Meaning
Momentary, microscopic opening of the mold halves along the parting line caused by the high pressure of the injected polymer melt. In injection molding, mold breathing allows trapped air and gases from the heated resin to escape through the parting line vents. This movement occurs during the filling and packing phases of the molding cycle.
Insufficient clamp force allows too much breathing, which leads to flash.
Mechanical Action
Injection pressure exerted by the advancing screw forces the mold halves apart against the clamping force of the press. Controlled mold breathing occurs on the order of microns, barely enough to let gas escape without allowing the viscous polymer melt to enter the gap. If the clamp force is set too high, the mold cannot breathe, trapping gases inside the cavity.
If the clamp force is too low, the parting line opens too wide, creating excess scrap.
Defect Mitigation
Prevention of cosmetic and structural flaws in molded parts depends on the correct balance of clamp force and injection pressure. By optimizing mold breathing, technicians avoid burn marks on the part surface caused by the compressed gas heating up. This release of trapped air also prevents short shots, where the resin cannot fill the entire cavity because of the air pocket.
Parts molded with proper breathing exhibit more consistent weight and density.
Vent Design
Machined channels along the parting line work in tandem with the mechanical flex of the mold to clear out vapors. While mold breathing provides a temporary escape route, permanent vents must be positioned where the melt flow fronts meet. These vents are typically designed with depths that prevent the specific polymer from flashing, ranging from five to twenty-five microns.
Regular cleaning of these vents prevents buildup of wax and resin residue that can block the gas escape pathway, which is why a thorough maintenance schedule is essential for high-speed packaging runs where plastic additives can quickly clog narrow vents.
