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Author Topic: Casting @ Design and Manufacturing  (Read 794 times)

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October 05, 2009, 11:23:43 AM
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Casting
Introduction

Casting is one of the easiest classes of process to understand. Casting is simply a process where a mould is filled with a fluid, which then solidifies in the shape of the mould cavity. Provided the liquid is capable of undergoing a liquid-to-solid transition, by freezing or chemical reaction for instance, then casting can be used.


Making ice cubes and jellies are useful analogies here. The production of the mould is one of the most important stages in making a casting. The casting, when solidified, must be of the right shape for the final product. In making the mould, often a ‘pattern’ made in the shape of the final component is used. This might be a wooden mock-up, for example.

Complex 3D shapes can be made using casting processes. Casting can be used to make a vast array of products, from gas-turbine blades to cheap plastic toys. Cast parts can range in size from fractions of centimetres and grams (such as the individual teeth on a zipper), to over 10 metres in length and many tonnes (such as the propellers of ocean liners).


Using one of the available casting processes almost anything can be manufactured. It is a matter of optimising materials to be cast, the mould material and the pouring method (see Properties for processing – casting).

Generally, during casting, the fluid flows into the mould under gravity, but sometimes the fluid may need some extra force to push it into the cavity.
Casting is not restricted to metals (or jellies).


Glass and plastics can also be cast using a variety of processes, each being dependent on the raw starting material, and the manner by which it can be made to flow when it is in its liquid state. Casting processes can be classified into three types depending on the nature of the mould used.

Properties for processing – casting
The casting (or pouring) group of processes is one of the most convenient for making three-dimensional shapes, especially if repeated copies are required. However, you do have to be able to get your material into liquid form, and it has then to be ‘runny’ enough to be poured.

What do these conditions require?
To get a liquid, you have to either melt the material; or dissolve it in a solvent which is subsequently evaporated off (the ‘solution route’); or pour liquid precursors into a mould where they react chemically to form a solid (the ‘reaction route’).

Some materials (e.g. thermosetting plastics) decompose rather than melt on heating. Others react with oxygen when heated, so need to be melted in inert atmospheres (which may prove expensive). Yet others have such high melting points (see the database) that the energy costs of heating them is only justified in special cases.

The solution route needs a suitable solvent, which you then have to be able to evaporate safely (many coatings such as paints are applied this way), but you can have shrinkage problems as the solvent is removed. The reaction route is used for both thermosets and thermoplastics and for concrete, but chemical reactions can produce considerable quantities of heat, so you must allow for this in the design of the process.

Once you have the liquid, can you pour it?

The physical property that determines the ‘runniness’ of liquid is called viscosity. This varies with temperature and is not all that useful for describing how well a mould will be filled if the temperature of the liquid is falling as it runs into the cold mould. In the casting of metals a more useful property is fluidity, which takes into account not only the viscosity changes but also the effects of cooling rate, surface tension of oxide films and the temperature range over which the alloy filling the mould actually freezes. Eutectic alloys have a high fluidity as they melt at a single temperature. Many of the alloys used for casting products are based on eutectic alloys.

Water and most liquids at room temperature have low viscosities, so can be poured easily, as can thermoset precursors. Molten thermoplastics, freshly-mixed concrete and clays have much higher viscosities. Although concrete can be poured, the others generally need to be pushed into their moulds, which is why injection-moulding machines for plastics are much ‘beefier’ than their pressure die-casting machine counterparts for metals.

Types of casting

Permanent pattern
This type of casting uses a model, or pattern, of the final product to make an impression which forms the mould cavity. Each mould is destroyed after use but the same pattern is used over and over again. Sand casting is a typical example of a permanent pattern process, where a pattern is placed into a special casting sand to form the right shape of cavity. Permanent pattern processes are usually cheaper than other methods, especially for small quantity production or ‘one-offs’, and are suitable for a wide range of sizes of product.
Permanent mould
In this method the same mould is used for large numbers of castings. Each casting is released by opening the mould rather than by destroying it. Permanent moulds need to be made of a material which can withstand the temperature fluctuations and wear associated with repeated casting. A good example of a product made with methods such of this is the ubiquitous ‘die-cast’ child’s toy (‘die’ is another word for ‘mould’).

 
Expendable mould and pattern
With this type of casting, a pattern is made from a low melting point material and the mould is built around it. The pattern is then melted or burnt out as the metal is poured in. The mould has to be destroyed to retrieve the casting.

This method is used to make moulds for casting high melting-point alloys like those used for jet engine turbine blades .A model (the pattern) of the blade is made in wax. The pattern is then coated in a thick slurry containing ceramic particles. The slurry dries, and is then fired in an oven: this hardens the ceramic (like firing a pot) and melts out the wax, leaving a hollow ceramic mould. The metal is then poured in to the mould, which is broken away after the metal has solidified and cooled.



Casting
Casting is a manufacturing process by which a molten material such as metal or plastic is introduced into a mold, allowed to solidify within the mold, and then ejected or broken out to make a fabricated part. Casting is used for making parts of complex shape that would be difficult or uneconomical to make by other methods, such as cutting from solid material.

Casting may be used to form hot, liquid metals or meltable plastics (called thermoplastics), or various materials that cold set after mixing of components such as certain plastic resins such as epoxy, water setting materials such as concrete or plaster, and materials that become liquid or paste when moist such as clay, which when dry enough to be rigid is removed from the mold, further dried, and fired in a kiln.
Substitution is always a factor in deciding whether other techniques should be used instead of casting. Alternatives include parts that can be stamped out on a punch press or deep-drawn, forged, items that can be manufactured by extrusion or by cold-bending, and parts that can be made from highly active metals.


The casting process is subdivided into two distinct subgroups: expendable and nonexpendable mold casting:


Expendable mold casting :

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

Sand casting :

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. Sand in most operations can be recycled many times and requires little additional input.

Preparation of the sand mold is fast and requires a pattern which can "stamp" out the casting template. Typically, sand casting is used for processing low-temperature metals, such as iron, copper, aluminium, magnesium, and nickel alloys. Sand casting can also be used for high temp metals where other means would be unpractical. It is by far the oldest and best understood of all techniques. Consequently, automation may easily be adapted to the production process, somewhat less easily to the design and preparation of forms. These forms must satisfy exacting standards as they are the heart of the sand casting process - creating the most obvious necessity for human control.

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.

Once used and cracked away, normal plaster cannot easily be recast. Plaster casting is normally used for nonferrous 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. 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 form is removed after the plaster sets.

Plaster casting represents a step up in sophistication and requires skill. The automatic functions easily are 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, multiple use piece molds, or molds made 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 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 later 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. 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.

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

initially preparing a metal-matched plate mixing resin and sand

heating pattern, usually to between 505-550 K

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)

curing shell and baking it.

removing investment.

inserting cores.

repeating for other half.

assembling mold.

pouring mold.

removing casting.

cleaning and trimming.

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

Investment casting (lost-wax process) yields a finely detailed and accurate product, but mechanical properties are not good since the process involves slow cooling.

Polystyrene foam is also used in investment casting—see lost-foam casting.

After a variable lead time, usually weeks, 1–1000 pieces/hour-mold can be produced in the mass range 2.3–2.7 kg. Items up to 45 kg and as light as 30 g are possible for unit production.

The process starts by creating an injection die to the desired specifications. This die will be used to inject wax to create the patterns needed for investment casting. The patterns are attached to a central wax sprue, creating an assembly, or mold. The sprue contains the fill cup where the molten metal will be poured into the assembly.
The wax assembly is now dipped multiple times in a ceramic slurry, depending on the shell thickness desired. A layer of fine sand (usually zircon) is added on top of each ceramic layer. This process will be repeated until the desired shell is created.

After the shell is created to the specifications desired, the wax must be removed; this is normally achieved using an autoclave. This is where the name "lost-wax process" comes from. This leaves an impression of the desired castings, which will be filled with metal. Before being cast, however, the shells must be heated in a furnace so they do not break during the casting process.

Next, the desired metal is poured into the hot ceramic shell. The metal fills each part on the assembly, and the central sprue cavity and fill cup. The individual parts will be removed after the mold cools and the shell is removed. The shell is generally removed with water-blasting, although alternate methods can be used. What remains are the cast metal parts, but they are still attached to the sprue assembly. The individual parts are removed by cold-break (dipping in liquid nitrogen and breaking the parts off with hammer and chisel) or with large cutoff saws.

Most investment castings need some degree of post casting machining to remove the sprue and runners, and improve surface finish. Grinding operations are perfomed to remove the gate. Parts are also inspected to make sure they were cast properly, and if not are either fixed or scrapped. Depending on the investment casting facility and specifications, more finishing work can be done on-site, sub-contracted, or not done at all.

Investment casting yields exceedingly fine quality products made of all types of metals. It has special applications in fabricating very high-temperature metals such as alloy steels or stainless steels, especially those which cannot be cast in metal or plaster molds and those which are difficult to machine or work.

Investment casting is often used in the aerospace and power generation industries to produce single crystal turbine blade, which exhibit superior creep resistance to equiaxed castings. A combination of slow cooling rates, seed crystals, and an elaborate sprue and runner system referred to as a "pigtail" are used to produce single crystal castings.

Nonexpendable mold casting :

Nonexpendable 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 refractory wash of acetylene soot before processing to allow easy removal of the workpiece and promote longer tool life. Permanent molds have a life which varies depending on maintenance of after which they require 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-, aluminium-, 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 the cavities of steel moulds. The moulds are called dies. Dies range in complexity to produce any non-ferrous metal parts (that need not be as strong, hard or heat-resistant as steel) from sink faucets to engine blocks (including hardware, component parts of machinery, toy cars, etc). In fact, the process lends itself to making any metal part that:

must be precise (dimensions plus or minus as little as 50 µm--over short distances),
must have a very smooth surface that can be bright plated without prior polishing and buffing,
has very thin sections (like sheet metal--as little as 1.2 mm),
must be produced much more economically than parts primarily machined (multicavity die casting moulds operating at high speed are much more productive than machine tools or even stamping presses),
must be very flexible in design; a single die casting may have all the features of a complex assembly.
If several machining operations would be required or assembly of several parts would be required (to make a finished part), die casting is probably far more economical. This level of versatility has placed die castings among the highest volume products made in the metalworking industry.

Common metals used in die casting include zinc and aluminum. These are usually not pure metals; rather are alloys which have better physical characteristics.

In recent years, injection-molded plastic parts have replaced some die castings because they are usually cheaper (and lighter--important especially for automotive parts since the fuel-economy standards). Plastic parts are practical (particularly now that plating of plastics has become possible) if hardness is not required and if parts can be redesigned to have the necessary strength.

Process :

There are four major steps in the die casting process. First, the mould is sprayed with lubricant and closed. The lubricant both helps control the temperature of the die and it also assists in the removal of the casting. Molten metal is then injected into the die under high pressure. The high pressure assures a casting as precise and as smooth as the mold. Typically it is around 100 MPa (1000 bar). Once the cavity is filled then the pressure is maintained until the casting has become solid (though this period is usually made short as possible by water cooling the mold). Finally, the die is opened and the casting is ejected.

Equally important as high-pressure injection is high-speed injection--required so the entire cavity fills before any part of the casting solidifies. In this way, discontinuities (spoiling the finish and even weakening the casting) are avoided even if the design requires difficult-to-fill very thin sections.

Before the cycle can be started the die must be installed in the die casting machine (set up) and brought to operating temperature. This set-up requires 1-2 hours after which a cycle can take anywhere between a few seconds to a few minutes depending on the size of the casting. Maximum mass limits for magnesium, zinc, and aluminium parts are roughly 4.5 kg, 18 kg, and 45 kg, respectively. A typical die set will last 500,000 shots during its lifetime with lifetime being heavily influenced by the melting temperature of the metal or alloy being used. Aluminum and its alloys typically shorten die life due to the high temperature of the liquid metal resulting in deterioration of the steel mold cavities. Molds for die casting zinc last almost indefinitely due to the lower temperature of the zinc. Molds for die casting brass are the shortest-lived of all. This is despite, in all cases, making the mold cavities out of the finest "hot work" alloy steel available.

A shot occurs every time the die is filled with metal. Shots are different from castings because there can be multiple cavities in a die, yielding multiple castings per shot. Also the shot consists not only of the individual castings but also the "scrap" (which, unlike in the case of scrap from machining, is not sold cheaply; it is remelted) that consists of the metal that has hardened in the channels leading into and out of the cavities. This includes, for example, the sprue, runners and overflows. Also there is usually some unplanned-for thin scrap called flash, the result of molds not fitting together tightly.
Molding (process) : Molding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a mold.
A mold or mould is a hollowed-out block that is filled with a liquid like plastic, glass, metal, or ceramic raw materials. The liquid hardens or sets inside the mold, adopting its shape. A mold is the opposite of a cast (see casting). The manufacturer who makes the molds is called moldmaker or mouldmaker. A release agent is typically used to make removal of the hardened/set substance from the mould easier.
« Last Edit: January 01, 1970, 04:00:00 AM by Guest »


 

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