Thermal Resistance Basics in Textured Plastic Injection Molds
Textured mold cavity thermal resistance stems from trapped micro-air gaps, reducing interfacial heat transfer coefficients down to 800 W/m²K and extending cooling cycle duration.
Viscoelastic Melt Recovery
Meaning
Tendency of a polymer melt to return to its original shape after being deformed during flow illustrates the complex nature of plastic rheology. Viscoelastic melt recovery occurs because polymer chains act like springs that store energy when they are stretched. This behavior is responsible for phenomena such as die swell in extrusion and part warpage in moulding.
Swell Effect
Expansion of the melt as it exits a restricted area like a gate or a nozzle is a direct result of the stored elastic energy. Viscoelastic melt recovery causes the plastic to become thicker than the opening it just passed through. This must be accounted for in the design of the tool to ensure the final part dimensions are correct.
Molecular Relaxation
Time required for the polymer chains to untangle and return to an equilibrium state affects the internal stress of the part. Viscoelastic melt recovery happens over a period of time, which means that the part may continue to change shape even after it has been ejected. High temperatures during the packing phase help the chains relax, which reduces the amount of stored energy.
Dimensional Precision
Stability of the finished product depends on managing the balance between the viscous flow and the elastic response. Viscoelastic melt recovery can lead to spring-back where a part bows or twists once the mould opens. Moulders often use longer hold times or higher melt temperatures to minimize these effects.
Precise control of the melt temperature allows for the production of parts that must fit together with tight tolerances in a final assembly.