18 Nov The Importance of Mold Venting in Injection Molding
Mold venting at the parting line is essential for both the quality of parts and the longevity of the mold, yet it is often misunderstood. Frequently, venting is treated as an afterthought, only receiving attention when cosmetic defects reveal themselves post-molding. Indeed, venting can be a treacherous path; if cut too deep, the only remedy may be welding, while neglecting vents altogether invites the dire consequences of steel erosion from trapped gases.
Before the plastic flows, the mold is not empty—it is filled with air that must escape for the molten material to mirror the mold’s contours perfectly. Vents facilitate this air release without allowing the hot plastic to escape, akin to air escaping from a hose before the water flows.
How much venting you need and how deep you make each vent depends on factors like material type, viscosity, part size, and injection speed. Too little venting can lead to air compression, causing high temperatures that could burn parts or damage the mold.
Common venting problems include burn marks, short shots, poor surface finish, and internal voids. These issues often arise when air can’t escape fast enough, leading to superheated air that ultimately combusts (dieseling). In addition to releasing the air in the cavity, many materials have additive packages such as flame retardants, which produce corrosive gases that must evacuate the mold to avoid combustion or mold erosion.
If venting isn’t part of your checklist during the mold design process, consider adding it to improve your success in achieving production-ready molds built to last.