Outside paint and mold release: where blemishes come from
What outside paint applied to the outer surface of the green tire does at mold entry and release after curing; how excess application fouls the mold and how to prevent sidewall blemishes.
Two different jobs: film on the product, film on the mold surface
Outside paint is applied to the surface of the green tire itself; mold release agent is applied to the mold surface. Their aims look close but their consumption and effects differ. Outside paint enters the mold with the tire and ensures clean release after curing.
Mold release agent is applied only to the mold surface and is renewed at regular intervals. Treating the two as the same product leads to the mold being constantly covered with excess film and the vent holes being clogged.
What outside paint solves
Outside paint has three functions: to ease seating by reducing friction when the green tire enters the mold, to ensure release after curing, and to prevent stains (blemish) on the sidewall surface.
Blemish usually arises from local temperature differences on the mold surface, from buildup of residue, or from an insufficient release film. It appears as irregular, matt stains on the sidewall and is a cause of visual rejection.
Water-based system and application
Today most outside paints are water-based: compared with solvent-based products, VOC, fire load and worker exposure are lower. In return, new process variables arrive, such as drying time and freezing sensitivity.
Application is by spray, dipping or brush. A product with a high solids content leaves a thicker film and needs less dilution; the risk of mold fouling rises in the same proportion.
The cost of excess application
Excess outside paint looks harmless at first glance: release improves. The cost appears a few days later. Mold vent holes and sidewall lettering fill with film, the mold cleaning interval shortens, and gloss differences and paint residue form on the sidewall.
Excessive film can also impair the legibility of DOT and size markings. For this reason, the target in outside paint is not the best release but the balance of sufficient release with a clean sidewall.
Checklist
Mold temperature and cleaning interval, application method and film thickness, dilution ratio, drying time, sidewall appearance and lettering legibility. If a release problem continues, first check the mold release regime and venting, then the outside paint dose.