3.1. Materials

Test materials were supplied by Minelco B.V. (The Netherlands). Minelco B.V. prepared in cooperation with RTP s.a.r.l (France) several polypropylene (PP) compounds with various fillers (Fe3O4, BaSO4, Cu, glass fibres, talc and SrFe12O19) in an extrusion process similar to that described in Ref. [2]. The filler materials are commonly used materials in industrial products. The filler particles do not have a surface coating which can affect thermal properties. Some selected properties of the filler materials  are  listed  in Table 1.

In Eq. (3), s denotes the wall thickness of the injection moulded part and T the temperature of the moulding    after

Fig. 1. Photograph of the used mould for the injection moulding experiments. The mould consists of a standard tensile test sample and a test bar for the measurement of thermal diffusivity.

B. Weidenfeller et al. / Composites: Part A 36 (2005)  345–351 347

Fig. 2. Mold with cavity for preparing test samples in an injection moulding machine. The position of the thermocouple for temperature measurements is marked by an arrow.

3.2. Thermal diffusivity measurements

The thermal diffusivity of the polymers is measured by a transient method [12], closely related to laser-flash experi- ments [11]. The used transient technique is especially optimized for measurements of polyphase aggregates. A temperature signal is transferred to the upper side of the sample and registered by a thermocouple. The transferred temperature signal starts a thermal equilibration process in the specimen, which is recorded by a thermocouple as the difference between sample’s rear surface and a constant temperature in a furnace and which is used for the evaluation of thermal diffusivity. A least squares algorithm is used to determine the thermal diffusivity, while varying systematically the thermal diffusivity value in an especially designed finite-difference scheme. A detailed description of the apparatus is given by Schilling [12]. The accuracy of the measurements of the polyphase aggregates is  3%.

For thermal diffusivity measurements, small cylinders of 10 mm diameter and 5–6 mm height were cut out of the injection-moulded rods (cf. Fig. 1).

3.3. Injection moulding

the cavity. Therefore, a good thermal contact between polymer and thermocouple even after shrinkage [10] of the moulding is ensured.

For a better comparison of the recorded temperature– time curves the same injection moulding parameters for all composite materials were chosen. The used injection moulding parameters are listed in Table 2. The resultant characteristic times of the injection moulding cycle are tabled in Table 3.

4. Results and discussion

In Fig. 3, the cooling behaviour of polypropylene without and with various fractions of magnetite filler are presented.

Table 2

Injection moulding parameters during preparation of sample rods for measurements of thermal diffusivity by transient technique

Mass (polymer) temperature (8C) 200

Injection pressure (Pa) 6!107

With an injection moulding machine (Allrounder   320C

600-250, Arburg, Germany) standard samples for measuring

tensile properties together with a rod for thermal measure- ments of 10 mm diameter and 130 mm length were prepared in one mould (cf. Fig. 1). In the cavity of the tensile test bar a chromel alumel (Type K) thermocouple was applied. During injection moulding experiments the  temperature was recorded every 0.5 s by a digital multimeter and stored in a personal computer. The position of the thermocouple at the sample surface and its position in the cavity of the ejector  are  shown  in Figs.  1   and   2, respectively. The  thermocouple  submerges  approximately  0.2 mm into