Keywords: injection molding, crystallization, simulation.

Commercial software for the simulation of injection molding has been available since 1980 and is commonly used in industry. Indeed simulation was recently voted by the editors of the trade journal Plastic Technology [1] as one of the key fifty ideas that changed plastics. Despite the success of simulation, there are several approximations commonly made to make simulation commercially viable. One such approximation is the use of a "no-flow" temperature - a temperature below which the material has an infinite viscosity.

For amorphous materials the no-flow temperature is set at around the glass transition temperature and presents little problem. However, for semi-crystalline polymers the rapid increase in viscosity at solidification is not captured by any known viscosity model for polymer melts.

Existing models such as the Cross-Carreau model can capture the behaviour of the melt as a function of shear rate. It also possible to incorporate the effect of pressure and temperature affects by setting the viscosity as follows:

where is the shear rate, T is temperature, and p is the pressure, and n, , , , , and are data fitted constants.

In order to capture the sudden increase in viscosity at solidification, the no-flow temperature, is introduced. For , the viscosity is given by equation (61) whereas for , the viscosity is assumed infinite and the material is solid. For semi-crystalline materials, this approximation can introduce some error. The effective temperature of solidification on cooling depends on the rate of cooling and also the flow conditions that the melt has experienced [2].

In this paper we show the error introduced by assuming a constant no flow temperature. We then briefly describe a model for crystallization [3] that accounts for the effect of shear on crystallization. This model relates the morphology of the injection molded material to processing conditions. The no flow concept is replaced with a suspension model [4] for viscosity that depends on the degree of crystallization. A description of the implementation of these models in an injection molding code is given. We then describe the model's calculation of the morphology of the molded part. To conclude, we present some experimental data to show the ability of the code to predict the pressure during the packing phase.

1

"50 Ideas that Change Plastics", Editorial Feature, Plastics Technology, Oct., 2005.

2

G. Eder and H. Janeschitz-Kriegl, "Crystallization" in H.E.H. Meijer (Ed.), Materials Science and Technology, Vol. 18, Processing of Polymers, Wiley-VCH, New York, 1997.

3

R. Zheng and P.K. Kennedy, "A Model for Post-flow Induced Crystallization: General Equations and Predictions", J. Rheology, 48, 823-842, 2004. doi:10.1122/1.1763944

4

R.I. Tanner, "A Suspension Model for Low Shear Rate Polymer Solidification", J. Non-Newtonian Fluid Mechanics, 102, 397-408, (2002). doi:10.1016/S0377-0257(01)00189-6

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