I have wanted to replicate a portion of a pattern so that I could make sodium silicate-bound pieces from the pattern without subjecting the pattern to the somewhat harsh treatment of the pattern associated with sodium silicate bound sand. So I bought some Vytaflex two-part urethane to make an impression copy of the relevant pattern portion. That step worked fine with virtually no shrinkage and excellent replication of pattern detail. The next step was to use the urethane impression and use polyester casting resin to make the "positive" pattern section. That worked OK after I found out that the density of the polyester was greater than the urethane density causing the urethane to float in the polyester---what a mess. The problem with the polyester is shrinkage. I ended up with 6" sections of polyester "positive" pattern pieces that were undersize by at least an eighth of an inch. I have since learned that using minimal kicker can reduce maybe by half but not eliminate shrinkage. I guess my next step is to order urethane and then use urethane to cast the positive mold piece. But are there other more immediately available materials that are tough enough to stand up to 50 or more mold cycles that I could use for the positive? Epoxy is not on the table as my understanding is that it shrinks less than poly but still shrinks on the order of 3%. Denis
I talked a bit with a guy who ran a casting kit company at one time, and I think he used Freeman products. There are quite a few videos on the Freeman's website that I think show what you are talking about. https://www.freemansupply.com/ If nothing else, call Freeman and have a chat with them about what you are trying to achieve. Its what they do so they should be able to guide you with products that will do what you want. .
Yah, polyester is very pour regarding dimensional accuracy but it's inexpensive. I've used a lot of Freeman liquid tooling materials for pattern making over the years. The general range for shrink on both Urethanes and Epoxies is .001"/in to .005"/in with the vast majority toward the lower end of that range. This is generally the case whether they are casting or laminating resins. Casting resins usually need some type of filler for thicker castings because they are poor conductors, generate heat as they kick, and this will cause additional shrink. This also makes them more conductive and typically higher working temperature range. For 90% plus of my pattern making using liquid tooling media I've just used Freeman fast cast urethanes, the repro product line. The only time I've resorted to higher cost epoxies is when I needed heat resistance. I've also used them in composite laminating resins instead of polyesters to avoid shrink on the actual composite parts. For foam bucks or build ups, epoxy wont attack polystyrene. Polyester will. For multi-cavity patterns I'd usually pull a silicone mold off the master and pop multiple cavity tools on a wooden match plate. Here was a an example of a 4-on pattern. http://www.alloyavenue.com/vb/showt...ourse-Tee-Box-Marker&highlight=tee+box+marker With a decent pattern paint and a modicum of care from the mold maker, and say >2-3 degrees of draft, 500-1000 sand molds on a Repro pattern is easily within reach. Deep draws with minimal draft will wear faster. The one in the example above has made 100 molds and other than the parting compound looks like the day I made it. If you were making a core box that would have media blown into it instead of hand packed, I'd use a hard Urethane elastomer for abrasion resistance. The stuff that is about as hard as a hard shoe heal is incredibly tough and great for sheet metal hammer forms too. Best, Kelly
Does using large amounts of inert filler powder in the polyester reduce shrinkage?. You'd think the more solid filler added the less shrinkage would occur: 2/3rds filler would eliminate 2/3rd of the polyester and 2/3rds of the shrinkage. I like to use aluminium dust off the bandsaw, it's a bit coarse but helps reduce heating of the resin when it cures. If you're using near net size castings that don't need any sanding, then black iron oxide used for concrete colour is a cheap filler that boosts hardness of the casting: I fill ping pong balls with polyester and iron oxide to make ball knobs for drill presses, lathes etc. and you can bounce them off the concrete floor with no damage like a golf ball. Yellow iron oxide powder badly retards the polyester cure, it must react chemically with the catalyst.
Fillers do retard shrinkage but they also retard FLOW of the resin. And they make it opaque. So, you’d get more voids and air bubbles and the opacity would prevent seeing them and needling them prior to setup. I have not had time to dig into the links provided by folks above. But I did see one data sheet on a resin that evidently shrinks .0008” per inch. That would be great. I think it was the Kwik-Cast above Denis
That's true, it may be possible to compensate with vibration as some additives are thixotropic and will be runny when vibrated and thick when static. Have you ever tried a cigarette lighter flame rather than a needle, it works great for instantly popping the tiniest bubbles in resins.
Some fillers act as insulators and increase the exothermic heat and decrease the cure time.. Better off buying a product intended for the purpose..
That's where aluminium dust is pretty hard to beat, it's thermally conductive and has a relatively high specific heat, so it takes more energy to heat it up (for a metal)....and if you are sawing runners off castings it's more or less free too. Another free source for me was cutting bronze runners off with an angle grinder, sweep the concrete floor clean before and sweep up the bronze dust afterwards, got about 8 lbs from one casting job. I've tested LG2: 85 copper, 5 tin, 5 lead, 5 zinc with clear polyester resin and it doesn't affect the cure time noticeably. This is in a hot climate where about half a cupful of pure catalyzed polyester will boil and even burn if not cooled during curing. Edit: Copper and it's alloys can speed up the cure of methacrylate resins (Loctite).
Mark, The problem with bubbles is not that bubbles rise to the surface where they can be popped with a propane torch—/I have experience with that. The bubbles that cause trouble are trapped at the interface of resin and pattern. If you cant see them, you can’t nudge them with a needle, they don’t rise and end up causing a void or many voids—-ugh. I don’t want to experiment, but want to find a proven and dependable product with known characteristics that will do the job. Denis
He Denis, what about using a pressure pot to rid the bubbles? I know that sounds strange, but I found some housewife on yt that casts crystals in resin. Where I thought a vac setup would have been correct, she instead is using a paint pot under pressure. No more bubbles.
The pattern is 18" in one dimension. Kinda needs a big pressure pot. This is a one time need I hope. So, looking for cheap and easy. Pressure pots and vacuums both evidently are used. But cost and availability are barriers. I think I need to find the right resin---a known 30 to 50 dollar solution. Denis
Isn't your pattern long and skinny?, stick it in a length of whatever pipe it'll fit in with a cheap USB camera to view the vacuum/pressure progress.
