Dimensional stability is an important consideration in polyurethane elastomer production, especially for precision components, industrial rollers, wheels, seals, molds, and other products where final dimensions are critical. A common question from manufacturers is: Why does a polyurethane elastomer continue to shrink or change size after demolding, and how can this problem be reduced?
One important reason is that the chemical reaction may not be completely finished when the product is removed from the mold. Polyurethane curing involves the reaction of NCO groups with reactive components in the curing system. Even when the material appears solid, residual reactive groups may continue reacting after demolding. This post-reaction can cause changes in hardness, mechanical properties, and dimensions.
Therefore, the first step in controlling shrinkage is to maintain an appropriate reaction index and curing ratio. An excessively high NCO index may leave additional reactive NCO groups for subsequent reactions. On the other hand, an excessively low index can lead to incomplete curing and poor mechanical properties. A balanced formulation is therefore required rather than simply increasing the amount of curing agent.
Good mixing and degassing are equally important. Poor mixing can create local areas with excess or insufficient reactive components. These differences may result in uneven curing and dimensional variation. Air bubbles can also become internal defects and may magnify dimensional changes during curing. Appropriate vacuum degassing before casting can help reduce this risk.
Another useful measure is mold temperature control. A mold that is too cold may cause uneven curing between the surface and the interior of the part. Maintaining a stable and suitable mold temperature helps the material cure more uniformly.
For some applications, pressure during the initial curing stage can also contribute to better dimensional control. Keeping the material under appropriate pressure while curing can help reduce defects and maintain contact with the mold surface.
Perhaps the most important step for dimensional stability is post-curing. After demolding, controlled heating can allow residual reactive groups to continue reacting in a more stable environment. Depending on the formulation, post-curing conditions may be developed around 60–80°C for several hours. The exact temperature and time should be established through testing because different polyurethane systems have different curing characteristics.
For production, a practical process may include preheating and degassing the prepolymer, preparing the curing agent at the required temperature, thoroughly mixing the components, vacuum degassing when appropriate, casting into a preheated mold, maintaining suitable pressure during initial curing, and then carrying out controlled post-curing.
The main principle is that dimensional stability is not controlled by one parameter alone. Formulation ratio, mixing quality, temperature, degassing, mold conditions, initial curing, and post-curing all work together. A properly optimized curing process can reduce post-cure dimensional changes and provide more consistent polyurethane elastomer performance.