400 lines
14 KiB
Markdown
Executable File
400 lines
14 KiB
Markdown
Executable File
---
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author: Alvie Rahman
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date: \today
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title: MMME1029 // Manufacturing
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tags:
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- uni
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- nottingham
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- mechanical
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- engineering
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- mmme1029
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- manufacturing
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uuid: b5fae4fd-32c3-4fd8-a05a-7440d6c44f9c
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---
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# Cost Modelling
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Key issues in selection:
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- Component function, including materials and shape, form, and assembly
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- Manufacturing process may greatly affect material properties, such as yield strength
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- Similarly, the material will likely decide the manufacturing process
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- Cost of a material and manufacture
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The main requirement for a product to be viable is
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$$cost < price < value$$
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Cost modelling equation:
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$$C = \frac{mC_m}{1-f} + \frac{C_t}{n} + \frac{1}{\dot n} \left[ \dot C_{oh} + \frac{C_c}{L\cdot t_{wo}} \right]$$
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# Shaping Processes
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## Casting
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- Can be used for large size range
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- Molten metal poured into solid mould to give shape
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- Heat removed leads to shrinkage
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- We need to be able to melt the metal and handlethe molten metal
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- Mould degradation by the liquid metal needs to be considered
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- Heat flowing from the molten metal into the mould causes a drop in temperature so solidification
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starts from outside inwards
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- Rate of solidification depends on rate of heat flow into mould
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### Types of Mould
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- Expendable mould (sand, plaster, ceramic)
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- The mould is used once, being broken to release the casting
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- Can have multiple use or single use pattern (investment and lost foam casting
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- Multiple mold casting
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- Die casting (pressure die casting)
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- Permanent mould casting (gravity die casting)
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#### Sand Casting
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- Wide range of metals can be cast
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- Almost no limit to size and shape of casting
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- Poorer tolerances than other proces, rough surface texture
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- Slow
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- Economic for a low number of castings
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- Applications include cylinder blocks and large pipe fittings
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#### Investment Casting
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- A high cost process
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- Used mostly for complex shapes, such as sculptures, jewellery, and gas turbine blades
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- Can be used for a wide range of metals
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- Very high precision and surface finish
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1. Make a master die
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2. Make wax pattern by casting wax into master die
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3. Coat wax pattern with investment material
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1. First with a slurry of water and fine ceramic to capture fine details
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2. Then coat with stucco, which is a thicker coating for strength
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4. Heat mould to melt wax out, bake and preheat mould
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5. Pour in molten metal
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6. Wait for solidification, break mould when done
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#### Permanent Mould Casting (Gravity Die Casting)
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- Mould cavity is machined into mating metal blocks
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- Molten material poured into mould
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- Mould material is cast iron, steel, bronze, graphite
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- Mould must disassebmble without locking
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- Mould is expensive but can be reused (typically around 25k times)
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- Mould life is reduce by casting high meling point metals
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- Good surface finish and dimensional accuracy
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- Cooling is rapid therefore high production rates
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- Example use is a piston
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#### Die Casting (High Pressure Die Casting)
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- Dies must be able to withstand high pressure
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- 0.1 mm slits at parting lines provide escape for air
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- Dies are made of expensive tool steels
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- High volume production is necessary to justify costs
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- Generally limited to low viscosity, low melting point, non ferrous metals like Al, Zn, Mg, and Pb
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- Good surface finish
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- Precision castings with thickness between 0.75 mm and 12 mm
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### Design of Castings
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- Distribute castings evently around parting planes
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- Need to be able to get patterns out of moulds and casting out of moulds where applicable
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- No re-entrants (complex multi-part moulds may be able to avoid this restriction)
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- Draft angle between surfaces
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- Need to be able to get solid patternout of mould in sand casting
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- Need to be able to get solid casting out of mould in die casting
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- Allow for shrinkage --- dimensions of casting mould/pattern needs to be made so that part is
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desired size after shrinkage
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- Avoid rapid change in section or direction:
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### Solifidification of Metals
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- How well the liquid fills detail depends on viscosity of liquid
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- During freezing, latent heat of fusion is removed
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- During freezing, material is a solid/liquid mixture
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- There is a significant (~7 %v) shrinkage during solidification
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- Heat flows down steepest thermal gradient so usually there is an actively cooled section
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- Thin sections freeze faster than thick sections
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### Castability
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- Low melting point
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- Low viscosity and surface tension
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- Low solidification contraction
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- Low thermal capacity and high conductibity
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- Low solubility
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- Not contaminated by air
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## Deformation
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When a metal is plastically deformed, dislocations move and multiply.
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Annealed aluminium may have a dislocatio density of around 200 m per mm$^3$.
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This is a very low amount.
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A heavily cold worked piece may have a density of up to 270 km per mm$^3$.
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As dislocation density increases, the dislocations impede the motion of other dislocations.
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This means that to continue plastically deforming, more stress has to be applied.
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The stress goes down towards the end of the graph due to the material necking, meaning the
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material gets thinner.
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This means that the engineering stress is lower as the true area is lower.
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The true stress, however, is going up:
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### Effect of Prior Deformation (*Work Hardening*)
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See
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[here](materials.html#work-hardening-and-cold-working)
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for more information
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### Effect of Temperature (Diffusion)
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In an alloy, atoms tend to migrate from regions of high concentration to low concentration.
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This is diffusion.
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More information on diffusion [here](materials.html#diffusion).
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### Annealing
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Annealing is a process by which a component is heated to reduce work hardening.
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These are diffusional processes and only occur at higher temperatures.
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When the temperature of a material, $T > 0.55T_m$, it is said to be hot.
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A material being worked on hot has its deformations eliminated as fast as they are created.
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A material is said to be cold when $T < 0.35T_m$.
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# Powder Processes
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Poweders can plowflow if forces between them are low
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With small amounts of binder, they can form "*plastic*" materials like clay.
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A *slurry* can be formed with a liquid carrier (where there is enough liquid to separate particles).
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In a slurry, often you want to reduce liquid content but avoid the particles touching or attracting
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each other.
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Adding a *deflocculant*[^d_deflocculant] results in the formation of a stable *slip*.
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Making the powders is often quite expensive when you have a controlled size distribution.
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## Clay and Ceramics
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Clay is an abundant raw material but it needs to be milled and screen for a controlled size
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distribution.
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When mixed with water it forms a *plastic* material.
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Structural clay products include bricks, tiles, and pipes.
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Other proucts include whitewares such as porcelain, pottery, and tableware.
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Ways to form the clay include pressing, isostatic pressing, extrusion, and machining.
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Engineering ceramics (e.g. silicon carbide, alumina) are shaped with small amounts of binder ---
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commonly pressed or isostatically pressed.
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## Slip Casting
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1. Pour slip into a mould (e.g. plaster of Paris)
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2. The mould is extremely water absorbing. This results in the remaining part developing some
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structural integrity.
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3. Remove the mould and place in the oven to reduce water content.
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4. Fire to harden
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5. Add glaze and fire again.
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Drying leads to shrinkage and potential cracking.
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It also gives strength and allows for handling and maybe machining.
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## Sintering of Metals and Ceramics
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Atoms diffuse to points of contact, creating bridges and reducing the pore size.
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Diffusion is driven by a desire to reduce the surface area as surfaces are regions of high energy.
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## Powdering Metallurgy
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- Competitive with processes like casting, forging, machining
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- Used when the melting point is too high, a chemical reaction occurs at melting point, the part is
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too hard to machine, or a very large quantity (on the order of 100 000) of the part is needed
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- Nearly 70% of parts produced is by powder metallurgy
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- Good dimensional accuracy
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- Controlloable porosity
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- Size range from balls in ball point pens to parts weighing 50 kg
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Basic steps of powder metallurgy:
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1. Powder production (commonly atomization) --- this is often a costly process and you must minimize
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oxidation of the metal
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2. Blending/mixing --- add binders to keep the particles together and lubricants to reduce damage to
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dies and aid consolidation
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3. Powder consolidation
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- Shaping in a die
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- 100-900 MPa of pressure applied
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- Fast process as no heat needs to be removed
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4. Sintering at $0.7T_m$ to $0.9T_m$
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Shaping equipment has no requirement to be able to withstand high temperatures and the sintering
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equipment does not have the need for complex designs.
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This separates problems, making them easier to design.
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The pressing equipment is costly but the time spent pressing is quite small, allowing for greater
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throughput.
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Additionally, the furnace can operate continuously and is simple and cost effective.
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### Green Density
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The *green density* is a fraction of the true density.
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A low green density will result in high shrinkage on sintering.
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## Moulding
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Moulding is a shaping process used for viscous materials (typically polymers and glasses).
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Here the material can hold a shape unsupported but not for very long or under even small stresses.
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In order to mould a material we must raise the temperature above the glass transition temperature,
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$T_g$.
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At this temperature, the C-C bond in the chapolymer chain are able to easily rotate around each
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other.
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Large side chains or molecules on the main chain make it harder to rotate these bonds, making
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$T_g$ higher.
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Polar groups (e.g. chloride, cynaide, and hydroxide) have also hinder bond rotation.
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More information about polymers
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[here](https://notes.alv.cx/notes/uni/mmme/1029_materials_and_manufacturing/materials.html#polymers-1).
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## Extrusion
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Extrusion produced parts of constant cross section, like pipes and rods.
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The process is used primarily with thermoplastics and 60% of polymers are prepared by extrusion.
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## Blow Moulding
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Blow moulding is a rapid process with low labour costs.
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It produces hollow components that do not require a constant thickness, such as
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bottles, petrol tanks, and drums.
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Common materials to blow mould are HDPE, LDPE, PP, PET, and PVC.
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There are three common types of blow moulding:
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- Extrusion blow moulding
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- Injetion blow moulding
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- Stretch-blow processes
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However, they involve the following stages:
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1. A tubular preform, called a *parison* (a word I haven't been able to remember since GCSE) is
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produced by either extrusion of injection moulding
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2. The *parison* is transferred into a cooled split-mould
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3. The *parison* is sealed and inflated to take up the shape of the mould
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4. The moulding is let to cool and solifidies under pressure
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5. The mould is opened and moulding is ejected
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## Injection Moulding
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1. Powder or pellets of polymer heated to liquid state (low viscosity)
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2. Under pressure, the liquid polymer is forced into a mould through a *sprue*, a small opening
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3. The pressurized material is held in the mould until it solidifies
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4. The mould is opened and the part is removed by ejector pins
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Selection was cancelled by keystroke or right-click.
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Theromoplastics are most common in injection moulding.
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A very high level of detail is attainable through this process and it produced little waste.
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Similar to [Die Casting](#die-casting-high-pressure-die-casting), you must consider corners (avoid
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sharp ones), draft angles (so you can get the part out), and section thickness (using ribs instead
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is preferable).
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Due to the high capital cost, injection moulding is only economical at high production volumes.
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### Co-Injection Moulding
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There is *sequential moulding* (one after the other) and *co-injection moulding* (together).
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These processes reduce assembly costs by integrating the parts and can use low grade recycled
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material for the inside of a component.
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It also allows for a part have to have multiple colours.
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This process requires special attention to be payed to shrinking/cooling.
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## Rotational Moulding
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Rotational moulding involves coating the insides of a heated mould with a thermoplastic.
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It is a low pressure alternative to blow moulding for making hollow components and is used
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for large components such as storage tanks, boat hulls, kayaks, and cones.
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## Moulding for Thermosetting Polymers
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There are two types:
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a. Compression moulding
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b. Transfer moulding
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### Compression Moulding
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For thermoplastics, the mould is cooled before removoal so the part will not lose its shape.
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Thermosets, however, may be ejected while they are hot so long as curing is complete.
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The process is slow but the material only moves a short distance and has lower mould pressures.
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It also does minimal damage to reinforcing fibres in composites and it is possible to make large
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parts.
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More manual labour is required and has longer cycle times than injection moulding.
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# Machining Processes
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## Advantages of Machining Processes
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- High precision of geometrical dimensions, tolerances, and surface finishes
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- Is able to make one off prototypes in production grade material
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- Creates high volume production tooling
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- Increasing hard/brittle/fragile/tough materials can only be machines
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- Some designs are so complex that machining is the only realistic process to make them with
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## Disadvantages of Machining Processes
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- Material is wasted (as it is a subtractive process)
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- Complex parts require expensive machines to make and making them can take a long time
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- Parts need to be set up using fixtures. These fixtures get increasingly complex with the part.
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- Faster production rates and harder materials wear down the tools.
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# Glossary
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[^d_deflocculant]: a substance which, when added to scattered particles in suspension, causes a reduction in apparent viscosity. Deflocculants are substances which prevent flocculation by increasing zeta potential and therefore the repulsive forces between particles. (<https://digitalfire.com/article/deflocculants%3A+a+detailed+overview>)
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