SLA printers: food-safe resin parts

Question

I've actually read that resin printed parts are not food-safe in Reddit and Formlabs guide. However, it also says that food-safe can be achieved applying some kind of food-safe coat to the printed parts, or even printing with ceramic resin.

As an owner of a Elegoo Mars Pro, I've searched for the second - ceramic resin - and I've found nothing, so I guess it's only suited for more expensive and professional printers.

But regarding the food-safe coatings, is there any recommendation on which and how to apply them for resin printed objects? Is there any other way to achieve food-safe resin printed parts? Maybe are there food-safe resins, even though they may be more expensive (I haven't been able to find any)?

The scenarios I think that have to be considered are four:

• Prints to be used for liquids: like mugs and coffee cups, or recipients to hold drinks in the fridge, for example.
• Prints to be used for solids: just recipients to put dried fruits, nuts, olives...
• Prints to be used while eating, which some tools may interact with: plates, bowls, which will be in contact with knifes, forks, and some other material that could scratch it.
• Prints to be in contact with the mouth: this would be maybe a mix of the other, but would include forks, spoons, mugs...

I guess that the cleaning and maintenance will depend on the process to make them food-safe.

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I've been searching for the net, and I've found these kind of epoxy resins. I see people use them and say it's food-safe, but I cannot see it stated anywhere. Maybe are those what I'm looking for? Maybe not because some of them are for wood (maybe they can be used in other suraces, too?)? Some samples:

• Fantasy craft
• Art Resin
• Dipoxy resin

Hope to find a little bit of knowledge to increase safety when printing some parts and being careful about the different applications that our prints may have.

Thank you!

Answer 1

Resin basics

Resins are tricky, but probably less tricky than FDM as the manufacturing process is much less likely to include contaminants in the shape of contaminated air, particles, or adding lead into the print. This is all due to the whole process of creating the polymer happening under the protection of the resin, which in its monomeric liquid state is so toxic that it is unlikely any bacteria can survive in it. But before going out on a spending spree, you need to be aware that there are basically 3 kinds of resins on the market, only one of them is for SLA. To know exactly what you get you'd need to read the MSDS, which is usually available by the manufacturer of the resin.

• Two-component resins as you found in the lower half of the question. You might know them as Epoxy or Polyurethane resins. They have a base material and a hardener, sometimes called Component A and B. If mixed perfectly, they cure fully, neither hardener nor resin remains. This means it is chemically inert.
• Air-curing resins. These two come as Polyurethanes and also Acrylates and you might know them better under the terms paint, lacquer or coating.
• Light-cured resins, as you find them in SLA resins and all coatings that are not air-curing. These are for SLA a strange chemical mixture of a photoinitiator and then some monomers that create a copolymer while coatings can be as easy as having some initiators and the rest is a single type of monomer.

Food safety?

There are some resins is on the list of FDA approved plastics for food contact. The list is exclusive: if your plastic doesn't fit one of the listed ones, you can not use it. One example would be polyacrylate, which is defined as being "formed by melt polycondensation of bisphenol-A with diphenylisophthalate and diphenylterephthalate" - which excludes any other method of getting a polyacrylate and it prescribes exactly what basic materials are allowed.

As a result, many two-component resins and light-cured resins that don't match the exact chemistry and method to create an approved plastic/coating will not match the FDA approval list and won't get approval on their own.

Coating

But there is often a way out by applying a proper coating, for example with a sufficiently thick food-grade Polyurethane coating for dry foods. This would render the item food safe under FDA standards for the approved appication. You'd need to adhere to the proper method to apply this coating (manufacturers add those to the labeling usually), which can at times be somewhat complicated.

Insert

Another way might be to add an insert that does follow FDA standards, for example, a steel cup for liquids.