Polyurethane Experiments

Sector67 was privileged to receive six barrels of two part polyurethane scrap resin from Isthmus Engineering (that’s 330 gallons for anyone counting, or about ~2871 lbs).  We’re very happy we turned down the remaining 20 barrels, as we still have 5.95 barrels left. Although the material is technically expired and moisture sensitive, it’s still able to cure correctly as long as it isn’t left to open air for more than a few weeks – even then it fortunately still cures, just with a few extra bubbles.

Lots of necessary dispensing and disposable handling equipment:

We initially had a lot of trouble figuring out an appropriate process to pouring and curing it bubble free as well as an appropriate mold material. Early on we determined that particle board and other woods didn’t work well, the polyurethane reacted vigorously with any moisture in the lumber and was an extremely strong adhesive to wood, even with various release agents and finishes.

Eventually we used wood-polyurethane to seal the moisture in the wood, and then added our two part polyurethane on top to give a thick coating, but it still wasn’t a solution for a mold, just for coating wood parts.  One of the largest strategies is mixing the polyurethane up, putting it in a temporary container and drawing a strong vacuum until it stops growing from the released air, and then pouring a beautiful bubble free part.  If you don’t vacuum pump the mixed materials, it makes an ugly bubble-filled part.

We quickly settled on two part flexible silicon rubber as a successful mold material, but this silicon is very costly and it doesn’t hold up well to repeated pours as it deteriorates after every part removal.  (silicon mold at left, polyurethane tank tread sections at right)

We were also able to pick up very detailed objects with a silicon mold:

Eventually, we arrived at Mann-Release combined with various plastics as a suitable mold making technique for round objects.  What might we be building out of round slugs of polyurethane you ask?  How about a giant Lite-Brite?

The polyurethane has been used around the shop as a go-to item for anything needing to be stuck down, sealed up, or generally made sticky (oops, too little hardener huh. . .)

Fixing a shattered plastic bicycle rim:

Contributing to a winning 100 Hour challenge entry:

(Absolutely nothing sticks to borosilicate lab glass, except our trusty polyurethane)  The project was eventually appropriately named the “Polymaster” and went on to win the most creative entry in the UW student competition and $300 for the creating team.

Snowboarding slider box first go didn’t work so well:

We’ve also stumbled on a creative strategy for lost foam polyurethane molding, by hot gluing together lots of foam chunks you can create very complex foam molds, pour polyurethane around them, and then dissolve out the foam with a solvent:

The latest experiments have centered around using heavy weight oil as a plasticizer, which prevents the polyurethane from fully hardening:

And toying with adding various materials, it’s pretty common to add fiberglass fill to polyurethane, but adding pet shavings can yield unexpectedly strong, lightweight parts because of the foaming reaction to the wood:

Vacuum pumping the polyurethane and carefully coating pink construction foam yields an extremely strong airplane wing that’s able to be bent well past the breaking point of uncoated foam:

We’ve also been very successful at carefully metering water into the mixed polyurethane to yield high density foam:

A City of Madison BLINK Art Grant was fullfilled by Sector67 member Tim Sprengelmeyer of Midwest Recycled Art, creating a large turtle that’s currently in Orton Park.  Tim didn’t want his turtle to rust while out in the weather, so it was suitably coated with dyed polyurethane:

A little testing was done before deploying the polyurethane on the turtle, yielding very good anti-rust properties (half coated):

Our recently christened vacuum former makes nice negatives to pour into, and when properly coated with Mann Release we’re able to remove the polyurethane from the styrene plastic:

You can see in this mold that it’s not the standard yellow, as we’ve been experimenting with polyester and other solid particle dyes, yielding very interesting wavelength absorption and transmission characteristics.  In this case, a change from aquamarine to red:

There’s always the old stand-by table by submerging interesting things in polyurethane.  In this case, we’ve finally found an off the shelf product that it won’t stick to – freezer paper!  It usually releases cleanly with no residue (without a mold release), and it’s cheap.

Of course when you’re done pouring, but have a little left over and your now very sticky gloves, what more to do than to make an extra hand to lend around the shop:

Yes, this stuff even bonds to nitrile, latex, vinyl, and any other glove materials we’ve tried. . .  It’s really really sticky stuff.

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