Introduction to Casting Process

The casting process is widely used in the manufacturing industry, hence it is very important for an NDT personnel to have a sound knowledge of the process and the steps involved in it. In this article will have a brief introduction to casting processes.

Need for this learning

Nondestructive testing is a fundamental and essential tool for control of quality of engineering materials, manufacturing processes, reliability of products in services, and maintenance of systems whose premature failure could be costly or disastrous. Analysis to provide accurate knowledge of the relation between a testable quality and one which cannot be tested directly without destruction is likely to require a great amount of knowledge, skill, and background experience together with good judgment which in a board sense can be described as an instinctive knowledge of the laws of statistical probability. Decisions to accept or reject following a test results must be based on a thorough knowledge of materials and the properties, processes, and their effect on properties, test techniques, design requirements, product applications service conditions, and suitable life expectancy.

NDT professional Requirements

It is important that an NDT supervisor be well versed in all the available NDT methods, their applications, and their limitations. In addition, the reliability of the methods and their correlation with desired material and product characteristics are very important. Knowledge of the product design, purpose, and function together with process details may enhance the application of test methods chosen. For Example, magnetic particle testing is only limited to only ferromagnetic materials, UT is not preferred for coarse-grain structure castings, radiography is not preferred for Forging materials etc.

For NDT personnel, continual improvement of entire knowledge-based is a must. Additional knowledge of NDT techniques particularly new developments is very important. Greater familiarity with engineering materials, manufacturing processes, changing designs, and service requirements, management policies, government requirements, environmental impacts, personal safety, and other areas are vital to the successful application of NDT.

The casting process is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods. Heavy equipment like machine tool beds, ships’ propellers, etc. can be cast easily in the required size, rather than fabricating by joining several small pieces. The casting industry supports other industries, small businesses, the global economy, the growth of the people who dedicate their time every day to building the industry and the environment.
Throughout history, metal casting has been used to make tools, weapons, and religious objects. Metal casting history and development can be traced back to Southern Asia. Since the beginning of metallurgy, the majority of castings were simple one to two-piece molds fashioned from either stone or ceramics. However, there is evidence of lost wax castings in numerous ancient civilizations.

The process:

Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various time setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster, and clay.

The casting process starts with a pattern. the casting process consists of a series of sequential steps performed in a definite order as shown in the figure below.

First, a pattern to represent the finished product must be chosen or constructed. Patterns can be of a number of styles but are always the shadow of the finished part and the roughly the same size as the finished part with slightly oversized dimensions to allow for shrinkage and additional allowances on surfaces that are to be machined.

A mold is constructed from the patterns. in some casting processes, the second step is to build a mold of material that can be made to flow into close contact with the pattern and that has sufficient strength to maintain that position. The mold is designed in such a way that it can be opened for removal of the pattern. The pattern may have attachments that make grooves in the mold to serve as channels for the flow of material into the cavity.

Mold cavity filled with molten material liquid metal is poured through the channels to fill the cavity completely. After time has been allowed for solidification to occur thee mold is open. The product is then ready for removing the excess metal that has solidified in the runners, cleaning for removal of any remaining mold part, and inspecting to determine if defects have been permitted by the process.

Fettling

Raw castings often contain irregularities caused by seams and imperfections in the molds, as well as access ports for pouring material into the molds. The process of cutting, grinding, shaving, or sanding away these unwanted bits is called “fettling”. In modern times robotic processes have been developed to perform some of the more repetitive parts of the fettling process, but historically fettlers carried out this arduous work manually, and often in conditions dangerous to their health. Fettling can add significantly to the cost of the resulting product, and designers of molds seek to minimize it through the shape of the mold, the material being cast, and sometimes by including decorative elements.

Foundry Technology

Although the casting process can be used to shape almost a metal, it has been necessary to develop a number of different methods to accommodate different materials and satisfy different requirements. Each method has certain advantages over the others, but all have limitations. Some are restricted to a few special applications.

Some common Casting Methods

Sand Molding:

It is a metal casting process characterized by using sand as the mold material. Molds made of sand are relatively cheap, and sufficiently refractory even for steel foundry use. In addition to the sand, a suitable bonding agent (usually clay) is mixed or occurs with the sand. The mixture is moistened, typically with water, but sometimes with other substances, to develop the strength and plasticity of the clay and to make the aggregate suitable for molding. The sand is typically contained in a system of frames or mold boxes known as a flask. The mold cavities and gate system are created by compacting the sand around models called patterns, by carving directly into the sand, or by 3D printing.

Permanent Mold Casting:

It is a metal casting process that employs reusable molds (“permanent molds”), usually made from metal. The most common process uses gravity to fill the mold, however gas pressure or a vacuum are also used. A variation on the typical gravity casting process, called slush casting, produces hollow castings. Common casting metals are aluminum, magnesium, and copper alloys. Other materials include tin, zinc, and lead alloys, and iron and steel are also cast in graphite molds. Typical products are components such as gears, splines, wheels, gear housings, pipe fittings, fuel injection housings, and automotive engine pistons.

Die casting

It is a metal casting process that is characterized by forcing molten metal under high pressure into a mould cavity. The mould cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mould during the process. Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter, and tin-based alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.

Investment Casting:

It is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. The term “lost-wax casting” can also refer to modern investment casting processes. Investment casting derives its name from the pattern being invested (surrounded) with a refractory material. Many materials are suitable for investment casting; examples are stainless steel alloys, brass, aluminum, carbon steel, and glass. The material is poured into a cavity in a refractory material that is an exact duplicate of the desired part. Due to the hardness of refractory materials used, investment casting can produce products with exceptional surface qualities, which can reduce the need for secondary machine processes. The process can be used for both small castings of a few ounces and large castings weighing several hundred pounds.

Centrifugal Casting :

Centrifugal casting or roto casting is a casting technique that is typically used to cast thin-walled cylinders. It is typically used to cast materials such as metals, glass, and concrete. A high quality is attainable by control of metallurgy and crystal structure. Unlike most other casting techniques, centrifugal casting is chiefly used to manufacture rotationally symmetric stock materials in standard sizes for further machining, rather than shaped parts tailored to a particular end-use.

Continuous Casting :

Continuous casting, also called strand casting, is the process whereby molten metal is solidified into a “semifinished” billet, bloom, or slab for subsequent rolling in the finishing mills. Prior to the introduction of continuous casting in the 1950s, steel was poured into stationary molds to form ingots. Since then, “continuous casting” has evolved to achieve improved yield, quality, productivity, and cost-efficiency. It allows lower-cost production of metal sections with better quality, due to the inherently lower costs of continuous, standardized production of a product, as well as providing increased control over the process through automation. This process is used most frequently to cast steel (in terms of tonnage cast). Aluminum and copper are also continuously cast.

Continuous casting. 1: Ladle. 2: Stopper. 3: Tundish. 4: Shroud. 5: Mold. 6: Roll support. 7: Turning zone. 8: Shroud. 9: Bath level. 10: Meniscus. 11: Withdrawal unit. 12: Slab. A: Liquid metal. B: Solidified metal. C: Slag. D: Water-cooled copper plates. E: Refractory material.

The reasons for the success of the casting process are:

• Owing to physical properties, some metals can only be cast since they cannot be re-modeled into bars, rods, plates, or other shapes.
• It’s a process highly adaptable to the requirements of mass production. Large numbers of a given casting can be produced quickly. For example; in the automotive industry there is a massive production of cast engine blocks and transmission cases.
• Certain light metal alloys because of their respective strength and weakness, can be produced only as castings.
• Shows excellent bearing qualities.

References

• ASNT material and Processes for NDT Technology.
• Wikipedia
• http://www.themetalcasting.com/