Abstract for ANTEC 1999

A stiffness-based ejection condition for injection molding is introduced and utilized for evaluation of different polymer materials. Comparison with industrial practice and other commonly used ejection criteria, such as molded part temperature, indicates that this approach not only can be an alternative when the ejection temperature of the polymer part is unavailable, but also is able to better model the complex behavior of polymeric materials. As such, the stiffness-based ejection criterion is effective in assisting the product development team to improve the design or reduce the cost by increasing the production rate while simultaneously ensuring injection molded part quality. Introduction Injection molding is capable of producing very complex components to tight specifications. The process consists of several stages: plastication, injection, packing, cooling, and ejection. In injection molding and its variants (coinjection, injection compression, gas assist molding, etc.), thermoplastic pellets are fed into a rotating screw and melted. With a homogeneous melt collected in front of the screw, the screw is moved forward axially at a controlled, time-varying velocity to drive the melt into an evacuated cavity. Once the melt is solidified and the molded component is sufficiently rigid to be removed, the mold is opened and the part is ejected while the next cycle’s thermoplastic melt is plasticized by the screw. Cycle times range from less than four seconds for compact discs to more than three minutes for automotive components. The cooling time typically occupies more than one third of the whole molding cycle. As such, cooling time offers the largest opportunity for cycle time reduction and increased production rates. In addition, the quality of the final product, production efficiency, and manufacturing cost are significantly affected by the cooling stage. Therefore, estimating and optimizing cooling time play an important role in manufacturing operations. Although cooling time is dependent on many factors, such as resin properties, cooling system and processing conditions, the cooling time estimate is based on some assumed ejection condition. The most common ejection condition is that the center-line or maximal temperature of the part is equal to or less than Te, the ejection temperature. Ballman and Shusman presented a cooling time model to predict the cooling time of a given injection molded part [1]: