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Cooling behaviour of particle fi lled polypropylene during injection moulding process Bernd Weidenfellera,*, Michael Ho ferb, Frank R. Schillingb aInstitute of Polymer Science and Plastics Processing, Technical University of Clausthal, Agricolastrasse 6, D-38678 Clausthal-Zellerfeld, Germany bGeoForschungsZentrum Potsdam, Section 4.1 Experimental Geochemistry and Mineral Physics, Telegrafenberg, D-14473 Potsdam, Germany Received 25 June 2004; accepted 4 July 2004 Abstract The effects of thermal properties of various fi llers (magnetite, barite, copper, talc, glass fi bres and strontium ferrite) in various proportions on the cooling behaviour of polypropylene matrix composites are investigated in an injection moulding process. A thermocouple in the cavity of the mould records the temperatures at the surface of the composite during injection moulding. From the slope of the cooling curves the thermal diffusivities of the composites are estimated and compared with thermal diffusivities at room temperature and elevated temperatures measured with a transient technique. The cooling curves show different merging sections affected by the after pressure, the diffusivity of the composite and the diffusivity of polypropylene matrix. The cooling behaviour depends on the anisotropic thermal diffusivity of the used composite, which is caused by the alignment of fi ller material due to the injection moulding process and the interconnectivity of the fi ller particles. The thermal diffusivity shows the highest value for 30 vol% talc fi lled polypropylene, whereas the shortest cooling time was found for 35 vol% copper fi lled polypropylene. The knowledge of the systematic variation of thermal transport properties of composites due to different fi ller material andfi ller proportionsallows to optimizethe mould process and tocustomize the heat fl ow properties. Furthermore,the strongly anisotropic thermal transport properties of talc fi lled polypropylene allow the design of composites with a predefi ned maximum heat fl ow capability to transport heat in a preferred direction. Keywords: A. Polymermatrix composites (PMCs); B. Thermal properties; E. Injection moulding; Particulate fi ller 1. Introduction Commonly used plastics, such as polypropylene and polyamide, have a low thermal conductivity. However, new applications, mainly in automotive industries, e.g. for sensors or actuators, require new materials with an enhanced or high thermal conductivity 1. By the addition of suitable fi llers to plastics, the thermal behaviour of polymers can be changed systematically up to signifi cant higher thermal diffusivity of O1.2 mm2/s from 0.2 mm2/s for unfi lled polypropylene 2,3. Such fi lled polymers with higher thermal conductivities than unfi lled ones become more and more an important area of study because of the wide range of applications, e.g. in electronic packaging 46. The higher thermal conductivity can be achieved by the use of a suitable fi ller such as aluminium 1, carbon fi bres and graphite 7, aluminium nitrides 6,8 or magnetite particles 2. Also, the cooling behaviour in the mould of the injection moulding machine is infl uenced by the thermal properties of the polymer-fi ller composite. However, published values of thermal conductivities of the same fi ller materials in different polymer matrices vary drastically and a comparison of different materials is diffi cult or at least impossible 2. Therefore, polypropylene samples with different com- mercially available fi llers (Fe3O4, BaSO4 , Cu, glass fi bres, talcandSrFe12O19)werepreparedbyextrusionandinjection moulding using various volume fractions (050%). Magne- tite and barite are generally used to increase the weight of Composites: Part A 36 (2005) 345351 * Corresponding author. Present address: Institute of Metallurgy, Robert- Koch-Str. 42, D-38678 Clausthal-Zellerfeld, Germany. Tel.: C49-5323- 723708; fax: C49-5323-723148. E-mail address: bernd.weidenfellertu-clausthal.de (B. Weidenfeller). polypropylene, e.g. for bottle closures (cosmetics industry, cf. Ref. 10), strontium ferrite is used in polymer bonded magnets, glass fi bres are used for the reinforcement of materials, and talc is an anti-blocking agent. However, copper was chosen as additional fi ller because of its high thermal conductivity compared to the other materials. The thermal properties of these injection moulded samples and the injection moulding behaviour were investigated and correlated to the amount and the kind of fi ller material. 2. Theoretical considerations The Fourier law of heat transport in one dimension is given by vT vt Za v2T vx2 (1) withtemperatureT,timet,positionxandthermaldiffusivitya. In an homogeneous body, thermal diffusivity a and thermal conductivity l are interrelated by specifi c density r and specifi c heat capacity cpaccording to l Zcpra(2) Assuming an injection moulding process with an isothermal fi lling stage for a polymer with a temperature TPand a constant temperature of the mould TMas well as a temperature independent thermal diffusivity a, an analytical solution of Eq. (1) results in 9 T ZTMC 4 p TPKTM ! X N nZ0 1 2nC1 exp Ka2nC1 2p2t s2 ? sin 2nC1px s ? (3) In Eq. (3), s denotes the wall thickness of the injection moulded part and T the temperature of the moulding after time t after injection. Neglecting higher order terms, Eq. (3) can be reduced for the position xZs/2 to T ZTMC 4 p TPKTM expK ap2t s2 ? (4) Eq. (4) gives a relation between cooling rate and thermal diffusivity in an injection moulding process, where high thermal diffusivities result in a higher cooling rate and shorter process cycles. 3. Experimental 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 fi llers (Fe3O4, BaSO4 , Cu, glass fi bres, talc and SrFe12O19) in an extrusion process similar to that described in Ref. 2. The fi ller materials are commonly used materials in industrial products. The fi ller particles do not have a surface coating which can affect thermal properties. Some selected properties of the fi ller materials are listed in Table 1. Table 1 Selected properties of fi ller materials Magnetite, Fe3O4Barite, BaSO4Talc, Mg3Si4O10OH2Strontium ferrite, SrFe12O19 Copper, Cu Glass fi bres Thermal conduc- tivity (W/(m K) a:4.61G0.42, a:5.10,l11:9.7 l11:2.07G0.02,l33: 2.92G0.07, a:1.72G0.04 l11:1.76G0.00,l33: 10.69G1.35, a:2.97,a:3.00G0.10, a:6.10G0.90 l11:401l:1.21.5 Reference1313131415 Mean particle diam- eter (mm) 91.52.01.51511 Particle shapeIrregularIrregularPlateletIrregularIrregularFibre Density (g/cm3)5.14.482.785.118.942.58 a denotes measurements on monomineralic aggregates. Directions of anisotropy are specifi ed by the thermal conductivity tensor (l11, l22, l33), where l11, l22 and l33are parallel to the crystallographic axes a, b and c, respectively. 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) 345351346 3.2. Thermal diffusivity measurements The thermal diffusivity of the polymers is measured by a transient method 12, closely related to laser-fl ash 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 samples 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 fi nite-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 56 mm height were cut out of the injection-moulded rods (cf. Fig. 1). 3.3. Injection moulding 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). Inthe 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 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 fi ller are presented. 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. 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)K8.52 Dwell time (s)29 Cooling time (s)954 Open/close time ejection time (s)5468 Total cycle time (s)76.5 These times defi ne the time axis (abscissa) of Figs. 3 and 6. Table 2 Injection moulding parameters during preparation of sample rods for measurements of thermal diffusivity by transient technique Mass (polymer) temperature (8C)200 Mould temperature (8C)20 Cycle time (s)76.5 Injection time (s)10.5 Dosing time (s)12.4 Holding pressure time (s)7.0 Injection pressure (Pa)6!107 B. Weidenfeller et al. / Composites: Part A 36 (2005) 345351347 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 solidifi ed. It can be seen in Fig. 3 that the slope of the curve changes signifi cantly 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 withincreasingmagnetitefraction.Toreachatemperatureof TZ60 8Ca temperature far below the solidifi cation of the samplethe polypropylene needs in the described exper- iment a time of tZ50.5 s, whereas cooling time of polypropylene with 50 vol% Fe3O4is reduced to tZ30.9 s (cf. Table 4). The reduced cooling time is in good agreement with the increased thermal diffusivity of magnetite fi lled composites due to the high thermal diffusivity of theparticles(cf.Table1)whichleads,regardingEq.(4),toan increased cooling rate. The temperature time dependence in Fig. 3doesnotfollow asimplelinear behaviour expected for temperaturetime curves by Eq. (4) in a logarithmic plot. Only for the unfi lled polypropylene the measured values can befi ttedwithasinglestraightlinebetweenapproximately15 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 fi tted 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 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 forPPwith30 vol%Fe3O4,anda1(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 a1of 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 a2of the lower temperature parts of the cooling curves meet the measured diffusivity values Fig. 3. Comparisonof coolingcurves ofunfi lledpolypropylene with polypropylene compositeswith variousfi llerfractionsof Fe3O4. The symbolsare measured values; the lines are regression lines (cf. text). Table 4 Time t to cool down a polypropylene-fi ller composite from a mass (polymer) temperature of TMZ200 down to 60 8C CompositeFiller fraction (vol%) t (from 200 to 60 8C) (s) PP050.5 PPCFe3O41546.4 PPCFe3O43040.5 PPCFe3O44534.6 PPCFe3O45034.9 PPCBaSO41544.3 PPCBaSO43040.7 PPCBaSO44535.6 PPCCu1540.5 PPCCu3033.8 PPCCu3529.0 PPCglass fi bres1546.0 PPCglass fi bres3041.8 PPCglass fi bres3540.8 PPCtalc1545.7 PPCtalc3042.5 PPCSrFe12O193040.9 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) 345351348 of unfi lled 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 unfi lled poly- propylene up to aZ0.48 (PPC50 vol% Fe3O4) with increasing magnetite loading. Therefore, the cooling time becomes shorter for higher magnetite fi ller 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% fi ller 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 a Z 1 3 vl(5) Above the solidifi cation temperature of the PP matrix (around110 8C,DSCmeasurements)thethermal diffusivity of the matrix is reduced due to the lowered bulk modulus K which results in a reduced phonon velocity (vz(K/r)0.5) and reduced mean free path length of phonons in a liquid (Einstein approximation). Furthermore, above solidifi cation temperature TSno crystallites in the poly- propylene matrix are present, but below TSa crystallization in the polypropylene matrix appears, and the degree of crystallization as well as the bulk modulus of the composite is dependent on the amount of fi ller 16. The presence or absence of crystallites affects the bulk modulus K and the phonon free path. Other reasons for the discrepancy between diffusivity values of the different experiments are the non-isobaric conditions in the injection moulding process and the non-isothermal conditions along the samples thickness. The cooling behaviour of magnetite, barite, glass fi bre, talc, hard ferrite and copper fi llers in comparison with the unfi lled polypropylene are plotted in Fig. 6. Only the cooling behaviour of the unfi lled and the copper fi lled polypropylene show signifi cant differences to the other composites. Table 5 Thermal diffusivity estimated from the cooling behaviour of injection moulded polypropylene-fi ller composites using the slope of the regression lines (a1, a2) (cf. Fig. 3) compared to thermal diffusivity values measured by the transient method (a) CompositeRegression linesTransient method a1(mm2/s)a2(mm2/s)a (mm2/s) PP0.21 (12055 8C)0.19 (26 8C) PPC15 vol% Fe3O40.24 (12067 8C)0.19 (6751 8C)0.27 (26 8C) PPC30 vol% Fe3O40.29 (12068 8C)0.19 (6845 8C)0.35 (26 8C) PPC50 vol% Fe3O40.33 (12577 8C)0.16 (6745 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. Fig. 4. Thermal diffusivity values of injection moulded polypropylene samples with different fi llers and various fi ller proportions measured by a transient technique at room temperature (cf. text). Solid lines are plotted to guide eyes. Fig. 5. Temperature dependence of thermal diffusivity of magnetite and barite fi lled polypropylene with a fi ller content of 45 vol%. The symbols represent measured values, the lines are deduced by linear regression. B. Weidenfeller et al. / Composites: Part A 36 (2005) 345351349 The copper fi lled composite cools down much faster than the other investigated composites. The temperature of the unfi lled polypropylene is during the whole injection moulding process higher than the temperature of the other investigated materials. The cooling behaviour of the other composite materials does not show large differences. The temperatures of the magnetite loaded PP is a little bit lower than the temperatures of the barite fi lled PP at the same cooling time. The temperatures of the strontium ferrite polypropylene composite again are a little bit lower than those o