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2022.10.22
About 6,000 years ago, humans figured out how to use a heated process to handle metal, and since then, casting has been an established industry. It is between 1700 and 1000 BC that China’s bronze casting technique reaches its peak. How are most castings made? How do their benefits and drawbacks stack up?
1.Typical sand casting
Casting sand and a molding sand binder are the foundational components of every sand mold. Siliceous sand, which is used for casting, is the most popular choice. Zircon sand, chromite ore sand, corundum sand, and other special sands are to be employed when silica sand’s high temperature performance falls short of application needs. The most common sand binder is clay, although other options include dry or semi-dry oils, water-soluble silicates or phosphates, and different synthetic resins. There are a few different kinds of external sand molds used in sand casting, and they are distinguished by the binder used in molding sand and the method in which it produces strength: clay wet sand molds, clay dry sand molds, and chemical hardening sand molds.
Advantage:
Clay is a low-cost material that is abundant in many ways. Following appropriate sand treatment, the vast majority of previously used clay wet sand may be reused; Rapid casting times and great productivity; The combined molding sand has a long shelf life; It may be molded to fit a variety of situations. Pieces of any size, complexity, or number (both individually and in bulk) can be employed;
Constraints and drawbacks:
Production efficiency is limited in sand casting because each sand mold may only be used once before it is destroyed by the casting process and must be rebuilt. Weak mold stiffness results in sloppy casting dimensions. It’s common for castings to have flaws including porosity, sand inclusion, and sand washing.
2.Casting for investments
Investment casting is more often known as “lost wax casting” when wax is utilized as a pattern. The term “investment casting” is commonly used to describe the process of creating a mold without a separating surface by first creating a pattern out of a fusible substance, then wrapping it in numerous layers of refractory material. It may then be filled with sand and roasted at a high temperature before being used for pouring. Investment casting is sometimes known as “lost wax casting” due to the widespread usage of wax in the pattern-making process. Investment casting may be used to create carbon steel, alloy steel, heat-resistant alloy, stainless steel, precision alloy, permanent magnet alloy, bearing alloy, copper alloy, aluminum alloy, titanium alloy, and ductile iron.
Advantage:
Superb precision in all dimensions. In most cases, it’s able to get to CT4-6 (CT10-13 for sand casting and CT5-7 for die casting); it can boost the metals’ usage rate. Investment casting allows for more similarity between the blank and the parts, and brings tremendous ease to the structural design of the parts, while simultaneously minimizing the processing amount of the formed surface and mating surface of the product. Investment casting is not restricted by alloy materials and may create combined and integrated castings as well as castings with wall thicknesses of 0.5mm and weights of less than 1g. Castings made by investment casting may be made out of a wide variety of materials, including but not limited to carbon steel, alloy steel, ductile iron, copper alloy, aluminum alloy, superalloys, magnesium alloy, titanium alloy, and precious metals. Precision casting is ideally suited for casting forging-, welding-, and machining-resistant alloys; Superior adaptability and versatility in manufacturing Investment casting may be used for any manufacturing run size, from large-scale to small-scale to even one-off.
Constraints and drawbacks:
No too massive castings are permitted. Castings require a lot of time to cool because of how complicated the process is. When compared to other blank forming techniques, investment casting has the most complicated process and the highest casting cost. Investment casting can be expensive, but it can be cost-effective if the product is chosen carefully, the component is designed sensibly, and the amount spent on cutting, assembly, and metal is reduced.
Liquid metal is pressed at high speed into a precise metal mold cavity using high pressure, and then the metal is cooled and solidified under pressure to create a casting using the die-casting process. Die casting is done in either a cold or hot chamber. The liquid metal is poured into the pressure chamber, either manually or automatically, and then the injection punch is moved forward to hydraulically push the metal into the cavity, completing the cold chamber die-casting process. Metal liquid flows automatically into the pressure chamber via the feeding port in a hot chamber die casting process, where the pressure chamber is perpendicular to the crucible. The liquid metal is forced into the cavity via the gooseneck as the injection punch descends. Die casting cycles are finished when the mold is opened after the metal has solidified and the casting is removed.
Advantage:
The standard of the goods is high. A casting’s dimensional correctness is often between Grade 6-7 and perhaps even Grade 4; Excellent gloss, on par with Grade 5–8; There is a fair amount of strength and toughness. In comparison to sand casting, the strength is often increased by 25–30%, but the elongation is decreased by roughly 70%; a manageable size and high degree of adaptability; Die casting complicated castings with thin walls; High rates of production; for instance, the average number of times a domestic J III 3 horizontal cold air die casting machine can die cast in eight hours is 600–700, and the typical rate of production for a small hot chamber die casting machine is 3000–7000. The die-casting mold will last a very long time. Die-casting bell alloy and molds may be used thousands of times, if not millions of times; Mechanization and automation are simple to implement, and the resulting economic benefits are substantial. Die castings’ benefits include a consistent size and finish. In most cases, it is not utilized immediately for machining, or just a very little amount of processing is required, which not only increases metal usage but also decreases the need for costly and time-consuming processing equipment and labor. The cost of casting is low; You can utilize combination die casting to create metals and nonmetals alike. Time and material are both conserved during assembly.
Constraints and drawbacks:
However, the castings are prone to developing air holes and cannot be heat treated because of the fast filling speed of liquid metal in the mold cavity and unstable flow pattern during die casting. Die casting is more challenging for castings with complicated shapes; Alloys with a high melting point (such copper and ferrous metal) have a short die casting life and are not suited for low-volume manufacturing runs. The major cause of this is the high die casting mold manufacturing cost, the high die casting machine production efficiency, and the uneconomical nature of producing in small batches.
Castings may be made using this technique by pouring molten metal into a metal mold, a process also known as hard mold casting. Permanent mold casting refers to using a metal casting mold for several castings (hundreds to thousands of times). Cast iron and cast steel are the most common metals used to make the mold. The casting’s core might be made of metal or sand. Metal mold architectures can range from horizontal parting to heavy straight parting to composite parting, just to name a few. The compound parting is formed by connecting the two halves of the vertical parting with hinges, and the lower half is a fixed horizontal bottom plate, which is primarily used for the casting of more complex castings. The vertical parting is convenient for opening the inner gate and taking out the casting.
Advantage:
Because of its high reusability and the fact that it may be “one type with several castings,” it cuts down on the need for both the time and resources spent on modeling. The metal mold’s powerful cooling capabilities allowed for a thick casting structure with excellent mechanical qualities. In terms of dimensions, castings excel, with a tolerance grade of IT12–IT14; surface roughness is minimal, at Ra 6.3m. Casting in metal molds eliminates or reduces the need for sand, leading to better working conditions.
Constraints and drawbacks:
There is a substantial initial investment in the metal mold, a lengthy production cycle, and stringent process requirements. It’s not a good fit for making castings in very tiny quantities. Aluminum pistons, cylinder bodies, cylinder heads, oil pump housings, and copper alloy bearing shells and sleeves are just few examples of the kinds of nonferrous alloy castings that may be made economically in large quantities using this method. Similar constraints apply to ferrous alloy castings, which are restricted to small and medium-sized castings of straightforward designs.
5. Casting under low pressure
The process of low pressure casting involves filling the mold with molten metal and then applying a low pressure (0.02 to 0.06 MPa) to force the metal to crystallize. After the molten metal has been poured into the heat-retaining crucible, the sealing cover has been installed, the liquid pipe has been lifted to connect the liquid metal with the mold, the mold has been locked, and dry compressed air has been slowly injected into the crucible furnace; the gas pressure causes the liquid metal to rise and fill the cavity along the lift pipe and the pouring system, where it then crystallizes. When the pressure is released in the crucible after the casting has been made, the level of liquid metal in the lift pipe returns to that in the crucible. Separate the casting from the mold.
Advantage:
Adjusting the rate at which the molten metal rises and the pressure at which it crystallizes as it is poured allows it to be used for casting a wide range of alloys and castings of varying sizes in a wide range of molds (including metal molds, sand molds, and so on); Castings with fewer flaws such porosity and slag inclusion are produced thanks to the use of bottom injection mold filling and the stability of liquid metal mold filling with no splash, which helps prevent the trapping of gas and the scouring of the mold wall and core. Particularly useful for casting big and thin wall components, the casting crystallizes under pressure, resulting in a dense structure, clear contour, smooth surface, and good mechanical qualities; Eliminating the feeding riser and increasing metal usage to 90%98%; With a small workforce, pleasant working surroundings, basic tools, and straightforward automation, your workday will go smoothly.
Constraints and drawbacks:
The riser has a short lifespan, and the liquid metal is susceptible to oxidation and slag inclusion during the heat preservation process. Thin-walled components, such as those used in high-speed internal combustion engines, such as cylinder blocks, cylinder heads, crankcases, and aluminum pistons, are often cast using this method.
In centrifugal casting, molten metal is poured into a revolving mold, which is then filled and solidified by centrifugal force. The most prevalent kind of centrifugal casting may be broken down into two subtypes determined by where the mold’s rotating axis is located in three dimensions: When the mold’s rotating axis is parallel to the horizontal line or when the angle between the mold’s rotational axis and the horizontal line is very small (4 °), the casting process is known as horizontal centrifugal casting. Centrifugal casting is referred to as vertical centrifugal casting when the mold’s rotating axis is perpendicular to the casting surface. When the rotational axis of the mold makes a sharp angle with the horizontal and vertical lines, we refer to this technique as centrifugal casting, but it is rarely employed.
Advantage:
Centrifugal casting allows for the production of hollow rotational castings without the need for a core, gating system, or riser; Feeding conditions are optimal, the casting structure is dense, and the mechanical properties are high because centrifugal force created by the liquid metal during rotation pushes the metal with high density to the outer wall, while the gas and slag with low density move to the free surface, forming directional solidification from outside to inside; Casting bearing sleeves and bearing shells made from a “bimetallic” material saves time and effort. The more expensive copper material can be preserved by casting a small layer of copper bushing in the steel sleeve. High volume filling capacity; No gate or riser required, cutting down on energy use.
Constraints and drawbacks:
It is not suitable for alloys with considerable density segregation (like lead bronze) and aluminum, magnesium, and other alloys due to the roughness of the inner free surface of the casting, as well as the huge size error and poor quality.