Frit Casting Project Help

[Ryan Blackburn]

Hey everyone,

I’m new to the forum, and to kiln/glass work in general. I’m planning on doing some frit casting to create a glass part for a project that I’m working on. I was wondering if anyone could give me some feedback on the plan I cooked up, and maybe point out any mistakes I’m about to make / anything I forgot.

I bought a small/test Cress kiln the other day for $225 and have been doing research all day, for the past 2 days. Here is the plan that I came up with.

Kiln I’ll be using: Cress BT711H


Finished part I’ll be creating: (with Bullseye 90coe frit powder)


Plaster mold I’ll be creating: (not sure which plaster/silica mix to use)


Mold Creation Process:
1. 3d print model in wax
2. Cast wax model in silicone (to create multiple wax models)
3. Secure wax model in mold box & cover it with plaster/silica/water mix
4. Let mold solidify, then cover plaster mold in plastic bag for 48 hours
5. Let plaster mold dry for 24 hours
6. Bake plaster mold at 225 degrees, upside down, until all wax melts out
7. Slowly heat mold in kiln to 1250F

Frit-Casting Process:
1. Spray Plaster mold with MR-97 (to keep glass from sticking to plaster – so I can re-use mold)
2. Fill plaster mold (& reservoir) full of frit powder & softly pack it down
3. Fire the mold

Firing Schedule:


A Few Questions: (if anyone feels like answering any)
1. Is there any way I could configure a PID controller/relay to where I can still control the ramp rate with the low/med/high power dial? (PID controller like this one -- https://www.youtube.com/watch?v=D4lVd5-VAyE )
2. Powder/fine frit is advisable when using MR-97 (to prevent scratching the layer of film), but will this present challenges as opposed to larger granules? Would it be smart to put some larger chunks on top, to melt down?
3. Is there a specific kind of wax, used for lost wax casting, that won’t leave a thin layer of wax behind on the plaster after most is melted out?

Thanks in advance.

[charlie]

1:yes
2: frit, when fired, can lose 50% or more of volume
3: any wax left over will burn off when heated up. usually it's steamed out, but that leaves traces that don't matter when firing.

[Jenna]

Wow. You've definitely done your research. Looks like an interesting project. Just out of curiosity, what is the part intended for? Here are some early thoughts...
If you make your molds out of plaster / silica, it is unlikely that you will get more than one piece of glass per mold. After a full fuse firing, the plaster becomes extremely fragile and brittle and hard to handle. (If I am lucky, I can get two firings per mold ) Even though you have plenty of draft on most parts of your pattern, it looks like the small hole has vertical sides. The glass will contract around that point and you most likely will not be able to just slide the part out of the mold without breaking off that area of plaster.
Continuing with the assumption of one piece per mold...think about making your pattern/model out of silicone instead of wax. This will save you the headache of getting the wax out of your mold. The plaster comes off the silicone without any residue and you can make endless molds from your one silicone model. Tip...spritz the silicone with some rubbing alcohol right before pouring your plaster to prevent air bubbles on mold face. Also, Don't forget to weigh your model to be able to calculate the amount of glass you'll need to make your part. You'll need the specific gravity of the silicone you're using.
As for the grade of frit...the finer the grade the more volume so consider adding a reservoir to your mold to hold the extra volume especially if youre using powder or fine. If you are careful, you can use a medium grade without scratching the MR-97. Or just use Kiln wash. The main consideration being whether the glass frit is small enough to reach the bottom of the mold cavity. It looks like your limitation is .1" for the bottom of the mold.
Also, step 7 of your mold creation process, why are you heating the mold to 1250 deg. At this point?

[charlie]

also address your annealing schedule. the one you have is for flat glass, not molds. you have something almost 2" thick. it has to cool pretty slowly.

[Ryan Blackburn]

Thanks for the informative responses you guys - that's going to help me out a bunch. I knew there were probably a few things I was overlooking.

@Charlie

I'm glad you mentioned something about my annealing schedule -- I assumed, naively, that since most of the walls were only about 1/8" thick that I could get away with a shorter annealing schedule. Would you suggest prolonging the period between 960 and 700, or the period between 700 and room temp? or maybe both?

@Jenna

Thanks for taking the time to type out that articulate response (and the PM). Seems like you really know what you're doing when it comes to the molding process. That was an awesome idea to just use a silicone mold instead of wax, by the way! I'll definitely try that -- seems like it'd save a LOT of time and effort, especially since I plan on making a bunch of these parts. In your experience, does silicone stick to the plaster at all? I'm just curious if there's a chance of breaking the cylinders that will create the holes, or pieces of plaster peeling off the mold, when removing the silicone.
The first one of these that I make is going to be part of a prototype that I'm working on -- I wish I could tell you all about it, but I don't have a patent for it yet. It's hard to be so excited about something, but not being able to talk about it, haha.
I'm really glad you told me more about how plaster is super brittle (especially after firing it) -- I guess I should just plan on throwing out each plaster mold after firing it. That might actually save me money if I don't have to buy the MR-97 (like $40/can), since I won't be reusing them. Do you think that not using MR-97 may create surface imperfections on the final part though? Nice observation about that little hole in the part too -- totally forgot to design that one with draft angles.
About that step 7 -- I was reading about creating plaster molds, from a few different online sources, and I remember one of them stating that the plaster needs to be fired to 1250 as part of the curing process. That does seem kind of odd now though -- since you said that the plaster becomes brittle after firing.. Do you think I should skip that step?
From the PM, it sounds like writing's a little tedious for you (though you write pretty articulately) -- if it's a hassle, I can give you a call if you want.

Thanks again.