- Email: [email protected]
Hydrodynamic wire coating using a tapered bore pressure unit
Journal of Materials Processing Technology 174 (2006) 384–388
Hydrodynamic wire coating using a tapered bore pressure unit S. Akter ∗ , M.S.J. Hashmi Material Processing Research Centre, Dublin City University, Dublin 9, Ireland Received 25 January 2004; received in revised form 13 February 2006; accepted 14 February 2006
Abstract In the conventional process of wire coating, the wire is either dipped in the coating material or is coextruded along with the coating material. Apart from dipping or coextrusion methods, there is another innovative method called hydrodynamic coating in which the polymer melt is used as a pressure medium in a pressure unit. The wire size is less than the smallest bore of the pressure unit. The wire coating thickness and quality depend on the wire speed, polymer viscosity, polymer melt temperature and the gap between the wire and exit end of the die. In this paper, results of experimental investigation are presented by comparing the coating quality on galvanized mild steel wire using molten Nylon 6 is used as the coating material in a tapered pressure unit at different temperatures and backpressures. The coating thickness and quality are also discussed for different wire speeds of up to 12 m/s. © 2006 Published by Elsevier B.V. Keywords: Hydrodynamic; Coating; Tapered bore pressure unit
1. Introduction Polymer coating is often applied to wires, strips, tubes or ropes for insulation or protection against corrosion. There are three different methods which are mostly used for this coating process. These methods are coaxial extrusion, dipping and electro-statical deposition process. The first two processes can be reasonably fast but bonding between the continuum and the coating material is not so strong. The third process offers much stronger bonding but is relatively slow. If the coating material can be forced onto the continuum uniformly the bonding can be improved significantly. However, a very high pressure is needed for the coating material to be forced. The hydrodynamic method is a process in which the relative velocity between the continuum and the polymer melt in a restricted space generates high pressure to create good bonding and the process also offers fast coating. The analysis for different coating processes could be found in the literature. In Refs. [1–4], coextrusion method for coating has been studied theoretically and experimentally. Refs. [5,6] present study of dip coating process. An experimental study of the electro-statical deposition process has been presented in Ref. [7]. For hydrodynamic technique, Thompson and Symmons [8] first experimented with polymer melt as a means of lubrication in ∗
Corresponding author. E-mail address: [email protected] (S. Akter).
0924-0136/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2006.02.011
wire drawing. Stevens [9] subsequently found that the polymer is very effective as lubricant and it also leaves a protective coating on the wire. Crampton et al. [10] further investigated plasto-hydrodynamic polymer lubrication of wire in drawing and coating. They showed in their analysis that the coating thickness on the wire depends on the wire speed, polymer melt viscosity and the die chamber configuration. This was further investigated by Parvinmehr et al. [11] who used a die-less unit smallest bore of which was slightly greater than the diameter of wire. For both of these works, the tests were carried out with wire of 1.6 mm diameter. Lamb and Hashmi [12] and Yu and Hashmi [13] reported the results of their works in relation to polymer coating of fine wires (Ø0.1–0.4 mm). They achieved fairly constant polymer coating for wire speeds of up to a maximum value of 0.6 m/s. Coating at this speed does not promise much for industrial scale production. The objectives of the present investigation are to achieve and assess the quality of coating in a tapered unit at a wire velocity in excess of 2 m/s. The effect of the polymer melt temperature and backpressure on the coating process is also investigated. 2. Experimental procedure and equipment Initially, theoretical studies were carried out to understand the hydrodynamic wire coating process and its principles. Based on the theoretical understanding of the process [14–17], the design of the rig in terms of the length of the melt chamber, wire preheating unit, argon backpressure, different pressure units, etc.,
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 174 (2006) 384–388
385
Fig. 1. Schematic diagram of hydrodynamic wire coating process. was completed. With this design, it was possible to obtain continuous and concentric coating on wire for speeds of up to 12 m/s. A brief description of this process is given here. The experimental set up consists of the drawing bench, the electrical installations, the wire feed mechanism, the drive system, the polymer feeding and melting unit and the pressure unit. A schematic diagram of the process is shown in Fig. 1. The polymer granules are poured in the hopper which is fitted with a heater band. The hopper is connected to the melt chamber and a gas bottle of pressurised argon is connected through a pressure line to the hopper which provides the backpressure in the polymer melt. The argon protects the polymer melt from thermal degradation and it also works against the irregularities of coated surface. During the coating process, the wire first enters the leakage control unit (which also acts as a wire preheating unit) attached to the melt chamber. The wire passes through the melt chamber and enters the pressure unit where coating forms on it due to the hydrodynamic pressure. The coated wire is then wound on the bull block which is driven by a continuously variable speed motor. To measure the drawing force during the wire drawing process a piezoelectric load cell is located at the exit end of the die unit. A proportional electric charge corresponding to the force is generated in the load cell during drawing and is converted by the charge amplifier into proportional voltage signal. The analog to digital converter receives the signal from the transducer via the charge amplifier. A computer digitizes the signal through a program written in BASIC language and displays the magnitude of the force through a printer output. For higher wire speeds, the polymer coating does not solidify completely by the time it reaches the bull block and the individual strands stick together and the polymer coating gets deformed. The sample of the coated wire is therefore taken from the part which does not reach the bull block after the wire is suddenly cut using a high speed snip. To measure the coating thickness, each sample is measured at five different positions along its length by a micrometer and the average diameter of the coated wire is noted. A small portion of the wire sample is cold mounted and polished. An optical microscope is then used to assess the actual coating thickness and the concentricity of the coating on the wire.
Fig. 2. Schematic diagram of a tapered bore pressure unit. Pressure unit characteristics: The schematic diagram of the pressure unit is shown in Fig. 2. The geometric parameters are presented as follows: Length of the unit, L = 22 mm Radial gap between the wire and the unit in the first part, h1 = 0.25 mm Radial gap between the wire and the unit in the second part, h2 = 0.0425 mm
3.2. Experimental work Experimental work was carried out at polymer melt temperatures of 230, 250 and 270 ◦ C. The backpressures of argon applied for each polymer melt temperature were 5 and 10 bar. For different combinations of polymer melt temperature and argon backpressure, experiments were carried out for wire velocities of 2, 4, 6, 8, 10 and 12 m/s. The results of these tests are presented in the following figures in terms of the coating thickness, drawing load, melt temperature, backpressure and drawing speed. Fig. 3 shows the coating thickness on the wire for velocities of up to 12 m/s for the polymer melt temperature 230 ◦ C.
3. Experimental results and discussions 3.1. Process parameters The process parameters for the investigations carried out were as follows: Polymer characteristics: Polymer type Polymer melt temperature
Durethan B 35F 9000/0 KYY (Nylon 6) 230–270 ◦ C
Wire characteristics: Wire diameter Wire material
0.7 mm Galvanized mild steel
Fig. 3. Coating thickness for polymer melt temperature of 230 ◦ C.
386
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 174 (2006) 384–388
Fig. 4. Coating thickness for polymer melt temperature of 250 ◦ C.
For the backpressure of 5 bar, the coating thickness at 2 m/s was 0.045 mm and at 12 m/s it was 0.0425 mm. At 10 bar backpressure, the coating thickness at 2 m/s was 0.053 mm and at 12 m/s it was 0.05 mm. From Figs. 4 and 5, it is apparent that at higher polymer melt temperatures viz. 250 and 270 ◦ C, the coating thickness decreases as the wire speed increases as in the case in Fig. 3. However, the overall coating thickness is higher for higher polymer melt temperatures. At polymer melt temperatures of 250 and 270 ◦ C, it was observed that a higher backpressure gives higher coating thickness (Figs. 4 and 5). As shown in Fig. 5, for 270 ◦ C temperature of the polymer melt, the backpressure seems to have no effect on the coating thickness at lower wire speeds. This type of thickening of the coating can be explained by the phenomena of die swelling an indication of the elasticity of the polymer. At the time of melt flows, the molecules slide past each other. The forces of attraction between the molecules cause
Fig. 5. Coating thickness for polymer melt temperature of 270 ◦ C.
a drag force, the magnitude of which is reflected in the viscosity of the melt. The chains of molten polymer with high-molecular weight tend to uncoil as they are sheared. But when the polymers enter a narrow capillary, because of high shear rate there is considerable uncoiling (orientation) and the oriented molecules then pass through the capillary. Two types of motion work on those molecules, one is the shear stress and the other is the Brownian motion, which tries to slide the molecules past each other. When the fluid is flowing along a channel which has uniform cross section (like the stepped parallel bore unit) then the fluid is subjected to shear stresses only. If the channel section changes (like the tapered bore unit) then tensile stresses will also be set up in the fluid. Therefore, the equivalent stress acting on the polymer melt in the case of a purely tapered unit is higher than any other units. They, when the polymer melt comes out from the pressure unit in the form of the coating, there is only Brownian motion as the stresses are absent outside the pressure unit. This Brownian motion expands or swells the coating in the radial direction and causes slight shrinkage in the longitudinal direction. The swelling increases with the increase in molecular weight and decreases for longer capillary length. Therefore, in this case, the swelling of the polymer coating is thought to be the combination of the following factors: (i) the tapered hydrodynamic pressure unit, (ii) the molecular weight of Nylon 6 – [NH(CH2 )5 CO]n – is higher than other conventional polymer used for hydrodynamic coating, and (iii) the length of the pressure unit is only 22 mm (the previous hydrodynamic coating researchers used units of length ranging from 57 to 180 mm). The argon backpressure adds up to the overall stress and therefore directly influences the swelling.
Fig. 6. Concentricity of coating on wire for wire velocity of 8 m/s, backpressure 5 bar and polymer melt temperature 270 ◦ C.
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 174 (2006) 384–388
387
Fig. 9. Drawing force for polymer melt temperature of 250 ◦ C.
Fig. 6 shows the coating on wire in a tapered unit for wire velocity of 8 m/s, backpressure 5 bar and polymer melt temperature 270 ◦ C. Fig. 7 shows the coating for wire velocity of 12 m/s, backpressure 10 bar and polymer melt temperature 250 ◦ C. The coating is reasonably concentric in both these cases. Fig. 8 shows the drawing force due to the pulling action of the wire through the for polymer melt at a temperature of 230 ◦ C. Experimental drawing forces for 5 and 10 bar backpressures have been plotted. The drawing force, which is a function of shear stress, changes with shear rate and viscosity. Though with the increase of shear rate the viscosity decreases, the overall effect is that the drawing force increases with the increase in the wire velocity. It also increases with the increase in argon
backpressure. For the backpressure of 10 bar, the drawing force is on average 30 N greater than for the backpressure of 5 bar. Fig. 9 presents the experimental drawing force for the polymer melt temperature of 250 ◦ C. It shows similar trend as in Fig. 8. Fig. 10 shows the experimental drawing force for polymer melt temperature of 270 ◦ C. However, it is evident from the above three figures that the increase in melt temperature in general causes the drawing load to decrease. As the temperature increase causes decrease in the viscosity of the polymer melt. It was found that no reduction in the cross sectional area of the wire took place and there was no discontinuity of the coating in any of these cases. For each of the test conditions, the adhesion of the coating to the wire was assessed qualitatively by attempting to scratch off the coating using a sharp-edged tool. It was quite difficult to debond the coating from the wire unless the tool edge sharpness is like that of a razor blade. As such, the coating applied should be good enough to withstand any
Fig. 8. Drawing force for polymer melt temperature of 230 ◦ C.
Fig. 10. Drawing force for polymer melt temperature of 270 ◦ C.
Fig. 7. Concentricity of coating on wire for wire velocity of 12 m/s, backpressure 10 bar and polymer melt temperature 250 ◦ C.
388
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 174 (2006) 384–388
subsequent processing steps involving contact with mechanical tools and dies. [8]
4. Conclusion [9]
Hydrodynamic wire coating with Nylon 6 using a tapered bore pressure unit has been presented. The experiments were carried out within the speed range between 2 and 12 m/s. The polymer coating on the wire is continuous and concentric for speeds of up to 12 m/s. The bonding quality of the coating with wire was very good. The coating thickness and drawing force in general is higher for higher backpressure. References [1] B. Caswell, R.I. Tanner, Wire coating die design using finite element methods, Polym. Eng. Sci. 18 (5(April)) (1978) 417–421. [2] C.L. Tucker, Computer Modelling for Polymer Processing, Hanser Publishers, Munich, Vienna, New York, 1989, pp. 311–317. [3] C.D. Han, D. Rao, Studies on wire coating extrusion. 1. The rheology of wire coating extrusion, J. Polym. Eng. Sci. 18 (October) (1978) 1019–1029. [4] S. Basu, A theoretical analysis of non-isothermal flow in wire-coating co-extrusion dies, J. Polym. Eng. Sci. 21 (December) (1981) 1128–1137. [5] H. Zhang, M.K. Moallemi, S. Kumar, Thermal analysis of the hot dipcoating process, journal of heat transfer in metals and container less processing and manufacturing, ASME (1991) 49–55. [6] S. Nakazawa, J.F.T. Pittman, A finite element system for analysis of melt flow and heat transfer in polymer process, in: Numerical Methods in Industrial Forming Process, Pineridge Press, Swansea, UK, 1982, pp. 523–533. [7] V.S. Mirnov, Increase of polymer coating wear resistance by electrophysical modification, in: P.K. Datta, J.S. Gray (Eds.), Surface Engineering:
[10]
[11]
[12]
[13] [14]
[15]
[16]
[17]
Engineering Applications, vol. II, The Royal Society of Chemistry, 1993, pp. 80–90. P.J. Thompson, G.R. Symmons, A plasto-hydrodynamic analysis of the lubrication and coating of wire using polymer melt during drawing, Proc. Inst. Mech. Eng. 191 (13) (1977) 115. A.J. Stevens, A plasto-hydrodynamic investigation of the lubrication and coating of wire using a polymer melt during drawing process. M.Phil. Thesis, Sheffield City Polytechnic, 1979. R. Crampton, G.R. Symmons, M.S.J. Hashmi, A non-Newtonian plastohydrodynamic analysis of the lubrication and coating of wire using a polymer melt during drawing, in: Proceedings of the International Symposium on Metal Working Lubrication, San Fransisco, USA, 1980. H. Parvinmehr, G.R. Symmons, M.S.J. Hashmi, A non-Newtonian plasto-hydrodynamic analysis of die-less wire drawing process using a stepped bore unit, Int. J. Mech. Sci. 29 (4) (1987) 239–257. R.E. Lamb, M.S.J. Hashmi, Polymer coating of superfine wires: a new technique to ensure quality, J. Mater. Process. Technol. 26 (1991) 197–205. J. Yu, M.S.J. Hashmi, Experimental results for polymer coating of fine wire, Key Eng. Mater. 86–87 (1993) 163–170. S. Akter, M.S.J. Hashmi, Modelling of velocity and temperature gradient boundary layer thickness for the drawing of a continuous wire through a polymer melt chamber, in: Proceeding of the 12th Conference of the Irish Manufacturing Committee (IMC 12), September 6–8, 1995, pp. 269–278. S. Akter, M.S.J. Hashmi, Formation and remelting of a solid polymer layer on the wire, in a polymer melt chamber, J. Mater. Process. Technol. 77 (1998) 314–318. S. Akter, M.S.J. Hashmi, Modelling for the pressure distribution within a hydrodynamic pressure unit: effect of the change in viscosity during drawing of wire coating, J. Mater. Process. Technol. 77 (1998) 32– 36. S. Akter, M.S.J. Hashmi, Finite element analysis for pressure and temperature distribution in a tapered hydrodynamic wire coating unit, in: Proceedings of the Application of Numerical Methods in Engineering (‘NUMETe-97), June 1997, pp. 224–232.
Hydrodynamic wire coating using a tapered bore pressure unit S. Akter ∗ , M.S.J. Hashmi Material Processing Research Centre, Dublin City University, Dublin 9, Ireland Received 25 January 2004; received in revised form 13 February 2006; accepted 14 February 2006
Abstract In the conventional process of wire coating, the wire is either dipped in the coating material or is coextruded along with the coating material. Apart from dipping or coextrusion methods, there is another innovative method called hydrodynamic coating in which the polymer melt is used as a pressure medium in a pressure unit. The wire size is less than the smallest bore of the pressure unit. The wire coating thickness and quality depend on the wire speed, polymer viscosity, polymer melt temperature and the gap between the wire and exit end of the die. In this paper, results of experimental investigation are presented by comparing the coating quality on galvanized mild steel wire using molten Nylon 6 is used as the coating material in a tapered pressure unit at different temperatures and backpressures. The coating thickness and quality are also discussed for different wire speeds of up to 12 m/s. © 2006 Published by Elsevier B.V. Keywords: Hydrodynamic; Coating; Tapered bore pressure unit
1. Introduction Polymer coating is often applied to wires, strips, tubes or ropes for insulation or protection against corrosion. There are three different methods which are mostly used for this coating process. These methods are coaxial extrusion, dipping and electro-statical deposition process. The first two processes can be reasonably fast but bonding between the continuum and the coating material is not so strong. The third process offers much stronger bonding but is relatively slow. If the coating material can be forced onto the continuum uniformly the bonding can be improved significantly. However, a very high pressure is needed for the coating material to be forced. The hydrodynamic method is a process in which the relative velocity between the continuum and the polymer melt in a restricted space generates high pressure to create good bonding and the process also offers fast coating. The analysis for different coating processes could be found in the literature. In Refs. [1–4], coextrusion method for coating has been studied theoretically and experimentally. Refs. [5,6] present study of dip coating process. An experimental study of the electro-statical deposition process has been presented in Ref. [7]. For hydrodynamic technique, Thompson and Symmons [8] first experimented with polymer melt as a means of lubrication in ∗
Corresponding author. E-mail address: [email protected] (S. Akter).
0924-0136/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2006.02.011
wire drawing. Stevens [9] subsequently found that the polymer is very effective as lubricant and it also leaves a protective coating on the wire. Crampton et al. [10] further investigated plasto-hydrodynamic polymer lubrication of wire in drawing and coating. They showed in their analysis that the coating thickness on the wire depends on the wire speed, polymer melt viscosity and the die chamber configuration. This was further investigated by Parvinmehr et al. [11] who used a die-less unit smallest bore of which was slightly greater than the diameter of wire. For both of these works, the tests were carried out with wire of 1.6 mm diameter. Lamb and Hashmi [12] and Yu and Hashmi [13] reported the results of their works in relation to polymer coating of fine wires (Ø0.1–0.4 mm). They achieved fairly constant polymer coating for wire speeds of up to a maximum value of 0.6 m/s. Coating at this speed does not promise much for industrial scale production. The objectives of the present investigation are to achieve and assess the quality of coating in a tapered unit at a wire velocity in excess of 2 m/s. The effect of the polymer melt temperature and backpressure on the coating process is also investigated. 2. Experimental procedure and equipment Initially, theoretical studies were carried out to understand the hydrodynamic wire coating process and its principles. Based on the theoretical understanding of the process [14–17], the design of the rig in terms of the length of the melt chamber, wire preheating unit, argon backpressure, different pressure units, etc.,
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 174 (2006) 384–388
385
Fig. 1. Schematic diagram of hydrodynamic wire coating process. was completed. With this design, it was possible to obtain continuous and concentric coating on wire for speeds of up to 12 m/s. A brief description of this process is given here. The experimental set up consists of the drawing bench, the electrical installations, the wire feed mechanism, the drive system, the polymer feeding and melting unit and the pressure unit. A schematic diagram of the process is shown in Fig. 1. The polymer granules are poured in the hopper which is fitted with a heater band. The hopper is connected to the melt chamber and a gas bottle of pressurised argon is connected through a pressure line to the hopper which provides the backpressure in the polymer melt. The argon protects the polymer melt from thermal degradation and it also works against the irregularities of coated surface. During the coating process, the wire first enters the leakage control unit (which also acts as a wire preheating unit) attached to the melt chamber. The wire passes through the melt chamber and enters the pressure unit where coating forms on it due to the hydrodynamic pressure. The coated wire is then wound on the bull block which is driven by a continuously variable speed motor. To measure the drawing force during the wire drawing process a piezoelectric load cell is located at the exit end of the die unit. A proportional electric charge corresponding to the force is generated in the load cell during drawing and is converted by the charge amplifier into proportional voltage signal. The analog to digital converter receives the signal from the transducer via the charge amplifier. A computer digitizes the signal through a program written in BASIC language and displays the magnitude of the force through a printer output. For higher wire speeds, the polymer coating does not solidify completely by the time it reaches the bull block and the individual strands stick together and the polymer coating gets deformed. The sample of the coated wire is therefore taken from the part which does not reach the bull block after the wire is suddenly cut using a high speed snip. To measure the coating thickness, each sample is measured at five different positions along its length by a micrometer and the average diameter of the coated wire is noted. A small portion of the wire sample is cold mounted and polished. An optical microscope is then used to assess the actual coating thickness and the concentricity of the coating on the wire.
Fig. 2. Schematic diagram of a tapered bore pressure unit. Pressure unit characteristics: The schematic diagram of the pressure unit is shown in Fig. 2. The geometric parameters are presented as follows: Length of the unit, L = 22 mm Radial gap between the wire and the unit in the first part, h1 = 0.25 mm Radial gap between the wire and the unit in the second part, h2 = 0.0425 mm
3.2. Experimental work Experimental work was carried out at polymer melt temperatures of 230, 250 and 270 ◦ C. The backpressures of argon applied for each polymer melt temperature were 5 and 10 bar. For different combinations of polymer melt temperature and argon backpressure, experiments were carried out for wire velocities of 2, 4, 6, 8, 10 and 12 m/s. The results of these tests are presented in the following figures in terms of the coating thickness, drawing load, melt temperature, backpressure and drawing speed. Fig. 3 shows the coating thickness on the wire for velocities of up to 12 m/s for the polymer melt temperature 230 ◦ C.
3. Experimental results and discussions 3.1. Process parameters The process parameters for the investigations carried out were as follows: Polymer characteristics: Polymer type Polymer melt temperature
Durethan B 35F 9000/0 KYY (Nylon 6) 230–270 ◦ C
Wire characteristics: Wire diameter Wire material
0.7 mm Galvanized mild steel
Fig. 3. Coating thickness for polymer melt temperature of 230 ◦ C.
386
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 174 (2006) 384–388
Fig. 4. Coating thickness for polymer melt temperature of 250 ◦ C.
For the backpressure of 5 bar, the coating thickness at 2 m/s was 0.045 mm and at 12 m/s it was 0.0425 mm. At 10 bar backpressure, the coating thickness at 2 m/s was 0.053 mm and at 12 m/s it was 0.05 mm. From Figs. 4 and 5, it is apparent that at higher polymer melt temperatures viz. 250 and 270 ◦ C, the coating thickness decreases as the wire speed increases as in the case in Fig. 3. However, the overall coating thickness is higher for higher polymer melt temperatures. At polymer melt temperatures of 250 and 270 ◦ C, it was observed that a higher backpressure gives higher coating thickness (Figs. 4 and 5). As shown in Fig. 5, for 270 ◦ C temperature of the polymer melt, the backpressure seems to have no effect on the coating thickness at lower wire speeds. This type of thickening of the coating can be explained by the phenomena of die swelling an indication of the elasticity of the polymer. At the time of melt flows, the molecules slide past each other. The forces of attraction between the molecules cause
Fig. 5. Coating thickness for polymer melt temperature of 270 ◦ C.
a drag force, the magnitude of which is reflected in the viscosity of the melt. The chains of molten polymer with high-molecular weight tend to uncoil as they are sheared. But when the polymers enter a narrow capillary, because of high shear rate there is considerable uncoiling (orientation) and the oriented molecules then pass through the capillary. Two types of motion work on those molecules, one is the shear stress and the other is the Brownian motion, which tries to slide the molecules past each other. When the fluid is flowing along a channel which has uniform cross section (like the stepped parallel bore unit) then the fluid is subjected to shear stresses only. If the channel section changes (like the tapered bore unit) then tensile stresses will also be set up in the fluid. Therefore, the equivalent stress acting on the polymer melt in the case of a purely tapered unit is higher than any other units. They, when the polymer melt comes out from the pressure unit in the form of the coating, there is only Brownian motion as the stresses are absent outside the pressure unit. This Brownian motion expands or swells the coating in the radial direction and causes slight shrinkage in the longitudinal direction. The swelling increases with the increase in molecular weight and decreases for longer capillary length. Therefore, in this case, the swelling of the polymer coating is thought to be the combination of the following factors: (i) the tapered hydrodynamic pressure unit, (ii) the molecular weight of Nylon 6 – [NH(CH2 )5 CO]n – is higher than other conventional polymer used for hydrodynamic coating, and (iii) the length of the pressure unit is only 22 mm (the previous hydrodynamic coating researchers used units of length ranging from 57 to 180 mm). The argon backpressure adds up to the overall stress and therefore directly influences the swelling.
Fig. 6. Concentricity of coating on wire for wire velocity of 8 m/s, backpressure 5 bar and polymer melt temperature 270 ◦ C.
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 174 (2006) 384–388
387
Fig. 9. Drawing force for polymer melt temperature of 250 ◦ C.
Fig. 6 shows the coating on wire in a tapered unit for wire velocity of 8 m/s, backpressure 5 bar and polymer melt temperature 270 ◦ C. Fig. 7 shows the coating for wire velocity of 12 m/s, backpressure 10 bar and polymer melt temperature 250 ◦ C. The coating is reasonably concentric in both these cases. Fig. 8 shows the drawing force due to the pulling action of the wire through the for polymer melt at a temperature of 230 ◦ C. Experimental drawing forces for 5 and 10 bar backpressures have been plotted. The drawing force, which is a function of shear stress, changes with shear rate and viscosity. Though with the increase of shear rate the viscosity decreases, the overall effect is that the drawing force increases with the increase in the wire velocity. It also increases with the increase in argon
backpressure. For the backpressure of 10 bar, the drawing force is on average 30 N greater than for the backpressure of 5 bar. Fig. 9 presents the experimental drawing force for the polymer melt temperature of 250 ◦ C. It shows similar trend as in Fig. 8. Fig. 10 shows the experimental drawing force for polymer melt temperature of 270 ◦ C. However, it is evident from the above three figures that the increase in melt temperature in general causes the drawing load to decrease. As the temperature increase causes decrease in the viscosity of the polymer melt. It was found that no reduction in the cross sectional area of the wire took place and there was no discontinuity of the coating in any of these cases. For each of the test conditions, the adhesion of the coating to the wire was assessed qualitatively by attempting to scratch off the coating using a sharp-edged tool. It was quite difficult to debond the coating from the wire unless the tool edge sharpness is like that of a razor blade. As such, the coating applied should be good enough to withstand any
Fig. 8. Drawing force for polymer melt temperature of 230 ◦ C.
Fig. 10. Drawing force for polymer melt temperature of 270 ◦ C.
Fig. 7. Concentricity of coating on wire for wire velocity of 12 m/s, backpressure 10 bar and polymer melt temperature 250 ◦ C.
388
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 174 (2006) 384–388
subsequent processing steps involving contact with mechanical tools and dies. [8]
4. Conclusion [9]
Hydrodynamic wire coating with Nylon 6 using a tapered bore pressure unit has been presented. The experiments were carried out within the speed range between 2 and 12 m/s. The polymer coating on the wire is continuous and concentric for speeds of up to 12 m/s. The bonding quality of the coating with wire was very good. The coating thickness and drawing force in general is higher for higher backpressure. References [1] B. Caswell, R.I. Tanner, Wire coating die design using finite element methods, Polym. Eng. Sci. 18 (5(April)) (1978) 417–421. [2] C.L. Tucker, Computer Modelling for Polymer Processing, Hanser Publishers, Munich, Vienna, New York, 1989, pp. 311–317. [3] C.D. Han, D. Rao, Studies on wire coating extrusion. 1. The rheology of wire coating extrusion, J. Polym. Eng. Sci. 18 (October) (1978) 1019–1029. [4] S. Basu, A theoretical analysis of non-isothermal flow in wire-coating co-extrusion dies, J. Polym. Eng. Sci. 21 (December) (1981) 1128–1137. [5] H. Zhang, M.K. Moallemi, S. Kumar, Thermal analysis of the hot dipcoating process, journal of heat transfer in metals and container less processing and manufacturing, ASME (1991) 49–55. [6] S. Nakazawa, J.F.T. Pittman, A finite element system for analysis of melt flow and heat transfer in polymer process, in: Numerical Methods in Industrial Forming Process, Pineridge Press, Swansea, UK, 1982, pp. 523–533. [7] V.S. Mirnov, Increase of polymer coating wear resistance by electrophysical modification, in: P.K. Datta, J.S. Gray (Eds.), Surface Engineering:
[10]
[11]
[12]
[13] [14]
[15]
[16]
[17]
Engineering Applications, vol. II, The Royal Society of Chemistry, 1993, pp. 80–90. P.J. Thompson, G.R. Symmons, A plasto-hydrodynamic analysis of the lubrication and coating of wire using polymer melt during drawing, Proc. Inst. Mech. Eng. 191 (13) (1977) 115. A.J. Stevens, A plasto-hydrodynamic investigation of the lubrication and coating of wire using a polymer melt during drawing process. M.Phil. Thesis, Sheffield City Polytechnic, 1979. R. Crampton, G.R. Symmons, M.S.J. Hashmi, A non-Newtonian plastohydrodynamic analysis of the lubrication and coating of wire using a polymer melt during drawing, in: Proceedings of the International Symposium on Metal Working Lubrication, San Fransisco, USA, 1980. H. Parvinmehr, G.R. Symmons, M.S.J. Hashmi, A non-Newtonian plasto-hydrodynamic analysis of die-less wire drawing process using a stepped bore unit, Int. J. Mech. Sci. 29 (4) (1987) 239–257. R.E. Lamb, M.S.J. Hashmi, Polymer coating of superfine wires: a new technique to ensure quality, J. Mater. Process. Technol. 26 (1991) 197–205. J. Yu, M.S.J. Hashmi, Experimental results for polymer coating of fine wire, Key Eng. Mater. 86–87 (1993) 163–170. S. Akter, M.S.J. Hashmi, Modelling of velocity and temperature gradient boundary layer thickness for the drawing of a continuous wire through a polymer melt chamber, in: Proceeding of the 12th Conference of the Irish Manufacturing Committee (IMC 12), September 6–8, 1995, pp. 269–278. S. Akter, M.S.J. Hashmi, Formation and remelting of a solid polymer layer on the wire, in a polymer melt chamber, J. Mater. Process. Technol. 77 (1998) 314–318. S. Akter, M.S.J. Hashmi, Modelling for the pressure distribution within a hydrodynamic pressure unit: effect of the change in viscosity during drawing of wire coating, J. Mater. Process. Technol. 77 (1998) 32– 36. S. Akter, M.S.J. Hashmi, Finite element analysis for pressure and temperature distribution in a tapered hydrodynamic wire coating unit, in: Proceedings of the Application of Numerical Methods in Engineering (‘NUMETe-97), June 1997, pp. 224–232.