The effect of the angle of the granulator knife edge on the efficiency of the extrusion process of granulated polyethylene

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1 Polimery, No. 3, 2002, pp. 196 The effect of the angle of the granulator knife edge on the efficiency of the extrusion process of granulated polyethylene J.W. Sikora Selected from International Polymer Science and Technology, 29, No. 5, 2002, reference PT 02/03/196; transl. serial no Translation submitted by E.A. Inglis INTRODUCTION In most conversion processes the material used is a solid plastic in the form of a granulate. This is obtained as a result of granulation of the powder product of polymerisation sometimes containing fillers and auxiliary agents. This process consists of the mechanical conversion of the plastic to form particles of predetermined shape and size, or granules. The process can be carried out cold (in the solid state) or hot (in a plastic or liquid state). Granulation can also be divided into primary granulation, occurring during the production of the plastic, and secondary, which is the final stage of plastics recycling (refs. 1 3). Granulation is one of the methods of machining or cutting of plastics (with multi-edged tools) used as a supplementary stage to processing which is necessary to achieve plastic products ready to use with the required quality (refs. 1, 4 6). Granulators for hot granulating are linked to extruder heads and designed in a variety of ways. The granulating extruder head is fixed to the barrel of a single- or twinscrew extruder, sometimes with an independent support used in view of the considerable weight of the extruder head and the small but unavoidable vibrations (refs. 7 9). Cold granulation involves extruding the plastic in the form of bars or rods 3 5 mm in diameter, cooling them and then cutting them up into small sections (granules). In this case the extruder heads used are straight (from the point of view of the direction of flow of the plastic from the head), with a die having several to tens of apertures. Extruder heads of this type are not complicated as regards design and technology and do not have to meet high requirements (refs. 10 and 11). Extruder heads for hot granulating have a more complex design (Figure 1), arising from the fact that in this case granulation takes place directly after the plastic leaves the head, when it is still in a plastic state. To obtain a granulate with the desired shape and dimensions the plastic must be cooled immediately after cutting into granules. Two cooling agents are currently employed air and water. The essence of hot granulation is the fixed rotary transverse cutting of the plastic rods. The fixed knife consists of a constructional resistance element a spinneret; the plastic is forced out through its die, and the edge of the die is the cutting edge. The rotary knife generally has a straight cutting edge perpendicular to the axis of the die and parallel to the working surface of the spinneret, separated from it by the knife aperture (ref. 12). Figure 1. Granulator for hot granulating connected to an extruder head: 1 spinneret, 2 rotating cutting knives, 3 extruder, 4 extruder head housing, 5 knife rotor, 6 head support (ref. 8) T/85

2 In constructional designs of granulating machines at least one cutting edge is sloped. The combined action of non-parallel cutting edges causes a considerable reduction in the cutting force, an extension of the path and time of contact between the plastic and the cutting edge and also a restriction of the mechanical vibrations and noise. It seems reasonable to carry out experimental studies of the process of extrusion with hot granulation of a plastic to determine the effect of changing the angle of the granulator knife edge on the basic values characterising the granulate and on the course of the extrusion process. The literature contains no data on this problem. In the present study we investigate an autothermal process of extrusion with hot granulation, using medium-density polyethylene (PE-MD) as the material. EXPERIMENTAL Material The medium-density polyethylene was ME2421 produced by Borealis in the form of granules measuring about 3 mm in diameter. The basic properties of this material are wellknown and documented in the literature (e.g. refs ). Research program On the basis of a survey of the literature and introductory studies, the factors listed below were included in the research program: Factors studied directly - the mass of the granulate specimen m w (in kg), - the time of extrusion of the granulate specimen, t w (sec), - time of measurement of the energy taken up by the extruder with the granulator and cooling equipment, t c (sec), - temperature of the extrudate leaving the die of the extruder head, T w ( C), - the mass of the measuring cylinder together with the granulate used to study the bulk density m 1 (kg), - the greatest height of the heaped cone h (m), - the diameter of the base of the heaped cone d (m). Factors studied indirectly - the mass rate of flow of the plastic in the plasticating system of the extruder Ġ (kg/h), - total power taken up by the extruder with the granulator and cooling device, Q c (W), - the specific enthalpy increment of the plastic being processed in the plasticating system of the extruder, i (J/g), - the total power absorbed by the plastic during extrusion with granulation, Q w (W), - the unit consumption of the total energy taken up by the extruder together with the granulator and cooling device, E JC (J/g), - the energy efficiency of the extruder with the granulator and cooling device k w (%), - the bulk density of the resulting granulate, ρ N (kg/m 3 ), - the angle of the natural heap of granulate ϕ (deg.). The main variable factor was the angle of the cutting edge (β) of the knife in the system: β 1 = 20, β 2 = 25, β 3 = 30, β 4 = 35, β 5 = 40. Constant factors - the geometric components of the screw, cylinder, extruder granulator head and the cooling device (except for the angle of the granulator knife edge); - the number of heating zones in the plasticating system, 2 in the case of the extruder and 1 for the extruder granulator head; - the frequency of the screw rotations ν = 2.95 s -1 (18.53 rad/s); - the frequency of rotation of the rotor of the granulating knives ν = 6.93 s -1 (43.54 rad/s); - the temperature of the individual zones of the plasticating system: T 1 = 115 C, T 2 = 194 C; - temperature of the extruder head T g = 164 C; - the nominal number of rotations of the electrical energy counter dial z = 20; - the mass of the empty measuring cylinder during study of the bulk density m = kg; - volume of the measuring cylinder V = 0.5 dm 3. The results of the investigations may be influenced primarily by the following disturbance factors: - the strength of the electric current (changes from about 215 to 225 V); - the ambient temperature (variations from 20 to 24 C); - the relative moisture content of the initial plastic (varying from 0.04 to 0.06%); - the relative air humidity (varying from 55 to 65%). It is assumed, however, that the influence of these disturbing factors is very small and can be ignored without affecting the results of the study. T/86

3 METHOD The studies were conducted in the laboratory of the Department of Polymer Processes of Lublin Polytechnic, using an experimental single-screw extruder W-25D, with a ratio of the working part of the screw to its diameter L/D = 25. An exact description of the design and operation of this extruder are given in the literature (refs. 13, 14 and 16). The extrusion process was autothermal. The extrusion and hot granulating line used for the studies are shown in Figure 2. It is additionally fitted with a granulating extruder head (Figure 3), consisting of an extruder head and granulator and a cooling device. The method used for carrying out the experimental investigations and the calculation methods are contained in earlier publications (refs. 13 and 14). The bulk density and the natural heaping angle were calculated in accordance with the standards PN-80/C and PN-89/C The granulator was equipped with a spinneret with four apertures, each 3 mm in diameter, located symmetrically with respect to the extruder axis, and a knife rotor with two flat single-edged cutting knives (Figure 4) situated opposite to each other in a plane perpendicular to the extruder axis. The axis of the knife rotor is displaced with regard to the axis of the plasticating system and the spinneret. This off-centre system of the granulator means that the knives come in contact with the plastic cyclically only in the region of the spinneret, whereas outside of this region they are cooled better. The knife rotor has an independent drive, allowing a smooth change in speed. Figure 2. Extrusion line with hot granulating, used for the studies: 1 barrel housing of the extruder W-25D, 2 polymer hopper, 3 granulating extruder head, 4 knife rotor drive, 5 device for cooling granulate, 6 supply pipe, 7 radial fan Figure 4. Granulator cutting knives Figure 3. Granulating extruder head (partly disassembled): 1 extruder plasticating system, 2 knife rotor, 3 front head housing, 4 knife grip, 5 rear head housing, 6 knife rotor drive After being cut, the granules are blown from the knife, fall down about 70 cm and are then transported pneumatically by a supply pipe leading about 115 cm vertically upwards to the cooling device. The device cooling the granulate is connected to the housing of the knife rotor of the granulator head and forms the final part of the processing line. The main part of this device is a container tank cooling the granulate deposited on the base. The tank has entry and exit locks, both powered by separate electric motors. The cooling agent is air which is conducted by an air pipe from a radial fan, also with an electric drive. Before switching on the extruder the temperatures of the various heating zones of the plasticating system and the extruder head were set. Then the drive of the extruder and the cooling device were started up, the heaters were T/87

4 switched off, and following heat stabilisation of the autothermally operating extruder the investigations were begun. The study was begun using a knife with the smallest edge angle (Figure 5). When we had obtained a sufficient amount of granulate for further measurements, we changed the cutting knife in accordance with the study program and the process was repeated from the beginning. Figure 5. Angles characterising the cutting knife and its position in relation to the plastic being processed: V main movement rate, U feed movement rate, a - tool clearance angle, b - edge angle, g - tool rake angle, 1 knife, 2 extruder head die, 3 head body, 4 processing plastic (plasticated rod) RESULTS After determining directly the values of the factors studied, we calculated the factors indirectly, and the results of these calculations for different angles of the granulator knife are shown in Table 1. It can be assumed that change in the angle of the granulator knife edge does not affect the power used by the extruder linked to the granulator and cooling device or on the total power absorbed by the plastic during extrusion and granulation. This change also does not have any effect on the total unit energy consumption or on the energy efficiency kω calculated with regard to the kinetic and potential energy of the plastic granules or on the increase in specific enthalpy of the plastic caused by the slight increase in temperature of the surface of the granules in relation to the temperature of the extrudate leaving the die of the extruder head. The values of Qc and E JC are high. This correlates with the calculated value of k ω, which must be assessed as low here it is about 20% less than the efficiency of the extruder itself operating autothermally (ref. 14). Despite the specific characteristics of autothermal extrusion with hot granulation, however, it is found that E JC and k are ω unsatisfactory. It must be concluded that if we use extruders with a grooved zone, with the correct design of the whole plasticating system, particularly the screw used for the autothermal process and for converting the given plastic the results will be more favourable. Such extruders are characterised by a higher rate of flow of the plastic owing to the higher pressure and friction of the plastic during processing in the plasticating system and by the forced direction of its movement along the grooves. This will have the effect of improving all of the energy factors of the extrusion process, despite the rapid cooling of the loading zone and parts of the feed zone. Also important is the need to optimise the whole granulator and cooling device with regard to energy consumption. A design involving as many as four drives for the knife rotor, the fan and entry and exit locks respectively is not the most effective design. Nor is the problem solved by using a different design of granulating extruder head and another method of transporting the granulate to the cooling device, e.g. involving the free dropping of the granules into a bath with cooling water, since it is then necessary to dry the granulate. However, a design using gravity feeding of the plastic granules into a cooling tank may improve the energy characteristics of the granulation process. Table 1. Results of calculations of factors studied directly Factors studied Angle of knife edge, β, deg Mass rate of flow of plastic, G, kg/ h 6.73 Total power taken up by the extruder. with the granulator and cooling device, Q c, W 2704 Increase in specific enthalpy of the plastic, i, J/ g 560 Total power absorbed. by the plastic during extrusion with granulation, Q, W 1082 w Total unit energy consumption, E, J/ g 1446 Energy efficiency of the extruder with the granulator and cooling device, k ω, % JC 3 Bulk density of granulate, ρ N, kg/ m Natural heaping angle of the granulate, ϕ, rad (deg) (46.2) (47.0) (47.5) (47.6) (48.1) T/88

5 The changes in the angle of the granulator knife edge (β) described in this study also do not have any substantial effect on the bulk density or the natural heaping angle of the granulate obtained. Increasing the angle β from 20 to 40, i.e. by 100%, causes a decrease in the bulk density of the granulate from 539 to 505 kg/m 3, which is about 6.3%. This relationship can be described by the following equation: 2 Q N = 0. 68β β (1) The same considerable increase in this angle increases the natural heaping angle from rad to rad, i.e. by about 4.1%. This corresponds to the equation: 5 2 ϕ = β β (2) In addition it is seen from the Table that under the conditions studied, as the value of β is increased there is an increase in the rate of its effect on the bulk density of the granulate, whereas the effect on the natural heaping angle ϕ decreases. In the range from 20 to 40 an increase of 1 degree in the angle β reduced the bulk density of the granulate by an average of 1.7 kg/m 3 and caused an average increase in the angle ϕ of rad (0.085 ). Changing the angle β alters the condition of the cutting surface of the plastic and the amount of heat generated during cutting. A small angle β gives rise to a less rough cutting surface, which has the effect that the granules are better packed in relation to each other during the formation of the conical heap, and this in turn results in a smaller natural heaping angle and a higher bulk density of the granulate. If the cutting angle is small there is less work of cutting (ref. 6), which is accompanied by less heat formation; this results in the formation of greater permanent plastic deformations and greater orientation and recrystallisation of the plastic in the surface layer nearest to the cutting surface. The effect of this is an increase in the density of the plastic (including the bulk density). CONCLUSIONS It is shown by the data presented that using smaller angles of the cutting edge has a positive effect on the characteristics of the granulate. With smaller angles β a somewhat higher bulk density and a more convenient natural heaping angle of the granulate is obtained, leading to higher quality products. Over a certain range, the angle of the knife edge of the granulator may be a constructional factor controlling the shape and size of the granules, and hence the effectiveness of the hot granulation process. On the other hand, as can be seen from ref. 17, lower values of β reduce the durability of the knife to a certain extent. It therefore seems reasonable to carry out further studies with a view to solving this problem. It may turn out that the production of granulate of a higher quality requires more frequent changing of more rapidly wearing knives. REFERENCES 1. R. Sikora, Machining of high-molecular plastics, Wydawnioctwo Edukacyjne, Warsaw 1996, pp. 168, M. Dominik, Kunststoffe, 87, 1997, p H. Schalles, Kunststoffe, 87, 1997, p M. Bielinski, Fundamentals and studies of the suitability of recycled thermoplastics for selected conversion processes, Wyd. Uczelniane ATR, Bydgoszcz M. Bielinski, Material and processing characteristics of selected recycled thermoplastics, Wyd. Uczelniane ATR, Bydgoszcz R. Konieczka, Bases of mechanical processes of recycling of films from low-density polyethylene, Wyd. Uczelniane ATR, Bydgoszcz D. Kuehlborn, Kunststoffe, 91, 2001, p Information and catalogues from Hans Weber Maschinenfabrik GmbH. 9. Anon., Plast. Techn., 47, 2001, p J.P. Gottberg, Kunststofftechnik, 9, 1970, p J. Marwick, Kunststoffe, 88, 1998, p Series of articles Granulation of thermoplastics, VDI-Verlag GmbH, Dusseldorf J.W. Sikora, Polimery, 43, 1998, p J.W. Sikora, Study of the autothermal nature of extrusion and a grooved extruder zone, Lublin Polytechnic, 2000, pp. 126 and Information and cataligues from the Borealis company. 16. Technical documentation of the experimental extruder W-25D, OBR Metalchem in Torun, Torun R. Sikora and P. Masztaleruk, Polimery, 37, 1992, p.421. (No date given) T/89