Functioning of Aluminium Die Casting Machines

Die-casting is a process that will be executed by pushing the metal in molten form under high pressure into the cavity of moulds. Two steel dies are used to create those cavities that are to be machined to get the perfect shape. Many die-castings are made up of non-ferrous metals like zinc, copper, aluminium, magnesium, lead, pewter and tin-based alloys.

Usually the preferred metals to be used in die casting are aluminium that has many favourable features such as lightweight, good dimensional stability for typical shapes and thin walls. It also exhibits high corrosion resistance, good mechanical properties, high thermal and electrical conductivity retaining strength at high temperatures.

The Aluminium Die casting is categorized as Cold chamber die casting that produces permanent moulds which are formed in the machine from two steel blocks with a great clamping force essential for holding two halves of the mould together. It refers to separate metal melting furnace.

In this process the molten aluminium is injected in the die-casting machine with considerable pressure into a steel mold to form moulds. However, the process in itself is so up to the mark that it produces excellent dimensional accuracy and smooth surfaces which requires no machining except the removal of flash around the edge with drilling and tapping holes. Also this process is fast and inexpensive relative to other casting processes.

Amongst all casting alloys aluminium is used in 80-90% of the high pressure die casting machine. Though steel can prove to be a good substitute for aluminium high-pressure die casting as it increases strength and reduces part weight. The high pressure die casting parts can be produced and shipped within two weeks of the receipt of the purchase order. If the right die casting tooling and proper high pressure die casting part design are used then the machine will have the ability to produce thousands of high pressure die casting parts in a single day.

Producing less number of Aluminium Die Casting parts can prove to be expensive than more number of aluminium die casting parts. Some of the high Pressure die casting tooling is generally made up of hardened steel that can bear high temperatures and extreme pressures. The tooling can range from simple inexpensive inserts to complete high pressure die casting dies but it is limited to only one part of the machine.

After producing a tool, the cost of making the parts of these machines will be very little. It is the dimensional accuracy of the moulds produced that makes this machine outstanding in performance.

High scope of Die-casting machines abroad

The die-casting machines incurs huge cost on their equipment, set-up and processing. However, because of the simple processing methods, the incremental cost of each item is kept low but only in the case where large production is involved. Hence the global demand of die-casting machines is increasing among the casting manufacturers provided the die-casting fulfills good surface finish and dimensional accuracy.

There are two options for the manufacturers to obtain the machine from Die-Casting Machine Exporters, those are:

Pore-free die-casting – eliminate gas porosity defects

Direct Injection die-casting – Zinc castings reduce scrap and increase yield

The first die-casting machine got patented in 1849 for being hand-operated. The two forms of die-casting machines are hot-chamber die-casting and cold-chamber die-casting.

The hot-chambers are known as ‘gooseneck’ machines in which the piston got retracted and used to fill the gooseneck with molten metal. Lot of advantages can be listed for hot-chamber casting machines such as short cycle-times and easy conversion of metal into castings within the die machine. But, at the same time the die-casting machine exporters need to make the buyers aware that aluminium cannot be used in this machine because due to its affinity towards iron, it picks up some iron from that surface that blends into it, when in molten form. Hence, these are mainly used in tin, zinc and lead-based alloys.

At the same time, in the cold-chamber machine, the molten metal enters an unheated chamber that pushes the metal into the die with the help of the hydraulic machine. Due to this long process, the cold chamber machine has a longer cycle time.

However, the buyers can chose any of them as per their applicability. But there is another factor over which the quality of die-casting depends and that is, its set-up. It is constituted with two dies – Cover die half and Ejector die half. They meet each other on parting lines.

The cover-die should be attached to the stationary platen whereas the ejector-die is on the movable platen covering the path from the sprue (hot-chamber machine) or shot hole(cold-chamber machine) to the mold cavity that has two cavity inserts that has two separate pieces which can be easily separated and bolt into the die halves.

For an effective die-casting, the ejector die half needs to accomodate the material and should ensure that the ejector pins push the casting out when the die gets opened. To accomplish this task, all ejector pins need to move together at the same time through the ejector pin plate. Cores and slides are other components that need to be taken care of.

The materialistic properties, which the buyers of these die-casting machines will look at, in the dies are thermal shock resistance and softening at elevated temperatures. Also, the demand for such machines will get influenced by the hardenability, machinability, heat resistance, weldability and most importantly cost.

How the High Pressure die-casting machine functions?

Die-casting is a process of pushing metal into the mold cavity with high pressure. However, this process is generally referred to as ‘High Pressure Die-casting’ where high-grade steel tool is used to accommodate high speed and pressure. Along with these two, high temperature also needs to be maintained around 700ᵒC. This method has a competitive advantage over the other processes as it produces large volumes, polished surface finish and improved tolerances.

It incurs a high tooling cost as it has implanted advanced technologies. The high pressure die-casting machine accounts to maintain considerable pressure while forcing metal into moulds. In this process, the casts possess the characteristics of splendid dimensional accuracy and smooth surfaces due to which it doesn’t require machining.

Almost every machine of this type employs Aluminium. The production rate of these ‘High-pressure die casting machines’ can move up to 1000 per day depending on the appropriate tooling of die-casting and developing designs maintaining an ideal pressure. With these standards a productivity of 20,000 to 30,000 Die Casting parts can be achieved. However, the cost of using high pressure die-casting machine is very high in comparison to low pressure die-casting machines.

The moulds of high pressure die-casting machine are made up of hard steel which are able to with stand extremities of the temperature and immense pressure. These can also be produced from inexpensive inputs that can reduce the overall cost of the machine.

Basically, die-casting machines are two types:

Horizontal cold chamber – Mostly for castings
Vertical – Mainly for electrical motor parts along with encast winding

However, using these high pressure die-casting machines causes turbulence resulting in air entrapment. Molten material enters with a speed of 20-45 m/s in the die. The speed is so high that the infusion takes place in a blink of the eye and there is no time left for the air to escape. Provision for air ventilation also couldn’t solve this problem. Moreover, impurities need to be introduced in the molten metal to avoid the stickiness of castings with moulds. The mouldings ranging from 50g to 50 kg can be produced from these conventional methods.

Irrespective of these drawbacks, HPDC machines are most preferred by the industries because of their merits.

The numerous processes undertaken by these conventional machines are as follows:

Slow shot die-casting process – Along with two sub-processes vis-a-vis Squeeze casting process that reduces shrinkage porosity and gas voids due to slow metal injection and Acurad process that minimizes air traps by slow injection through the first plunger and then pushes it hard through the second plunger.

Vacuum die-casting process – Involves two processes: Ultra-high vacuum process with air and gas content below 5cc per 100g of metal and Vacural process that is similar to the previous but employs low pressure.

Pore-free process – This process replaced air with oxygen gas that reduced the air blow holes, which is called as Otivac process.

Minimum Fill-time process – This uses a multi-runner system that incurs minimal time in filling the cavity.

Semi solid-liquid metal casting process – With the advantages of conducting operations at low-temperature and long-lasting dies.

Thixocasting process – Reheats the solidified die to convert it into a semi-liquid slurry to impart high strength to the end-product.

Rheotech Casting process – Involves an electro-magnetic stirrer in the injection process to prepare a solid-liquid mixture.