanks guys, that some great feedback.

When I said a structural Silicon piece, I meant one that is acting as the housing and not an overmolded grip or plug for a port. Similar to how the Knog light wraps its hard components in silicon housing which makes up most of the structure.

Having taken several knog lights apart, it’s pretty trick some of the stuff they got going on in there. The tool must have been pretty wild or they just pulled the part right off undercut and all. They also have some pretty unique features on the hard assembly to help keep the parting line between hard housing and silicon from slipping off.

If we decide to use major silicon components, I think I will definitely go down the SLA master and create a mold to pour silicon into, I haven’t researched this yet but I believe a bunch of RP houses do this.

Thanks again for the help.

Just a small addition:

For master you should consider MJM/3DP instead of SLA, gets you a better surface finish.
You can also do actual silicon molding in a silicon mold. Normally the materials stick together, but it IS doable by experienced firms with solvents and experimenting on their part :slight_smile: However, a soft PU will probably get you a long way, parts like button arrays are common to do this way.

In production, you can pull it off the mold. I have a part right here with about 15mm 90 degree undercut. BUT - according to my plastics expert the tooling is really complex and hard to get right, and probably much more expensive than typical IM. I mean not just the layout of slides, but cooling and all that stuff… Just passing along…

M3rik, we are working on starting up molding department as part of the 3D printing service I have created. I would like to give you a quote to do prototypes. drop me a PM or email