Injection temperature is a critical factor influencing injection pressure. The injection molding machine's barrel typically features five to six heating zones, and each raw material has an optimal processing temperature range (specific details can be found in the supplier's data). Injection temperature must be strictly controlled within this range. If the temperature is too low, the melt will not plasticize properly, compromising part quality and complicating the molding process; conversely, if the temperature is too high, the raw material is prone to thermal degradation. In actual production, the actual melt temperature is often higher than the set barrel temperature-sometimes by as much as 30°C, depending on injection speed and material properties. This temperature rise is caused by the intense heat generated by shear forces as the melt passes through the injection nozzle. When performing mold flow analysis, this temperature difference can be accounted for in two ways: by measuring the melt temperature during an "air shot" (injecting into open air) or by including the nozzle geometry in the simulation model.
As the injection phase nears completion, the screw stops rotating and moves only forward, marking the transition to the holding pressure phase. During this stage, the machine's nozzle continues to feed material into the mold cavity to compensate for the volume loss caused by part shrinkage. Without a holding pressure phase, the part would shrink by approximately 25% after filling; excessive shrinkage-particularly in ribbed areas-would result in visible sink marks. Holding pressure is typically set at around 85% of the peak filling pressure, though the exact value should be determined based on specific process conditions.
