The previous article talked about the various types of casting. Casting requirements have increased with the advancement of science and technology. We require the production of more accurate, better performance, and lower cost of casting. To meet these requirements, casting workers have invented many new casting methods. These methods are collectively known as special casting methods, that is, special casting.
Special casting: casting with less sand or sand, using special technology and equipment for special casting methods, such as fusion casting, metal casting, pressure casting, low-pressure casting, centrifugal casting, ceramic casting, and solid casting. (This article will take a closer look at the various types of special casting)
The primary characteristics of special casting
- From the casting process point of view, special cast castings have a better surface finish.
- Special cast changes the liquid metal filling and cooling conditions, with a more dense and more refined grain structure.
- Reduce metal consumption and casting scrap rate, high production efficiency
- The special casting process can simplify (except for investment casting), and it is easy to mechanize and automate.
- Special casting can lower production costs, improve labor conditions and increase labor productivity.
- Special casting has better ductility than castings
Special casting steps
- Mold cavity production (considering the size and shape of the part and shrinkage)
- Melting process
- Pouring technique (proper pouring speed to minimize erosion and defects)
- Solidification process
- Mold removal
- Cleaning, finishing, and inspection operations
Advantages of special casting process compared to sand casting
- Higher-dimensional accuracy
- Higher metallurgical quality
- Lower production costs
- Ability to cast very thin parts
- High productivity
- The better surface finish of special castings
- Minimal need for further machining of the casting
- Special castings might have a finer and denser grain structure
- Castings are slightly more durable and ductile than solid mold castings
Types of casting
Plaster casting works in the same way as sand casting. But the difference between plaster casting and it is mold material. Metal casting uses sand molds, while plaster casting uses gypsum. This special casting technique is suitable for casting complex parts and molds that require high accuracy. Even in complex designs, this casting method ensures the finish of the casting surface.
Plaster casting process
- Pattern spraying: The special casting mold is sprayed with a release agent on the pattern before casting. This forms a thin coating to prevent the plaster from adhering to the design.
- Molding: The plaster is mixed and wrapped around the design. Spread the plaster on the design and shake it to cover all corners. After 15 minutes, remove the pattern. Afterward, we can heat the mold between 120°C and 260°C to eliminate excess moisture.
- Metal Casting: After we finish making the plaster casting model, we can fill the mold with molten steel.
- Plaster casting: Plaster casting offers a cost-effective alternative to traditional casting methods. This is unsuitable for non-ferrous low melting metals like copper, magnesium, or zinc.
The process is based on a thermosetting resin and sand mixture rapidly hardened by heat to produce a shell mold. You can use shell molding to produce simple and special castings. This casting has tight tolerances and high dimensional stability. One method of producing high-quality castings is shell mold. This mold material is mixed with resin to produce a smoother, faster heating cavity. This is why shell casting is more accurate and automated than sand casting.
Shell casting process
- Mold making: We heat the special cast iron design to 230-315°C. And allow the sand to sit on it for a few minutes to cure partially. Then the special cast mold is turned over, and the excess sand is removed. After that, we get a shell with a wall thickness of 10-20 mm. The model and the shell will have a tensile force of 350-450 PSI after curing the sand.
- Pattern Removal: After curing the shell, remove the pattern.
- Mold Assembly: Clamp the two half molds together to complete the shell casting mold. If any cores are needed, they are inserted before the mold is closed. The shell mold is then placed in the flask, supported by the backing material.
- Pouring: The molds are firmly clamped together as the molten metal is poured from the ladle into the pouring system and fills the mold cavity.
- Shell Casting Application: After the molten metal has cooled, it can break the mold, and the special casting remove. A trimming and cleaning process is required to remove excess metal from the feed system and any sand in the mold.
Advantages of shell casting
- Good types of casting details and dimensional accuracy
- The lightweight of mold for long-term storage
- Better design flexibility than die casting.
- Cheaper than investment casting, with lower basic equipment costs than mechanized wet sand molding.
- Higher metal yield.
- It provides superior surface finish and higher dimensional accuracy and is less expensive to refinish than conventional sand castings.
Disadvantages of special cast
- Higher cost
- Limited casting size
- Severe dust and fumes problems
- Carbon absorption from steel
Centrifugal casting involves pouring molten steel into a rotating mold. The mold’s centrifugal force acts to position and feeds the metal. After the metal has cooled, the mold rotates. Centrifugal Casting produces a denser, cleaner metal. This is because the heavier metal can throw into the part of the mold away from the center of rotation. And the lighter impurities can extrud to the center. Types of castings resulting from this process have a tight grain structure and excellent detail. They also have superior mechanical properties.
Types of centrifugal casting
Three types of the centrifugal cast can classify: centrifugal, semi-centrifugal, and accurate centrifugal.
True centrifugal casting
True centrifugal casting produces hollow castings with round holes. The process is characterized by centrifugal force alone to make the hole without using a core. The mold rotates around the hole axis, which is kept horizontal, inclined, or vertical.
In a semi-centrifugal cast, there is no attempt to create holes without a core. Any round or other hole is made using a core. Use the centrifugal force created by the rotation of a mold to feed the casting properly to create a clean casting with fine grain. This process is suitable for large asymmetric castings such as gear blanks, flywheels, and track wheels.
Metal is poured through a central gate along this axis of rotation and flows into the mold cavity through a radial internal gate cut in the mold interface. Gentrification helps to produce the casting correctly, resulting in clean, fine casting.
Investment casting is also known as lost wax casting. In investment casting, wax is used for molding, which facilitates the production of thin-walled patterns with complex geometric designs. This process creates precise and adaptable parts in various metals and high-performance alloys. This special casting procedure is suitable for casting delicate and precise parts but at a higher cost. However, as mass manufacturing increases, the unit cost decreases. This special casting method is proper when details with complex designs are produced.
Investment casting process
- Making the master pattern: This is a modified version of the product you want, made from metal, wood or plastic.
- Create the wax mold for your investment casting. This stage is usually done using high-grade casting wax.
- Assemble the wax tree: Melt casting manufacturers can assemble the wax tree to make more goods. The shell is constructed on the wax tree and cured for use in the casting process.
- Prep the mold for pouring. Heat the mold to 550 to 1100 degC. This allows molten metal to retain heat and flow easier into all sections
- Pouring the molten metal: In addition to simple pouring, you can use several methods to ensure complete filling of the mold
- Wax removal: removes the wax from the shell to create a space for the molten metal.
Advantages of special cast
- The smoother surface of castings from investment casting
- Can cast complex shapes
- Spexial castings do not contain any distorted parting lines
- Can eliminate machining operations
Disadvantages of special casting
- The production cycle is longer
- The process is relatively more expensive than other processes
- The pattern is scalable
- The size of the part to be cast is limited. Most castings weigh less than 0.5 kg
Ceramic Mold Casting
We can perform ceramic mold casting for permanent patterns using plaster, plastic, wood, or metal. Like investment molds, ceramic molds are consumable. However, ceramic molds are made from a combination of upper and lower molds. This is different from the monolithic molds used in investment casting.
Advantages of special casting
- Excellent surface finish and precision.
- Due to the slow cooling speed, this process doesn’t require rising, venting, or cooling.
- You can use any wood, metal, or plastic pattern.
- Can use the process on all metals, even highly reactive titanium and uranium.
Disadvantages of special casting
- High cost
- Difficult to control dimensional tolerances
Permanent Mold Casting
Permanent mold casting also goes by the name non-consumable mold casting. This type is very similar to die casting, except that the mold is permanent. And it is often repeated for casting using reusable molds. This special ops casting is suitable for mass production. The molds are metal or graphite, which can use to cast different metals and alloys. This is why most permanent mold casting processes produce base metal parts.
Advantages of the permanent mold casting process
- The smoother surface of the casting compared to sand casting
- Smaller dimensional tolerances and better mechanical properties of the special ops castings
- Short and fast cycle times for the production of molds
Gravity casting is a common method of metal casting. Molten metal is poured using gravity alone without vacuum or centrifugal force. It also requires a miniature mold to create the mold cavity. The mini-mold usually consists of sand, gypsum, or tuff.
Gravity casting process
- Mini-molds are made to complete the voids in the metal parts. This is why they call sand molds.
- Pouring the ingredients into them, heating them, and cooling them completes the mini-mold.
- Pouring molten metal: the small pieces are placed in the final mold and filled with molten metal.
- The metal solidifies to produce a part with a cavity.
- The mini-mold should remove after opening the mold and removing the metal parts. Most manufacturers get rid of the pattern.
Gravity casting applications
This special casting process fabricates large, heavy metal parts such as turbochargers, brake calipers, and engine cylinder heads. You can use the technology in other areas such as lighting and kitchen equipment.
Die casting is one of the types of casting. Special ops casting is accomplished by producing a mold cavity with two hardening tools. This special casting can create non-ferrous metals like zinc, brass, and aluminum. Typical applications of the die casting process include automotive parts, household appliances, various hardware, and other similar parts.
The die casting process
- Close the mold
- Clamping the half-die firmly together
- Press the liquid metal into the mold. Close the mold and inject molten steel under high pressure
- Eject the casting: Use the ejector pin to eject the component from the mold after cooling and curing.
- Separate the scrap from your workplace by opening the half-die
- Spray lubricant into the mold cavity to help regulate mold temperature and remove debris
Advantages of die casting process.
- Long die life and high production efficiency
- Tight castings and bright surface.
- Fast production speed
- Fragile parts can cast, and excellent details can obtain a good surface finish.
Disadvantages of the die casting process.
- High cost of molds and machines.
- The size limitation of castings is around 100 kg for zinc alloy and 30 kg for aluminum alloy.
- Only non-ferrous metals like zinc, aluminum, brass, and magnesium can ever be die cast.
- Weak mechanical properties of die cast products
Extrusion Die Casting
Extrusion die casting is a mixed metal forming method. This method combines die casting with forging. The molten alloy is poured into a mold after it has been heating, greased, stamped, and hardened under pressure.
Benefits of the extrusion die casting.
- Elimination of internal defects such as porosity, shrinkage, and shrinkage
- Excellent dimensional accuracy and minimal surface roughness
- Prevents special casting fractures
- Mechanization and automation are simple.
- Energy saving
Resin Sand Casting
Unlike wet sand, naturally bonded sand, resin sand casting requires a catalyst and binder. Sand, resin, and motivation are mixed, and the chemical reaction hardens the sand bond.
Forming resin sand compositions
- The following recipe is the process of making resin sand casting molds.
- Sand (SiO2) (40-60 % ); 7.7-1.3% resin, depending on the quality of the casting. Depending on the casting requirements and the part’s structure, little or no nitrogen may be required.
- The casting process is resin-dependent. It comprises 75 % furfuryl, 11 % formaldehyde, and 9 % Urea. The proportion of formaldehyde and urea influences the cure time, strength, and heat resistance.
- Curing agent and adhesion agent (30-50 %)
Advantages of resin sand casting
- Ability to create a smooth surface
- Minimizes sand holes, porosity, and shrinkage
- Resin sand casting produces huge objects weighing several tons
Disadvantages of resin sand casting
- It is more costly than other special ops casting methods.
- Particular requirements for raw materials
- It pollutes the environment
Metal casting is the most popular type of casting. Special casting is the most common type of casting. Metal casting uses either expendable or non-expendable molds. We can use disposable resin sand, ceramic shells, plaster encapsulation, or foam molds for metal casting. These include lost wax, fuse molding, and sand casting. Can use these molds for reuse are made from cast iron or steel. Die casting and enamel casting use non-replaceable molds.
Metal Casting Processes
There are many molds for a variety of tasks. First, decide if you need a disposable or reusable mold. We form a clay mold by dipping the wax model into a clay slurry layer. The wax is then burned off, and the metal is cast into it. The special cast mold is then placed in a frame. We then carve the sand cast mold with a model, impression, or directly into the sand. Plaster fusion molds are also available with wooden or metal frames.
Benefits of metal casting
- It is possible to create complex-shaped products.
- Internal chambers and hollow parts are easy to produce
- Can cast large parts in one piece
- Casting can make materials that are hard or expensive to manufacture.
- Casting is less expensive than other production techniques
- Can cast most metals
- Little to no post-polishing or polishing required
Disadvantages of metal casting
- A rough surface finish requires greater tolerances, not suitable for mating contacts.
- Metal casting, such as shell molding, has size and design limitations
- The production of patterns requires a lot of time and money. For example, it requires using additive manufacturing techniques such as adhesive injection.
- Due to the high tooling costs, die casting can be expensive for small to medium quantities.
- The special ops casting process determines the size and material of the part. For example, permanent mold castings require non-ferrous metals.