Polyurethane sheets and rods are widely used in industrial applications because of their high wear resistance, elasticity, impact resistance and mechanical strength. However, air bubbles or small voids may sometimes be found inside cast polyurethane products. Understanding why these bubbles occur and how they can be reduced is important for improving product quality, especially when polyurethane sheets and rods are used for demanding mechanical applications.
The main reason is related to the casting process used to manufacture many polyurethane products. Unlike thermoplastic polyurethane, which is commonly processed by heating and molding, cast polyurethane elastomers are produced by mixing liquid components and then pouring the material into a mold. During mixing, pouring and molding, a certain amount of air can become trapped in the liquid material. Chemical reactions during curing can also affect the formation and movement of small bubbles.
Several factors can contribute to air entrapment. Mixing speed, material temperature, viscosity, mold design and the condition of the raw materials can all influence the final result. If the material is highly viscous, trapped air may have difficulty escaping before the polyurethane begins to cure. Moisture can also be an important factor in polyurethane processing because isocyanates can react with water and generate carbon dioxide gas. For this reason, controlling moisture in raw materials, molds, equipment and the production environment is important.
For polyurethane sheets and rods, controlling internal bubbles is particularly important because these products are often machined into finished components. Large or concentrated voids may affect mechanical strength, dimensional stability and the appearance of the machined surface. In applications involving continuous load, impact or wear, internal defects can also influence the service life of the final component.
To reduce air entrapment, manufacturers can improve the casting process through better control of mixing, pouring and curing conditions. One approach is to use centrifugal casting equipment for suitable polyurethane sheet applications. During centrifugal molding, the mold rotates at high speed after the polyurethane material is introduced. The centrifugal force helps move the liquid material toward the mold wall and can encourage a significant portion of the trapped air to escape from the material.
This method can be particularly useful when producing polyurethane sheets with relatively uniform thickness and high surface quality requirements. However, centrifugal casting is not suitable for every product geometry, so the production method needs to be selected according to the size, shape and formulation of the polyurethane component.
Additional thermal treatment can also be used when appropriate. A controlled short-time heating or post-curing process can help remove or reduce some remaining small bubbles and improve the overall condition of the material. The temperature and treatment time must be carefully controlled because excessive heating may affect the polyurethane's mechanical properties or dimensional stability.
In practice, completely eliminating every microscopic void from a cast polyurethane product can be difficult, and the acceptable level depends on the intended application. For critical components, production parameters and inspection standards should therefore be defined according to the actual operating requirements.