After the melt is injected into the mold cavity, a thin shell is formed on the wall of the mold cavity. When the shell is pressed by the subsequent molten material in the process of mold filling, the melt will break.
Once a very thin surface has been torn or moved, the surface of the plastic will produce scratches or wrinkles.For example, in the melt index of low density polyethylene plastic parts, the surface diameter often can see the light and shade alternate bar area, its part of the general a certain distance away from the gate, and all over the entire surface, especially the thin wall parts are most likely to produce this kind of failure, this is mainly due to the molten material before the end of the not small molten filling larger pressure, lead to the melt fracture, the formation of a surface defects.
Usually, slow cooling rate of molten material in the process of mold filling and the formation of the crust layer rate is the best way to eliminate this kind of failure, can improve mold temperature by appropriate or improve the local temperature of the melt broken parts to eliminate the fault.For the local heating of the cavity surface, a small tubular electric heater can be installed near the gate and at the broken part of the melt.
The flow characteristics of the melt are related to its rheological properties and also to the cross-sectional area of the gate which determines the shear rate of the melt at the entrance of the mould.When the gate is small in size and injection rate are high, molten material is fine and bending state of jet injection mold, if the cooling speed of melting, soon will be irregular flow of material and subsequent molding fusion and surface opacity and markings that generates near the gate.
Sometimes, a small amount of cold material will move along the surface of the mold cavity, causing the surface turbidity and speckle to occur far away from the gate.
Usually, the surface of crystalline polymer injection was cloudy and markings are difficult to rule out, because this kind of resin is of very high melting temperature, compared with an amorphous type polymer, crystallization of polymer curing speed, narrow area, processing temperature and a sharp change in wall thickness and sudden change of flow direction in the melting of irregular flow of molten material with the rest of the molten material in the cavity of the fusion time is short, it is easy to produce surface opacity and markings.
In order to eliminate this kind of fault, the temperature of mould, barrel and nozzle should be increased to reduce the advance speed of screw during injection.
In the mold operation, should enlarge the gate size, first choose the fan gate, if the tunnel gate, the top size is too small will make gate in the residual impurities affect molding, intensifying irregular flow, material flow should be appropriate increase the top size.If the mould exhaust is not good, it will also affect the regular flow of the material, which should be improved.