Flash is the excess material that forms on the surface of a molded product. They are considered to be cosmetic defects and does not affect the performance of the product. It is almost impossible to mold parts that are 100% free of flash. During injection molding, rubber materials usually leak along the parting lines.
Photo: Rubber Mounting with flash
For rubber molders, flash can happen in any meeting point of metals within the mold. There are many causes of flash. These causes can be categorized in five major sub-groups: Improper venting, parting line mismatch, low viscosity, insufficient sprue bushing support, and finally poor and inadequate clamping techniques. Improper venting can occur when the rubber material is excessively stiff or the rubber material is very liquid. Stiff material requires a very deep vent which might be impossible to install. A very liquid material requires the vents to be very thin which in some cases might not be possible.
Photo: Rubber Stopper Mounting with flash
Mismatching of the parting lines are normally caused by dirt or any residual material that prevents the mold to close properly. Wearing out of the molds can also lead to improper fitting hence resulting to flash formation. Molding requires sufficient clamping forces and adequate sprue bushing. Injection molding involves high pressures and the material can push apart the mold along the parting lines. When the sprue is not supported properly, it tends to transfer pressure to the mold’s parting lines.
During processing, the resulting rubber material may have lower viscosity than the recommended range. Causes of low viscosity includes excess temperature during melting, inadequate drying and long times of residence. Low viscosity causes the rubber material to flow excessively leading to formation of flash.
Photo: Rubber bellow hose with minimal flash, only mold line visible
Although flash is considered a purely aesthetic problem, the greatest concern about flash occurring is the wastage in material. Flash reduces production efficiency and increases production costs thus it should be minimised as much as possible.
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