Influence of the Dwell Time and Dwell Temperature on the Glass Transition of Injection Molded Parts

Introduction

In injection molding, the liquid polymer mass is forced into the mold and held under pressure at a particular temperature (the dwell or mold temperature) for a certain period (the dwell or molding time) before cooling. The dwell temperature is usually appreciably higher than the glass transition temperature of the material. The question therefore arises as to what extent the raw material used changes during the injection molding process, and what influence the process parameters (dwell time and dwell temperature) have on possible changes in the final product.

This was investigated using a blend of ABS (acrylonitrile-butadiene-styrene) and PC (polycarbonate) as an example. The study shows how a simple procedure can be used to characterize the individual and conjugated influence of the process conditions on the glass transitions of this material. The samples available were prepared at dwell temperatures of 260 °C and 300 °C with dwell times of 0 min and 5 min.

Experimental Details

Samples were measured in a DSC822e at 10 K/min under a nitrogen flow of 50 mL/min using the light 20-µL crucibles. Figure 1 displays the first and second heating runs of a sample that had been held at 260 °C for 0 min. Both curves clearly show two glass transitions: the first originates from the polystyrene in the ABS and the second from the PC. Also clearly visible is a melting peak at about 40 °C. The curve from the first heating run also exhibits a broad endothermic peak due to the evaporation of a volatile component. This was confirmed by weighing, which showed a mass loss of 0.4% after the first heating run. 

Figure 2 displays the curves of the second heating runs for the different samples. The sample name (e.g. 260–0) means a dwell temperature of 260 °C and a dwell time of 0 minutes. The values obtained for the evaluation of the glass transitions are summarized in Table 1. They show that for polystyrene, both the glass transition temperature and the step height of the glass transition are hardly influenced by the process conditions in the range investigated. With polycarbonate, however, there is a marked changed in the glass transition temperature and in the step height. The differences in the glass transition temperature are relatively small but nevertheless significant. The change in the step height of the glass transition is only about 0.05 J/(gK) in absolute terms; in relative terms, however, this corresponds to a change of 40%. In the following sections, the influence of the process parameters on the glass transition of the PC component will be studied more closely. 

Evaluation of Results

In Table 1, the measurements differ in pairs only with respect to one factor. The mean value of these differences is a measure of the influence of this factor on the experimental result and is called the effect of this factor. 

Conclusions

The results show that the glass transition of polystyrene is hardly influenced by the process conditions. In contrast, the glass transition temperature of the PC component is shifted to lower values, and the step height decreases by about up to 40% with higher dwell temperatures or dwell times. This indicates that the amorphous part of PC that contributes to the glass transition decreases significantly depending on the dwell temperature and dwell time. It is well known that the polymer chains of PC exhibit increasing depolymerization above the glass transition temperature with the formation of oligomers that no longer contribute to the glass transition.

An analysis of the dwell temperature, dwell time and the interaction of these two parameters on the glass transition temperature and the step height of the glass transition shows that, of the parameters considered here, dwell temperature has most influence. In the case of the step height, the combined influence of dwell time and dwell temperature has the same order of magnitude as the two individual parameters; the effect of interaction of dwell temperature and dwell time is therefore not negligible in this case. In contrast, the dwell temperature and dwell time affect the glass transition independently of one another; an effect of an interaction of these two parameters could not be detected.

_{Influence of the Dwell Time and Dwell Temperature on the Glass Transition of Injection Molded Parts | Thermal Analysis Application No. UC 272 | Application published in METTLER TOLEDO Thermal Analysis UserCom 27}