Calculating Cavity Thermal Contact Conductance for Etched Tool Surfaces
Calculated thermal contact conductance on etched tools drops up to 80 percent during cooling, requiring dynamic pressure modeling to avoid warpage and cycle delays.
Polymer Microhardness
Meaning
Measure of a plastic material’s resistance to localized deformation at the micron scale which indicates the degree of crystallinity and mechanical strength. Evaluation of polymer microhardness is performed by pressing a diamond indenter into the surface with a small, precisely controlled load. The resulting indentation is measured to calculate the hardness value, which provides insight into the local molecular structure and the effects of processing conditions.
This measurement is particularly useful for characterizing the skin layer of a moulded part, which often has different properties than the core due to the rapid cooling at the tool interface. It is a non destructive way to assess the quality and the consistency of a finished part.
Indentation Technique
Precision in measuring the surface resistance requires specialized equipment and careful sample preparation. To determine the polymer microhardness, a Vickers or Berkovich indenter is typically used, applying loads in the range of millinewtons to newtons. The depth of the indentation is monitored in real time, allowing for the calculation of both the elastic and the plastic deformation of the material.
This method is sensitive enough to detect changes in the material properties caused by environmental aging or chemical exposure. Because polymers are viscoelastic, the hardness value can depend on the rate at which the load is applied and the duration of the hold. Standardized testing protocols ensure that the results are comparable across different laboratories and material grades.
Crystallinity Link
Mechanical properties of semi crystalline resins are strongly tied to the size and the distribution of the crystalline regions. A higher polymer microhardness usually indicates a higher degree of crystallinity, as the ordered lattice structures are more resistant to deformation than the amorphous regions. In an injection moulded part, the cooling rate varies from the surface to the center, leading to a gradient in the crystal structure.
Microhardness testing across the cross section of a part can reveal this gradient and show how the cooling conditions in the mould have affected the final part properties. This is especially important when sourcing resins that are sensitive to cooling rates, such as polyamides or polyesters. By optimizing the tool temperature, moulders can achieve the desired hardness and wear resistance on the part surface.
Surface Integrity
Durability of a moulded component in its final application often depends on the strength of its outer layer. Measuring the polymer microhardness provides a way to verify that the skin of the part has the necessary resistance to scratching and wear. If the melt temperature is too high or if the resin is degraded during processing, the surface hardness may be lower than expected, leading to premature failure.
Sourcing high quality virgin resin ensures a consistent starting point for these properties, whereas regrind can introduce variability that affects the surface integrity. The test can also identify if a part has been properly annealed or if it contains high levels of residual stress. Regular monitoring of microhardness on the production line serves as a reliable check on the stability of the moulding process.