Ejection forces in deep tubular moldings: theoretical and experimental approach

Abstract

Injection molding is the most used process to produce plastic products, many of them requiring the use of complex molds. During the injection molding process, molten polymer is injected into a cold mold; once the mold is filled, the pressure is held high to force additional material inside the cavity, thereby compensating for the density increase due to cooling and crystallization. After solidification of the gate, the pressure decreases while the polymer cools down and solidifies. When a sufficient degree of solidification is reached, the mold opens and the part is ejected. The accurate prediction of the forces generated during the ejection phase, gives an important contribute for the design of the ejection system. Upon this prediction it will be possible to develop less expensive and lighter ejection systems. There are many factors governing the ejection force. In general, the ejection force is regarded as the result of the interaction between part geometry, mold geometry and material, molding material and processing conditions. In this work it is analyzed these factors in terms of a thermo mechanical model developed to predict the ejection forces in a tubular molding. Also, the prediction results are compared with experimental results in several thermoplastics (reinforced, unreinforced, amorphous and semicrystalline) obtained with a fully instrumented injection mold specially developed to assess the ejection forces in controlled processing conditions.