Die Casting Manufacturing Report Engineering Essay

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The aim of this report is to provide information regarding one of the solidification process of choice along with the brief understanding of solidification and solidification processes, based on the research that was conducted. Therefore, this document contains the details of solidification processes, how and why are they used. Die casting, a type of solidification process, is the chosen topic of this report; details about Die casting have been discussed in depth.

Solidification is the transformation of a liquid into a solid. Casting is the oldest solidification process; it was first used to shape metals in around 4000BC. In casting of metals a molten metal is poured into a mould and upon cooling it retains the shape of the mould. In a nutshell, we can say; melt the metal, pour it and let it solidify.

To make a product using casting we must begin with making a suitable mould, the mould is the inverse of the actual part to be made. The easiest way to make a mould is to look at an existing example of the part to be made and create a mould that looks exactly like it; however the size of the mould must be larger than of the actual part as the metal shrinks upon cooling.

Casting can be performed on any metal that can be heated to a liquid state. In today's age and time casting still remains a popular manufacturing technique. It is because casting has got various advantages such as, complex shapes can be produced easily, very large parts can be produced provided that the size of the mould is large enough and abundant molten metal is available. In some processes casting produces finished products which are of good surface finish, so not lot of machining is required.

Despite the many advantages there are a few disadvantages of casting, a metal can lose its strength or hardness because of the process of heating and cooling. Casting can also make a material weaker as bubbles or cavities may form inside the casting. Casting cannot produce parts which require high tolerances and in some casting processes i.e. sand casting the finished products do not have good surface finish. Health and safety wise casting is considered as a dangerous manufacturing technique, as molten metal has to be poured in large quantities into the mould and a single mistake can lead to deadly consequences. Casting is also not very environmentally friendly because of the amount of energy used and gases released.

Types of Solidification Processes

Listed below, are the different types of casting:

Sand Casting

Investment Casting

Shell Moulding

Die Casting

What are they?

Sand Casting

Sand casting is most widely used casting process. A pattern is created out of silica based sand, which is basically the replica of the original part to be created. Metal is melted in a furnace and then by the help of a ladle it is poured into the cavity of the sand mould. A gating system is used, for metal to enter and exit, the molten metal is inserted into the cavity through sprue or runner and then the excess metal is collected in the riser, which is later cut off from the solidified part. The pattern is held between the sand mould, the sand mould is divided between two parts drag and cope. Drag is the lower part and Cope is the top part. The cope and drag creates a parting line between the sand mould for the solidified part to be removed. Both small and large parts can be produced by sand casting e.g. gears, pulleys, crankshafts, propellers engine blocks and transmission cases etc.

Figure 1 shows a typical sand mould, used in sand casting.

Figure 1

casting_main.gif

Investment Casting

Investment casting or lost wax process is the oldest manufacturing process known to man. In this process a mould is made by making a pattern using wax .This wax pattern is dipped in ceramic slurry, which coats the wax pattern and forms a skin. This is dried and the process of dipping in the slurry and drying is repeated until a robust thickness is achieved. After this, the entire pattern is placed in an oven and the wax is melted away. This leads to a mould that can be filled with the molten metal. Because the mould is formed around a one-piece pattern, very intricate parts and undercuts can be made. [1] Investment casting leads to a very good surface finish and high accuracy so minimum machining is required at the end product. Products that are made from investment casting are turbine blades, armament parts, pipe fittings, lock parts, hand tools and jewelry etc.

Figure 2 shows the processes involved in Investment Casting.

investment-casting.png

Figure 2

Shell Moulding

The first step in shell moulding is to produce a shell mould. A shell mould basically consists of small grain sand and a thermosetting resin mixture. The metal pattern is preheated to 350 degrees and then it is mixed with the mixture. The mixture forms a thin shell over the surface of the pattern; the desired thickness of the shell is dependent upon the strength requirements of the mold for the particular metal casting application. The pattern is then placed in an oven; this allows the shell on the pattern to harden and then it is separated from the pattern. The shells are joined together by clamping or gluing and then the molten metal is poured and after the metal is cooled, the shells are broken, giving us a fine finished product. Typical products manufactured from shell moulding are gear housings, cylinder heads and connecting rods.

Figure 3 illustrates the steps involved in shell moulding.

Figure 3shell-mold-casting.png

Die Casting

Die casting is dated back to the mid-1800s. It was first used for the purpose of mechanised printing type production. [2] In die casting molten metal is forced into the mould cavity under high pressure. The mould cavity is created using two hardened tool steel dies which have been machined into shape. [3] These dies or mould are of permanent shape and are used for mass production of specific parts.

Molten metal is injected into the mould under high pressure of up to 200MPa which forces the metal in to all parts of the mould; the pressure is maintained during solidification so that the metal doesn't move inside the mould. Two methods are used for die casting, hot chamber or cold chamber. A complete die casting cycle can vary from less than one second for small components weighing less than an ounce, to two-to-three minutes for a casting of several pounds, making die casting the fastest technique available for producing precise non-ferrous metal parts. [4]

In a hot chamber machine, metal is melted in a furnace which is attached to the machine, by a metal feed system called a gooseneck. Molten metal is forced through the gooseneck using a plunger and a nozzle into the die cavity. After the metal has solidified in the die cavity, the die opens and the casting is ejected. Low melting point alloys are used in this process to prevent damage to the machine.

In a cold chamber the metal is melted in a furnace first then it is poured in the machine using a ladle, a plunger is used to inject the metal under high pressure into the die. Alloys with high melting point are used in this process.

Equipment Used

Hot and Cold chambers are needed in die casting. Figure 4 shows the detailed diagram of a hot chamber and Figure 5 shows a detailed diagram of a cold chamber.

hot-zink.jpg

Figure 4

Figure 5diecasting_machine8.jpg

Advantages and Disadvantages of Die Casting

Advantages

Parts produced are durable and dimensionally stable, while maintaining close tolerances.

It is an economical method to produce large number of products.

As a permanent mould is used, production rate is rapid.

Quick cooling means small grain sizes inside the metal, meaning metal is of high strength.

Wide range of possible shapes can be made.

Parts produced are with smooth surfaces, so minimal machining is required.

Disadvantages

Cost of setting up the whole machine/die setup is high, initially.

Production rate must be high in order to be cost effective.

Some porosity is common with die casting.[5]

As the moulds are metallic so only low melting point metals can be used, e.g. zinc, aluminum, magnesium, copper, lead and tin.

Products Manufactured

Products manufactured through die casting are everywhere around us, they range from everyday household objects to automotive and marine industry. Some of the area where die casting products are used, are mentioned below [6]:

Marine Components

Sports Equipment Components

High Voltage Electric Controls Components

Automotive components

Pressure tight components for medical equipment

Heat sinks for electronics and motor drive controllers

Pneumatic, gas and hydraulic valves

Radio communication handset chasses and frames

Figure 6 shows some of the products manufactured through die casting.

Figure 6aluminum-die-casting.jpg

Figure 7 shows a breakdown of parts made through die casting in North America.

http://www.diecasting.org/faq/Images/Figure04.gif

Figure 7

Conclusion

All of the solidification processes discussed in this report are of utmost importance in manufacturing, but we can safely say that die casting is one of the fastest and most cost-effective methods for producing a wide range of components. Die casting is an efficient, economical process offering a broader range of shapes and components than any other manufacturing technique.

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