The influence of processing parameters on injection-moulded PET

The correlation of microhardness and morphology for injection-moulded PET will be highlighted in this section as a second example of the application of the microhardness technique. 

The iiucrohardness variations that occur across the moulding thickness have been measured, and the effect of thermal treatment of pellets of the original material on the properties of the mouldings has been analysed. The effect of an annealing treatment on the microhardness of the mouldings has also been examined (Balta Calleja etal, 1993). 

The experiments described were performed using pellets of PET with a weight average molecular weight of My, = 29 800 which were used to prepare the injection moulded materials. 

The influence of the mould temperature on the H measured at the outer surface of the PET bar is shown in Fig. 7.6. The stepwise increase of H with Tp can be explained because below and near Tg, H values between 120-130 MPa have been ascribed to the incipient sphemlite stmcture of the polymer (see Section 4.2.2, 

The H profile in the y direction supports the presence of an outer softer amorphous layer (H = 120 MPa) in the samples with Tmouid below 120 °C (Pig. 7.8). However, on increasing Tmouid above 120 °C, the amorphous layer crystallizes and hardens. It should be noted that for the samples prepared at Tmouid 120 °C, H shows a distinct maximum value at both surfaces, suggesting an enhancing nucleation effect of the metallic walls of the mould on the crystallizing material. 

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