Troubleshooting flow marks problems

A flow mark or halo, is a surface defect in which circular ripples or wavelets appear near the gate.

Ripples, a similar defect, appear as small fingerprint-like waves near the edge or at the end of the flow.

Causes

• Material freezing near the gate. Low melt or mold temperature, and low ram speed can result in cold material entering the cavity. This can cause the partly solidified material to take on the form of the flow pattern.

  

  Figure 1 Ripples caused by low temperature. a normal fountain flow with no ripples, b flow causing ripples (R).

• Insufficient material compensation. Early gate freeze-off or low packing pressure may not pack the cavity properly. The material near the gate then freezes while maintaining the form of the flow pattern.

Remedies

• Optimize the cold well. Design the cold well in the runner system to trap the cold material during the filling phase. The proper length of the cold well is usually equal to that of the runner diameter.
• Optimize the runner system design. A restrictive runner system design can result in premature gate freeze-off. It can however, increase shear heating for better melt flow.
• Increase the mold and melt temperature.
• Optimize packing pressure.

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.