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ArthurG

Opinions on Discovery universal paste extruder?

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We need to 3d print realistic organ models and wanted to use silicone as a material. Has anyone got the paste extruder to work well with PVA supports? We need to make complicated structures with overhangs. Molds are out of the question because we are using a CT or MRI scan to create the model. 

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Have you considered printing an inverse (negative) of your model? For example a solid block, where the model is subtracted from, leaving the shape of the model as hollow canals in the block? And then fill those canals with liquid silicone? If you print that model in PVA, you can dissolve it, and you get the positive silicone model out.

 

I am not sure it would work for your models, but it might be worth trying?

 

The main issues would probably be:

- Big overhangs would cause sagging of the top layer. Thus causing rough top sides in the casted silicone model.

- You may need to design venting canals in the model, to allow entrapped air to escape. This may require some thinking and trial and error.

- A cured silicone model is hard to post-process, at least the silicones I used, because they are too flexible. It's hard to grind and polish.

 

Advantages are:

- You can use any liquid silicone of your choice, as long as its viscosity is low enough to fill the canals.

- You can add any color to the silicone mix.

- Curing time is no problem, you can use silicone that takes all night to cure (these are often stronger than fast-curing ones).

- Degassing to remove bubbles is no problem (unless this would damage the PVA support?)

- And shaking the mould to remove bubbles is no problem.

 

If the overhangs would be too large to print well without distortion, maybe it might be possible to cut the (negative of the) model in multple pieces, print these separately, and then stick them together? Wet the edges of the PVA, and glue parts together? PVA is water-based glue, so it might work?

 

First try this concept on small models which do contain all critical aspects: overhangs, entrapped air, fine details... But which don't waste too much material and printing time, until you get it right...
 

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So what your saying is print the positive space of the model out of PVA and the negative space and box outside out of PLA to make a mold, then dissolve the PVA? I'm having a hard time visualizing what you are saying but this is what it sounds like. Wouldn't that make any PLA surrounded by PVA be left without support when the PVA dissolves?

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No, I rather thought about only printing the negative (thus the surrondings) in PVA. No PLA at all. So you would have a hollow dissolvable PVA mould, with empty canals. In which you could pour the liquid silicone.

 

But this would only work if you can orientate the model so that there are no steep overhangs. For example let's take the "heart" symbol shown next to each post here on this forum, which you click to like. Imagine this is a 3D-model. Then you would only print the grey ball in PVA, and leave the heart itself hollow. So you can fill that hollow heart with silicone. But if you print the heart as shown, it would not work because of the overhangs. However, if you print it upside down, with the point facing upwards, and add a canal for filling at that point, it could be done. At least, it would be worth trying.

 

If you would want to print the model in PVA, and the surroundings in another removable material, then maybe some "breakaway" material might work as surrounding? Then the PVA-model could be dissolved, leaving the breakaway-material as a stable mould for pouring the silicone in? And then, after curing, break away the "breakaway" material?

 

Or maybe print the model in PVA, and the surrounding mould in wax, which is water resistant? Then you could first dissolve the PVA, next pour the silicone model, and then melt away the wax?

 

Or something else along these lines? An imperfect method that does exist and can be used immediately, might be better than a perfect method that does not yet exist.

 

If not clear, feel free to ask. I know what I mean when writing this, because I have the pictures in mind. But I am not sure the meaning comes across. If not, I will make a quick drawing (but not today).

 

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I do not think silicone will stick to the PVA enough to use as the support material for your models. Many have suggested using the printer to make something to use in casting the silicone. I would go with that option as well as seriously consider what @geert_2 has said. Could eliminate one step between print and final casting.

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10 hours ago, kmanstudios said:

I do not think silicone will stick to the PVA enough to use as the support material for your models.

This is a good point, I hadn't thought of that. Sticky silicone like used in bathrooms and pipings (=often acid, and cured by moisture in the air) might work, but this has a curing time of 24h or more. But I don't think you can print in sticky silicone: it would string like mad. Mould-making silicone definitely won't stick, indeed, otherwise it would be useless for mould-making: you would never be able to get the model out of the mould. Here too, printing will be difficult: if liquid, it will leak away, if paste, it will string.

 

Mould-making silicone has curing times between a few minutes (for dental use, but these do not have much strength) and 24h (for traditional mould making, much stronger). If you would print in silicone, then you would have to wait after each layer until the previous layer is cured enough so that it does not sag or leak away. But it must still be tacky, otherwise the next layer won't bond. If you would print with a syringe system with mixing tip, then the silicone in the mixing tip will be cured too during this waiting cycle, and you would have to replace and prime the mixing tip for each layer. Unless you keep printing somewhere else to keep the flow going, wasting a lot of material.

 

All these things add a lot of complexity to the system.

 

Another point to consider: do the models really have to be in silicone? Or would that just be the prefered material, because of its resistance against fracture when dropping? In the last case, maybe you could consider printing in nylon too? Nylon may not be easy to print, but it is compatible with PVA supports.

 

You might even be able to make the mould in PLA, if you pour in a silicone that is heat resistant enough. Most silicones can handle 250°C for a short time. Then you could melt the PLA away. Or heat it to 100°C and peel it away.

 

This is why making a traditional mould (in PVA, breakaway, wax, PLA, whatever...), and then pouring non-stick silicone in it, seems the best option to me, if it has to be silicone. If you use a low-viscosity silicone with long curing time, you have plenty of time to remove the bubbles by shaking and vacuuming.

 

I would suggest you watch a couple of "silicone mould making and casting" videos on Youtube. There are lots of good ones, made by real professionals, and they give a wealth of info.

 

Anyway, we would appreciate if you could give feedback on the results, of whatever method you chose.

 

DSCN5778b.thumb.jpg.dc7d69b86eb2cef753a4e63fdfb734cf.jpg

Fast curing silicone (1...5min), with mixing tips. These are a paste when extruded, like slimy pudding.

 

DSCN5780b.thumb.jpg.186a07939344cfe24ac13a6ad6ecdd0a.jpg

Fast curing (ca. 5min), paste-like when extruded. See the string at the back where the mixing tip took off. It was a long string, but it folded back on itself.

 

DSCN5784b.thumb.jpg.01a44dd724a596732d34f50108979af2.jpg

Medium curing (15...60min) liquid silicone for pouring moulds. This is liquid like thin sirup or liquid honey. These come in bottles of 1 liter, to be mixed 50-50. The orange took ca. 1h to cure and was more liquid. The blue was a bit more viscous, and took ca. 15 min to cure: this was too fast to allow the bubbles to rise to the surface, so they got trapped. I poured it in the hard shell, and around the model (not shown). So you would need a similar liquid silicone to pour into empty canals in the mould.

 

Shown here are dental silicones, but you are cheaper off buying artist silicones. It has to be platinum cured silicone. (No tin-cured silicone, which may decompose over time).

 

 

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