My urethane foam did not rise as expected (collapsed). What happened?

Article courtesy of Smooth-On, Inc.

Both flexible and rigid urethane foams are balanced systems that are sensitive to cold weather, contaminants and other variables. Carefully following directions (technical bulletin) is essential to being successful with foams. Some reasons for foams not performing as expected include:

  • Wrong Release Agent – Silicone based release agents will collapse foam and should not be used. Instead, use a wax based release such as Ease Release™ 2831 as directed.
  • Casting into a Tin-Cured Silicone Mold that is Not Fully Cured – A freshly made tin-cured silicone mold can cause your foam to collapse. To remedy, post cure the mold with mild heat of 150°F (60°C) for 4 to 8 hours before use.
  • Not Pre-Mixing Parts A & B – It is essential to pre-mix the containers of Parts A & B thoroughly by vigorously shaking the containers before dispensing in order to re-disperse chemicals that may have settled or separated (especially Part B).
  • Not Measuring Parts A & B Accurately – These are balanced chemical systems that require accuracy when measuring Parts A & B and should only be mixed at the specified Mix Ratio.
  • Inadequate Mixing of Parts A & B – You must mix quickly and aggressively by hand or by using a drill with mechanical mixer attachment such as a "turbine" mixer, making sure to scrape the sides and bottom of the mixing container several times.
  • Temperature – When weather is cold, we receive calls from people complaining that their foam is collapsing or not rising as expected. If the material is exposed to less than 60°F (15°C) for any period of time it will separate, especially Part B. To remedy, bring material back to room temperature of at least 73°F (23°C) and shake containers vigorously for 3 minutes before dispensing. Depending on the amount of material in the containers, it may take several hours to uniformly acclimate.
  • Cold Mold – If the material is mixed and poured into a cold mold, it will not rise as expected. If needed, you can pre-heat your rubber mold at 150°F (60°C) for 1 hour in a commercial oven prior to casting foam.
  • Age of Material – Old material will not react and rise as expected. For best results, use materials as soon as possible (especially after opening) as they have a limited shelf life.
  • Demolding Too Soon – Removing your foam casting from the mold before the specified cure time can cause foam collapse.
  • Adding Fillers – You cannot add powdered fillers (URE-FIL™, Quarry Tone™, etc.) to castable rigid or flexible foams. They are balanced systems that cannot have anything of substance introduced during mixing and pouring. Any filler will disturb that balance and the foam will not rise.

This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.