I want to 3d print some moulds in PETG to make plaster objects. Before I can pour the plaster, I need to remove the layer marks on the PETG mould.
What would be the best way to make it smooth, spray paint, spray primer or car body filler? I must have those somewhere in the house but I fear that it will not adhere to the PETG.
Are there any special plastics primers or can I use the ordinary car stuff?
PETG has low surface energy, which means adhesives generally don’t stick well. Why PETG for this application?
That is what I have available, no particular reason.
You might be able to sand it. However, PLA and ABS are less gummy and easier to sand, and also hold adhesives and paint better.
I would advise against using fillers or primers on the inside of a mould as the tend to come off or break up if you put too much pressure on them. sanding is best.
generally speaking petg doesn’t make a very good mould for a soft substance like plaster as it’s not very flexible, it may be hard to remove the plaster object without breaking the mold or cracking the plaster.
You normally want your mould to be slightly more flexible. I tend to make objects on my printer in pla, petg or resin, and then use those to make a mould in silicone or latex. then put the plaster in that softer mould.
or am i just getting confused and your sayo g that you want to make box out of petg to pour plaster in, and and your object is something seperate made from another material?
either way, if you plan on refusing your moulds multiple times, fexibity is king. Silcone for small detailed objectives latex for larger ones with less detail
I am trying to 3d print a plastic mould to make plaster moulds. The plaster mould can be used only so many times. When working with it, the plaster will chip at some moment.
I want to make ceramic soup bowls. I made one by throwing on a pottery disc 13 years ago. That first bowl was used to make a plaster mould. The bowl chipped when I tried to get it out of the plaster as you can see on these pictures.
After 13 years of use the glazing is worn and I want to make a new series.
It is not a real problem when the paint on the PETG is not durable. I can always put on a new layer but it would be more practical when it stays on.
There is a slight taper in the PETG mould to release the plaster and when that is not sufficient I will create some cuts in the PETG with a saw blade and put duck tape over it for the next run.
have you considered investing in a flexible filiment. in the long run the outlay on filiment might offset the time required to get things right another way.
can you make your moulds in two parts, it might make removing them easier, but would cause a line that would need sanding on the end product.
I usually deal with much smaller models so my issues tend to be diff8from the kind that you’re having.
I have been looking at TPU for something else but that needed a second extruder which I don’t have 
With TPU I would still have the problem of 3d printed layers visible in the print. I guess it is more problematic to remove that from TPU
I can sand a line in the plaster mould, but it will never get the same surface finish as plaster poured on a painted surface.
have you considered making a mould with petg, so and then using that mould to make a latex or silicone mould to make your final product?
You could try coating the inside of a petg mould with food safe resin and then sand g that. my experience with sanding is that it can leave a dust residue that’s hard to fully clean off.
sanding interior surfaces is also notoriously a chore.
In an ideal world it use a vacuum former, but 99 percent of people don’t have access to one.
for the wealthy, injection moulding.
roll on the star trek replicator, so I say.
Not affordable for us poor makers!
I am thinking about splitting one of the printed moulds in two. I can leave the line out of the part that touches the ceramic surface.
silly question, but have you considered printing your mould slightly too large, and then putting a sacrefisial liner in it, maybe clay or liquid rubber. that way no layer lines.
You can see in the design drawing that the top edge of the bowl is somewhat thicker than the one in the photograph. I had already planned to use some filler/primer/paint.
When throwing pottery on a disk, the most difficult part is to get clay to the top edge. At least for a beginner like me. For a terracotta cup the edge is a bit thin, it chips easily. A higher temperature makes the ceramic a bit stronger. To achieve that I would like to fire the next batch at 1200C but I have no access to a kiln at the moment.
The old bowls were fired in a kiln at 1100 C. I am planning to use a wood fire next time. That is uncontrollable in terms of temperature but gives nice unexpected effects in the glazing. When it doesn’t get hot enough there will be a third batch very soon
I use epoxy on my 3d printed PLA molds. Most of the time I get away with one coat and no sanding. I have included a bell mold that I use.
I didn’t know that epoxy was used for coatings. Is it a two part composition like epoxy adhesive?
Epoxy might work for me. I have glued PETG parts already and that makes a strong bond.
After seeing the topic, this was what I was looking for. Too cast the bowl in silicone. I have seen far more complex things cast in silicone. Then replicated.
Any existing bowl that is to be used as a reference will need to undergo a process to repair chips and smooth it off. UV resin us a great way to make quick patches. Epoxy and other materials make great long term patches for large gaps. Sand and polish to finish. The material then needs to be treated so it wont stick to the casting silicone.
I am assuming the OP works with plaster or ceramic clay so probably knows this, but for the rest of us this is how I would address the issue:
A material often referred to as slip, a liquefied clay can be poured into the casts. If done right they can be closed up making a otherwise solid two part cast. Might take some tinkery to get just the right results. A cast of this size should fit nicely into very commonly accessible evap tanks (large pot with a vacuum lid). As the air is removed, the water in the clay can be also removed. I wont claim to know the best rate to do this. The result would be a rapid dried cast. Otherwise maybe a week or more of waiting for the slip to dry on it’s own. Also the more water the more shrinkage. So the more clay mass to start the better the results.
The finished part can be trimmed and sanded, and glazed before kiln fired.
The internet says two part epoxy. Works on PETG, but I have not tested. Google also tries to sell a kits that I guess some like. I have not done much in terms of coats. But I would assume that various lacquers would work.
Google warns that some materials with strong solvents might damage PETG parts. Though I personally have seen very few things that stick to PETG. I know because I have tested and generated a chart. Things that do work on PETG I can tell you I have not personally tested and given how dangerous many of the solvents are, and most should not be handled without considering the risks.
Xometry posted that For PETG, “…Solvents like ethyl acetate, MEK, or dichloromethane are required to produce a high gloss surface.” Google remarks “PETG is resistant to many common solvents, so only certain chemicals, such as dichloromethane or tetrahydrofuran, are effective, and they must be handled under strict safety protocols due to their toxicity and volatility” But google says this too is posted by Xometry.
Personal experience is that I do not recall testing MEK on PETG, though I am fairly certain I have and the results were limited effect. The same for Dichloromethane, ie Plastiweld or Bondine. I would not by memory recommend either for bonding. Though I suppose in large enough quantities it may be possible to use either one for surfacing…
I have just tested the effect of butanon (MEK) on PETG and I’ve noticed that it does something to it. I cannot tell whether it dissolves or disintegrates it but when i wet the PETG and scrape with my fingernail over it, i get a rubber like substance on my nails.
This information is not really the best news. Something adheres to it but I was not planning to start mixing my own paint. I had the lazy approach in my mind, to just by some cans of spray paint.
Since sites that reference use in part of their production process dont normaly tell us how they do it. I have to assume they use a process related to how we smooth other thermo plastics. This is usually by missing or vapor in a sealed container.
I have no memory of any reaction with MEK but then I would not have sealed a sample in a container long enough to see it rubberized.
My own testing with chemicals was mostly using cut samples with an amount of a solvent spilled in the samples. I looked for residue washed from the samples, discoloration and any bonding.
So in my opinion, it is very likely the sample will need to be exposed to a high enough concentration over a larger period of time. If the surface is rubberized. Clearly there is a reaction. I have no memory of seeing this. So likely a larger reaction will produce better results.
Though, in my opinion. Repairing the bowl and making a cast from that object would be better.
