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Wire coating using different sizes of stepped parallel bore pressure units
Progress in Organic Coatings 51 (2004) 67–76
Wire coating using different sizes of stepped parallel bore pressure units S. Akter∗ , M.S.J. Hashmi Material Processing Research Centre, Dublin City University, Dublin 9, Ireland Received 23 February 2004; accepted 15 June 2004
Abstract Polymer coating on wire gives protection against corrosion. The dip coating or the co-axial extruded coating can be reasonably fast but the bonding between the wire and the coating material is not so strong. The bonding can be improved by applying the coating material on the wire under pressure. Hydrodynamic coating is such a technique where it is possible to achieve consistent coating with strong bond. In this process, the wire is pulled through an orifice of stepped bore which is filled with a polymer melt. The pulling action of the wire through the viscoelastic fluid gives rise to drag force and generates hydrodynamic pressure, the magnitude of which depends on the type of the pressure medium, melt temperature, wire speed and the geometry of the unit. The objectives of the present investigation are to achieve and assess the quality of coating at wire velocities of between 1 and 12 m/s using three different sizes of stepped parallel bore pressure units. The effects of the polymer melt temperature and back pressure on the coating process are also investigated. © 2004 Published by Elsevier B.V. Keywords: Hydrodynamic; Coating; Stepped parallel bore pressure unit
1. Introduction Hydrodynamic wire coating using a die-less drawing unit was first carried out by Parvinmehr et al. [1–4] who used a die-less drawing unit whose smallest bore was slightly greater than the diameter of the undeformed wire. For their work the tests were carried out with wire of 1.6 mm diameter. Relatively recently Symmons et al. [5] reported the results of their work on coating of fine wire (diameter 0.45 mm) in relation to plasto-hydrodynamic drawing and coating. The drawing speed range was from 0.05 to 0.8 m/s. Also Lamb and Hashmi [6] and Yu and Hashmi [7] carried out their research for polymer coating of fine wires (diameter 0.1–0.4 mm). They achieved a fairly constant polymer coating for wire speeds of up to a maximum of 0.6 m/s. In all these works, the wire speed in the process was rather low. Coating at this speed is not appropriate for industrial scale production. ∗ Corresponding author. E-mail address: [email protected] (S. Akter).
0300-9440/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.porgcoat.2004.06.003
The main objective of this paper is to present the quality of coating at high wire velocities of between 1 and 12 m/s. Results of experimental investigation are presented comparing the coating quality on galvanized mild steel wire when molten Nylon 6 is used as the coating material at different temperatures and back pressures for three different sizes of stepped parallel bore pressure units.
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 [8–14], the design of the rig in terms of the length of the melt chamber, wire preheating unit, argon back pressure, different pressure units etc. 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.
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scope is then used to assess the actual coating thickness and the concentricity of the coating on the wire.
3. Experimental results and discussions 3.1. Process parameters The process parameters for the investigations carried out were as follows. 3.1.1. Polymer characteristics Polymer type Fig. 1. Schematic diagram of hydrodynamic wire coating process.
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 back pressure 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 at first the wire 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 piezo-electric 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 micro-
Polymer melt temperature
Durethan B 35F 9000/0 KYY (Nylon 6) 230–270 ◦ C
3.1.2. Wire characteristics Wire diameter Wire material
0.7 mm Galvanized mild steel
3.1.3. Pressure unit characteristics Three different types of stepped parallel bore pressure units were used to carry out the experiments. The schematic diagram of the pressure unit is shown in Fig. 2. The total length of all the pressure units was 22 mm. Other geometric parameters are presented as follows: Length of the first part (before step), L1 = 17 mm; Length of the second part (after step), L2 = 5 mm; Radial gap between the wire and the pressure unit in the first part, h1 = 0.5 mm; Radial gap between the wire and the pressure unit in the second part, h2 = 0.05, 0.12 and 0.03 mm. 3.2. Experimental works Experimental work was carried out at polymer melt temperatures of 230, 250 and 270 ◦ C. The two back pressures of argon applied in each case were 5 and 10 bar. For different combinations of polymer melt temperature and argon back
Fig. 2. Schematic diagram of a stepped parallel bore pressure unit.
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Fig. 3. Coating thickness for polymer melt temperature of 230 ◦ C (h2 = 0.05 mm).
pressure experiments were carried out for wire velocities of 2, 4, 6, 8, 10 and 12 m/s for all pressure units except stepped parallel bore pressure unit with h2 equal to 0.05 mm. For this unit wire velocities were 1, 4, 6, 8, 10, 12 and 12.5 m/s and the argon back pressures were 0, 5 and 10 bar. The results of these tests are presented in the following figures in terms of the coating thickness, drawing load, melt temperature, back pressure and drawing speed. 3.2.1. Stepped parallel bore pressure unit (h2 = 0.05 mm) Fig. 3 shows the thickness of Nylon 6 coating at the melt temperature of 230 ◦ C. From the graph it can be seen that the coating thickness is uniform at about 0.05 mm for velocities of up to 12m/s. Visual inspection of the coating also showed that the coating was continuous. The coating thickness of 0.05 mm is equal to the radial gap between the wire and the exit of the pressure unit. In these experiments it was observed that there was no deformation of the wire, as such theoretically the coating thickness should be equal to the gap between the wire and the exit of the pressure unit. The experimental results confirm this. It is also evident from the figure that the thickness and continuity of the polymer coating on the wire are not affected by the back pressure of the polymer melt. The results in Fig. 4 show the coating thickness and continuity of Nylon 6 at the melt temperature of 250 ◦ C. For
Fig. 4. Coating thickness for polymer melt temperature of 250 ◦ C (h2 = 0.05 mm).
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Fig. 5. Coating thickness for polymer melt temperature of 270 ◦ C (h2 = 0.05 mm).
Plate 1. Concentricity of coating on wire for wire velocity of 4 m/s, back pressure 5 bar and polymer melt temperature 270 ◦ C (h2 = 0.05 mm).
argon pressures of 5 and 10 bar as well as without any back pressure tests were carried out at up to 12.5 m/s. The coating thickness was constant at 0.05 mm for wire velocities of up to 6 m/s for no back pressure. At wire velocity of 12 m/s it was 0.048 mm. At 5 bar back pressure the coating thickness was 0.05 mm up to 12 m/s. However at 12.5 m/s the coating thickness was found to be 0.049 mm. This decrease in coating thickness is thought to be due to two reasons - (i) absence or lack of back pressure means less pressure on the melt layer for adequate coating formation, (ii) inelastic response of the polymer material that works against die swell elastic behaviour. For the argon pressure of 10 bar the coating was continuous within the entire velocity range and the coating thickness was uniform at 0.05 mm. With all these back pressure cases the coating was found to be continuous. Fig. 5 shows the coating thickness for Nylon 6 at the melt temperature of 270 ◦ C. These tests were carried out at wire velocities of up to 12.5 m/s for argon pressure of 5 and 10 bar as well as at zero back pressure. The coating was continuous and the thickness was uniform at 0.05 mm over the entire range of velocities except for at zero back pressure. Argon creates inert environment for the polymer melt and with zero back pressure the Nylon 6 melt started to burn as the melt temperature increased beyond 230 ◦ C. This degradation of polymer led to slightly lower quality coating at higher wire velocities. However, the coating on the polymer for different ranges of velocities, back pressures and melt temperatures appeared to be concentric. Some samples were prepared and polished for examination under an optical microscope. The cross section of the wire was observed to be circular and the coat-
ing was reasonably concentric. Plate 1 shows the quality of concentricity of the coating on wire with 5 bar back pressure and wire velocity of 4 m/s at 270 ◦ C. Plate 2 shows the concentricity of coating with zero back pressure at 230 ◦ C and wire velocity of 10 m/s. Results from these plates show that the coating is reasonably concentric. Plate 3 illustrates the coating concentricity at 250 ◦ C for zero back pressure and wire velocity of 12 m/s which appears to be slightly eccentric.
Plate 2. Concentricity of coating on wire for wire velocity of 10 m/s, back pressure 0 bar and polymer melt temperature 230 ◦ C (h2 = 0.05 mm).
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Plate 3. Concentricity of coating on wire for wire velocity of 12 m/s, back pressure 0 bar and polymer melt temperature 250 ◦ C (h2 = 0.05 mm).
Fig. 6 shows the drawing force during the coating process for the polymer melt temperature of 230 ◦ C. The drawing force generally increases with the increase in the wire velocity. Also it increases with the increase in argon back pressure.
Fig. 7. Drawing force for polymer melt temperature of 250 ◦ C (h2 = 0.05 mm).
For the back pressure of 10 bar the drawing force is on average about 25 and 50 N greater than that for 5 bar and zero back pressure respectively. Fig. 7 shows the drawing force for the polymer melt temperature of 250 ◦ C. It shows a similar trend as in Fig. 6. Fig. 8 shows the drawing force for the polymer melt temperature of 270 ◦ C. The drawing force increases for up to 7 m/s drawing speed after which it decreases. This is thought to be due to reaching critical shear stress that causes slip at the surface of the wire and the polymer melt. It is evident from the there three figures that the increase in melt temperature in general causes the drawing load to decrease. This is because as the melt temperature increases the viscosity of the polymer melt decreases, which leads to a decrease in the shear stress and hence in the drawing force.
Fig. 6. Drawing force for polymer melt temperature of 230 ◦ C (h2 = 0.05 mm).
3.2.2. Stepped parallel bore pressure unit (h2 = 0.12 mm) The results in Fig. 9 show the coating thickness and continuity with Nylon 6 at the melt temperature of 230 ◦ C. For argon pressures of 5 and 10 bar the test was carried out at up to 12 m/s. The coating thickness was about 0.09 mm for wire velocities of up to 12 m/s for both back pressures. The coating thickness was smaller than the exit gap between the wire and the unit. The reasons for the thinner coating could be (i) due to formation of an initially melt deposit at the edge of the pressure unit which solidifies and may partially obstruct
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Fig. 8. Drawing force for polymer melt temperature of 0.05 mm).
270 ◦ C
(h2 =
Fig. 10. Coating thickness for polymer melt temperature of 250 ◦ C (h2 = 0.12 mm).
the exit flow, (ii) the result of inelastic behaviour of melt flow or (iii) as the exit gap between this pressure unit and wire (0.12 mm) was larger than other units (0.05 and 0.03 mm) it took more time to solidify the coating and the pulling action of the coated wire might drawdown the coating. Figs. 10 and 11 presents similarly decrease in the coating thickness for higher polymer melt temperatures viz. 250 and 270 ◦ C respectively. With all these back pressures and all these temperatures the coating was found to be continuous. Plate 4 shows the quality of concentricity of coating at 250 ◦ C and wire velocity of 12 m/s. The coating thickness is almost uniform. Figs. 12–14 show the drawing force generated due to the pulling action of the wire through the pressure medium for polymer melt temperatures of 230, 250 and 270 ◦ C respectively. With the increase in wire velocity the drawing force also increases except for Fig. 14 where it decreases after 8 m/s which is also thought to be due to reaching the critical shear stress of the polymer melt. The drawing force increases with the increase in argon back pressure.
Fig. 9. Coating thickness for polymer melt temperature of 230 ◦ C (h2 = 0.12 mm).
3.2.3. Stepped parallel bore pressure unit (h2 = 0.03 mm) Tests were also carried out with a stepped parallel bore pressure unit with a very narrow exit gap (h2 = 0.03 mm) for both back pressures, 5 and 10 bar, and for polymer melt temperatures of 230, 250 and 270 ◦ C. Although in all these tests the coating appeared to be continuous, under the microscope the coating was found to be broken in some cases
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Fig. 11. Coating thickness for polymer melt temperature of 270 ◦ C (h2 = 0.12 mm).
where the coating was not reasonably concentric. This may have resulted during the preparation of the sample. Fig. 15 shows the coating thickness and continuity with Nylon 6 at the melt temperature of 230 ◦ C. The coating thickness was about 0.025 and 0.026 mm for wire velocities of up to 6 m/s for the argon back pressure of 5 and 10 bar respectively. At 12 m/s the coating thickness was about 0.022 mm for both the back pressures. The coating thickness was smaller than
Plate 4. Concentricity of coating on wire for wire velocity of 12 m/s, back pressure 10 bar and polymer melt temperature 250 ◦ C (h2 = 0.12 mm).
Fig. 12. Drawing force for polymer melt temperature of 230 ◦ C (h2 = 0.12 mm).
Fig. 13. Drawing force for polymer melt temperature of 250 ◦ C (h2 = 0.12 mm).
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Fig. 16. Coating thickness for polymer melt temperature of 250 ◦ C (h2 = 0.03 mm). Fig. 14. Drawing force for polymer melt temperature of 270 ◦ C (h2 = 0.12 mm).
Fig. 15. Coating thickness for polymer melt temperature of 230 ◦ C (h2 = 0.03 mm).
the exit gap between the wire and the unit. The exit gap between the wire and the unit was so small that it scratched the coating in some cases. It is evident that at the back pressure 10 bar the coating thickness seems to be slightly thicker than back pressure 5 bar. Fig. 16 shows the coating thickness for different velocities at 250 ◦ C. At 5 bar back pressure the coating thickness is 0.023 mm for wire velocity of up to 8 m/s and at 12 m/s it is 0.021 mm. Also at 10 bar for velocity of up to 8 m/s it is 0.024 mm and at 12 m/s wire speed it is 0.022 mm. Fig. 17 shows the coating thickness at polymer melt temperature of 270 ◦ C. For both the back pressures the coating thickness at 2 m/s velocity is 0.023 mm and at 12 m/s the thickness is 0.022 mm. Plate 5 shows the quality of concentricity of coating at 270 ◦ C, back pressure 10 bar and wire velocity 12 m/s. The coating thickness is not radially uniform, possibly because of the narrow gap between the wire and the pressure unit and the resulting restricted flow. Figs. 18–20 show the drawing force generated due to the pulling action of the wire through the pressure medium for polymer melt temperatures of 230, 250 and 270 ◦ C respectively. With the increase in the wire velocity the drawing force also increases generally except for Fig. 18 where it decreases after 6 m/s. The drawing force is generally higher for higher argon back pressure. It may be mentioned again that no reduction in the cross sectional area of the wire was observed 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
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Fig. 17. Coating thickness for polymer melt temperature of 270 ◦ C (h2 = 0.03 mm).
using a sharp edged knife or cutting 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 subsequent processing steps involving contact with mechanical tools and dies.
Fig. 18. Drawing force for polymer melt temperature of 230 ◦ C (h2 = 0.03 mm).
Plate 5. Concentricity of coating on wire for wire velocity of 12 m/s, back pressure 10 bar and polymer melt temperature 270 ◦ C (h2 = 0.03 mm).
Fig. 19. Drawing force for polymer melt temperature of 250 ◦ C (h2 = 0.03 mm).
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References
Fig. 20. Drawing force for polymer melt temperature of 270 ◦ C (h2 = 0.03 mm).
4. Conclusion Experimental results of hydrodynamic high-speed wire coating with Nylon 6 have been presented. Due to some limitation in the present experimental set up the drawing speed is limited to about 13 m/s. Therefore, experiments were carried out within the speed range of up to 12 m/s. The polymer coating on the wire is continuous for speeds of up to 12 m/s. The bonding quality of the coating with wire was found to be very good. Concentricity is better in the case of pressure unit with h2 equal to 0.05 and 0.12 mm but poor for the pressure unit with h2 equal to 0.03 mm. Application of back pressure generally improves the quality of the coating.
[1] 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. [2] M.S.J. Hashmi, G.R. Symmons, H. Parvinmehr, A Novel technique of wire drawing, J. Mech. Eng. Sci. Inst. Mech. Engrs. 24 (1982). [3] G.R. Symmons, M.S.J. Hashmi, H. Parvinmehr, Plasto-hydrodynamic die less wire drawing: theoretical treatment and experimental results, in: Proceeding of International Conference on Developments in Drawing Metals, Metals Society, London, May 1983, pp. 54–62. [4] G.R. Symmons, M.S.J. Hashmi, H. Parvinmehr, Aspects of product quality and process control in plasto-hydrodynamic die-less wire drawing, in: Proceeding of 1st Conference on Manufacturing Technology, Irish Manufacturing Committee, Dublin, March 1984, pp. 153–172. [5] G.R. Symmons, A.H. Memon, Z. Ming, M.S.J. Hashmi, Polymer coating of wire using a die-less drawing process, J. Mater. Process. Technol. 26 (1991) 173–180. [6] 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. [7] J. Yu, M.S.J. Hashmi, Experimental results for polymer coating of fine wires, Key Eng. Mater. 86–87 (1993) 163–170. [8] 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 Twelfth Conference of the Irish Manufacturing Committee (IMC 12), 6–8, September 1995, pp. 269–278. [9] 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. [10] S. Akter, M.S.J. Hashmi, Modelling for the pressure distribution within a hydrodynamic unit: effect of the change in viscosity during drawing of wire coating, J. Mater. Process. Technol. 77 (1998) 32–36. [11] S. Akter, M.S.J. Hashmi, Modelling the pressure distribution within a combined geometry hydrodynamic pressure unit during wire coating process, in: Proceedings of the XIIth International Congress on Rheology, Canada, 18–23 August 1996, pp. 689–690. [12] S. Akter, M.S.J. Hashmi, Finite element temperature and pressure analysis for pressure distribution in a combined hydrodynamic unit, Design and Control (PEDAC’97), Egypt, February 1997, pp. 41–50. [13] 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), 1997, pp. 224–232. [14] S. Akter, M.S.J. Hashmi, Plasto-hydrodynamic Nylon coating of wire and prediction of pressure inside the stepped parallel bore unit, in: Thirteenth Conference of the Irish Manufacturing Committee (IMC 13), Limerick, 1996, pp. 123–134.
Wire coating using different sizes of stepped parallel bore pressure units S. Akter∗ , M.S.J. Hashmi Material Processing Research Centre, Dublin City University, Dublin 9, Ireland Received 23 February 2004; accepted 15 June 2004
Abstract Polymer coating on wire gives protection against corrosion. The dip coating or the co-axial extruded coating can be reasonably fast but the bonding between the wire and the coating material is not so strong. The bonding can be improved by applying the coating material on the wire under pressure. Hydrodynamic coating is such a technique where it is possible to achieve consistent coating with strong bond. In this process, the wire is pulled through an orifice of stepped bore which is filled with a polymer melt. The pulling action of the wire through the viscoelastic fluid gives rise to drag force and generates hydrodynamic pressure, the magnitude of which depends on the type of the pressure medium, melt temperature, wire speed and the geometry of the unit. The objectives of the present investigation are to achieve and assess the quality of coating at wire velocities of between 1 and 12 m/s using three different sizes of stepped parallel bore pressure units. The effects of the polymer melt temperature and back pressure on the coating process are also investigated. © 2004 Published by Elsevier B.V. Keywords: Hydrodynamic; Coating; Stepped parallel bore pressure unit
1. Introduction Hydrodynamic wire coating using a die-less drawing unit was first carried out by Parvinmehr et al. [1–4] who used a die-less drawing unit whose smallest bore was slightly greater than the diameter of the undeformed wire. For their work the tests were carried out with wire of 1.6 mm diameter. Relatively recently Symmons et al. [5] reported the results of their work on coating of fine wire (diameter 0.45 mm) in relation to plasto-hydrodynamic drawing and coating. The drawing speed range was from 0.05 to 0.8 m/s. Also Lamb and Hashmi [6] and Yu and Hashmi [7] carried out their research for polymer coating of fine wires (diameter 0.1–0.4 mm). They achieved a fairly constant polymer coating for wire speeds of up to a maximum of 0.6 m/s. In all these works, the wire speed in the process was rather low. Coating at this speed is not appropriate for industrial scale production. ∗ Corresponding author. E-mail address: [email protected] (S. Akter).
0300-9440/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.porgcoat.2004.06.003
The main objective of this paper is to present the quality of coating at high wire velocities of between 1 and 12 m/s. Results of experimental investigation are presented comparing the coating quality on galvanized mild steel wire when molten Nylon 6 is used as the coating material at different temperatures and back pressures for three different sizes of stepped parallel bore pressure units.
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 [8–14], the design of the rig in terms of the length of the melt chamber, wire preheating unit, argon back pressure, different pressure units etc. 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.
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scope is then used to assess the actual coating thickness and the concentricity of the coating on the wire.
3. Experimental results and discussions 3.1. Process parameters The process parameters for the investigations carried out were as follows. 3.1.1. Polymer characteristics Polymer type Fig. 1. Schematic diagram of hydrodynamic wire coating process.
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 back pressure 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 at first the wire 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 piezo-electric 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 micro-
Polymer melt temperature
Durethan B 35F 9000/0 KYY (Nylon 6) 230–270 ◦ C
3.1.2. Wire characteristics Wire diameter Wire material
0.7 mm Galvanized mild steel
3.1.3. Pressure unit characteristics Three different types of stepped parallel bore pressure units were used to carry out the experiments. The schematic diagram of the pressure unit is shown in Fig. 2. The total length of all the pressure units was 22 mm. Other geometric parameters are presented as follows: Length of the first part (before step), L1 = 17 mm; Length of the second part (after step), L2 = 5 mm; Radial gap between the wire and the pressure unit in the first part, h1 = 0.5 mm; Radial gap between the wire and the pressure unit in the second part, h2 = 0.05, 0.12 and 0.03 mm. 3.2. Experimental works Experimental work was carried out at polymer melt temperatures of 230, 250 and 270 ◦ C. The two back pressures of argon applied in each case were 5 and 10 bar. For different combinations of polymer melt temperature and argon back
Fig. 2. Schematic diagram of a stepped parallel bore pressure unit.
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Fig. 3. Coating thickness for polymer melt temperature of 230 ◦ C (h2 = 0.05 mm).
pressure experiments were carried out for wire velocities of 2, 4, 6, 8, 10 and 12 m/s for all pressure units except stepped parallel bore pressure unit with h2 equal to 0.05 mm. For this unit wire velocities were 1, 4, 6, 8, 10, 12 and 12.5 m/s and the argon back pressures were 0, 5 and 10 bar. The results of these tests are presented in the following figures in terms of the coating thickness, drawing load, melt temperature, back pressure and drawing speed. 3.2.1. Stepped parallel bore pressure unit (h2 = 0.05 mm) Fig. 3 shows the thickness of Nylon 6 coating at the melt temperature of 230 ◦ C. From the graph it can be seen that the coating thickness is uniform at about 0.05 mm for velocities of up to 12m/s. Visual inspection of the coating also showed that the coating was continuous. The coating thickness of 0.05 mm is equal to the radial gap between the wire and the exit of the pressure unit. In these experiments it was observed that there was no deformation of the wire, as such theoretically the coating thickness should be equal to the gap between the wire and the exit of the pressure unit. The experimental results confirm this. It is also evident from the figure that the thickness and continuity of the polymer coating on the wire are not affected by the back pressure of the polymer melt. The results in Fig. 4 show the coating thickness and continuity of Nylon 6 at the melt temperature of 250 ◦ C. For
Fig. 4. Coating thickness for polymer melt temperature of 250 ◦ C (h2 = 0.05 mm).
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Fig. 5. Coating thickness for polymer melt temperature of 270 ◦ C (h2 = 0.05 mm).
Plate 1. Concentricity of coating on wire for wire velocity of 4 m/s, back pressure 5 bar and polymer melt temperature 270 ◦ C (h2 = 0.05 mm).
argon pressures of 5 and 10 bar as well as without any back pressure tests were carried out at up to 12.5 m/s. The coating thickness was constant at 0.05 mm for wire velocities of up to 6 m/s for no back pressure. At wire velocity of 12 m/s it was 0.048 mm. At 5 bar back pressure the coating thickness was 0.05 mm up to 12 m/s. However at 12.5 m/s the coating thickness was found to be 0.049 mm. This decrease in coating thickness is thought to be due to two reasons - (i) absence or lack of back pressure means less pressure on the melt layer for adequate coating formation, (ii) inelastic response of the polymer material that works against die swell elastic behaviour. For the argon pressure of 10 bar the coating was continuous within the entire velocity range and the coating thickness was uniform at 0.05 mm. With all these back pressure cases the coating was found to be continuous. Fig. 5 shows the coating thickness for Nylon 6 at the melt temperature of 270 ◦ C. These tests were carried out at wire velocities of up to 12.5 m/s for argon pressure of 5 and 10 bar as well as at zero back pressure. The coating was continuous and the thickness was uniform at 0.05 mm over the entire range of velocities except for at zero back pressure. Argon creates inert environment for the polymer melt and with zero back pressure the Nylon 6 melt started to burn as the melt temperature increased beyond 230 ◦ C. This degradation of polymer led to slightly lower quality coating at higher wire velocities. However, the coating on the polymer for different ranges of velocities, back pressures and melt temperatures appeared to be concentric. Some samples were prepared and polished for examination under an optical microscope. The cross section of the wire was observed to be circular and the coat-
ing was reasonably concentric. Plate 1 shows the quality of concentricity of the coating on wire with 5 bar back pressure and wire velocity of 4 m/s at 270 ◦ C. Plate 2 shows the concentricity of coating with zero back pressure at 230 ◦ C and wire velocity of 10 m/s. Results from these plates show that the coating is reasonably concentric. Plate 3 illustrates the coating concentricity at 250 ◦ C for zero back pressure and wire velocity of 12 m/s which appears to be slightly eccentric.
Plate 2. Concentricity of coating on wire for wire velocity of 10 m/s, back pressure 0 bar and polymer melt temperature 230 ◦ C (h2 = 0.05 mm).
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Plate 3. Concentricity of coating on wire for wire velocity of 12 m/s, back pressure 0 bar and polymer melt temperature 250 ◦ C (h2 = 0.05 mm).
Fig. 6 shows the drawing force during the coating process for the polymer melt temperature of 230 ◦ C. The drawing force generally increases with the increase in the wire velocity. Also it increases with the increase in argon back pressure.
Fig. 7. Drawing force for polymer melt temperature of 250 ◦ C (h2 = 0.05 mm).
For the back pressure of 10 bar the drawing force is on average about 25 and 50 N greater than that for 5 bar and zero back pressure respectively. Fig. 7 shows the drawing force for the polymer melt temperature of 250 ◦ C. It shows a similar trend as in Fig. 6. Fig. 8 shows the drawing force for the polymer melt temperature of 270 ◦ C. The drawing force increases for up to 7 m/s drawing speed after which it decreases. This is thought to be due to reaching critical shear stress that causes slip at the surface of the wire and the polymer melt. It is evident from the there three figures that the increase in melt temperature in general causes the drawing load to decrease. This is because as the melt temperature increases the viscosity of the polymer melt decreases, which leads to a decrease in the shear stress and hence in the drawing force.
Fig. 6. Drawing force for polymer melt temperature of 230 ◦ C (h2 = 0.05 mm).
3.2.2. Stepped parallel bore pressure unit (h2 = 0.12 mm) The results in Fig. 9 show the coating thickness and continuity with Nylon 6 at the melt temperature of 230 ◦ C. For argon pressures of 5 and 10 bar the test was carried out at up to 12 m/s. The coating thickness was about 0.09 mm for wire velocities of up to 12 m/s for both back pressures. The coating thickness was smaller than the exit gap between the wire and the unit. The reasons for the thinner coating could be (i) due to formation of an initially melt deposit at the edge of the pressure unit which solidifies and may partially obstruct
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Fig. 8. Drawing force for polymer melt temperature of 0.05 mm).
270 ◦ C
(h2 =
Fig. 10. Coating thickness for polymer melt temperature of 250 ◦ C (h2 = 0.12 mm).
the exit flow, (ii) the result of inelastic behaviour of melt flow or (iii) as the exit gap between this pressure unit and wire (0.12 mm) was larger than other units (0.05 and 0.03 mm) it took more time to solidify the coating and the pulling action of the coated wire might drawdown the coating. Figs. 10 and 11 presents similarly decrease in the coating thickness for higher polymer melt temperatures viz. 250 and 270 ◦ C respectively. With all these back pressures and all these temperatures the coating was found to be continuous. Plate 4 shows the quality of concentricity of coating at 250 ◦ C and wire velocity of 12 m/s. The coating thickness is almost uniform. Figs. 12–14 show the drawing force generated due to the pulling action of the wire through the pressure medium for polymer melt temperatures of 230, 250 and 270 ◦ C respectively. With the increase in wire velocity the drawing force also increases except for Fig. 14 where it decreases after 8 m/s which is also thought to be due to reaching the critical shear stress of the polymer melt. The drawing force increases with the increase in argon back pressure.
Fig. 9. Coating thickness for polymer melt temperature of 230 ◦ C (h2 = 0.12 mm).
3.2.3. Stepped parallel bore pressure unit (h2 = 0.03 mm) Tests were also carried out with a stepped parallel bore pressure unit with a very narrow exit gap (h2 = 0.03 mm) for both back pressures, 5 and 10 bar, and for polymer melt temperatures of 230, 250 and 270 ◦ C. Although in all these tests the coating appeared to be continuous, under the microscope the coating was found to be broken in some cases
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Fig. 11. Coating thickness for polymer melt temperature of 270 ◦ C (h2 = 0.12 mm).
where the coating was not reasonably concentric. This may have resulted during the preparation of the sample. Fig. 15 shows the coating thickness and continuity with Nylon 6 at the melt temperature of 230 ◦ C. The coating thickness was about 0.025 and 0.026 mm for wire velocities of up to 6 m/s for the argon back pressure of 5 and 10 bar respectively. At 12 m/s the coating thickness was about 0.022 mm for both the back pressures. The coating thickness was smaller than
Plate 4. Concentricity of coating on wire for wire velocity of 12 m/s, back pressure 10 bar and polymer melt temperature 250 ◦ C (h2 = 0.12 mm).
Fig. 12. Drawing force for polymer melt temperature of 230 ◦ C (h2 = 0.12 mm).
Fig. 13. Drawing force for polymer melt temperature of 250 ◦ C (h2 = 0.12 mm).
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Fig. 16. Coating thickness for polymer melt temperature of 250 ◦ C (h2 = 0.03 mm). Fig. 14. Drawing force for polymer melt temperature of 270 ◦ C (h2 = 0.12 mm).
Fig. 15. Coating thickness for polymer melt temperature of 230 ◦ C (h2 = 0.03 mm).
the exit gap between the wire and the unit. The exit gap between the wire and the unit was so small that it scratched the coating in some cases. It is evident that at the back pressure 10 bar the coating thickness seems to be slightly thicker than back pressure 5 bar. Fig. 16 shows the coating thickness for different velocities at 250 ◦ C. At 5 bar back pressure the coating thickness is 0.023 mm for wire velocity of up to 8 m/s and at 12 m/s it is 0.021 mm. Also at 10 bar for velocity of up to 8 m/s it is 0.024 mm and at 12 m/s wire speed it is 0.022 mm. Fig. 17 shows the coating thickness at polymer melt temperature of 270 ◦ C. For both the back pressures the coating thickness at 2 m/s velocity is 0.023 mm and at 12 m/s the thickness is 0.022 mm. Plate 5 shows the quality of concentricity of coating at 270 ◦ C, back pressure 10 bar and wire velocity 12 m/s. The coating thickness is not radially uniform, possibly because of the narrow gap between the wire and the pressure unit and the resulting restricted flow. Figs. 18–20 show the drawing force generated due to the pulling action of the wire through the pressure medium for polymer melt temperatures of 230, 250 and 270 ◦ C respectively. With the increase in the wire velocity the drawing force also increases generally except for Fig. 18 where it decreases after 6 m/s. The drawing force is generally higher for higher argon back pressure. It may be mentioned again that no reduction in the cross sectional area of the wire was observed 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
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Fig. 17. Coating thickness for polymer melt temperature of 270 ◦ C (h2 = 0.03 mm).
using a sharp edged knife or cutting 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 subsequent processing steps involving contact with mechanical tools and dies.
Fig. 18. Drawing force for polymer melt temperature of 230 ◦ C (h2 = 0.03 mm).
Plate 5. Concentricity of coating on wire for wire velocity of 12 m/s, back pressure 10 bar and polymer melt temperature 270 ◦ C (h2 = 0.03 mm).
Fig. 19. Drawing force for polymer melt temperature of 250 ◦ C (h2 = 0.03 mm).
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References
Fig. 20. Drawing force for polymer melt temperature of 270 ◦ C (h2 = 0.03 mm).
4. Conclusion Experimental results of hydrodynamic high-speed wire coating with Nylon 6 have been presented. Due to some limitation in the present experimental set up the drawing speed is limited to about 13 m/s. Therefore, experiments were carried out within the speed range of up to 12 m/s. The polymer coating on the wire is continuous for speeds of up to 12 m/s. The bonding quality of the coating with wire was found to be very good. Concentricity is better in the case of pressure unit with h2 equal to 0.05 and 0.12 mm but poor for the pressure unit with h2 equal to 0.03 mm. Application of back pressure generally improves the quality of the coating.
[1] 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. [2] M.S.J. Hashmi, G.R. Symmons, H. Parvinmehr, A Novel technique of wire drawing, J. Mech. Eng. Sci. Inst. Mech. Engrs. 24 (1982). [3] G.R. Symmons, M.S.J. Hashmi, H. Parvinmehr, Plasto-hydrodynamic die less wire drawing: theoretical treatment and experimental results, in: Proceeding of International Conference on Developments in Drawing Metals, Metals Society, London, May 1983, pp. 54–62. [4] G.R. Symmons, M.S.J. Hashmi, H. Parvinmehr, Aspects of product quality and process control in plasto-hydrodynamic die-less wire drawing, in: Proceeding of 1st Conference on Manufacturing Technology, Irish Manufacturing Committee, Dublin, March 1984, pp. 153–172. [5] G.R. Symmons, A.H. Memon, Z. Ming, M.S.J. Hashmi, Polymer coating of wire using a die-less drawing process, J. Mater. Process. Technol. 26 (1991) 173–180. [6] 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. [7] J. Yu, M.S.J. Hashmi, Experimental results for polymer coating of fine wires, Key Eng. Mater. 86–87 (1993) 163–170. [8] 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 Twelfth Conference of the Irish Manufacturing Committee (IMC 12), 6–8, September 1995, pp. 269–278. [9] 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. [10] S. Akter, M.S.J. Hashmi, Modelling for the pressure distribution within a hydrodynamic unit: effect of the change in viscosity during drawing of wire coating, J. Mater. Process. Technol. 77 (1998) 32–36. [11] S. Akter, M.S.J. Hashmi, Modelling the pressure distribution within a combined geometry hydrodynamic pressure unit during wire coating process, in: Proceedings of the XIIth International Congress on Rheology, Canada, 18–23 August 1996, pp. 689–690. [12] S. Akter, M.S.J. Hashmi, Finite element temperature and pressure analysis for pressure distribution in a combined hydrodynamic unit, Design and Control (PEDAC’97), Egypt, February 1997, pp. 41–50. [13] 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), 1997, pp. 224–232. [14] S. Akter, M.S.J. Hashmi, Plasto-hydrodynamic Nylon coating of wire and prediction of pressure inside the stepped parallel bore unit, in: Thirteenth Conference of the Irish Manufacturing Committee (IMC 13), Limerick, 1996, pp. 123–134.