Sunday, March 17, 2013

Mold Making

I've been making molds for vacuum forming recently. My UROP advisor, Steve Leeb, is having me build a portable electronics prototyping station. The biggest design requirement is that Freshmen considering a major in course 6 (EE/CS) be able to fabricate the entire thing quickly and safely. The project has been dubbed the Nerd Kit.

The lab I'm in has a small vacuum former. The max part size it can produce is 9" by 11". It's a good machine. Here are the molds I've made for it so far:

The first mold I made for this project. It's a good example of things to not do. The draft angle is way to low, the holes to provide suction are way too big and originally I didn't add relief holes to the underside. Ideally a male mold like this should have a draft of at least 7 degrees. The suction holes should be about half your material thickness. Relief holes on the underside provide additional sucking power to pull the material down.

As you can see, the parts did not come out well. Lots of warping and overall poor definition. They stuck to the mold and releasing them was very difficult, I had to reheat, apply positive pressure, wiggle, and pull and I still ended up tearing the corner to get my mold back. On to the second try.

I don't seem to have a picture of this mold on hand. But as you can see, the second version of the mold produced a very nice part. Smooth edges and surfaces all around. It popped right out of the mold just like it should. It's kind of a bummer that it won't actually get used for anything. After Prof. Leeb reviewed the overall design I decided that laser cutting the lower insert was the way to go. It's precise and fast and I was eager to try out my new laser cutter access gained though 2.007.

Laser cutters also let you put some very nice raster images on to almost any flat material. You can't really see it in the picture, but there are six cutouts for some keyed banana jacks. A variety of boards with plugs on the back will be developed to fit in that space. There are holes above the breadboards to run wires to your circuit from the jacks. So, no more vacuum forming on the bottom. What about the top? Any proper Nerd Kit needs some tool and part storage, so that's where they'll go. Vacuum forming is back in the equation.

I wanted to make a female mold for this part. I went to the MIT Hobby Shop hoping to find a bearing bit and a router to make the mold by hand again, but by the time I left I had made a MasterCam tool path file from my original SolidWorks model. Turns out it's not that hard to convince me that letting a robot do my work for me is worthwhile. The big CNC router took care of the job for me the next day.

Here are the results, not too shabby. Very accurate molds that require minimal finishing. It's safe to say I'm a fan of this technique.

This time around I made some very small suction holes using a piece of stiff wire chucked in a hand drill. 

Here are the reverse side relief holes, nice and big for (hopefully) good flow.

I haven't had a chance to draw a part from these molds yet. One small problem I've run into is how to get the darn thing below the draw plane. It appears the "Formech Compac Mini" was designed for male molds only. I'll either have to trim the molds and come up with a clever way to seal them or partially disassemble the machine to change molds. A pretty big oversight on my part in retrospect.

More banana jacks are on their way and with a little luck I'll have a complete nerd kit by the end of the week.

Until next time.