In the point gate design of the vertical part direction, a radiation system appears on the surface of the part during injection molding. It’s composed of different color depths and glosses and it centers on the gate. And we call it the light line. There are roughly three manifestations of blanching, namely, dark lines on a fuscous background, fuscous lines on a dark background, and dense and whitish dark lines around the gate.
Most of these defects appear in the injection of polystyrene and modified polystyrene mixtures. And they are related to the following factors:
- The two materials have differences in rheology, coloring, etc..
- There are differences between the flow rate of the stratosphere and turbulent layer of the gating systemin the flow rate and heating conditions.
- Plastics generate charred filaments due to thermal decomposition.
- The interference of gaseous substances when plastics enter the mold.
(1) When using mixed plastics, we should mix the plastic well. And the particle size of the plastics should be the same and uniform.
(2) The plastic and coloring agent should be mixed uniformly. Besides, appropriately dispersing agent should be added if necessary and mixed by machinery.
(3) Plasticization must be complete. And the machine must have a good plasticization performance.
(4) Reduce injection pressure and speed, shorten injection and pressure holding time. At the same time, increase mold temperature and nozzle temperature, reduce forehearth temperature.
(5) Prevent the melting and coking materials with increased viscosity caused by the degradation of plastics.
Pay attention to the following situation:
Whether the screw and barrel are worn out with dead corners.
Whether the heating system is out of control.
Improper processing operations will cause the plastic to be decomposed by long-term heating. The inner surface of the front end of the screw and barrel can be polished.
(6) Improve the gate design, such as enlarging the gate diameter, changing the gate position, changing the gate to a fillet transition. Besides, try to heat the gate locally, and add a cold slug well at the end of the runner.