Comparison of metal casting processes
12

Mar 2023

Comparison of metal casting processes

The process of casting metal (also known as liquid forming) is a method of forming metal products of a certain shape and properties after solidification by melting and casting the metal into a pre-made mold (a container that makes the liquid metal a solid casting). Liquid forming was one of the earliest manufacturing processes and although the history of casting over the centuries has made the process less mysterious, it is still a fundamental forming technology.

Advantages of casting: low production costs, great process flexibility, adaptability, suitability for producing castings of different materials, shapes, and weights and for mass production.

Disadvantages: large tolerances (maximum limit size minus minimum limit size), susceptibility to internal defects, and the need for more materials and equipment. At the same time, casting production generates dust, harmful gases, and noise, which pollute the environment.

The casting process usually consists of the following three stages.

1. Casting preparation: Casting according to the materials used can be divided into sand, metal, ceramic, clay, graphite, etc., according to the number of times users can be divided into one-time casting, semi-permanent casting, and permanent casting. One-time casting is generally cast in the process of taking out the casting is destroyed, its cavity is commonly used wax, wood, plastic, or metal made of mold to manufacture. Permanent casts are a more costly but reusable type of casting. Because of the high cost and the extreme expense of on-the-spot modifications, some of which are not even repairable, it requires more design preparation time to prepare the cast.

The quality of the mold preparation is a major factor in the quality of the casting.

2. melting and pouring of casting metals: casting metals (casting alloys) are mainly cast iron, cast steel, and casting non-ferrous alloys.

3. Casting processing and inspection: casting processing, including the removal of the core and casting surface foreign matter, removal of pouring mouth, shovel grinding burrs and phi seams and other protrusions, as well as heat treatment, shaping, rust treatment, and rough machining.

There are many types of casting, divided into the following according to the modeling method.

1. Ordinary sand casting, including wet and dry sand type, and chemical hardening sand type two categories.

2. Special casting, according to the modeling material can be divided into natural mineral sand and stone as the main modeling material of special casting (such as investment casting, shell casting, solid casting, ceramic casting, etc.) and metal as the main casting material of special casting (such as metal casting, pressure casting, continuous casting, low-pressure casting, centrifugal casting, etc.) two categories.

(a)Ordinary sand casting

1. Wet and dry sand casting

Wet sand type is damp sand compacted into a sandbox with wooden molds or metal halves, the assembled cast can be with or without a core, molten metal cast into the final formation of the cavity, remove the parts when the cast is destroyed. The sand used for dry sand casting has a slightly higher moisture content in the wet state than the sand used for wet casting. After the sand is made, the surface of the cavity should be coated with refractory paint, and refractory materials to avoid overheating of the sand, and then placed in the oven to dry, after it has cooled down can be combined and poured. It takes a long time to dry the clay sand type, consuming a lot of fuel, and the sand type is easy to produce deformation in the drying process so the accuracy of the casting is subject to typecasting.

2. Chemically hardened sand

The chemically hardened sand type used in the sand is called chemically hardened sand. Its binder is generally in the role of hardener can occur under molecular polymerization and thus become a three-dimensional structure of the material, commonly used in a variety of synthetic resins and water glass. The chemically hardened sand-casting process is characterized by.

(1) the mold can more accurately reflect the size and contour shape of the mold sample, and the resulting casting has high dimensional accuracy.

(2) Lightweight equipment structure, low power and high productivity, and treatment work part can be simplified.

(3) When using chemically hardened sand molding, the mold material can be selected according to the production requirements, such as wood, plastic, and metal.

(b) Special casting

1. Investment casting

Investment casting, also known as lost wax casting, precision casting, or overall casting, is a copy of the final product, paraffin-like materials pressed into a precise and clean wax mold, in the wax mold coated with multiple layers of refractory materials, to be dried and hardened to form a shell, and then the solidification of the wax mold in the shell melted to make it flow, and then the shell roasted to make it strong and dry, and then finally the molten liquid metal poured into the shell, liquid metal After cooling and solidifying in the shell, the liquid metal becomes a precise and clean casting. It is mainly used for the production of blades for steam and gas turbines, impellers for pumps, cutting tools, and small parts for aircraft, cars, tractors, wind tools, and machine tools.

Features of investment casting

(1) casting high precision, good surface quality. Such as the investment casting of turbine engine blades, casting accuracy has reached no machining allowance requirements;

(2) can be manufactured in the shape of complex castings, the combination of several parts into a complex component, available for investment casting once cast out.

(3) casting alloy type is not limited, for high melting point and difficult-to-cut alloy, more significant superiority.

(4) the production batch is basically unlimited, both in bulk, and mass production, but also single, small batch production.

(5) complicated processes, long production cycles, high production costs, castings should not be too large, or too long.

2. Shell Casting

Shell casting is a casting method that uses a thin shell cast to produce castings.

The process is to use heat-hardened sand (generally using resin sand) covered in a heated metal mold (heating temperature is generally about 300 ℃) plate so that it hardens into a thin shell, thin shell thickness is generally 6 ~ 12 mm, with sufficient strength and rigidity, so the upper and lower two pieces of shell with clamps or resin glued firmly, without sandbox can constitute the casting, pouring castings

Shell casting has the following characteristics.

(1) The amount of molding sand used can be significantly reduced.

(2) The obtained castings have a clear outline, a clean surface and precise dimensions, and can be made without mechanical processing or with only a small amount of processing. Particularly suitable for the production of castings of various alloys with large batches, high dimensional accuracy requirements, thin walls, and complex shapes.

(3) The resin used in shell casting is expensive, the template must be precision machined, the cost is high, and there is also an irritating odor when pouring, which to some extent limits the widespread use of this method.

3. Solid casting

Solid casting is the use of polystyrene foam plastic mold samples instead of ordinary mold samples, to make a good model without taking out the mold samples into the metal liquid, the role of the metal liquid, plastic mold samples burning, gasification, disappearance, the metal liquid to replace the original plastic mold space occupied by the position, cooling and solidification to obtain the required casting method.

Solid-type casting has the following characteristics.

(1) Due to the use of a foam mold pattern that vaporizes when it meets the metal liquid, there is no need to start the mold, no parting surface, no core, and therefore no flying burrs.

(2) Foam molds can be bonded and molded as a whole for complex castings of various shapes, reducing processing and assembly time, reducing casting costs by 10% to 30%, and providing full freedom in the structural design of the casting.

(3) simplifies the production process of castings, shortens the production cycle, and increases the efficiency of molding by 2 to 5 times compared to sand casting.

(4) solid casting mold sample can only be used once, and foam density is small, and low strength, mold sample is easy to deform, affecting the casting size accuracy, and the gas generated by the mold sample when casting pollutes the environment.

4. Ceramic casting

Ceramic casting is a new process developed on the basis of sand investment casting. Ceramic type is the use of pure texture, the high thermal stability of refractory materials for modeling materials, with ethyl silicate hydrolysis solution as a binder, under the action of catalyst, by the grouting, gluing, starting mold, baking, and other processes and made. The castings made by this casting method have high dimensional accuracy and surface finish, so this method is also called ceramic precision casting.

(1) sand sleeve modeling: the first water glass sand made of the sand sleeve. Manufacturing sand jacket mold B and then casting mold A should be larger than the thickness of a ceramic material. The sand jacket is manufactured in the same way as sand casting.

(2) grouting and gluing: the process is to fix the casting mold sample metal type casting is the liquid metal under the action of gravity poured into the gold on the bottom plate of the mold, brush on the parting agent, buckle on the sand sleeve, will be prepared to cast a good genus to obtain a method of casting. Casting type made of metal, the ceramic slurry from the pouring mouth filled with sand sleeve, after a few minutes can be used repeatedly hundreds of times to thousands of times. Once the material for the metal mold has been manufactured, the ceramic paste begins to gel. The ceramic slurry consists of refractory materials (e.g. corundum powder, bauxite, etc.), binder (e.g. ethyl silicate) hydrolysate), etc.

(3) start mold and spray firing: slurry pouring for 5 ~ 15 minutes, while the slurry still has a certain elasticity can be started out of the mold sample. In order to accelerate the curing process to improve the strength of the mold, the whole cavity must be sprayed with an open fire.

(4) Roasting and molding: to be heated to 350-550°C and roasted for 2-5 hours to burn away the residual moisture and improve the strength of the mold.

(5) Pouring: the pouring temperature can be slightly higher in order to obtain a clear outline of the casting.

Characteristics of ceramic type casting.

(1) The ceramic surface layer starts the mold in an elastic state, while the ceramic surface layer is resistant to high temperatures and has little deformation, so the dimensional accuracy and surface roughness of the castings are similar to those of investment mold casting.

(2) The weight of ceramic castings is almost unlimited and can range from a few kilograms to several tonnes.

(3) in single-piece, small batch production conditions, low investment, short production cycle, in general foundry can be produced.

(4) Not suitable for the production of large batches, lightweight or complex shape castings, the production process is difficult to achieve mechanization and automation.

5. Metal casting

Metal molding is a method of pouring liquid metal into a metal mold under gravity to obtain a casting. The castings are made of metal and can be used hundreds to thousands of times over and over again. The melting point of the material used to make the metal type should generally be higher than the melting point of the alloy poured, such as pouring tin, zinc, magnesium, and other low melting point alloys, available grey cast iron manufacturing metal type; pouring aluminum, copper and other alloys, it is necessary to use alloy cast iron or steel metal type.

Characteristics of metal type casting.

(1) High dimensional accuracy, small surface roughness, and small machining allowances.

(2) fine grain of the casting, good mechanical properties.

(3) can achieve a type of multi-casting, improve labor productivity, and save molding materials.

(4) high manufacturing cost of metal type, not suitable for the production of large, complex shape and thin-walled castings.

(5) Due to the rapid cooling rate, the surface of cast iron parts is easy to produce white mouth, cutting processing difficulties.

(6) Restricted by the melting point of the metal type material, the alloy with a high melting point is not suitable for casting with metal type. It is mainly used for the mass production of copper alloy, aluminum alloy, and other castings, such as pistons, connecting rods, cylinder heads, etc. Cast iron parts of the metal type casting are also developed, but its size is limited to 300mm, and the mass does not exceed 8kg, such as the bottom plate of the electric iron.

6. Pressure casting

Pressure casting is a casting method in which the liquid or semi-liquid metal is filled at high speed under high pressure and solidified into a casting under pressure, the pressure used is 4~500MPa, and the metal filling speed is 0.5~120m/s. The die casting machine used for pressure casting is divided into two types: hot chamber die casting machine and cold chamber die casting machine. Hot chamber die-casting machine on the pressure chamber immersed in liquid metal, pressure injection piston in the highest position when the metal into the pressure chamber, the piston down, the pressure chamber of the metal through the grackle neck channel pressed into the tight die-casting cavity and quickly solidified into shape. The cold chamber die-casting machine has a separate chamber and holding furnace. When casting, the liquid metal is first poured into the chamber and then pressed into the cavity by the injection piston. The hot chamber dies casting machine is suitable for die casting of lead and zinc alloys with low melting points, and can also be used for die casting of magnesium alloys. Cold chamber die-casting machines are suitable for die-casting aluminum, copper, or magnesium alloys. Common die-castings within more pores are not easy to heat treat and welding, affecting the use of performance, so most of the production of several special pressure casting methods, commonly used vacuum die-casting, inflatable die-casting, and fine speed dense die-casting.

(1) vacuum die-casting: die-casting the air in the cavity of the mold in advance.

(2) inflatable die-casting; die-casting first in the casting cavity filled with oxygen, so that the liquid metal and oxygen form a solid oxide, diffuse distribution in the casting internal;

(3) Fine speed dense die casting: liquid metal fills the casting cavity at low speed, after the metal fills the cavity, supplemented by small pistons to pressurize.

Pressure casting allows the manufacture of complex castings, saving material, energy, and machining time. The weight of a die casting can be as small as a few grams or as large as several tens of kilograms. The pressure casting method is suitable for the mass production of castings, and with high production efficiency, the production process is easy to achieve mechanization and automation, in the automotive, instrumentation, agricultural machinery, electrical appliances, medical equipment, and manufacturing industries that are widely used.

7. Continuous casting

Continuous casting is a casting method that uses a through crystallizer to continuously pour liquid metal at one end and continuously pull out the molding material from the other end. The crystallizer is generally made of materials with good thermal conductivity and certain strength, such as copper, cast iron, graphite, etc. The molded material is cast in square, rectangular, round, flat, tubular, or various-shaped sections.

A schematic diagram of a horizontal continuous ingot casting, with the crystallized in the lower part of the ladle, through which the steel is continuously drawn out into ingots, with the surface cured ingots being completely solidified by the secondary cooling of the jet water below the crystallizer. When the ingot material is drawn to a certain length, it is cut by a cutter into segments for further processing.

The characteristics of continuous casting are.

(1) It is suitable for the production of castings with constant section shapes and large lengths of alloys such as iron, steel, copper, lead, and magnesium.

(2) continuous casting using equipment and processes are very simple, high production efficiency and metal utilization, used and rolling mill to form a production line, but also substantial energy savings.

8. Low-pressure casting

In low-pressure casting, the metal liquid is pressed from the heated crucible into the mold through the riser tube under certain air pressure, filling the bottom without the turbulence that typically occurs in gravity-filled casting. After the casting has solidified, the pressure is reduced, allowing the remaining metal to flow back into the crucible.

The process of low-pressure casting: dry compressed air is slowly introduced into the crucible furnace; the metal liquid is pressurized by the gas, filling the cavity from the bottom up along the riser and pouring system; the mold is opened and the casting is removed.

Characteristics of low-pressure casting.

(1) The pressure and speed of pouring can be adjusted, so it can be applied to different casting types (e.g. metal type, sand type, etc.), casting various alloys and various sizes of castings.

(2) The metal liquid fills the mold smoothly and without splashing, which can avoid the involvement of gas and the scouring of the mold wall and core, and improves the qualified rate of the casting.

(3) castings crystallized under pressure, castings with dense organization, clear contours, clean surfaces, and high mechanical properties, particularly beneficial for the casting of large thin-walled parts.

(4) Increased metal utilization rate from 90% to 98%.

(5) low labor intensity, good labor conditions, simple equipment, easy-to-achieve mechanization, and automation.

9. Centrifugal casting

Centrifugal casting is a method of casting in which molten metal is poured into a rotating mold so that the liquid metal fills the mold and solidifies under the action of centrifugal force. The liquid metal is injected into the high-speed rotating drum-shaped mold and the dense metal is pressed against the outer wall, resulting in a pipe with a uniform density of grain tissue. The process is mainly used for the production of cast iron tubes, cylinder liners, copper liners, bimetallic bearings, seamless billets of special steels, paper machine drums, and other castings.

Features of centrifugal casting.

(1) The liquid metal can form a hollow free surface in the casting pattern, and hollow castings can be cast without a core, simplifying the production process of sleeves and tube castings.

(2) due to the role of centrifugal force generated by the liquid metal when rotating, centrifugal casting can improve the ability of the metal to fill the casting, so some poor mobility of the alloy and thin-walled castings can be produced by centrifugal casting.

(3) due to the role of centrifugal force, gas, and non-metallic inclusions are also easy to discharge from the metal liquid, producing shrinkage, shrinkage, and porosity; and other defects are less likely.

(4) gas, slag, and other inclusions in the metal, because of the lighter density and concentration on the inner surface of the casting, so the size of the inner hole is not accurate, the quality is also poor, the casting is easy to produce composition and density segregation.