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PMC-746 rubber is expensive - this is large and it will use a lot. The displacer (left) will reduce the pitcher walls to 1in thick (rather than a large solid piece of rubber). Following is how I calculated how much rubber is needed. First, I used Fusion 360 to generate a solid model of the upper part of a pitcher (this mold will forms that). I sent it to the slicer and it displays the volume in mm3 (as shown in the screenshot inset). To calculate the total volume of rubber needed I did this:
Outer Box volume (250mm x 205mm x 107mm = 5484cm3) -
Inner Box volume (231mm x 180mm x 96mm = 3991cm3) +
Pitcher volume / 2 (1817cm3/2 = 908cm3) -
Displacer volume (292cm3)
= 2109cm3 + 10% error = ~2300cm3
PMC-746 has a specific gravity of 1.0 and the mix requires 2 parts yellow resin to 1 part blue hardener. Thus:
33.3% of 2300 = 765g blue
66.6% of 2300 = 1531g yellow.
This is a Medalta Potteries medium-sized ball pitcher block mold, version 2.0 - it has a more oval body shape. Upper left is the top section (actually, it is half of the top section, a base will fit on to create a three-piece working mold. There is to 3D print something this large in one piece in a consumer 3D printer. Even if it did it would require 50 hours of print time! Also, working surface quality is affected by the orientation of printing (especially if a surface is down-facing and on top of printed support). Further, a large print would almost certainly warp and corner-lift during printing. Cutting it into four pieces and hollowing them individually (in the CAD software, lower right) solved all the problems. Each of the pieces is still quite large, taking 10+ hours to print. But they can each be hollowed individually and rotated to the optimum position for the best finished surface (and print speed). It is amazing how well these four pieces fitted together. This approach really paid off because I made a mistake - and I only needed to reprint one of the pieces!
This is PMC-746 mold rubber being poured into a 3D printed mold (which has been made more rigid with glued on cardboards). The displacer is weighed down with rocks as the rubber is poured in. It reduces both the cost and the weight of the rubber mold. This is one of our early ventures into this method, going forward we will design-in the displacers from the start (this one could have displaced considerably more).
Glossary |
Displacer
Displacers are used to minimize the amount of plaster or rubber needed to make a block or case mold for ceramics. |
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Projects |
Medalta Ball Pitcher Slip Casting Mold via 3D Printing
A project to make a reproduction of a Medalta Potteries piece that was done during the 1940s. This is the smallest of the three sizes they made. |
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