Meaning
The progressive increase of the solidified polymer layer formed against the cooled wall of a mould cavity determines skin thickness growth. This phenomenon dictates the mechanical structural integrity and the final dimension of a part during the injection moulding cycle. When the molten resin enters the cavity, a shell begins to solidify immediately upon contact with the metal surface.
Moulding Dynamics
Thermal conductivity properties within the thermoplastic material govern the rate at which the skin expands toward the core. High viscosity grades or lower injection temperatures accelerate this solidification process. A premature transition to the solid phase results in a thick skin that restricts the flow of molten resin to remote sections of the geometry.
Flow hesitation occurs when the velocity of the plastic front drops below a threshold established by the building resistance of the rigid layers.
Thermal Resistance
Convection occurs at the molten core while the solidified skin acts as an insulator against heat transfer. The thickness of this insulating barrier grows proportional to the square root of time based on the Fourier number of the specific resin. Cooling circuits in the tooling maintain the wall temperature to stabilize this rate of solid accumulation.
Disruption in heat extraction forces uneven growth across the part geometry which promotes internal stress or warpage.
Economic Impact
Virgin material specifications often define a melt flow index to keep this solidification rate predictable throughout the production run. Incorporating regrind material introduces molecular weight distribution changes that alter the rate at which the skin thickens. Excessive variations in layer formation lead to dimensional drift that forces costly adjustments to injection pressure or packing time.
Consistent control of the cooling interface minimizes scrap rates by ensuring the skin reaches sufficient strength before the ejection sequence begins.
