Effects of mold surface treatment on flow behavior and flash formation in BMC injection molding

BMC is a highly filled, polyester-based thermoset composite, which can be processed by reaction injection molding. This process is characterized by short injection times and high mold temperature, in order to avoid crosslinking during the filling phase and foster it after filling, respectively. Further, to reduce the risk of void formation or charring, related to air or solvent entrapment during the process, the mold has to be equipped with complex venting and evacuation systems. The combination of low viscosities, due to high shear rates and temperatures, high pressures, related to flow in long, thin cavities, and high thickness venting systems promotes the formation of flashes, the removal of which requires expensive post-molding operations. In the present work, the main causes for flash formation during injection are examined, and the possibility of applying an appropriate surface treatment to reduce this phenomenon is investigated. To this aim, a prototype mold has been designed. A section of the mold has interchangeable surfaces that underwent different surface treatments. Further, this section is equipped with pressure and temperature sensors to assess the BMC flow behavior in real processing conditions. The mold was then used in a moulding campaign, and data acquired during the molding operation were analyzed in combination with those from a fundamental rheological characterization of the BMC and information on the extent of flashing measured on the samples after molding. The analysis highlighted a significant effect of the surface treatments in reducing the flash extent, probably related to an increase of the resistance to flow in the thin area where flash occur. Surprisingly, no significant effect on flow is observed from data measured in the instrumented section of the mold. These observations suggest complex effect of the flow channel thickness, still under investigation.