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120 140 160 180 200 0 200 400 600 800 1000 1200 1400 Time [s] Temperature [C] 0.8 1.2 1.6 2 2.4 2.8 3.2 Pressure [bar] temperature expamsion end model pressure Fig. 7 Temperature and pressure inside the mould, 8 mm thickness sample, mass fraction of the starch 0.52, mass of sample 250 g, sample did not fill the mould Potatoes starch, weight 250g, 52% of starch, 8 mm thick 0 20 40 60 80 100 120 140 160 180 200 0 100 200 300 400 500 600 700 800 900 1000 Time [s] Temparature [C 0.8 1.3 1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3 P ressure [bar temperature expansion end model pressure Fig. 8 Temperature and pressure inside the mould, 8 mm thickness sample, mass fraction of the starch 0.52, mass of sample 250 g, sample filled the mould
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Type 9 The essential feature of thermal forming technology is a considerable volumetric expansion and an elevated pressure inside the mould during heating. After pouring the sample onto the lower plate and closing the mould the sample expands immediately. End of expansion is marked by vertical lines in Figs. 5, 6, 7 and 8. The expansion rate was measured by a network of needle thermocouples located inside the mould (the measuring junctions were outside of the sample border). When the periphery of the expanding sample reached a thermocouple, the temperature was changed and this sudden alternation indicated the exact time and place of the sample border. The measured time courses of expansion are shown in Fig.9 for the 4 mm thickness sample and for different mass of the sample 100, 120 and 130 g. The averaged time from closing the mould to expansion is around 50 s for the 4 mm sample. Expansion of 4 mm thick sample 0 50 100 150 200 250 0 10 20 30 40 50 60 time [s] volume [cm3] 1 120g 2 120g 3 130g volume of sample volume of mould 1 100g 2 100g 3 100g 4 100g 5 100g Fig. 9 Expansion rate of the 4 mm sample Expansion rate of the 8 mm sample is shown in the Fig.10 only for one sample mass 150 g. The average expansion time is around 130 s for 8 mm sample.
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Skočilas Jan et al. Expansion of 8 mm thick sample 130 180 230 280 330 380 430 0 20 40 60 80 100 120 140 160 180 time [s] Volume [cm 3 ] 1. 250 g 2. 250 g 3. 250g 4. 250g 5. 250g volume of mould volume of sample Fig. 10 Expansion rate of the 8 mm sample The mathematical model presented in the chapter 2 (integral model) was implemented into the program FEMINA, which involves optimization procedures. The model parameters were identified using experimental data by this program. Values of five parameter are presented in table 1, the other two parameters were assumed constant – initial radius of the bubble R b0 = 0.2 mm and flow index n = 0.672. Tab.1 Parameters of the model Parameter 4 mm * 4 mm 8 mm * 8 mm *10 -8 0.65 0.65 0.80 0.30 h 1 0.80 0.80 0.80 0.80 h 2 0.80 0.35 0.80 0.35 h 3 0.67 0.50 0.80 1.40 h 4 0.75 0.57 0.85 1.50 * - sample not fill the mould 4.3 Direct ohmic heating The following figure 11 shows the result of preliminary experiment with the direct ohmic heating using the ohmic heating cell. The power, voltage, current and temperature were measured, whereas the presented conductivity of the sample was computed. The mass of sample was 48 g, mass fraction of the starch 50 %, and the 0.5 g of salt was added for improving conductivity. The maximum value of electrical power applied into the sample was not higher than 140W.
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Type 11 Resulting temperature time course shown in Fig.11 demonstrates the fact, that the
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