PP 0 50.5

PPCFe3O4 15 46.4

PPCFe3O4 30 40.5

PPCFe3O4 45 34.6

PPCFe3O4 50 34.9

PPCBaSO4 15 44.3

PPCBaSO4 30 40.7

PPCBaSO4 45 35.6

PPCCu 15 40.5

PPCCu 30 33.8

PPCCu 35 29.0

PPCglass  fibres 35 40.8

PPCtalc 15 45.7

PPCtalc 30 42.5

PPCSrFe12O19 30 40.9

The cooling is measured in situ within a cavity of the mould by a K-type thermocouple.

B. Weidenfeller et al. / Composites: Part A 36 (2005)  345–351 349

Table 5

Thermal diffusivity estimated from the cooling behaviour of injection moulded polypropylene-filler composites using the slope of the regression lines (a1, a2) (cf. Fig. 3) compared to thermal diffusivity values measured by the transient method   (a)

Composite Regression lines Transient method

a1 (mm2/s) a2 (mm2/s) a (mm2/s)

PP 0.21 (120–55 8C) – 0.19 (26 8C)

PPC15 vol% Fe3O4 0.24 (120–67 8C) 0.19 (67–51 8C) 0.27 (26 8C)

PPC30 vol% Fe3O4 0.29 (120–68 8C) 0.19 (68–45 8C) 0.35 (26 8C)

PPC50 vol% Fe3O4 0.33 (125–77 8C) 0.16 (67–45 8C) 0.48 (26 8C)

The temperature values in parenthesis give the temperature region of the regression lines and the ambient temperature during the measurement with the transient technique.

of unfilled polypropylene quite  well  (cf.  Table  5  and Fig. 4).

Fig. 4 shows the measured thermal diffusivity data of the investigated samples at ambient conditions. It can be seen that the thermal diffusivity of the magnetite-polypropylene composite is increased from aZ0.19 for unfilled poly- propylene up to aZ0.48 (PPC50 vol% Fe3O4) with increasing magnetite loading. Therefore, the cooling time becomes shorter for higher magnetite  filler  fractions (Fig. 3).

One reason for the change in the slope of the cooling curves shown in Fig. 3 is a change of the thermal diffusivity with temperature which is shown in Fig. 5 for magnetite  and  barite   polypropylene   composites   with 45 vol% filler fraction. With increasing temperature thermal diffusivity decreases. Therefore, the values derived from mould experiments should  be  smaller than the measured values of the composites at room tempera- tures. Thermal diffusivity of the PP matrix is  mainly caused by phonons and is related to the mean sound velocity v and mean free path length l  of phonons according to