extrusion process

Manufacturing Processes 1

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The University Of Lahore

What is Extrusion?

Extrusion is the process by which a block/billet of metal is reduced in cross section by forcing it to flow through a die orifice under high pressure.

What is Extrusion?

In general, extrusion is used to produce cylindrical bars or hollow tubes or for the starting stock for drawn rod, cold extrusion or forged products. 

Most metals are hot extruded due to large amount of forces required in extrusion. Complex shape can be extruded from the more readily extrudable metals such as Aluminium.

The products obtained are also called extrusion.

Drawing:

In drawing, the cross-section of a long rod or wire typically is reduced or changed by pulling it through a die called adraw die.Thus, the difference between drawing and extrusion is that in extrusion the material is pushed through a die, whereasin drawing it is pulled through it.

EXTUSION AND DRAWING A billet, usually round, is forced through a die

to create solid or hollow cross-sectional work pieces with elongated grain structures.

Common analogy – squeezing toothpaste A semi-continuous or batch process At room temperature or elevated Typical products:  Door & window frames  Tubing  Railings  Brackets

Classification of extrusion process

Forward / backward extrusion Direct/Indirect Extrusion Hot / cold extrusion

Forward extrusion:

Metal is forced to flow in the same direction as the punch. The punch closely fits the die cavity to prevent backward flow of the material.

Backward Extrusion:

Metal is forced to flow in the direction opposite to the punch movement. Metal can also be forced to flow into recesses in the punch, see Fig.

Direct Extrusion:

The metal billet is placed in a container and driven through the die by the ram.  The dummy block or pressure plate, is placed at the end of the ram in contact with the billet.  Friction is at the die and container wall requires higher pressure than indirect extrusion.

Indirect Extrusion:

The hollow ram containing the die is kept stationary and the container with the billet is caused to move.  Friction at the die only (no relative movement at the container wall) requires roughly constant pressure. Hollow ram limits the applied load.

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 maThe biggest disadvantage of this process is its cost for machinery.

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

Hydrostatic Extrusion:In the hydrostatic extrusion process the billet is completely surrounded by a pressurized liquid, except where the billet contacts the die. This process can be done hot, warm, or cold, however the temperature is limited by the stability of the fluid used. The process must be carried out in a sealed cylinder to contain the hydrostatic medium.

Extrusion Equipment

Horizontal Hydraulic Press Can control stroke & speed Can apply constant force over long stroke Vertical Hydraulic Press Used for cold extrusion Lower capacity, smaller footprint

Extrusion Design

The whole manufacturing and production process starts with the design. It is here that the extrusion takes shape and features are built in to reduce weight, simplify assembly, add functionality and minimise finishing costs. Here we take advantage of the unique benefits of aluminium, in combination with the extrusion process, to make a cost-effective product with optimal functionality and an attractive appearance.

Wall Thickness

When deciding how thick the walls of a profile should be, strength and optimum cost-efficiency are two of the main considerations.

Profiles with a uniform wall thickness are the simplest to produce. However, where necessary, wall thickness within a profile can easily be varied. For example, a profile’s bending strength can be increased by concentrating weight/thickness away from the centre of gravity.

Cost-efficient production

To optimize cost-efficiency, a profile’s design should always be as production-friendly as possible. To achieve this, the profile should:

– have a uniform wall thickness– have simple, soft lines and radiused corners– be symmetrical– have a small circumscribing circle– not have deep, narrow channels

Uniform Wall ThicknessIt is often acceptable to have a large range of wall thicknesses within a single profile. However, a profile with uniform wall thickness is easier to extrude.

Exceptions

It is of course perfectly acceptable for a profile to have walls of different thicknesses. For example, for strength reasons, it may be best to concentrate weight/thickness away from the centre of gravity.

Working Principle

The reaction of the extrusion billet with the container and die results in high compressive stresses which are effective in reducing cracking of materials during primary breakdown from the ingot.

This helps to increase the utilisation of extrusion in the working of metals that are difficult to form like stainless steels, nickel-based alloys, and other high-temperature materials.  

Similar to forging, lower ram force and a fine grained recrystallised structure are possible in hot extrusion.

However, better surface finish and higher strengths (strain hardened metals) are provided by cold extrusion.

Example of Extrusion

ALLUMINIUM EXTRUSION

Extrusion occurs when a pre-heated aluminium billet is forced through shaped orifices in hardened steel dies by means of high longitudinal pressure. Special dies are heated and loaded into the press.

The appropriate size billet is preheated to exact specifications and introduced into the press. Initial heating is the first of several quality control points in the extrusion procedure.

The billet is forced through the die opening, by means of a ram to obtain the desired hollow or solid profile. The pressure and frictional forces acting on the billet cause it to reach temperatures in excess of 500 deg. C. As the extrusion exits the press, it is cooled at a specific rate to ensure quality finish.

Uses

Typical parts produced by extrusion are trim parts used in automotive and construction applications, window frame members, railings, aircraft structural parts.

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Aluminium extrusions are used in commercial and domestic buildings for window and door frame systems, prefabricated houses/building structures, roofing and exterior cladding, curtain walling, shop fronts, etc. Furthermore, extrusions are also used in transport for airframes, road and rail vehicles and in marine applications.

Advantages:

Low Tooling Costs Design and Change Flexibility Good Quality Minimal wasted material

Limitations:

Limited complexity of parts. Uniform cross sectional shape only.

Applications in Pakistan Industries

Pakistan Cables is the only alluminium extrusion company in Pakistan certified for ISO (Environmental Management System (Occupational Health & Safety Management System) and the first alluminium extrusion company in Pakistan certified (Quality Management System) since 1996.

Pakistan Cables Limited in Lahore, Rawalpindi, Quetta, Multan, Abbotabad, Peshawar and Muzaffarabad.

Alluminium Industries in Pakistan

Pakistan Alco as one of the largest manufacturer of Pakistan in the Aluminium industry. Alco industries are in Lahore and Karachi mainly.

Lucky Alluminium in Karachi. Shaheen Alluminium Industries in Lahore

Comparison with other Techniques

EXTRUSIONINJECTION MOLDING

Extrusion is a method by which molten plastic or any other material is pushed through a two-dimensional die opening.

The finished products have a two-dimensional form which are continuous in length. The extrusion method produces linear shapes.

The injection molding process is basically based on the molten die-casting method.

Injection molding forms three-dimensional shapes.