9 Different types of Metal Casting Process Advantages and Disadvantages
Casting is the process of casting parts with required shape and performance by pouring molten metal liquid into the mold and cooling and solidification. Casting is a commonly used manufacturing method with low manufacturing cost and great process flexibility. It can obtain complex shapes and large castings, which accounts for a large proportion in mechanical manufacturing, such as 60-80% for machine tools, 25% for automobiles, and 50-60% for tractors. The following introduces the classification, advantages and disadvantages of common casting processes, and the principles of casting process selection
1. Sand casting
Sand casting: Steel, iron and most non-ferrous alloy castings can be obtained by sand casting.
(1) Process :
(2) Technical features:
① It is suitable for making blanks with complex shapes, especially with complex cavities;
② Wide adaptability and low cost;
③ For some materials with poor plasticity, such as cast iron, sand casting is the only forming process for manufacturing its parts or blanks.
(2) Application: automobile engine cylinder block, cylinder head, crankshaft and other castings.
2. Investment casting
Investment casting: usually refers to a casting scheme in which a pattern is made of fusible materials, a mold shell is made by covering several layers of refractory materials on the surface of the pattern, and then the pattern is melted out of the mold shell, so as to obtain a mold without parting surface, which can be filled with sand after being baked at high temperature. It is often called "lost wax casting".
(1) Process flow:
① High dimensional and geometric accuracy;
② High surface roughness;
③ It is able to cast castings with complex appearance, and the cast alloys are not limited.
(3) Disadvantages: complicated process and high cost.
(4) Application: It is applicable to the production of small parts with complex shape, high precision requirements, or difficult to carry out other processing, such as turbine engine blades.
3. Die casting
Die casting: It is to use high pressure to press liquid metal into a precision metal mold cavity at high speed, and the liquid metal is cooled and solidified under pressure to form a casting.
(1) Process flow:
① During die casting, the metal liquid bears high pressure and fast flow rate;
② Good product quality, stable size and good interchangeability;
③ High production efficiency and more die casting dies used;
④ It is suitable for mass production with good economic benefits.
① The castings are easy to produce small air holes and shrinkage porosity;
② Die castings have low plasticity and should not work under impact load and vibration;
③ When high melting point alloy die casting, the mold life is low, which affects the expansion of die casting production.
(4) Application: Die castings were first used in automobile industry and instrument industry, and then gradually expanded to various industries, such as agricultural machinery, machine tool industry, electronic industry, national defense industry, computers, medical devices, clocks, cameras, and daily hardware.
4. Low pressure casting
Low pressure casting: refers to the method of making liquid metal fill the mold under low pressure (0.02~0.06MPa) and crystallize under pressure to form castings.
(1) Process flow:
(2) Technical features:
① The pressure and speed during pouring can be adjusted, so it can be used for casting various kinds of castings (such as metal mold, sand mold, etc.), various alloys and castings of various sizes;
② The bottom injection mold filling is adopted, which makes the liquid metal mold filling stable and free of splash, avoiding the entrapment of gas and the scouring of mold wall and core, and improving the qualification rate of castings;
③ The casting crystallizes under pressure, with dense structure, clear contour, smooth surface and high mechanical properties, which is particularly beneficial to the casting of large and thin wall parts;
④ The feeding riser is omitted, and the metal utilization rate is increased to 90~98%;
⑤ Low labor intensity, good working conditions, simple equipment, easy to realize mechanization and automation.
(3) Application: Mainly traditional products (cylinder head, wheel hub, cylinder frame, etc.).
5. Centrifugal casting
Centrifugal casting: It is a casting method in which molten metal is poured into a rotating mold, and the mold is filled and solidified under the action of centrifugal force.
(1) Process flow:
① There is almost no metal consumption in the gating system and riser system, which improves the process yield;
② The core can not be used in the production of hollow castings, so the metal filling capacity can be greatly improved in the production of long tubular castings;
③ The casting has high density, less porosity, slag inclusion and other defects, and high mechanical properties;
④ It is convenient for manufacturing barrel and sleeve composite metal castings.
① There are some limitations when it is used to produce special-shaped castings;
② The inner hole diameter of the casting is inaccurate, the inner hole surface is relatively rough, the quality is poor, and the machining allowance is large;
③ Castings are prone to specific gravity segregation.
Centrifugal casting was first used to produce cast pipes. Centrifugal casting process was adopted in metallurgy, mining, transportation, irrigation and drainage machinery, aviation, national defense, automobile and other industries at home and abroad to produce steel, iron and non-ferrous carbon alloy castings. Among them, the production of centrifugal cast iron pipe, cylinder liner and shaft sleeve of internal combustion engine is the most common.
6. Gravity die casting
Metal mold casting: refers to a molding method in which liquid metal is filled with metal mold under the action of gravity and cooled and solidified in the mold to obtain castings.
① The thermal conductivity and heat capacity of the metal mold are large, the cooling rate is fast, the structure of the casting is dense, and the mechanical properties are about 15% higher than those of the sand mold casting;
② The castings with higher dimensional accuracy and lower surface roughness value can be obtained, and the quality stability is good;
③ Because sand cores are not used or rarely used, the environment is improved, dust and harmful gases are reduced, and labor intensity is reduced.
① Since the metal mold itself has no permeability, certain measures must be taken to derive the air in the mold cavity and the gas generated by the sand core;
② The metal mold is non yielding, and the casting is easy to crack during solidification;
③ The manufacturing cycle of metal mold is long and the cost is high. Therefore, only in mass production can good economic results be achieved.
Metal mold casting is not only suitable for mass production of nonferrous alloy castings such as aluminum alloy and magnesium alloy with complex shape, but also suitable for production of iron and steel castings, ingots, etc.
7. Vacuum die casting
Vacuum casting: It is an advanced die casting process that eliminates or significantly reduces the porosity and dissolved gas in the die casting by removing the gas in the die casting mold cavity during the die casting process, thereby improving the mechanical properties and surface quality of the die casting.
(1) Process flow:
① Eliminate or reduce the air hole inside the die casting, improve the mechanical properties and surface quality of the die casting, and improve the plating performance;
② Reduce the back pressure of the mold cavity, use the alloy with low specific pressure and poor casting performance, and it is possible to use small machines to cast larger castings;
③ The filling condition is improved, and thinner castings can be diecasted.
① The die sealing structure is complex, and the manufacturing and installation are difficult, so the cost is high;
② If the vacuum die casting method is not properly controlled, the effect is not very significant.
8. Squeezing die casting
Squeeze casting: It is a method to make liquid or semi-solid metal solidify under high pressure, flow to form, and directly obtain parts or blanks. It has the advantages of high utilization rate of liquid metal, simplified process and stable quality. It is an energy-saving metal forming technology with potential application prospects.
(1) Process flow:
① Direct squeeze casting: coating spraying, alloy casting, mold closing, pressurization, pressure maintaining, pressure relief, mold parting, blank demoulding and resetting;
② Indirect squeeze casting: coating spraying, mold closing, feeding, mold filling, pressurizing, pressure maintaining, pressure relieving, mold parting, blank demoulding, and resetting.
(2) Technical features:
② Low surface roughness and high dimensional accuracy;
③ It can prevent casting cracks;
④ It is easy to realize mechanization and automation.
(3) Application: It can be used to produce various types of alloys, such as aluminum alloy, zinc alloy, copper alloy, ductile iron, etc
9. Lost wax casting
Lost Wax Casting (also known as full mold casting): it is a new casting method that combines paraffin wax or foam models similar to the size and shape of castings into model clusters, brushes fire-resistant coatings and dries them, buries them in dry quartz sand for vibration molding, pours them under negative pressure, vaporizes the models, liquid metal occupies the mold position, and solidifies and cools them to form castings.
(1) Process flow: pre foaming → foaming molding → coating dipping → drying → molding → pouring → sand falling → cleaning
The classification, advantages and disadvantages of common casting processes are introduced in detail!
(2) Technical features:
① High casting precision, no sand core, reduced processing time;
② No parting surface, flexible design, high degree of freedom;
③ Clean production, no pollution;
④ Reduce investment and production costs.
It is suitable for the production of precision castings of various sizes with complex structures, with unlimited alloy types and production batches. Such as gray cast iron engine box, high manganese steel elbow, etc.
Principles of casting process selection
1. Sand casting is preferred
(1) According to statistics, 60~70% of all castings in China or internationally are produced by sand mold, and about 70% of them are produced by clay sand mold. The main reason is that sand casting has lower cost, simpler production process and shorter production cycle than other casting methods. Therefore, castings such as engine cylinder block, cylinder head and crankshaft of automobiles are all produced by clay green sand process. When the wet mold cannot meet the requirements, the clay sand surface dry sand mold, dry sand mold or other sand molds shall be considered. The casting weight of clay green sand casting can be from several kilograms to tens of kilograms, while the casting weight of clay dry mold production can be up to tens of tons.
(2) Generally speaking, for medium and large castings, iron castings can be produced by resin self hardening sand mold, and steel castings can be produced by water glass sand mold. Castings with accurate dimensions and smooth surface can be obtained, but the cost is high. Of course, the precision, surface finish, material density, metallographic structure, mechanical properties and other aspects of castings produced by sand casting are often poor. Therefore, when these properties of castings are required to be higher, other casting methods should be used, such as investment casting (wax loss), die casting, low pressure casting, etc.
2. The casting process should adapt to the production batch
(1) For example, sand casting, mass production plants should create conditions to adopt advanced molding and core making methods. The production line of the old shock molding machine or shock compression molding machine is not productive enough, the labor intensity of workers is high, the noise is large, and it does not meet the requirements of mass production, so it should be gradually transformed. For small castings, the production line of box less high-pressure molding machine with horizontal parting or vertical parting can be used. The production efficiency of full mold molding is high and the floor area is small; For medium parts, various high-pressure molding machine production lines with boxes and air impact molding lines can be selected to meet the requirements of fast and high-precision molding production lines. The core making method can be selected as: cold core box, hot core box, shell core and other efficient core making methods. Resin self hardening sand can be used for molding and core making of large castings in medium batch.
(2) For heavy castings produced in a single small batch, manual molding is still an important method. Manual molding is flexible and can adapt to various complex requirements without requiring many process equipment. Water glass sand mold, VRH water glass sand mold, organic ester water glass self hardening sand mold, clay dry mold, resin self hardening sand mold and cement sand mold can be used; For heavy castings produced by single piece, the pit molding method is low in cost and fast in production. It is appropriate to adopt multi box modeling and split box modeling for finalized products in batch production or long-term production. Although the initial investment in mold and sand box is high, it can be compensated in terms of saving molding hours and improving product quality.
(3) Low pressure casting, die casting, centrifugal casting and other casting methods are only suitable for batch production because of the high price of equipment and molds.
3. The casting process shall be suitable for the factory conditions
For example, for the same production of castings such as large machine tool bed, the core assembly molding method is generally adopted, instead of making patterns and sand boxes, the core assembly is conducted in the pit; In other factories, the sand box molding method is used to make patterns. Different enterprises have different production conditions (including equipment, site, staff quality, etc.), production habits, and accumulated experience. We should consider what products are suitable for doing and what products are not suitable (or not) for doing according to these conditions.
4. The precision requirements and cost of castings should be considered
The casting accuracy obtained by various casting processes is different, the initial investment and productivity are also different, and the final economic benefits are also different. Therefore, in order to achieve more, faster, better and less, we should give consideration to all aspects. A preliminary cost estimate shall be made for the selected casting method to determine the casting method with high economic benefits and meeting the casting requirements.
There are many kinds of casting processes, each of which has its own advantages and is suitable for processing in different fields. Therefore, everyone must be careful when selecting the casting process, and select the most appropriate casting process according to their own needs, so that it can play its role. Due to the improvement of the requirements for casting quality, casting precision, casting cost and casting automation, the casting technology is developing towards the direction of precision, large-scale, high-quality, automation and cleaning. For example, China has developed rapidly in precision casting technology, continuous casting technology, special casting technology, casting automation and casting simulation technology in recent years.