During the injection and filling process, the plastic melt enters into the clearance of parting line,clearance of sliding surface of the slide, gap between inserts and holes of ejector pin which are related with mold cavity structure, and overflow thereby occurs

These surplus materials overflowing from cavity shall attach to the injection products after ejection, thereby forming flash. The occurrence of flash not only increases the workload of operators but also reduces productivity and influences ejection and the quality of products. The specific causes are as follows:

1 Insufficient clamping force.
2 Unbalanced mold clamping and fluctuated clamping force.
3 Overlow or overhigh viscosity of plastic raw materials. Overlow viscosity shall increase the

possibility of occurring flash despite the high precision of fit clearance; if the viscosity is too high, the flow and filling resistance of melt will increase, and consequently the molding pressure within the cavity shall rise and flash will form due to the insufficient clamping force.

4 Overhigh injection pressure, overfast injection rate, excessively small section dimension of gate, overhigh temperature of barrel, nozzle and mold, which altogether result in the overlow flow viscosity of melt.

5 Deflected position of mold cavity, which result in unilateral tension of the mold during injection and thereby forming flash.

6 Overlarge fit clearance of relevant parts of mold cavity due to deformation, abrasion and foreign matters existing on the parting line, or due to the overlow design precision for the clearance of moving or fixed part of mold as well as the inadequate processing and assembly.

7 Overlarge or overdeep exhausting clearance.