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VAM2132 - Manufacturing materials:
Laboratory 1

Name: Santoss
Student ID: XXXXX
High Pressure die Casting: Aluminium

High Pressure Die Casting Aluminium:

High Pressure Die Casting (HPDC) is an industrial metal casting process categorized by forcing molten metals under high pressure in a mould cavity. Mould cavity is made of two hardened tool steel dies which have been properly machined into desired shape and work similarly to an injection during the process. HPDC is most widely used casting process for aluminium casting alloys.

There are two types of HPDC: Hot-chamber and Cold-chamber die casting. Variations on these two types include: 1. Hot-chamber die casting: is the more popular of the two die casting processes. In this process, the cylinder chamber of the injection device is completely dipped in the molten metal immersion. A gooseneck metal feed system draws the molten metal into the die cavity.

Fig. 02 Hot Chamber High pressure die casting. http://www.engineerstudent.co.uk

Direct immersion in the molten bath allows for quick and appropriate mould injection, it also results in increased corrosion proneness. Due to this point, the hot-chamber die casting process is best appropriate for applications that utilize metals with low melting points and high fluidity. Good metals for the hot-chamber die casting process include lead, magnesium, zinc and copper. 2. The cold-chamber die casting process is very similar to hot-chamber die casting. With a design that emphases on minimizing machine corrosion rather than production productivity, the melted metal is automatically or hand-ladled into the injection system. This eliminates the need for the injection mechanism to be submerged in the pool of molten metal.

Fig. 03 Cold Chamber Die casting: http://www.dynacast.co.uk
Cold-Chamber die casting can be an alternative for application that is highly corrosive for immersion design of hot-chamber die casting. These applications include the casting of metals with high melting temperatures such as aluminium and its alloys.

Table 01: Temperatures for HPDC (Aluminium)
Holding furnace temperature (°C) 640~700 640 670 700
Die temperature (°C) 180~260 180 220 260
Plunger velocity, 1st stage (m/s) 0.05~0.35 0.05 0.2 0.35
Plunger velocity, 2nd stage (m/s) 1.5~3.5 1.5 2.5 3.5
Multiplied pressure (bars) 200~280 200 240 280
Steps involved in HPDC are: * Die preparation * Filling * Ejection of shape * Shake out of shape

The dies are prepared by spraying the mould cavity with die lubricant. The lubricant both helps control the temperature of the die and it also assists in the removal of the casting. The dies are then closed and molten metal is injected into the dies under high pressure; between 10 and 175Mpa. Once the mould cavity is filled, the pressure is maintained until the casting solidifies. The dies are then opened and the shot (shots are different from castings because there can be multiple cavities in a die, yielding multiple castings per shot) is ejected by the ejector pins. Finally, the shakeout involves separating the scrap, which includes the gate, runners, spruces and flash, from the shot

Advantages of High Pressure Die Casting: * HPDC is of a particular advantage in the volume production of parts where the requirement is on high surface quality and the least possible machining. Special submission during casting enables castings to be welded followed by heat treatment which fully exploits the property potential displayed by the casting alloy. The special properties are achieved by shaping the metal during the solidification liquid phase. * Lowest Cost mass production casting process * Very easy to automate * Very high dimensional tolerances are possible * Excellent surface finish * High solidification rate of i.e., < than 1second.

Disadvantages of High pressure of die casting. * Expensive tooling which make it unfordable by small * There is usually high percentage of porosity/defects. * Huge sum of capital is needed to set up a HPDC process as die casting Machines cost a lot of money. * Casting machine capacity is limited which makes it difficult for large castings. * It is only suitable for materials with low melting points. * Defects are very common and heat treatment is usually difficult and costly. * To avoid soldering, secondary alloys are used and this makes materials lack good mechanical properties and fatigue life. * Highly skilled workers are needed to operate the machines.

Factors affecting High Pressure die casting. * Impurities in Metals * Metal quality (oxides, hydrogen content, sludge, dross, other inclusions) * Metal temperature, treatment, transfer, delivery to shot sleeve * Die-casting machine (size, type, equipment) * Clamp/platen: clamp pressure/platen programmable * Shot end: shot speeds/profile, pressure, closed loop control * Monitoring/Control system: * for all critical process parameters / full machine diagnostics * graphical user interface (HMI) provide SPC

High-pressure die-casting (HPDC) is used in moulding motor vehicles chassis and small hard to machined components, but the process is also important for other industries. While other cast and wrought aluminium alloy parts can be heat treated to improve their mechanical properties (such as strength, hardness, toughness, fatigue and ductility), this has not been possible until now for high-pressure die-castings. High-pressure die-castings contain trapped gas pores (porosity) which expand, blister and change the casting when heat treatment is applied. This makes the parts unworkable. Previous attempts at making them heat-treatable have tried to remove porosity from the cast parts, but these attempts have proved costly and time-consuming.
From the economic aspect, for mass produced engines and parts in automotive, construction and defence industries, high pressure die casting is mainly competing with the highly automated sand core package process whose cycle time is not bound to the solidification. In the highly competitive, cost sensitive market segment of the four-cylinder and six-cylinder engines, however, high pressure die casting is incomparable in cost effectiveness due to an outstanding productivity based on appropriate component design currently, worldwide quantities of automotive engines and components of aluminium are made by this method. The aluminium high pressure die casting method is now also used in the process of establishing itself from many industrial businesses worldwide.

International environmental aftermaths of High pressure die casting industries are not so positive. In spite of Aluminium high die castings being amongst the highest volume items manufactured by the metal working industry, the influence of high pressure die casting (HPDC) process parameters on greenhouse gas (GHG) emissions remains largely unreported. High Pressure Die casting is becoming increasingly important worldwide. Increase in demand for (HPDC) comes from internal combustion engines, diesels generators, Valves and fitting applications. The increased demand for die cast parts is directly connected to the increase of the total environmental burden of die casting. Regulation, economic advantage, or environmental factors may drive these businesses to more nonthreatening practices throughout the manufacturing process.

Emissions from High Pressure Die Casting are divided into two categories:
Emission from Extrusion process:
After extrusion, the parts are cooled and hardened, and then surface treated if necessary. Emission usually results from treatment of surface; as a result of using electrolytic baths where extrusions are given protective coating that contain aluminium compounds. Emission from the liquid baths can contribute to green house emission by noticeable percentage.
Emission form casting process:
Largely reported, melting and casting in Permanent moulds does not have negative effects on environment. Emission is only present when coating is involved.
Waste products during casting:
The best thing about aluminium is that, there is no waste involved since all defected parts and scraps of Aluminium are directly recycled straight away by throwing them back into the furnaces.

Conclusion:
High Pressure die casting is one of the best processes suitable for aluminium and its alloys. Cost effective and can handle large scale production in a shortest period of time possible. It can be use to produces parts that are too small to machine. It applications are limited to aluminium and its alloy only. Economically, the process has slashed man power into third compare to sand castings. Environmentally, waste products are recycled again and again and green house emission is less than other casting practices in the die casting industry.

References: 1. Title Die Castings, Volume 5 Publisher Technical Publishing Company, 1947 Original from the University of Michigan Digitized 14 Dec 2006

2. Title The Engineering Index
Contributor American Society of Mechanical Engineers
Publisher American Society of Mechanical Engineers, 1948
Original from the University of Michigan
Digitized 3 Feb 2011 3. http://www.engineerstudent.co.uk 4. http://www.dynacast.co.uk/