What is Die Casting?
Die casting is a permanent mold casting in which molten metal fills the cavity due to gravitational force or external pressure. The mold cavity is made up using two hardened steel dies which are shaped according to the desired product. Die casting is generally made of non-ferrous material like zinc, Tin based alloy, aluminum, copper, etc. It works similarly to injection molding.
Process of Die Casting
The die casting process is divided into five steps as mentioned below:
Preparation and Clamping
The preparation and clamping work of dies is done in this step of Die Casting. At first two halves of the dies are cleaned and then lubricated, so it is easier to remove the casted part after casting. Also, lubricant helps to control the temperature of the die. In general, we lubricate the part after 2-3 cycles, depending on the specification of the casted part.
After the lubrication process, we attach the die halves to the die casting machine and clamp it together rigidly, so that when the injection process is started they can securely be enclosed together, otherwise, we see some leakage, and the final product will have defects.
Filling and Injection Process
Molten metal from the furnace is poured into a chamber from where it is injected into the die cavity. This procedure can be different for different types of casting processes (i.e. hot chamber or cold chamber). The injection pressure is in between 10 and 175 MPa (1,500 and 25,400 psi), and the same pressure is maintained until the casting solidifies.
The amount of molten metal which is injected into the die cavity is called a Shot. The injection time of the molten metal is very short generally less than 0.1 second. If it’s more than that we can see early solidification of the molten metal, which brings the defect in the product. Sometime the product will be wasted.
When the injection process is completed and the die cavity is filled, we have to keep the system untouched, so that sufficient cooling and solidifying happens. The cooling and solidification time depends on the property of the material.
Ejection and Shakeout Process
After keeping the system for sufficient time for cooling and solidification, now we have to eject the casted product. There is an ejection mechanism (ejector pin) that helps us to eject the product easily. So after ejection, we will go for the trimming process where, excess materials (i.e. gates, runners, sprues, and flash) from the casted part are removed.
Now we have the casted product without any excess material. The casted part is now undergoing defect inspection. It includes X-ray, surface crack detection, etc. The very common defect we can see after die-casting is stagnation and cold heading. After the inspection process, the casted part goes for post-processing like grinding, polishing, honing, etc. if needed.
Types of Die Casting
Die casting is classified into the following types:
- Gravity Die Casting
- Pressure Die Casting
Gravity Die Casting
Here the molten metal is being poured into the mold cavity due to the ‘gravitational force’. A liquid (slurry coating) is used so that the molten metal does not adhere to the metallic mold. The slurry coating is poured before pouring the molten metal.
In this process, half of the pouring basin, sprue, runner, and cavity are in one mold box, and another half is made in the other molding box. Half of the core is placed in one mold box, and another half is placed in other boxes.
Gravity Die Casting Process
Pressure Die Casting
Here the molten metal is injected by the application of external pressure. Permanent metallic molds are used in this casting process.
Pressure Die Casting can be classified into the following types:
Cold Chamber Pressure Die Casting
The molten metal is poured into a ‘hot chamber’ from the ladle. After that, the ram pushes or injects the molten metal, and then the cavity inside the movable die half and fixed die half is filled. After solidification, the casting component is removed from the dies. Here the metal is melted away from the chamber. The molten metal brings by the ladle and then poured into the hot chamber. Here the furnace is not an integral part of the cold chamber pressure die-casting machine.
Hot Chamber Pressure Die Casting
Here the furnace is an integral part of the hot chamber pressure die-casting machine. The molten metal is pushed by the hydraulic shot cylinder and then the pressure increases. Due to the increase of pressure, the molten metal goes into the cavity between the two die halves. After solidification, the part is removed by separating the dies.
Die Casting Defects with solutions
|Defects||Causes||Remedies or Solution|
|Gas Porosity||The temperature of pouring metal is low. A slow pouring rate can cause gas porosity.||Increase the metal pouring temperature. The fast pouring rate of molten metal can eliminate gas porosity.|
|Shrinkage Porosity||The volume of molten metal should be reduced during solidification. Improper design of the runner and gating system can produce shrinkage porosity.||Extra tolerance is required during the design of the die. The use of simple geometry in casting can reduce shrinkage porosity.|
|Sink||Insufficient metal can be a major reason for sinks. The seal time of the gate is slow. The speed of injection should be low.||Extra molten metal is required to avoid the sink. The seal time of the gate should be increased. Increase the injection speed so that viscosity can decrease.|
|Blister||A high rate of injection may produce a blister. Removal of parts that are at high temperature during ejection from the die. High heat concentration at the die surface also produces a blister.||Low rate of injection. The vacuum should be used during filling the die cavity. Temperature sensors are required to place near the die. Lower heat concentration at the die surface can eliminate blister.|
|Segregation||The non-equilibrium composition can produce segregation.||Segregation can be removed or eliminated by the heat treatment process.|
|Inclusions||When sand and dirt are not properly cleaned from molds, inclusions are produced. An improper gating system produces inclusions.||Regular cleaning of the die is mandatory. Improvement of the gating system helps to avoid inclusions.|
|Dross||The high rate of reaction can forms dross. Thermite reaction causes the dross. The high temperature of molten metal may also produce dross.||Lower the reaction time which reduces the formation of dross. Avoid the thermite reaction and oxidation to eliminate dross. The low temperature of the molten metal can avoid dross.|
|Cracks||More stress concentration in a specific area may cause a crack in that particular region. High thermal concentration may also lead to crack formation. The high rate of injection forces results in cracks in the casting. Damaged die cavities can form a crack. Improper drafts in the die section can produce cracks. A faulty die design can produce cracks. Shrinkage of casting in the die leads to cracks in casting.||Proper design of casting can eliminate cracks. Use chillers in casting areas. Give the exact time for cooling to the molten metal for solidification. Reduce the temperature of molten metal. Avoiding sharp corners, providing fillets, and rounding-off the corners can eliminate crack formation. Add saw or coal dust to reduce die strength.|
|Cold Shut||Low fluidity of mold helps to arise cold shut problems. A thin section of casting leads cold shut. Low pouring temperature of molten metal can produce cold shuts. Improper gating system produces cold shut.||Use different compositions of metal to improve the fluidity. Avoid thin sections in the casting. Increasing the pouring temperature helps to eliminate cold shuts. Use the proper gating system to avoid cold shut.|
|Hot Tears||The thermal contraction in casting can produce hot tears. Improper placement of gates may also produce hot tears. High pouring temperature causes hot tears. Faulty solidification procedure or method forms hot tears. Thin sections play a major role in the formation of hot tears.||To avoid hot tears, provide a fillet radius at junctions. Uniform thickness throughout the section of the casting can eliminate hot tears. The gates should be accurately placed near the die. Using the exact molten metal pouring temperature helps to avoid hot tears. Use standard solidification procedures or methods to eliminate hot tears. Use the minimum permissible amount of thickness which depends upon the casting process.|
Applications of Die Casting
Die casting is used to make the following products:
- Valve bodies
- Transmission housings
- Hand tools
- Power tools etc
Advantages of Die Casting
Die casting has several advantages some of them are listed below:
- Die casting is an economical process as we can use the die for making several product.
- The surface finish we are getting from the casted product is smooth.
- As the surface finish of the casted product is smooth, we can eliminate the post-processing machining.
- Casted products have greater dimensional accuracy.
- Tensile strength of casting is high. It reaches up to 465 MPA.
- Production rate is faster.
Disadvantages of Die Casting
Disadvantages of Die Casting process are listed below:
- The initial cost of the machine is high, and also cost of the die is quite high.
- Thickness of the material should be less than 13mm, however now a days we are eliminating this restriction.
- We need to manufacture in bulk to counter the high cost as of other processes.
Video on Die Casting
More Resources for You
- Die Casting Images are made by Saubhik Roy, Design team LM.
- Feature image is designed by the Author.