Wire drawing

WIRE DRAWING

Calculation of lubricant film thickness between wire and die
Area reduction (A₁-A₂)/A₁	%
diameter d₁	mm
reduction angle φ	deg
speed v₁	m/s
viscosity η₀	Pa.s
pressure-viscosity coefficient α_p	1/GPa
extrusion pressure p_e	MPa
bearing length ratio L_b/d	-

diameter d₂	mm
speed v₂	m/s
viscosity ratio η/η₀	-
reduction length L_r	mm
bearing lengthe L_b	mm
lubricant film thickness h_min	µm
tensile stress	MPa
computation based on Reynolds wedge effect with pressure-viscous lubricant, full-film lubrication assumed (reference ). For full-film lubrication a film thickness / roughness ratio λ=h_min/Ra'>2...2.5 is required, where Ra'=(R_a,1+R_a,2)/2. With smooth surfaces, i.e. Ra'<0.2 a film thickness of 0.5 µm suffices for full film elasto hydrodynamic lubrication.

Wire drawing in brief

In drawing wire, the required deformation is accomplished by drawing the wire through the conical bore section of the die, hereby reducing the diameter through plastic deformation. During deformation, a thin film of lubricant between wire surface and die surface is essential to minimise friction, to reduce die wear, and to keep the die cool.

For good wire deformation, it is necessary to select a drawing die tool with the appropriate profile, designed for either ferrous or non-ferrous materials.

Entry Lubrication is introduced, and the material is guided to the deformation zone (reduction/bearing) of the die.

Reduction Generally speaking, harder (ferrous) materials require smaller reduction angles; softer (non-ferrous) materials require larger reduction angles. Included angles (2-alpha) may vary within the range 8°-30°, zone length (0.5...1)d.

Bearing In this cilindrical part the deformed wire is calibrated to the desired size. The bearing length depends on the materials drawn and is specified as a percent of the bore hole diameter (0.3...0.6)d. As a general rule, harder materials demand longer bearings than softer ones. Wear on the bearing only occurs when heavy wear in the reduction zone was not remedied in timely fashion.

Exit This zone, in which the deformed wire leaves the die tool, must provide sufficient support for the axial mechanical wire drawing stress which occurs.

Some purposes for which the drawing die tools were designed require that transitions among the various zones are perfectly blended, with smaller radii for harder materials, larger radii for softer. Back relief, so-called, is often provided as a high-polish transition phase to allow the wire to exit smoothly from the bearing of the die.

Die wear takes place mainly in the reduction zone, at first by abrasion at the point at which the incoming wire contacts the die. Initially the mirror-polished die surface there will show only small signs of wear. Then rapid wear sets in, characterised by development of a "wear or drawing ring." Such a die tool must be withdrawn for refurbishment at the onset of wear at the impact point.

Wire area reduction

In products where subsequent draws are needed to reach the desired finish diameter an average area reduction per die of about 20-30% is usual. Steel wire work hardens during plastic deformation and the ductility (the degree of elasticity) is reduced while the tensile strength increases. The degree of total area reduction possible without intermediate annealing depends on the composition of the steel, i.e. the work hardening characteristic of the steel quality (=grade). In general it is possible through subsequent or sequential passes through ever smaller dies to reduce the cross section area of a wire between 85-95%. Further area reduction will require an intermediate anneal to restore ductility.

Die Materials Overview

Tungsten Carbide:

Lowest cost, shock resistance, ease of production, large sizes available.
Lower life expectancy.

Natural Diamonds:

Wear resistance, gives excellent wire surface, high thermal conductivity, longer life expectancy
Susceptible to fractures from shock or wear, limited availability in required high quality and quantity, constantly escalating price.

Synthetic Single Crystal:

Consistently uniform material, gives excellent wire surface, high thermal conductivity, predictable wear schedule, uniform wear pattern gives longer life expectancy.
Larger size ranges are still costly at this time.

Polycrystalline Diamond:

Excels in life expectancy, wear resistance of diamond, shock resistance of carbide, high availability, cost effectiveness
Higher drawing force, smaller fines requires more filtration, may be damaged by temperatures above 700ºC, wire surface condition less than from natural diamond.