Processes
Investment Casting
Investment Cast
Tolerances
|
Dimension mm |
Tolerance |
|
Up to 12.5 |
±0.10 |
|
12.5 - 25 |
±0.20 |
|
25 - 50 |
±0.30 |
|
50 - 75 |
±0.40 |
|
75 - 100 |
±0.50 |
|
100 - 125 |
±0.60 |
|
150 - 175 |
±0.70 |
|
175 - 200 |
±0.80 |
Investment casting occasionally called the Lost Wax process is a very good technique for producing complicated shapes in many metals and alloys. Surface finish is very good and in some situations complicated internal cores can be utilised.
Investment casting is usually the most expensive casting process but is able to give good tolerances.
The process begins with the injection of wax into a die cavity.
This wax is then assembled onto a wax tree and subsequently coated in layers of ceramic slurry and dried.
The wax is removed by Autoclaves and firing in an oven.
The fired ceramics are then filled with the desired metal.
After cooling the ceramic is removed and discarded leaving a copy of the original wax injections.
Sand Casting
Sand Cast
Tolerances
|
Dimension mm |
Tolerance |
|
Up to 12.5 |
±0.20 |
|
12.5 - 25 |
±0.50 |
|
25 - 50 |
±1.00 |
|
50 - 75 |
±1.50 |
|
75 - 100 |
±2.00 |
|
100 - 125 |
±2.00 |
|
150 - 175 |
±2.50 |
|
175 - 200 |
±2.50 |
|
200 - 225 |
±3.00 |
|
225 - 250 |
±3.00 |
|
250 - 275 |
±3.00 |
|
300 - 325 |
±3.50 |
|
325 - 350 |
±3.50 |
|
375 - 400 |
±4.00 |
|
400 - 425 |
±4.00 |
|
425 - 450 |
±4.50 |
Patterns are made from wood, plastic or metal. Designs tend to be simpler but again complicated internal shapes can be achieved. Surface finish is usually very pitted.
Sand casting is cheapest technique to begin casting but does suffer from dimensional accuracy.
The patterns are packed in sand with a binder to give green strength. The patterns are removed and the two piece mould clamped together.
Metal is poured into the cavity left by the patterns and allowed too cool.
The sand is then knocked off the metal leaving the castings.
Die Casting (Gravity and Pressure)
Gravity Die Cast
Tolerances
|
Dimension mm |
Tolerance |
|
Up to 12.5 |
±0.26 |
|
12.5 - 25 |
±0.28 |
|
25 - 50 |
±0.32 |
|
50 - 75 |
±0.36 |
|
75 - 100 |
±0.40 |
|
100 - 125 |
±0.50 |
|
150 - 175 |
±0.60 |
|
175 - 200 |
±0.60 |
Die Castings are made by injecting or forcing molton metal into a harden steel die.
The advantage of die casting is that you can rapidly create thin wall complex shapes with high dimensional accuracy.
The disadvantage is the limitation to low melting point alloys. The dies also do not last as long due to high wear from the metals.
MIM (Metal Injection Moulding)
The MIM process is used in manufacture of high volume thin section parts.
Almost all metal that can be produced in a suitable powder form can be utilised in the MIM technique.
The powdered metal is forced into the cavity of a die whilst being heated.
The green binder is then removed from the metal and further heating to sinter the metal particles together.
Great care is taken in reducing the amount of oxide that can be produced during the sintering as this can reduce strength of the part.
The great advantage of MIM is the lack of waste and very little production of scrap parts.
A selection of parts using the MIM process.
Registered N°. 786649 England