Metal Forming, Casting and Cutting
Category : Railways
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify.
The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods.
Casting processes have been known for thousands of years, and widely used for sculpture, especially in bronze, jewelry in precious metals, and weapons and tools. Traditional techniques include lost-wax casting, plaster mold casting and sand casting.
The modem casting process is subdivided into two main categories: expendable and non-expendable casting.
It is further broken down by the mold material, such as sand or metal, and pouring method, such as gravity, vacuum, or low pressure.
Sand casting is one of the most popular and simplest types of casting, and has been used for centuries. Sand casting allows for smaller batches than permanent mold casting and at a very reasonable cost.
Not only does this method allow manufacturers to create products at a low cost, but there are other benefits to sand casting, such as very small-size operations. From castings that fit in the palm of your hand to train beds it can all be done with sand casting.
Sand casting also allows most metals to be east depending on the type of sand used for the molds.
Sand casting requires a lead time of days, or even weeks sometimes, for production at high output rates and is unsurpassed for large-part production. Green (moist) sand has almost no part weight limit, whereas dry sand has a practical part mass limit of \[2,300-2,700\,kg\] \[(5,100-6,000\,lb).\]
Minimum part weight ranges from \[0.075-0.1\,kg\,(0.17-0.22\,lb).\] the sand is bonded together using clays, chemical binders, or polymerized oils (such as motor oil). Sand can be recycled many times in most operations and requires little maintenance.
Plaster casting is similar to sand casting except that plaster of parts is substituted for sand as a mold material. Generally, the form takes less than a week to prepare, after which a production rate of \[1-10\] units/hr-mold is achieved, with items as massive as 45 kg (99 Ib) and as small as 30 g (1 oz) with very good surface finish and close tolerances.
Plaster casting is an inexpensive alternative to other molding processes for complex parts due to the low cost of the plaster and its ability to produce near net shape castings. The biggest disadvantage is that it can only be used with low melting point non-ferrous materials, such as aluminium, copper, magnesium, and zinc.
Shell molding is similar to sand casting, but the molding cavity is formed by a hardened "shell" of sand instead of a flask filled with sand. The sand used is finer than sand casting sand and is mixed with a resin so that it can be heated by the pattern and hardened into a shell around the pattern.
Because of the resin and finer sand, it gives a much finer surface finish. The process is easily automated and more precise than sand casting. Common metals that are cast include cast iron, aluminium, magnesium, and copper alloys. This process is ideal for complex items that are small to medium sized.
Investment casting is a process that has been practiced for thousands of years, with the lost-wax process being one of the oldest known metal forming techniques.
From 5 000 years ago, when beeswax formed the pattern, to today's high technology waxes, refractory materials and specialist alloys, the castings ensure high-quality components are produced with the key benefits of accuracy, repeatability, versatility and integrity.
Investment casting derives its name from the fact that the pattern is invested, or surrounded, with a refractory material. The wax patterns require extreme care for they are not strong enough to withstand forces encountered during the mold making. One advantage of investment casting is that the wax can be reused.
The process is suitable for repeatable production of net shape components from a variety of different metals and high performance alloys. Although generally used for small castings, this process has been used to produce complete aircraft door frames, with steel castings of up to 300 kg and aluminum castings of up to 30 kg.
Compared to other casting processes such as die casting or sand casting, it can be an expensive process, however the components that can be produced using investment casting can incorporate intricate contours, and in most cases the components are cast near net shape, so require little or no rework once cast.
A durable plaster intermediate is often used as a stage toward the production of a bronze sculpture or as a pointing guide for the creation of a carved stone. With the completion of a plaster, the work is more durable (if stored indoors) than a clay original which must be kept moist to avoid cracking.
With the low cost plaster at hand, the expensive work of bronze casting or stone carving may be deferred until a patron is found, and as such work is considered to be a technical, rather than artistic process, it may even be deferred beyond the lifetime of the artist.
In waste molding a simple and thin plaster mold, reinforced by sisal or burlap, is cast over the original clay mixture. When cured, it is then removed from the damp clay, incidentally destroying the fine details in undercuts present in the clay, but which are now captured in the mold.
The mold may then at any later time (but only once) be used to cast a plaster positive image, identical to the original clay. The surface of this plaster may be further refined and may be painted and waxed to resemble a finished bronze casting.
This is a class of casting processes that use pattern materials that evaporate during the pour, which means there is no need to remove the pattern material from the mold before casting. The two main processes are lost- foam casting and full-mold casting.
Lost-foam casting is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of foam to simplify the investment casting process by removing the need to melt the wax out of the mold.
Full-mold casting is an evaporative-pattern casting process which is a combination of sand casting and lost-foam casting. It uses an expanded polystyrene foam pattern which is then surrounded by sand, much like sand casting. The metal is then poured directly into the mold, which vaporizes the foam upon contact.
Non-expendable mold casting differs from expendable processes in that the mold need not be reformed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting. This form of casting also results in improved repeatability in parts produced and delivers Near Net Shape results.
Permanent mold casting is a metal casting process that employs reusable molds .usually made from metal. The most common process uses gravity to fill the mold, however gas pressure or a vacuum are also used.
Permanent molds, while lasting more than one casting still have a limited life before wearing out.
The die casting process forces molten metal under high pressure into mold cavities. Most die castings are made from nonferrous metals, specifically zinc, copper, and aluminum based alloys, but ferrous metal die castings are possible.
The die casting method is especially suited for applications where many small to medium sized parts are needed with good detail, a fine surface quality and dimensional consistency.
Semi-solid metal (SSM) casting is a modified die casting process that reduces or eliminates the residua] porosity present in most die castings. Rather than using liquid metal as the feed material, SSM casting uses a higher viscosity feed material that is partially solid and partially liquid.
Used commercially mainly for aluminum and magnesium alloys, SSM castings can be heat treated to the T4, T5 or T6 tempers. The combination of heat treatment, fast cooling rates and minimal porosity provides excellent combinations of strength and ductility.
Other advantages of SSM casting include the ability to produce complex shaped parts net shape, pressure tightness, tight dimensional tolerances and the ability to cast thin walls
Pattern: An approximate duplicate of the final casting used to form the mold cavity.
Molding material: The material that is packed around the pattern and then the pattern is removed to leave the cavity where the casting material will be poured.
Flask: The rigid wood or metal frame that holds the molding material.
Cope: The top half of the pattern, flask, mold, or core.
Drag: The bottom half of the pattern, flask, mold, or core.
Core: An insert in the mold that produces internal features in the casting, such as holes.
Core print: The region added to the pattern, core, or mold used to locate and support the core.
Mold cavity: The combined open area of the molding material and core, where the metal is poured to produce the casting.
Riser: An extra void in the mold that fills with molten material to compensate for shrinkage during solidification.
Gating system: The network of connected channels that deliver the molten material to the mold cavities.
Pouring cup or pouring basin: The part of the gating system that receives the molten material from the pouring vessel.
Sprue: The pouring cup attaches to the sprue, which is the vertical part of the gating system. The other end of the sprue attaches to the runners.
Runners: The horizontal portion of the gating system that connects the sprues to the gates.
Gates: The controlled entrances from the runners into the mold cavities.
Vents: Additional channels that provide an escape gases generated during the pour.
Parting line or parting surface: The interface between the cope and drag halves of the mold, flask, or pattern.
Draft: The taper on the casting or pattern that allow it to be withdrawn from the mold
Core box: The mold or die used to produce the cores.