Molding Defects | Mold Flash

Five Reasons for Flash on Plastic and Rubber Parts


What Is Residual Mold Flash?

Flash is excess plastic or rubber material that forms on the surface of molded parts. Flash, or flashing, affects the part’s surface finish and may also interfere with sealing. Flash is usually a cosmetic defect, but molders want to avoid the appearance of thin, unwanted lines that can suggest poor part quality or manufacturing defects.

Flash forms during the molding process. With injection molding, plastic or rubber material can leak between the mold’s surfaces, typically along the parting line. Flash may occur during the filling or first injection stage, or during the pack/hold phase. Neither conventional parts nor thin-walled parts are immune. With overmolding, material leaks between the base material and the mold instead.


Why Do My Parts Have Flash?

For plastic and rubber molders, it’s important to understand that flashing can occur anywhere that metal meets metal. The causes of flash are numerous, but can be divided into five main categories:

1. parting line mismatches
2. improper venting
3. problems with clamping pressure
4. inadequate sprue bushing support
5. processing issues that lower viscosity


1. Parting line mismatches can be caused by dirt, dust, contaminants, or residues that keep the mold from closing properly. Old or worn mold cavities that no longer fit together tightly can also cause flashing. With injection molding, the pressure that’s exerted on mold plates can deform these metal surfaces. Complex part geometries can cause parting line mismatches, too.

2. Improper venting also causes flash. If a rubber or plastic material is stiff, the mold’s vents may not be deep enough. If the material is fluid, the vents may not be thin enough. Problems with mold design aren’t always to blame. Rubber and plastic flashing can also be caused by old or worn vents that no longer meet required tolerances.

3. & 4. Problems with clamping pressure and inadequate sprue bushing support can cause flash, too. With injection molding, the pack/hold phase may push the mold’s parting line apart. Inadequate support for the sprue bushing can transfer pressure to the parting line, too. Thermal expansion in the sprue bushing can hold the mold open and cause flash to develop.

5. Processing issues that lower viscosity also cause flashing in molded parts. The specific causes include excessive melt temperature, overly-long residence times, moisture from inadequate drying, and the use of too much coolarant with lubricating vehicle.

For more information about flash, the prevention of flash, and cryogenic deflashing with Nitrofreeze®, contact the Cryogenic Institute of New England.