Metal Mould System 1. Introduction

Metal Mould System 1. Introduction Moulds for these purposes can be used many times and are usually made of metal, although semi-permanent moulds of graphite have been successful in some instances. The permanency of the metal mould is due principally to the ability of the metallic structure to resist heat shock. Although the coefficient of thermal expansion of metals is higher than that of most other materials, uneven expansion does not lead to rupture since it can be compensated for by plastic flow. Thermal conductivity is also an important property in metallic moulds. The high thermal conductivity of graphite is a significant factor in the successful use of this material. The mould cavity (or die cavity) in a metallic mould is often cast to its rough contour and then machined to its finished dimensions. The gating system is also machined. The machined mould makes it possible to obtain very good surface finish and dimensional accuracy in the casting. Usually, cast irons and steels are used as dies or moulds for lower melting point aluminium-, magnesium-, zinc-, and copper-base alloys. However, ferrous alloys have also been successfully poured into water-cooled aluminium dies. For very high production runs, tool steels or low-carbon steel tools may be used. Dies made of wrought materials can also be used and, in fact, they are the only solution for the production of refractory material dies, which, at present, cannot be precision cast. Cores for metal mould castings may be made up of metal (alloy steels etc.) or dry sand. Shell, plaster and soluble salt cores can also be used. Dies are provided with various arrangements for pin locators, clamping devices and ejection systems for casting removal. The moulds often contain internal cooling passages through which water flows to remove heat from the die, and the dies are usually coated with mould wash between pours. To reduce the rate of cooling, dies are generally pre-heated to about 200-400 C for aluminium and magnesium alloys and 300-500 C for copper alloys. Only initial heating is necessary. Afterwards, the rate of production and time cycle are so maintained that before next casting, die temperature reaches the equilibrium preheating temperature. The pouring temperature should be as low as possible and the casting solidifies within 30 to 40 seconds. Mould coatings are used to prevent premature freezing of the metal in the mould, to control the rate and direction of casting solidification, to minimize thermal shock to the mould, and to prevent reactions between the metal being poured and the metal mould (aluminium alloys, in particular, tend to solder to ferrous dies). Coatings must have low thermal conductivity and be inert to the metal being cast, non-corrosive to the metal mould material, and adhere to the mould. A primer coat may be applied to the mould first. Coatings are usually sprayed on the mould. They may be brushed on by hand, but this is a slower process, and does not produce as uniform a coating as spraying. Coatings are usually thin water-based slurries of mixtures of refractory particles; the high temperature of the mould evaporates the moisture in the coating before pouring takes place. Coating thickness must be controlled to maintain casting dimensions, and it is usually thicker on gates and risers. Coatings are not always applied after each cycle, but they must be refreshed periodically through the production run. The choice of coating composition is usually dictated by the alloy being cast. The most common mode of die failure is thermal fatigue, although mechanical erosion and chemical attack may impose severe problems. Severe thermal shock due to repetitive loading and unloading of high temperature molten metal results in thermal fatigue in the die material, causing hairline cracks in the die, which in turn are transferred to the casting. Factors that influence mould life are those that affect thermal fatigue, either by increasing the temperature difference between areas of the mould or by increasing the stresses these temperature differences cause. These factors include alloy pouring temperature (higher temperatures increase thermal fatigue), casting weight and shape (heavy sections contain more heat than thin sections), cooling methods (the proper placement of cooling lines is essential to minimize heat checking), and mould coatings. These factors control the temperature of the mould. Note that the mould temperature will probably not be uniform over all surfaces of the mould. Minimizing this difference can substantially increase mould life. Mould design also plays an important role in mould life and casting quality. Variation in mould wall thickness causes excessive stress to develop during heating and cooling, which in turn causes premature mould failure from cracking. Abrupt changes in thickness without generous fillets also cause premature mould failure. Small fillets and radii concentrate stresses, again decreasing mould life. Aluminium, magnesium, zinc, lead, copper-base alloys, and cast irons are the principal alloys to cast. The extremely high temperatures of casting and consequent mould erosion usually make it unsuitable for most steel

castings. The process is limited to volume production, and usually requires a continuous cycle of mould preparation, pouring and casting ejection. This is necessary so that all steps can be timed and the mould thus kept within a fixed operating temperature range at the start of the pour. Operating temperature of the mould is one of the most important factors in successful permanent-mould casting. Automatic machines have been developed to obtain a continuous cycle. 2. Gravity Die Casting The process is known as gravity die casting in England and as permanent mould casting in USA. The terms are used interchangeably to describe both the process and the product. This process is widely used for aluminium and copper alloys and for a limited output of magnesium alloys and certain grades of cast irons. This process can also cast steels. The molten metal is poured into the mould under gravity only; no external pressure is applied to force the liquid metal into the mould cavity. So gravity die castings are much thicker in cross section than high pressure die castings, as gravity alone does not provide enough pressure to force molten metal into moulds in sections thinner than 5 mm or less. Permanent moulds are made into two halves in order to facilitate the removal of casting from the mould. They may be designed with a vertical parting line, as in Fig. 1, or with a horizontal line as in ordinary sand moulding. The mould material is usually a good grade of cast iron, although die steel, graphite, copper and aluminium have also been used. Fig. 1 Solidified permanent mould casting with gate and metal core shown in left half of mould with completed casting in front. The sequence in operations in gravity die casting includes: 1. cleaning the mould and maintaining it at proper casting temperature, 2. painting or spraying the mould surface with a thin layer of refractory wash, or blacking it with carbon shoot, 3. inserting the core, if necessary, and closing the mould by hand or by automatic action, 4. pouring the metal from a hand ladle, 5. ejecting the casting from the mould automatically or by hand after allowing sufficient time for it to solidify. Permanent moulding is easily automated (Fig. 2). Pouring may be automated using tilting pour ladles, or the moulding may be placed in a cradle. As the metal is poured into the moulding, the cradle is lowered at a rate consistent with the desired rate of fill, effectively filling the moulding at a constant level. If the metal source, rather than the moulding, is in the cradle, it is raised, and the moulding remains stationary. This method virtually ensures controlled fill and the elimination of turbulence and dross entrapment in the casting. Another method used to fill large permanent mouldings is to fill reservoirs at one end of the moulding with metal, and then tilt or

rotate the moulding so that the metal runs smoothly through the moulding, filling it in a uniform and consistent manner. The most important advantages of gravity die casting over sand casting are: 1. closer dimensional tolerance and accuracy 2. smoother surface finish and better appearance; less machining 3. fine-grained structure due to the chilling effect of metal mould 4. lesser defects (microporosity etc.) which are commonly found in sand castings 5. mass production Fig. 2 Schematic of a 12-machine turntable for automatic gravity die casting. The most important advantages of gravity die casting over sand casting are: 1. closer dimensional tolerance and accuracy 2. smoother surface finish and better appearance; less machining 3. fine-grained structure due to the chilling effect of metal mould 4. lesser defects (microporosity etc.) which are commonly found in sand castings 5. mass production Some notable disadvantages include: 1. cost of die is high 2. castings of all sizes and shapes cannot be cast 3. once prepared, the gating system cannot be changed; less flexible operation 4. unsuitable for steel castings due to high melting temperature 5. uneconomical for small run 3. Pressure Die Casting In pressure die casting the metal is injected into a die under pressure and solidifies under the same conditions. This enables the production of intricate castings at a higher production rate and at a low cost. The pressure is generally obtained by compressed air or hydraulically and it varies from 20 kpa to 500 MPa. The process is best suited for zinc alloys, copper alloys like bronzes, and aluminium alloy castings. Nowadays even cast irons can be cast in pressure die casting machine. The pressure die casting methods are classified as high-pressure and low-pressure die casting methods depending on the amount of pressure used.

3.1 High-pressure die casting In high-pressure die casting, the metal is injected into the mould at high pressures, 10-500 MPa. The process is primarily used for high-volume production of zinc, aluminum, and magnesium alloys, although ferrous and copper-base alloys may also be cast. The process is capable of high production rates. Production rates are related to the size and complexity of the part. Parts as complex as automotive engine blocks and transmission housings are routinely made by this process. There are two principal types of high-pressure die casting machines: hot-chamber and cold-chamber. Both vertical and horizontal injection systems can be used. In the hot-chamber or Goose-neck type (Fig. 3) machine a reservoir of molten metal is maintain at a temperature well above the melting point. For casting, metal from the chamber is forced through the gooseneck into the die cavity at 10-20 MPa. This type of machine is suitable for zinc, tin, lead and other low-melting alloys; aluminium and other high temperature alloys are not used due to contamination with the metallic holding furnace and for air entrapment. Fig. 3 Schematic showing the principal components of a hot chamber die casting machine. In the cold chamber process (Fig. 4), the molten metal is usually maintained separately at constant temperature at an adjacent holding furnace. A measured quantity of liquid metal from the holding furnace is poured into a chamber in the die casting machine and the piston is activated to force the liquid metal into the die cavity. The entire operation is completed in a few seconds so that contamination with mould is eliminated. Here the pressure attainable may be as high as 200 MPa. Because of the high pressure available, intricate castings can be made and the smaller amount of liquid metal used ensures less solidification shrinkage losses and less air entrapments. The cold-chamber machine is used for die casting aluminium, magnesium, copper-base, and other high-melting alloys. Fig. 4 Schematic showing the principal components of a cold chamber die casting machine.

The die consists of at least two parts, the cover half and the ejector half. The dies are made of heat-resistant tool steels. The metal enters the die through the nozzle or shot sleeve, which is located in the cover half. The cover half is stationary during casting. Ejector pins, which are activated when the ejector half retracts after the casting is solid, are located in the ejector half of the die. The gating is also generally placed in the ejector half of the die. The die also contains vents and overflow areas where excess metal flows during injection. As in gravity die casting, proper placement of the water-cooling lines is critical to obtaining good quality castings, especially for multiple-cavity dies. The sequence of the die casting process begins when the die is open before the shot. The die is first sprayed with a lubricant, usually an aqueous solution. This spray coats the die with a thin layer of mould release, and the water evaporates, cooling the die surface. The die is closed, and metal is either injected into the die (hot chamber) or ladled into the shot sleeve (cold chamber). The shot proceeds in three stages. In the first stage, the piston moves slowly, to fill the shot sleeve so that air is not entrapped in the metal before injection begins. In the second stage, the piston moves very quickly, forcing the metal into the mould cavity. The dies are filled in less than 0.15 s. Pressure builds up on the metal during this phase. In the third stage, pressure is intensified to minimize the formation of porosity. The high pressures require elaborate locking mechanisms to keep the dies closed during the cycle; locking forces of up to 45,000 kn are used in the largest machines. After the casting is solid, the dies open, and the casting and its runner system are removed from the die. Slides and cores are usually retracted first, and then the dies open. The gates are trimmed off on a separate trim press. High production die casting operations are highly automated, with the cycle controlled by computer and robots handling the metering of metal into the shot sleeve and removing the casting and gating from the die. After the casting is removed, the die is sprayed again, and the cycle is repeated. Risers are rarely used in die casting because the metal freezes so quickly that feeding does not have time to occur. The gating system is designed to fill the mould as quickly and efficiently as possible. The area where the shot plunger comes to rest in the gating system is called the "biscuit." Molten metal may be transferred to the shot sleeve by means of hand ladling, automated ladling, or by being pumped from the melting furnace to the shot sleeve, using pumps made of ceramics, or using electromagnetic force. High-pressure die casting machines are complex. They must be capable of rapid, repetitive motion. The dies must maintain alignment during operation to avoid being damaged, and the cycle of the machine must be controlled accurately. The rapid injection of metal into the die cavity inevitably traps air in the casting. This air expands during heat treatment, forming blisters on the surface of the casting. For this reason, most conventional high-pressure die castings are not heat treated. However, there are a number of methods that may be used to minimize the entrapment of air in the die cavity. In one, the air in the cavity is evacuated prior to making the shot. In another method used for aluminium alloys, the die cavity is filled with oxygen before the shot. During the shot, the oxygen reacts with the aluminium alloy, forming tiny particles of aluminium oxide, which are dispersed in the casting. Because all of the gas in the cavity is used in this reaction, there is no gas to form gas bubbles in the casting. Die casting as a production casting process has certain advantages, some of which are: 1. very high production rate can be achieved 2. close dimensional tolerance of 0.001 to 0.0005 inch can be obtained 3. thin sections, down to 0.015 inch in small castings can be cast 4. rapid cooling rate produces high strength and quality. Important limitations of die casting processes are: 1. not suitable for ferrous castings 2. die castings may contain some porosity, so that they should no be machined 3. chemical treatment (electroplating, anodising, etc.) after casting is difficult 4. intricate shaped casting is not possible because of ejection problem.

3.2 Low-pressure die casting Low-pressure die casting is a quite different process from high-pressure die casting. By nature of its design, it always involves vertical injection. The pressure involved are normally only in the range of 20-60 KPa compared to upto 200 MPa for high pressure die casting. In this process, sand moulds can be used as well as metal dies. In low-pressure die casting, the die is placed in a casting device above a sealed airtight chamber that contains a crucible holding molten metal. A fill tube or riser tube extends from the die down into the metal bath, and the casting is made by pressurizing the chamber and forcing metal up the fill tube into the casting. This process usually operates at lower die temperatures and shorter cycle times than conventional gravity die casting. Vacuum casting is similar, except that a vacuum is created in the die cavity and the metal is pulled rather than pushed into the die cavity. These processes are compared schematically in Fig. 5. Although the process is most widely used for the casting of aluminium alloys, particularly automotive wheels, and cylinder heads and blocks it has been used for the casting of steel railway wheels, for cast iron for the production of baths, and, recently, for magnesium alloys for aerospace castings. The low pressures have in general given low injection velocities, so that, together with the uphill flow of the metal, metal quality has been maintained. The process has therefore earned for itself a reputation for delivering castings of constantly high quality. Fig. 5 Schematics of low-pressure (a) and vacuum casting (b) units used with permanent molds.