Casting ProcessesDr Ajay Batish

Shell MoldingIt is a process in which, the sand mixed with thermosetting resin is allowed to come in contact with the heated metallic pattern plate, so that a thin and strong shell of mould is formed around the pattern, Then the shell is removed from the pattern and the cope and drag are removed together and kept in a flask with necessary back up material and then the molten metal is poured.

Shell MoldingShell molding process offers better surface finish, better dimensional tolerances, and higher throughput due to reduced cycle times. A heated (200 C / 392 F) metal pattern is covered with a mixture of sand and thermoset plastic. This causes a skin of about 3.5 mm (0.125 in) of sand/plastic mixture to adhere to the pattern. This skin is removed from the pattern to form the "shell mold". The two halves of the shell mold are secured together and the metal is poured in the shell to form the part. Once the metal solidifies, the shell is broken.

Shell Molding

Shell Molding

Shell MoldingThis process can produce complex parts with good surface finish 1.25 m to 3.75 m (50 in to 150 in) rms, and good dimensional tolerance of +/- 0.25mm.Size limits of 30 g to 12 kg (1 oz to 25 lb). Minimum thicknesses can be as low as 1.5 mm (0.062 in) to 6.25 mm (0.25 in), depending on the material. A good surface finish and good size tolerance reduce the need for machining.

Shell MoldingA fairly high capital investment is required, but high production rates can be achieved. The process overall is quite cost effective due to reduced machining and cleanup costs. The materials that can be used with this process are cast irons, and aluminum and copper alloys. Typical parts made with this process are connecting rods, gear housings, lever arms etc.

Shell MoldingApplications-Crankshaft fabrication -Steel casting parts, fittings -Molded tubing fabrication -Hydraulic control housing fabrication -Automotive castings (cylinder head and ribbed cylinder fabrication).

Shells removed from the heated patternShells matched to form a mould. (Mould preheated to avoid moisture)

Shells formed on the heated metal pattern

Copyright Amit M Joshi

Precision Investment Castings through Lost Wax Process

Investment Casting uses a mold that has been produced by surrounding an expendable pattern with a refractory slurry that sets at room temperature. The pattern (usually of wax) is then melted or burned out, leaving the mold cavity. Investment casting is also known as the "lost-wax process" and as "precision casting".In Investment casting, a metal pattern die is used to produce the patterns, which, in turn, are used to produce ceramic molds. Both the pattern and molds are expendable. Ceramic cores are used, as required, and these also are expendable.

Precision Investment CastingsInvestment casting is the most flexible of all the precision casting process with respect to attainable intricacy, precision and the variety of alloys that may be cast within its inherent size limitations.Many exaggerated claims were made regarding the "precision" casting process.Many technical articles purported to establish the dimensional tolerances for production castings to limits as extreme as 0.0001 in. In reality, however, liner tolerances of 0.002-0.005 in. per inch where difficult to obtain

Advantages of precision castingComplex shapes can be made as there is no need to withdraw the patternVery fine details and thin sections can be obtainedReasonably close tolerances and good finish can be achievedCastings require little or no machiningSince there is no parting line the dimensions across will not vary

Limitations and applicationsSize cannot exceed 5kgExpensiveApplications:Vanes and blades of Gas turbinesClaws of movie cameras, wave guides for radarsTriggers for fire armsStainless steel valve bodies

Permanent mold castings

Instead of using sand as the mold material, a metal is used as a mold. Typically cast iron is used as the mold material and the cores are made from metal or sand. Cavity surfaces are coated with a thin layer of heat resistant material such as clay or sodium silicate. The molds are pre-heated upto 200 C (392 F) before the metal is poured into the cavity. The cavity design for these molds do not follow the same rules for shrinkage as in sand casting molds, This is because the metal molds heat up and expand during the pour, so the cavity do not need to be expanded as much as in the sand castings.

Permanent mold castingsGates and Risers are similar to that in sand castingMoulds are coated with refractory material to a thickness of about 0.8mm forPreventing the soldering of metal to moldMinimizing the thermal shock to mold materialControlling the rate and direction of solidification

Permanent mold castingsCoatings may be applied by spraying or brushing and must be thicker at surfaces which need to be cooled slowly e.g. sprues, runners, risersThe usual considerations of minimum wall thicknesses (such as 3mm for lengths under 75 mm), radius (inside radius = nominal wall thickness, outside radius = 3 x nominal wall thickness), draft angles (1 to 3 on outside surfaces, 2 to 5 on inside surfaces) etc all apply. Typical tolerances are 2 % of linear dimensions. Surface finish ranges from 2.5 m to 7.5 m (100 in to 250 in).

Permanent mold castingsTypical part sizes range from 50 g to 70 kg (1.5 ounces to 150 lb). Typical materials used are small and medium sized parts made from aluminum, magnesium and brass and their alloys. Typical parts include gears, splines, wheels, gear housings, pipefitting, fuel injection housings, and automotive engine pistons.

Permanent mold castingsPermanent mold castings, while not as flexible as sand castings in allowing the use of different patterns (different part designs), lower the cost of producing a part. At a production run of 1000 or more parts, permanent mold castings produce a lower piece cost part. Of course, the break-even point depends on the complexity of the part.

Die CastingDie-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 CastingDie-casting can be done using a cold chamber or hot chamber process. 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.

Die Casting

Die Casting 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

Die Casting

Die Casting

Dies

Dies consists of two parts:A cover die fixed on the stationery platen of the die casting machine. It consists of sprue, runners and gates and is also in contact with the nozzle of the gooseneck in the case of hot chamber and with the shot chamber in case of cold chamber processEjector die is fixed on the moving platen. Ejector pins move thru the moving die to free the casting from the ejector die

Die CastingCores used are metallic and are of two types:Fixed core are fixed to the die halves and parallel to the die movementMoving cores are not parallel with the die movement and are to be removed before the casting is to be ejected from the dieDie casting molds 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

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 AlloysAluminum is cast at a temperature of 650 C (1200 F). It is alloyed with Silicon 9% and Copper about 3.5%. 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; A high silicon alloy is used in automotive engines for cylinder castings, with 17% Silicon for high wear resistance

Zinc alloysZinc can be made to close tolerances and with thinner walls than Aluminum, due to its high melt fluidity. Zinc is alloyed with Aluminum (4%), which adds strength and hardness. The casting is done at a fairly low temperature of 425C (800F) so the part does not have to cool much before it can be ejected from the die. Zinc alloys are used in making precision parts such as sprockets, gears, and connector housings.

Copper alloys are used in plumbing, electrical and marine applications where corrosion and wear resistance is important. Minimum wall thicknesses and minimum draft angles for die casting are

MaterialMin. Thickness mm (in)Min. Draft Angle ()Aluminum alloys0.9 mm (0.035 in)0.5

Zinc alloys

0.6 mm (0.025 in)0.25

Copper alloys (Brass)1.25 mm (0.050 in)0.7

Die CastingDie-castings are typically limited from 20 kg (55 lb) max. for Magnesium, to 35 kg (77 lb) max. for Zinc. Large castings tend to have greater porosity problems, due to entrapped air, and the melt solidifying before it gets to the furthest extremities of the die-cast cavity. From a design point of view, it is best to design parts with uniform wall thicknesses and cores of simple shapes. Heavy sections cause cooling problems, trapped gases causing porosity. All corners should be radiused generously to avoid stress concentration. Draft allowance should be provided to all for releasing the parts-these are typically 0.25 to 0.75 per side depending on the material.

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 solidification takes place from the outside surface towards the axis of rotation resulting in directional solidification, which takes care of shrinkage. Centrifugal forces continue to feed molten metal, compensating for shrinkage, so no risers are required for centrifugal castings.

Centrifugal Casting

No core or mold is needed to shape the interior of the castings, which will always have a round profile because the molten metal is uniformly distributed by the centrifugal forces. When rotation is about the horizontal axis, the inner surface is always cylindrical. In case a vertical axis of rotation is used, gravitational forces cause the inner surface to take shape of a parabola and the exact shape is a function of the speed of rotation. 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 like dross which have lower density and are thus lighter, which can be machined away.

Centrifugal Casting

Advantages Mechanical properties are better because of inclusions such as slag and oxides are segregated towards the centre and can be easily removed by machiningThe pressure acting on the metal through the segregation causes the porosity to be eliminated.There is no need for gates or runners which increases the yield reaching almost 100%.

Centrifugal CastingOnly 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.

Centrifugal CastingTypical 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.

Semi-Centrifugal Casting

Semi-Centrifugal CastingIs used for jobs which are more complicated than those possible in true centrifugal castings but are asymmetrical in natureThe moulds made of sand or metal are rotated about a vertical axis and the metal enters the mould through the central pouring basinFor larger production rates, the moulds can be stacked one over the other all feeding from the same central pouring basin

CentrifugingCentrifuging 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 details. This method is often used for the pouring of investment casting pattern.

Centrifuging