Excellent info from p-kimberley above. What is said, is absolutely true: any layer lines, blobs or irregularities will show up in the cast, and they will make it *much* more difficult to remove the cast from the mould.
I have made silicone moulds from 3D-printed models. It goes as follows: first 3D-print a model, including what will become the filling canals and venting holes. Print in multiple parts and assemble as required. Carefully remove all blobs and irregularities, and smooth the model, as any defects will show up. Stick the model to a plastic base plate, or provide some form of support or hanging it stable, so it does not go swimming away when you pour silicone in later on. Using plasticine, wood, or plastic plates, build a container around the model. Make sure the container is absolutely watertight at the bottom and sides. Make sure it has pouring and venting openings (or leave the top open). And then pour silicone in the container around the model, and let cure. Then cut the model out of the silicone by cutting zig-zag lines, so both mould parts align themself correctly later on. Now you have an empty silicone mould, in which you can pour resin. Spray the silicone with silicone oil as release spray, prior to casting. Silicone is watertight because it repells water, but it is *not* oil-tight and not solvent-tight. So the solvents of resins creep into the silicone and destroy it soon, after a few times of use. Therefore you really need to saturate the silicone with release spray, prior to casting. As with any moulding and casting, keep aware of undercuts, use slanted side-walls, and avoid enclosed features and stuff that could make removal of the cast difficult.
I have also done the opposite: cast silicone models in a 3D-printed mould. Thus: 3D-print a mould in PLA, carefully remove defects, smooth it. Then carefully seal all openings in the seams in side-walls and bottom. Pour silicone in and let cure. If you do not smooth the layer lines, and close tiny gaps in the model from the 3D-printing process, it will be almost impossible to get the model out of the mould. Don't ask how I know. :-)
You can tap or shake the silicone to remove bubbles, or gently (!) blow on bubbles. Gently pouring from a high distance in a thin stream also helps. Vacuum degassing is best, but only works for slow-curing silicones, not the fast ones I have, and you need professional vacuum equipment.
On Youtube there are tons of excellent tutorial videos, showing the techniques. Search for: moulding and casting. You find info on really anything: thick silicone paste (like for body masks), thin liquid silicone, slow and fast curing, epoxy and PU-resins, coloring, info about avoiding bubbles, and so on. Be sure to study a lot of these, even if it takes a few days: you won't regret it, and you will very soon recuperate that invested time. Preferably download good videos and store them locally, so they won't be affected by "linkrot" and error-404.
For smoothing PLA-prints, you can use the "cloadfiend method", optimised by user cloakfiend. See his topic on this forum.
A few photos (I have them, so I can as well show them):
1. Silicone moulds, with a hard shell for more stability. The original models are not shown here, and were made from plexiglass (right), and very hard wax (left), but you could use a 3D-printed original as well. This was before we had 3D-printers. Notice the zig-zag cuts, so both parts align well. Also notice the notches in the shell, for alignment. And see the pouring and venting openings. The blue model has a base-plate in the shell, so it stays upright.
2. Orange 3D-printed models, and green silicone impressions of them. Top one is as-printed, bottom one is smoothed with acetone (using the "cloadfiend-procedure"). The difference in the silicone impressions is obvious.
3. Same model 3D-printed, but with different post-processing. Left = smoothed with acetone. Center = smoothed with heat-gun: don't do this: it causes craters due to internal bubbles exploding when heating the plastic. Right = as-printed, untreated.
4. Idem, different camera angle.
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p-kimberley 4
There are a couple of techniques to mould printing. I've made both one-part and two-part moulds for use with epoxy resin (ArtResin).
One approach is you can design a two-part mould in CAD, creating a shaft through which you pour the medium, such as resin. Check out my model on Thingiverse for an example: https://www.thingiverse.com/thing:3945872.
The other approach is to print the actual object you want to model and then embed it in something like moulding silicone. I've used https://www.barnes.com.au/addition-cured/pinkysil-silicone-2209 with good results.
Layer lines and porous surfaces are an issue with 3d printing moulds. From an aesthetic perspective, without smoothing the part/mould before pouring, the layer lines and micro holes will be replicated in the final object. Even if you sand the part/mould first, you'll at best, end up with a rough or matte appearance, which isn't ideal if you're going for a smooth injection-moulded look. An unsmoothed mould will also tend to stick to whatever medium you cast in it, making removal a pain.
To smooth your part/mould, you can print in ABS and give it the acetone treatment, or use something like Polymaker's Polysmooth filament. I've tried both and the Polysmooth filament produced some excellent molds, with good surface detail and smooth appearance. See the attached pic for an example. The surface bubbles are from the resin, not the mould itself. ABS was harder to get an evenly smoothed surface and detail was lost in the process due to running/melting of the plastic. Also, printing a complex ABS object with supports can be tricky as PVA is basically a no-go due to the bed temp.
Surprisingly, I haven't been able to find much about 3d printing moulds online. My guess is the setup cost is fairly high and for a casual hobbyist, it's probably easier to find an existing mould on eBay or similar, instead of working through the trial-and-error and learning curve associated with 3d printing and casting.
Hope this helps!
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