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  #1  
Old 05-24-2007, 04:52 PM
Paul Paul is offline
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How much shrinkage takes place from prototype through bronze casting?

I am planning a bas relief that has to be 24"x36"x1". I'll be building the prototype out of plasticine on a plywood support. I m not sure how much shrinkage to calculate for to make the end result as close to the desired dimension as possible. I have heard that bronze will shrink linearly about 2%. I am not sure how much more size to add to compensate for the wax shrinkage. I'll be making a silicone mold, so there will be minimal shrinkage to consider there. Any advice would be greatly appreciated.

Thanks, Paul
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  #2  
Old 05-24-2007, 08:19 PM
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fritchie fritchie is offline
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Re: How much shrinkage takes place from prototype through bronze casting?

If I understand your situation correctly, you shouldn't have to include any compensation for wax shrinkage at all. The way I've seen this done it to pour liquid wax into the mold slowly, with slight tipping of the mold as the wax enters and cools, so that the wax, when solid, is in full comtact with edges of the mold. (The wax should be poured as cool as possible - just above its coagulation temperature, so it flows smoothly but shrinks a mimimum amount.)

So, if you get a cooled, solidified wax in full contact with the mold, the only shrinkage problem left is with the bronze. Your figure of 2% linearly is pretty much exactly what I have seen personally from my waxes to my bronzes (by an experienced founder who has done most of my casts).

The possible flaw in what I have said is that you might find, after you have removed the wax from the mold, areas of wax will not have merged with neighboring areas because of too low a wax temperature when it is added. If you have that problem, the only remedy is to pour a new wax, with a slightly higher temperature or faster pouring. It's a balancing act.
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Old 05-25-2007, 05:44 PM
dilida dilida is offline
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Re: How much shrinkage takes place from prototype through bronze casting?

Fritchie,

Your answer was very enlightening to me. My limited experience has been that one should expect shrinkage with the wax. In order to avoid the "chill lines" or separations, the wax should be sufficiently hot, the mold as well. Can one pour the wax cool enough so there is no shrink at all? Would most foundries be experienced enough to deal with this?

lisa
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Old 05-26-2007, 08:02 PM
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fritchie fritchie is offline
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Re: How much shrinkage takes place from prototype through bronze casting?

Both Lisa and Paul; My apologies in giving what was only a partly correct response. Later I realized what I had omitted, but this is my first chance to get back. I spoke of pouring molten, but cool wax into the mold. Much more common with a mold of this size, or even smaller ones, is PAINTING the wax into the mold, with a cheap, throw-away brush. I have used both horse-hair and synthetic, ordinary small paintbrushes for this purpose. In my experience, the natural bristle brushes work better, but they may lose a bristle on occasion, and those should be removed from the wax. These brushes actually can be reused repeatedly.

The proper first reply should have been something like this: Put the mold in an accessible, flat location, and carefully but fairly quickly paint in wax that is uniformly warmed and mixed but almost cooled to the congealing point. I use a small-point soldering iron to correct tiny troublesome spots such as bubbles that may develop during application of the wax. Sometimes it also is helpful to go over the inside surface of the waxed-filled mold with an electric, pressing-type iron. That's generally not required, but the inside surface should be fairly smooth, as tiny bits of mold material might break away and cause problems if it is too rough.

Painting the mold this way allows vertical, or even overhanging edges, to be coated uniformly with wax. Uniformity of thickness is important, as it will affect the cooling rate of the cast. If some portions cool much faster than others, the chilled area will "steal" metal from hot areas, even to the point of tearing the setting metal and producing holes in the cast. In general, my founder recommends a thickness of about 3/16 inch, or about 3 mm. If you paint the mold this way, be sure to get fully thickened wax at the edges. The wax is smoothed easily on the edges also with an iron.
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Old 05-29-2007, 09:17 AM
Paul Paul is offline
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Re: How much shrinkage takes place from prototype through bronze casting?

Thanks for your advice and expertise, Fritchie! I'll keep your reccomendations about the wax in mind. It's great to have a forum like this to access people with more experience.

Best regards, Paul
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