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First, a little background. A couple of years ago, I was researching making my own candy, and I came across this page: Lego brick shaped gummy candies, describing how to use real Lego bricks as a positive to make silicone molds for Lego brick shaped candy. Now that I have a 3D printer, and inspired by the usual description of ABS filament ("It's the same plastic used to make Lego bricks") it occurs to me that I can now make any positive I want.

The question is, would that be safe? I know the filament I'm using is not food safe, but if I create positives for a food safe silicone mold, would toxins leech into the mold? And if so, is there a barrier I can use to prevent this, such as some kind of coating?

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In principle, ABS is safe for contact with (cold or room-temperature) food. The two main concerns specific to 3D printing are, assuming you start with a filament that is not itself contaminated:

  1. Pores and holes in the printed part which may harbor bacteria

  2. Impurities introduced into the plastic during the printing process

I doubt that the silicone mold will capture the pores and holes with sufficient detail to be of any concern (it certainly won't capture the internal structure, only the surface).

That leaves us with 2. It has been noted that brass nozzles contain trace amounts of lead. This lead can contaminate the printed part, which may in turn contaminate your mold, which may in turn contaminate your food. I don't think this is of realistic concern, since we're looking at trace amounts of trace amounts of lead. The nozzle might also have burnt plastic stuck to it (which might be carcinogenic) so you should make sure to do the print with a very clean nozzle and at a temperature that is not too high.

ABS is food safe for contact with cold or room-temperature food. It is however not food safe for contact with hot food, because at higher temperatures the food may leach certain chemicals out of the plastic. Your application is one of low temperature, but silicone is not food and might perhaps leach some contaminants out of the plastic, regardless of temperature. However, this concern is not specific to 3D printing, as it applies to the method of making moulds out of Lego bricks as well. Therefore, making moulds from 3D printed positives does not appear to be different in a food safety perspective from making them out of LEGO blocks.

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    $\begingroup$ The same grades of brass used to make nozzles are often used to make potable water plumbing fittings... the lead-leaching concern seems really tenuous to me in general with direct FDM part food contact, and even less so for silicone molds. $\endgroup$ Jun 8 '16 at 15:39
  • $\begingroup$ I'd really like it if this answer included some info on PLA, but I didn't specifically ask for it in my question, and the relative novelty of the material would likely make solid info hard to find.I would assume, however, that PLA would be at least as safe as ABS for this application.. $\endgroup$ Jun 9 '16 at 17:38
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The plastic is not quite your main concern (though it still can be). You should worry about the cracks and crevices in FDM prints. Bacteria loves to hide there. For most people, this is the first concern when it comes to "Is X 3D printed food-safe?" If the end product is a hard material, you should sand or smooth your print to prevent the layer crevices from appearing in the end use product. Also consider food-safe epoxies for filling in gaps. If the end product is made of a flexible silicone, then this is less of a concern.

ABS is not food safe. PLA as a material is considered food safe by the 3D printing community, but I have not seen a scientific study on this. However, many filament makers do not extrude pure PLA. Therefore, the answer to your question is that it depends on the manufacturer. You will need to contact the company to know what is in the filament first. Beware of untrustworthy manufacturers that claim food safety without backing that up.

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    $\begingroup$ This seems to pass by the main point of the question entirely: whether it is safe to create silicone molds from 3D printed parts (which would presumably not copy these cracks and crevices from the original part). Note that ABS is a food-safe material if used properly, so your claim about ABS being considered not food safe while PLA is, is wrong. $\endgroup$ Jun 8 '16 at 6:24
  • $\begingroup$ When comparing PLA vs ABS for this application, basic features like printability and the possibility for acetone smoothing might be of interest. If cracks and crevices turn out to be an issue, then perhaps smoothened ABS parts could be an option provided that the acetone itself doesn't turn the mold toxic. $\endgroup$ Jun 8 '16 at 14:18

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