Meaning
A microscopic separation occurs along the parting line or between mold inserts during the high-pressure injection phase of the molding cycle. The presence of micro-gap cleavage indicates that the clamping force of the machine is insufficient to overcome the cavity pressure generated by the polymer melt. This phenomenon occurs momentarily during peak injection pressure.
It allows polymer molecules to penetrate the seam, creating a thin film of plastic.
Tool Wear
Continuous flexing and micro-movement of the mold components under cyclic loading accelerate the degradation of the tool steel. When micro-gap cleavage occurs repeatedly, the edges of the mold inserts rub against each other during each cycle, causing fatigue and chipping. This wear reduces the precision of the tool and leads to permanent damage that requires expensive tool repair.
Steel selection must favor materials with high yield strength to resist this deformation.
Processing Defects
The immediate result of this momentary parting line separation is the formation of flash on the molded part. As the melt enters the tiny opening created by micro-gap cleavage, it cools and solidifies into a thin plastic protrusion that must be manually or mechanically trimmed. This increases labor costs and cycle times, while reducing the overall yield of acceptable parts.
In severe cases, the flash can prevent the mold from closing fully in subsequent cycles, triggering machine safety stops.
Prevention Strategy
Eliminating this defect requires either increasing the machine clamp tonnage or adjusting the injection process parameters to reduce peak cavity pressure. Process engineers can modify the injection speed or use a lower viscosity resin to lower the pressure generated inside the cavity. They can also optimize the holding pressure profile to avoid pressure spikes that exceed the mold’s holding capacity.
Regular maintenance and calibration of the mold’s support pillars ensure that the clamping force is distributed evenly across the parting line. This prevented separation maintains the integrity of the molded part and avoids the costly secondary operations associated with flash removal, ensuring a highly efficient production run.
