Tuesday, February 7, 2012

Lye Bath! - How to Clean Prints

As discussed before, when parts come out of the printer they're covered in a rubbery, wax-like support material that has to be cleaned off.  Typically a water jet is used for cleaning but even after that I've had a lot of residue left.

This is a particular annoyance with my current tentacle design.
After some research, I found that the printer manufacturer recommends a sodium hydroxide bath which will loosen and remove the support material.  I have access to an ultrasonic cleaning tank which would work great with the solution.

The problem is sodium hydroxide, also known as lye, is kind of hard to find due to the fact that it's used in crystal meth production. Yay!  Typically, you can use drain cleaner but I couldn't find any that was pure lye.  The good news is it's also used in darkrooms and I was able to pick some up at B&H Photo. (Pick up only, no mail order).

Anyone who has seen Fight Club and/or made soap knows that you can get a nasty chemical burn from lye, so be careful.  While vinegar technically will neutralize lye, it sounds like it takes a lot of it, so the general consensus seems to use a lot of running water for an extended period. Lye in water is also exothermic and will give off a lot of heat.

Objet, the printer manufacturer, is recommending a 2% lye solution which should be relatively low-key, but still be cautious.  (For example, this concentration is about the same strength as what pretzels or bagels are dipped in to develop the delicious crust when baking)

Here's what I have gathered:


- Use protective eyewear and gloves.
- Use cold water and slowly add powder to water, NOT water to powder.
- If you spill any powder, clean in up with a brush, NOT water.
- Lye solution will eat aluminum and brass! OK for steel and plastic.
- From what I understand, the solution should be odorless.
- Powder will absorb water from air, so keep it tightly sealed. Do NOT use desiccants in the container.
- Further research indicates that it's ok to use in ultrasonic cleaners although you may not want to use
the heater. Thoroughly rinse parts in water when done.

I had a total brain-fart when trying to figure out how to make a 2% solution. But by dumb luck I found this great article from the wine industry, of all places, on mixing sodium hydroxide solutions.

I forgot that in most cases, 1 gram weight = 1 mL volume. So, to make 1000mL of a 2% solution, mix 20 mL sodium hydroxide crystals with 980 mL water. Please correct me if I'm wrong.

I found this thread on an Objet user forum that addresses both lye solutions and ultrasonic cleaning.

For those of you not familiar with ultrasonic cleaners, they clean by immersing the object in a liquid bath which is vibrated at... ultrasonic frequencies which forms micro-bubbles that get into all the nooks and lifts dirt away.  Great for cleaning small parts, hard to reach areas and jewelry.

I tested this on one of the tentacles, the computer panel and tire with great success.  I left them in the lye bath for about 15-20 minutes, then put them in a water bath for another 10.  It really cleaned them up and revealed a lot more detail.  I'll be sticking with this method.

UPDATE:  I have stopped using the water bath since it seems unnecessary.  After the lye bath I scrub the parts with nail & tooth brushes under running water. You'll know they're clean when they don't feel slimy and stop producing foam when brushed.