Swirls/Streaks in Parts

Occasionally, swirls and streaks will show up in a finished part. This generally manifests itself as multicolored swirls throughout the part in a fairly random pattern.

Swirls caused by inadequate mixing.

Potential Causes

Improper Mixing
Even without pigment, the prepolymer and curative are generally different colors, or different shades of the same color. If these two (or more) materials are not completely mixed together, prepolymer-rich or curative-rich pockets will remain in the elastomer. This will lead to poor physical properties since the pockets will alter the stoichiometry of the elastomer. This phenomenon is usually fairly easy to spot when a pigment is in one stream of the mixture. The drastic difference between the colors in the streams is an excellent indicator of poor mixing.

Unevenly Dispersed Filler
Many softer polyurethane formulations contain fillers which in some instances can be very difficult to mix into the formulation. Insufficient mixing of fillers will cause swirls and streaks in the resulting part.

Knit Lines
In the production of some parts, most notably vertically cast rollers, one can observe knit lines. These lines occur when material flows around a core or insert and the material is partially cured when it meets at the opposite side of the insert. Depending upon the degree of cure at the point of contact, this knit line area can be a weak point in the elastomer.

Solutions

Ensure Sufficient Mixing of All Components
Use of a pigment will help highlight areas where materials are mixed inadequately.

Disperse Fillers in Plasticizers
It is often more effective to dissolve or suspend fillers in a plasticizer before adding to the prepolymer. 

Change Particle Size for Fillers
Smaller particles are generally more difficult to disperse in the mixture. However, if the problem appears to be particles settling out of the mixture, then going to a smaller particle may help.

Use Materials with High Viscosity
Again, if settling appears to be the issue, the use of prepolymers with higher viscosities could solve the problem.

Use Systems with Increased Pot Life
The general way to minimize knit lines is to either pour material into the mold at a higher flow rate or to use a system which has a longer pot life, such as an Airthane prepolymer.