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Old 07-16-2015, 04:13 AM   #13
Scotophor
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Join Date: Aug 2014
Location: La Puente, CA U.S.A.
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You've found the same resin and tint products that I've been planning to use. However, Crystal Clear 202 might be a little too fast-setting for this kind of fiddly work, so I was thinking of using #200 and post-curing it with a heat treatment if needed.

For the LEDs, you can use any shape of regular through-hole component LED, because the epoxy encapsulation is not critical to their operation. You can grind it away to any shape/size within reason, as long as you don't damage the metal or semiconductor parts inside... especially the spiderweb-thin bond wire that connects from the top of the side post (usually the positive or anode lead) to the top contact of the diode chip. So I would start by finding LEDs of the correct pale transparent tints and emitted colors, and not worry about what shape they are.

The hard part for me is making the master patterns from which the mold will be made. There are several reasons not to use original LEDs for the master patterns:
  1. The RTV silicone from which the mold will be poured shrinks slightly as it cures.
  2. The clear resin with which the replica LEDs will be cast also shrinks slightly as it cures. Together, these shrinkages mean that the master pattern should be in the range of about 0.3-1.5% oversized, depending upon the actual products used, and may take some iteration to get right. I know that percentage range doesn't seem like much, but remember there will be several clusters of several LEDs each in the prop and differences add up.
  3. Because the original LEDs were cast in metal molds, they were made with what is known as a "draft angle" of about 1 on their sides to allow them to be removed easily from the molds. This is almost unnoticeable in single LEDs, but when you put several of them together in a cluster, they either don't butt up together flush, or don't conform to a flat surface on their tops and bottoms. While I'm not sure how the original prop makers dealt with this issue (mainly due to lack of a high-resolution, close up image of one of the original props from a particular angle), my guess is that they used a belt or disc sander with fine grit to "square up" many of the prop's LEDs before assembly. To avoid this step, I'll make master patterns without any draft angles. The flexibility of the silicone mold will allow for easy removal of the patterns and castings.
To make the casting process efficient, I would make a multi-cavity open-top block mold in a jig so the LEDs can be accurately held in proper position as the resin cures. I have already designed and acquired the materials for the jig, as well as a supply of the LEDs I intend to use. What I don't have is a means to easily make a bunch of small rectangular-prism shapes of identical precise dimensions, with optically flat surfaces.

I've thought about these issues and the solution I've come up with is to first use a milling machine to make a flat plate of acrylic the same thickness as one of the dimensions. Toward this end I've taken a basic mill operation class at one of my local maker spaces. Then the plate will be cut into the individual master pattern blocks, possibly by a laser cutter. Once milled and cut, the blocks will be drilled to hold common 3 mm round-top LEDs glued in with leads in the same positions as the cast replica LEDs' leads will eventually be, for precise alignment in the casting jig. Then the pattern blocks may need to be annealed (if a laser cutter was used), and finally if the surfaces are smooth enough, the blocks can be quickly vapor-polished.

Then it's simply a matter of putting ~20 or however many of the master patterns into the jig, pouring the mold, letting it cure, and removing the patterns from the mold and jig. For casting, I assemble a bunch of prepared LEDs into the jig, pour or inject the color-matched resin into the mold and let it cure in a pressure pot.

I welcome any input of alternatives to or improvements upon these ideas.
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Last edited by Scotophor; 07-16-2015 at 05:39 AM.
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