Poor Surface Quality

Several processing conditions can cause surface irregularities in cast parts. This is a concern since many applications for polyurethane parts require aesthetically pleasing surfaces.

MDI Elastomer that has surface mottling due to a cold mold.

Potential Causes

Improper Mold Release Application
Using either too much or not enough mold release will lead to poor part surface characteristics. Many processors try to push their release agent to get several releases with one application. This will eventually result in parts sticking in the mold and ruined part surfaces. It may also be tempting to try to use a heavy coat of mold release to get several turns from the mold. However, this too can lead to surface quality problems. Not all mold releases work with all systems; wax-based mold releases, for example, may work for MDI systems, but not for TDI systems.

Dirty Mold
Buildup of elastomer material on the mold will cause poor surface quality on the molded parts. This buildup generally occurs because of inadequate mold release, which in turn continues to allow elastomer material to build up on the mold surface. The molded parts subsequently take the shape of the mold, which now has deposits of polyurethane on it.

Excessively Cold Mold
Cold molds can lead to shrink marks on the surface of parts. This is particularly true with MDI prepolymer systems. The long "green state" of these systems, combined with excessively cold molds, will lead to shrinkage marks.

Solutions

Apply the Proper Amount of Mold Release Evenly
Following the manufacturer's directions for mold release application will solve most surface quality problems. Also make sure that the mold release is applied after the appropriate number of turns.

Clean Molds Regularly
Sandblasting the mold is one method of cleaning molds whenever a buildup is noticed and part surface imperfections have begun to appear.

Heat the Mold Thoroughly
Thorough heating of the mold will reduce the likelihood of cold mold marks and will help avoid several other potential problems. The mold should also be removed from the oven as close to pour time as possible.

Choose the Best Mold Material
Steel molds retain heat better than aluminum; therefore, it is easier to keep the temperature of steel molds uniform versus aluminum molds.