What is Ceramic mold casting?
Ceramic mold casting, also known ambiguously as ceramic molding, is a group of metal casting processes that use ceramics as the mold material.
It is a combination of plaster mold casting and investment casting. There are two types of ceramic mold casting: the Shaw process and the Unicast process.
These casting processes are commonly used to make tooling, especially drop forging dies, but also injection molding dies, die casting dies, glass molds, stamping dies, and extrusion dies.
Process of Ceramic mold casting
Ceramic slurry is poured over a relief pattern and left to solidify, forming a cavity mould in the shape of the component to be cast. Cope and drag parts are produced.
Each ceramic part is hardened by high temperature firing, then fitted together to make one hollow mould in which to cast the molten metal.
Shaw process
The Shaw process, also known as the Osborn-Shaw process, uses a mixture of refractory aggregate, hydrolyzed ethyl silicate, alcohol, and a gelling agent to create a mold.
This slurry mixture is poured into a slightly tapered flask and a reusable pattern (i.e. the item used to create the shape of the mold) is used.
The slurry hardens almost immediately to a rubbery state (the consistency of vulcanized rubber). The flask and pattern is then removed.
Then a torch is used to ignite the mold, which causes most of the volatiles to burn-off and the formation of ceramic microcrazes (microscopic cracks).
These cracks are important, because they allow gases to escape while preventing the metal from flowing through; they also ease thermal expansion and contraction during solidification and shrinkage.
After the burn-off, the mold is baked at 1,800 °F (980 °C) to remove any remaining volatiles. Prior to pouring metal, the mold is pre-warmed to control shrinkage.
Unicast process
The Unicast process is very similar to the Shaw process, except it does not require the mold to be ignited and then be cured in a furnace.
Instead, the mold is partially cured so the pattern can be removed and it is then completely cured by firing it at approximately 1,900 °F (1,040 °C).
If a metal with a low melting point is cast then the firing can be skipped, because the mold has enough strength in the “green state” (un-fired).
Manufacturing process of Ceramic mold casting
#1. Pattern Creation.
A pattern, often made of wax or a similar material, is produced to replicate the desired shape of the final ceramic object. The pattern can be created by hand or using computer-aided design (CAD) software and 3D printing.
#2. Mould Production.
The pattern is then embedded in a material called investment, which is a refractory material capable of withstanding high temperatures.
The investment material is usually a mixture of fine ceramic powders, such as silica, zircon, or alumina, suspended in a binder. The investment material is poured around the pattern and allowed to harden, forming a ceramic mould.
#3. Pattern Removal.
Once the investment material has solidified, the mould is heated to melt or burn out the pattern. This leaves behind a cavity in the mould that matches the shape of the original pattern.
#4. Ceramic Pouring.
The ceramic material, typically in a liquid or slurry form, is poured or injected into the mould cavity. The mould is carefully heated to encourage the ceramic material to solidify and take the shape of the mould.
#5. Cooling and Solidification.
The ceramic material is allowed to cool and harden within the mould. This can be done by controlled cooling or by placing the mould in a furnace.
#6. Mould Removal.
After the ceramic material has solidified, the investment mould is broken or dissolved away to reveal the final ceramic object. This can be done mechanically, through vibration or water blasting, or chemically with the use of solvents.
#7. Finishing.
The ceramic object may require additional finishing processes, such as sanding, polishing, or glazing, to achieve the desired appearance and surface quality.
Materials Needed For Ceramic Mold Casting
Plaster sands are only suitable for low melting point materials such as Aluminium and Copper based alloys, and gold and silver jewellery alloys.
Insufficient drying of plastic moulds can lead to hydrogen pick-up in aluminium alloys, causing porosity.
Refractory moulds (Shaw process) are suitable for almost all castable alloys (see lost wax process) but are usually used with high melting point alloys such as cast iron, carbon and alloy steels, and Cu/Be alloys.
The Shaw process can produce castings without the usual casting defects found in other processes. Benefits include:
- High resistance to hot tearing
- Casting free from gas holes
- Inclusion-free castings
- Process permits “natural feeding” of the casting.
Properties And Considerations Of Manufacturing By Ceramic Mold Casting
Manufacturing by ceramic mold casting is similar to plaster mold casting in that it can produce parts with thin sections, excellent surface finish, and high dimensional accuracy. Manufacturing tolerances between .002 and .010 inches are possible with this process.
To be able to cast parts with high dimensional accuracy eliminates the need for machining, and the scrap that would be produced by machining.
Therefore, precision metal casting processes like this are efficient to cast precious metals, or materials that would be difficult to machine.
Unlike the mold material in the plaster metal casting process, the refractory mold material in ceramic casting can withstand extremely elevated temperatures.
Due to this heat tolerance, the ceramic casting process can be used to manufacture ferrous and other high melting point metal casting materials. Stainless steels and tool steels can be cast with this process.
Ceramic mold casting is relatively expensive.
The long preparation time of the mold makes manufacturing production rates for this process slow.
Unlike in plaster mold casting, the ceramic mold has excellent permeability due to the microcrazing, (production of microscopic cracks), that occurs in the firing of the ceramic mold.
Applications of Ceramic Mold Casting
Using stainless steel and bronze, ceramic mold castingis best suited for casting a wide variety of products ranging from house hold goods to industrial tools.
Some of casted products are kitchenwares like kettles, industrial products like impellers, complex cutting tools, plastic mold tooling etc.
Advantage of Ceramic Mold Casting
- High temperature pours possible therefore suitable for steels and other alloys
- Creative complex designs can be made
- Can be used for mass production
- Casting with accurate dimensional accuracy possible
- Little machining is required therfore difficult-to machine alloys can be cast
- Supports both industry and home foundry operations
- Complicate and innovative designs can be casted.
Disadvantages Of Ceramic Mold Casting
The main disadvantages are: It is only cost effective for small- to medium-sized production runs and the ceramic is not reusable.
Ferrous and high-temperature non-ferrous are most commonly cast with these processes; other materials cast include: aluminum, copper, magnesium, titanium, and zinc alloys.