Metal Casting Zone Info Blog

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

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

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

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.

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