Cosmetic Defects Terminology Used in Plastic Injection Molding

Plastic injection molding is a manufacturing process using various thermoplastic materials that are fed into a heated barrel, mixed and forced into a mold cavity where it cools and hardens to the cavity configuration. Plastic injection molding has evolved into a complex technology for various industries and components therefore the Society of Plastics Industry produced a list of terms and definitions for typical cosmetic defects in plastic injection molding. Below are a few of the more common terms:


o Void pockets, typically seen only in transparent parts. May appear as a bulge or protrusion in an opaque part.


o Stress induced splitting or fissures causing separation of material.


o Any change from the original color standard. Unintended, inconsistent color.


o Excess plastic at parting line or mating surface of the mold

o Normally very thin and flat protrusion of plastic along an edge of a part

o Can also appear as a very thin string or thread of plastic away from the edge of a part.

o Often found at vents, knock outs and other shut-off areas.


o Surface imperfection due to an abrasion that removes small amounts of material

o Depth is measurable.


o Cloudiness on an otherwise transparent part


o Surface imperfection due to abrasion that removes small amounts of material.

o Depth is not measurable.

o Differs from scratch in mold which leaves a consistent mark.

SHORTS (short shot, non-fill)

o Missing plastic due to incomplete filling of the mold cavity.

o Parts are not completely formed.

o Can usually be identified by smooth, shiny and rounded surfaces.


o Surface depression caused by non uniform material solidification and shrinkage.

o Most often noted at interface between differing wall thicknesses.


o Small discolored points of matter embedded in the surface.

o Typically black, caused by material contamination or material degradation


o Off colored streaking.

o Usually appears silver-like.

o Splay is caused by moisture in the material or thermal degradation of the resin during processing.

o A similar look can be caused by cold material skipping across the surface during a fast fill.

o This is commonly called "jetting".


o Witness line where 2 or more fronts of molten plastic converge.

o Also called knitlines or flow lines.

Learn more molding terminology by downloading Crescent's SPI complete Glossary of Terms for Cosmetic Defects of custom injection molded plastic components.

Crescent Industries is a single source solution for your plastic injection molded components providing mold building and custom injection molded components to the medical, pharmaceutical, defense, safety, and aerospace industries. For more information on Crescent Industries capabilities, please call 1-800-411-3844 or visit our website


All Topics