Meaning
Microscopic separation of the mould halves during the high-pressure injection phase allows for the escape of trapped air. This action, referred to as cavity breathing, occurs when the internal plastic pressure momentarily exceeds the machine clamping force. While a small amount of movement is necessary for venting, excessive separation leads to the formation of flash on the part.
Proper control ensures that the air exits without the resin following it into the parting line.
Venting Efficiency
Air trapped in the mould must find a path out to prevent burn marks or short shots. Cavity breathing assists this process by opening a temporary path at the parting line during the peak pressure moment.
Clamping Force
Balancing the machine tonnage against the projected area of the part prevents uncontrolled opening of the tool. If the tonnage is too low, the cavity breathing becomes excessive and produces thick flash that requires manual trimming. Adjusting the clamp pressure allows the moulder to find the minimum movement required for quality parts.
Part Quality
Consistent movement of the mould halves ensures that every part in the run has the same dimensions and finish. If the cavity breathing fluctuates, the part weight and thickness will vary between shots. Monitoring this movement helps the operator detect changes in resin viscosity or machine performance.
Excessive breathing can also indicate that the mould base is deflecting under the injection load. Sensors mounted on the platens provide real-time data regarding the magnitude of this separation.
