Table 3

Characteristic times in one injection moulding cycle starting with the injection of the polymer into the cavity at time tiZK8.5 s until the ejection of the mould at tfZ68   s

Injection time (s) K8.5–2

Dwell time (s) 2–9

Cooling time (s) 9–54

Open/close time ejection time (s) 54–68

Total cycle time (s) 76.5

These times define the time axis (abscissa) of Figs. 3 and  6.

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

Fig. 3. Comparison of cooling curves of unfilled polypropylene with polypropylene composites with various filler fractions of Fe3O4. The symbols are measured values; the lines are regression lines (cf.  text).

At a time t0Z0 s the temperature measured by the thermocouple reaches a maximum value around 200 8C. With increasing time the observed temperature decreases. After tZ54 s the mould opens and the cooling behaviour recorded with the thermocouple changes because it is no longer in contact with the injection moulded material. Due to the large diameter of the rod, the time (54 s) until the mould is opened and the injection moulded parts are ejected is chosen relatively high to ensure that the parts are surely solidified.

It can be seen in Fig. 3 that the slope of the curve changes significantly after tz9 s, which corresponds to the time where the after pressure is removed. Additionally, Fig. 3 points out that the composite in the cavity cools down faster with increasing magnetite fraction. To reach a temperature of

are a1(15 s!t!40 s)z0.24 mm2/s and a2(41 s!t! 54 s)z0.19 mm2/s for PP with 15 vol% Fe3O4, a1(12 s! t!33 s)z0.29 mm2/s and a2(34 s!t!54 s)z0.19 mm2/s for PP with 30 vol% Fe3O4, and a1(9 s!t! 22 s)z0.33 mm2/s and a2(28 s!t!54 s)z0.16 mm2/s for PP with 50 vol% Fe3O4 (cf. Table 5).

It is remarkable that the calculated thermal diffusivities a1 of the higher temperature parts  of the  cooling  curves are a little bit lower than the diffusivities measured with the transient technique, while the calculated thermal diffusivities a2 of the lower temperature parts  of  the cooling   curves   meet   the   measured   diffusivity   values

Table 4

Time t to cool down a polypropylene-filler composite from a mass (polymer) temperature of TMZ200 down to 60   8C

TZ60 8C—a temperature far below the solidification of the

sample—the polypropylene needs in the described exper- iment   a   time   of   tZ50.5 s,   whereas  cooling  time of

Composite Filler fraction (vol%)

t (from 200 to 60 8C) (s)

polypropylene with 50 vol% Fe3O4 is reduced to tZ30.9 s (cf. Table 4). The reduced cooling time is in good agreement with the increased thermal diffusivity of  magnetite filled composites due to the  high  thermal  diffusivity  of the particles (cf. Table 1) which leads, regarding Eq. (4), to an increased cooling rate. The temperature time dependence in Fig. 3 does not follow a simple linear behaviour expected for temperature–time curves by Eq. (4) in a logarithmic plot. Only for the unfilled polypropylene the measured values can be fitted with a single straight line between approximately 15 and 54 s. The slope of this line leads to a diffusivity of az0.21 mm2/s (cf. Eq. (4)). The other measured cooling curves of the polypropylene-magnetite composites are fitted in each case with two straight lines, for the high temperature (a1) and low temperature (a2) region. The thermal diffusiv- ities  estimated  from  the  slopes  of  the  regression    lines