The escape of a thin web of material at parting lines,around ejector pins or at other parts of the mould，can be caused in several ways. It could be the material，the machine, the mould, or the process that is at fault. Taking these in order:
Material at fault.
Manufacturers are very careful about the quality of their products, but very occasionally it is possible for a quantity of material of the wrong melt flow index to be included in a consignment. If flashing occurs shortly after recharging the feed hopper and if no other changes have been made，the material can be suspected. Too easy a flow or too high a temperature are common causes of flashing. If re-work has been used，the quantity should be checked because passage through the machine can raise the melt flow index of the material. The fault may also occur when changing colour for，as previously discussed; some colours seem to take up heat in the cylinder more quickly than others.
Machine at fault
If flashing occurs，the first thing to check is the mold locking. A change of temperature with a toggle locking machine might be sufficient to reduce the mould locking force，but it may also be that the mould parts are binding on their guide pins and preventing them from coming together with adequate force. This should always be checked before making.
Any alteration to the locking pressure on the hydraulic system. Before doing this the cause of flashing should always be ascertained
Having made sure that the locking of the mould is satisfactory, attention can then be turned to the heating cylinder. It may be that one of a pair of heater bands has broken down and that the other one is over-running. This can occur where two bands are controlled by one thermocouple. A more likely and possibly more frequent cause is that a thermocouple has been pulled out of its pocket，either fully or partly, and that the heater band it controls is over¬running. A still further, often unsuspected, cause of flashing is that too much mechanical heat is being put into the material because screw back is taking too long，is too fast, or is variable.
Finally，a check should be made on the injection pressure setting and the injection rate. It may be that the flow control valve has become heated and that injection is taking place too fast.
Mould at fault
A well-designed and constructed mold will，of course,give flash-free mouldings unless the machine has been wrongly set. After a time most moulds become slightly worn and may show a tendency to flash, but the signs of this starting may be the appearance of a small burn mark— perhaps better described as an incipient burn mark—appearing at a parting line. If this develops into a bad blemish it will be necessary to repair the mould. Slight flashing at ejector pins may occur and may be acceptable but if they develop ejection becomes difficult; this，in turn, causes the cycle to be lengthened，allowing more time for plasticization in the cylinder and making the flashing worse. With hot-runner moulds, any malfunctioning of heater circuits can alter the viscosity of the melt and cause flashing.
Process at fault
It must always be remembered that the injection molding process involves the balancing of a number of variables in order to achieve optimum conditions. The appearance of flash is often the sign that something is out of balance. For example, a lowering of viscosity of material entering the mould causes more pressure to be exerted within the mould. This reacts against the mould locking force and if this is at its maximum, the injection pressure has to be reduced .Conversely,reducing the temperature increases the viscosity of the melt; pressure is then increased to counteract this，resulting in an increase in the rate of injection. The material is sheared more rapidly and then the pressure, acting on the more highly sheared，less viscous material，is sufficient to cause flashing. Attention to one variable only in treating flashing，as is the case with many other faults，may lead to further difficulties and it is always the best policy to try to discover exactly what is the cause of the trouble. In the end，it might simply be a small piece of a previous moulding preventing proper mould locking.