Processing metals

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Blanking and Piercing (Wasting process)

  • Sheet metals can be cut into the required shapes using punches.
    • These cut through metal using a shearing action.
    • A guillotine is often used to cut large sheet metal into smaller, more useable sheets.
    • These sheets are then passed through machines (manual/automatic) and punches are used to cut the sheet into the correct shape.
  • A piece of metal which has had blanks punched from it has been pierced.
  • A piece of metal which has been punched from a sheet is called a blank.
  • Some products such as aluminium cans use the blanks punched from aluminium sheets; maximising efficiency and minimising waste. 
  • Some products require blanking and piercing; such as casings for computers.

(http://upload.wikimedia.org/wikipedia/commons/thumb/3/34/Blanking_vs_piercing.svg/440px-Blanking_vs_piercing.svg.png)

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Plasma cutting (Wasting process)

  • Plasma cutting uses an electric arc to generate enough energy to blast through sheet metal.
  • The energy comes from the electric arc, as well as inert gas and compressed air.
  • This process produces very little waste material.
  • A fine cut is achieved with little or no finishing required to remove burrs.

(http://www.plasma-cutter.org/images/plasma2.gif)

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Laser cutting (Wasting process)

  • Laser cutting can produce cuts which are much more detailed than plasma cutting; as the width of the cut is much narrower.
    • This also prouces less waste material. 
  • This and plasma cutting can be controlled using CNC machines to ensure consistent quality during mass production. 
  • A broader range of materials can be cut using laser cutting, including paper, card, plastics and certain manmade boards.
    • Plasma cutting on the other hand is restricted to metals because of its electrical conductive properties.
  • The amount of energy emitted by the laser can be controlled, and the detail of the cut can be controlled by changing the speed at which the laser moves over the material.
    • Altering the speed can also determine whether the material is to be cut or engraved; another advantage of laser cutting over plasma cutting.
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Laser cutting (Diagram)

(http://www.engineerstudent.co.uk/Images/Laser_cutter_head_sml.png)

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Press forming (Redistribution process)

  • Press forming simple means pressing a metal into a more 3D shape.
  • It is carried out with the material at room temperature and relies upon the ductility of the material; if a material is insufficiently ductile it means it will require annealing before press forming.
  • Press forming is carried out using a press and a die, the punch presses the material into the die so it takes its shape.
  • Both the punch and the die are made from toughened die steels; which is hard enough to reshape the material without the material reshaping it and is also resistant to impact and wear from repeatedly contacting material to be pressed.

[+] Can form sheet materials into 3D shapes; such as panels for car bodies.

[+] The act of folding a material gives it more stiffness and rigidity. 

[+] Stretching the material acts as work hardening; the material becomes less ductile and gains structural strength. Therefore a car body achieved its strength and rigidity by the joining together of a large number of press-formed steel components; whilst weight is kept to a minimum.

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Press forming: Embossing and deep drawing

Embossing

  • Embossing is similar to press forming in the way that it uses dies and punches to make a material a more 3D shape.
  • Embossing however is used to make decorative features on the surface of a material and can create quite intricate details.
  • The detail to be applied to the material is on the end of the punch; much like a stamp.
  • Examples of products using embossing include biscuit tins, paper and card products such as greeting cards and posh paper.

Deep drawing

  • Deep drawing involves pushing a blank through a series of rings to form it into a cup shape.
  • This process is called 'cupping'. 
  • It is used in the manufacture of drinks cans.
  • It uses the material's ductility to deep draw without fracture.
  • Deep drawing results in a reduction of wall thickness, the stretching results in sides up to a third thinner than the base.
  • The base of the can is formed by press forming whilst other processes are used to shape the neck of the can and seal the top.
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Spinning (Redistribution process)

  • Spinning is a traditional process used for forming hollow 3D objects from sheet material.
  • A circular metal sheet is gripped in a machine similar to a lathe and is formed around a mandrel using a forming tool as it spins.
  • Products manufactured in this way include saucepans and woks.

(http://www.quia.com/files/quia/users/brownfieldreview/Spinning.jpg)

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Sand casting (Redistribution process)

1. A pattern is made, from wood, metal, polymer or other ranges of materials. They can be made into complex shapes.
2. Each half of the pattern is placed on a base board and a mould half box is placed over it.
3. Green sand is 'tamped' around the pattern, forcing it to take its shape; followed by backing sand. 
4. The pattern is removed from the mould half and riser and runner gates are cut into the sand.
5. The mould halves are then joined together using locating pins to ensure alignment.
6. The molten metal is poured into the runner gate and the riser indicates when the mould is full.
7. Once the mould has solidified the mould is broken open; leaving the material with the runner and riser gates still attached; later removed using a band saw or other means depending on material.

[+] Complex 3D shapes can be produced.
[+] Cores can be made for hollow sections.
[+] Appropriate for small production runs.
[-]  Poor surface finish; therefore some machining is necessary.
[-]  Not as accurate as die or investment casting.
[-]  Low rate of output; therefore only suitable for small production runs.

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Sand casting (Diagram)

(http://core.materials.ac.uk/repository/ou_manufacturing/t173_2_020i.jpg)

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Gravity die casting (Redistribution process)

Die casting is used for casting metals with low melting points into alloy steel/die steel moulds.

  • Gravity die casting is similar to sand casting in the way that it relies solely on gravity for the metal to reach every part of the mould; poured in using a runner gate.
  • The moulds are made from alloy steel and split to allow removal of the product (unlike sand casting where the mould has the be broken).
  • Gas rings are located on the outside of the mould to keep it heated and ensure even cooling.
  • Fluxes are used to prevent the oxidisation of metals during processing.

Products made using gravity die casting tend to be large, undetailed objects, such as car and motorbike wheels.

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Pressure die casting (Redistribution process)

Die casting is used for casting metals with low melting points into alloy steel dies.

Cold chamber high pressure die casting

  • This uses a hydraulic ram to produce pressure; ensuring that the molten metal meets all parts of the die.
  • Metal is poured into the cyclinder and then forced by the hydraulic ram into the closed die.
  • The dies are then water cooled; resulting in rapid cooling of the product.

Hot chamber high pressure die casting

  • This involves the same action from the hydraulic ram.
  • However instead of the metal being poured into a cyclinder, the ram is fed directly from a 'hot chamber' reservoire of molten metal.

Products produced using high pressure die casting are usually small, highly complex components, including those for locks and uPVC sliding doors.

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Pressure die casting - Diagrams

Cold chamber high pressure die casting

(http://2.bp.blogspot.com/_7UH1P0Umjhk/SLD63eBWrYI/AAAAAAAAAlk/IIcE-bmJjzQ/s320/die+casting.jpg)

Hot chamber high pressure die casting

(http://upload.wikimedia.org/wikipedia/commons/thumb/6/6f/Cold-chamber_die_casting_machine_schematic.svg/1170px-Cold-chamber_die_casting_machine_schematic.svg.png)

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Industrial die casting (Redistribution process)

  • Industrial die casting is essentially a development of hot chamber high pressure die casting.
  • Instead of using two mould halves it uses up to four slides with different component moulds which all fit together to make one big complex mould (like a jigsaw). 
    • Each of the slides contains either a core or a cavity which when closed together with all the other slides makes the desired shape.
    • Each slide moves independently from one another and is controlled by computers. 
    • Mechanical locking systems hold them together whilst the material is being injected.
  • This type of casting is used for the rapid manufacture of small zinc and magnesium components. 
    • Products include door locks and the internal components for domestic plug sockets.

(http://www.ceatec.com/2011/pgm/images/products/160.jpg)

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Advantages of die casting

  • The surface finish of die cast products is far superior to that of a sand casted product.
    • It is as smooth as the surface of the die (alloy). 
  • The shape of the die determines the shape of the product; therefore the accuracy of size and detail of the die are important. 
  • The structure of a product which is die cast tends to be much better than other means of processing; due to the structural effects of rapid cooling which strengthens the material.
  • Rapid cooling of die cast products makes this process suitable for large-scale production as it makes things quicker.
    • This is necessary to cover the cost of the dies and for the manufacturer to make a profit.
  • Alloys with a low melting point require less energy to process; resulting in low energy costs.
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Investment casting (Redistribution process)

This is a very old process, and is used for the processing of metals with very high melting points.

  • A wax pattern is created to a high degree of accuracy.
  • This is then coated with high temperature ceramic material, by dipping it into ceramic slip. When the sufficient thickness of ceramic coating is achieved, it is left to dry.
  • Once dried it is fired in a kiln; causing the wax to melt leaving a cavity for metal to be cast into.
  • When the ceramic mould has cooled, the molten metal is poured in; usually using gravity.
  • When the metal has cooled, the ceramic mould is destroyed leaving only the finished product.

Typical products of investment casting include turbine blades for jet engines, tools and dies, motorcycle steering head components. 

[+] Good finishes with high degrees of accuracy can be achieved.

[+] Complex shapes which can't be achieved with other casting processes can be achieved.

[+] Complex shapes can be made using metals which can't be machined.

[-]  Cost is very high because of the constant need to remould the wax and remake the ceramic mould.

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Sintering (Redistribution process)

Sintering is the process used to form metals which are difficult to form in any other way.

  • It relies on crushing the material into a powder and then compacting it in the die which will give the product its final shape.
  • The compacted shape is heated to promote bonding between the particles of the material.
  • This process is exactly the same for processing metals and cermets.
    • Clay powder with a low moisture content is formed into a compacted shape in a hydraulic kiln and then fired in a press; where all the particles of the clay become bonded.

Examples of products manufactured in this way are magnetic products made from cobalt.

(http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/apps/sinter1.gif)

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Forging (Redistribution process)

Forging can be done by hand or machine and is done when the metal is hot. This is to reduce the risk of work hardening and make the metal easier to manipulate (using less energy). Basic hand processes are carried out using hammers and anvils. Great force can be generated by using mechanical hammers.

Bending 
Bends are achieved when the metal is being worked. Cold metal = Gradual bend. Hot metal = sharper bend.

Drawing down
The metal is hammered into a thinner section; resulting in increased length of the section.

Punching and drifting
Hammering a spiked tool into the piece to produce a hole of various shapes and sizes. Drifts are used to tidy up the hole.

Twisting and scrolling
Literally twisting and manipulating the metal. This can be done when the metal is hot or cold.

Products made using forging include wrought iron gates and horseshoes.

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Drop forging (Redistribution process)

Done when large numbers of similarly shaped objects are required; e.g. spanners. It is a refining process; where metal is placed between a pair of drop forging dies.

  • The upper die is attached to the vertical sliding hammer and large forces are exerted onto the metal blank between the dies; forcing it into the right shape.
  • The blank will often pass through the die a number of times before the right shape is achieved.
  • Products made include cams and gears used in the automotive industry.

The spanner

  • The function of a spanner is to fit over a nut and loosen it using a twisting, pressure motion. 
  • Failure to do so would render the spanner useless.
  • A cast iron spanner has a grain with no overall grain; meaning that sufficient force would make it break at the handle or the jaw.
  • A spanner cut from sheet has a parallel grain orientation (due to the rolling process forming the sheet). This means it would be stronger than a cast iron spanner but could still fracture around the head.
  • A drop forged spanner would have a refined grain which follows the shape of the die and therefore the product; making it much stronger than either other spanner.
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Gravity die casting (Diagram)

(http://ustudy.in/sites/default/files/Gravity.jpg)

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