Extruder, material and operating conditions

The layout of the extruder is represented in Figure 18. The three-zone screw has a diameter, Dscrew = 25 mm, a L/D ratio of 25 and a square pitch. The lengths of the feed, compression and metering zones are 8D, 8D and 9D long, respectively. The internal screw diameter is 16.6 mm and 22.0 mm, respectively, in the feed, Di1, and metering, Di3, zones; the longitudinal length of the grooves, Lg, is 100 mm. A simple circular die is coupled at the end.

Fig. 18. Extruder layout

Table 5 shows the diverse geometries utilized. They could also correspond to different positions of devices allowing to adjust continuously the geometry of the grooves. In such a case, G1a represents the position when the grooves have the maximum value for the initial depth, hN0, while G1d represents the case when this depth is nil.

Table 6 presents the relevant properties of the polymer used, a Low-Density Polyethylene (grade Malen E FGAN 18-D003 produced by LyondellBasell). The melt viscosity (Figure 19) was measured by capillary rheometry, the data being fitted to a power law with an Arrhenius temperature dependence:

  



e

37

Table 5. Geometry of the grooves studied

Geometry bN (mm) hN0 (mm) NN L (mm) B = NN*bN

Table 6. Main properties of LDPE (Basel Malen E FGAN 18-D003)

Properties LDPE Unity

Density

38

Fig. 19. Rheological data for LDPE (Basel Malen E FGAN 18-D003)

In all calculations, the operating conditions were fixed. The screw speed was 120 rpm; the barrel set temperature profile during the first 5D (corresponding to the length of the grooves) varies linearly from 30 ºC and 70ºC in order to prevent melting of the polymer and is constant and equal to 170ºC in the remainder of the barrel and die.

6. Results

Figure 20 presents the axial development of pressure, P, solids width (X/W, where X is the width of the solids and W is the channel width), and maximum temperature in the solids, Ts max, together with the barrel temperature profile imposed, TBarrel, for the geometry G1a. Table 7 shows the values of global extruder responses: output, average melt temperature at die exit, mechanical power consumption, length required for melting, degree of distributive mixing (quantified by WATS, a measure of the average total of the melt in the extruder [13]), and viscous dissipation (ratio between the maximum and the barrel temperatures). Observation of the temperature graphs in Figure 20 shows that Ts max raises faster than TBarrel due to the high rate of heat generated by friction.

Consequently, the length of the solids conveying stage reduces and so does the pressure generation, which reaches only less than 9 MPa. As expected, the maximum pressure is attained near the end of the compression zone of the screw.

Most of the melting stage develops along the compression zone and is completed before the metering zone.

The Potente model used to calculate fef is only sensitive to values of hN/bN <

0.1 (see Figure 14). For geometries G1a to G1c, B = 24, thus hN/bN = 0.3 at the

39

beginning of the grooves (hN0 = 2); this ratio becomes small only towards the end of the grooves, where the effect of fef on the pressure profile is minor.

Therefore, the results for these three cases are very similar (Table 6).

Fig. 20. Results obtained when using grooves with the geometry G1a (Table 5)

40

Table 7. Results for the geometries G1 in Table 5

Geometry Output (kg/hr)

Tmelt

(°C)

Power consumption

(W)

Lmelting

(L/D) WATS Viscous Dissipation

G1a 5.43 188.5 2647 14.21 302.6 1.53

G1b 5.38 188.6 2597 14.17 303.1 1.54

G1c 5.38 188.6 2630 13.69 299.5 1.54

G1d 5.34 188.9 2278 14.64 308.0 1.46

G1e 5.47 188.3 2728 14.21 301.8 1.53

G1f 5.46 188.3 2669 13.83 299.2 1.51

G1g 5.35 188.3 2709 13.88 301.2 1.53

G1h 5.34 188.9 2278 14.64 308.0 1.46

Geometry G1e exhibits grooves with bigger depth and total width. As seen in Figure 21, the pressure can reach approximately 10 MPa, as expected.

Conversely, G1d and G1h, where grooves are absent, are capable of inducing lower pressures than the remaining (Figure 22). As demonstrated in Figure 23, the various groove geometries will induce distinct pressure generations. Values between 6 MPa and 10 MPa will be obtained at the end of solids conveying. In turn, they will influence the remaining thermomechanical history of the material in the extruder, yielding the differences summarized in Table 7.

Fig. 21. Results obtained when using grooves with the geometry G1e (Table 5)

41

Fig. 22. Results obtained when using grooves with the geometries G1d and G1h (Table 5)

Figure 23. Pressure profiles for the geometries G1 (in Table 6)

7. Conclusions

The presence of grooves in the initial part of the barrel can affect significantly the performance of single screw extruders. Modelling their effect would enable the development of suitable and reliable process modelling routines. One possible approach is to calculate the global friction coefficient between barrel and polymer, fef, that is created by the presence of the grooves. The more sensitive fef

would be to the geometrical parameters of the grooves, the better. This chapter assessed the suitability of four calculation methods of fef. It was shown that the equation developed by Potente considers both the width and depth of the grooves, whilst the model of Goldaker has the strongest influence on the plasticating sequence.

The examples discussed showed that the presence of the grooves does indeed improve the performance of the extruder in terms of pressure generation and output, but their geometry, particularly depth and the total width, greatly influences the behaviour.

42 Acknowledgements

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 734205 – H2020-MSCA-RISE-2017.

8. References

[1] H. Darnell, E.A.J. Mol, SPE J., 12, 20, 1956

[2] Tadmor, Z.; Klein, I., "Engineering Principles of Plasticating Extrusion", Van Nostrand Reinhold, Ney York, 1970

[3] Rawendaal, C., "Polymer Extrusion", Hanser Publishers, Munich, 1986 [4] Boes, D.; Krämer, A.; Lohrbäccher, V.; Scheneiders, A., Kunststoffe Germ.

Plast., 80, 6, 659, 1990

[5] Potente, H., Kunststoffe Germ. Plast., 75, 7, 439, 1985

[6] Rautenbach, R., Peiffer, H., Kunststoffe Germ. Plast., 72, 3, 137, 1982 [7] Rautenbach, R., Peiffer, H., Kunststoffe Germ. Plast., 72, 5, 262, 1982 [8] Grünschloss, E., Kunsttoffe Germ. Plast., 74, 7, 405, 1984

[9] Potente, H., Kunststoffe Germ. Plast., 78, 4, 355, 1988

[10] Potente, H., Koch, M., Intern. Polym. Process., 4, 4, 288, 1989 [11] Goldacker, E., Diss. RWTH Aachen, 1971

[12] Rosenbrock, H.H., The Computer Journal, 3, 3, 175, 1960 [13] G. Pinto, Z. Tadmor, Polym. Eng. Sci., 10, 279, 1970

43

José A. Covas¹, A. Gaspar-Cunha¹

MODELLING OF FLOW AND HEAT TRANSFER, MIXING AND MORPHOLOGY DEVELOPMENT IN PLASTICATING SINGLE