Metal Extrusion Practice For Manufacturing

Metal extrusion practice, in manufacturing industry, must take into consideration a variety of factors, many of which will be specific to each particular operation. The type of material, size of work piece, geometric cross section of extruded part, ram speed, temperature of work and type of metal extrusion process, are all important elements in the design and analysis of an extrusion operation. The main goal is to enact the right metal flow through the correct application of force. The force is applied through a ram, powered by some sort of press. Most extrusions are performed horizontally, by hydraulic presses. Hydraulic presses can deliver a constant force, at a constant speed, over a long stroke, making them ideal for extruding metal parts; however, in some instances mechanical presses may be used. The ram's speed affects the forces involved during the operation. Ram speeds can be as low as a few feet every minute, or may be as high as 15 feet per second, though most are under 2 feet per second. The length of extruded metal product in common manufacturing practice is generally up to 25 feet, but much longer lengths, as high as 90 feet, have been created. Many of the extruded sections produced in industry require bending or straightening after the completion of the extrusion process. When performed correctly, metal extrusion can be very economical for both small and large batch production.

Metal Flow During Extrusion

During a metal extrusion process, metal from a work piece of a certain cross section is forced to flow through a die of smaller cross section, forming an extruded part. It is important to understand the flow of material that occurs as the part is being formed. In some ways it is similar to fluid flowing from one channel into another channel of decreasing width. The metal is deformed and forced to flow together as it progresses towards, and through, the die. As the work travels through the die, the outer layers are deformed more than the ones closer to the middle. The outer sections, further from the central axis, will experience greater material displacement and will have more turbulent metal flow characteristics. The material closer to the center will move faster through the mold, meaning it will have the higher velocity relative to the die. With square die, which are die with 90 degree angles, sections in the material close to the mold opening, but adjacent to the die, may not move. These areas, termed dead zones or dead metal zones, are indicative by stagnation of metal flow. Note that there will be a type of shearing of the material occurring between layers, at the interfaces of dead zones.

Extrusion Ratio

The extrusion process is capable of creating a tremendous amount of metal deformation of the work. The size of the cross section of the work billet may be much larger than the size of the cross section of the extruded part. For example, in figure 214 the starting work billet has a certain diameter, say 10 inches. It is formed into a round extrusion with a diameter of 5 inches. We can relate the size of the work's cross section with that of the extruded part by comparing their diameters. It can be said that the extrusion has a diameter of 1/2 the original work, thus measuring the cross sectional reduction that occurred during the metal manufacturing process.

This is an easy relationship to make, since both the work and the metal extrusion are round. If the work and the extruded part have a different profile, another means will be needed to relate their sizes. For example, in figure 215 a round billet is extruded into a smaller u-channel profile.

To relate the cross section of the work piece to that of the extruded product, the extrusion ratio was established. The extrusion ratio is the ratio of the area of the work's cross section (A_o) to that of the extrusion's cross section (A_f). The extrusion ratio, or reduction ratio, can be expressed as (A_o/A_f).

Obviously, since the starting work's cross section will be greater than that of the metal extrusion, the extrusion ratio will always be more than 1. In manufacturing industry, extrusion ratios typically range from about 4 to 100, although they can be even higher in certain special cases.

Extrusion Shape Factor

The exact geometric profile of a metal extrusion's cross section will have an effect on the force required to extrude the work. A completely round circle cross section requires the least amount of work to extrude. Generally the more complex a shape, the more force that will be needed to extrude a cross section of that shape. In order to quantify the effect that different cross sectional profiles have on metal extrusion force requirements, the extrusion shape factor was established. The lower the shape factor, the lower the relative pressure needed to extrude that cross section. A completely round circle profile has a shape factor of 1, the shape factor increases as the part becomes more complex. The actual shape factor calculation is relative to the ratio between the perimeter of the extruded cross section and the perimeter of a circle of the same area.

Circumscribing Circle Diameter

As noted, the geometry of an extruded metal profile is a large factor in force requirements for that manufacturing operation. As in all processes, there are always limitations on the size of parts that may be manufactured based on the physical natures of the process. The work material is an important characteristic in determining the size limitations for an extruded part. Stronger materials require more pressure to form, therefore the maximum size of an extrusion will be lower for more difficult to shape metals.

Another method, used in manufacturing industry, to quantify the geometry of a metal extrusion's profile, particularly with regard to size, is the circumscribing circle diameter. The circumscribing circle diameter is simply the diameter of the smallest circle that the profile of the extruded cross section can fit. Aluminum is one of the easiest to shape metals for extrusion. The range of circumscribing circle diameters for extruded aluminum parts, for industrial manufacturing production, typically spans from 1/4 inch to 10 inches. Although much larger aluminum parts have been extruded in certain operations. Figure 217 shows some different cross sectional profiles produced by metal extrusion, with their circumscribing circles and circumscribing circle diameters.

Metal Extrusion Die

Metal extrusion die, used in manufacturing extruded sections, must have certain mechanical characteristics. Extrusion die must be strong and hard, capable of holding their dimensional accuracy throughout the high stresses created during the manufacturing process. They must also be resistant to wear, which is always an issue when extruding metal in large quantities. Dies for hot extrusion must have high thermal resistance and be able to maintain strength and hardness at elevated temperatures. Tool steels are a common type of material for metal extrusion molds. Extruding dies may be coated to increase wear resistance. Carbides are sometimes used for a mold material, carbides do not wear easy and can provide accurate part dimensions.

Extrusion die angle is an important factor in the manufacturing process, as it is a large determinant in the flow of material. The amount of force necessary to form a certain cross section will vary with different die angles. A lower angle will create more friction at the work-die interface. Friction is a factor that increases the force necessary to extrude a part. High die angles create more material movement, particularly in the outer regions away from the center. The greater metal displacement gives a greater turbulence in the metal flow. Increased turbulence in the flow also increases the amount of force necessary for the operation. All factors must be calculated in the design of a metal extrusion process.

The optimum die angle will balance out the more extreme friction of lower die angles with the more extreme turbulence of higher die angles, and be somewhere between the two extremes. The exact optimum die angle is difficult to determine for any metal extrusion process due to the influence of other operational factors, such as temperature and lubrication. The manufacturing engineer must try to provide the best angle based on all the considerations of a given operation.

Lubrication

Lubrication is used, in manufacturing industry, to assist in metal flow over the work-mold surfaces as a part is being extruded. Soaps, oils, graphite immersed in oil and many other special lubricants are all used in manufacturing industry to extrude parts. Some materials can be problematic in that they tend to stick to the tooling. To prevent sticking, a softer metal may be used for lubrication. In this case, the softer metal will be jacketed around the work. For manufacturing practice, particularly in high temperature processes, molten glass is often employed as an effective lubricant in the extrusion of tougher materials.