Flow‐induced crystallization in the injection molding of polymers: A thermodynamic approach

Abstract

ABSTRACT:The prediction of the crystallinity and microstructure that develop in injection molding is very important for satisfying the required specifications of molded products. A novel approach to the numerical simulation of the skin-layer thickness and crystallinity in moldings of semicrystalline polymers is proposed. The approach is based on the calculation of the entropy reduction in the oriented melt and the elevated equilibrium melting temperature by means of a nonlinear viscoelastic constitutive equation. The elevation of the equilibrium melting temperature that results from the entropy reduction between the oriented and unoriented melts is used to determine the occurrence of flowinduced crystallization. The crystallization rate enhanced by the flow effect is obtained by the inclusion of the elevated equilibrium melting temperature in the modified HoffmanLauritzen equation. Injection-molding experiments at various processing conditions were carried out on polypropylenes of various molecular weights. The thickness of the highly oriented skin layer and the crystallinity in the moldings were measured. The measured data for the microstructures in the moldings agree well with the simulated results.

Model Development and Validation of Crystallization Behavior in Injection Molding Prototype Flows

“…The predicted spherulitical volume for condition MPR2 is also found to be smaller then condition MPR1, further favoring a higher volume fraction of oriented crystals. The time evolution of the radial growth rate G(T, p), given by Equation (27), at the same position close to the wall after wall quenching is shown in Figure 20(a) for conditions MPR1 and MPR2. One can clearly see that the radial growth for condition MPR1, due to differences in cooling history, see Figure 20(b), is higher, explaining the larger spherulitical volume predicted for this condition.…”

“…The parameters used in Equation (27)(28)(29) are taken from [15] and given in Table 3. Only for the highest cooling rate used (32.5 8C Á s À1 ) the model gave a poor result.…”

“…Isayev et al [26] studied in a similar way iPPs polymerized using metallocene and Ziegler-Natta catalysts. Kim et al [27] modified the crystallization rate constant in the Nakamura model, making it dependent on the shear rate. They tried to predict the thickness of the oriented shear-induced layer using a thermodynamic approach, in which viscoelastic stresses were computed to find changes in entropy associated with the flow.…”

“…In ref. [27] the transition from spherulitical core to oriented shear layer was assumed to take place when the local temperature would equal the calculated T m . Even though their results qualitatively describe the experimental data, they still fall short from providing morphological detail.…”

Determination of non-isothermal crystallization rate constant for pseudo-experimental calorimetric data

“…Using the calculated K(T) and assuming n equal to 3, the differential form of the Nakamura equation 10 , given by Equation 7, can be used to simulate the dθ/dt vs. T curves, which are integrated to obtain almost exclusively to the Nakamura equation, mostly in Isayev's various scientific works [6][7][8][9] . In this respect, nonlinear regression is more flexible; moreover, it can be applied to other types of equation, not only to those that fit into the form of Equation 1.…”