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inversion-casting

Casting

[kas-ting, kah-sting]

Casting is a manufacturing process by 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 solid casting is then ejected or broken out to complete the process. Casting may be used to form hot liquid metals or various materials that cold set after mixing of components (such as epoxies, concrete, plaster and clay). Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.

Casting is a 6000 year old process. The oldest surviving casting is a copper frog from 3200 BC.

The casting process is subdivided into two distinct subgroups: expendable and non-expendable mold casting.

Expendable mold casting

Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster, and investment (lost-wax technique) moldings. This method of mold casting involves the use of temporary, non-reusable molds.

Waste molding of plaster

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 prosperous 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.

Sand casting

Sand casting is one of the most popular and simplest types of casting that has been used for centuries. Sand casting allows for smaller batches to be made compared to 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 (one casting can create the entire bed for one rail car), it can all be done with sand casting. Sand casting also allows most metals to be cast depending on the type of sand used for the molds.

Sand casting requires a lead time of days for production at high output rates (1-20 pieces/hr-mold) 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 2300-2700 kg. Minimum part weight ranges from 0.075-0.1 kg. The sand is bonded together using clays (as in green sand) or chemical binders, or polymerized oils (such as motor oil). Sand can be recycled many times in most operations and requires little additional input.

Plaster casting (of metals)

Plaster casting is similar to sand molding except that plaster is substituted for sand. Plaster compound is actually composed of 70-80% gypsum and 20-30% strengthener and water. 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 and as small as 30 g with very high surface resolution and fine tolerances. Parts that are typically made by plaster casting are lock components, gears, valves, fittings, tooling, and ornaments. Plaster casting is an inexpensive alternative to other molding processes due to the low cost of the plaster and the mold production. It may be disadvantageous, however, because the mold quality is dependent on several factors, "including consistency of the plaster molding composition, mold pouring procedures, and plaster curing techniques. If these factors are not closely monitored, the mold can result in distorted dimensions, shrinking upon drying and poor mold surfaces.

Once used and cracked away, normal plaster cannot easily be recast. Plaster casting is normally used for non-ferrous metals such as aluminium-, zinc-, or copper-based alloys. It cannot be used to cast ferrous material because sulfur in gypsum slowly reacts with iron. The plaster itself cannot stand temperatures above 1200oC, which also limits the materials to be cast in plaster. Prior to mold preparation the pattern is sprayed with a thin film of parting compound to prevent the mold from sticking to the pattern. The unit is shaken, so plaster fills the small cavities around the pattern. The plaster sets, usually in about 15 minutes, and the pattern is removed. The plaster is dried at temperatures between 120o and 260oC. The mold is preheated and the molten metal poured in.

Plaster casting represents a step up in sophistication and requires skill. The automatic functions are easily handed over to robots, yet the higher-precision pattern designs required demand even higher levels of direct human assistance.

Casting of plaster, concrete, or plastic resin

Plaster itself may be cast, as can other chemical setting materials such as concrete or plastic resin - either using single-use waste molds as noted above or multiple-use piece molds, or molds made of small ridged pieces or of flexible material such as latex rubber (which is in turn supported by an exterior mold). When casting plaster or concrete, the finished product is, unlike marble, relatively unattractive, lacking in transparency, and so it is usually painted, often in ways that give the appearance of metal or stone. Alternatively, the first layers cast may contain colored sand so as to give an appearance of stone. By casting concrete, rather than plaster, it is possible to create sculptures, fountains, or seating for outdoor use. A simulation of high-quality marble may be made using certain chemically-set plastic resins (for example epoxy or polyester) with powdered stone added for coloration, often with multiple colors worked in. The latter is a common means of making attractive washstands, washstand tops and shower stalls, with the skilled working of multiple colors resulting in simulated staining patterns as is often found in natural marble or travertine.

Shell molding

Shell molding is also similar to sand molding except that a mixture of sand and 3-6% resin holds the grains together. Shell molding also uses sand with a much smaller grain than green-sand. Set-up and production of shell mold patterns takes weeks, after which an output of 5-50 pieces/hr-mold is attainable. Aluminium and magnesium products average about 13.5 kg as a normal limit, but it is possible to cast items in the 45-90 kg range. Shell mold walling varies from 3-10 mm thick, depending on the forming time of the resin.

Shell molding is used for small parts that require high precision. Some examples include gear housings, cylinder heads and connecting rods. It is also used to make high-precision molding cores. This process makes it so complex parts can be cast with less labor.

There are a dozen different stages in shell mold processing that include:

  1. Initially preparing a metal-matched plate
  2. Mixing resin and sand
  3. Heating pattern, usually to between 505-550 K
  4. Inverting the pattern (the sand is at one end of a box and the pattern at the other, and the box is inverted for a time determined by the desired thickness of the mill)
  5. Curing shell and baking it
  6. Removing investment
  7. Inserting cores
  8. Repeating for other half
  9. Assembling mold
  10. Pouring mold
  11. Removing casting
  12. Cleaning and trimming.

The sand-resin mix can be recycled by burning off the resin at high temperatures.

Investment casting

Investment casting (known as lost-wax casting in art) is a process that has been practised for thousands of years, with the lost-wax process being one of the oldest known metal forming techniques. From 5000 years ago, when bees wax 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 it 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 aluminium 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 requiring little or no rework once cast.

Non-expendable mold casting

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.

Permanent mold casting

Permanent mold casting (typically for non-ferrous metals) requires a set-up time on the order of weeks to prepare a steel tool, after which production rates of 5-50 pieces/hr-mold are achieved with an upper mass limit of 9 kg per iron alloy item (cf., up to 135 kg for many nonferrous metal parts) and a lower limit of about 0.1 kg. Steel cavities are coated with a refractory wash of acetylene soot before processing to allow easy removal of the workpiece and promote longer tool life. Permanent molds have a limited life before wearing out. Worn molds require either refinishing or replacement. Cast parts from a permanent mold generally show 20% increase in tensile strength and 30% increase in elongation as compared to the products of sand casting.

The only necessary input is the coating applied regularly. Typically, permanent mold casting is used in forming iron, aluminum, magnesium, and copper based alloys. The process is highly automated.

Die casting

Die casting is the process of forcing molten metal under high pressure into mold cavities (which are machined into dies). 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.

Centrifugal casting

Centrifugal casting is both gravity- and pressure-independent since it creates its own force feed using a temporary sand mold held in a spinning chamber at up to 900 N (90 g). Lead time varies with the application. Semi- and true-centrifugal processing permit 30-50 pieces/hr-mold to be produced, with a practical limit for batch processing of approximately 9000 kg total mass with a typical per-item limit of 2.3-4.5 kg.

Industrially, the centrifugal casting of railway wheels was an early application of the method developed by German industrial company Krupp and this capability enabled the rapid growth of the enterprise.

Small art pieces such as jewelry are often cast by this method using the lost wax process, as the forces enable the rather viscous liquid metals to flow through very small passages and into fine details such as leaves and petals. This effect is similar to the benefits from vacuum casting, also applied to jewelry casting.

Continuous casting

Continuous casting is a refinement of the casting process for the continuous, high-volume production of metal sections with a constant cross-section. Molten metal is poured into an open-ended, water-cooled copper mold, which allows a 'skin' of solid metal to form over the still-liquid centre. The strand, as it is now called, is withdrawn from the mold and passed into a chamber of rollers and water sprays; the rollers support the thin skin of the strand while the sprays remove heat from the strand, gradually solidifying the strand from the outside in. After solidification, predetermined lengths of the strand are cut off by either mechanical shears or travelling oxyacetylene torches and transferred to further forming processes, or to a stockpile. Cast sizes can range from strip (a few millimetres thick by about five metres wide) to billets (90 to 160 mm square) to slabs (1.25 m wide by 230 mm thick). Sometimes, the strand may undergo an initial hot rolling process before being cut.

Continuous casting is used due to the lower costs associated with continuous production of a standard product, and also increases the quality of the final product. Metals such as steel, copper and aluminium are continuously cast, with steel being the metal with the greatest tonnages cast using this method.

Cooling rate

The rate at which a casting cools affects its microstructure, quality, and properties.

The cooling rate is largely controlled by the molding media used for making the mold. When the molten metal is poured into the mold, the cooling down begins. This happens because the heat within the molten metal flows into the relatively cooler parts of the mold. Molding materials transfer heat from the casting into the mold at different rates. For example, some molds made of plaster may transfer heat very slowly, while a mold made entirely of steel would transfer the heat very fast. This cooling down ends with (solidification) where the liquid metal turns to solid metal.

At its basic level a foundry may pour a casting without regard to controlling how the casting cools down and the metal freezes within the mold. However, if proper planning is not done the result can be gas porosities and shrink porosities within the casting. To improve the quality of a casting and engineer how it is made, the foundry engineer studies the geometry of the part and plans how the heat removal should be controlled.

Where heat should be removed quickly, the engineer will plan the mold to include special heat sinks to the mold, called chills. Fins may also be designed on a casting to extract heat, which are later removed in the cleaning (also called fettling) procees. Both methods may be used at local spots in a mold where the heat will be extracted quickly.

Where heat should be removed slowly, a riser or some padding may be added to a casting. A riser is an additional larger cast piece which will cool more slowly than the place where it is attached to the casting.

Generally speaking, an area of the casting which is cooled quickly will have a fine grain structure and an area which cools slowly will have a coarse grain structure.

Shrinkage

Castings shrink when they cool. Like nearly all materials, metals are less dense as a liquid than a solid. During solidification (freezing), the metal density dramatically increases. This results in a volume decrease for the metal in a mold. Solidification shrinkage is the term used for this contraction. Cooling from the solidification temperature to room temperature also involves a contraction. The easiest way to explain this contraction is that is the reverse of thermal expansion. Compensation for this natural phenomenon must be considered in two ways.

Solidification shrinkage

The shrinkage caused by solidification can leave cavities in a casting, weakening it. Risers provide additional material to the casting as it solidifies. The riser (sometimes called a "feeder") is designed to solidify later than the part of the casting to which it is attached. Thus the liquid metal in the riser will flow into the solidifying casting and feed it until the casting is completely solid. In the riser itself there will be a cavity showing where the metal was fed. Risers add cost because some of their material must be removed, by cutting away from the casting which will be shipped to the customer. They are often necessary to produce parts which are free of internal shrinkage voids. One method that assists in keeping the metal molten in the riser longer is the utilisation of an exothermic sleeve. http://www.gw-svr-a.org.uk/4566_castings.html

Sometimes, to promote directional solidification, chills must be used in the mold. A chill is any material which will conduct heat away from the casting more rapidly that the material used for molding. Thus if silica sand is used for molding, a chill may be made of copper, iron, aluminum, graphite, zircon sand, chromite or any other material with the ability to remove heat faster locally from the casting. All castings solidify with progressive solidification but in some designs a chill is used to control the rate and sequence of solidification of the casting.

Patternmaker's shrink (thermal contraction)

Shrinkage after solidification can be dealt with by using an oversized pattern designed for the relevant alloy. Pattern makers use special "contraction rulers" (also called "shrink rules") to make the patterns used by the foundry to make castings to the design size required. These rulers are 1 - 6% oversize, depending on the material to be cast. These rulers are mainly referred to by their actual changes to the size. For example a 1/100 ruler would add 1 mm to 100 mm if measured by a "standard ruler" (hence being called a 1/100 contraction ruler). Using such a ruler during pattern making will ensure an oversize pattern. Thus, the mold is larger also, and when the molten metal solidifies it will shrink and the casting will be the size required by the design, if measured by a standard ruler. A pattern made to match an existing part would be made as follows: First, the existing part would be measured using a standard ruler, then when constructing the pattern, the pattern maker would use a contraction ruler, ensuring that the casting would contract to the correct size.

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