Process of Wire Drawing - Equipment and Stages

🔗Types of Metal Drawing

What is Wire Drawing?

Wire drawing is a metalworking process that involves reducing the cross-sectional area of a metal wire or rod by pulling it through a die or series of dies. As the material is pulled, it elongates and thins, ultimately achieving the desired shape and thickness. The wire drawing process can reduce the diameter by up to 90% from the original coil or rod. For example, a 20mm rod could be reduced to a 2mm wire.

Wire drawing is typically a cold working process, but higher temperatures may be employed to hot work large wires, rods, or hollow tubes to reduce forces. This process is similar to metal extrusion, with the key distinction being in the application of force. Unlike extrusion, which pushes material through an opening, drawing pulls it through.

🔗Applications and Advantages of Wire drawing process

Wire Drawing Equipment and Setup

The wire drawing manufacturing process requires various tools to achieve the desired results. The fundamental concept of the metal drawing is depicted in the following figure.

Raw Material: The wire drawing process begins with a metallic rod, usually copper, aluminium, or steel alloys. First, a hot rolled rod is produced through other metal-forming processes such as forging, extruding, and centrifugal casting. The initial input rod has a larger diameter, ranging from a half inch to several inches, depending on the final wire size.

Drawing dies: Wire drawing involves pulling a metal wire through a small circular opening called a die. Dies are made of hard materials like carbide or diamond and have a hole through the middle. The hole is slightly smaller in diameter than the incoming wire bar. As the bar is pulled through, its diameter is reduced to fit through the die.

🔗Wire Drawing Die: Material, types, and sections

Wire Pointing Machines: These machines are used to shape the end of the wire so that it can be easily fed through the drawing dies. A pointing machine is used to taper the end of the rod to a sufficient size to fit the initial dies. This machine usually consists of two motor-driven rollers that have a number of grooves of decreasing size. By rolling the rod through these grooves, the end tapers down to fit the first die.

Wire Drawing Machine: This machine pulls the metal rod or wires through the series of dies. It consists of a motor that drives the wire through the dies and a cooling / lubricating system that lubricates the wire and dies. This machine may have six or more dies and utilizes power-driven pulleys or rotating drums to accomplish the task. As the pulleys/drums spin, the rod is fed through the dies, each narrowing the wire until the rod emerges as wire. Although the machines are based on the same principle, they are broadly classified as "non-slip" and "slip types" for practical reasons due to their different designs.

Capstans and Motors: Capstans and motors are used to pull the wire through the dies. They must be powerful enough to handle the force required to draw the wire and must be capable of maintaining the correct speed and tension.

Tension Control System: This system maintains a constant tension on the wire as it is being drawn through the machine. It ensures that the wire is drawn evenly and consistently, producing high-quality wire.

The strength of the material limits the pulling force; if the wire is pulled too hard, it will break. The force required to pull the wire through the die is determined by the degree of reduction in the cross-sectional area; the greater the reduction, the greater the force needed. It is evident that the wire's yield strength limits the maximum feasible reduction in diameter.

Lubricants: Lubricants reduce friction between the wire and dies and prevent the wire from overheating. They are applied to the wire and dies using a spray or immersion method. Various lubricant powders are also available for lubrication.

Annealing Equipment: Annealing equipment is used to heat treat the wire after drawing to relieve stress and improve its ductility.

Stages of Wire Drawing Process

The wire drawing process can be divided into several stages, each with its own specific tasks and requirements. Here we discuss about how to draw metal.

1. Preparation of the Wire

Before the wire is drawn, it must be prepared. This involves cleaning the wire to remove dirt or impurities and applying a lubricant to reduce friction during the drawing process. Initially, the rods used for wire drawing are pickled in acid to eliminate any scale buildup and then electrically butt welded. Iron hydroxide, copper, or tin coating may be applied to the rod during or after the pickling process.

Large hot rolled coils around 10mm in diameter undergo preparatory treatment prior to actual wire drawing. The preparation treatment for steel wire consists of, 

1) Cleaning. This may involve acid pickling, rinsing, drying the coil, or mechanical flexing.

2) Neutralization. The raw material is then immersed in a lime bath to neutralize residual acid.

2. Pointing the Wire

The wire's end must be pointed so that it can be easily inserted into the die. This is typically done using a wire pointing machine or by hand using a file. 

3. Drawing the Wire

The wire is fed through the first die and pulled by a powerful motor or capstan. As the wire is drawn through the die, its diameter is reduced, and its length increases. The process is repeated through a series of dies until the desired diameter is achieved.

4. Annealing 

As the wire is drawn, it becomes harder and stronger. It can also become more brittle. In some cases, the wire may need to be annealed to relieve the stress and strain caused by the drawing process. Annealing involves heating it to a specific temperature and then cooling it slowly at room temperature or quenching it. Annealing is done to the wire to soften the metal and make it more ductile for further drawing or fabrication steps. The annealing process reduces the strength of the wire but makes it less brittle.

5. Finishing and Final Steps

Depending on the application, the wire will have a consistent diameter ranging from a fraction of a millimeter to several millimetres. The properties of the wire, like strength, conductivity, and hardness, can be tailored based on the initial material and specifics of the wire drawing process. Once the wire has been drawn to its final diameter, it may undergo additional finishing processes, such as coating or plating, to enhance its properties or appearance. Then wire undergoes final cleaning, cutting, and packaging for sale and distribution. To prevent rust, some wires may receive additional finishes, such as galvanization. The wire may be spooled for sale as is or fed into another manufacturing line as input.