A while back I shared some things you should know about urethane castings. I focused mainly on when you should use them, and not so much on some of the actual design considerations.
So here you go… design considerations.
The overarching theme is that rapid prototyping houses like to do things a certain way. Sometimes though, that way won’t cut it. Ask yourself these questions to determine if you have to call out extra detail, or if you can just shoot off some files.
how accurate do you need these parts?
I’ve mentioned before that the accuracy of urethane castings is poor. If you’re rapid prototyping a toothbrush, that tolerance stack is probably not a big deal.
But if you’re trying to prototype fine detail features like press-fit pins, sloppy might not cut it.
Choices: Have them use a machined master to cut down on the tolerances, or oversize the pins and be prepared to core out the holes by hand with a drill bit.
do you have mating surfaces adjacent to aesthetic surfaces?
The way most RP houses achieve a smooth finish is by painting the aesthetic surfaces of the SLA master. It’s a little sloppy but remember, they are aesthetic surfaces, not functional surfaces.
No harm no foul.
The place this can potentially become harmful is if that paint finds its way to surfaces that are used to mate to other parts. Then the extra thickness of the paint can throw things off.
Even a thin coat of paint on a mating surface can make the assembly look not-quite-right enough. It’ll look hacked.
Choices: Ensure your parts fit nicely by calling out what faces need to be masked, or skipping the painting process all together and have the master machined and blasted.
do the internal surfaces need to be smooth?
Most RP houses won’t paint the inside of the master, because it is likely to wreak havoc with the way parts go together (see above).
Fair enough, but the result is that your internal surfaces will have the stratified look of an SLA.
Does it matter? Usually not. But if your internal surfaces are functional (maybe you want a nice clean surface for adhesive), you have a decision to make.
Choices: Use an SLA master and file the surface down manually, or have the master machined.
bottom line: SLA or machined master?
Both. But usually SLAs.
SLAs are faster, cheaper, and more versatile from a design standpoint. Machined parts are more accurate.
Usually SLAs will get the job done, but don’t hesitate to push your vendor if you need it machined.
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