Microscopic Air Entrapment Mechanics in Textured Mold Surfaces
Microscopic air entrapment in textured molds stems from rapid melt front advancement trapping gas in surface valleys, manageable via controlled fill rates and porous micro-venting.
Dieseling Burn Marks
Meaning
Localized carbon deposits and surface discoloration occur when trapped air inside a mould cavity reaches its ignition point during the injection phase. These dieseling burn marks indicate a failure to vent gas from the mold before the polymer melt seals the flow path. The presence of these marks usually correlates with rapid injection speeds and inadequate venting channels.
Visual Identification
Dark spots or charred regions appear at the last area of the cavity to fill, particularly in deep ribs or blind pockets. Often, dieseling burn marks are accompanied by a distinct smell of burnt plastic and a dull finish on the surrounding polymer. The physical damage can range from a light brown shadow to a deep black crater.
Root Cause
High injection pressure forces the trapped air into a tiny volume, raising its temperature to the point of spontaneous combustion. As a consequence of this compression, dieseling burn marks form because the polymer undergoes rapid thermal decomposition in the presence of oxygen. The trapped gas acts as a diesel engine cylinder, compressing the mixture until it burns.
Mitigation Method
Lowering the injection velocity or modified vent positioning provides the primary solution to this thermal defect. Molders can reduce the boost pressure or establish a multi-stage injection profile that slows the flow front as it approaches the end of the cavity. Clean venting channels must be maintained to allow gas to escape freely, which reduces the chance of pressure buildup.
If the defect persists, the insertion of porous steel inserts or specialized ejector pins can evacuate the air from dead-end regions.