The importance of curing, heat, and time.

July 12, 2021
The importance of curing, heat, and time.


A history can be seen in heat lamps.
Tens of thousands of cured polyurethane sheets, blocks, meshes and everything in between have came to life under their gentle heat.
While we’ve moved forward to a larger setup now, it’s functionally the same as this vintage Engineering Plastics picture.

Time and heat requirements change with every material. Your polyurethane cure time may be as little as an hour for tiny pieces, or may take closer to a month for large pieces to “truly” cure throughout.
If you plan on curing multiple jobs in the same oven, keep in mind these small changes can have a big effect on your finished product.

A leading cause of failure in cured polyurethane products is not being allowed to cure fully before use.
Heat and time are needed for the almost organic building of a long molecular chain from the prepolymer components.

Heat’s secondary purpose, is removing moisture from the air and material before it cures; This is just as important as the cure itself.
Moisture is everywhere.
The smallest amounts of moisture lead to voids in the product, and larger amounts will prevent a cure altogether.
All moisture possible should be removed prior to mixing through generous use of heat, and a vacuum if possible.


Not curingCold molds and tooling are the usual culprit if there is no contamination present. Especially with filled resin molds and inserts which do not retain heat as well, try heating them for longer and increase the heat if possible.

Common Heat Woes
Issue Solution
Slow to cure Mold or material may be too cold, heat further
High shrinkage Material is too hot, try lower temperature
Pot life too short Material is too hot, try lower temperature or use part A (TDI base) at room temperature when mixing to squeeze another 30 seconds of time
Voids in parts Temperature of the material/mold may be too high, less time in the oven before heating or lower oven temperature required
Streaks Material may be curing as you pour. If not a pot life issue, it may be a cold mold disrupting flow
Bubbles Part B of your TDI base may have absorbed environmental moisture. Heat to at least 85c if using a vacuum chamber for 1-2 hours at 30hg.
Some materials may handle 100c and not require the chamber, check with your prepolymer supplier