cemoore

Thanks Peter, I read the links and will check out bone china. Sounds like a material that's outside my current skill level (everything is at the moment with slip casting), but maybe aspirational. I like the idea of being able to glaze after grinding and knowing there won't be discernible shrinkage.

I'm new to slip casting and want to understand an issue I'm having - hoping its an easy fix. I made some one piece shallow bowls - finished size estimated at 5" diameter x 1" height, soft rounded bottom profile. Everything seems to working great with my commercial stoneware casting slip but I'm noticing raised areas/bumps at inside bottom of casting that look like they might be coming from where slip first hits the mold. I'm trying to move my pour stream around the bottom to avoid pouring all in one place, but still noticing. I read something about pouring out too fast could be creating a glug (not sure if this is slang or a term of art?) where mold might pull off plaster prematurely. I definitely have been pouring out fast. I am not noticing any imperfections on outside of casting (that was against plaster surface), rather just the inside, built-up wall. I don't have a picture at moment because I sanded them out. Once I've poured out (currently at about 25 minutes time to get a bit over 1/8" wall), keeping item at about a 30 degree angle, I prop it up for about 10 minutes to where there doesn't seem to be a surface shine. Then I turn the mold right-side up. After roughly 1 hour I trim the sprue/excess material off the top rim using the trim ring and a knife, then waiting another hour or so before the item wants to release from the mold, which it does cleanly. I'm using a 3d printed raised plate to put down into the trim ring area so that my piece doesn't fall out and deform the edge or other parts. Then I sandwich the casting with another plate on bottom to turn it over to right-side up and gently push onto on a metal drying rack. I'm using some cloth rather than plastic over the casting for draft-less drying, cleaning up the rim and overall surfaces with a scotch-brite pad once bone dry. Any tips for this newbie would be most welcome!

Thanks Tom/(Hulk)! I like your idea of unglazed and hollow but with enough extra thickness to account for removal. It seems there would be so many variations, cutting it down seems the most predictable and practical. Without a glaze, the body would have to be semi-vitreous or vitreous. As to what I make from concrete to the narrow specs - they are OEM parts that go in a bathroom/damp setting, that are attached to cast metal parts. The part shape looks about like an ice hockey puck - nothing truly fancy in shape. At the end of the day, while my 3d printed models looked round to my eyes, they were slightly off using caliper measures (eg, not perfectly round). A partial solution to that might be spending thousands on machined models, but that's not a financial option. As well, the casting process is always introducing some variations. Milling them after casting made them beautifully round and within spec. As they were small I ended up using a drill press and attaching into a hole on their underside a blank drill bit with a flexible plastic gasket to get tight on the bit. I used a velcro-backed flexible diamond hand pad affixed to a 3d printed arc'd pad holder to get the curved sidewall surface to 90 degrees. A sort of poor-man's milling. Just thinking about that project in ceramic but also future projects where I might need to meet these kind of specs and ways I might be able to machine/grind my own designs. Your input is greatly appreciated! Always learning.

I am new to slip casting but not to mold making and other mediums. In another medium (exposed aggregate concrete), I've made custom wholesale products that required me to make a larger-than-spec round item from a mold, then grind it down to an exacting diameter (within +/-.02") standard. With that medium, grinding of 1/32 to 1/24" was required anyway to expose the decorative stones in the concrete body. So, I'm thinking as bisqueware (in my case stoneware) can theoretically be sanded, are there opportunities to make a thicker walled item and grind it down after bisque firing to the required diameter with wet diamond pads? If so, how thick is too thick? Are solid castings an option? I'd also have to take into account any thickness added by glazes when grinding. Just thinking out loud, would love to hear from slip casting ceramicists on this idea. PS. I think the answer on the keeping the final item within spec is that the amount of additional shrinkage during glazing might be too difficult to pinpoint or distort the shape?

I've been making 3d printed models for molds for glass casting, concrete casting, etc, for about seven years and now delving into plaster mold making for slipcasting. I've never had success making a plaster cast directly from a plastic model. Also, plastic models printed on low to mid-priced FDM printers are always going to need 'post-processing' to smooth out the surface (especially the tell-tale 3d printer striations) and seal it properly. I use automotive primer/filler paints or Smooth-On's XTC-3d epoxy to fill and sand, then shellac, wax, etc. It's an iterative and time-consuming process but can't be skipped. Once I print the model (I use Rhinoceros software to design - and check for undercuts within the software), I post-process significantly to get a 'perfect' surface as mentioned, cast a rubber mold (I use mostly Smooth-On Vytaflex 40 urethane rubber), then cast the plaster duplicates from the rubber mold. If I have a concave surface model that is small, say a teacup where it would be difficult to post-process the printed model and/or you don't want to make your knuckles bleed, I take one more step and make a convex positive instead, where I'm then able to post-process more easily an exposed outside/convex surface. From that I then cast two rubber molds - the first rubber mold from the plastic model, and then the second and final rubber mold from the first rubber mold. When casting the rubber molds, I always use a release agent recommended by the rubber manufacturer. No release is necessary to cast the plaster from the rubber mold. The process is much work, but it's worth the effort, if not for the learning when the failures happen (usually design-related)!