casting process

casting process 

The Permanent Mold Process

Permanent mold casting is a cost effective approach to today's requirements for near net shape castings. Because of its flexibility and design freedom, permanent mold has become a basic foundry process and should be considered as a readily acceptable alternative casting method for components made from sand castings, welded assemblies, forgings, investment castings, and even complex configurations machined from bar stock.

The Permanent Mold process utilizes a metal casting die in conjunction with metal or sand cores, where applicable. The basic casting sequence is as follows: The Metal Mold is preheated prior to production to elevate the mold temperature to operable conditions. The mold, consisting of two or more parts, is then assembled and closed. Molten metal is introduced at the top of the mold with steel cores, if present, being removed shortly thereafter. After solidification, the mold is opened and the casting ejected. The mold is reassembled and the cycle repeated.

 

Design Data and General Information

Permanent Mold, also referred to as gravity die casting, has several advantages over other processes.

1. IMPROVED PROPERTIES - ALLOYS: Since the molten metal is poured into a steel die, which acts like a heat sink, improved physical and mechanical properties are realized. This is due to the finer grain structure gained by the rapid solidification of the alloy. This allows for reduced section thicknesses and weight while maintaining physical strengths.
2. DESIGN CRITERIA - Since parts are produced utilizing rigid steel dies, parts can be cast to size with excellent dimensional repeatability. This allows for holes and shapes to be cast ready for tapping and/or reaming. Insert casting is easily accommodated.
3. WEIGHT - With parts cast to tighter tolerances, section thicknesses can be reduced as well as machining stock, allowing for reductions in secondary operations.
4. SURFACE FINISH - Greatly improved versus a sand casting, usually 125 RMS, again due to the steel die offering a superior surface condition capable of offering a reduction in finishing operations.

ALLOYS - Aluminum Bronzes, Yellow Brasses, Silicon Brasses, Copper, and Aluminimum.

WEIGHT - 1 ounce to 20 pounds

TOLERANCES - +/- .005 possible in production on some dimensions if less than 1", +/- .010 would be a more practical tolerance and less costly to maintain. Tolerance of +/- .010 applies on dimensions 1" to 6". Dimensions greater than 6" can be held to +/- .015.
Centrifugal casting as a category includes Centrifugal Casting, Semi-Centrifugal Casting and Centrifuging.
Centrifugal Casting: In centrifugal casting, a permanent mold is rotated about its axis at high speeds (300 to 3000 rpm) as the molten metal is poured. The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling. The casting is usually a fine grain casting with a very fine-grained outer diameter, which is resistant to atmospheric corrosion, a typical situation with pipes. The inside diameter has more impurities and inclusions, which can be machined away.

Introduction

Only cylindrical shapes can be produced with this process. Size limits are upto 3 m (10 feet) diameter and 15 m (50 feet) length. Wall thickness can be 2.5 mm to 125 mm (0.1 - 5.0 in). The tolerances that can be held on the OD can be as good as 2.5 mm (0.1 in) and on the ID can be 3.8 mm (0.15 in). The surface finish ranges from 2.5 mm to 12.5 mm (0.1 - 0.5 in) rms.

Typical materials that can be cast with this process are iron, steel, stainless steels, and alloys of aluminum, copper and nickel. Two materials can be cast by introducing a second material during the process. Typical parts made by this process are pipes, boilers, pressure vessels, flywheels, cylinder liners and other parts that are axi-symmetric.
Semi-Centrifugal Casting: The molds used can be permanent or expendable, can be stacked as necessary. The rotational speeds are lower than those used in centrifugal casting. The center axis of the part has inclusion defects as well as porosity and thus is suitable only for parts where this can be machined away. This process is used for making wheels, nozzles and similar parts where the axis of the part is removed by subsequent machining.

Centrifuging: Centrifuging is used for forcing metal from a central axis of the equipment into individual mold cavities that are placed on the circumference. This provides a means of increasing the filling pressure within each mold and allows for reproduction of intricate

Centrifugal Casting

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Centrifugal casting, sometimes called rotocasting, is a metal casting process that uses centrifugal force to form cylindrical parts. This differs from most metal casting processes, which use gravity or pressure to fill the mold. In centrifugal casting, a permanent mold made from steel, cast iron, or graphite is typically used. However, the use of expendable sand molds is also possible. The casting process is usually performed on a horizontal centrifugal casting machine (vertical machines are also available) and includes the following steps:

1.      Mold preparation - The walls of a cylindrical mold are first coated with a refractory ceramic coating, which involves a few steps (application, rotation, drying, and baking). Once prepared and secured, the mold is rotated about its axis at high speeds (300-3000 RPM), typically around 1000 RPM.

2.      Pouring - Molten metal is poured directly into the rotating mold, without the use of runners or a gating system. The centrifugal force drives the material towards the mold walls as the mold fills.

3.      Cooling - With all of the molten metal in the mold, the mold remains spinning as the metal cools. Cooling begins quickly at the mold walls and proceeds inwards.

4.      Casting removal - After the casting has cooled and solidified, the rotation is stopped and the casting can be removed.

5.      Finishing - While the centrifugal force drives the dense metal to the mold walls, any less dense impurities or bubbles flow to the inner surface of the casting. As a result, secondary processes such as machining, grinding, or sand-blasting, are required to clean and smooth the inner diameter of the part.

Centrifugal casting is used to produce axi-symmetric parts, such as cylinders or disks, which are typically hollow. Due to the high centrifugal forces, these parts have a very fine grain on the outer surface and possess mechanical properties approximately 30% greater than parts formed with static casting methods. These parts may be cast from ferrous metals such as low alloy steel, stainless steel, and iron, or from non-ferrous alloys such as aluminum, bronze, copper, magnesium, and nickel. Centrifugal casting is performed in wide variety of industries, including aerospace, industrial, marine, and power transmission. Typical parts include bearings, bushings, coils, cylinder liners, nozzles, pipes/tubes, pressure vessels, pulleys, rings, and wheels.

OVERVIEW OF THE INVESTMENT CASTING PROCESS

Investment casting, often called lost wax casting, is regarded as a precision casting process to fabricate near-net-shaped metal parts from almost any alloy. Although its history lies to a great extent in the production of art, the most common use of investment casting in more recent history has been the production of components requiring complex, often thin-wall castings. While a complete description of the process is beyond the scope of the discussion here, the sequential steps of the investment casting process will be briefly described, with emphasis on casting from rapid prototyping patterns.
The investment casting process begins with fabrication of a sacrificial pattern with the same basic geometrical shape as the finished cast part (Fig. 10.1a). Patterns are normally made of investment casting wax that is injected into a metal wax injection die. Fabricating the injection die often costs tens of thousands of dollars and can require several months of lead time. Once a wax pattern is produced, it is assembled with other wax components to form a metal delivery system (Fig. 10.1b), called the gate and runner system. The entire wax assembly is then dipped in a ceramic slurry, covered with a sand stucco (Fig. 10.1c), and allowed to dry. The dipping and stuccoing process is repeated until a shell of ~6-8 mm (1/4-3/8 in) is applied.

Once the ceramic has dried, the entire assembly is placed in a steam autoclave to remove most of the wax (Fig. 10.2a). After autoclaving, the remaining amount of wax that soaked into the ceramic shell is burned out in a furnace (Fig. 10.2b). At this point, all of the residual pattern and gating material is removed, and the ceramic mold remains. The mold is then preheated to a specific temperature and filled with molten metal, creating the metal casting (Fig. 10.3a). Once the casting has cooled sufficiently, the mold shell is chipped away from the casting. Next, the gates and runners are cut from the casting, and final postprocessing (sandblasting, machining) is done to finish the casting (10.3b). Fig. 10.4 shows the CAD solid model, the shell, and the pattern produced in the QuickCast process.

The major impact rapid prototyping processes have had on investment casting is their ability to make high-quality patterns (Fig. 10.5) without the cost and lead times associated with fabricating injection mold dies. In addition, a pattern can be fabricated directly from a design engineer's computer-aided design (CAD) solid model. Now it is possible to fabricate a complex pattern in a matter of hours and provide a casting in a matter of days. Investment casting is usually required for fabricating complex shapes where other manufacturing processes are too costly and time-consuming. Another advantage of rapid prototyping casting is the low cost of producing castings in small lot sizes.

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, standardised 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). Aluminium and copper are also continuously cast.
Sir Henry Bessemer, of Bessemer converter fame, received a patent in 1857 for casting metal between two contra-rotating rollers. The basic outline of this system has recently been implemented today in the casting of steel strip.

continuous casting

In continuous casting, the molten steel from the steelmaking operation or ladle metallurgy step is cast directly into semifinished shapes (slabs, blooms, and billets). Continuous casting represents a tremendous savings in time, labor, energy, and capital. By casting the steel directly into semifinished shapes, the following steps are eliminated: ingot teeming, stripping, and transfer; soaking pits; and primary rolling. Continuous casting also increases yield and product quality.

Thin slab casting is the centerpiece of a new technology that could revolutionize the competitive structure of steelmaking both in the U.S. and worldwide by making flat rolling accessible to minimills. Unlike conventional casting that produces a slab with up to a 10" section, thin slab casters produce a slab from 2"-3.5" thick that is integrated with a strip mill. The technology eliminates the large roughing mills required to work the thick slabs, and integrates slab production and sheet and strip rolling, greatly reducing reheating requirements.

Die-casting is similar to permanent mold casting except that the metal is injected into the mold under high pressure of 10-210Mpa (1,450-30,500) psi . This results in a more uniform part, generally good surface finish and good dimensional accuracy, as good as 0.2 % of casting dimension. For many parts, post-machining can be totally eliminated, or very light machining may be required to bring dimensions to size. Die-casting can be done using a cold chamber or hot chamber process.   • In a cold chamber process, the molten metal is ladled into the cold chamber for each shot. There is less time exposure of the melt to the plunger walls or the plunger. This is particularly useful for metals such as Aluminum, and Copper (and its alloys) that alloy easily with Iron at the higher temperatures.   • In a hot chamber process the pressure chamber is connected to the die cavity is immersed permanently in the molten metal. The inlet port of the pressurizing cylinder is uncovered as the plunger moves to the open (unpressurized) position. This allows a new charge of molten metal to fill the cavity and thus can fill the cavity faster than the cold chamber process. The hot chamber process is used for metals of low melting point and high fluidity such as tin, zinc, and lead that tend not to alloy easily with steel at their melt temperatures.

Die casting molds (called dies in the industry) tend to be expensive as they are made from hardened steel-also the cycle time for building these tend to be long. Also the stronger and harder metals such as iron and steel cannot be die-cast

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Common Alloys in Die Casting

Aluminum, Zinc and Copper alloys are the materials predominantly used in die-casting. On the other hand, pure Aluminum is rarely cast due to high shrinkage, and susceptibility to hot cracking. It is alloyed with Silicon, which increases melt fluidity, reduces machinability. Copper is another alloying element, which increases hardness, reduces ductility, and reduces corrosion resistance. Aluminum is cast at a temperature of 650 ºC (1200 ºF). It is alloyed with Silicon 9% and Copper about 3.5% to form the Aluminum Association 380 alloy (UNS A03800). Silicon increases the melt fluidity, reduces machinability, Copper increases hardness and reduces the ductility. By greatly reducing the amount of Copper (less than 0.6%) the chemical resistance is improved; thus, AA 360 (UNS A03600) is formulated for use in marine environments. A high silicon alloy is used in automotive engines for cylinder castings, AA 390 (UNS A03900) with 17% Silicon for high wear resistance. Common aluminum alloys for die casting are summarized as follows:

How To Slush Cast

To slush cast is to pour a fraction of the amount of resin into your mold (typically of a bigger piece that has a lot of volume) and coat the sides with it by rotating the mold while the resin cures. Once the sides are coated, you can back fill the piece or leave it to be hollow

To start, pick an original that you would like to slush cast. This turtle is approximately 6'x4'x3' which would require a large amount of resin to fill ... approximately 40 ounces of resin.
Pick a piece of plastic or wood to use as a base for the mold box. You will want the base to be approximately double the size of the piece to make sure you have room to seal the mold box thoroughly to prevent leakage.
Make a mold box around the turtle. Ideally you want your piece to be 1/4' to 3/8' away from the mold box but no more than that to prevent wasting silicone rubber.
As you can see we fastened the turtle in the mold box off center to reduce the amount of wasted space. To eliminate the wasted space we cut a piece of corrugated plastic and sealed it with modeling clay. We also sealed the PVC pipe to the base using the synthetic modeling clay.
Once the mold box has been sealed, mix and pour the silicone rubber. We have chosen to use the Dow Corning's HS III silicone rubber because of it's low durometer and high tear strength which will aid in demolding the piece.
Cover the highest point of the turtle by 1/4' to 3/8', then let the silicone cure overnight.
Once the silicone rubber has cured, remove the rubber from the mold box, and demold your original.
Here you can see the silicone rubber and the perfect original unaffected by the molding rubber. The moldmaking rubber will not bond to your master. Silicone rubber only bonds to other silicone rubbers. Therefore, as long as your original is not made out of a silicone rubber, the moldmaking material will not bond to it or affect the original in any way.
We recommend warming your mold prior to casting to ensure the thin section of the slush cast cures properly. Here we are adding a touch of green dye to the A side of Alumilite Regular to give us a light green finished color. Once we've mixed the green dye in thoroughly, we add the B side to the green A side.
Once we've thoroughly mixed the B side with the green A side, we slowly pour it into our mold.
This is where the slush cast term comes from. The amount of mixed resin is a fraction of the amount required to fill the mold. Next, we pick up the mold and continually tilt it, forcing the resin to coat all of the sides in the mold. We continue to slowly slush the resin around in the mold until the resin sets up (approximately 2 minutes with Alumilite Regular).
Once the resin has stopped flowing and has set up, you can set the mold back on the table. You can see where some of the resin spilled out of the mold by the feet of the turtle as we slushed the resin. Be careful not to get the resin onto your clothes or carpet. The other option is to put a flat piece of wood or glass that has been mold released over the back side to prevent it from spilling out. I would recommend slush casting it over a cardboard box, garbage can, or anything else that would catch the resin to prevent any unwanted messes. The green resin that has coated the outside of your mold will give you an exact replica of the surface of your original turtle.
Because the outside surface is so thin and fragile, we chose to use our 610 Foam to reinforce the turtle's skin and to fill the hollow void on the inside of it's body. The 610 Foam expands 10 times the original liquid volume to give you a very durable 6 lb density foam backing.
Once we've mixed equal amounts of the 610 Foam, we poured it into our mold. Because of the fine detail and multiple layers of the the turtle's head, neck, body, and feet we slushed cast the foam a couple of times to make sure we had all of the surfaces in the mold covered.
Once the foam starts to rise, we simply put the mold back down and let the foam take full form inside the turtle's hollow green slush cast. To ensure a good bond between the Alumilite Regular and the 610 Foam, we recommend pouring the 610 Foam within 5-10 minutes of pouring your slush cast layer. The sooner you pour one to another, the better the bond. They would still bond to one another even after waiting a long period of time, but the sooner the better.
Once the 610 Foam is cured (approximately 5 minutes), flex the HS III moldmaking rubber and remove your reinforced slush cast. Notice where the foam expanded out the bottom of the mold making a rounded bottom to the turtle. You can simply sand that down to make it perfectly flat or you could have used a flat piece of mold released glass or wood to lay on top of the mold as it was expanding to compress or not allow the foam to expand out giving you a flat bottom when you went to demold it.
Here you can see the original turtle along side of our slush casting that was backed with the 610 Foam. The original turtle on the left used approximately 40 ounces of resin, whereas the slush cast turtle on the right used 4 ounces Alumilite Regular and 3 ounces of the 610 Foam. Therefore we cut the amount of resin used from approximately 40 ounces to 7 ounces.

Investment casting is an industrial process based on and also called lost-wax casting, one of the oldest known metal-forming techniques.[1] From 5,000 years ago, when beeswax formed the pattern, to today’s high-technology waxes, refractory materials and specialist alloys, the castings allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys. Lost foam casting is a modern form of investment casting that eliminates certain steps in the process.
The process is generally used for small castings, but has produced complete aircraft door frames, steel castings of up to 300 kg and aluminium castings of up to 30 kg. It is generally more expensive per unit than die casting or sand casting but with lower equipment cost. It can produce complicated shapes that would be difficult or impossible with die casting, yet like that process, it requires little surface finishing and only minor machining.

Process

Casts can be made of the wax model itself, the direct method; or of a wax copy of a model that need not be of wax, the indirect method. The following steps are for the indirect process which can take two days to one week to complete.

1. Produce a master pattern: An artist or mould-maker creates an original pattern from wax, clay, wood, plastic, steel, or another material.[2]
2. Mouldmaking: A mould, known as the master die, is made of the master pattern. The master pattern may be made from a low-melting-point metal, steel, or wood. If a steel pattern was created then a low-melting-point metal may be cast directly from the master pattern. Rubber moulds can also be cast directly from the master pattern. The first step may also be skipped if the master die is machined directly into steel.[2]
3. Produce the wax patterns: Although called a wax pattern pattern materials also include plastic and frozen mercury.[2] Wax patterns may be produced in one of two ways. In one process the wax is poured into the mold and swished around until an even coating, usually about 3 mm (0.12 in) thick, covers the inner surface of the mould. This is repeated until the desired thickness is reached. Another method is filling the entire mould with molten wax, and let it cool, until a desired thickness has set on the surface of the mould. After this the rest of the wax is poured out again, the mould is turned upside down and the wax layer is left to cool and harden. With this method it is more difficult to control the overall thickness of the wax layer.[citation needed]
   If a core is required, there are two options: soluble wax or ceramic. Soluble wax cores are designed to melt out of the investment coating with the rest of the wax pattern, whereas ceramic cores remain part of the wax pattern and are removed after the workpiece is cast.[2]
4. Assemble the wax patterns: The wax pattern is then removed from the mould. Depending on the application multiple wax patterns may be created so that they can all be cast at once. In other applications, multiple different wax patterns may be created and then assembled into one complex pattern. In the first case the multiple patterns are attached to a wax sprue, with the result known as a pattern cluster, or tree; as many as several hundred patterns may be assembled into a tree.[3] Foundries often use registration marks to indicate exactly where they go.[citation needed] The wax patterns are attached to the sprue or each other by means of a heated metal tool.[2] The wax pattern may also be chased, which means the parting line or flashing are rubbed out using the heated metal tool. Finally it is dressed, which means any other imperfections are addressed so that the wax now looks like the finished piece.[4]
5. Investment: The ceramic mould, known as the investment, is produced by three repeating steps: coating, stuccoing, and hardening. The first step involves dipping the cluster into a slurry of fine refractory material and then letting any excess drain off, so a uniform surface is produced. This fine material is used first to give a smooth surface finish and reproduce fine details. In the second step, the cluster is stuccoed with a coarse ceramic particle, by dipping it into a fluidised bed, placing it in a rainfall-sander, or by applying by hand. Finally, the coating is allowed to harden. These steps are repeated until the investment is the required thickness, which is usually 5 to 15 mm (0.2 to 0.6 in). Note that the first coatings are known as prime coats. An alternative to multiple dips is to place the cluster upside-down in a flask and then liquid investment material is poured into the flask. The flask is then vibrated to allow entrapped air to escape and help the investment material fill in all of the details.[2][5]

Common refractory materials used to create the investments are: silica, zircon, various aluminium silicates, and alumina. Silica is usually used in the fused silica form, but sometimes quartz is used because it is less expensive. Aluminium silicates are a mixture of alumina and silica, where commonly used mixtures have an alumina content from 42 to 72%; at 72% alumina the compound is known as mullite. During the primary coat(s), zircon-based refractories are commonly used, because zirconium is less likely to react with the molten metal.[5] Chamotte is another refractory material that has been used.[citation needed] Prior to silica, a mixture of plaster and ground up old molds (chamotte) was used.[6]

The binders used to hold the refractory material in place include: ethyl silicate (alcohol-based and chemically set), colloidal silica (water-based, also known as silica sol, set by drying), sodium silicate, and a hybrid of these controlled for pH and viscosity.

6. Dewax: The investment is then allowed to completely dry, which can take 16 to 48 hours. Drying can be enhanced by applying a vacuum or minimizing the environmental humidity. It is then turned upside-down and placed in a furnace or autoclave to melt out and/or vaporize the wax. Most shell failures occur at this point because the waxes used have a thermal expansion coefficient that is much greater than the investment material surrounding it, so as the wax is heated it expands and induces great stresses. In order to minimize these stresses the wax is heated as rapidly as possible so that the surface of the wax can melt into the surface of the investment or run out of the mold, which makes room for the rest of the wax to expand. In certain situations holes may be drilled into the mold beforehand to help reduce these stresses. Any wax that runs out of the mold is usually recovered and reused.[7]
7. Burnout & preheating: The mold is then subjected to a burnout, which heats the mold between 870 °C and 1095 °C to remove any moisture and residual wax, and to sinter the mold. Sometimes this heating is also as the preheat, but other times the mold is allowed to cool so that it can be tested. If any cracks are found they can be repaired with ceramic slurry or special cements.[7] The mold is preheated to allow the metal to stay liquid longer to fill any details and to increase dimensional accuracy, because the mold and casting cool together.[8]
8. Pouring: The investment mold is then placed cup-upwards into a tub filled with sand. The metal may be gravity poured, but if there are thin sections in the mold it may be filled by applying positive air pressure, vacuum cast, tilt cast, pressure assisted pouring, or centrifugal cast.[8]
9. Removal: The shell is hammered, media blasted, vibrated, waterjeted, or chemically dissolved (sometimes with liquid nitrogen) to release the casting. The sprue is cut off and recycled. The casting may then be cleaned up to remove signs of the casting process, usually by grinding.[8]

A sand casting or a sand molded casting is a cast part produced by forming a mold with the help of a model or pattern pressed into a sand mixture and then removed, after which molten liquid metal is poured into the cavity in the mold. The mold is then cooled until the metal has solidified. In the last stage, the casting is separated from the mold. There are six steps in this process:

1. Place a pattern in sand to create a mold.
2. Incorporate the pattern and sand in a gating system.
3. Remove the pattern.
4. Fill the mold cavity with molten metal.
5. Allow the metal to cool.
6. Break away the sand mold and remove the casting.

There are two main types of sand used for molding. Green sand (the name is due to its unfired or green state, not its colour); it is a mixture of silica or olivine sand, clay, moisture and other additives. The air set method uses dry sand bonded with materials other than clay, using a fast curing adhesive. The latter may also be referred to as no bake mold casting. When these are used, they are collectively called "air set" sand castings to distinguish them from "green sand" castings. Two types of molding sand are natural bonded (bank sand) and synthetic (lake sand); the latter is generally preferred due to its more consistent composition.
With both methods, the sand mixture is packed around a master pattern, forming a mold cavity. If necessary, a temporary plug is placed in the sand and touching the pattern in order to later form a channel into which the casting fluid can be poured. Air-set molds are often formed with the help of a two-part mold having a top and bottom part, termed the cope and drag. The sand mixture is tamped down as it is added around the pattern, and the final mold assembly is sometimes vibrated to compact the sand and fill any unwanted voids in the mold. Then the pattern is removed along with the channel plug, leaving the mold cavity. The casting liquid (typically molten metal) is then poured into the mold cavity. After the metal has solidified and cooled, the casting is separated from the sand mold. There is typically no mold release agent, and the mold is generally destroyed in the removal process.[1]
The accuracy of the casting is limited by the type of sand and the molding process. Sand castings made from coarse green sand impart a rough texture to the surface, and this makes them easy to identify. Air-set molds can produce castings with much smoother surfaces. Surfaces can also be later ground and polished, for example when making a large bell. After molding, the casting is covered with a residue of oxides, silicates and other compounds. This residue can be removed by various means, such as grinding, or shot blasting.
During casting, some of the components of the sand mixture are lost in the thermal casting process. Green sand can be reused after adjusting its composition to replenish the lost moisture and additives. The pattern itself can be reused indefinitely to produce new sand molds. The sand molding process has been used for many centuries to produce castings manually. Since 1950, partially-automated casting processes have been developed for production lines.


Semi permanent mold casting:

Semi-permanent mold is a casting process - producing Aluminum alloy castings - using re-usable metal molds and sand cores to form internal passages within the casting. Molds are typically arranged in two halves - the sand cores being put into place before the two halves are placed together. The molten metal flows into the mold cavity and surrounds the sand core while filling the mold cavity. When the casting is removed from the mold the sand core is removed from the casting leaving an internal passage in the casting.

The re-usable metal molds are used time and again, but the sand cores have to be replaced each time the product is cast, hence the term semi-permanent molding.

Semi-permanent molding affords a very high precision quality to the casting at a reduced price compared to the sand casting processes.

A single crystal or monocrystalline solid is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. The absence of the defects associated with grain boundaries can give monocrystals unique properties, particularly mechanical, optical and electrical, which can also be anisotropic, depending on the type of crystallographic structure. These properties, in addition to make them precious in some gems, are industrially exploited in technological applications, especially in optics and electronics.
Because entropic effects favor the presence of some imperfections in the microstructure of solids, such as impurities, inhomogeneous strain and crystallographic defects such as dislocations, perfect single crystals of meaningful size are exceedingly rare in nature, and are also difficult to produce in the laboratory, though they can be made under controlled conditions. On the other hand, imperfect single crystals can reach enormous sizes in nature: several mineral species such as beryl, gypsum and feldspars are known to have produced crystals several metres across.
The opposite of a single crystal is an amorphous structure where the atomic position is limited to short range order only. In between the two extremes exist polycrystalline , which is made up of a number of smaller crystals known as crystallites, and paracrystalline phases.

Introduction: Slush Casting is a traditional method of permanent mold casting process, where the molten metal is not allowed to completely solidify in the mold. When the desired thickness in obtained, the remaining molten metal in poured out. Slush casting method is an effective technique to cast hollow items like decorative pieces, components, ornaments, etc.

Process: Mostly pewter is casted using the slush casting technique. Firstly, a pattern is made using plaster or wood. Now the pattern is placed on a cardboard or wooden board. A mold box is kept around the pattern. The unwanted space that is formed is the mold box can be eliminated by placing a board. Once the pattern is set the molding material is poured on the pattern and allowed to set with the molding aggregate. When the mold is set, the pattern is withdrawn from the mold.

The metal melted completely and poured into the mold which is shaped in the desired form. Rotate the mold to coat the sides. When the metal settles in the mold, remaining liquid metal is poured out of the mold. Thus, a hollow skin metal is formed inside the mold.

If the cast needs to be more thicker, once again molten metal is poured into the mold and poured out. This process is repeated until the desired thickness is achieved. In some slush castings, bronze molds are used. When the metal hardens, the mold is broken to remove the castings. The inside of each cast retains molten textures while the exterior is smooth and shiny. Bowls and vases are serially produced by this technique that ensures no two are ever the same.

Similarly, to cast metals a bowl, a new process designed to capture the beauty of Pewter and its unique characteristics. Recycled molten Pewter is swirled inside amould to form a fine skin. The inside of each cast retains molten textures whilst the exterior is smooth and shiny. Bowls are serially produced by a technique that ensures no two are ever the same.

Application: Some casting of pewter is cast using slush casting method. Using pewter and other metal s mainly hollow products are casted. Decorative and ornamental objects that are casted are as vase, bowls, candlesticks, lamps, statues, jeweleries, animal miniatures, various collectibles,etc. Small objects and components for industry like tankard handle, handles for hollow wares, etc.

Advantage:

• Slush casting is used to produce hollow parts without the use of cores
• The desired thickness can be achieved by pouring our the left over molten metal
• A variety of exquisitely designed casting can be casted for decorative and ornamental purpose.

Squeeze casting, also known as liquid metal forging, is a combination of casting and forging process.
The molten metal is poured into the bottom half of the pre-heated die. As the metal starts solidifying, the upper half closes the die and applies pressure during the solidification process. The amount of pressure thus applied is significantly less than used in forging, and parts of great detail can be produced. Coring can be used with this process to form holes and recesses. The porosity is low and the mechanical properties are improved.
Both ferrous and non-ferrous materials can be produced using this method.
Vacuum Casting

A form of RTV moulding known as Vacuum Casting is widely used for producing accurate silicone tools for casting parts with fine details and very thin walls.

Vacuum castings are precise replicas of the patterns, dimensionally accurate without blemishes, with all profiles and textures faithfully reproduced.

Process

The MCP vacuum casting requires initial investment in a vacuum chamber with two sections. The upper section is for mixing the resin and the lower is for casting the resin into the mould.

The process includes nine steps:

1. The first step is to produce a pattern using any of the available RP processes (SLA, SLS, FDM, etc.)
2. The pattern is fit with a casting gate and set up on the parting line, and then suspended in a mould casting frame.
3. Once the two-part silicone-rubber is de-aerated and then mixed, it is poured into the mould casting frame around the pattern.
4. The mould is cured inside a heating chamber.
5. The pattern is removed from the silicone mould by cutting along the parting line.
6. The urethane resin is measured, dye is added for coloured components and casting funnels placed. Then, the mould is closed and sealed.
7. The computer-controlled equipment mixes and pour the resin inside the vacuum chamber. Because this takes place in a vacuum, the mould is filled completely without leaving any airpockets or voids.
8. After casting the resin, the mould is moved to the heating chamber for two to four hours to cure the urethane part.
9. After hardening, the casting is removed from the silicone mould. The gate and risers are cut off to make an exact copy of the pattern. If required, the component can be painted or plated.

Application

Vacuum casting is an alternative technique for the production of prototype castings which mimic injection moulded parts such as ABS, PP, PMMA, PA and different grades of rubber. Using the MCP Heated Cup Module, complex, accurate wax models for investment casting can be produced.

In-house Facilities to Manufacture EP Tools and RTV Moulds

• SL 3500 (maximum size - L 350 mm x W 350 mm x H 400 mm),
• MCP Vacuum mixer (casting capacity - 15 Kg)
• MCP Vacuum casting machine 4/10 (castable tool size - L 400 mm x W 425 mm x H 450 mm)

Pro/Engineer, Pro/mold software, Magic RP software