CASTING Plaster Investments vs. Ceramic Shell?????


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Posted by bruce paul fink on January 15, 1999 at 16:48:15:

Having helped develop and done approx 100 ceramic shell castings and possibly 500 plus investment casts in an in-house studio/foundry and having also taught the processes to students on the college level... the comparisons for myself have become quite evident.

I offer this as a discourse for added comparisons by others as the best system will be the one that works for you.

There have always been investers that just couldn't get the feel of sensitivity for one or the other media. For myself the personal bottom line has been to give up ceramic shell unless the specific part was extremely delicate and in need of a stainless steel or silver pour. If I was set up for shell molds though there are many times I would choose it over the plaster based systems, especially if the works were fairly small and easy to handle.

The plaster based/sand investment has proven for me to save time,

cost less, give more consistant results and over all fit the extreme detailing or interior core needs that many of my forms push themselves into.

When doing life sized figurative shapes I find there is also no match in control and ease of handling. That also has to be defined though as I have two one ton bridge cranes, a four ton moving trailer lift and several placed chain falls to assist and many of these finished molds start in the 400 to 2000 plus lb. range. The kiln is lifted off its base so investments can be set in carefully and turning over a 3000 lb. mold is no problem for one person (It's a one man foundry and I always work alone). Without these systems in place any comparative equation could/might have other conclusions.

As for the venting and spruing: stryofoam core sprues and steel

rod cored vents will leave an opening to the investment interior before the burnout even starts. (Styrofoam is often removed just before going in the kiln by pouring in a cup of lacquer thinner and the steel rods are pulled out with a vice grip on a hoist while the plaster is setting and still warm so the wax around it acts a grease). In addition the plaster investment first coat is put on with a heavy retarder so it is still soft when the second pour coat is set in place. Yes, I pour the units up as one mass and abandoned the age old 'fleck it on over a several hour period' system as I've seen done many other places.

The investment is also mixed with as HOT a water as I can get away with, without the wax starting to alter shape or move before solidification.

This makes the wax also expand prior to the investment setting up

and flashing will be kept to a stricter minimum, or often

to none at all.

Another key to this (occassionally used) is to whip the wax in a mixer as it starts to cool and prior to being used in modeling. It then goes on like a peanut butter spread but hardens with microscopic air bubbles that are not visable in the surfaces but will give further expansion room to the wax before melting out.

I always burn out WET molds and if they are drying before

burnout, I rewet them. Water is a great heat conductor to evenly bring the mold temperatures up and the flashing is once again kept to a minimum.

Finally the 6 to 7 day burnout (I only go to 900F by choice)

that is so often quoted as a distinct disadvantage is just

one more of the ways I heat the studio as the burner is

a standard oil fired heater system in a down draft kiln.

Burn out of the wax is complete and little smell is evident in the process.

I would much rather invest a plaster mold in 2 to 4 hours

start to finish and be done and then have 6 or 7 days to

do other things than to spend many short dips and dries

over the 6 to 7 days and then burn out in 15 minutes. This offers me time to focus on other levels and unrelated works without returning to the dips at hand.

(Many of my pieces have had 50 lb of wax per mold and that would make for quite a burn in that short period of time with shell molds or would add another step as the wax was partially removed via another system.)

Investment mold total costs of both equipment, loss of material and initial setup are a fraction of the shell system and I also highly prefer the detail and ease of cleaning off the investments.

While hand removal, sandblasting or high pressure air blasts will do the job, I recently switched completely to power washing off the entire investment with a 2500 psi cold water jet which takes literally minutes to remove to a perfect clean state with no abrasive sand alterations. This requires equipment in the $1000. range but saves days and efforts worth the whole outlay. It also spoils one due to the ease of use and lack of barked knucles or sandy clothing.

Cores are also possible with hollowed pieces and on some occassions I start with a cast core investment and build the wax over it in the very first stage.

Only place I give the shell an advantage plus would

be in less mess during investing

and when casting larger silver quantities when I want the mold to be as hot as the silver that flows in. I don't do stainless steel. Many of these works get a bit large and surface details are a bit deep so that is also a factor.

bruce paul fink


Woodstock, CT USA

I most always work larger than the comforts of smaller ceramic shell molds afford me so I hope others with counter or additional opinions will add to this dialoge to aid the thoughts of persons trying to make first try comparisons. Thanks. bpfink

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