notes on casting

uni/mmme/1029_materials_and_manufacturing/manufacturing.md

@@ -21,3 +21,113 @@ $$cost < price < value$$
 Cost modelling equation:
 
 $$C = \frac{mC_m}{1-f} + \frac{C_t}{n} + \frac{1}{\dot n} \left[ \dot C_{oh} + \frac{C_c}{L\cdot t_{wo}} \right]$$
+
+# 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
+
+![](./images/vimscrot-2022-02-21T16:36:48,640821636+00:00.png)
+
+- 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
+
+1. Make a master die
+2. Make wax pattern by casting wax into master die
+3. Coat wax pattern with investment material 
+
+   1. First with a slurry of water and fine ceramic to capture fine details
+   2. Then coat with stucco, which is a thicker coating for strength
+
+4. Heat mould to melt wax out, bake and preheat mould
+5. Pour in molten metal
+6. 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:
+
+  ![](./images/vimscrot-2022-02-21T17:12:20,117329889+00:00.png)
+
+### 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