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Cost Modelling
Key issues in selection:
- Component function, including materials and shape, form, and assembly
- Manufacturing process may greatly affect material properties, such as yield strength
- Similarly, the material will likely decide the manufacturing process
- Cost of a material and manufacture
The main requirement for a product to be viable is
Cost modelling equation:
Shaping Processes
Casting
- Can be used for large size range
- Molten metal poured into solid mould to give shape
- Heat removed leads to shrinkage
- We need to be able to melt the metal and handlethe molten metal
- Mould degradation by the liquid metal needs to be considered
- Heat flowing from the molten metal into the mould causes a drop in temperature so solidification starts from outside inwards
- Rate of solidification depends on rate of heat flow into mould
Types of Mould
Expendable mould (sand, plaster, ceramic)
- The mould is used once, being broken to release the casting
- Can have multiple use or single use pattern (investment and lost foam casting
Multiple mold casting
- Die casting (pressure die casting)
- Permanent mould casting (gravity die casting)
Sand Casting
- Wide range of metals can be cast
- Almost no limit to size and shape of casting
- Poorer tolerances than other proces, rough surface texture
- Slow
- Economic for a low number of castings
- Applications include cylinder blocks and large pipe fittings
Investment Casting
- A high cost process
- Used mostly for complex shapes, such as sculptures, jewellery, and gas turbine blades
- Can be used for a wide range of metals
- Very high precision and surface finish
Make a master die
Make wax pattern by casting wax into master die
Coat wax pattern with investment material
- First with a slurry of water and fine ceramic to capture fine details
- Then coat with stucco, which is a thicker coating for strength
Heat mould to melt wax out, bake and preheat mould
Pour in molten metal
Wait for solidification, break mould when done
Permanent Mould Casting (Gravity Die Casting)
- Mould cavity is machined into mating metal blocks
- Molten material poured into mould
- Mould material is cast iron, steel, bronze, graphite
- Mould must disassebmble without locking
- Mould is expensive but can be reused (typically around 25k times)
- Mould life is reduce by casting high meling point metals
- Good surface finish and dimensional accuracy
- Cooling is rapid therefore high production rates
- Example use is a piston
Die Casting (High Pressure Die Casting)
- Dies must be able to withstand high pressure
- 0.1 mm slits at parting lines provide escape for air
- Dies are made of expensive tool steels
- High volume production is necessary to justify costs
- Generally limited to low viscosity, low melting point, non ferrous metals like Al, Zn, Mg, and Pb
- Good surface finish
- Precision castings with thickness between 0.75 mm and 12 mm
Design of Castings
Distribute castings evently around parting planes
Need to be able to get patterns out of moulds and casting out of moulds where applicable
No re-entrants (complex multi-part moulds may be able to avoid this restriction)
Draft angle between surfaces
- Need to be able to get solid patternout of mould in sand casting
- Need to be able to get solid casting out of mould in die casting
Allow for shrinkage — dimensions of casting mould/pattern needs to be made so that part is desired size after shrinkage
Avoid rapid change in section or direction:
Solifidification of Metals
- How well the liquid fills detail depends on viscosity of liquid
- During freezing, latent heat of fusion is removed
- During freezing, material is a solid/liquid mixture
- There is a significant (~7 %v) shrinkage during solidification
- Heat flows down steepest thermal gradient so usually there is an actively cooled section
- Thin sections freeze faster than thick sections
Castability
- Low melting point
- Low viscosity and surface tension
- Low solidification contraction
- Low thermal capacity and high conductibity
- Low solubility
- Not contaminated by air
Deformation
When a metal is plastically deformed, dislocations move and multiply.
Annealed aluminium may have a dislocatio density of around 200 m per mm. This is a very low amount. A heavily cold worked piece may have a density of up to 270 km per mm.
As dislocation density increases, the dislocations impede the motion of other dislocations. This means that to continue plastically deforming, more stress has to be applied.
The stress goes down towards the end of the graph due to the material necking, meaning the material gets thinner. This means that the engineering stress is lower as the true area is lower. The true stress, however, is going up:
Effect of Prior Deformation (Work Hardening)
See here for more information
Effect of Temperature (Diffusion)
In an alloy, atoms tend to migrate from regions of high concentration to low concentration. This is diffusion.
More information on diffusion here.
Annealing
Annealing is a process by which a component is heated to reduce work hardening.
These are diffusional processes and only occur at higher temperatures.
When the temperature of a material, , it is said to be hot. A material being worked on hot has its deformations eliminated as fast as they are created.
A material is said to be cold when .
Powder Processes
Poweders can plowflow if forces between them are low
With small amounts of binder, they can form “plastic” materials like clay.
A slurry can be formed with a liquid carrier (where there is enough liquid to separate particles). In a slurry, often you want to reduce liquid content but avoid the particles touching or attracting each other. Adding a deflocculant1 results in the formation of a stable slip.
Making the powders is often quite expensive when you have a controlled size distribution.
Clay and Ceramics
Clay is an abundant raw material but it needs to be milled and screen for a controlled size distribution. When mixed with water it forms a plastic material.
Structural clay products include bricks, tiles, and pipes. Other proucts include whitewares such as porcelain, pottery, and tableware.
Ways to form the clay include pressing, isostatic pressing, extrusion, and machining.
Engineering ceramics (e.g. silicon carbide, alumina) are shaped with small amounts of binder — commonly pressed or isostatically pressed.
Slip Casting
- Pour slip into a mould (e.g. plaster of Paris)
- The mould is extremely water absorbing. This results in the remaining part developing some structural integrity.
- Remove the mould and place in the oven to reduce water content.
- Fire to harden
- Add glaze and fire again.
Drying leads to shrinkage and potential cracking. It also gives strength and allows for handling and maybe machining.
Sintering of Metals and Ceramics
Atoms diffuse to points of contact, creating bridges and reducing the pore size. Diffusion is driven by a desire to reduce the surface area as surfaces are regions of high energy.
Powdering Metallurgy
- Competitive with processes like casting, forging, machining
- Used when the melting point is too high, a chemical reaction occurs at melting point, the part is too hard to machine, or a very large quantity (on the order of 100 000) of the part is needed
- Nearly 70% of parts produced is by powder metallurgy
- Good dimensional accuracy
- Controlloable porosity
- Size range from balls in ball point pens to parts weighing 50 kg
Basic steps of powder metallurgy:
Powder production (commonly atomization) — this is often a costly process and you must minimize oxidation of the metal
Blending/mixing — add binders to keep the particles together and lubricants to reduce damage to dies and aid consolidation
Powder consolidation
- Shaping in a die
- 100-900 MPa of pressure applied
- Fast process as no heat needs to be removed
Sintering at to
Shaping equipment has no requirement to be able to withstand high temperatures and the sintering equipment does not have the need for complex designs. This separates problems, making them easier to design.
The pressing equipment is costly but the time spent pressing is quite small, allowing for greater throughput. Additionally, the furnace can operate continuously and is simple and cost effective.
Green Density
The green density is a fraction of the true density. A low green density will result in high shrinkage on sintering.
Moulding
Moulding is a shaping process used for viscous materials (typically polymers and glasses). Here the material can hold a shape unsupported but not for very long or under even small stresses.
In order to mould a material we must raise the temperature above the glass transition temperature, . At this temperature, the C-C bond in the chapolymer chain are able to easily rotate around each other.
Large side chains or molecules on the main chain make it harder to rotate these bonds, making higher. Polar groups (e.g. chloride, cynaide, and hydroxide) have also hinder bond rotation.
More information about polymers here.
Extrusion
Extrusion produced parts of constant cross section, like pipes and rods. The process is used primarily with thermoplastics and 60% of polymers are prepared by extrusion.
Blow Moulding
Blow moulding is a rapid process with low labour costs. It produces hollow components that do not require a constant thickness, such as bottles, petrol tanks, and drums. Common materials to blow mould are HDPE, LDPE, PP, PET, and PVC.
There are three common types of blow moulding:
- Extrusion blow moulding
- Injetion blow moulding
- Stretch-blow processes
However, they involve the following stages:
- A tubular preform, called a parison (a word I haven’t been able to remember since GCSE) is produced by either extrusion of injection moulding
- The parison is transferred into a cooled split-mould
- The parison is sealed and inflated to take up the shape of the mould
- The moulding is let to cool and solifidies under pressure
- The mould is opened and moulding is ejected
Injection Moulding
- Powder or pellets of polymer heated to liquid state (low viscosity)
- Under pressure, the liquid polymer is forced into a mould through a sprue, a small opening
- The pressurized material is held in the mould until it solidifies
- The mould is opened and the part is removed by ejector pins Selection was cancelled by keystroke or right-click.
Theromoplastics are most common in injection moulding. A very high level of detail is attainable through this process and it produced little waste.
Similar to Die Casting, you must consider corners (avoid sharp ones), draft angles (so you can get the part out), and section thickness (using ribs instead is preferable).
Due to the high capital cost, injection moulding is only economical at high production volumes.
Co-Injection Moulding
There is sequential moulding (one after the other) and co-injection moulding (together).
These processes reduce assembly costs by integrating the parts and can use low grade recycled material for the inside of a component. It also allows for a part have to have multiple colours.
This process requires special attention to be payed to shrinking/cooling.
Rotational Moulding
Rotational moulding involves coating the insides of a heated mould with a thermoplastic. It is a low pressure alternative to blow moulding for making hollow components and is used for large components such as storage tanks, boat hulls, kayaks, and cones.
Moulding for Thermosetting Polymers
There are two types:
- Compression moulding
- Transfer moulding
Compression Moulding
For thermoplastics, the mould is cooled before removoal so the part will not lose its shape. Thermosets, however, may be ejected while they are hot so long as curing is complete.
The process is slow but the material only moves a short distance and has lower mould pressures. It also does minimal damage to reinforcing fibres in composites and it is possible to make large parts.
More manual labour is required and has longer cycle times than injection moulding.
Machining Processes
Advantages of Machining Processes
- High precision of geometrical dimensions, tolerances, and surface finishes
- Is able to make one off prototypes in production grade material
- Creates high volume production tooling
- Increasing hard/brittle/fragile/tough materials can only be machines
- Some designs are so complex that machining is the only realistic process to make them with
Disadvantages of Machining Processes
- Material is wasted (as it is a subtractive process)
- Complex parts require expensive machines to make and making them can take a long time
- Parts need to be set up using fixtures. These fixtures get increasingly complex with the part.
- Faster production rates and harder materials wear down the tools.
Glossary
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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