Rickresource Rickenbacker Forum

jingle_jangle wrote:Technically not 100% reliably so, as the mold shrinking actually opens up the cavity. You'll get mostly expansion, dimensionally, but as I said, it's hard to predict.

If the wet-sand casting molds experience shrink, then you're right that the cavity will get noticeably bigger. I'm not expert with sandcasting though, so I can't go beyond theory.

But in the lost wax method (which would really be a poor choice, financially speaking, for this job) the modern ceramic shell/stainless wire molds for bronze that I've made are pretty much impervious to anything until you bust 'em up. The more traditional solid investment material used in the traditional large flask method also for lost-wax casting is, simply speaking, a special gypsum plaster. Now, that material does expand minutely in the curing process but you'd be hard put to notice. The investment flasks and the ceramic shells are two ways to do the same thing, which is to house the cavity created by the loss of the wax pattern, and finally to receive and hold the molten metal until it cools. I've never experienced expansion in the lost-wax process, but only shrinkage due to the nature of cooling liquids.

As you know, it's not rocket science, as they've been doing it for 5000 years or more with scarce few essential changes in the process. YYMV is a big understatement.

Well since we've led everyone this far down the road, maybe I should back up and provide a Wikipedia article or two on metal casting

http://en.wikipedia.org/wiki/Lost_wax_casting
http://en.wikipedia.org/wiki/Sand_casting


Sorry to run on so geekily about it, but I used to find it fascinating when I worked in the art foundries. Sadly, though, I never got to work in sand casting. Thanks for giving me an airing.

Airing not finished yet, Kira. My own personal experience in sourcing precision lost wax caused me to write. However, far beyond that is my experience with RTV silicone and urethane tooling of largish parts, in which mold expansion and shrinkage is a sort of black art. When I ran parts of up to 36" X 60" X 3" (prototype Xerox machine bases, back in 1984), we would do two tooling runs, with the first being measured to determine amount of shrink. Numbers indicated that, though our cold-casting material had 3.5% shrinkage--typical for the era and material--we only experienced about 1% in the final part. This was still substantial in a 60" long part. We credited this lessened shrinkage to the enlargement of the cavity due to mold shrinkage, and over many years and hundreds of molds, had no experiences to contradict these findings.

In any event, I came to the same conclusion as you did when I measured a Selmer B5 and a later, newly-tooled one--what a cheapo thing to do (make a mold from an existing part, so that duplicates were 7% smaller). Making a new pattern would not have been that expensive, especially amortized over many thousands of units...

jingle_jangle wrote:Airing not finished yet, Kira. My own personal experience in sourcing precision lost wax caused me to write. However, far beyond that is my experience with RTV silicone and urethane tooling of largish parts, in which mold expansion and shrinkage is a sort of black art. When I ran parts of up to 36" X 60" X 3" (prototype Xerox machine bases, back in 1984), we would do two tooling runs, with the first being measured to determine amount of shrink. Numbers indicated that, though our cold-casting material had 3.5% shrinkage--typical for the era and material--we only experienced about 1% in the final part. This was still substantial in a 60" long part. We credited this lessened shrinkage to the enlargement of the cavity due to mold shrinkage, and over many years and hundreds of molds, had no experiences to contradict these findings.

In any event, I came to the same conclusion as you did when I measured a Selmer B5 and a later, newly-tooled one--what a cheapo thing to do (make a mold from an existing part, so that duplicates were 7% smaller). Making a new pattern would not have been that expensive, especially amortized over many thousands of units...

Oh goody. You were on the other side of the tracks in the precision department, I guess. I used to watch our patternmakers go through the same trial and error process on archtectural hardware duplications (fancy doorknobs, sash lifts, everything up to chandeliers). We also used those same mold materials. I did get a little experience with pattern making with those jobs but no measure of real expertise. Now, on the fine art side of the tracks we didn't worry about a little shrink as it was an acceptable part of the process for nearly all our artists (who didn't need to pay extra to achieve the absolute tolerances). If the RTV or urethanes shrunk during curing, I never could tell it, and the resulting wax castings were always smaller than the models, of that much I'm sure. Expansion in the lost-wax process? That's a new one to me. But like you say, it is indeed a black art with new discoveries to be made on every different job.