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I've a friend who is expecting. There are several adorable weapon themed rattles on thingiverse. I am, however, concerned about safety associated with such a product in the hands of a baby who will gnaw on it. To me the safety concerns here are much larger than for most food handling applications.

What steps should be taken to ensure such a print is safe for use?

This includes: filament selection, pea material selection, wall thickness, smoothing, construction, etc.

My current thoughts are as follows: "food grade PLA", dried peas, sanding, and single piece construction

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You're on the right track. Since you asked for "steps" here you go:

Step 1. Choose a safe material:

Consider chemical safety and physical safety. Food grade PLA should be chemically safe, but could be too brittle depending on the design you choose. PETG, T-Glase, or similar filaments (depending on dye) are normally also chemically safe and are less brittle than PLA so may be a better alternative. ABS is NOT typically considered safe for food contact.

Step 2. Choose a safe design:

If using PLA, be sure the design is robust enough to ensure it won't break. Broken rattles with sharp edges make baby...sad. Even less brittle filaments can still break with jagged edges if the design is fragile.

Step 2a. Choose a single piece design:

Choose a design that requires you to add the peas (or other safe filler) during the print such that the finished rattle is fully enclosed. This will minimize parts becoming loose or peas spilling and minimize choke hazards.

Step 3. Consider post processing to improve safety:

Sanding could reduce ridges and minimize crevices that could harbor bacteria, but sealing it with a food-safe sealant may be more effective. There are many sealants that the FDA considers safe, but polyurethane or food-safe epoxy finishes will work well with PLA. (If you use a different material, test to verify good adhesion.)

Step 4. Test:

Make a test rattle and run it through the paces. I guess you could chew on it, but since babies don't have teeth, this might invalidate your data...

Step 5. Consider alternatives such as professional printing services:

If your tests don't inspire confidence, professional services can offer additional materials (metals, ceramics, etc.) that could be safer than a typical fused filament printer.

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    $\begingroup$ +1 for the food safe sealant comment. If it has dried peas (or other perishables) inside of it, I would be worried about bacteria getting inside the object and festering. $\endgroup$ – BrainSlugs83 Oct 8 '16 at 5:36
  • $\begingroup$ Welcome to the site. Nice answer! $\endgroup$ – Tom van der Zanden Oct 8 '16 at 8:38
  • $\begingroup$ Several ideas I didn't have. Good. Are dried peas good or is there something safer? $\endgroup$ – kaine Oct 8 '16 at 12:31
  • $\begingroup$ Good point, BrainSlugs. Thanks, Tom. Kaine, you may consider something more stable than dried peas or the like...maybe clip off little bits of the same filament you print with or even print little beads or pellets for filler. That might be a waste of time if you were manufacturing these, but for a single item it's probably not too much more time. That would eliminate any chance of chemical interaction between the beads and shell...probably not a concern anyway, but it would eliminate one more potential risk. $\endgroup$ – Chris Thompson Oct 9 '16 at 18:09
  • $\begingroup$ Having loose plastic inside scares me incase it somehow breaks. Dried peas or raw beans are digestible...if barely. I will print a single large pea of the same material. This pea will be too large to be a choking hazard. $\endgroup$ – kaine Oct 11 '16 at 18:09
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Also consider the nozzle on your printer. Most nozzles are made of brass, which is not considered food safe due to the presence of small amounts of lead. Stainless steel nozzles are available which will not perform quite as well as brass but are food safe.

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