Looking for a better castable mold material

Jim Bryant · Post by **Jim Bryant** » Sat Jan 03, 2004 10:10 pm

My situation is that I need to form 200 or more parts and the castable and release I am using do not last very long.

Currently, I am using Harbison Walkerâ€™s Kastolite 26 with pure talc as my release. The glass I am using is Guardianâ€™s 3.3mm clear float, tin side down on the mold at about 800Â°C for a period of time.

After making a new mold, I will get several good parts, then small particles of mold material from the raised areas will first start to stick to the glass. Once this starts, the problem only gets worse. I reapply the talc after every firing.

I will only get 15 to 25 usable parts per mold before I have to make a new mold. My molds range in size from 8" to 30" wide by 30" to 74" long. The larger molds can be very heavy, about 175 lbs. Having to make these molds over and over is too expensive and time consuming to get all the parts done I need.

Does somebody have a solution to my problem? Any input would be appreciated. Thanks.

Jim Bryant

Brock · Post by **Brock** » Sat Jan 03, 2004 10:12 pm

Tell us more about the shapes of the molds. Could you use metal? Brock

Jim Bryant · Post by **Jim Bryant** » Sat Jan 03, 2004 10:33 pm

Brock,

The shapes are primarily geometric shapes. The designs duplicate three dimensional images within those shapes, also an imitation bevel effect and a variety of textures.

Jim

Brock · Post by **Brock** » Sat Jan 03, 2004 10:40 pm

Hmmm . . . Mold Mix 6? Brock

Jim Bryant · Post by **Jim Bryant** » Sat Jan 03, 2004 10:59 pm

Brock, I have tried mold mix #6 for patching bad spots on molds, and have found that it too will stick to the glass in spots. This is a production item and I am trying to keep costs down if I can. The Kastolit 26 I use costs about $.60 per pound.

Brock · Post by **Brock** » Sat Jan 03, 2004 11:00 pm

Sorry, I can't help you then, casting is not really my forte. Brock

Tom White · Post by **Tom White** » Sun Jan 04, 2004 12:06 am

A couple of thoughts on this, first 899 C = 1472 F. Could you get the results you want with a longer soak at a slightly lower temp? 1400 F = 760 C as a starting point. In my experience the hotter glass is heated the worse it sticks to anything it touches. Second, I have had good results with several different glasses with a release of 85% finely ground alumina hydrate and 15% EPK kaolin. I use 100 grams of this to make a pint of release and add a bit of cmc gum to give it some strength before it is fired. The alumina hydrate is ball milled very fine before mixing with the kaolin. I apply several coats to the shelf or mold surface with an airbrush. I have used this for pot melt cabs with System 96 glass at about 1600 F with very smooth backs and minimal sticking to the glass. If I only go to full fuse with float glass I can get several firings without removing the release and reapplying. I have been guilty of just airbrushing another thin coat on the shelf every three or four firings.

Best wishes,
Tom in Texas

Jim Bryant · Post by **Jim Bryant** » Sun Jan 04, 2004 1:19 am

Tom, first, I have a boss who is already on me about the length of time for a firing cycle. His background is in tempering, and although he's begining to understand the reasons for the time it takes for forming glass, he still would like the firing cycle to be as short as possible.

Next, is the release not multi layered because of buildup? And, how is the release removed for reaplication?

We do firing cycle after firing cycle, without allowing the molds to cool below 160 degrees C, 24 hours a day, five days a week.

Thanks for your input.

Jim

watershed · Post by **watershed** » Sun Jan 04, 2004 1:21 am

anything wrong with straight Earthenware? I've been getting pretty good at popping the excess glass out of pots, and salvageing them.

Many of the cheapest kiln shelves are just high strength, high grog stoneware. Might require re washing/sanding after each fire, but should be worth some testing. Especially if you don't want to play with your temps.

OH shoot, Here's more. misc quick thoughts

If you reinforced the edges of the molds, steel etc, you should get fewer cracks. Castable likes to heat up and stay up, any ups and downs mess with it a bit. How about rigidized fiber?

You mention costs, but if you have to make 2x the molds, vs paying + 1.5x the cost, you still make good because of your man hours...

Greg

hoknok · Post by **hoknok** » Sun Jan 04, 2004 2:02 am

I like the idea of clay also.

Ceramics will last much longer and you can also have the texture too. You can bisque fire the clay molds yourself if your kiln goes above 2250F. You can also make many copies of the same master clay mold.

gone · Post by **gone** » Sun Jan 04, 2004 4:49 am

Michael Dupille has a moldmix out that is supposed to last many firings, although I don't personally know anything about it.

Lauri Levanto · Post by **Lauri Levanto** » Sun Jan 04, 2004 4:58 am

Charles Brad's Dictionary of glass
mentions Ciment Fondu - that is alumina cement - and Secar
as materials for glass blowing molds. Those are
prefired to 900 C before use.

No personal experience with them.

-lauri

hoknok · Post by **hoknok** » Sun Jan 04, 2004 6:32 am

I have made two molds from a product a South Eastern refractory company has mixed for me that they claim is more updated than Cement Fondu. It is made up of Calcium Aluminate and various meshes of TAB. Once mixed, I take it to 300F to burn out the water. I made two different castings of two inches deep and 4 inches deep and covered with kiln wash. It lasted me about 6 kiln fires and the seventh (and last) is cooking now. It started to tear at the edges and come up with the glass. The mold was about 2x3 feet and requires an electric wench to lift and move. I can post a photo once I figure put how to.

I am also testing a plastic-like Alumina-silica-phosphate mold currently called P9FF which is moldable like clay but when fired to 900F gets hard like cement. I have not casted anything with it yet so I can not comment on the results.

Bert Weiss · Post by **Bert Weiss** » Sun Jan 04, 2004 11:23 am

I think that your problem lies in using talc as the release. I would use Bullseye wash or make your own either 80/20 alumina hydrate/kaolin or 50/50. The 80/20 is easier to remove. Another dry powder release would be plaster of paris. I never tried bentonite, but we're told it works as well.

The Kastolite has lots of large pits in it. I would try and fill them with some other material, maybe even plaster/ silica or plaster/ silica/ grog and then kiln wash over that.

I have made large molds with kastolite. My biggest problem was lack of green strength. Geting the mold from where it was formed to the kiln was my problem. After it was fired it was stronger.

Ceramic is only useful if you are not draping at all. Glass shrinks more than ceramic.

Whatever you use, shrinkage rates need to be calculated in to the design. The mold does not usually end up the same size as the original.

Tom White · Post by **Tom White** » Sun Jan 04, 2004 11:28 am

Jim, I have a couple of questions about the talc you are using as a release. First, is it raw (gray color) or calcined (white color)? Second, are you applying it by dusting it on the mold dry or are you applying it in a slurry form?

There has been some discussion on this board in the past about the reason to change (replace) the kilnwash on shelves used for fusing firing each time being due to the fact that at those temps some of the fluxing agents in the glass are leaching out of the glass and depositing in/on the kilnwash. This is sometimes shown by a change in color of the kilnwash after firing where the glass laid on it. That "contaminated" kilnwash is more likely to stick tightly to glass fired on it in later firings so the whole coat of kilnwash is removed and replaced with fresh.

If you are only dusting dry talc on your molds it would seem logical that the highest points of your molds would be covered by the least talc and that the fluxing agents from the glass are being deposited on the mold material in those areas first. Subsiquent firings increase the buildup of these materials in/on the mold itself until the glass begins to stick in those areas. Would it be possible for you to mix your talc to a thin slurry in water and apply this to your mold with an ordinary compressed air paint spray gun? The sprayed slurry should deposit more talc on the highest points of the mold and at 160 C the water should evaporate off about as fast as you can spray.

Another thought to consider would be doing a small scale test using finely ground alumina hydrate instead of talc in a sprayed slurry application. You might have to grind the alumina hydrate yourself as I do if you cannot find some at about 325 mesh. This is normally mixed with kaolin as a binder at rates of 15% to 50% depending on the specific use. The more kaolin the harder the release after firing and the more difficult it is to remove from the shelf/mold.

IMHO the highest points of the texture in your mold are always going to be the most subject to contamination by the fluxing agents from the glass and anything you can do to protect them with heavier application of any release agent you use will prolong the useful life of the mold.

Best wishes,
Tom in Texas

Barbara Cashman · Post by **Barbara Cashman** » Sun Jan 04, 2004 8:08 pm

I use a refractory cement made for boilers. Cost about $18.00 for 100 lb bag. Does not shrink, but weighs a ton. Mixes like cement with water, and I don't use a kiln wash for slumping molds. I don't know if the material is what you are looking for, especially since it is so heavy, but email me and I'll look up the manufacturer when I get back to the shop. - Barbara

Jim Bryant · Post by **Jim Bryant** » Sun Jan 04, 2004 9:27 pm

Barbara C.

Yes, I would like the info on this material. I am currently testing a material that is like you say, "weighs a ton." I have also tried a pure boron nitride as a release agent, which at first showed lots of promise, but after about ten firings or more would start to stick to the glass. Reapplication would, like kiln wash, build up and create more problems than solve. Boron nitride is also very dificult to remove from the mold. You also mentioned that you do not use kiln wash. What do you use, anything?

Thank you barbara.

Jim

watershed · Post by **watershed** » Mon Jan 05, 2004 2:44 am

Just a nosy. OT question. Are you making enough money on these pieces, or did you get caught in a low bid situation?

What I mean to say is this; Is this crisis management, or planning in process?

Greg

Jim Bryant · Post by **Jim Bryant** » Mon Jan 05, 2004 9:34 pm

Watershed:

Right now, where the bulk of our costs are, is in inspection of the parts. Since we still get some release agent on glass or mold material, and with a customer who is quite particular about the quality of the parts, we spend alot of time inspecting and clean of each part. If we can improve the process, then there won't be as much time in the inspection process. We also are always trying to fine ways to cut costs, like any manufacturer would.

Jim