What is Ceramic mold casting?
Ceramic mold casting, sometimes referred to simply as ceramic molding, describes a set of metal casting techniques where ceramics form the mold material. This approach essentially bridges the methods used in plaster mold casting and investment casting.
Within ceramic mold casting, two main methods are typically recognized: the Shaw process and the Unicast process.
These techniques are frequently chosen for producing various types of tooling. For instance, ceramic mold casting is often used to create drop forging dies, but you’ll also find it in the manufacture of dies for injection molding, die casting, glass molding, stamping, and extrusion.
The versatility of this process makes it a reliable option across several industries where precise and durable tooling is essential.
Process of Ceramic mold casting
A ceramic slurry is carefully poured onto a relief pattern and allowed time to solidify, resulting in a cavity mould that matches the intended shape of the component. This process produces both the cope and drag sections needed for the mould.
Once formed, each ceramic piece undergoes high-temperature firing to achieve the necessary hardness. After firing, the separate parts are assembled to create a single, hollow mould, ready to receive the molten metal during casting.
Shaw process
The Shaw process, sometimes referred to as the Osborn-Shaw process, involves preparing a mold using a blend of refractory aggregate, hydrolyzed ethyl silicate, alcohol, and a gelling agent. This combination forms a slurry, which is then poured into a flask that has a slight taper. A reusable pattern—the object that shapes the mold—is placed inside during this step.
What stands out about this method is how quickly the mixture sets; within moments, it takes on a rubbery texture, similar to that of vulcanized rubber. Once the mixture has firmed up, both the flask and the pattern are carefully removed, leaving behind the mold.
At this point, a torch is used to ignite the surface of the mold. This isn’t just for show—the heat burns off most of the volatile components and creates what are known as ceramic microcrazes, or fine cracks on a microscopic level. These tiny cracks are actually quite useful: they provide escape routes for gases, preventing pressure build-up, yet are small enough to stop molten metal from seeping through. On top of that, these microcrazes help accommodate the natural expansion and contraction that occurs when the metal solidifies and shrinks.
Following this burn-off, the mold is subjected to baking at around 1,800 °F (or 980 °C), ensuring that any residual volatile materials are driven out. Before metal is finally poured into the mold, it is preheated—this extra step helps manage shrinkage during casting, leading to a more precise final product.
Unicast process
The Unicast process closely resembles the Shaw process, with one notable distinction: it eliminates the need to ignite the mold or cure it inside a furnace. In practice, the mold undergoes only a partial cure—just enough to allow the pattern to be withdrawn without damage. After this step, complete curing is achieved by firing the mold at about 1,900 °F (1,040 °C).
Interestingly, if the casting involves a metal with a relatively low melting point, this firing stage isn’t always necessary. In such cases, the mold retains sufficient strength in its so-called “green state” (meaning it hasn’t been fired) to support the casting process without any issues.
Manufacturing process of Ceramic mold casting
#1. Pattern Creation.
To replicate the intended form of the ceramic piece, a pattern—typically fashioned from wax or a comparable substance—is first created. This step can be carried out manually, allowing for hands-on shaping, or alternatively, designers may turn to computer-aided design (CAD) tools and 3D printing technologies to achieve precise results.
#2. Mould Production.
The pattern is next encased in a substance known as investment, a heat-resistant material specifically designed to endure the high temperatures involved in the process.
Typically, investment is composed of finely ground ceramic powders—such as silica, zircon, or alumina—blended with a binder to hold the mixture together. This material is carefully poured around the pattern, and once it sets, it forms a durable ceramic mold ready for the subsequent steps.
#3. Pattern Removal.
After the investment material has hardened, the mould undergoes a heating process to either melt or burn away the original pattern. This step creates a hollow cavity within the mould, perfectly replicating the form of the initial pattern.
#4. Ceramic Pouring.
Ceramic materials, often prepared as a liquid or slurry, are introduced into the mold cavity either by pouring or injection. Once inside, the mold is gradually heated, allowing the ceramic to solidify and accurately conform to the contours of the mold.
#5. Cooling and Solidification.
The ceramic is left to cool and solidify inside the mould, a process that may involve either carefully managed cooling conditions or the use of a furnace to ensure the material hardens as intended.
#6. Mould Removal.
Once the ceramic material has set, the investment mould is removed to uncover the finished ceramic piece. This step can be carried out by physically breaking the mould, using vibration or water blasting, or alternatively, by dissolving it with appropriate chemical solvents.
#7. Finishing.
Sometimes, achieving the right look and feel for a ceramic piece means going beyond the basic forming process. Additional steps like sanding, polishing, or applying a glaze can help refine the surface and bring out the qualities you’re aiming for. These finishing touches are often essential for reaching the desired appearance and surface quality.
Materials Needed For Ceramic Mold Casting
Plaster sands are typically reserved for casting metals with lower melting points, such as aluminium and copper-based alloys, as well as precious metals like gold and silver used in jewellery. When it comes to plastic moulds, failing to dry them thoroughly can introduce hydrogen into aluminium alloys, which often results in unwanted porosity.
For a broader range of castable alloys, refractory moulds created by the Shaw process offer much more versatility. These moulds are particularly favored for high melting point materials, think cast iron, carbon and alloy steels, or copper-beryllium alloys, as also seen in the lost wax process.
One of the distinctive advantages of the Shaw process is its ability to sidestep many of the typical casting defects encountered in other methods. Some clear benefits include a strong resistance to hot tearing, the production of castings without gas holes, and castings that are free from inclusions.
Another practical aspect of this process is that it allows for “natural feeding” of the casting, streamlining the overall operation.
Properties And Considerations Of Manufacturing By Ceramic Mold Casting
Ceramic mold casting shares many similarities with plaster mold casting, particularly in its ability to create components featuring thin walls, a high-quality surface finish, and impressive dimensional precision. With this method, tolerances as tight as 0.002 to 0.010 inches are achievable.
One significant advantage of such accuracy is that it can often remove the need for secondary machining, along with the material waste that machining tends to generate. For that reason, processes like ceramic mold casting are especially practical when working with precious metals or alloys that are notoriously challenging to machine.
A notable distinction from the plaster mold technique lies in the mold itself. Ceramic molds are made from refractory materials, so they’re capable of withstanding much higher temperatures. Because of this heat resistance, ceramic mold casting becomes a viable option for manufacturing ferrous alloys and other metals with high melting points—including stainless steels and tool steels.
It’s worth noting, however, that ceramic mold casting tends to be on the costly side. The extensive time needed to prepare each mold means production rates are generally slow.
There is, though, an upside in terms of mold performance: unlike plaster molds, fired ceramic molds develop microscopic cracks—a phenomenon known as microcrazing—which gives them excellent permeability during the casting process.
Applications of Ceramic Mold Casting
Ceramic mold casting, particularly when working with materials such as stainless steel and bronze, proves to be highly versatile. This method is well-suited for producing a broad spectrum of items, from everyday household goods to specialized industrial tools.
For instance, manufacturers often use ceramic mold casting to create kitchenware like kettles, as well as industrial components such as impellers. The technique is equally effective for fabricating complex cutting tools and molds used in plastic manufacturing.
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.