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3D Printing For Plaster Molds

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I have been 3D printing some forms to cast in plaster but found it time consuming setting up cottle boards every time I wanted a new mold. Started working on getting a form that I could pour plaster directly into to make the mold.

They turned out quite good, I just need to find the right release agent to use, tried a beeswax polish and soft soap but neither seem to work as well as I would like. Going to buy some proper mold release or other spray that will let the plaster release nicely but my initial tests are promising ^_^ If anybody knows a good product I could use as a release agent please let me know, using PLA to print.




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51 minutes ago, High Bridge Pottery said:

I have been 3D printing some forms to cast in plaster but found it time consuming setting up cottle boards every time I wanted a new mold. Started working on getting a form that I could pour plaster directly into to make the mold.

I was reminded of a different approach to the same 3D printing idea. It used sacrificial 3D prints, and produced moulds with minimal  excess plaster.

3D Printing a Mold for a Slipcasting Mold

Obviously horses for courses, but maybe something for your notebook.

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42 minutes ago, High Bridge Pottery said:

Thanks Peter, I wonder what plastic they are using, maybe PETG as they don't seem apply anything to help the plaster release. I have done some printing with PETG but it's not as easy to print with as PLA.

Why not ask her https://www.instructables.com/member/Charlotte_J/instructables/

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Tony Hansen strikes again.
in https://digitalfire.com/picture/2463
The incredible utility of 3D printing master handle molds

As a parting-agent I use Murphy's Oil Soap on the 3D-printed PLA mold, this makes it fairly easy to extract the freshly-cast plaster molds (the sidewalls have a draft of about 5 degrees).

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I did try using soft soap which is a "non-hazardous composition of potassium soap and vegetable oil" and I think similar to Murphy's Oil Soap. I was letting it dry completely before pouring in plaster so maybe I should pour before it dries. I have ordered some PTFE/Teflon spray to see if that works any better.

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When I've used Murphy's soap I used a quite damp sponge and applied on a thin layer. I let that dry then repeat with a bit of light buffing between coats. Keep adding coats until water beads off the surface. I then bought some mold soap, for me it works better. (I used Murphy's on a whole fish once, as the plaster heated up it made a nasty smell of hot stinky fish combined with Murphy's, Can't use the stuff now without gagging a bit, probably too much info)

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The key to Murphy's is to create a lather. (with a brush) Then wiping away the excess lather with a sponge, then wiping away excess bubbles with a dry brush. Do this three times and you'll have a nice "oil-like" surface. (Which flakes off/dissolves in a few days.)

My concern with pouring plaster against wet Murphy's is that the wet substance MAY absorb into the plaster surface. You might have a different experience, however?

Another option is to use a food safe silicone spray. In the US there is a product called PAM. In another mold making forum this was mentioned as an effective release material when pouring plaster onto nonporous surfaces.

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Speculation ... based on plaster apparently left adhering to your 3D printed mould.

Could some of your issues be caused by the "texture" generated by the 3D printing process. Are you are pulling the plaster out of the mould "against the grain" rather than normal to it at the points retaining plaster. (Is the issue texture-scale undercuts rather than PVA/plaster adhesion?)

If that's the case, these seem to be some of the ways other people have used to get a smoother/less-textured surface.

Sanding 3D Prints: How to Sand PLA & More
... includes How to Print Smooth Parts

PLA Smoothing: How to Smooth 3D Prints

UK 3D printing epoxy ads

PS The sacrificial mould technique would probably tend to remove the PLA from the plaster in a direction more normal to the local plaster surface and its texture, which might reduce damage. in some circumstances -- but I don't think it would work well at the edges of the handle.

PPS At one point I found an article advising that auto-restart (after power failure) was turned off to get a smoother 3D print, but I cannot find it again.
...  this seems to mention the issue.
Power Loss Recovery Might Make 3D-Printed Blobs

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I hate to say it but I would suggest you abandon this project. It doesn't look like the molds are friendly to the plaster.

See where all the plaster remains in the mold? Those are undercuts. In the casting process those will be little bits of clay that need to be cleaned with a fettling knife. That's time consuming.

Do you like the surface texture resulting from the 3D printing process? If not I would suggest you use the plaster molds you just made and make plaster block molds with them. Now you have a plaster mold that can be easily sanded. Use this new plaster block mold as your new mold.

Ten years ago I made urethane rubber block and case molds, from a plaster mold, and they have beautiful smooth surfaces and they flex just enough for new plaster castings to pop out easily. They make mold making super simple and the molds come out perfect. (Because I took time to make the plaster originals perfect as well.)

Good Luck otherwise. I appreciate the effort.

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I don't think there's any undercuts as some places it sticks and others come out well. Big chunks left in with PTFE spray as it was hard to apply evenly across the form.


Here's a bigger photo of the mold, to me there's some clean edges and other ones that didn't release properly.



I don't mind the texture but I think after shrinking 10% and being glazed it will be difficult to notice. 


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Just or the record, there are several 3D printing pages at digitalfire.

3D Printing a Clay Cookie Cutter-Stamper https://digitalfire.com/video/3d+printing+a+clay+cookie+cutter-stamper
3D-Printing https://digitalfire.com/glossary/3d-printing
3D Design https://digitalfire.com/glossary/3d+design
3D Printer https://digitalfire.com/glossary/3d+printer
Project: 2019 Jiggering-Casting Project https://digitalfire.com/project/15

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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)! 

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I might have to look into making rubber master molds but it is just another process to learn so I am trying to avoid it. I have no problem with the FDM surface having striations.


Going to try some full size prints as these have been 25% / 37.5% the actual size I want while I tested mold release. 

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Looking at the residue left in your printed masters, I think Peter and Cenmoore are correct about smoothing the surface.  The striations are acting like finger prints and creating small vacuums that grip the plaster. Even filling the print with something like automotive filler or spackle, sanding and sealing it with a good waterproof finish might save doing a rubber cast. 

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  • 5 months later...

Hi everyone! I recently discovered something really cool about this topic.

Challenges with FDM printing a PLA box: 1) resolution of the layers creates a rough texture that makes everything harder. 2) the PLA has high enough stiffness that it can't flex to release only one small area at a time as you remove the mold. That's like pulling a piece of tape directly off a surface instead of peeling one little corner at a time.

Our theoretical goal for the best master mold is a mold that is stiff when the plaster is curing and then flexible when you want to remove the plaster. For this reason, silicone master molds are a great choice, especially with a removable plastic support band that can help hold the shape of the outer walls while the plaster pours and cures. Silicone master molds require quite a few steps which is great if you have a skills, equipment, and tolerance for messes.

Alright, so how do we get around these issues? My recent finding is that there is a new type of 3D printing called Multi-Jet Fusion (MJF) and you can print using a rubber called TPU88 and use a technique called vapor smoothing to create an excellent production-ready surface finish. Plus, there are printing services where you can submit a CAD model and someone else can print the parts as a service since these printers are capital-intensive. I just tried this method on a part and I don't have it quite right yet, but it shows promise.

I think this is an important and interesting problem to work on. What if doing this well could move forward the whole slipcasting industry? What's the impact if you can go straight from a CAD file to a rubber master mold in one week with no mold-making skills/equipment/mess?

I can post some pictures at some point if that is beneficial. Any interest from anyone in learning more or talking about this?

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I've had online vendors do 3D printing for me and it worked well. For me, and for the volume I did, it was better than buying a printer and going through the learning curve. The printing itself is only half of the learning curve. I also had to learn how to create the file for the 3D forms, which was not too bad because I've used Photoshop a lot and there are similarities, and my forms were fairly simple. 

I was advised that printing with 'resin' would give smoother results. But, resin printing is messy and requires what could be described as a darkroom. Printing with a rubbery material sounds really promising. I'd love to hear more about your progress down this path.

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38 minutes ago, Min said:

it would probably be a good idea to start a new thread on this, would make it easier to find in a search.

+1 for starting another thread.

I'm used to seing rather chunky rubber block moulds on the net. As from 6:53 in this video.

How thick are you printing the walls? Would these rubber moulds be self-supporting when casting into them, or would they need a "backup" mould to prevent distortion?

...  which I believe is common practice in some areas.
How to Make Backup Mother Moulds

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Interesting video. Thanks for posting, Peter.

I disagree with a few of his methods. For some reason he sprays a release substance into the silicone mold before pouring plaster. If I'm not mistaken there's no need to do that. My urethane rubber molds have lasted 10 years and they release easily without any release substance.

My other disagreement is with the wooden frame concept.  (For the rubber molds.) Rubber comes in a variety of hardnesses.  Polytek recommended using a urethane rubber with a hardness of 55 and it works very well. (7455) It keeps its shape and it flexes when it's time to remove the plaster mold. I've used the 10 yr old molds to make 60 molds to date.  I'm not patient enough to gently remove a wooden frame 60 times and have it retain its original shape and precision. (If this gent does than he's miles ahead of me in that regard.)

I do appreciate that he has had the fortitude to make videos to show the process to those getting started. 


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On 6/27/2023 at 1:07 PM, High Bridge Pottery said:

That's pretty much the technique I used with soft soap/mould makers size for the first photo, it does work pretty well and I am sure my application can improve.

I also tried using a PTFE dry spray yesterday but had worse and better results as it's tough to get an even covering. 


One of the reasons you are having difficulty with mold release is due to your print lines.  They add a texture that will cause the plaster to grip your 3D printed surface.  So any roughness needs to be filled in, sanded out, or smoothed over.  It doesn't mean that you can't have the texture but at least coat it with varnish or something to help smooth it out and fill in micro undercuts.  All those little places you have plaster sticking inside of the print lines are forming tiny undercuts.  The main problem with 3D printing, IMO, is that there is a TON of cleanup.    I 3D print resins of my horse sculptures and no matter how clean my prints, how much I fuss over no print lines, temperature and other issues still cause so many nit picky things that I can literally spend hours cleaning up a single print afterwards before it's usable for mold making.  

By the way, you can also 3D print originals or mold masters in rubber!  This is super nice for pouring plaster molds and works a lot better than using the hard plastic.  But again, you need super perfect printing because you won't be able to clean up rough areas in rubber as easily as you can with plastic resin.  So as the technology is still evolving and not nearly good enough unless you really, really know what you are doing and that's a steep learning curve.  I've been 3D printing two years now and there's still a ton I'm trying to fine tune.  So mostly I still use the old fashioned methods of pouring rubber and building mold boxes.

Pardon me - I see belatedly that others also pointed this out so I'll just agree with them!  ;)  (edited to add this comment)

Edited by Hyn Patty
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  • 1 month later...

I finally got around to doing a few glaze tests and brushed some on these mugs. Can't feel any texture through the glaze so pretty happy with the results as I didn't do any cleanup on the mold print.


Need gum for brushing, seem to have lost mine.

Need to go back and work on my clay, maybe.


The clay is great except it still takes 1.5-2 hours to cast the larger mug and it likes to hang onto bubbles.

Fires like a dream, bisque in 4 hours (20 min to 100c, hold for 20 min then 3h to 800c and hold for 20 min) and glaze in 5.5. I could go faster on the glaze but after 800 my kiln stops climbing at 250 c/h. At 1000 to 1100 it can only manage 80 c/h but that's ok for hitting 1100 cone03.


Edited by High Bridge Pottery
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