Lecture Series on Die Casting Process
By Dr. D. Benny Karunakar, Department of Mechanical and Industrial Engineering, National Programme on Technology Enhanced Learning, India
Good morning friends. In our previous lectures, we have learned about sand casting. We have seen how to make the sand mold and how to get the sand casting. Then, we have learned about casting defects. Then, we have learned about melting and pouring practices. Then, we have learned about important cast metals and alloys. Now, today, we will switch over to another important phase of metal casting that is the die casting process. Now, what is this die casting? Why should we go for die casting? Let us see. First of all, what is the drawback of sand molding process and why we should switch over to die casting?
In the case of the sand casting, a separate mold has to be made in each and every case. If we want to make sand casting, initially we have to make the pattern, then we have to make the mold, then we pour the mold and metal. Once we put the mold and metal, once the mold and metal is solidified, we break that sand mold, and the mold is no more permanent. If we want to make another casting, again we have to make another mold, and we have to pour. Likewise, in each and every case, we have to make a separate mold. This involves a lot of labor and also it increases the cost of production and also productivity will be lesser.
Now, to overcome these drawbacks, die casting has been developed. Now, let us come to the specialty of the die casting process. Now, in the case of the die casting process, instead of your sand mold, your metallic mold is used in the place of sand mold with which a large number of castings are made. It means this is a metallic mold, remember. This metallic mold, we are not going to break after the solidification of the casting. Same mold will be used again and again for making several castings. This metallic mold is known as die. Now, here, the mold material is metal or alloy. Now, we have seen that in the case of the die casting process, a metallic mold will be used and the same metallic mold will be used to make thousands and thousands of castings.
Now, types of die casting process. What are the types? One is the gravity die casting process. This is also known as permanent molding process. Means, here, no doubt, the metallic molds will be used. There won't be any sand molds, but molten metal will be poured into these metallic molds by virtue of gravity, so that's why it is known as the gravity die casting. It means we won't apply any external pressure so that is the gravity die casting. The second one is the pressure die casting. In the case of the gravity die casting, the molten metal enters into the metallic molds by virtue of gravity. Whereas here in the case of the pressure die casting, the molten metal will be entering into the metallic molds because of the external pressure we apply. That's why these two are different.
Again, the pressure die casting is two types. One is the cold chamber pressure die casting. Second one is the hot chamber pressure die casting. In the case of the cold chamber pressure die casting, yes, there will be metallic molds will be there. Yes, we inject the molten metal into the metallic molds, but we need molten metal. We need to prepare the molten metal. For that, we need a furnace. This furnace is separate from the machine, cold chamber die casting machine. It will be away from the machine. Whereas in the case of the hot chamber pressure die casting machine, yes, again here, there will be metallic molds and we apply external pressure to inject the molten metal inside the die, but the furnace in which we make the molten metal is an integral part of the die casting machine. That way, hot chamber pressure die casting machine is different from cold chamber pressure die casting machine.
Now, we'll be seeing all these one by one. First, we'll see gravity die casting or it is also known as the permanent molding process. What is the history of the permanent molding process? The first permanent mold castings were made using stone molds sometime around 2000 BC. This was in practice even during BC, but that time metallic molds were not used. People used sand molds. Between the sand molds, they are pouring the molten metal by virtue of gravity. This process was used to cast tools out of copper, those days. Now, the discovery of iron and the ability to forge the cast iron led to permanent molds made from iron, which were mostly used to cast bronze.
Now, later, man has learned how to extract iron, how to melt iron and how to forge iron components. With that, they even made permanent molds, permanent metallic molds made of iron. These were used to cast bronze. Now, what is the status today? Today, permanent molds are made from the range of materials including iron, steel, and graphite. These are the molding materials today. The mold materials are made of iron, steel, and graphite. A wide range of metals are cast in these permanent molds. What are the cast metals that are largely cast in these molds? Several materials, several non-ferrous materials can be cast using these parts of molds.
Now, first, we'll be learning about the gravity die casting, what are the steps involved in this process. Here, we can see there are two molds will be there. These are the metallic molds. One mold is stationary mold. The other mold is a movable mold. Here, this was the stationary mold. Here, this is the cavity. This is the cavity. Now, this is the movable mold and this is also the cavity. Now, when we close these two, what will happen? Inside there will be cavity. That cavity resembles the shape of the casting, which we are going to produce.
Now, initially, these two parts of dies will be kept apart. Then you can see here a spray will be put on the cavity surface. Now, here we can see this is the hydraulic cylinder to open and close the movable mold. This cylinder operates hydraulically and it exerts intense pressure so that this mold will be moving forward and backward. Initially, the mold will be heated and coated. Next, what will happen? These two molds will be closed. Then what happens? See, there is a cavity. This will be further closed. You can see here. Now, it is about to close so this will be further closed. Then, there will be cavities there. You can see this is the cavity. This is the shape of the casting, which we want.
Now, if we want a recess inside the casting, we can place a core, so this is a core. Cores, if used, are inserted and the mold is fully closed. Then, the die will be fully closed. Again, you see here, pressure will be applied to the die. Two dies will be closed fully. Then, what will happen? Here, you can see the dies are fully closed and there is no gap between the fixed mold and the movable mold. Here, you can see there is no gap. Now, the molten metal is poured into the mold where it solidifies. This is the partly crucible in which there is molten metal. Now, the molten metal is being poured. Now, the molten metal enters into the cavity. Yes, it flows around the core, then it comes down. Then, after some time, the molten metal will be solidifying. Here, we can see pouring of molten metal into the gravity cast mold. It is being poured. This is one die and this is one die.
Now, this is the ejected casting permanent mold. Here you can see aluminum pistons so these are made by with the permanent molding. We can see here automotive engine blocks produced by gravity die casting. These are engine blocks, which are used in automobiles. These engine blocks are made by permanent molding process. Now, let us see the advantages of gravity die casting. Better mechanical properties. Why? The molten metal is being poured into the metallic molds. Then what will happen? The cooling rate will be faster. Once the cooling rate is faster, we get the mechanical properties. Better mechanical properties are obtained in the gravity die casting.
Next one, homogenous grain structure and chemical composition can be obtained. Why? Because we exert pressure and because of this pressure, the molten metal will be trapped between the two dies. That is how we get the homogenous grain structure and chemical composition. Next one, again, low shrinkage and gas porosity. In the case of the conventional sand castings, what will happen? There is always a chance for the shrinkage cavities. Now, here, the shrinkage cavities cannot form, rarely they form. Why? Why there is no shrinkage cavity? Because we are exerting external pressure on the molten metal. See, if there is any shrinkage, that will be nullified. That is the shrinkage cavity and also even if there is a gas porosity, same thing can happen because we are applying external pressure. These are tiny gases that are present in the mold will be, inside the metal will be escaping, so low shrinkage and gas porosity.
Next, we get the good surface quality. Why? If we use the sand mold, what will happen? Once a mold surface will have a rough friction because how this sand mold is made. It is made up of sand. Because of the sand grains present on the surface, the texture will be rough. Finally, even this rough texture will be transferred to the casting. That is so the surface roughness of your sand casting will be very poor, but here we are not using a sand mold. We are using a metallic mold. Metallic mold, the inner surface will have a very good fine surface finish. Then into such a mold, when we pour the molten metal, the casting will also have a very good surface quality.
Next one, low dimensions tolerances. In the case of the sand casting, because there is a rough surface on the casting, so we need to machine. We give the machining elements. That is so there will be lesser dimensional tolerance. Whereas here, in the case of the die casting, already the metallic mold is very smooth, so minimum machining will be done, sometimes no machining. Then what will happen? Then there will be closer dimensional tolerance. There will be very good dimensional accuracy. These are the advantages of gravity die casting.
Next one, we'll see the limitations of gravity die casting. One is the costly mold. Making this mold is not that much easy, not that much cheap. If it is a sand mold, within one hour one can make. Let's say, the cost involved may not be very high, but here the material is costly. We use special alloys so that the material is costly and making is not that much easy. Sometimes that alloy will be tough to machine. We need to use advanced machining process in which case the machining would be expensive, machining would be difficult and it will be laborious and it will be time-consuming. That is how the cost of making the mold will be going up so that how we can conclude that it's a costly mold.
Next one, only for components of simple shapes. Complex shapes cannot be obtained using gravity die casting or the permanent molding process. Next one, we have completed the gravity die casting. Next one, we will see the pressure die casting. Again, the pressure die casting is subclassified as cold chamber pressure die casting. Second one is the hot chamber pressure die casting. First, we'll see the pressure die casting. Pressure die casting is believed to have begun sometime during the middle of the 19th century. It is not very well-evolved. It is only about 100 years old. According to the records, Sturges patented the first manually operated pressure die casting machine during 1849. This man has patented the pressure die casting.
Various compositions of tin and lead were the first die casting alloys. Initially, pressure die casting was meant for casting tin and lead components. Their importance and use declined with the development of zinc alloys. Later, zinc alloys were developed so slowly people started even making zinc castings using pressure die casting process. Next, finally, magnesium and copper alloys were used with the pressure die casting process.
Now, pressure die casting again is subclassified into two types. One is the cold chamber pressure die casting and the second one is the hot chamber pressure die casting. In the case of the cold chamber pressure die casting, the furnace which is meant for melting metal is away from this machine. It is not an integral part of the machine. Whereas in the case of the hot chamber pressure die casting machine, the furnace which is meant for melting the metal is an integral part of the machine, so that way these two are different.
First, we'll see cold chamber pressure die casting process. Now, this is the schematic diagram of cold chamber pressure die casting process. What is there here? Here, we can see and there will be metallic molds will be there. Die means metallic mold. Here, this is the fixed die and this is the movable die and each die has the cavity. Both the fixed die and the movable dies, they have cavities engraved inside. When we close these dies together, inside there will be cavity. Here we can see there is a cavity. This is the shape of the casting which we want.
Now, these are the ejector pins and this is the cavity. Now, this is the shot chamber. It's a cylindrical shot chamber. Now, here we put the molten metal and here we can see this is the ram. After the molten metal is poured into the shot chamber, the ram will exert pressure, external pressure on the molten metal and the molten metal will be squeezed inside the cavity. Because of the external pressure that we apply, the molten metal will be squeezed and it will be going inside the cavity like this, then it solidifies. This is the simple principle of cold chamber pressure die casting process.
Here we can see the animation of the cold chamber pressure die casting process. Here we can see this is one die. This is the fixed die and this is the movable die. Here we can see this is the cylindrical chamber in which molten metal is being poured and this is the ram or the piston. It is exerting pressure inside the die, inside these two dies and then it solidifies and the casting will be taken out. Here we can see, yes, this is the casting. Again, it will be pulled. Again, it exerts pressure into the die and the casting solidifies and the casting will be taken out.
Now, here we can see a cold chamber pressure die casting machine. Another cold chamber pressure die casting machine we can see here and these are the dies we can see here. Now, let us see the advantages of cold chamber die casting process. Simple construction. Thus, cost of the equipment is not very high. What is there? Two metallic molds are there and, yes, there will be ejector pins will be there and the cylindrical chamber is there and the piston and the ram. Just put the molten metal and push it, so it's a simple construction.
Then what are the disadvantages of cold chamber die casting process? Slower cycle time due to the need to transfer the molten metal from the furnace to the cold chamber machine. What is this? Sometime back I told you in the case of the cold chamber die casting process, the furnace will be away from this machine. It is not an integral part of this machine. Then what will happen? Every time we have to go to that furnace, tap some molten metal, carry it in your ladle, then bring it to the cold chamber die casting machine, pour it, then push it.
In this process, what is happening? There is the time delay. Every time we have to go to the furnace, carry some molten metal, then you pour. That is so it's the slower cyclic time due to the need to transfer the molten metal. Hence, the rate of production will be less naturally, so that is the disadvantage of cold chamber die casting process. Now, let us see the alloys cast with cold chamber die casting process. A360, A380, A383, A384, A390, A413, and A518. These are the alloys which are cast using the cold chamber die casting process. Next one, among the zinc alloys, what are those alloys? Let us see. Among the zinc, ZA-12, ZA-27 are the zinc alloys, which are cast using the cold chamber die casting process.
Coming to the magnesium alloys, among the magnesium alloys, AZ91D, AM60A, AS41A. These are the magnesium alloys, which are cast using cold chamber die casting process. Of course, coming to the copper alloys, most of the copper alloys can be cast using the cold chamber die casting process. Now, these are the applications of cold chamber die casting process. This process is used to make automotive components. You can see here this is an automotive component part. This is made up of cold chamber die casting process. Next one, it is used for making electrical motor frames and housings and here we can see this electric motor housing, so this is made by cold chamber die casting process.
Next one, complex shapes and thin walls. Here, we can see there are complex shapes and also thin walls so this component is also made by cold chamber die casting process. Next one, we'll see hot chamber pressure die casting process. First of all, again, it has got two subclassifications. Before going to the subclassifications, again, I must remind you that hot chamber pressure die casting means the furnace for melting the molten metal is an integral part of that machine. Again, it is subclassified as Gooseneck air injection type and the second one is submerged plunger type. First, we'll see the Gooseneck air injection type hot chamber pressure die casting machine.
Now, here, again, as usual, what is the hot chamber pressure die casting process? The furnace meant for melting the metal is an integral part of the machine. This is that machine, hot chamber pressure die casting machine and here we can see this is the furnace. You can see here is the molten metal and initially the metallic blocks will be kept inside this furnace in this metal part. Then, here there is a fire box. Using this fire box, there will be cook or oil. It will be bond and because of that the molten blocks will be melted and molten metal will be there. Here, we can see that the furnace is an integral part of the machine.
Next, we can see here there is a Gooseneck cylindrical chamber is there. Gooseneck chamber is there. Now, the molten metal will be entering into this cylinder from this part it enters. Then what will happen? Air will be coming and the air exerts pressure on the molten metal. The molten metal will be slowly it will be transferred. In fact, it will be pushed and it will be coming up to here you can see. Here, we can see this is the stationary die and this is the movable die. Of course, we can see ejector pins are also there. This is the Gooseneck injector. Initially, the molten metal from the metal part will be transferred to the Gooseneck injector.
Because of the pressure exerted by the air, the molten metal will be injected into the cavity between the two dies. What will happen then? Molten metal solidifies between the two dies and there will be a cooling system. Because of the cooling system, the molten metal quickly solidifies and after cooling is over, this is the movable die and because of the ejector pins, it comes out and the casting will be taken out. Again, it will be closing, same thing will happen. Again, the molten metal from the metal part enters into the Gooseneck injector. The air will come and because of the pressure of the air, the molten metal will be injected into the cavity between the two dies. This process goes on and no need to carry the molten metal from a faraway place, then you pour it, no. Such business is not there. That is how the rate of production will be faster in the case of the hot chamber pressure die casting process.
Now, let us go to the second type that is the plunger type hot chamber pressure die casting process. In the case of the Gooseneck air injection, we are sending pressurized air to inject the molten metal into the metallic dies. Sometimes there is a risk that this air contains oxygen and this oxygen may react with the molten metal and causes oxidation. That's why people have also developed the another type that is the plunger type. Here, no air injection will be there. There is no air. Instead of air, the plunger will be there. Now, how to study this? Again, we must know that hot chamber pressure die casting means the furnace meant for making the molten metal is an integral part of the machine. Now, this is that furnace. Here we can see this is that furnace. This is that furnace part, so this is the furnace and here we can see this is the refractory or the lining will be there, so that it can withstand very high temperature.
Now, this is the part. Inside that part we place the metallic blocks initially. Now, there will be heating system will be there and because of that heating system, the metallic blocks will be melting and there will be molten metal will be there. Now, how to study the next parts? Here there is a hydraulic shock cylinder is there. This comes down and goes up hydraulically. It can exact very high pressure. Now, you can see here the molten metal enters into here there is a cylindrical chamber is there, cylindrical chamber. The molten metal enters into the cylindrical chamber. Once it enters into the cylindrical chamber, here we can see the Gooseneck plunger will be coming down because of the hydraulic pressure.
Once it comes down, what will happen? The molten metal is trapped between the plunger and the dies. Then what will happen? The molten metal will be injected at very high pressure. It will be injected between the cavity of the two dies. Here, we can see there are two dies. Here, this is the fixed die and this is the movable die. Between the fixed die and the movable die, there's cavities there, you can see. This is also known as ejector die. Movable die is also known as the ejector die. Here, there's a cavity. Now, once the Gooseneck plunger, once the pusher is pushed down, the molten metal is trapped and it will be injected into the cavity here.
Once the molten metal fills the cavity, yes, cooling system will be there. This cooling system flows around the molten metal and the molten metal enter the dies will be solidifying at a very fast rate. Then what will happen? The movable die will be moving this side, to the left side. Then, the casting will be ejected. Again, it will be closing. Again, the molten metal will be entering into this cylindrical chamber. Again, the plunger will be coming down. The molten metal will be injected into the cavity of the dies. This process keeps ongoing. Now, what is the benefit here? The benefit is the chamber is an integral part of the machine. No need to carry the molten metal from a faraway place and then you pour, no. There is no such business. Automatically, the molten metal will be entering into the cylindrical seal.
Once the casting is taken out, immediately closes, the molten metal enters into the two cylindrical chamber. It will be pushed. The plunger will be coming down and it will be injected. The benefit is the rate of production will be very fast. What is the per hour speed? You can see here 900 to 18,000 shots per hour. In one hour, we can produce 900 to 18,000 castings we can produce. What a great speed it is. Even if it is sand casting, in one hour, we can produce one casting. Here, in one hour, we can produce 900 to 18,000 castings, so that is the great benefit of hot chamber pressure die casting process.
Here, we can see the animation. This is the furnace and here we can see the molten metal is going inside. Here, we can see the Gooseneck plunger is coming down and these two are the dies. The molten metal will be injected inside. Immediately, there will be a cooling system to cool the casting and it will be ejected and the casting will be taken out. This is the animation of the hot chamber pressure die casting process. Here, we can see another hot chamber die casting machine. Here, we can see eight-ton hot chamber die casting machine. This is the largest hot chamber machine in the world and it cost about $1.25 million. You can see its height is double than the height of an average man. You can see here. Maybe the height must be at 12 feet and this is the length. This is the world's largest hot chamber die casting machine.
These are typical die casting molds, so these are the dies. Dies means metallic molds, which are used in the hot chamber die casting process. When we close this, we can see here this is the male die and this is the female die. The male die has got some pins are there, four pins and here there are four holes are there. When we close, so these four pins will be going inside the four holes and there will be a cavity between these two dies. Again, these are the metallic molds, which are used in the hot chamber pressure die casting process. Metallic molds, nothing but these are the dies. Dies means metallic molds. When we close this two, there is a cavity. These are the dies. Now, alloys cast with hot chamber die casting process. Zinc alloys can be cast. Tin alloys can be cast. Lead alloys can be cast and finally magnesium alloys also can be cast using hot chamber die casting process. What are those? Among the zinc alloys, three, five, seven, so these are the Zamak zinc alloys. These can be cast using hot chamber die casting process and also ZA-8 alloy can be cast using the hot chamber die casting process.
Next one, important applications of pressure die casting process. Carburetor bodies. These carburetor bodies are very important components in the automotive, especially in the petrol vehicles. This carburetor mixes petrol and yield uniformly. This carburetor body is made up of pressure die casting. Next one, hydraulic brake cylinders. These are made up of pressure die casting. Connecting rods and automotive pistons, oil pump bodies, aircraft components, sports goods, household utensils, plumbing good. In fact, these are some examples. In fact, many more