A. M. Brito

Isothermal and nonisothermal crystallization of polymers : analysis with a shear differential thermal analyzer

The working principle of an instrument developed for studying the effects of controlled shear pulses on the isothermal and nonisothermal solidification of polymers is presented. The device combines a capillary rheometer and a differential thermal analyzer (DTA). The capillary rheometer part of the system allows the production of shear pulses with controlled duration and intensity at any prescribed temperature up to 300°C, and the DTA records the thermal effects resulting from the solidification. Results obtained for quiescent isothermal and nonisothermal crystallizations compare well with those obtained from power compensation differential scanning calorimetry. The effect of controlled shear pulses on the overall isothermal crystallization kinetics enables the evaluation of a critical strain responsible for the saturation of crystallization in sheared polymer melts. Additional shear (increase of the shear rate or of the shearing time) does not accelerate the crystallization kinetics. The shear-induced non...

Effect of Melt Viscosity on the Ejection Force in Injection Moulds

This paper shows data on the ejection force of injection mouldings. The tubular mouldings are pin gated. Two polypropylenes with different melt viscosities (melt flow indices) were investigated. Experimental focus is put on the effect of the holding pressure, mould temperature and the cooling time on the measured ejection force. Pressure and temperature were continuously monitored with transducers during the filling process. The ejection force was directly measured with load cells. The results show that higher viscosity lends to higher ejection force. Introduction The development and manufacture of injection moulds for high quality technical parts are complex tasks involving the knowledge of the injection moulding process and the material changes induced by processing. In the case of some specific shapes (boxes, cylinders) the shrinkage is partially restricted by the mould. The moulding shrinks against the core, inserts or pins. Thus, upon ejection, it will be necessary to overcome the frictional forces resulting from the shrinkage. This aspect has significant implications in the mechanical design of the injection moulds especially when deep cavities are involved. In the design of these moulds, the accurate prediction of the ejection force may contribute for a more precise and economically efficient decision of the ejection system. In fact, if the designer knows the force likely to be required during the moulding ejection, it will be possible that a proper decision can be made for a less complex and cheaper ejection system: mechanical, pneumatic, or hydraulic [1,2]. The capability of predicting the ejection force may also help to optimising the mould design and to guaranteeing the structural integrity of the mouldings. In this work a study on the influence of the viscosity was envisaged. The effect of processing variables (mould temperature, holding pressure and cooling time) on the ejection force for deep tubular mouldings was assessed using two polypropylenes with different melt flow indices.