Optical Profilometry Metrology Protocols for Indirect Tool Cavity Topography Verification
Calibrated silicone replicas combined with optical profilometry yield sub-micron tool cavity topography metrics without destructive sectioning.
Volumetric Cure Shrink
Meaning
The reduction in volume that occurs as a thermoset resin or a crosslinking elastomer transforms from a liquid to a solid state affects the final dimensions of a part. This phenomenon, known as volumetric cure shrink, results from the formation of chemical bonds that pull the polymer chains closer together. It is distinct from the thermal contraction that happens during cooling.
Managing this change is necessary for maintaining tight tolerances and avoiding internal voids in thick sections.
Stress Accumulation
When a material is constrained within a mould, this reduction in volume creates internal tension. Volumetric cure shrink can lead to warping or cracking if the part geometry prevents the material from moving freely. In composite moulding, this shrinkage can cause the resin to pull away from the fibers, leading to delamination or a poor surface finish.
Designers often add fillers like calcium carbonate to the resin to reduce the total amount of shrinkage and improve dimensional stability.
Tool Compensation
Mould designers must oversize the cavity to account for both the chemical and thermal components of the size change. A failure to accurately predict the volumetric cure shrink will result in parts that are consistently too small.
Process Optimization
Controlling the rate of the chemical reaction through temperature and catalyst concentration helps manage the impact of these changes. A slower cure often allows for better stress relaxation and more uniform shrinkage across the part. Monitoring the pressure inside the cavity during the cycle provides a direct way to see when the shrinkage begins and when the reaction has reached completion.