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Hey all,

 

I really, really suck at making plaster molds. I have tried twice but messed up badly each time. However, I think I've got it down now, after doing a load of research and reading.

 

Now for my question-

The way I am going to be casting is using 3d printed plastic forms and cast them in plaster. I have seen other people do this (Kurt Hammerly is best known for it), however they use a silicon mold and then cast the cup from the silicone. Is there a reason why? I am using PETG which can handle temps of 200+ºc. 

 

I figure the basic way of making slip casting molds, covering half in clay, pouring plaster, etc, (you know the drill), would be substantially easier than making another mold from silicon. Do people purely do the silicon so they can make a large number of molds?

Attached are pictures of what will be my first 3D printed mold attempt. The black line is the middle mark, if you are wondering.

 

 

Anyway, I will let you know how this goes later in the week. It's ironic because I always thought of slip casting as "Lazy Pottery," but I make so many test pieces now a days, (I prefer to test on cups and such rather than regular test pieces, it helps me vision the glazes better, especially crystalline), it is too time consuming to spend hours throwing "disposable" pieces.

 

IMG_4263.jpeg.806ac8f31d10eea5a54c250b533bccba.jpegIMG_4261.jpeg.90dae971404e265b05e830edfc8bd825.jpeg

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On 5/18/2020 at 2:36 AM, Mark C. said:

Neil is spot on I still have all my masters from an old slip business and they are all in silicon rubber as well as hydra call positives.

You can them make all working molds from that setup.

If you 3 d is rough sand it smooth or coat it smooth

Do you have any tips on getting the plaster to not stick to the 3d prints?

 

The one other time I tried this, it was incredibly hard to pry the plaster from the print. I was just thinking about using mold soap as a barrier.

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Mold soap (real mold soap not super market stuff) is the best if the  surface is smooth-if its scaly or rough then its needs smoothing or a coating like clear polyurathane for  wood

I once made some wooden forms and coated them. I have  seen 3 d stuff super smooth and also rough-whats yours??They can be sanded as well.

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2 hours ago, Mark C. said:

Mold soap (real mold soap not super market stuff) is the best if the  surface is smooth-if its scaly or rough then its needs smoothing or a coating like clear polyurathane for  wood

I once made some wooden forms and coated them. I have  seen 3 d stuff super smooth and also rough-whats yours??They can be sanded as well.

I sanded mine down, but have had some sticking issues in the past. I could probably coat the plastic in polyurethane too, that might help. I will try it without first. Worse comes to worse, I re-print and coat in something.

 

What I am most concerned with is trying to get the piece in the exact middle of the slip form. I have had issues in my very limited past attempts where the center of the piece was 1 mm off and it would just rip the clay as I took it out of the mold. I am thinking about doing 5 piece forms, doing 4 pieces for the body and 1 piece for the foot, while that seems like the better option, I don't want to get way over my head. I think the 2 piece forms will be better to learn and experiment with.

 

This is super exciting for me, I have never been into this side of ceramics. 

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Looking at that form its a two or three piece -the third piece would be the bottom

No need for any more-finding center could be done with water or calipers or ?? you have to get creative . Its a simple form so making it a 4-5 poiece is just more seems to clean up and more work. Coating in a poly finish will be super smooth-then use mold soap.

Mold soap has no structure to smooth the form thats what poly or paint will do. If its finding cener line is the issue address that and get past that.

Have you read any books on mold making? they mostly are old totles but all is still 100% relavent today .

You can even use spray poly as it dries fast build up thin coats not one heavy one.

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3 hours ago, Mark C. said:

Looking at that form its a two or three piece -the third piece would be the bottom

No need for any more-finding center could be done with water or calipers or ?? you have to get creative . Its a simple form so making it a 4-5 poiece is just more seems to clean up and more work. Coating in a poly finish will be super smooth-then use mold soap.

Mold soap has no structure to smooth the form thats what poly or paint will do. If its finding cener line is the issue address that and get past that.

Have you read any books on mold making? they mostly are old totles but all is still 100% relavent today .

You can even use spray poly as it dries fast build up thin coats not one heavy one.

I ordered a book off amazon, coming tomorrow. I have done some online reading and watched every youtube video I could find.

https://www.amazon.com/gp/product/1600590772/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

What do you mean by using water to find the center? The black lines I marked out on this one are pretty close to center, I am hoping they are close enough. Im about 90% sure they are.

 

These pics below are what I am currently designing. These are "Mother Molds" for a two part mold. I think this will be the easiest route, assuming they release from the plaster easily. The second one is the "female" piece of the mold, still being printed at the moment. 

 

About the multiple piece molds- I found this video and it seems fairly easy. https://www.youtube.com/watch?v=m4rDS5rBWb8&list=PLN1iBYK2Y2X_dpUZvWZlwk17XxBErWkpy&index=2&t=0s 

 

 

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Edited by Brandon Franks
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Just now, liambesaw said:

For something small like that it is.

This French guy here is really good, I've watched all his videos: https://www.youtube.com/channel/UCs9HKK7tAyZ9ZLHggWDRLRQ

How funny, was just watching one of his videos.

Don't understand a word he's saying, but I feel like I am learning something.

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On 5/20/2020 at 12:27 AM, Brandon Franks said:

What I am most concerned with is trying to get the piece in the exact middle of the slip form. I have had issues in my very limited past attempts where the center of the piece was 1 mm off and it would just rip the clay as I took it out of the mold.

Instead of two pieces for the main cylinder, which have to be pretty exact so they will part properly, you could do that with 3 parts, then no matter how unequal they are, it should part without tearing.

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On 5/22/2020 at 5:22 AM, Chilly said:

Instead of two pieces for the main cylinder, which have to be pretty exact so they will part properly, you could do that with 3 parts, then no matter how unequal they are, it should part without tearing.

Below are my first attempts. Came out better than I expected. My clay was not perfect yet, did not deflocculate it completely since there was a lot of water to being with, just wanted to try it out. Obviously, a bit rough. Nothing a little practice and a sponge cant fix, however.

These were done with both 2 piece molds and three piece molds (respectively)

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IMG_4367.jpeg

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Looks like plaster was a bit too hard when poured?  I've found if I wait too long to pour I'll get small cavities like this and big bubbles that end up collapsing later.  I now pour mine in when it's more like whole milk and get a much better result.

Did you use cottle boards or something else to make the mold?  Seems like a lot of flash at the junctions for a new mold as well, did you have a hard time separating them?  My trick for separation is first to use plenty of soap, mold soap or Murphy's or green soap, whatever works for you.  Then once it's time to separate it's important to take a rubber mallet and tap the outside of the plaster all around, it'll gently break that tension that has built up around the master.  Less chipping and stronger junctions that way.

I don't know if you're doing this for cone 10 or 6, but if it's cone 6, I really like the top two recipes here.  I mix the casting slip (but use veegum-t instead of bentonite), and then use the clay body recipe for attachments (also using veegum-t).  So far it's held up very nicely.  In the bucket, working time, etc.  Fires very white, plays nicely with many glazes as well.

 

Edited by liambesaw
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I agree that plaster was poured late without tapping the bubbles out

So next time power mix the plaster for a few minutes and then I drop the bucket a few times on floor to let the bubbles rise to surface.You will get feel for this over time

The next tip is pouring paying attention to letting the bubbles rise to top. You can vibrate the mold ones poured by taping the table or beating on it with rubber mallet or any vibration method you choose. The idea to start with is get the bubbles out of the plaster when its thin  as they rise easy the -not later as it sets off and they get trapped like in your form in photos.

Pour the plaster sooner than that pour you did in photos.

The trick is get everything together before mixing plaster -everything ready 

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11 hours ago, Mark C. said:

I agree that plaster was poured late without tapping the bubbles out

So next time power mix the plaster for a few minutes and then I drop the bucket a few times on floor to let the bubbles rise to surface.You will get feel for this over time

The next tip is pouring paying attention to letting the bubbles rise to top. You can vibrate the mold ones poured by taping the table or beating on it with rubber mallet or any vibration method you choose. The idea to start with is get the bubbles out of the plaster when its thin  as they rise easy the -not later as it sets off and they get trapped like in your form in photos.

Pour the plaster sooner than that pour you did in photos.

The trick is get everything together before mixing plaster -everything ready 

The molds are actually smooth, I think it's the clay that caused the bubbles. I think my clay had too much water in it, and it was drying uneven. Im about to pour new molds now, A different method and going to be way smoother. Ill take a picture and post them on here in about an hour.

 

I am using my normal cone 6 body, just formulated with less Bentonite- only .5% instead of 4.5%. 

 

I tapped all the bubbles out pretty vigorously. I know for a fact one of the pure was a little late, but the others felt good to me.

 

 

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18 minutes ago, Brandon Franks said:

The molds are actually smooth, I think it's the clay that caused the bubbles. I think my clay had too much water in it, and it was drying uneven. Im about to pour new molds now, A different method and going to be way smoother. Ill take a picture and post them on here in about an hour.

 

I am using my normal cone 6 body, just formulated with less Bentonite- only .5% instead of 4.5%. 

 

I tapped all the bubbles out pretty vigorously. I know for a fact one of the pure was a little late, but the others felt good to me.

 

 

Nice!  What's your new method?  I use cottle boards and clamps, and some old porcelain that wasn't good for throwing as the block out.

I've heard of people using Play-Doh, but I worry about the oils in it doing something funky.

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27 minutes ago, Mark C. said:

Ok its a slip issue-read up on making the slip just right-you will need a special cup (like a funnel) ,timer and darvan 7  and slip hygometer or are you buying premade slip???

Making slip. Already have darn and all the clay components. I have read up on the slip testing methods. Just rushed the first few pieces because I was eager.

 

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2 hours ago, liambesaw said:

Nice!  What's your new method?  I use cottle boards and clamps, and some old porcelain that wasn't good for throwing as the block out.

I've heard of people using Play-Doh, but I worry about the oils in it doing something funky.

I made a 3d model of a really simple and small planter, then cut the model in half. I can cast the bottom of the mold straight on MDF so its completely flat. Then put the whole model in and cast the top portion.

Only bad part is I cant make circular keys that easy, I don't have the bit to do it. I just chiseled out some sloped lines that will work equally well.

 

I designed this so the mold lines will line up with the pot lines - should be little to no clean up. (its not cleaned up yet, just de-molded it)

 

IMG_4429.jpeg.4d9dac264aa9abac07e9bb31f3146bee.jpeg

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