The surface defects of moldings obtained by injection method are frequently encountered in the form of overlaps, also called flashings. They appear when the alloy is pressed between the gaps of the mold surface halves or the cores. They can be small, in the form of a perceptible burr, or more extensive, resembling membranes or fish fins. The formation of flashes in a few processing cycles can even damage the sealing surfaces. By taking appropriate remedial measures, we can quickly eliminate this defect.
How are the flashes formed?
Flashes most often appear in the vicinity of the gaps between the mold surface halves, cores, on the sealing surfaces, venting channels or at the ejectors. They have the shape of more or less shaped edges made of plastic, resembling a kind of membranes or fish fins. Small burrs in the form of a burr that can be felt with your fingertips are often imperceptible, while extensive membranes may protrude even several centimeters beyond the edges of the proper shape of the detail.
Where do the flashes come from?
The various causes of risers can be summarized in the following points:
1) The permissible joint widths have been exceeded – the mold is insufficiently tight, the processing tolerances are too high, the sealing surfaces are damaged or the clamping force is too low;
2) Incorrectly selected parameters of the machine clamping phase – the clamping force is too low to resist the buoyancy forces or as a result of too much clamping force, significant deformation of the plates for fixing the detail or the mold itself occurs;
3) Too high pressure of the mass in the mold – the pressure force shaping the shape of the molded part is so high that it presses the mass into even the smallest gaps;
4) Too low melt viscosity at high injection pressure, hold down pressure or at too high melt temperature favor the formation of risers.
A step-by-step analysis of the provided diagram may be helpful in eliminating this surface defect.
The article was prepared on the basis of materials from the Dopak Research and Development Center and the KraussMaffei company
If you are interested in details about risers and other defects of moldings and how to eliminate them, we invite you to our training at the Research and Development Center in Wrocław .
Also read another article about defects in moldings: Where do underfills come from and how to eliminate them quickly?