Die Casting
What is die casting?
 |
| (a) Die Casting |
The die casting (dc) is a special casting process that consists of forcing molten metal into a die cavity under pressure and filling the die sections to form a finished casting.
The steps required for producing casting are lesser if we compare it with sand casting. This is a cost-effective casting process for mass-producing small to medium-sized casting at a faster rate and less time.
Surface finish, fine details, complex shape and dimensional accurate casting products can be obtained from this casting process better than sand casting because of the use of metal dies and molten metal being forced into the narrow sections of the mould through pressure.
The mould used to make metal dies is made from tool dies and cast iron for non-ferrous metals such as zinc, aluminium, lead, brass, bronze, copper, pewter, magnesium, tin and alloys.
{tocify} $title={Die Casting Process Table of Contents}
Construction of Die-Casting Machine
Die casting machine can be divided into three parts or units and they are as follows:
- Clamping unit.
- Die assembly unit.
- Injection unit.
Clamping Unit: The clamping unit involves a mechanism to clamp the moving die by pushing the die towards the fixed die using hydraulic actuators.
These clamping units consist of fixed platen and moving platen that slides along the tie bar. The moving platen has a moving die attached to it.
The toggle unit mechanism is actuated by a hydraulic cylinder forwarding the moving platen along the tie bars with the help of the toggle mechanism while closing the die and locking the dies with pressure.
When the die is to be opened the piston in the hydraulic cylinder is retracted, retracting the toggle mechanism and sliding the platen backwards towards the fixed rear platen.
Die Assembly Unit: The die assembly unit consists of moving die and stationary die. Fixed die is called stationary die or cover die. The moving die is called the ejector die.
The moving die is assembled to the moving platen and the fixed die is assembled to the stationary platen.
Dies need to be assembled properly to the platen making sure there is no misalignment between both dies resulting in solid uniform casting along the parting line.
Air at higher pressure is blown here and lubrication is sprayed on the dies through automatic blowing and spraying mechanism.
Injection Unit: The fixed die is attached to the stationary platen and does not have an ejector mechanism attached to it. The fixed die has an inlet where molten metal enters the die when the dies are closed.
Ejector mechanisms are placed on the moving die which ejects the solidified casting out with the help of ejector pins.
These dies have a cooling system attached to them for solidification of the casting. Casting is solidified on the fixed (cover die) and sprue formation of biscuit takes place on the die which helps the robot to take out casting seamlessly.
Die Casting Step By Step Process
What is the step-by-step process for producing die casting?
Step 1: Meling Ingots For Hot And Cold Chamber Casting Process
Depending upon the type of die-casting process ingots are selected for melting. These ingots are melted in a furnace and molten metal is kept ready until the die is heated.
For hot chamber die casting high-temperature metals such as aluminium, steel and brass are selected while for cold chamber die casting metals such as zinc, tin, lead and its alloys are selected as high-temperature metals are not suitable for the process.
Step 2: Pre-Heating Metal Die For Casting
Before heating dies, any remaining early casting sections and fins are removed and cleaned the die is blown with pressurized air and the die is lubricated for a better surface finish. This operation is done when the moving die is in a retractable position.
Both cover platen/cover die/moving die and stationary platen (ejector die) need to be heated to manufacture sound casting.
The die needs to be heated to a sufficient temperature so that molten metal does not suddenly solidify when it comes in contact with the cold die surface, this will disturb the surface texture of casting and result in molten metal sticking to the die. This will not happen if molten metal comes in contact with the pre-heated surface of the die.
After the die is pre-heated it is lubricated by automated spray with water-based lubricants to prevent molten metal from sticking to the metal die. A lubrication mixture of water and oil is mostly preferred as it is safe, cost-effective and environmentally friendly.
This lubricant creates a film between metal and the die surface reducing contact between the metal and the die.
This step is necessary to get a good clean even smooth surface finish casting and reduces surface-based casting defects. This step is repeated before dies are locked for every shot.
Step 3: Injecting Molten Metal In The Closed Die
Once the molten metal and die are ready for injection, the die is closed. Die slides with the help of a tie bar and closing the die with a toggle mechanism.
The locking force is applied on the dies and the die is allowed to close by applying the die locking force. Die locking force should be sufficient to lock the moving die and stationary die.
Molten metal is filled with the ladle and a slug is injected into the dies (In case of cold chamber die casting) and in case of hot chamber die casting molten metal is injected into the closed die through a gooseneck.
Sprue, runners, cavities, sections and chills are present on the moving die allowing molten metal to flow in it while the stationary die or ejector die serves as a supporting member in this casting process.
Molten metal is allowed to fill all sections of the die because molten metal is injected at high pressure. The pressure of molten metal in the casting machine depends upon the type of metal used, the complexity of casting, size and shape of the casting process.
Alignment between dies needs to be perfect to avoid flash defects in casting. When there is a mismatch between molten metal pressure, volume and die alignment excess molten metal is pushed away in the parting plane.
It is ensured that molten metal fills all sections of the die avoiding misrun and cold shuts defects.
Solidification of molten metal in the die should be gradual and uniform to avoid shrinkage defects.
Retractable metal cores serve as a tool to produce holes and cavities in the casting process.
Step 4: Solidification of Molten Metal In The Die Casting Process
Once molten metal fills all the sections of the die, molten metal starts solidifying uniformly and gradually. Metal dies have a water cooling system in the die to cool the casting, depending upon the thickness, design, shape and size of the section to be cooled.
Uniform cooling of the molten metal takes place during the solidification operation reducing defects such as misrun, shrinkage cavity defects, hot spots, warpage, hot tears and cold shuts.
Metal inserts are used in this process which becomes part of the final casting. Metal inserts are parts of the casting of different materials which are attached to the die before the metal is injected into the die so that it becomes part of the final casting after the metal has solidified.
Step 5: Ejecting Casting From The Die
The function of ejector pins is to separate the casting from both dies. Ejector pins are projected out from ejector plates which induce equal uniform pressure on the final casting to separate casting that has stuck to the die during the soldification process.
Ejector pins are placed uniformly around all dies making sure there is no uneven force induced on the casting causing casting sections to break, bend or damage.
Many big foundries have automated systems to remove casting out of die-casting machines using the robotic arm.
The end of the robotic arm has clamps to hold the biscuit to withdraw casting out. The biscuit on the casting is designed to hold the casting and remove the casting out of the machine.
Once the solidified casting is taken out and the operator removes all the remaining fins, metal remains from the previous casting on the die.
At times pneumatic blowers are used to remove any metal remaining on the die. This process is repeated after every casting is taken out.
Step 6: Secondary Manufacturing of Die Casting Solidified Casting
Inspection of Die-Casting
After the casting is taken out of the machine by the robotic arm, the gating system such as the sprue and runner is broken, cut and separated from the actual casting. This process is called fettling in the casting process.
The solidified casting is inspected for blowholes, misrun defects, shrinkage, hot tears, porosity, flow marks and cold-shut defects. Geometry, surface finish, accuracy and dimensions are also checked for the casting.
X-ray machine, ultrasonic inspection, visual inspection, die penetration and magnetic inspection, pressure test, magnetic particle test and radiographic test.
Cold shut defects accords due to casting solidifying faster before it completes its entire path in the die, thin sections resulting in an incomplete casting.
Blowhole defects happen when the die does not have proper ventilation and the die locking force is high blocking any internal gases from escaping out of the tight die.
Flow marks are the result of excess lubricants and improper gating system design.
Hot tears in this casting process happen when a thinner section of the casting cools faster as a thicker section creates uneven stress during cooling resulting in a defect called hot tears.
This can be eliminated by designing a die with the cooling system having a lower cooling rate for thinner sections balancing both the thick and thin section solidification rates of the casting equally.
Secondary Manufacturing Operations
The first step in this process is to get rid of overflows, flash and fins by trimming the edges and sides of the casting. Trimming of flash is done manually or on a trimming machine similar to sheet metal operations.
Operations such as machining, tapping, reaming, boring and drilling operations are carried out before sending it for the heat treatment process.
The heat treatment process is carried out through annealing, hardening, normalizing and galvanizing to improve the mechanical properties of the final casting.
Other operations such as inspection, testing, correction and repairs are carried out to make sure casting has proper dimensional tolerance, accuracy and surface finish before casting is powder coated, polished, buffed, lapped, painted and ceramic coated.
Die-casting products can be coated, electroplated, anti-corrosive painted and surface-finished easily to achieve aesthetic and functional value.
Classification And Types of Die Casting
How are die casting classified and what are their types?
Die casting is classified according to the relation between molten metal injection, pressure and use of vacuum in the die casting process.
Types of die casting process and classification can be done in the following types.
- Hot chamber die casting.
- Cold chamber die casting.
- High-pressure die casting (HPDC).
- Low-pressure die casting (LPDC).
- Vacuum high-pressure die casting (VHPDC).
- Vacuum low-pressure die casting (VLPDC).
I have discussed all types of die-casting (DC) in the following section of this article.
Hot Chamber Die Casting
 |
| (b) Hot Chamber Die Casting |
Hot chamber die casting is a process used for manufacturing casting with a low melting point using a gooseneck machine.
Low-temperature metals such as zinc, tin, lead and alloys are used in this process.
High-temperature metals such as aluminium and magnesium are not used in this process as they catch iron.
In this process die is directly connected to the pot containing molten metal where ingots are melted in a combustion chamber.
When the piston in the molten metal is advanced molten metal from one open end of the cylinder is pushed towards the gooseneck that is connected to the die cavity.
The gooseneck is connected to the sprue in the die mould. The gooseneck used in this process is made from cast iron, ductile iron and alloy. This is an important element in hot chamber casting and serves as an entry point for the mould cavity.
The volume of molten metal that enters the cavity through the gooseneck is fixed, equivalent to the volume in the cylinder and how much the piston actuates in the cylinder.
This process is repeated for every casting injecting molten metal directly into the mould.
Hot chamber die casting has a shorter cycle time as compared to cold chamber die casting as molten metal is melted at the same station and ejected in the die cavity through the gooseneck without waiting for the molten metal to be filled in the chamber before the molten metal is shot in the die cavity with pressure.
This improves the travel time of molten metal in the foundry giving better production output and reduction in the wastegate of the molten metal.
The advantage of this process is metal is melted at the same spot reducing any heat loss.
Die-casting products manufactured from hot chamber die-casting have less pinhole porosity and blisters.
Die life in this casting process is longer due to the use of low-temperature molten metal and shorter cycle time.
Cold Chamber Die Casting
 |
| (c) Cold Chamber Die Casting |
This process is best suited for high-temperature metals such as aluminium, brass, copper, magnesium and alloys.
In this process metal ingots are melted in a separate furnace and molten metal is transferred from the furnace to the holding pot where liquid metal is stored for further operation.
This liquid metal storage is continuously heated electrically to maintain the pouring temperature of the molten metal.
The process starts from pouring molten metal through a ladle into a shot chamber. This process of pouring molten metal into a horizontal passage called a shot chamber is repeated for every casting, metal in the ladle is the exact amount of metal that is required for the final casting.
This shot chamber has a plunger at one end. The plunger (piston in a cylinder) is then advanced and molten metal is then forced into the die cavity at high pressure. This metal travels through sprue into the die at high velocity filling all sections of the die cavity.
The disadvantage of the cold chamber casting process is that it has a lower cycle time, ingots are not melted at one working station and the time that it takes to transfer molten metal from the melting furnace to the holding furnace is large compared to hot chamber die casting resulting into heat loss and increase travel time.
Also, heat loss during this casting process is higher due to the transfer of molten metal from the ladle to the shot-injecting chamber.
Dies used in this process have shorter cycle times due to the use of high-temperature metals for producing casting which reduces the life of the die.
In recent years ferrous dies have been used in the die casting process for high-temperature metals. Ferrous dies are made from wrought dies, graphite and molybdenum metals.
The use of these ferrous dies in the casting process gives higher flexibility to cast steel and cast iron metal products which were difficult to cast early due to their high-temperature properties resulting in scaling, soldering and surface defects in the final casting.
Ferrous dies give better surface quality casting, better dimensional tolerance, longer die life, draft angle, ribs and wall thickness.
These dies have low thermal expansion with high thermal conductivity, resistance to shock and vibration and a higher melting point for cast iron and steel with resistance to thermal fatigue.
High-Pressure Die Casting (HPDC)
High-pressure die casting is used for manufacturing casting at higher volumes using a horizontal die casting machine.
Casting is produced by injecting molten metal in the die by passing molten metal at a higher speed through piston and cylinder arrangement. High-pressurized gas pushes the piston forward at high speed injecting molten metal into the cavity.
Dies are locked together during this whole process with mechanical toggles and actuators. Molten metal is pushed between the locked die. The die used in this process is made from tool steel.
This molten metal is solidified between the two dies which have cooling channels in them solidifying the final casting efficiently producing a lightweight net shape complex casting with an excellent surface finish, mechanical properties, close tolerance and accuracy, strength and uniformity.
Thin section components with smooth surface finish can be produced in a short cycle time as compared to low-pressure die casting (LPDC).
HPDC machine is simple in design and casting steps are automated. These high-pressure die-casting machines are costly and should be only used when casting is to be produced in mass production.
This machine has limitations in the size of casting, locking force and weight of casting it can handle.
Low-Pressure Die Casting (LPDC)
 |
| (d) Low Pressure Die Casting |
Casting in the low-pressure die casting machine is done vertically as compared to high-pressure die casting the casting is manufactured horizontally on the machine.
This process is suitable for metals such as magnesium and aluminium.
The process starts with applying compressed pressurised air to the molten metal that is stored in the crucible, pressurized air pushes the molten metal at low velocity upwards through the riser tube that is connected to the metal die mould.
In the LPDC process the molten metal is injected vertically into the metal dies that are locked together using die locking force and molten metal is injected at low pressure against gravity into the mould cavity.
Cycle time for this process is longer as molten metal needs to be injected slowly at lower pressure without any feeders through only one inlet.
The low-pressure die-casting process gives better casting quality because metal enters the die at a slower rate reducing the turbulence of molten metal in the die and liquid metal is solidified using water jets.
After the casting is solidified dies are opened and the final casting is taken out of the mould. It is easier to take out casting out of HPDC mould than the LPDC machine.
Casting engineers ensure that heavy and thick sections of the casting are located at the entry of the die cavity and serve as a riser.
LPDC machines are cost-effective, simple in design, operation-friendly, high density and dimensional accurate small to large-size castings are produced.
Vacuum High Pressure Die Casting (VHPDC)
 |
| (e) Vacuum Die Casting |
Vacuum high-pressure die casting consists of injecting molten metal into a die cavity that does not have air in it. The entire air from the mould cavity is taken out through a vacuum pump.
Due to the vacuum, there is less turbulence and obstacles for the molten metal entering into the die resulting in casting with a better surface finish with porosity and gas defects.
As in VHPDC molten metal is injected at higher pressure there is no back pressure due to the absence of air in the mould reducing the molten metal turbulence in the die providing better quality casting having high dimensional accuracy.
As molten metal is injected into the die without any air with high-pressure casting formed during this process, it does not have casting defects such as pinhole porosity and air inclusions as compared to normal die casting which is exposed to these gas defects.
As casting is free from surface defects polishing, grinding, honing operation, painting, UV resrstance shielding and corrosion resistance applications can be carried out on the surface of the final casting to improve casting life.
Vacuum Low Pressure Die Casting (VLPDC)
Vacuum low-pressure die casting consists of injecting molten metal into a die at lower pressure in the absence of the air in a vacuum resulting in casting with lower open hole defects and shrinkage cavity depression.
Molten metal enters through a ceramic riser tube in the vacuum mould cavity but this time in the absence of air giving better quality casting due to less turbulence of molten metal and backpressure of air in the die.
As gas defects such as porosity reduce casting becomes available for surface finish operation, heat treatment process, milling, drilling and welding operation.
Die Casting Application
What are the applications of the die-casting process?
Die-casting products are used in the automobile industry, electrical, electronic, household industry, machinery, machine tool industry, industrial and toy industries.
Automobile Industry
- Cylinder, engine blocks, engine brackets, valves.
- Filtration components and cylinder heads.
- Clutch housing, instrumental panel, engine housing and switch housing.
- Passenger and commercial vehicles have small engines.
- Transmission, fuel pump casing, carburettors and crankcase cover.
- Handles, handlebar, handle cover and top.
- Motor hub, magneto and fuel pump hub.
- Clutch, brake and show housing.
- Gear, wheels, bushes, glad hands and transmission rods.
- Gearbox setup, fuel pump, power steering components.
- Parts of stands, front view mirror, rearview mirror and headlamp bezels.
- Windshield cover, frame and channels.
- Switch gears, button frame and switches.
- Instrumental panels, enclosures and circuit breakers.
- Circuit panel boards, connectors.
- Alternator, generator and rotor fans.
- Heat sinks for heat dissipation.
- LED light parts and connectors.
- Electric terminal sockets.
- Model die-cast cars, buses, bikes and trucks.
- Model aeroplanes, spacecraft and game equipment.
- Die-cast metal model train toys, mono-rail and electric trains.
- Die-cast utensils, stove stand and can openers.
- Window frames and furniture frames.
- Decorative products.
- Sculptures, crafts and art statues.
- Bathroom fitting, fixtures, faucets, sinks and shower head.
- Drawers and cabinet, latches, knobs and sturdy handles.
- Kitchen tabs, valves, fans and fixtures.
- Stove burners, hob tops and regulators.
- Machine tools.
- Machine structure, panels and supporting unit.
- Lifting types of equipment.
- Impellers, blades, motors and rotors.
- Mechanical parts of vacuum cleaners, air conditioners, dishwashers, washing machines and blenders.
- Mechanical components of lifts, hoisting machines, cranes and actuators.
Die Casting Advantages and Disadvantages
Discuss the advantages and disadvantages of the die-casting process in detail?
Die Casting Advantages
What are the advantages of the die-casting process?
- This process can manufacture casting with smaller variable wall thicknesses as molten metal can fill the die cavities at high pressure.
- Die casting machines are available in various sizes and capacities from small, medium to large size casting. Low-cost semi-automated machines to fully automated die-casting machines. Low-pressure die-casting machines to high-pressure die-casting machines.
- Able to produce casting at higher production volume as the process is automated with the advantage of permeant mould which can solidify casting faster. At least a production rate of 100 units/hour is possible using this process. The production cycle of this process is better than another casting process like sand casting.
- Die life for the Al and Cu casting is a minimum of 1,00,000 units with little die maintenance.
- The machining operations and finishing processes required for die-casting products are less. This process is best for lightweight casting having thin complex walls curved and seam parts.
- Mechanical properties such as tensile strength, surface finish and fine grain surface texture of die-casting products are better than those of traditional sand-casting processes. Fine-grained surface finish up to 1 micron can be manufactured.
- Convenient to place inserts such as thread plugs, screws and bearings. Easy to produce cavities/holes in the casting with movable cores in the die.
- Life of die used in die casting has at least 100,000 casting shots. At least 100,000 casting products can be produced in this process.
- Cold chamber die casting machines are suitable for materials such as aluminium, copper and their alloys having high melting points and also for low melting point metals such as zinc, lead and tin for producing large-size casting.
- Better geometrical dimensional accuracy and surface finish from 1μm up to 2.5 μm is possible with die-casting products.
- Full control over manufacturing parameters such as temperature, solidification rate, molten metal pressure, die temperature, turbulence of molten metal in the die, die locking force, and molten metal temperature is possible in this process.
- Hot chamber die casting machines, cold chamber die casting machines and energy-saving servo machines for materials such as aluminium and magnesium casting products are possible to be used in the foundry.
- The safety of the operator operating the machine is highest in this casting process as compared to the sand casting process reducing the plant accidents and increasing the productivity of the plant.
- The die-casting process is more efficient and easy to control as compared to the sand-casting process. Die-casting machines have higher overall equipment effectiveness.
- The robotic arm can be employed in bigger foundries to withdraw casting from the die-casting machine and place it on the conveyors for a higher production rate.
- Labour cost is reduced in this process as steps such as spraying the coating on the die, pouring operation and withdrawing solidified casting are done automatically.
- Defects such as porosity and blowholes are minimized in this casting process as compared to the sand casting process but if process parameters are not controlled there can be the possibility of micro-cracking and pores.
- A separate holding heating furnace is employed in the cold chamber die-casting process reducing the travel time from the actual furnace where ingots are melted to the cold chamber die-casting machine.
- A highly efficient way to cool the die using a water cooling system is possible reducing defects related to solidification and improving the quality of the casting.
- These intricated castings are used in the automobile sector, construction, mining, electrical and electronic industry, aerospace and household appliances.
- The cost of production of casting reduces as the volume for manufacturing casting increases. Labour cost for this process is slow as one semi-skilled operator can operate two fully die-casting machines.
- Corrosion resistance coating application on casting products is easy on this type of casting as the surface of the casting is smooth.
- Single solid intricate complicated casting can be manufactured without assembly requirements at high-speed production.
- Labour cost is lower than the sand moulding process as the process is automated reducing the working load on the operator.
- Thin-section casting walls can be produced with this casting process as compared to sand casting.
- Movable metal cores can produce complex sections, cavities and holes.
Die Casting Disadvantages
What are the disadvantages of the die-casting process?
- The cost of production of die casting is higher for low-volume production. Quantity lesser than 1000 units production are avoided.
- This process is not suitable for all metals, unlike sand casting. Aluminium and magnesium low-temperature metals are the best-suited metals for the die moulding process. High-temperature ferrous metals such as cast iron and steel are not suitable for this casting process.
- Die castings are prone to porosity, blister, blowholes and pinhole defects, to eliminate these defects vacuum die-casting machines should be used.
- Large-size casting cannot be produced if we compare this size of casting that is made from the dry sand moulding process. The process is best for casting small to medium size casting as there is a limitation on the size of the die.
- Die-casting machines can be purchased only when there is confidence that casting can be sold at higher demand justifying the cost of die-casting machines as the cost of this machine is very high.
- When fixed and movable die are closed, there is the possibility of air being trapped in the die resulting in back-pressure when molten metal is injected in the die at high pressure. These gases create porous structures in the final casting reducing the strength of the final component, to eliminate this defect vacuum die-casting method is used.
- High pressure during the casting process with the loose gap between the closed die can result in flash defects.
- Die needs to be replaced for every new casting batch product that is to be made. If casting is made in small batches, the cost of the replacement of the die should be justified. The time taken to change the die can take 8 hours.
- Die-casting machines used in this process should have maintainability, serviceability and reparability or else it will reduce the productivity output of casting production.
- The molten metal used in this process should have a high fluidity rate for molten metal to pass through thin sections of the mould.
- Scrap in this process is higher as metals such as aluminium and magnesium are used which have high fluidity. Lengthier runner, sprue, gate and thicker biscuit need to be accommodated while designing the gating system.
- Control over die heating and cooling needs to be controlled to avoid defects such as misrun and cold shuts as molten metal can solidify in thin sections of die for complex castings.
- The cost of die, machines and automation setup is higher. The life of dies used in this process is reduced resulting in erosion and wear over the period due to sudden changes in thermal characteristics of die material in every casting cycle due to the die being subjected to high and low temperatures, pressure built during the injecting molten metal in the cavity and die locking/clamping force.
Die Casting Vs Permanent Mould Casting
| Die Casting | Permanent Casting |
|---|---|
| 1) Sand cores are not used but movable metal cores are used in die casting process. | 1) Sand cores are used in the permanent mould-casting process. |
| 2) Mass production of casting is possible as the process is automated from melting metal to removing finished casting from the die. | 2) Mass production is not possible as the process is still not fully automated and the majority of work is done manually like pouring operation. |
| 3) Molten metal is injected in the die through low to high pressure depending upon the final application. | 3) Molten metal is not injected through pressure but through gravity. This process is also called gravity permanent mould casting or gravity permanent die casting. |
| 4) In processes such as hot chamber die casting molten metal is directly injected into the die cavity without pouring through a ladle. | 4) Ingots are melted in the furnace and molten metal is poured into the metal cavity through a ladle. |
| 5) The process starts with cleaning dies through an automated pneumatic spray and blowers, lubricating the die surface, injecting molten metal in the die at pressure, solidification process and casting shakeout by a robotic arm in the foundry. | 5) This process is time-consuming and requires a skilled operator to clean the die, lubricate the die, solidify the molten metal and remove the casting from the die. |
| 6) The cost of production is lower per casting for large volumes (quantity) of casting but not suitable for smaller orders because of the die-casting machine. | 6) The cost of production becomes lower per casting for small volume (quantity) casting because this step only requires a die and clamps. |
Die Casting Vs Investment Casting
| Die Casting | Investment Casting |
|---|---|
| 1) Molten metal is forced into a closed die against gravity with pressure in the die-casting process. | 1) Molten metal is poured with the help of gravitational force in the plaster ceramic mould in the investment casting process. |
| 2) This process does not require a pattern made from wax to produce a mould cavity. This process can be called a patternless casting process. | 2) This process requires a wax pattern to create a mould cavity using a ceramic slurry mould. |
| 3) Mould is made from metal and can produce at least 100000 to 150000 casting in a lifetime. | 3) Moulds are non-expandable moulds and need to be broken for every casting cycle. |
| 4) Surface finish and smoothness achieved by this casting process are less as compared to investment casting because of the metal die. | 4) The surface finish, texture and smoothness achieved by this casting process are of better quality than the die-casting process because of the wax pattern and ceramic mould. |
| 5) The manufacturing steps involved in this process are fewer producing more casting per hour. | 5) The manufacturing steps involve making wax patterns first, then ceramic mould and then the pouring operation starts consuming more time and restricts the higher production output of the foundry. |
| 6) This process is best for the repetitive production of casting in mass. | 6) This process is best for customized, small scale and batch production of casting. |
Die Casting Vs Squeeze Casting
| High-Pressure Die Casting | Squeeze Casting |
|---|---|
| 1) Molten metal is injected in the close die for making die casting. | 1) Molten metal is injected in the open die for making squeeze casting. |
| 2) After the liquid metal is injected, it is allowed to solidify by cooling the dies designed with water-cooled channels. | 2) Molten metal is allowed to solidify when the pressure is applied by the plunger. |
| 3) Products produced from this process do not have forging properties. | 3) Products produced from this process have forging properties. |
| 4) Molten metal is injected in a closed die which is why it is called the die-casting process. | 4) Molten metal is injected into the die and squeezed by the plunger until the metal solidifies that is why it is called the squeeze casting process. |
| 5) Machines, equipment, set up and process are complex as compared to squeeze casting. | 5) The machine, equipment, setup, mechanism and process are simple compared to the die-casting process. |
| 6) This casting process is more sustainable for mass production as compared to squeeze casting. | 6) Mass production in the squeeze casting process is time-consuming as semi-solid metal needs to be squeezed between the plunger and die. |
| 7) Die life is longer as it is not subjected to the pressure of the plunger. | 7) Die life is shorter as the die is subjected to the pressure of the plunger when the metal starts to cool and is forged in all directions. |
Die Casting Vs Centrifugal Casting
| Die Casting | Centrifugal Casting |
|---|---|
| 1) The die cavity is injected with molten metal with force through a piston that actuates in the shot chamber during an injection operation. | 2) Force is not generated through the piston as metal is not injected with the piston but centrifugal force pushes metal outwards towards the circumference of the die. |
| 2) Slag and inclusions stay at the centre of the final casting and are turned out with machining operation. | 2) Slag and inclusions can be found in any part of the final casting as metal is not pushed to the circumference of the die. |
| 3) Large lengthy uniform pipes, bores and hollow sections cannot be produced with this casting as there are limitations over the length of the die. | 3) Dies here are built with the intent to manufacture pipes, long hollow channels and components. |
| 4) The die does not rotate along its own axis to create centrifugal force in the die to produce casting products. | 4) The die rotates along its own axis here along its own axis to create centrifugal force in the die to produce casting products. |
| 5) In this casting process, molten metal enters into small sections of the die cavity at pressure creating intricate complex casting. | 5) In this casting process, complex shape casting cannot be created. |
| 6) There are limitations on the size of the die of the die mould for producing large-size heavy-weight casting. | 6) In this casting process, there are no limitations on the die size as large-size casting can be produced. |
| 7) A symmetrical casting can be easily manufactured along the parting line. | 7) Mostly symmetrical casting can be produced along the parting line. |
Casting Defects In Die Casting
Cold Shut Defect:
When two sections of the casting part do not connect together and freeze before connecting in the die mould resulting in incomplete casting is called a cold shut defect.
Remedies for cold shut defects are proper pre-heating of the die cavity, maintaining the required temperature of molten metal in the die and sufficient supply and pressure of molten metal in all sections of the die.
Misrun Defect:
A misrun defect is caused when the molten metal used in the die is insufficient to fill all sections of the die leaving some sections of the casting incomplete. Some other causes of the misrun defect can also be due to faster cooling of the molten metal in the die, the injection pressure is not sufficient and less plasticity of molten metal in the die.
Remedies to reduce defects in casting are injecting molten metal in the die at the required pressure and temperature, preheating the die to reduce premature solidification of the casting sections, and proper distribution of cooling and heating rates in the thermal system of the die.
Warpage In Casting:
Final casting when solidified tends to have a deformed structure due to the difference in solidification of casting sections. This happens when joints of two sections of casting have different solidification rates due to different thicknesses in the casting sections.
This will cause deformation in the final casting leading to a change in the dimension and geometry of the casting products.
Flash:
When molten metal escapes out from the die capacity and ends up on the edges and solidifies that metal is called a flash. Flash looks like thin uneven non-uniform metal that has solidified at the sides of the casting and needs to be removed out from the casting by trimming the sides.
This defect is caused due to misalignment between moving and fixed dies, the clamping force acting on the dies is insufficient, high injecting force, molten metal has been injected at high temperatures and worn-out dies.
The remedy to reduce this casting defect is to align dies properly, the project required clamping forces on the dies, providing desirable injecting pressure, repairing and reworking dies reducing the gap between them.
Heat Marks:
The causes of heat mark defects are low-temperature casting temperature, casting in colder dies, improper cooling and solidification rate, the temperature difference between die and casting and casting in cold dies results in this defect which leaves a visible heat mark on the die.
Inclusions Defect:
When ingots are melted in the furnace there are inclusions, foreign materials, impure metals and dirt that remain in the molten metal. These foreign materials end up in the final casting if they are not properly filtered in the casting process.
Die Soldering Defect:
A soldering defect is caused when a solidified casting tries to stick to the uneven surface of the die due to the high temperature of the die, and excess lubrication between the casting and die surface.
The remedy to reduce this defect is to cast casting having a die with a smooth surface at the required temperature, reducing excess lubrication between die and casting, having required iron content in the molten metal reducing the sticking phenomena of casting and die surface. This defect results in the formation of cracks on the casting surface.
Scaling:
Due to chemical reactions, oxides scales are formed on the casting causing scaling defects. This happens when the lubrication application applied on the die is insufficient or not effective. Selecting proper lubrication for the die is the remedy for scaling defects.
Mismatch:
The cause of mismatch defects in final casting is caused because moving to die and fixed die are not alight together causing a change in the dimension of the final casting. This defect is caused when there is no proper calibration between dies when a new die is fit on the platen.
Blister, Porosity, Pinhole and Blowhole Defect:
Porosity in die casting takes place due to the presence of air in the die due to insufficient venting. Molten metal when passes through all channels of the die absorbs air present in the metal and ends up solidifying during the solidification process. This causes casting products to form porosity defects.
The remedy to reduce porosity, pinhole and blowhole defects in casting is to have proper ventilation for the dies. Connecting the vacuum pump to the die cavity and producing a casting in a vacuum environment will result in casting free from porosity, blowholes and pinhole defects.
Chill And Flow Lines:
Chill defects are caused in the entire section of the casting where molten metal freezes at all sections of the die cavity before the required complete geometrical shape of the casting is formed. Causes of chill and flow lines are similar to misrun and cold shut defects.
Improper solidification, premature cooling of molten metal in casting sections and the difference between the solidification rate between two adjacent solidifying casting sections.
The remedy to reduce this defect is to cool casting sections properly using the cooling system in the die uniquely for every segment of the casting parts, maintaining the required temperature of the die before injecting molten metal in the casting.
Injecting only molten metal at the required temperature keeping in mind heat loss during heat transfer, especially in the cold chamber die-casting process.
Damaged During Ejection/Casting Withdrawal Casting:
When casting is solidified between dies, this solid casting needs to be ejected from the dies to process it for secondary manufacturing operation.
While ejecting this casting from the die with ejector pins the casting can get damaged due to excess ejector pins forces on the casting. The main cause for this defect is excess ejector pin force on the fragile, thin and weak casting sections.
The remedy to reduce this casting defect is to change the forces acting on the casting sections from ejector plates or change the location of the pins to stronger casting. This defect can also because when the metal has not solidified completely and is ejected from the die causing the solidified casting to bend and break.
Cracks In Casting:
Casting is subjected to high thermal stress, change injection pressure, thermal shock, change in temperature inside the die cavity and change in solidification rate.
Important Die Material Properties
What properties and characteristics should die-casting die should have to be used for the die-casting process?
Materials such as tool steel, hot working steel, cast iron and alloy steel are used to manufacture dies. Dies used in this process should have the following properties to be fit for the die-casting process:
Thermal Shock- Dies used in this process are subjected to thermal fatigue due to changes in temperature before pouring operation till the final casting is taken out. Dies should have the thermal capacity to sustain sudden changes in temperature during preheating when molten metal is injected and during the solidification process.
Durability- Dies are subjected to changes in temperature, shock, pressure from molten metal, pressure included in the cavity due to air/gas being trapped and vibration when the shot is applied to the dies. Dies should have the durability to sustain for at least 100,000 cycles.
Machinability- When manufacturing dies safe positive tolerance and allowances are given to the dies keeping in mind the wear, tear and damage the dies will be subjected to during casting operation. To regain dimensional tolerance, accuracy, surface finish, texture and smoothness dies must have machinability properties.
Hardenability- Dies must be hard enough to sustain the locking force and impact of molten metal on small thin sections of the die. After going through different cycles, die sections should sustain hardenability for machining operations during repair and service.
Availability And Cost- Cost-effective die material should be available.
Maintainability and serviceability- The life of the die reduces gradually with casting cycles as the die is subjected to shock, change in temperature, vibration and impact during casting.
Dies should be designed in a manner for operators to do service and maintain the die as the die is subjected to wear, tear, damage and modification in the design.
The design of the die should allow machining tools and equipment to access all sections for repair and service for grinding, lapping, honing, welding and drilling.
Die Casting Process Cost
What are the costs associated with the die-casting process?
Cost of Manufacturing Die Mould
- Mould (Die) production cost depends upon the type of machining operation done on the die. The first step for producing a die is to cast a metal block using the sand casting process.
- This metal block is given the shape of the cavity using wire electric discharge machining, electrochemical machining, abrasive jet machining, laser beam machining, plasma arc machining, electron beam machining, chemical machining and grinding.
- The cost of manufacturing of die depends upon the costs of the machining process carried out on the metal block to make the die cavity, cores, core inserts, sprue, runner, gating channels and casting ejector die systems such as ejector plate and pins.
- After this, the die goes through a heat treatment process to improve the hardness, toughness and durability of the final die.
- Material cost, labour cost for producing the die, material handling cost of die mould on shop floor, tooling cost, inspection cost, testing cost, quality checks and resources used for producing the die should be considered while producing the die for the die casting process.
Production Cost of Die Casting Process
- Size of the casting: As the size of the casting goes on increasing cost of metal, pouring time, clamping force, metal injecting force, solidification time and time for removal of casting increases thus, increasing the cost of production. The larger the solidification time lesser the number of casting produced per hour.
- Complexity of casting: Dies used in this process need to be designed for different sectional thicknesses of the casting products. Thinner sections require special cooling channels as compared to thicker sections to reduce defects such as cold shuts and misruns. Maintenance, service and finishing of complex cavities in the die for complex intricate casting increase the production cost due to the downtime.
- Solidification time: Solidification operation in the casting process increases the production time and cost. To solve this problem efficient cooling system needs to be set up in the die so that the die can be cooled at a faster rate keeping in mind all sections of the casting.
- Clamping force: The clamping force used in the die-casting machine is directly proportional to the size of the casting products. Larger casting parts require larger machines as the clamping force requirement is higher increasing the production cost.
Labour Cost In Die Casting Process
- Labour costs in this process for producing casting are low as compared to sand casting once the mould is manufactured.
- The steps for producing castings are automated such as pouring molten metal through a ladle, injecting molten metal in the die, inducing pressure, the solidification process and removing the final casting out with a robotic arm reducing the number of operators requirement.
- As sound defect-free casting is produced using automated machines having good control over the process parameters the labour cost is reduced for this process to do rework, repair, re-testing, re-inspection and handling the defective castings.
Casting Components Rejection Cost
- The overall efficiency of the foundry increases once the process is automated reducing the rejection cost of the final casting because human errors are reduced over some time as fatigue on the machine operator reduces.
- As rejection cost reduces, the cost of rework, material handling costs, re-testing, re-inspection and quality checkups reduces improving the profit of the foundry.
Raw Material Costs
- The material used in the casting depends on the complexity of the casting design, weight, size, shape, tolerance, surface finish, strength-to-weight ratio, finishing allowance, machining allowances, casting density requirement and the size of the gating system w.r.t thickness of the casting.
- The material used in this process plays an important role in selecting the type of machine and equipment used in the die moulding process.
- The selection of types of equipment, tools, hot chamber and cold chamber machines is based on the type of metal used. For low-melting metals hot chamber die-casting machine is used and for high-temperature metals col chamber die-casting machines are used. The ladle, die material, metal pouring and melting types of equipment and costs change accordingly.
- The cost of the metal used depends upon the current market price. Maximum efforts are taken to use the metal effectively to produce casting and improve casting yield by designing a mould cavity such that it reduces waste.
Material Handling Cost
- This cost depends upon the size of the casting, the type of machine used in the process, the distance between the furnace and the die mould, the distance from the die machine to the inspection station, the types of robots used in the process to remove the casting from the die.
- The more automated the foundry larger the cost of material handling equipment.
Finishing And Secondary Manufacturing Operation Cost
- The cost of the secondary manufacturing operation is less in the die-casting process as compared to the sand moulding process because the surface of the final casting has a better finish and casting is produced in a controlled environment keeping in control the process parameters.
- This cost depends upon the type of lathe machine, reaming machine, NC machine, CNC machine, drilling machine, milling machine, saw cutting machine, antique coating machine, trimming machine, electroplating machine, chemical coating machine, ceramic coating, painting machine, polishing machine, powder coating, buffering machine, lapping machine, grinding machine and anti-corrosive painting machine used in the process.
- Small minute holes are difficult to produce for that purpose laser machines are used increasing secondary manufacturing costs as metal inserts cannot be used for this type of casting design.
Design Of Die Casting Die/Mould
- Dies should have a sufficient amount of vents to allow gases/air to exit when the shot operation takes place during the process in the machine, if gases fail to escape the die it will result in defects in casting due to backpressure and turbulence in the process.
- Die locking in this process should be sufficient to disallow molten metal to get out of the die.
- Dies used in this process should not have sudden changes in sections leading to turbulence of molten metal during the casting process.
- The die surface should be smooth enough for molten metal to flow seamlessly on its surface.
- Dies should have sufficient and efficient cooling systems to create a gradual and uniform solidification on the final casting.
- The ejector pin plate should have sufficient ejector force to push the final solidified casting out of the die with the help of the ejector pin. The location of the ejector pins should be on the stationary and movable die. Before moving the die retracts to its original position ejector pin is used to separate the die and casting while the die remains on the fixed die. Once the moving die goes back to its original position ejector pins are ejected from the stationary die allowing the robot to remove casting from the die-casting machine. This entire process should not cause any damage to the final casting that is why draft or slope and projections are provided on the casting for easy ejection.
- A parting line is a line that divides casting along the stationary die and moving die and serves as a reference line.
- Bosses are supporting stand-offs used to mount casting which brings strength to the casting.
- Ribs are small sections of metal strips that are provided on the final casting to provide strength and support without making changes in the wall thickness.
Die Casting Conclusion
Die-casting (dc) methods have improved over time increasing the precision, accuracy and efficiency of the process by using programmable robots for casting withdrawal, trimming, lubrication coating and quenching operation.
The use of a vacuum in this process reduces porosity and gives dense casting with fine grain structure in the final casting reducing the rejection cost of defective casting and increasing the productivity of the foundry.
This process is efficient having a lower cycle time. Lead time for delivering the casting to the customer is short making this casting process suitable for the automobile industry, construction, aerospace industry, mining industry, agricultural industry, space programmes, marine industry and telecommunication.
Multiple alloys can be used in combination to produce sound casting using aluminium, lead, tin, zinc, copper alloy, stainless steel and magnesium alloy. Producing highly dense, lightweight, corrosive resistance-free, strong, durable, longer life, hard and reliable casting products.
The cost of casting per piece is lower as compared to sand casting due to the high volume casting production of identical parts using reusable metal moulds.
This process does not require a pattern for manufacturing castings because of the permanent die used in this moulding process.
This casting process is more accurate than sand casting and has repeatability as one casting is produced multiple times with the same die in controlled process parameters cycle time of this process is shorter than the sand casting process.
This process has repeatability resulting in eliminating defects, improving the process parameters and method of production, increasing the efficiency of the process, reducing the cost of manufacturing, collecting data points for the process, reducing labour cost and safety and increasing profit and growth of the foundry.