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METAL EXTRUSION PROCESS & EQUIPMENT

METAL EXTRUSION

METAL EXTRUSION TYPES

METAL EXTRUTION

  • HOT EXTRUSION
  • COLD EXTRUSION
  • WARM EXTRUSION

is a metal forming process in which a work piece, of a certain length and cross section, is forced to flow through a die of a smaller cross sectional area, thus forming the work to the new cross section. The length of the extruded part will vary, dependant upon the amount of material in the work piece and the profile extruded.

The two main advantages of this process over other manufacturing processes are its ability to create very complex cross-sections, and to work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms parts with an excellent surface finish.

HYDROSTATIC EXTRUSION

  • The pressure required in the chamber is supplied via a piston through an incompressible fluid via a piston through through an incompressible incompressible fluid medium surrounding the billet
  • The fluid in contact with die surfaces reduces friction
  • A cold process

„ The viscosity of the fluids used (vegetable oils such as castor oil) does not change with heat

  • Can use to extrude brittle materials Can use to extrude extrude brittle brittle materials materials

„ Ductility is increased

  • Limited applications

„ Complex tooling; specialized equipment; uneconomic

Schematic illustration of the Schematic Schematic illustration illustration of the impact-extrusion process extrusion process.

The extruded parts are stripped by the use of a stripper

plate, because they tend to stick to the punch.

Indirect extrusion can also be used to produce hollow parts. In this process, a ram is forced into the work material. The ram gives the internal geometry to the tubular part, while the material is formed around it. Difficulties in supporting the ram limit this process and the length of tubular metal extrusions that may be manufactured.

Metal Flow During Extrusion

EXTRUSION RATIO

(a) Flow pattern obtained at low friction or in indirect extrusion.

(b) Pattern obtained with high friction at the billet (b) Pattern Pattern obtained obtained with high friction friction at the billet–chamber interfaces chamber interfaces.

(c) Pattern obtained at high friction or with cooling of the outer regions

of the billet in the chamber. This type of pattern, observed in metals

whose strength increases rapidly with decreasing temperature, leads to whose strength strength increases increases rapidly rapidly with decreasing decreasing temperature, temperature, leads to

a defect known as pipe (or extrusion) defect.

Metal Extrusion Practice For Manufacturing

HOT EXTRUSION

INDIRECT EXTRUSION

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.

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.

METAL EXTRUSION PROCESS

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.

  • DIRECT (or FORWARD) EXTRUSION
  • INDIRECT (or REVERSE) EXTRUSION
  • HYDROSTATIC EXTRUSION
  • LATERAL (or SIDE) EXTRUSION
  • IMPACT EXTRUSION

Indirect extrusion is a particular type of metal extrusion process in which the work piece is located in a chamber that is completely closed off at one side. The metal extrusion die are located on the ram, which exerts force from the open end of the chamber. As the manufacturing process proceeds, the extruded product flows in the opposite direction that the ram is moving. For this purpose the ram is made hollow, so that the extruded section travels through the ram itself. This manufacturing process is advantageous in that there are no frictional forces between the work piece and the chamber walls. Indirect extrusion does present limitations. Tooling and machine set up are more complicated, hollow rams are not as strong and less ridged and support of the length of the metal extrusion's profile, as it travels out of the mold, is more difficult.

Metal Extrusion Die

A. DIE FOR NON-FERROUS METALS

B. DIE FOR FERROUS METALS

Extrusion Shape Factor

Hot extrusion is a hot working process, which means it is done above the material's recrystallization temperature to keep the material from work hardening and to make it easier to push the material through the die. Most hot extrusions are done on horizontal hydraulic presses that range from 230 to 11,000 metric tons (250 to 12,130 short tons). Pressures range from 30 to 700 MPa (4,400 to 101,500 psi), therefore lubrication is required, which can be oil or graphite for lower temperature extrusions, or glass powder for higher temperature extrusions. The biggest disadvantage of this process is its cost for machinery and its upkeep.

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.

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 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.

(a)

(b)

B. HYDROSTATIC C. LATERAL

A. INDIRECT EXTRUSION EXTRUSION EXTRUSION

Lubrication

  • Useful in hot extrusion:

„ Material flow during extrusion Material Material flow during extrusion extrusion

„ Surface finish & integrity

„ Product quality

„Extrusion forces

  • Glass is excellent lubricant for:

„ Steels

„ Stainless steels

„ High-temperature metals & alloys temperature metals & alloys

  • Glass applied as powder to billet surface or Glass applied applied as powder to billet surface surface, or
  • Insert glass pad at die entrance; when heated, melted glass lubricates die surface

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.

Hot extrusion temperature

for various metals

EXTRUSION EQUIPMENT

EXTRUSION DEFECTS

1. Horizontal Hydraulic Press

„ Can control stroke & speed

„ Can apply constant force over long stroke

2. Vertical Hydraulic Press

„ Used for cold extrusion

„ Lower capacity, smaller footprint

1. SURFACE CRACKING

- High extrusion temperature, friction, speed

cause high surface temperatures.

- Caused by hot shorting: local cooling cooling of

constituents or impurities at grain boundaries.

2. INTERNAL CRACKING

- also known as center cracking, centre-burst, arrowhead fracture, chevron cracking.

- Increases with increasing die angle.

- Increases Increases with increasing amount of impurities.

- Decreases with increasing extrusion ratio &

friction .

THANK YOU & GOD BLESS! :)

ADVANTAGES OF COLD vs. HOT EXTRUSION

  • Improved mechanical properties due to work hardening
  • Good control of dimensional tolerance
  • Improved surface finish Improved surface finish
  • Competitive production rates & costs

COLD EXTRUSION

DIRECT EXTRUSION

Cold extrusion is done at room temperature or near room temperature. The advantages of this over hot extrusion are the lack of oxidation, higher strength due to cold working, closer tolerances, better surface finish, and fast extrusion speeds if the material is subject to hot shortness.

Materials that are commonly cold extruded include: lead, tin, aluminum, copper, zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel.

Examples of products produced by this process are: collapsible tubes, fire extinguisher cases, shock absorber cylinders and gear blanks.

WARM EXTRUSION

Hollow, or semi hollow, parts can be directly extruded with the use of a mandrel attached to the dummy block. A hole is created through the work, parallel to the axis over which the ram applies the force to form the extrusion. The mandrel is fitted within this hole. Once the operation begins, the ram is forced forward. The extruded metal flows between the mandrel and the die surfaces, forming the part. The interior profile of the metal extrusion is formed by the mandrel, while the exterior profile is formed by the extruding die.

In direct, or forward extrusion, the work billet is contained in a chamber. The ram exerts force on one side of the work piece, while the forming die, through which the material is extruded, is located on the opposite side of the chamber. The length of extruded metal product flows in the same direction that the force is applied.

During direct extrusion, metal flow and forces required are affected by the friction between the work piece and the chamber walls. Particularly in hot working, oxide scale build up on the outer surfaces of the work piece can negatively influence the operation.

For these reasons, it is common manufacturing practice to place a dummy block ahead of the ram. The dummy block is of slightly smaller diameter than the chamber and work piece. As the metal extrusion proceeds, the outermost surface of the work is not extruded and remains in the chamber. This material will form a thin shell, (called skull), that will latter be removed. Much of the skull will be comprised of the surface layer of oxidized scale from the work metal.

Warm extrusion is done above room temperature, but below the recrystallization temperature of the material the temperatures ranges from 800 to 1800 °F (424 to 975 °C). It is usually used to achieve the proper balance of required forces, ductility and final extrusion properties.

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