What Is Extrusion?- Working, Types, And Application

What is Extrusion?

Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section.

Its two main advantages over other manufacturing processes are its ability to create very complex cross-sections; and to work with brittle materials, because the material encounters only compressive and shear stresses.

It also creates an excellent surface finish and gives considerable freedom of form in the design process.

Drawing is a similar process, using the tensile strength of the material to pull it through the die.

It limits the amount of change that can be performed in one step, so it is limited to simpler shapes, and multiple stages are usually needed. Drawing is the main way to produce wire. Metal bars and tubes are also often drawn.

Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). It can be done with hot or cold material.

Commonly extruded materials include metals, polymers, ceramics, concrete, modeling clay, and foodstuffs. Products of extrusion are generally called extrudates.

Also referred to as “hole flanging”, hollow cavities within extruded material cannot be produced using a simple flat extrusion die because there would be no way to support the center barrier of the die.

Instead, the die assumes the shape of a block with depth, beginning first with a shape profile that supports the center section.

The die shape then internally changes along its length into the final shape, with the suspended center pieces supported from the back of the die. The material flows around the supports and fuses to create the desired closed shape.

The extrusion of metals can also increase their strength.

Working Principle

Extrusion is a simple compressive metal forming process. In this process, piston or plunger is used to apply compressive force at work piece. This process can be summarized as follow.

• First billet or ingot (metal work piece of standard size) is produced.
• This billet is heated in hot extrusion or remains at room temperature and placed into a extrusion press (Extrusion press is like a piston cylinder device in which metal is placed in cylinder and pushed by a piston. The upper portion of cylinder is fitted with die).
• Now a compressive force is applied to this part by a plunger fitted into the press which pushes the billet towards die.
• The die is small opening of required cross section. This high compressive force allows the work metal to flow through die and convert into desire shape.
• Now the extruded part removes from press and is heat treated for better mechanical properties.

This is basic working of extrusion process.

Characteristics of Extrusion

• Able to create complex cross-sections and will be uniform over the entire length of the extrudates
• Factors that affect the quality of extrusion are die design, extrusion ratio, billet temperature, lubrication, and extrusion speed. Check out the detailed design guide for metal extrusion, “How to design parts for direct metal extrusion” to understand the 5 key design variables of metal extrusion and design for manufacture (DFM) extrusion design tips.
• Similar to any other metal forming processes, it can be performed either hot or cold, although the process generally is carried out at elevated temperatures in order to reduce the extrusion force and improve the ductility of the material
• Low cost due to reduced raw material wastage and high production rate
• Brittle material can be deformed without a tear as it only exerts compressive and shear forces in the stock part
• Parts that are formed have an excellent surface finish which minimizes post-processing machining
• Metal extrusion tends to produce a favorable elongated grain structure in the direction of the material.
• The minimum wall thickness of ~1mm (aluminum) to ~3mm (steel) could be achieved

Types of Extrusion Processes

There are quite a few different types of metal extrusion forms to choose from. While the core principles remain invariable, we apply them differently in order to achieve a myriad of different products.

We can classify all the metal extrusion processes into one of the following types.

#1. Hot extrusion.

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.

#2. Cold 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.

#3. Warm extrusion.

When extrusion is carried out at temperatures between room temperature and recrystallization temperature of the metal, we call it warm extrusion.

Compared to the aforementioned manufacturing processes, warm extrusion offers benefits such as higher control over extrudate properties such as ductility.

The temperatures for warm extrusion range from 424°C to 975°C. The temperature never exceeds the critical melting point at any stage of the metal-forming process.

#4. Friction extrusion.

Friction extrusion was invented at the Welding Institute in the UK and patented in 1991. It was originally intended primarily as a method for production of homogeneous microstructures and particle distributions in metal matrix composite materials.

Friction extrusion differs from conventional extrusion in that the charge (billet or other precursor) rotates relative to the extrusion die. An extrusion force is applied so as to push the charge against the die.

In practice either the die or the charge may rotate or they may be counter-rotating. The relative rotary motion between the charge and the die has several significant effects on the process.

First, the relative motion in the plane of rotation leads to large shear stresses, hence, plastic deformation in the layer of charge in contact with and near the die. This plastic deformation is dissipated by recovery and recrystallization processes leading to substantial heating of the deforming charge.

Because of the deformation heating, friction extrusion does not generally require preheating of the charge by auxiliary means potentially resulting in a more energy efficient process.

Second, the substantial level of plastic deformation in the region of relative rotary motion can promote solid state welding of powders or other finely divided precursors, such as flakes and chips, effectively consolidating the charge (friction consolidation) prior to extrusion.

#5. Micro-extrusion.

The microextrusion process creates products at the submillimeter range for special applications.

The process remains the same but it requires small dies and rams which is a challenge with tight accuracy requirements. The products of microextrusion fit within a 1 mm square.

Several other issues also come into play when dealing with small products such as grain boundaries and structure, deformation defects, and forming stability.

Application of Extrusion

• Extrusion is widely used in production of tubes and hollow pipes.
• Aluminum extrusion is used in structure work in many industries.
• This process is used to produce frames, doors, window etc. in automotive industries.
• Extrusion is widely used to produce plastic objects.

Advantages of Extrusion

• It is possible to get a consistent cross-section of the parts.
• Compared to other methods, the extrusion method is inexpensive.
• Because the extruded component is very heated after the extrusion process, the post-extrusion works can be completed very quickly.
• A broad range of part complexity and variety can be produced using extrusion.
• To complete the process, the extruder must only be run once.
• Extrusion is a simple process that can be automated.
• An extruded product’s smooth surface, exact measurements, and clearly defined shape all serve as indicators of its quality.
• It is simple to make items with a broad range of diameters and wall thicknesses using the extrusion method.
• Rolling cannot provide the benefits of the extrusion technique.

Disadvantages of Extrusion

• After extrusion, undesirable residual tensions may remain.
• The extruded component is extremely hot as a result, and this causes dimension variations after extrusion.
• The same cross-sections must run the full length of the object.
• Size variations are brought on by hot materials and production.
• Squeezing requires a lot of effort.
• High startup or establishment costs.
• Not suitable for small-scale manufacturing.
• The manufacturers must accept a substantial degree of deviation from the original part because of this unpredictable expansion.
• It requires heavier tools.

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