Casting Caveats

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Casting Caveats

Post by guest »

If you're considering using casting resin to make a Hornyak button or BC-720 type detector, be careful when you pick up the casting resin in the store. Shake the container to make sure it gurgles. This is a sign that the resin is still good. I was burned this weekend, finding that the can of resin I had set aside to make a detector had hardened in the can. A quick trip to the plastics store just before closing time yielded another can of resin, which unfortunately had also gone bad. I'll be doing no casting this weekend as a result, but it gives me time to prune my roses and build another preamp.
Jon Rosenstiel
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Re: Casting Caveats

Post by Jon Rosenstiel »

I believe I read somewhere that casting resin has a 6 month shelf life.

I'll pass on a couple of things I learned while casting Hornyak buttons.
1. Don't use more than the recommended amount of hardener. (Amount of hardener is based upon the thickness of the mold).
2. Don't try to "hurry up" the hardening process with heat. This stuff is like the proverbial "watched pot"! After it is poured it seems to take forever to harden, but if it's too warm when it does decide to "go off" it then hardens too quickly, creating a lumpy surface.
3. It seems to take 3 to 4 days before it is not sticky to the touch.
4. Consider casting one or two "buttons" using just the plain resin, (and hardener), to get a feel for what is happening before you mix in the valuable ZnS.
5. Patience, patience, patience!

Jon Rosenstiel
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Re: Casting Caveats

Post by DaveC »

I have been using a marine epoxy resin that's available in the Home Depot stores, and also in auto supply stores. It is intended for use with fillers, and or fiberglass cloth. What is particularly nice about it is that it mixes at 1:1 with the hardener, and sets at room temperature in about a half hr.

But as already noted in this thread, slow setting generally gives better optical qualities. You can retard the cure rate by putting the casting in the cool part of your frig. Also, the sticky exterior, is usually quite slow in hardening, taking up to a week. But you can usually just wipe the casting down with a little solvent to remove the sticky layer. The casting itself will be fully cured.

Dave Cooper

Dave Cooper
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Re: Casting Caveats

Post by guest »

When using epoxy be aware that you can get "bloom" that is curative that leaches to the surface. This most certainly could affect your optical properties. Most boat building and even woodworking sites have a lot of information about epoxies. You can easily vary the setting time by adjusting the curatives, usually amines, singly or in blends. If you make it to fast it can actually spontaneously combust from heat of reaction.
This is usually only a problem with larger castings and thick sections.

If you have a sticky surface be sure you are using epoxy. By the one to one ratio I am sure you are correct. However, many resins (e.g. polyester, are peroxide cured (a few drops of catalyst). In these cases the sticky outer surface is cause by oxygen interference. It will react with the peroxide and inhibit cure to form the sticky surface. If you can exclude oxygen this will not be a problem.
Jon Rosenstiel
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Joined: Thu Jun 28, 2001 1:30 am
Real name: Jon Rosenstiel
Location: Southern California

Re: Casting Caveats

Post by Jon Rosenstiel »

I wonder if the optical properties are that big of a deal when ZnS is mixed in. Even at 2% the resin is quite translucent.

Jon Rosenstiel
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Re: Casting Caveats

Post by guest »

The optical properties of the base resin are still important, even though the addition of the ZnS phosphor turns it translucent. You want to start with a base resin that is water -clear. There are enough problems with scintillation self-absorbtion by the ZnS without throwing in off-color resins (especially yellow) or turbidity into the mix. The self absorbtion by the ZnS is another reason you don't want to overload the resin with phosphor, hence the optimized formula posted elsewhere in this section.
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