Flash, also known as flashing, is excess material attached to a molded, forged, or cast product, which must usually be removed. This is typically caused by leakage of the material between the two surfaces of a mold (beginning along the parting line) or between the base material and the mold (in the case of overmolding). Molding flash is seen when the optimized parameter on cull height is not calibrated. Proper design of mold parting surfaces can reduce or eliminate flash.
Molding flash can be caused from old or worn mold cavities that no longer fit tightly together. Other times, the complexity of the part requires so many mating pieces with such precise geometries that it is almost impossible to create a perfect fit on every impression. Most often, the type of material being molded, and its attendant viscosity in its liquid form, is the primary factor that leads to the creation of the unwanted mold flash.
Witness mark 
In plastic injection, a faint mark called witness mark (or witness line) will occur along the parting line. This is unavoidable and is usually accepted despite minor aesthetics issue. However, some part surfaces (e.g. when used for sealing) cannot tolerate witness mark, and thus either the mark must be removed post-molding or the mold redesigned.
See also 
- David Kazmer (2007). Injection mold design engineering. Hanser Verlag. p. 70. ISBN 978-1-56990-417-6. Retrieved 15 April 2011.
- "NX I-deasVGX Core/Cavity". Siemens PLM Software. 2007. Retrieved 2009-07-01.
- Bozzelli, John "An Expert Tells How to Stop Flash", Plastics Technology, July 2004, accessed May 9, 2011.
- US patent 6939500, Bernard, Edward Helmut, "Method and apparatus for eliminating a parting line witness mark from a molded part", issued 2005-09-06
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