Shrinkage Prediction for Slowly-Crystallizing Thermoplastic Polymers in Injection Molding

Abstract

The prediction of shrinkage and warpage of crystalline polymers is quite difficult. This is because of the complications of the crystallization process and associated material property changes. In typical injection molding of semi-crystalline polymers, it is difficult to calculate crystallinity accurately because of its complicated dependence on temperature, time and stress. In the case of slowly-crystallizing polymers, typically the crystallinity of the polymer remains small during the process. In this case, therefore, the importance of accurate calculation of crystallinity is decreased. The difficulty in this case is the measurement of material properties under variable crystallinity because of the possible crystallization during measurement. In the present study, methods have been developed to obtain material properties which include the effect of crystallinity. The material properties measured include viscosity, thermal conductivity, heat capacity, PvT relation and crystallization kinetics.Injection-molding experiments have been conducted to measure the shrinkage of a part using a slowly-crystallizing polymer. Shrinkage has been measured by comparing the mold cavity dimension and the dimension of the molded sample. The polymer used in this study is PET (polyethylene terephthalate).A simulation program has been developed to analyze the injection-molding process. This program is based on quiescent crystallization kinetics. The predicted shrinkage and measured shrinkage are found to agree reasonably well.

Crystallinity and Linear Rheological Properties of Polymers

“…1 in terms of the normalized rheological function (NRF) C = viscosity of the crystallizing material/viscosity of the melt measured at the same conditions (temperature, shear rate or oscillatory frequency) versus the degree of space filling n g , which is equivalent to the relative crystallinity. Apart from the data of Han and Wang (1997) all the data are for different iPPs. From Fig.…”

“…Pantani et al (2000) performed similar isothermal tests (T = 140 and 143°C), working with a different iPP and measuring complex viscosity (instead of viscosity) for one frequency, x = 1 rad/s. Han and Wang (1997) measured the viscosity of a PET during a cooling test (from 290 to 230°C) using a rotational rheometry in steady motion ( _ c c = 0.1 l/s), and simulated the crystallinity evolution during the test with a kinetic model tuned using DSC, operated under isothermal conditions. Boutahar et al (1998) investigated the isothermal crystallization behavior of a PE (at T = 126.2°C) and of an iPP (at T = 135°C) by DSC, and the rheological behaviors at same temperatures using dynamical oscillatory tests, at various frequencies (x = 0.316 to 100 rad/s).…”

Suspension-like hardening behavior of HDPE and time-hardening superposition

“…However, many of these models are not applicable to crystallizing polymer melts. Empirical models can also be found in abundance in literature (Boutahar et al 1998;Doufas et al 2000;Han and Wang 1997;Hieber 2002;Katayama and Yoon 1985;Khanna 1993;Shimizu et al 1985;Tanner 2003;Titomanlio et al 1997;Yarin 1992;Ziabicki 1988;Zuidema et al 2001), but there is little agreement between these models. Thus, there is need for a model that is able to accurately describe rheological properties of semi-crystalline polymers during solidification.…”

“…A vast amount of experiments investigating the change of rheological properties can be found in the literature, cf. Acierno and Grizzuti (2003), Boutahar et al (1998), Han and Wang (1997), Lamberti et al (2007), Pantani et al (2001), Pogodina and Winter (1998), and Titomanlio et al (1997). To quantify the hardening behavior of the material, a normalized rheological function (NRF) is commonly used.…”

Thermoplastic Polymer Shrinkage in Emerging Molding Processes

“…Different thermoplastic materials will also exhibit different shrinkage characteristics. For example, models have been developed that describe how different material properties, such as crystallinity and viscosity, can affect shrinkage [13].…”