The basics: what are drawing dies?

Drawing dies are the basic tools used in drawing or similar processes to reduce the cross-section of the starting material and, if necessary, give it a specific shape, such as profiles or sections with complex geometries. Dies are generally mounted on structures made of either electro-welded steel or cast iron, known as die holders, serving a dual purpose: containing the lubricant (either liquid or solid) and managing a water cooling system. The latter is crucial for maintaining the dies at an optimal operating temperature during the drawing.
 
In today's industrial landscape, there are two main types of drawing dies: those made of metal carbides, also known as hard metal alloys, and those made of diamond. Both offer distinct advantages in terms of durability and precision, but the choice between them depends on the specific application and manufacturing process requirements. Carbide dies, with their wear resistance, are often used in demanding industrial applications, while diamond dies are known for their ability to provide exceptional surface finishes on more delicate materials. A careful choice can significantly affect production quality and efficiency.

Carbide dies

Carbide dies represent a pinnacle of excellence in metalworking technology. These tools are produced through a sintering process, in which exceptionally strong alloys are created from carbides formed by the reaction of carbon powders with metal powders with extremely high melting points. The size of the carbide grains and the binder is crucial in this process, typically ranging from 3 to 1 micron. This meticulous preparation results in extremely hard materials that can often challenge even the hardness of diamond. As a result, these drawing dies offer unparalleled wear resistance. Carbide dies find a wide range of applications in the processing of various steel wire rods, including mild steel, hard steel, and alloy steel. Thanks to their exceptional hardness and strength, these dies allow for the processing of wire with extraordinarily thin diameters, as fine as 0.20-0.25 mm. On the market, it is possible to find a variety of sintered alloys, each with specific characteristics suited for different industrial needs: 

- WIDIA: Composition with 5.5-6% carbon, 0.2-5% chromium, 0.5-1% iron, 89-98% tungsten, and 5-6% cobalt. 
- TITANIT: This alloy features 9-11% carbon, 0.4-2% chromium, 0.5-1% iron, 35-40% Molybdenum, nickel, etc.
- Other alloys like ELMARID, CARBOLOY, CARBORAM, WALLRAMIT, and many others, meet specific production requirements. 

Ultimately, carbide dies represent a milestone in the metalworking industry, enabling the production of extraordinarily thin wires and the processing of hard materials with unrivaled precision. These tools embody innovation and versatility at the core of modern industrial production.

Diamond dies

Diamond dies are at their best when processing fine, ultra-fine, and capillary metal wires with diameters less than 0.7 mm. These tools leverage the unique properties of diamond to allow for larger cross-section reductions without exceeding the material's strength limits. In the core of diamond dies, the diameter can reach an astonishing size of 6-7 microns. Diamond comes with a range of characteristics that translate into significant advantages for wire drawing:

Extremely long calibration life
Thanks to its extraordinary hardness, diamond maintains its structural integrity over time, ensuring precise and consistent calibration.

Minimal friction
Diamond offers near-zero friction, meaning that the wire flows smoothly through the die without excessive heat generation or wear.

Low coefficient of thermal expansion
Diamond has an extremely low coefficient of thermal expansion, meaning it remains stable even at high temperatures, maintaining process accuracy.

Good thermal conductivity
Diamond's ability to dissipate heat helps prevent overheating during wire drawing, ensuring the quality of the final product.

Diamond dies find application in drawing a wide range of materials, including steel, copper, bronze, brass, aluminum, and special alloys. These tools are known for their incredible versatility and precision. Diamond dies can be manufactured in two main methods. 

- Mechanical drilling using diamond drills: this manufacturing process is known for its precision, but requires extended times due to the highly specialized machining involved. 
- Electrical discharge machining (EDM) system: this technique combines chemical and electrical erosion to create diamond dies with greater efficiency and precision.

Temperature control during wire drawing

Temperature control is essential during the wire drawing process, especially at the die core diameter, where the actual wire reduction occurs. Despite generous lubrication applied to the wire, the process involves a significant thermal increase. This phenomenon results from the combination of two main factors: the material deformation friction and the sliding friction between the wire and the die. In the case of mild steel wire drawing, especially when working at high speeds with significant percentage section reduction, the temperature can exceed 200°C. This situation requires an effective cooling system to keep the drawing dies in optimal conditions. A further temperature increase occurs due to thermal radiation when the wire is drawn through straight-line multipass dies, where multiple section reductions are performed in sequence. To ensure perfect processing, it is necessary to intensify cooling, often through the addition of water. Careful temperature control during wire drawing not only ensures the quality of the final product, but also contributes to extending the tool life, reducing wear, and maintaining high production performance.

Die wear in wire drawing: a detailed analysis

In the complex world of wire drawing, one of the most significant challenges is die wear. This industrial process, which is essential for creating precision wire and components, is subject to three main types of wear, each with its unique characteristics.


1. Abrasive wear: it results from the sliding friction between the wire material, which is typically softer, and the die material, which is harder. Over time, this friction leads to gradual erosion of the die, resulting in surface irregularities. When observed under a microscope, these irregularities appear as scratches and grooves in the direction of the wire feed.

2. Adhesive wear: it is the result of the formation of micro-welds between the wire material and the die surface. These micro-welds form due to the pressure and high temperatures reached during the wire drawing process. When examined under a microscope, particles torn from the die surface display the grain structure of the material.

3. Erosive wear: it occurs mainly due to the interaction between the wire material and the die itself. Proper lubrication can limit – but not completely – eliminate this phenomenon.

It is important to note that die wear is not uniform over their entire surface, but tends to concentrate in specific parts of the die profile. These areas include the die cone zone, especially at the entrance (zone 1) and exit (zone 2), and the bearing zone (zone 3), with the cylindrical guide. Maximum wear at the entrance cone position is commonly referred to as the "wear ring." Advanced research, conducted through numerical simulations using the Finite Element Method (FEM), has confirmed these observations. In particular, it has highlighted that the most intense wear occurs where the highest stress values are also concentrated. Employing an analytical approach is crucial for understanding and managing die wear, thus helping to enhance the efficiency and durability of this critical process in industrial manufacturing. In conclusion, accurate analysis of die wear in wire drawing is essential to maintain the high quality and reliability of end products, helping to optimize this key process in many industries.

Literature

Angelo Frascio, “Il filo metallico: la trafilatura dei metalli, volume primo”, Milano, Hoepli, 1970, terza edizione

Roger N. Wright, “Wire Technology: Process Engineering and Metallurgy“, Oxford, Butterworth-Heinemann, 2010, 1st

The information has been collected and reviewed by Davide Dell'Oro, founder of expometals.net and former drawing machine designer with over 30 years of experience in the industry.