Linear Shrinkage/Shrinkage

All thermoset materials shrink as they cure. The source of this shrinkage is three-fold. First, thermoset materials tend to exotherm, or heat up as they cure. The materials flow into their final shape as a liquid that is warm and then set to a solid while warm. Upon cooling back to room temperature, the solid goes through normal contraction experienced by solids as they cool. This results in a part that is smaller than the mold or part it was originally poured into.

A second source of shrinkage in thermoset materials is due to the molecular reorganization that results in the cure of the thermoset material. When a mixture of molecules reacts chemically into a single, new molecule, the distances between atoms are reduced. This decrease in molecular length results in shrinkage.

Third, in general, liquids are less dense than solids. A thermoset material begins as a lower density liquid and ends as a higher density solid. This change in density results in shrinkage.

Linear shrinkage specifically refers to the change in length of an extended volume of thermoset material on cure.

Higher exotherm materials like rigid epoxies tend to shrink more than softer ones like urethanes. Filled materials display less shrinkage than unfilled ones.