Parting line

A parting line, or seam in moldmaking is the place where two or more parts of the mold meet.^[1] Parting lines occur due to minute inherent gaps between two mating faces of halves of the die. The material filling into these gaps is generally called molding flash or simply flashing. Parting lines generally show up as a raised line around the product (witness mark) on commodity products such as toys.

Many applications (seals, tight running molded parts) that require precision for shape control, call for removal of flashes. Secondary operations are thus employed, in addition to high precision molding dies to remove these flashes. Many molders will repair or even replace the mold tooling so that the flash is reduced to an acceptable tolerance or eliminated altogether. Secondary operations to remove parting line flash include hand trimming, vibratory tumbling, media blasting and cryogenic deflashing.^[2]

Flashes are known to improve sealing between mating faces in some instances (Fuel tank seals, hydraulic power steering gear, etc.), and thus flashes are permitted to some extent in special cases.

In engineering drawing, parting line is often abbrivated as PL. ASME's Y14.8 standard specifies a symbol for parting line.^[3]

References