Metal Casting Zone Info Blog

Steve Manik asked:


Best pressure die casting

This relates to a pressure die casting machine and in particular to an apparatus and method for moving a die of a pressure die casting machine.

Pressure die casting is the injection of melted metal or plastic under high pressure into a mould cavity.

Before injecting the melted metal into the cavity, the mould is “closed” i.e. the two halves of the mould, called dies are brought together, after which dies are held together while the melted metal is forced into the cavity they form. The metal is allowed to solidify in the shape of mould cavity and then the dies are pulled apart so that solidified object may be ejected and the cycle repeated.

To manufacture die castings free of pores and shrink holes it is normal practice to fill the mould at high pressure and to let the metal solidify while under high pressure so as to effect compression of die casted metal. The apparatus which closes the dies and holds them together needs to have capacity to withstand this high pressure and it is required to work for long hours.

To simplify the clamping procedure for holding the dies together the apparatus uses a die casting machine which has one die fixed on the machine base and second die half is removable into and out of apparatus. Thus movement and control apparatus might be given for one half of die. This has an additional advantage that the hot melted metal can be fed into the mould cavity or chamber between the dies through a sprue in the fixed die.

Aluminium and copper alloys that attack and erode machine components with which they are in regular contact are generally produced in what is called cold chamber machine, whereas tin, lead and zinc die castings are processed in a chamber called hot chamber machine. The pressure die casting machine is equally well used in hot and cold chamber pressure die casting.

A die casting machine uses a fixed and moveable die. Four tie bars or guideways are rigidly connected to two plates upstanding from the machine lease; one die is placed on one plate while the other die is slidably mounted on the tie bars. Pressure oil is given to an operating cylinder containing an operating piston which is linked to a displacement yoke linked in turn with guide spars these are externally threaded to receive nuts between which the yoke is clamped.

The dies are closed and melted metal is entered into cavity of dies from the sprue in the fixed die and required pressure is given. After the metal solidifies the pressure on the dies is released and two halves of dies are separated and finished product is taken out and process of pressure die casting is repeated.

The same process of pressure die casting is used in plastic moulded die casting products the only difference being that instead of melted aluminium, melted plastic is poured into the die from a sprue fixed in lower die and required pressure is given. After the plastic cools down and solidifies the pressure is released, and die opened and finished product is taken out and process repeated.

Metal Casting DIY

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The zinc-aluminium die casting alloys

New high performing zinc-aluminium ZA casting alloys (zA-8, ZA-12, ZA-27) give superior mechanical properties which designers can apply utilizing die casting technology. In general the ZA alloys are stronger, harder and offer more creep resistance than standard zinc alloys and can be used where bearing properties are important.

Aluminium alloys with 0.5-0.9% Fe content have largely replaced 1350 EC alloy for making electrical circuits because the latter continuously suffered from gradual loosening at terminals, which led to overheating. This problem has been totally removed in new conductor alloys without sacrifice of conductivity.

To get economic benefit of weight advantage of aluminium wire should be capable of attaching securely to standard fixtures without special handling techniques. But EC wire on binding screw terminals tightened to a standard torque may become loose, when the wire heats due to being overloaded. The wire gets expanded more than the Cu-alloy fixture and creeps to relax the added stress.

On getting cool it contracts to a smaller dimension, whereby the area of contact is reduced and it permits oxide to form at interface. On a subsequent current overflow, the overheating increases which leads to further loosening of wire. EC wire annealed for adequate bend ability gets sub structurally loosened at 200°C and ultimately fails due to repetitions of these cycles.

The new alloys (800 series) of 0.5-0.9% Fe have much better microstructural stability and creep resistance and, therefore, they are not prone to these failures.

While annealed to the same ductility or bend ability, the high Fe alloys are double strong. This capability has been established by practical field use of many years in USA, Europe and South Africa after these alloys were introduced in 1968.

Better and latest alloys which not only provide high integrity to terminations but are suitable for magnet wire after normal hot annealing have been made after adding a third alloy to improve its performance examples are 0.5% Fe with 0.5% Co and 0.5% Fe with 0.2-0.4% Si.

Processing and microstructure:

In continuous casting a bar of 50cm2 is made at 16 m/min on a 2.5m diameter copper wheel. The quick solidification results in a 20 μm dendrite arm spacing and eutectic red cpacing of about 0.2 μm with a supersaturation of about 0.1% Fe. These very fine particles play a significant role in giving stability to substructure while being incapable of nucleating crystallization.

The presence of sub grains has been known in hot worked aluminiums but without quantitative determinations of the dimensions or the effects on properties. As the temperature rises from 200-450°C, the cold yield strength of the hot worked product decreases greatly from the strengthening made by 97.5% cold rolling.

As has been seen in many hot worked metals, the yield strength is inversely proportional to sub grain diameter. Because the temperature is less and strain rate is high in a given pass than those in the previous one, substructure “inherited” from i.e., carried forward from, the latter is altered by dislocations to the existing walls to raise their density and by formation of new walls to subdivide the sub grains lessening their size.

Casting Furnace

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Iron casting

Cast iron generally means grey cast iron, but is identifies a group of ferrous alloys which solidify with a eutectic.

Overview:

Iron accounts for more than 95% the alloy material, while the main alloying elements are carbon and silicon. The amount of carbon in cast iron is 2.1-4% while ferrous alloys with less carbon are called carbon steel by definition. Cast iron has appreciable amount of silicon normally 1.3%. Therefore, these alloys should be considered ternary Fe-C-Si alloys.

In spite of this, the principles of cast iron solidification are understood from the binary iron carbon phase diagram, where the eutectic point lies at 1154 °C and 4.3 wt% carbon. Because cast iron has this composition, its melting temperature of 1150 to 1200 °C is about 300 degrees less than the melting point of pure iron. Cast iron tends to be brittle, though the name of particular alloy may suggest opposite. The color of a fracture surface may be utilized to identify an alloy; carbide impurities allow cracks to pass straight through resulting in a smooth “white” surface, while graphic flakes deflect a passing crack and initiate countless new cracks as the material breaks, leading to a rough surface that looks grey with its low melting point, good fluidity, castability, excellent machinability and wear rising resistance, cast irons have become an engineering material with a wide range of uses like pipes, machine and auto parts.

Products:

Cast iron is produced by remelting pig iron, normally with large quantities of scrap iron and steel and initiating steps to remove unwanted contaminants like phosphorus and sulfur. Depending on use carbon and silicon content are lessened to the required levels which may be anywhere from 2% to 3.5% and 1% 3% respectively. Other elements are then added to the melt prior to the final form being made by casting.

Iron is generally melted in a small blast furnace called cupola. After melting is over the melted iron is ladled from the forehearth of blast furnace. This system was developed by the Chinese whose innovative ideas brought revolution in field of metallurgy. Before that iron was melted in an air furnace, which is a type of reverberatory furnace.

Some advantages of cast iron in engineering uses:

a) A family of metals having capacity of being used for engineering and production needs.

b) You can have it in a wide range of mechanical and physical properties.

c) Good strength to weight ratio.

d) Generally cheaper than other competing metals and lower financial cost per unit of strength compared to other metals.

e) Lesser density and higher thermal conductivity then steels at comparable tensile strength levels.

f) Easily mechniable, allows high speeds and feeds and less energy due to free graphite being presence.

g) Many iron castings may be utilized without heat treatment (as cast) but when required may be heat treated to increase overall properties or local property like surface hardness.

h) Very good damping capability especially in grey irons.

i) Chemical analysis may be changed to give improved special properties like corrosion resistance, oxidation and wear resistance.

j) Quickly changes from design to finished goods.

k) Capable of having highly complex sizes from ounces to 100 tons.

l) Of flexible pattern and capacity to improve appearance for sales appeal.

m) You can make intricate shapes as well as very thin to very thick sections.

n) Capable of redesigning and combining two or more parts from metals into a single casting thereby lessening assembly cost and time.

o) Capable of being cast with inserts of other metals.

p) Many casting systems for low, medium or high production.

q) Less tendency toward residual stress and warpage than other competing metals.

Metal Casting Blog

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Sand moulding

Green sand is a permanent favorite for metal casting because it is very easy to use and you can also foretell what the result will be for using it. It can retain moisture for many days continuously if you pack it in a plastic container.

Green sand is an efficient, economical means of making moulds. But one problem is that you require a Muller to make the first batch. Green sand needs maintenance and care if you wish for best results, but it lasts for years and can be reused many times for hobby purposes.

CO2 gas systems and dry sand:

The CO2 process is an easy process for hobby use. It is regularly utilized in technical schools and colleges for giving practical training in foundry practices and due to basic equipment needs small batches of moulding sand may be easily made up.

Normally it is easily usable and repeatable results are easily achievable. But there is one thing which might harm its results and i.e., if there is more moisture in sand say above 0.5% it will lead to poor moulding results.

Another minus point is rental costs of gas bottles and expenditure of cylinder regulators needed for proper gas pressure. Sand used in moulding is useless after every use, which creates a problem if casting is continuous.

The self set moulding system:

The self set system is easily usable; all that you require is clean foundry grade sand, a silicate resin for mixing with sand and a catalyst to induce reaction in silicate, which will take about 10 minutes on a hot day.

The equipment needed is:

A mixer for this a small handheld power drill with a point mixer will do it, and an accurate scale to measure chemicals being used.

The silicate is a costly item which comes in drums of 20 or 200 liters. The hobby foundry worker will need to be on friendly terms with a commercial foundry operator of your area.

There are many separate processes which might be used, but they are rather complex to give details here. Suppliers like Foseco have their free guidelines for using their products and you can use chemicals quite safely so long as you follow their producers instructions.

The EPS or Full mould system:

EPS means Expended Poly-Styrene it is similar to investment casting because a single part flask is utilized with result that no lines appear on final casting. It is necessarily a ‘one off’ system, because the consumable design is made from expanded polystyrene.

This is a polymer taken from benzene and ethylene and in its expanded form it has only 2% real solid polystyrene. Readers may know this substance as it is utilized in producing ceiling tiles, and as packaging for audio and electronics equipment.

An expendable design, complete with runners and risers, is cut from expanded polystyrene and is totally surrounded with clean dry sand in a box or can. The melted metal is then poured on the design, which melts and burns fast and leaves a cavity which is occupied by melted metal. No residue is formed; the carbon dioxide and water vapor evolved by burning of polystyrene don’t dissolve in the melted metal, but escape via the permeable mould sand as gas. The EPS system produces a very strong smell.

Moulding may be done by pouring dry sand around the pattern. As the polystyrene burns it makes a tacky land among the sand grains long enough for metal’s skin to be formed.

This moulding process is largely used in engineering companies. It is used to make press-tool die holders and small parts in ‘one off’ category. Casting tolerance is about same as that of investment casting. This process has extensive range of uses for experienced hobby caster.

Back Yard Metal Casting

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Steps of plaster mould casting

In mould casting, melted aluminium is poured into a cavity in the required shape of product. When the hot aluminium comes into contact with mould its temperature goes down very fast. This causes the quick solidification of product being produced. Mould makers face the challenge of making sure that all the parts of mould cavity are properly filled, whatever the local temperature of aluminium might be or the hole might be narrow from which melted metal is to be filled.

Fluent goods give chance to mould designers to keep track of melted aluminium as it is poured into the mould. This gives them time to identify probable defects due to trapped air, extreme cooling of metal and possible defects of mould due to combined action of high temperature and high pressure. Mould designers are also able to detect folding free surface that can entrap air bubbles, break free surfaces which may cause excess oxidation of the metal and the progressive solidification of the aluminium.

What is plaster mould casting?

Plaster mould casting, it is also known as Rubber Plaster Moulding (RPM) is a system of making aluminium or zinc castings by pouring liquid metal into plaster (Gypsum) moulds.

Step – 1: Model or master pattern:

a) Made from client’s drawing or CAD file.

b) Stereolithography, traditional hand made or machined.

c) Model is engineered to include:

a. Metal shrinkage

b. Mould taper (if needed)

c. Machine stock (if needed)

d) You may “clone” or adapt client supplied model if requested.

Step – 2: Foundry Pattern equipment:

a) Negative moulds are made from model

b) Core plugs are made from moulds

c) A positive resin cope and drag design is now made from negative moulds.

d) Core boxes are made from core plugs

e) Gating, runner system and flasks are joined as needed.

f) Duplicate sets of tooling might be made from the master negative.

Step – 3: Plaster mould:

a) A liquid plaster slurry is poured around the cope and drag pattern and into the core boxes.

b) The plaster mould is next removed from the cope and drag pattern.

c) The plaster mould and cores are then baked to remove moisture.

Step – 4: Pour Casting:

a) Melted metal is made by degassing, and a spectrographic sample is taken to check the chemical analysis.

b) The melted metal is then poured into the assembled plaster mould.

c) The plaster is removed by mechanical knockout and high pressure waterjet.

d) After cooling of the casting, the gates and risers are then removed.

Step – 5: Secondary operations:

a) The raw castings are inspected and serialized.

b) Castings may then need (as per client specifications):

a. Heat treatment

b. X-Ray

c. Penetrant inspection

c) After finish inspection, casting is ready for:

a. Machining

b. Chemical film, chromate conversion, paint or special finishes

c. Assembly

d. Form-in-place gasketing.

Plaster craft is a hobby, which gives you chance to make a few pennies worth of material into beautiful wall hangings and sculptures.

Plaster crafting is quite safe if you observe safety rules while mixing plaster. Once it is hardened three is little or no hazard from handling plaster things. Plaster is usually Plaster Of Paris. It is called by various names depending on its producer-viz.-Plaster Of Paris, casting plaster or just plaster.

DIY Metal Casting

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Aluminium rainwater guttering

The type of guttering on a building gives an indication of the period in which it was built. On older buildings you will find iron or cement pipes and on buildings built recently or in recent past you will find plastic guttering pipes. The latest is seamless aluminium guttering.

The older systems of iron pipes, rolled steel and cement guttering have almost same problems regarding their fixings and joints. But modern plastic rainwater pipes are lightweight, durable and are normally self colored. Each producer has his own system of joining and fixing. Some use brackets and separate joining clips while there are others who use brackets having joining clips.

Maintenance of guttering:

Defective pipes and rainwater guttering is a source of many damp spots in the building. The guttering and rainwater pipework are installed to carry rainwater away from the building but if not properly maintained, the water runs along the wall and dose to the building eventually damaging the main structure. Therefore, it is necessary to keep it well maintained.

The most appropriate time to carry out repairs is in autumn when rains are over and weather is dry and slightly pleasant.

If water is seen coming over the side of pipe during heavy rain it means that some rubbish is blocking its opening. Scoop the rainwater pipe with a trowel or a cardboard piece which can bend into face of guttering pipe. After removing all the solid rubbish and soft rubbish underneath flush the guttering with water. The best method is using a hosepipe.

Be careful while clearing a downpipe. If you try to push the blockage downward you might block it instead of opening it. Place a bowl at the bottom of the pipe so that the rubbish does not go down the gutter at ground level and block it. Get the rubbish out by using a stiff wire of steal.

Guttering is normally fixed in a way as to give it a little slope. If water is collecting along the run of the guttering then you might need to reposition it or change its slope. If the affected position is plastic guttering it is probably easy to fix it. Inspect cast iron and steel pipes if there is any rust; if you find it then clean it with a wire brush and paint the bare areas with anti rust paint. Fill any hairline cracks with two coats of anti rust paint.

If a joint is leaking and you are unable to dismantle it then you need to carryout repairs. Seal the leaking joint internally with waterproof mastic after cleaning the surface. If one joint is leaking there might be others also so do a thorough job and apply the mastic to all joints. If a leak has developed in a plastic rainwater pipe, just replace that part of it, and apply master sealer at both ends to prevent any leakage. While buying a replacement section of guttering, remember to take along an old piece of guttering so that you buy new pipe of correct diameter and shape.

Metal guttering and rainwater pipes should be painted to keep them safe. If the paint of your guttering has spoiled just clean it with a sand paper apply a primer and after it dries paint it up again.

Basically all these jobs should be done by a professional because they have expertise to handle such jobs. But if you wish to do it yourself you can do it too by following the above mentioned guidelines.

Metal Casting

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Permanent mould castings

A metal is used as mould in place of sand. Basically cast iron or Meehanite (a dense cast iron) are utilized for making moulds and cores are made of metal or sand. Cavity surfaces can be covered with a thin layer of heat proof material of clay or sodium silicate.

The moulds need to be preheated up to 200 degree Celsius prior to metal being poured in cavity. The cavity pattern of those moulds does not follow similar rules of shrinkages as in sand casting moulds. The reason is, metal moulds heat up and expand during the pour therefore, the cavity does not expand as in sand castings. Though care must be taken to keep correct thermal balance; by having outside water cooling.

Permanent mould castings are not as flexible as sand castings in using various designs that are cost effective. While producing 1000 or more parts the cost per piece is less. But breaking even is subject to complexity of part. More complex parts are suited to permanent moulds.

Normal part size may vary from 50 grams to 70kg. Basic materials are aluminium magnesium and brass plus their alloys. The parts are gears, splines, wheels, gear housings, pipefitting and vehicle pistons.

Slush casting:

It is a special method of permanent mould casting in which melted metal is not completely solidified. After getting the required wall thickness, the not yet solid melted metal is poured out. This is used for making hollow ornamental objects like lamps and statues made.

Corthias casting:

This is another method of permanent mold casting. In this a plunger is utilized to push down the melted metal to make the sprue hole. This makes it possible for you to get thinner walls and greater details to be made.

Low pressure permanent mould casting:

It is yet another version of permanent mould casting. In this gravity is not used for pouring melted metal into the mould but a low pressure of up to 1 atmosphere gas is put to melted metal. This continuous pressure on metal causes the mould to be completely filled and as a result there is no shrinking after cooling. In this process thin wall castings can be made. Mechanical properties are 50% better than permanent mould casting. As no riser is utilized the yield is usually higher as the metal in pressurized feed tube is still melted and the mould is ready for next shot right away.

Vacuum permanent mould casting:

This is another method of permanent mould casting. It is similar to low pressure casting, in which vacuum is utilized in place of pressure. Thin walls can be achieved as in low pressure mould casting. Along with it yields are better as no risers are utilized. As vacuum is utilized in its place, the purity of metal is kept. The mechanical properties are 10-15% better than permanent mould casting. The size of castings ranges from 4.5kg-200kg.

It is a permanent mould casting powdery mould releasing agent. As per this discovery has a granulated or powdery mixture of a lubricant, an organic polymer and a metal soap, in which the lubricant is coated with polymer or metal soap. Due to this the mould releasing agent permits production of high quality in a permanent mould casting.

In a spray system for spraying and applying a powdery mould casting agent on to both inside surfaces of a moveable metal mold and fix metal mould for a permanent mould casting tool. A spray method of permanent mould casting agent of which a spray head is given to electrostatic spray gun is fitted to a feeding port so as to shut it tightly under a stage where a moveable and stationery metal mould are closed.

Iron Metal Casting

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ALUMINUM Some Facts

Aluminum is the third most abundant element in the earth’s crust. It is never found in metallic form, and is always found combined with other elements. The chief source of aluminum is bauxite ore. Bauxite is a combination of hydrated aluminum oxides, usually containing oxides of silicon and iron. Bauxite is soft and clay like in texture.

Although, the ancient Greeks and Romans used Alum as an astringent and for dyeing, Friedrich Wohler is generally credited with having isolated the metal in 1827. An impure form was prepared by Hans Christian Oersted two years earlier. In 1807, Humphry Davy proposed the name alumium for the metal, undiscovered at that time, and later agreed to change it to aluminium. Shortly thereafter, the name aluminium was adopted to conform to the “ium” ending of most elements, and this spelling is now in use elsewhere in the world.

Aluminum is strong, flexible, lightweight, recyclable, and resistant to corrosion. Transportation represents the largest market for Aluminum in North America. It is the second most widely used material in new cars and trucks in the world. Mainly aluminum is used to improve gas mileage in vehicles. Aluminum can be processed many different ways, cast, rolled, extruded, and forged. One of the first statutes to be cast in aluminum is the statue known as “Eros”, erected in 1893, at London’s Piccadilly Circus. When aluminum is rolled under pressure, it becomes thinner and longer. The three most widely used forms, is aluminum plate, aluminum sheet, and aluminum foil.

Aluminum plate is one quarter inch thick or more. Aluminum sheet is .249 to .006 inch thick, and aluminum foil is less than .006 inch thin (the thickness of a human hair). Aluminum sheet, plate, and foil, represent the industries major products. Aluminum sheet is the most widely used form of aluminum. It is used for containers, packaging, transportation, and building construction.

Aluminum plate is used for the skins of airliners, spacecraft fuel tanks, and other heavy duty applications. Aluminum foil is used in building insulation, electrical, and packaging. Aluminum foil reflects heat and is widely used in cooking. Do you remember the aluminum foil beanies of the 1950’s? They were used to prevent mind control and deflect radiation and cosmic rays from Outer Space.

Aluminum coil is one third the density of and weight of steel and has excellent heat and electric conductivity. Aluminum strip is almost three times lighter than steel because of it’s low density. Aluminum strip is second only to steel strip in world usage.

The largest producer of aluminum in the world is Alcoa. In 1999, Alcoa acquired Reynolds Metal Company. In 1963 Reynolds introduced the aluminum beverage can, and in 1968 Reynolds pioneered the can recycling plan that gave the industry a cheap source of aluminum, and was cheered by the environmentalists

According to the I.A.I. (International Aluminum Institute), their future goals are to reduce greenhouse gas emissions from aluminum production; increase energy efficiency in aluminum production; maximize used-product collection, recycling and reuse; and promote the light-weighting of vehicles.

Backyard Metal Casting

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Sand casting

Sand casting is utilized to make large components of iron, Bronze, Brass and Aluminium. Melted metal is poured into a mould cavity made out of sand.

Patterns:

The cavity in the sand is made by utilizing a pattern (a rough duplicate of original) which are generally made of wood and rarely of metal. The cavity is contained in an aggregate housed in a box called the flask. Core is a sand shape put into the mould to produce the inner features of the component like holes or internal passages. Core is put in cavity to form holes of the desired shapes. Core print is the portion added to pattern core or mould which is utilized to find and support the core within the mould. A riser is an extra cavity made in mould to hold excess molten material. The reason of this is to feed the molten metal to the mould cavity and as metal solidifies and shrinks and therefore, prevents holes in the main casting.

In a two part mould, the upper half is called cope and the lower half is called drag. The parting line separates cope and drag. The drag is filled with sand in part and the core print, the cores and the gating process are put close to the parting line. The cope is then joined to drag and sand is poured covering the pattern, core and the gating system. The sand is made compact by vibration and mechanical method. After that the cope is separated from drag and the pattern is removed carefully. The purpose is to remove the pattern without breaking the mould cavity. This is facilitated by designing a draft, a slight angular offset from the vertical to vertical surface of the pattern. This is generally of 1° or 1.5 mm (0.060 in) whichever is more. The rougher the surface of design, the more the draft to be given.

Sprues and runners:

The melted material is poured in the pouring cup, it is part of gating process that provides the melted metal to the mould cavity. The vertical part of gating process connected with pouring cup is called sprue and the horizontal part is called the runners and lastly to the multiple points where it connects to the mould cavity is called gates. Alongside there are extensions to the gating process called vents that give the way for built-up gases and the displaced air to vent to the atmosphere.

The cavity is generally made oversize to allow for metal contraction as it cools down to normal temperature. These are linear factors and apply in each direction. These shrinkages for allowance are approximate because the exact allowance is decided by shape and size of casting. Besides different components of casting need separate shrinking allowances.

Sand casting is a way of making rough metal castings utilizing a mould generally made of sand made around a duplicate of the part to be cast that is removed once the sand becomes compact. Castings made this way may be refined by hammer preening, shot preening, polishing forging, plating, rough grinding machining sand castings not refined further are easily recognizable by sand like texture given by the mould. Therefore, the accuracy of part has to be improved by polishing or grinding. And the mould is broken after every casting to take out the part, a fresh mould is to be made for every casting.

Paths for metal pourin during pouring into mould casting are called the runner system and include sprue, various feeders which maintain a good metal ‘feed’ and ‘runners’ and ingates which attach the runner system to cavity. Gas and steam made during casting exit via the permeable sand and riser are added either in the pattern itself or a separate pieces.

Home Metal Casting

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Plaster casting

Plaster casting is similar to sand moulding the difference being that plaster is used in place of sand. Plaster is 70-80% gypsum and 20-30% strengthener and water. Normally it takes 4-6 days to prepare after which a production rate of 1-10 units/hr. mould is possible with items as big as 45kg and as small as 30 grams having very high surface resolution and fine tolerances.

Once used and cracked, it cannot be easily recast. Plaster casting is generally utilized for nonferrous metals like aluminium, zinc or copper based alloys. It cannot be utilized to cast ferrous metals because sulfur in gypsum slowly reacts with iron. Before mould is prepared the pattern is sprayed with a thin film of parting compound to prevent the mould from sticking to pattern. The unit is shaken so that the plaster fills the small cavities around the pattern. The form is extracted after the plaster sets. Plaster casting means a step up in sophistication and needs skill. The automatic functions may be given to robots but the higher precision pattern designs need even greater level of direct human assistance.

In plaster mould casting, a plaster, usually gypsum or calcium sulfate, is mixed with talc, sand, asbestos, sodium silicate and water to form a slurry. This slurry is sprayed on the polished surfaces of pattern halves (generally brass). The slurry sets in less than 15 minutes to form a mould. The mould halves are removed carefully from the pattern and dried in oven.

The mould halves are carefully joined, along with the cases. The melted metal is poured in moulds. After the metal cools down, the plaster is broken and cores cleaned out.

Parts cast are generally small to medium size, having weight of 30 grams to 7kg. the section thickness maybe as small as 0.6mm and tolerances are 0.2% linear. The draft allowance is 0.5-0.1 degree. The surface finish is 1.25 μm to 3 μm (50 μm to 125 μm) rms.

Low temperature melting materials like aluminium, copper, magnesium and zinc may be cast utilizing this system. This system is utilized to make fast prototype components as well as limited production parts.

Plaster casting as a sculpture system is of three types. One uses a waste mould, another a piece mould (both Plaster Of Paris) and the third a gelatin mould, all remake the original clay or wax model made by the sculptor. The waste mould is broken to free the hardened cast, which was poured in as a liquid plaster. The gelatin mould being reusable may be sprung from the cast with care and extracted intact and utilized for replicas. The piece mould can also be utilized again, being so divided as to be easily drawn away from the undercutting of the cast without damaging either of them. Plaster casts are utilized not only for creations of new sculptures, but also for the many replicas of famous marble and stone statues. The ancient Egyptians utilized models of plaster made directly from human body. The Romans cast in plaster many thousands of copies of Greek Statues. In another sense of the term, plaster casting refers to the surgical process of encasting in a Plaster-Of-Pairs cast any part of body in which bones are broken so that the bones set well without interference of motion or jarring or physical shock.

Metal Casting Zone