Testing Release Agents for FOAM-iT

Testing Release Agents for FOAM-iT

December, 2016

I’m working on my next robot and need a sturdy, but very lightweight material for the body.  I came across Smooth-On’s FOAM-iT Series of castable, rigid urethane foam and felt it would be a good fit for my needs.  I chose FOAM-iT 4 to get the balance of rigidity and weight I was seeking.

In a nutshell, this is a 2 part formula that, when mixed, create an exothermic chemical reaction that rises and fills a cast.  One of the key challenges with this material is that it is VERY sticky and sometimes difficult to remove from a mold without destroying the foam or the mold.  Because of the size and shape of my mold, I chose plywood for the cast material – an especially challenging material.  I attempted to do some research to find the best release agent, but was disappointed to find a general lack of information on the topic.  So, I’m writing this blog to document the testing I did in hopes it will help someone else in the future.


Release Agents Tested:

  1. Epoxy – A helpful employee at Home Depot recommended this stating “almost nothing sticks to Epoxy and it’s really inexpensive”.  Worth a try?  The Rust-Oleum 12oz. Epoxy Gloss Appliance Spray was recommended.  Epoxy.jpg
  2. Finishing Wax – I stumbled across this at Home Depot as well.  I like the idea of wax because I saw that the foam was easily removed from the wax paper cups I use for measuring.  I selected the Minwax 1 lbs. Paste Finishing Wax.Wax.jpg
  3. Mann’s Ease Release 2831 – In my research, this stuff was highly recommended by Smooth-on and seemed to be specifically designed to do what I wanted.  It comes in pint and 1 gallon and 5 gallon sizes.  Ease Release 2831 information is readily available online.Mann.jpg
  4. Petroleum Jelly – My friend recommended this and said it worked well with silicone molds.  She recommended it for my project as well.  I selected Vaseline 3.75 oz container size since I don’t need much.  Vaseline.jpg

There were other materials considered but not tested.  First, Car Wax (like Turtle Wax) was eliminated because it specifically warns against using with wood.  Also not tested was Polyurethane wood finishes since the foam is also a urethane material and would likely bond with it.  A Shellac or Lacqure was also on my list, but I halted the testing once I found my winner and did not try these.


Test Method:

I built 4 identical molds that would result in 4 inch foam cubes using 1/2″ plywood.  I secure the sides, bottom and top with 1 inch wood screws.  I drilled a 5/8″ hole into the top of the mold so I could pour in the FOAM-iT and also give it an overflow spout.  I coated the inside of the mold with the release agent and screwed on the lid.  I followed all application instructions and exceeded the recommended wait time for product drying before testing.  I used liberal amounts of product in each case.


Using some mixing cups, I measured out 2 oz. of Part A and Part B of FOAM-IT, mixed them together, poured them into the top and waited for the “magic” to happen!  I repeated this 4 times to allow a side-by-side comparison.

I did not use a control scenario for the test, but in an early attempt, I over-estimated the amount of FOAM-iT required and it came up through the overflow spout and ran down the sides of the container.  The effort of cleaning up that mess was considerable because I had not put any release agent on the outside of the boxes.  I had to use a knife and remove the foam in segments.  Even after the foam was removed, there was lots of it stuck to the wood that I had to use a razor blade to scrape off.  A release agent would have prevented that, so I considered that my baseline control result.


Uh oh!  I think I poured too much in!



I used the following criteria when judging the results:

  • Must allow the end resulting foam cube to have a smooth, clean-looking finish
  • Must not stick to the inside of the plywood cast
  • Must allow plywood to release with minimal effort/tools
  • Must help to fill cracks where the plywood joins to reduce foam leakage
  • The form must be reusable after the foam cube is removed

Other important factors:

  • Low Cost
  • Easy of application
  • Avoidance of dangerous chemicals


Test Results

Results 1.JPG

Results 2.JPG


Test Result Details

  1.  Epoxy – oh, man, what a nightmare!  It turns out Epoxy was a terrible choice for a release agent.  The foam stuck to the inside of the form and created a very strong bond that would not allow the form walls to release.  I took a flat-head screwdriver and rubber mallet and ended up destroying the form in an attempt to remove the foam block.  I stopped after removing the top and first wall – there is no way the foam block would survive the removal process!  Never mind that it was easiest to apply and the lowest cost – it is just not suited to be a release agent for FOAM-iT.  Epoxy Mold.jpg                                                Epoxy is not a good choice as a release agent!
  2. Finishing Wax – This turned out to have the best results for the foam block.  Very little foam stuck to the plywood and it was quick and easy to knock the form walls off the foam block.  The fact that it was low cost was a nice bonus.  Reaching inside the form to apply the wax with a cloth took some effort, and I imagine a more detailed form would have required additional time and effort to spread the wax.  However, the effort paid off because it acted like a filler for many of the small gaps between the form walls and turned out to be the best at keeping the foam inside. Wax Mold a.JPG                                                                   Finishing Wax Results
  3. Mann’s Ease Release 2831 – This turned out to be the biggest disappointment.  First the warning label really had me nervous about using this stuff – all kinds of precautions and restrictions on usage.  I used latex gloves and threw out the brush when I was done.  This is a milky, highly viscous liquid that I poured into the small cap.  I used a foam brush to absorb the liquid and apply it to the inside of the form.  It dries quickly (maybe 10 minutes) and results in a powdery crystalline residue.  It does nothing to help fill the cracks because it is so viscous – it just runs right out the bottom.  I applied 2 coats in a 73 degree room, as instructed.  This stuff also has a limited shelf life after opening, so now the clock is ticking on this stuff.   The end result was some foam sticking to the plywood, which resulted in a bit more effort required to pull the foam block out of the form.  A knife was required to cut free some of the walls.  The end result looks uneven and is not smooth.  I’m surprised since this stuff was specifically made for this purpose.Mann Mold a.jpg              Ease Release 2831 Results – note the foam on the boards and splintered wood
  4. Petroleum Jelly –  I applied a liberal amount of Vaseline onto the inside of the form with a folded paper towel.  I carefully filled the cracks with this goo and screwed on the lid.  The release was relatively easy – and I was expecting a good result.  Although the foam did not stick to the sides of the plywood, the end result was not too different from the Mann’s Ease Release 2831 – uneven and not smooth.  The Vaseline didn’t effectively fill the cracks as I still had a lot of seepage out the bottom and sides.  I suspect it was because the foam gets hot and the Vaseline did not hold a seal when it got warm.  Also, unlike the other release agents tested, there was a sticky mess both before and after the foam was applied since Vaseline does not harden.Vaseline Mold a.jpg                                                           Petroleum Jelly Results



The clear winner, for me, is the Minwax Paste Finishing Wax.  It met all my primary requirements and did well with my secondary requirements.  Upon careful inspection of the image below, you’ll see a nice smooth, uniform finish without pits or spots on the 2nd block.  4 blocks a.jpg


Lessons Learned

  • There is no release agent that will just let the foam “slide out” of the form.  It will always require some prying to coax the foam out.
  • One of the key challenges in getting the sides of the form to release is when there is seepage through the gap between the plywood pieces.  Minimizing the seepage will result in least effort.
  • FOAM-iT clean-up can be reduced by taking action while it is still warm and pliable.  I used some wooden craft sticks to stir and cleanup.  They were effective and disposable.
  • Estimating the amount of FOAM-iT is quite challenging – part art/part science.  I attempted to measure the volume and calculate the amount and I was waaay off.  Unfortunately, this results in some waste which is frustrating since this stuff is quite expensive.



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