Reaction injection molding: Filling of a rectangular mold

The present trend in fascia reaction injection molding is to ward larger, more complex parts and faster reacting systems. This has lead to short shot problems. To study this, slow motion movies were taken of filling and flow around objects in a transparent plaque mold with fascia type urethane formulations. Mold pressure and temperatures were recorded simultaneously. For fill times less than a characteristic reaction time, pres sures can be predicted assuming constant viscosity during filling. For filling times greater than the characteristic reaction time, weld lines and short shots can occur. Criteria for good filling based on the dimensional analysis of the flow field equations are developed. Flow front instability was ob served for high flow rates.