- Email: [email protected]
Experimental results of high speed wire coating using a combined hydrodynamic unit
Journal of Materials Processing Technology 92±93 (1999) 224±229
(8)
Experimental results of high speed wire coating using a combined hydrodynamic unit S. Akter, M.S.J. Hashmi* Material Processing Research Centre, School of Mechanical & Manufacturing Engineering, Dublin City University, Dublin 9, Ireland
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. Different experimental works on this new process have been found in literature. In all of these works, the wire speed in the process was rather low. Coating at this speed does not promise much for industrial scale production. In this paper the results of experimental investigation using a combined parallel and tapered bore unit have been presented comparing the coating quality on galvanized mild steel wire when molten Nylon 6 was used as the coating material at different temperatures and back pressures. The coating thickness and quality and drawing force due to the pulling action of the wire through the pressure media have also been discussed for different wire speeds of up to 12 m sÿ1. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Plasto-hydrodynamic; Wire coating; Pressure distribution
1. Introduction
2. Equations for hydrodynamic pressure
Hydrodynamic wire coating was ®rst carried out by Parvinmehr et al. [1±4] who used a die-less pressure 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. Symmons et al. [5] reported the results of their works on coating of ®ne wire (diameter 0.45 mm) in relation to plastohydrodynamic drawing and coating. The drawing speed range was from 0.05 to 0.8 m sÿ1. Also Lamb and Hashmi [6] and Yu and Hashmi [7] carried out their research on polymer coating using ®ne wires of diameter between 0.1 and 0.4 mm. They achieved a fairly constant polymer coating for wire speeds of up to a maximum of 0.6 m sÿ1. In all these works, the wire coating speed was rather low with little prospect of industrial scale production. The main objective of this paper is to present the results of research carried out on the quality of coating at wire velocities between 2 and 12 m sÿ1. The effects of the polymer melt temperature and back pressure on the coating process have also been investigated.
Figs. 1 and 2 show the schematic diagram of a wire coating machine and a combined geometry pressure unit, respectively. The combined geometry unit consists of two parts. The ®rst is a parallel part which is followed by a tapered part. Detailed analysis of the process has been published elsewhere [8] and will not be repeated here and only the salient equations are presented. The wall shear stress in the parallel section of the unit can be expressed as 0 i ÿ 1V c1 ÿ@ 2Kh1 0 i ÿ 1V @ÿ 2kh1
1 !1=2 1=3 i ÿ 12 V 2 1 1 1 0 2 3 A P 2h 27 k 4 1 1 4K 2 h21 1 !1=2 1=3 i ÿ 12 1 1 1 0 2 3 A P 21 h1 ÿ 4k2 h21 27 k 4
1 ÿ P01 h1 2
(1)
and the volumetric ¯ow rate is given by Q1
*Corresponding author. Fax: +353-01-704-5345 E-mail address: [email protected] (M.S.J. Hashmi) 0924-0136/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 1 2 0 - X
P01 ih31 c1 h21 k 6 i ÿ 1 2 i ÿ 1 i ÿ 1
P01 i3 h51 c31 h21 P01 i2 h41 c1 c21 P01 ih31 20 2 4 2
! Vh1 (2)
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 92±93 (1999) 224±229
P2 j P2 j ÿ 1
6 j ÿ 1V
"
1
225
#
M1 3k 2 j2 h j2 " # 6 j ÿ 1 1
M1 3K 2 j2 h j3 3 2 2 P1 L1M1 3k 2 j2 =6 j ÿ 1 V 1=h3 ÿ 1=h2 5DX 4 1=h23 ÿ 1=h22
(8) where, M (h2ÿh3)/L2. 3. Experimental equipment and procedure Fig. 1. Schematic diagram of a hydrodynamic wire coating process.
The volumetric ¯ow rate and shear stress in the tapered part of the unit are respectively, P02 h3 K 22 P02 h3 Vh ÿ (3) Q2 ÿ 12 4 2 0 1 " 2 #1=2 1=3 _ _ j ÿ 1 j 1 3 j ÿ 1 j @ A 2 j 2k 3k 2k 0
_ ÿ j ÿ 1 j ÿ@ 2k
1 " 2 #1=2 1=3 _ 1 3 j ÿ 1 j A 3k 2k (4)
The viscosity can be expressed as i i ÿ 1 e ÿ T iÿBDPÿT iÿ1
(5)
where, T i T i ÿ 1 ÿ C DX
(6)
Eqs. (1)±(4) may be solved simultaneously. Numerical values of Pstep (pressure at the step) and hence P10 and P20 may be substituted in the above equations, using an iteration technique, until the condition Q1 Q2 is satis®ed. Thus shear stresses on the wire and volumetric ¯ow rate can be determined. The pressure distribution in the parallel section and in the tapered section thus can be determined from Eqs. (7) and (8), respectively: P1 i P1 i ÿ 1 P01 i DX
(7)
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 set-up is shown in Fig. 1. The wire ®rst enters the leakage control unit attached to the melt chamber. The leakage control unit also acts as a preheating unit for the wire. The wire then passes through the melt chamber and enters the plasto-hydrodynamic pressure unit. The coated wire is then wound in the bull block which is driven by a continuously variable speed motor. Polymer granules are poured in the hopper which is ®tted with a heater band. The hopper is connected to the melt chamber and a pressurised argon gas bottle is connected through a pressure line into the hopper which provides the back pressure in the polymer melt. To measure the drawing force during the wire drawing process a piezo-electric load cell is located at the exit end of the pressure unit. A proportional electric charge corresponding to the force is generated in the load cell during drawing and is converted by the charge ampli®er into voltage signal. The analog to digital converter receives the signal from the transducer via the charge ampli®er. 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 get solidi®ed completely by the time it reaches the bull block and the individual strands stick together and the polymer coating gets deformed. The samples of the coated wire were therefore taken from the part which does not reach the bull block after the motor is suddenly stopped.To measure the coating thickness each sample is measured at ®ve 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 was cold mounted and polished. An optical microscope was then used to assess the actual coating thickness and the concentricity of the coating on the wire. 4. Experimental results and discussions 4.1. Process parameters
Fig. 2. Schematic diagram of a combined parallel and tapered bore unit.
The process parameters for the investigations carried out were as follows.
226
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 92±93 (1999) 224±229
Polymer characteristics: polymer type polymer melt temperature
Durethan B 35F 9000/0 KYY (Nylon 6) 2308C±2708C
Wire characteristics: wire diameter wire material
0.7 mm galvanized mild steel
Pressure unit characteristics: The schematic diagram of the combined pressure unit is shown in Fig. 2. The total length of the pressure units is 22 mm. Other geometric parameters are presented in the following; length of the ®rst part (parallel) before step, L1 17 mm; length of the second part (tapered) after step, L2 5 mm; radial gap between the wire and the unit in the ®rst part, h1 0.7 mm; radial gap between the wire and the unit in the step h2 0.25 mm; radial gap between the wire and the unit in the exit of the second part, h3 0.051 mm. Polymer melt properties: Viscosity at polymer melt temperature 2708C at different wire velocities and shear rates: at 4 m sÿ1 the shear rate is 5714 sÿ1 and the polymer melt viscosity is 100 N s mÿ2 (1 N s mÿ2 10 Poise); at 8 m sÿ1 the shear rate is 11 428 sÿ1 and the polymer melt viscosity is 65 N s mÿ2; at 12 m sÿ1 the shear rate is 17 142 sÿ1 and the polymer melt viscosity is 50 N s mÿ2; whilst other factors are: B 0.328C Nÿ1 mm2, G 0.18C mmÿ1, 0.0114 per 8C; and K 10.1 10ÿ11 m4 Nÿ2.
Fig. 3. Coating thickness on wire in combined pressure unit for polymer melt temperature 2308C.
theoretically the coating thickness should be equal to the gap between the wire and the exit end of the pressure unit. The experimental results con®rm this. Upon visual inspection the coating on the polymer appeared to be concentric. To con®rm this a number of samples were prepared and polished for examination using an optical microscope.
4.2. Experimental and theoretical work Experimental work was carried out at polymer melt temperatures of 2308C, 2508C and 2708C. The two different back pressures of argon applied in the polymer melt chamber were 5 and 10 bar (1 bar 105 N mÿ20.1 MPa). For different combinations of polymer melt temperature and argon back pressure, experiments were carried out for wire velocities of 2, 4, 6, 8, 10 and 12 m sÿ1. The results of these tests are presented in the following ®gures in terms of the coating thickness and drawing load. Figs. 3±5 present the coating thickness on wire deposited using a combined parallel and tapered bore unit. The variables were drawing speed (up to 12 m sÿ1), argon back pressure (5 and 10 bar) and polymer melt temperature (2308C, 2508C and 2708C). In all these cases the coating was found to be continuous. The thickness was almost equal to the exit gap between the wire and the unit (0.051 mm). In these experiments it was observed that there was no deformation of the wire. If there is no deformation in the wire then
Fig. 4. Coating thickness on wire in combined pressure unit for polymer melt temperature 2508C.
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 92±93 (1999) 224±229
Fig. 5. Coating thickness on wire in combined pressure unit for polymer melt temperature 2708C.
227
Fig. 7. Drawing force on wire in combined pressure unit for polymer melt temperature 2508C.
Fig. 6 shows the drawing force generated by the wire in the combined pressure unit for the polymer melt temperature of 2308C. This ®gure illustrates that with the increase in wire velocity and back pressure, the drawing force also increases. With the increase in the wire velocity, the hydrodynamic pressure and the viscosity of the polymer increases. Subsequently the drawing force due to the shear force on the wire
also increases. For the back pressure of 10 bar the drawing force is on average 25 N greater than those for the back pressure of 5 bar. Figs. 7 and 8, respectively represent the generated drawing force for polymer melt temperatures of 2508C and 2708C. They show similar trend as in Fig. 6 i.e., with the increase in wire velocity the drawing force increases. As the viscosity decreases with the increase in
Fig. 6. Drawing force on wire in combined pressure unit for polymer melt temperature 2308C.
Fig. 8. Drawing force on wire in combined pressure unit for polymer melt temperature 2708C.
228
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 92±93 (1999) 224±229
Fig. 10. Concentricity of coating on wire in a combined unit (h2 0.051 mm, wire velocity 10 m sÿ1, polymer temperature 2308C and back pressure 5 bar).
Fig. 9. Theoretical pressure distribution within the combined unit for different wire velocities.
the polymer melt temperature, the drawing force also decreases which has been con®rmed by these three ®gures. The hydrodynamic unit was too small to incorporate pressure transducers at different locations and thus, it was not possible to measure the experimental pressure distribution within the unit. Fig. 9 shows the theoretical hydrodynamic pressure within the unit for polymer melt temperature of 2708C and wire velocities of 4, 8 and 12 m sÿ1. In this solution, the theoretical pressure has been computed for each small increment of length in the plasto-hydrodynamic unit. The initial pressure in the ®rst section of the unit and the ®nal pressure in the second section of the unit are zero. The pressure in the parallel section increases linearly, in the tapered section it increases gradually upto certain point and then decreases. The pressure increases with the increase in the wire velocity. From these pressure pro®les it can be seen that the maximum hydrodynamic pressure at wire velocity of 4 m sÿ1 and polymer melt temperature of 2708C is about 140 bar. According to von Mises theory of yielding, the deformation of the wire starts when the combined effect of hydrodynamic pressure and axial stress equals or exceeds the elastic limit of wire material. In this case the drawing force at polymer melt temperature of 2708C, back pressure 10 bar and wire velocity 4 m sÿ1 is about 35 N (Fig. 9). The axial stress due to this load is 906 bar. The combined effect of hydrodynamic pressure and axial stress is therefore 1046 bar (104.6 MPa) which is less than the elastic limit
of mild steel which is 280 MPa. So in this case the combined effect of the pressure and the drawing load is not suf®cient to cause any plastic deformation in the wire. Experimentally it was also observed that there was no plastic deformation of the wire at wire velocity of 4 m sÿ1 or even at 12 m sÿ1. The coating thickness, mentioned in this paper therefore is accurate and represents the gap between the wire and the exit end of the pressure unit. The cross section of the wire was observed to be circular and the coating was reasonably concentric. Fig. 10 shows the concentricity of the coating on wire formed using a combined unit. The wire velocity was 10 m sÿ1, back pressure was 5 bar and the melt temperature was 2308C. Fig. 11 represents the coating concentricity for wire velocity of 6 m sÿ1, polymer melt temperature of 2308C and back pressure of 10 bar. Both plates show a concentric coating. It may be mentioned again that 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 dif®cult 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. 11. Concentricity of coating on wire in a combined unit (h2 0.051 mm, wire velocity 6 m sÿ1, polymer temperature 2308C and back pressure 10 bar).
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 92±93 (1999) 224±229
5. Conclusions Hydrodynamic wire coating with Nylon 6 using a combined geometry pressure unit has been presented. The experiments were carried out within the speed range of 2 and 12 m sÿ1. The polymer coating on the wire was continuous and concentric for speeds of upto 12 m sÿ1. The bonding quality of the coating with wire was very good. 6. Nomenclature B C DP DX K P0 Q T V h L
_ c
temperature±pressure coefficient temperature±length coefficient change in hydrodynamic pressure increment in length non-Newtonian factor pressure gradient flow of polymer temperature wire velocity radial gap in the unit and the wire length of section shear rate ln (/0)/(T ÿ T0) viscosity shear stress wall shear stress
Subscripts 0 1
reference parallel section of the unit
2 i j
229
tapered section of the unit denotes the increment of length in the parallel section denotes the increment of length in the tapered section
References [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) 1±4. [3] G.R. Symmons, M.S.J. Hashmi, H. Parvimehr, Plasto-hydrodynamic dieless wire drawing: theoretical treatment and experimental results, in: Proc. Int. Conf. on Developments in Drawing Metals, Metals Soci., 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: Proc. 1st Conf. on Manufacturing Technology, Irish Manufacturing Committee, Dublin, March 1984, pp. 153± 172. [5] G.R. Symmons, A.H. Memon, Zhang Ming, M.S.J. Hashmi, Polymer coating of wire using a die-less drawing process, J. Mats. Proc. Tech. 26 (1991) 173±180. [6] R.E. Lamb, M.S.J. Hashmi, Polymer coating of superfine wires: a new technique to ensure quality, J. Mats. Proc. Tech. 26 (1991) 197± 205. [7] J. Yu., M.S.J. Hashmi, Experimental results for polymer coating of fine wires, Key Eng. Mats. 86±87, 163±170 (1993). [8] S. Akter, M.S.J. Hashmi, Modelling the pressure distribution within a combined geometry hydrodynamic pressure unit during wire coating process, in: Proc. 12th Int. Cong. on Rheology, Canada, 18±23 August 1996, pp. 689±690.
(8)
Experimental results of high speed wire coating using a combined hydrodynamic unit S. Akter, M.S.J. Hashmi* Material Processing Research Centre, School of Mechanical & Manufacturing Engineering, Dublin City University, Dublin 9, Ireland
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. Different experimental works on this new process have been found in literature. In all of these works, the wire speed in the process was rather low. Coating at this speed does not promise much for industrial scale production. In this paper the results of experimental investigation using a combined parallel and tapered bore unit have been presented comparing the coating quality on galvanized mild steel wire when molten Nylon 6 was used as the coating material at different temperatures and back pressures. The coating thickness and quality and drawing force due to the pulling action of the wire through the pressure media have also been discussed for different wire speeds of up to 12 m sÿ1. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Plasto-hydrodynamic; Wire coating; Pressure distribution
1. Introduction
2. Equations for hydrodynamic pressure
Hydrodynamic wire coating was ®rst carried out by Parvinmehr et al. [1±4] who used a die-less pressure 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. Symmons et al. [5] reported the results of their works on coating of ®ne wire (diameter 0.45 mm) in relation to plastohydrodynamic drawing and coating. The drawing speed range was from 0.05 to 0.8 m sÿ1. Also Lamb and Hashmi [6] and Yu and Hashmi [7] carried out their research on polymer coating using ®ne wires of diameter between 0.1 and 0.4 mm. They achieved a fairly constant polymer coating for wire speeds of up to a maximum of 0.6 m sÿ1. In all these works, the wire coating speed was rather low with little prospect of industrial scale production. The main objective of this paper is to present the results of research carried out on the quality of coating at wire velocities between 2 and 12 m sÿ1. The effects of the polymer melt temperature and back pressure on the coating process have also been investigated.
Figs. 1 and 2 show the schematic diagram of a wire coating machine and a combined geometry pressure unit, respectively. The combined geometry unit consists of two parts. The ®rst is a parallel part which is followed by a tapered part. Detailed analysis of the process has been published elsewhere [8] and will not be repeated here and only the salient equations are presented. The wall shear stress in the parallel section of the unit can be expressed as 0 i ÿ 1V c1 ÿ@ 2Kh1 0 i ÿ 1V @ÿ 2kh1
1 !1=2 1=3 i ÿ 12 V 2 1 1 1 0 2 3 A P 2h 27 k 4 1 1 4K 2 h21 1 !1=2 1=3 i ÿ 12 1 1 1 0 2 3 A P 21 h1 ÿ 4k2 h21 27 k 4
1 ÿ P01 h1 2
(1)
and the volumetric ¯ow rate is given by Q1
*Corresponding author. Fax: +353-01-704-5345 E-mail address: [email protected] (M.S.J. Hashmi) 0924-0136/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 1 2 0 - X
P01 ih31 c1 h21 k 6 i ÿ 1 2 i ÿ 1 i ÿ 1
P01 i3 h51 c31 h21 P01 i2 h41 c1 c21 P01 ih31 20 2 4 2
! Vh1 (2)
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 92±93 (1999) 224±229
P2 j P2 j ÿ 1
6 j ÿ 1V
"
1
225
#
M1 3k 2 j2 h j2 " # 6 j ÿ 1 1
M1 3K 2 j2 h j3 3 2 2 P1 L1M1 3k 2 j2 =6 j ÿ 1 V 1=h3 ÿ 1=h2 5DX 4 1=h23 ÿ 1=h22
(8) where, M (h2ÿh3)/L2. 3. Experimental equipment and procedure Fig. 1. Schematic diagram of a hydrodynamic wire coating process.
The volumetric ¯ow rate and shear stress in the tapered part of the unit are respectively, P02 h3 K 22 P02 h3 Vh ÿ (3) Q2 ÿ 12 4 2 0 1 " 2 #1=2 1=3 _ _ j ÿ 1 j 1 3 j ÿ 1 j @ A 2 j 2k 3k 2k 0
_ ÿ j ÿ 1 j ÿ@ 2k
1 " 2 #1=2 1=3 _ 1 3 j ÿ 1 j A 3k 2k (4)
The viscosity can be expressed as i i ÿ 1 e ÿ T iÿBDPÿT iÿ1
(5)
where, T i T i ÿ 1 ÿ C DX
(6)
Eqs. (1)±(4) may be solved simultaneously. Numerical values of Pstep (pressure at the step) and hence P10 and P20 may be substituted in the above equations, using an iteration technique, until the condition Q1 Q2 is satis®ed. Thus shear stresses on the wire and volumetric ¯ow rate can be determined. The pressure distribution in the parallel section and in the tapered section thus can be determined from Eqs. (7) and (8), respectively: P1 i P1 i ÿ 1 P01 i DX
(7)
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 set-up is shown in Fig. 1. The wire ®rst enters the leakage control unit attached to the melt chamber. The leakage control unit also acts as a preheating unit for the wire. The wire then passes through the melt chamber and enters the plasto-hydrodynamic pressure unit. The coated wire is then wound in the bull block which is driven by a continuously variable speed motor. Polymer granules are poured in the hopper which is ®tted with a heater band. The hopper is connected to the melt chamber and a pressurised argon gas bottle is connected through a pressure line into the hopper which provides the back pressure in the polymer melt. To measure the drawing force during the wire drawing process a piezo-electric load cell is located at the exit end of the pressure unit. A proportional electric charge corresponding to the force is generated in the load cell during drawing and is converted by the charge ampli®er into voltage signal. The analog to digital converter receives the signal from the transducer via the charge ampli®er. 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 get solidi®ed completely by the time it reaches the bull block and the individual strands stick together and the polymer coating gets deformed. The samples of the coated wire were therefore taken from the part which does not reach the bull block after the motor is suddenly stopped.To measure the coating thickness each sample is measured at ®ve 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 was cold mounted and polished. An optical microscope was then used to assess the actual coating thickness and the concentricity of the coating on the wire. 4. Experimental results and discussions 4.1. Process parameters
Fig. 2. Schematic diagram of a combined parallel and tapered bore unit.
The process parameters for the investigations carried out were as follows.
226
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 92±93 (1999) 224±229
Polymer characteristics: polymer type polymer melt temperature
Durethan B 35F 9000/0 KYY (Nylon 6) 2308C±2708C
Wire characteristics: wire diameter wire material
0.7 mm galvanized mild steel
Pressure unit characteristics: The schematic diagram of the combined pressure unit is shown in Fig. 2. The total length of the pressure units is 22 mm. Other geometric parameters are presented in the following; length of the ®rst part (parallel) before step, L1 17 mm; length of the second part (tapered) after step, L2 5 mm; radial gap between the wire and the unit in the ®rst part, h1 0.7 mm; radial gap between the wire and the unit in the step h2 0.25 mm; radial gap between the wire and the unit in the exit of the second part, h3 0.051 mm. Polymer melt properties: Viscosity at polymer melt temperature 2708C at different wire velocities and shear rates: at 4 m sÿ1 the shear rate is 5714 sÿ1 and the polymer melt viscosity is 100 N s mÿ2 (1 N s mÿ2 10 Poise); at 8 m sÿ1 the shear rate is 11 428 sÿ1 and the polymer melt viscosity is 65 N s mÿ2; at 12 m sÿ1 the shear rate is 17 142 sÿ1 and the polymer melt viscosity is 50 N s mÿ2; whilst other factors are: B 0.328C Nÿ1 mm2, G 0.18C mmÿ1, 0.0114 per 8C; and K 10.1 10ÿ11 m4 Nÿ2.
Fig. 3. Coating thickness on wire in combined pressure unit for polymer melt temperature 2308C.
theoretically the coating thickness should be equal to the gap between the wire and the exit end of the pressure unit. The experimental results con®rm this. Upon visual inspection the coating on the polymer appeared to be concentric. To con®rm this a number of samples were prepared and polished for examination using an optical microscope.
4.2. Experimental and theoretical work Experimental work was carried out at polymer melt temperatures of 2308C, 2508C and 2708C. The two different back pressures of argon applied in the polymer melt chamber were 5 and 10 bar (1 bar 105 N mÿ20.1 MPa). For different combinations of polymer melt temperature and argon back pressure, experiments were carried out for wire velocities of 2, 4, 6, 8, 10 and 12 m sÿ1. The results of these tests are presented in the following ®gures in terms of the coating thickness and drawing load. Figs. 3±5 present the coating thickness on wire deposited using a combined parallel and tapered bore unit. The variables were drawing speed (up to 12 m sÿ1), argon back pressure (5 and 10 bar) and polymer melt temperature (2308C, 2508C and 2708C). In all these cases the coating was found to be continuous. The thickness was almost equal to the exit gap between the wire and the unit (0.051 mm). In these experiments it was observed that there was no deformation of the wire. If there is no deformation in the wire then
Fig. 4. Coating thickness on wire in combined pressure unit for polymer melt temperature 2508C.
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 92±93 (1999) 224±229
Fig. 5. Coating thickness on wire in combined pressure unit for polymer melt temperature 2708C.
227
Fig. 7. Drawing force on wire in combined pressure unit for polymer melt temperature 2508C.
Fig. 6 shows the drawing force generated by the wire in the combined pressure unit for the polymer melt temperature of 2308C. This ®gure illustrates that with the increase in wire velocity and back pressure, the drawing force also increases. With the increase in the wire velocity, the hydrodynamic pressure and the viscosity of the polymer increases. Subsequently the drawing force due to the shear force on the wire
also increases. For the back pressure of 10 bar the drawing force is on average 25 N greater than those for the back pressure of 5 bar. Figs. 7 and 8, respectively represent the generated drawing force for polymer melt temperatures of 2508C and 2708C. They show similar trend as in Fig. 6 i.e., with the increase in wire velocity the drawing force increases. As the viscosity decreases with the increase in
Fig. 6. Drawing force on wire in combined pressure unit for polymer melt temperature 2308C.
Fig. 8. Drawing force on wire in combined pressure unit for polymer melt temperature 2708C.
228
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 92±93 (1999) 224±229
Fig. 10. Concentricity of coating on wire in a combined unit (h2 0.051 mm, wire velocity 10 m sÿ1, polymer temperature 2308C and back pressure 5 bar).
Fig. 9. Theoretical pressure distribution within the combined unit for different wire velocities.
the polymer melt temperature, the drawing force also decreases which has been con®rmed by these three ®gures. The hydrodynamic unit was too small to incorporate pressure transducers at different locations and thus, it was not possible to measure the experimental pressure distribution within the unit. Fig. 9 shows the theoretical hydrodynamic pressure within the unit for polymer melt temperature of 2708C and wire velocities of 4, 8 and 12 m sÿ1. In this solution, the theoretical pressure has been computed for each small increment of length in the plasto-hydrodynamic unit. The initial pressure in the ®rst section of the unit and the ®nal pressure in the second section of the unit are zero. The pressure in the parallel section increases linearly, in the tapered section it increases gradually upto certain point and then decreases. The pressure increases with the increase in the wire velocity. From these pressure pro®les it can be seen that the maximum hydrodynamic pressure at wire velocity of 4 m sÿ1 and polymer melt temperature of 2708C is about 140 bar. According to von Mises theory of yielding, the deformation of the wire starts when the combined effect of hydrodynamic pressure and axial stress equals or exceeds the elastic limit of wire material. In this case the drawing force at polymer melt temperature of 2708C, back pressure 10 bar and wire velocity 4 m sÿ1 is about 35 N (Fig. 9). The axial stress due to this load is 906 bar. The combined effect of hydrodynamic pressure and axial stress is therefore 1046 bar (104.6 MPa) which is less than the elastic limit
of mild steel which is 280 MPa. So in this case the combined effect of the pressure and the drawing load is not suf®cient to cause any plastic deformation in the wire. Experimentally it was also observed that there was no plastic deformation of the wire at wire velocity of 4 m sÿ1 or even at 12 m sÿ1. The coating thickness, mentioned in this paper therefore is accurate and represents the gap between the wire and the exit end of the pressure unit. The cross section of the wire was observed to be circular and the coating was reasonably concentric. Fig. 10 shows the concentricity of the coating on wire formed using a combined unit. The wire velocity was 10 m sÿ1, back pressure was 5 bar and the melt temperature was 2308C. Fig. 11 represents the coating concentricity for wire velocity of 6 m sÿ1, polymer melt temperature of 2308C and back pressure of 10 bar. Both plates show a concentric coating. It may be mentioned again that 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 dif®cult 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. 11. Concentricity of coating on wire in a combined unit (h2 0.051 mm, wire velocity 6 m sÿ1, polymer temperature 2308C and back pressure 10 bar).
S. Akter, M.S.J. Hashmi / Journal of Materials Processing Technology 92±93 (1999) 224±229
5. Conclusions Hydrodynamic wire coating with Nylon 6 using a combined geometry pressure unit has been presented. The experiments were carried out within the speed range of 2 and 12 m sÿ1. The polymer coating on the wire was continuous and concentric for speeds of upto 12 m sÿ1. The bonding quality of the coating with wire was very good. 6. Nomenclature B C DP DX K P0 Q T V h L
_ c
temperature±pressure coefficient temperature±length coefficient change in hydrodynamic pressure increment in length non-Newtonian factor pressure gradient flow of polymer temperature wire velocity radial gap in the unit and the wire length of section shear rate ln (/0)/(T ÿ T0) viscosity shear stress wall shear stress
Subscripts 0 1
reference parallel section of the unit
2 i j
229
tapered section of the unit denotes the increment of length in the parallel section denotes the increment of length in the tapered section
References [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) 1±4. [3] G.R. Symmons, M.S.J. Hashmi, H. Parvimehr, Plasto-hydrodynamic dieless wire drawing: theoretical treatment and experimental results, in: Proc. Int. Conf. on Developments in Drawing Metals, Metals Soci., 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: Proc. 1st Conf. on Manufacturing Technology, Irish Manufacturing Committee, Dublin, March 1984, pp. 153± 172. [5] G.R. Symmons, A.H. Memon, Zhang Ming, M.S.J. Hashmi, Polymer coating of wire using a die-less drawing process, J. Mats. Proc. Tech. 26 (1991) 173±180. [6] R.E. Lamb, M.S.J. Hashmi, Polymer coating of superfine wires: a new technique to ensure quality, J. Mats. Proc. Tech. 26 (1991) 197± 205. [7] J. Yu., M.S.J. Hashmi, Experimental results for polymer coating of fine wires, Key Eng. Mats. 86±87, 163±170 (1993). [8] S. Akter, M.S.J. Hashmi, Modelling the pressure distribution within a combined geometry hydrodynamic pressure unit during wire coating process, in: Proc. 12th Int. Cong. on Rheology, Canada, 18±23 August 1996, pp. 689±690.