What are Causes of Flash in Plastic Injection Molding?
Flash is a thin layer of plastic that flows outside of the cavity where the two halves of the injection mold meet. Flashing on a part can occur for several reasons - from variations in the process to material to tooling trouble.
Flash can appear on the part’s edge along the parting line of the mold. It can also occur anyplace where the mold has metal meeting metal to form the boundary of the part.
Flash during the injection molding of a part can occur during the filling (first injection stage) or during the pack/hold (second stage). It can occur with conventional parts - and with thin-walled parts.
Preventing Parting Line Mismatch With The Tool
To best way to prevent parting line mismatch in the tool is to clean off any plastic, dirt or contaminants that could keep the mold from closing properly, especially behind slides and inside the leader pin bushings.
Solving potential flash issues means ensuring correctly fitting mold plates by making sure the mold is sealing properly and cleaning the molding press from any obstructions. Since molding pressure can deform mold plates during injection and packing, sometimes pillar supports need to be added and mold plates need to be thickened to reduce or eliminate flash.
The mold plates should have adequate surface area that seal off at the parting line, evenly distributing the clamp force during molding. This is especially important in the area around the part cavity, where a sufficient "land" of seal-off area should be provided. Any situation where the seal-off area is uneven or inadequate should be avoided.
Flash Can Be Caused By Improper Venting
If flash is present, the mold should be checked for adequate vents. Check with the supplier of the plastic resin being used for their recommendations of vent size. On older tools that are showing wear, vents may have to be re-machined.
The viscosity of the plastic being molded determines the depth of the vent. Stiff materials can utilize deeper vents - but fluid materials require thinner vents. In either case - the concept is to remove air from the mold as fast as possible with as deep a vent as the material viscosity will allow.
Clamping Pressure Challenges
If the part passes the first stage short shot test without flash, the pack/hold phase may be pushing the parting line apart. Increasing the clamp tonnage may be the solution. Size the mold to run in the proper machine to ensure that adequate clamp pressure will be used.