Your system processes beautifully, but when you demold, you notice surface defects ranging from shrink marks, to voids, to mottling and knit lines. Why does this happen?

There are many causes, but improper mold release application and a dirty mold are common culprits. Using too much mold release is as bad as using too little. Too much leads to a buildup on the molds, and if they're not cleaned and conditioned properly prior to being put back into use, your parts will have voids, mottling and craters.

Shrink marks due to a cold mold.

Cold molds are a leading cause of surface shrink marks, particularly with MDI systems, which are quite temperature sensitive. Cold molds are often an issue late in the day when you may not allow enough time for the mold to reheat. Problems can also occur during seasonal changes if adjustments aren't made to your preheating schedule. Also, different types of molds have different thermal properties. For instance, steel molds take longer to heat up than aluminum, but steel also retains heat better.

Knit lines occur when a material starts to gel before it has completely flowed around an insert or core. This can be avoided by using a prepolymer with a longer pot life and lower viscosity, such as an Airthane® prepolymer.

For further information on how to avoid elastomer problems, go to our on-line Trouble Shooting Guide, accessible from the navigation at the left of this page.