Meaning
Thermal fluctuations at the interface between the molten polymer and the mould wall during the injection cycle determine the final appearance and skin structure of a part. This transient surface temperature rises rapidly upon contact with the melt and then decays as heat transfers into the cooling system. It is a dynamic variable that differs from the steady state temperature of the bulk tool.
Crystallinity Control
Rapid cooling at the surface can freeze the molecular chains before they can organize into a structured lattice. If the transient surface temperature remains high for longer, the polymer develops a more crystalline skin which improves chemical resistance and surface hardness. This effect is particularly pronounced in semi crystalline resins like polypropylene.
Defect Prevention
Insufficient heat at the interface often leads to visible flaws such as flow lines or a dull finish. By managing the transient surface temperature through variotherm heating, a moulder can eliminate knit lines in aesthetic components. This ensures the part meets high visual standards without secondary painting.
Moulding Efficiency
Cycle time is directly tied to how quickly the interface returns to the ejection temperature. Excessive transient surface temperature increases the time the part must stay in the tool to reach structural rigidity. Finding the minimum effective heat is the key to high volume production.
