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Quoted: There are also some great RTV urethanes available for mold making. There is one manufacturer in Houston that makes it specifically for casting...I need to dredge up the literature on it. What was really cool was the shrinking/expanding line. You could take your pattern, cast the mold and then soak the mold in water to get a mold that is 50-100% larger than the pattern.
For lead jigs, neither silicones nor urethanes will work.
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I am only making plasic heads out of casting resin. Does RTV urethanes work good with casting resin? I would like the molds to keep the same shape.
Sorry a little confused which product would I want from industrialpolymers to make molds of existing plastic lure heads and then using casting resin to reproduce them? It would also be cool If I could make the same heads but in a soft rubber plastic from. Does anything they sell do that?
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Yes, nearly anything is possible. Too bad you are in NY and cannot take a trip to see how it is all done. For molds, it is best to have some rigidity in the mold material but to also have a good deal of elasticity to remove the cast product. This limits the amount of draft necessary in the pattern, meaning you can cast round objects with relative ease. The mold will flex to remove the cast object and not damage the mold.
The project starts with a mold box. This is where you pour the mold compound. It needs to be twice as thick as the object you are replicating, also known as the pattern. The mold box needs to be very smooth and coated with a release agent. Silicone is good for all but silicone molds, these are best with a wax. But the important element is very smooth walls. Plastic works well. A "tree" or casting channel is also needed. This is just a rod or "tree" of rods connecting all patterns in the mold. This is used to fill the mold with the product used in casting the finished product.
The first step is to cast the bottom of the mold. Coat mold box and pattern with release agent. The mold box is half-filled with the mold compound and then the pattern is set onto the surface of the molding compound and carefully forced HALFWAY into the compound along with the tree, also coated with release agent and also only halfway submerged in the mold compound. It is allowed to cure completely without moving the pattern. Then the top surface of the molding compound is coated with release agent and the mold box is completely filled with molding compound. After a full cure, the top is removed and the pattern/tree are removed. THe mold should now fit together, allowing highly detailed replicas to be made. Care is needed when fitting the mold halves together to match the sides and also to achieve the proper clamping force. Too little and the mold leaks. Too much and the molds are deformed, producing a mishshapen product. Experiment.
Now for the specifics. Hardness and or flexibilty is measured using the Shore Durometer scale. Anything in the 40-70 "A" scale will be adequate. This range is a soft rubber but for your application, it will be fine. Working time is important, longer is better so go with the Truecast GTS65 Slow.
For making the actual product, you could use your casting resin or go with any of the RTV urethanes for more flexible patterns.. Yes, the GTS could be used although they are not as durable. The softest is like that of plastic worms (0 Shore A) and it works up to hard rubber (80 Shore D). If you chose a flexible product, you will need a "backbone" in the mold to attach the hooks and tie eye. But since the molds are flexible, knife slots can be cut in the mold to hold these items. Just make sure plenty of release agent coats these modifications to the mold.
And some important notes. Cured urethane will NEVER come out of clothes. No solvent will touch it. It is on your skin until it wears off. And it takes a LONG time to wear off.
For highest quality, vacuum degassing is a big help. I mix the two components and then give them a 2 minute vacuum degassing step before pouring. All bubbles expand, rise and break, leaving NO bubbles in the finished product. I use a Mity Vac hand pump and home made vacuum chamber.
Give them a call. The president was a most interesting fellow and taught me most of everything I know. He even took me through my first urethane casting.