There are many types of casting, which can be traditionally divided according to the molding methods into:
General sand casting, including wet sand mold, dry sand mold, and chemically hardened sand mold.
Special casting, subdivided according to the molding materials into special casting primarily using natural mineral sand as the molding material (such as investment casting, clay mold casting, shell molding, vacuum casting, full mold casting, ceramic mold casting, etc.) and special casting primarily using metal as the mold material (such as die-casting, pressure casting, continuous casting, low-pressure casting, centrifugal casting, etc.).
From the above, it can be seen that precision casting is a category of special casting.
Die-casting: This is a shorthand for pressure casting, which involves pouring molten alloy into a pressure chamber, filling the steel mold cavity at high speed, and allowing the alloy to solidify under pressure to form the casting. The main characteristics distinguishing die-casting from other casting methods are high pressure and high speed.
Investment casting: Precision casting is a special kind of casting. Parts obtained by this method generally do not require further machining, such as investment casting or pressure casting. Precision casting is a casting method relative to traditional casting technology. It can obtain relatively accurate shapes and higher casting accuracy.
The common practice is to design and manufacture molds according to product requirements (with very small or no allowances), cast wax to obtain the original wax pattern, repeat the coating and sanding process on the wax pattern, harden and dry the shell, melt the internal wax pattern (known as dewaxing) to obtain the mold cavity, fire the shell to obtain sufficient strength, pour the required metal material, and after decasting, clean the sand to obtain a high-precision final product. Heat treatment or cold working may be done according to product requirements.
Wax casting: Also known as the lost-wax casting method, which first appeared during the Spring and Autumn Period, and the entire manufacturing process is done manually.
The process involves making a wax model in the desired shape, then pouring high-temperature-resistant fine clay slurry onto the wax model surface, and sprinkling fine gauze on the slurry surface repeatedly to form a complete shell.
After drying, the shell is heated to melt out the wax, creating the mold cavity, which is used for pouring the copper liquid (i.e., a solution of copper added with lead, zinc, tin and other metal elements).
After the casting is completed, the shell is removed, the product is polished and aged, and a beautifully exquisite antique bronze is displayed before you. The lost-wax method of precision casting is now referred to as investment precision casting, a cutting-less or non-cutting precision casting process, which is an excellent technology in the precision casting industry and has a wide range of applications.
It is not only suitable for precision casting of various types and alloys, but the castings produced also have higher dimensional accuracy and surface quality than other precision casting methods.
Even complicated, high-temperature resistant, and difficult-to-process castings that are hard to obtain by other precision casting methods can be cast by investment precision casting.
Differences in Current Casting Processes
Presently, casting processes can be roughly divided into three categories: gravity casting, pressure casting, and sand casting.
1. Casting can be classified into gravity casting and pressure casting based on the metal filling process.
Gravity casting involves pouring molten metal into a mold under the influence of gravity, also known as casting. In a broad sense, gravity casting includes sand casting, metal casting, lost wax casting, and clay mold casting; in a narrow sense, gravity casting specifically refers to metal casting.
Pressure casting refers to the process where molten metal is forced into a mold under pressure (excluding gravity). Broadly speaking, pressure casting includes die casting, vacuum casting, low-pressure casting, and centrifugal casting; in a narrow sense, pressure casting specifically refers to die casting.
These casting processes are the most commonly used in non-ferrous metal casting and are also the most cost-effective.
2. Sand casting is a traditional casting process that mainly uses sand as the primary mold material.
Sand molds generally use gravity casting, but can also use low-pressure casting, centrifugal casting, etc., when special requirements are needed. Sand casting is very versatile and can be used for small, large, simple, complex, single, and mass production.
Previously, molds used in sand casting were mostly made from wood, commonly referred to as wooden molds. To overcome the disadvantages of wooden molds, such as easy deformation and damage.
These molds have higher dimensional accuracy and longer service life. Although the price has increased, they are still much cheaper than the molds used in metal casting, especially in small batch and large piece production.
Furthermore, sand molds have higher fire resistance than metal molds, making them suitable for materials with high melting points, such as copper alloys and ferrous metals.
However, sand casting has its drawbacks: each sand mold can only be used once and is destroyed after casting, resulting in low production efficiency.
Also, due to the soft and porous nature of sand, sand castings have lower dimensional accuracy and rougher surfaces.
3. Metal casting is a modern process that uses heat-resistant alloy steel to make hollow casting molds.
Metal molds can be used for both gravity casting and pressure casting. Metal molds can be used repeatedly, with each pour of molten metal producing a new casting, resulting in high production efficiency.
Metal castings not only have good dimensional accuracy and smooth surfaces, but also have higher strength and are less prone to damage than sand castings when casting the same metal.
Therefore, metal casting is usually the first choice for mass production of small and medium non-ferrous metal castings, as long as the melting point of the casting material is not too high.
However, metal casting also has its drawbacks: the cost of heat-resistant alloy steel and machining the hollow cavity is relatively high, making metal molds expensive, although they are still much cheaper compared to die casting molds.
For small batch production, the mold cost per product is usually too high and generally unacceptable. Also, due to the size limitations of the mold material and the capabilities of the machining and casting equipment, metal molds are not suitable for exceptionally large castings.
Therefore, metal casting is rarely used in small batch and large piece production.
Additionally, despite being made of heat-resistant alloy steel, metal molds still have limited heat resistance. They are mainly used for casting aluminum alloys, zinc alloys, and magnesium alloys, and are less commonly used for casting copper alloys and almost never for ferrous metals.
4. Die casting is a type of metal pressure casting performed on a die casting machine and is currently the most efficient casting process.
Die casting machines are divided into hot chamber and cold chamber machines. Hot chamber machines have a high degree of automation, less material waste, and higher production efficiency than cold chamber machines.
However, due to the heat resistance limitations of the machine parts, they can currently only be used for casting low-melting-point materials such as zinc and magnesium alloys.
Today’s widely used aluminum alloy die castings, due to their higher melting point, can only be produced on cold chamber machines.
The main characteristic of die casting is that the molten metal fills the mold cavity at high pressure and high speed, and solidifies under high pressure. The drawback of die castings is that the high pressure and high speed filling process inevitably traps air in the casting, forming subcutaneous pores.
Therefore, aluminum alloy die castings are not suitable for heat treatment, and zinc alloy die castings should not be spray-painted (although they can be painted). Otherwise, the internal casting pores will expand when heated during these processes, causing the casting to deform or blister.
Furthermore, the machining allowance for die casting should be kept small, generally around 0.5mm, to reduce casting weight, reduce cutting workload, lower costs, and avoid penetrating the dense surface layer to expose subcutaneous pores, resulting in scrapped workpieces.