The Frozen layer fraction at end of fill result represents the thickness fraction of the frozen layer at the end of filling.
It ranges from 0.0 to 1.0. A higher value indicates a thicker frozen layer (or thinner flow layer) and higher flow resistance. A polymer is considered frozen when the temperature falls below the transition temperature (Ttrans).
During filling, the frozen layer should maintain a constant thickness for those areas with continuous flow, because the heat loss to the mold wall is balanced by the hot melt coming from upstream. Once the flow stops, the heat loss through the thickness is completely dominant in that area. A rapid increase in the thickness of the frozen layer results.
Frozen-layer fraction has very significant effects on the flow resistance. The viscosity exponentially increases with decreasing temperature. The thickness of the flow layer is also reduced as the thickness of the frozen layer increases.
The effect of the thickness reduction can be roughly estimated with the definition of fluidity, as with representative shear rate. The fluidity is proportional to the cubic power of the part thickness. A 50% reduction in part thickness reduces the fluidity by a factor of eight (or increases the flow resistance by a factor of eight). Moreover, a 50% reduction in thickness in runners reduces the fluidity by a factor of 16. It is not surprising that excessive high pressure is required to fill parts in which hesitation occurs early in the filling stage. The flow layer becomes very thin in areas of hesitation, which are filled last.
The frozen layer fraction generally will be very low near the injection location and the end of fill. The maximum frozen layer fraction at the end of fill should be less than 0.20–0.25. Higher values will make the part difficult to pack out. Areas of the mold that are filled early in the cycle, but have little subsequent flow, normally have the highest frozen layer fraction.