Permanent Mould Casting | Gravity Die Casting |
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| (a) Permanent Mould Casting Diagram |
What is the permanent mould casting process?
The permanent mould casting process is the process of making complex solid fine-grain casting using a solid metal permanent die mould as shown above in diagram (a).
Molten metal is poured into the metal die using a ladle without any external pressure and allowed to solidify in the die mould.
This process is different from the die-casting process as the molten metal used here is poured into the die at low pressure and not injected at higher pressure as we see in die-casting.
The mould used in this process is simple and does not have an efficient cooling and monitoring system as we have it in the die-casting process making this process restricted to produce casting with simple shape and size as compared to die casting.
This process is fairly simple where the operator pours metal and solidified casting is taken out once the molten metal is solidified in the metal mould, the mould is not broken as we see in other casting processes such as sand casting, shell casting and investment casting.
This process is better than the sand casting process in terms of productivity, operator safety, production rate, accuracy, tight tolerances of casting products, ease in manufacturing, surface finish and texture.
What is the permanent mould casting process also called?
Permanent mould casting is also a gravity permanent casting or gravity die casting or gravity die casting process or permanent mould gravity die casting process or low-pressure mould casting process as shown below in diagram (b).
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| (b) Gravity Die Casting Process |
Permanent Mould Casting Or Gravity Die Casting Step-By-Step Process
Explain the step-by-step gravity/permanent mould or gravity die casting process with the diagram, method and working to produce permanent mould casting or gravity casting products?
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| (c) Permanent Mould Casting Process Animation |
Steps to produce casting using permanent mould or by gravity casting process are shown above in animation (c) in permanent mould casting animation.
A flow chart of this process is given below in diagram (d) which explains in snipped every step from melting molten metal to taking the solidified casting out of the mould and further carrying secondary manufacturing processes on the solidified casting such as machining, coating and super finishing on the casting before dispatching.
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| (d) Permanent Mould Casting Process Flow Chart |
The step-by-step gravity or permanent mould casting process is explained below with principles with desirable diagrams in detail and are as follows:-
STEP 1: Melting Ingots And Pre-Heating Permanent Mould
Melting Ingots In Furnace
In this process, ingots which are small metal blocks of desirable required metal are melted in the crucible furnace to the required temperature to pour in the permanent metal mould. Ingots are selected based on the final application of the casting.
Based on the melting material, the mould type, tool type, pouring temperature, solidification rate, mould material and properties of mould material are considered and metal ingots are selected.
Metal is heated above the melting point to the pouring temperature before pouring the liquid metal into the die mould cavity.
Pre-Heating Of Permanent Mould
Pre-heating starts with heating the die mould in an oven or with a heating torch. Mould is heated at high temperatures to avoid defects such as soldering, misrun defects and cold shuts as these defects can produce incomplete castings.
These defects will occur when the high-temperature molten metal comes in contact with the cold die which is why this step of pre-heating die becomes important.
For small dies pre-heating can be done with a flame torch gun where all internal and external sections of the cold dies need to be manually heated in the foundry before pouring the hot molten metal but for larger dies heating mould in the oven becomes necessary.
Pre-heating in this process is very different from the die-casting process where internal heating elements are attached to the die for accurate heating of all sections of the molten metal.
After the mould surface is heated the mould is coated with the carbonaceous paste to avoid surface defects in the final casting.
This paste has insulating and lubricating properties improving the flow of molten metal from the gating system to the mould cavity giving high quality of the surface finish.
This refractory coating consists of other ingredients such as talc, sodium silicate and clay used to eliminate the soldering of molten metal to the metal mould at high temperatures improving directional solidification and reducing thermal shock in casting.
Graphite mould coating reduces mechanical shock, thermal stress, thermal resistance and cracks, improves surface finish and grain refinement, provides an insulating layer between molten metal and die surface and improves microstructure.
This mould coating further increases dimensional accuracy, reduces surface defects, reduces the soldering of molten metal and die mould and improves the mechanical properties of the casting products.
STEP 2: Pouring Operation In Metal Die
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| (e) Pouring Operation In Permanent Mould |
A ladle is used to pour the liquid metal into the die mould slowly, this is done manually in the permanent mould casting process as shown above in diagram (e).
The operator keeps filling the molten metal in the mould until the operator sees the molten metal from the other end of the mould which is the top riser.
Care is taken that the pouring rate does not cause turbulence in the die and metal flows slowly in all sections of the die.
Before the pouring operation degassing of molten metal is carried out removing hydrogen from the molten metal that can cause defects in the final casting.
All inclusions, oxides, impurities, foreign material and unwanted slag are moved before pouring the molten metal into the pouring basin.
The pouring basin is the first element of the gating system at which molten metal comes in contact, from there molten metal starts flowing towards the sprue.
Molten metal falls at the gravitational acceleration and is slowed down at the bottom of the sprue. Sprue is made tapered in cross-section to reduce
Sprue is a vertical tapered section of the gating system that connects the pouring basin and horizontal passageway called a runner.
The runner is a horizontal passageway that is connected to the in-gate. Any slag and oxides present in the molten metal get trapped in the runner system.
Molten metal from the runner gets into the gate and fills the mould cavity. After this metal starts filling the riser.
The riser is filled fully and then the metal filling stops through the ladle once the operator sees the liquid metal on the top riser side.
STEP 3: Solidification Process In Permeant Mould
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| (f) Solidification Operation In Permanent Mould |
Once molten metal is filled all sections of the mould cavity and the riser molten metal are allowed to solidify as shown in diagram (f) above.
Mould at this point is left untouched and molten metal is allowed to solidify.
Before creating a cavity in the die mould shrinkage of the molten metal is considered and shrinkage allowance is provided on the die. A shrinkage allowance is provided considering solid shrinkage during the solidification process.
Cooling in permanent mould or gravity die casting is done by providing air-cooled dies. Cold air is blown on the dies to reduce their temperature slowly. This air-cooling system is best for casting with thin sections.
Water-cooling dies are best for thicker casting sections casting. Water-cooled dies can also be used where cold water is circulated in the die through channels reducing the molten metal temperature.
While metal solidifies both liquid shrinkage and solid shrinkage are considered. In liquid shrinkage liquid changes from liquid state to solid state. In solid shrinkage solidified metal shrinks as it loses its temperature.
Uniform directional solidification results in uniaxial grain growth in all directional producing fine-grain uniform solid casting.
Padding and chills are provided inside the die mould for uniform directional solidification of casting sections.
As the solidification process takes place in the die metal mould surface texture that we get from this process is smooth as compared to the sand casting.
We get better surface finish and texture in this process because metal is solidified on the finished smooth surface of the die instead of being solidified on the rough course grain structure.
After the metal is solidified in the die, the die is opened and the solidified casting is withdrawn.
STEP 4: Shakeout And Secondary Manufacturing Process
Material handling equipment is used to withdraw the casting from the die mould.
After this sprue, the runner,in-gate and riser are broken and separated from the actual final casting with a saw, cutter, welding cutter, hammers and table vibrators.
This solidified part of the gating systems such as the solidified runner, sprue, in-gates and riser is re-melted in the furnace and reused again for creating other casting products.
The process of removing gating elements, finishing, rough edges, burr and flash is called fettling.
These castings go through heat treatment processes such as normalizing, tempering, nitriding, quenching, annealing, quenching, carburizing, case hardening, induction hardening and austempering.
Trimming is done to remove any flash that gets out between the dies and shot blasting is used to remove any excess metals on the casting and give the casting an aesthetic look.
These casts are shot-blasted and cleaned before spraying it with coating the surface of the casting.
How is the permanent mould-casting (PMC)or graviy die-casting (GDC) process different from than sand-casting process?
This process is different than sand casting where moulds are made from silica sand, clay, moisture, binding agents and additives.
Gas casting defects such as air inclusions, blow holes, open hole defects, pinhole defects, and porosity are reduced in this process due to the absence of moisture as the die is made from tool steel material.
Surface finish, surface texture, accuracy, tolerance, defects and material wastage is reduced in this process as compared to sand casting.
Mould made from metal gives the added advantage of having a higher functional life, multiple casting products can be produced with one die and highly accurate tight tolerance components can be manufactured.
The die mould-making process is a resource, time, cost, and energy-consuming process as compared to sand casting and shell moulding where mould needs to be made and broken every time a new casting cycle starts for producing any components.
It requires highly skilled experienced operators in the foundry to produce sand mould every time for every casting, keeping the mould-making process dependent on the operator completely.
Dependency on the skilled operators on the foundry floor is reduced once the die moulds are produced as compared to the sand casting process.
The production rate of this process is better than the sand casting process as there are no steps of making a pattern, pattern withdrawal, sand mould making and breaking the mould to remove the final casting.
How is the permanent casting process or gravity die-casting different from the die-casting process?
In the permanent mould gravity die-casting process, molten metal is allowed to pass through a sprue, runner, gates and riser without any external forced pressure.
Gravity is allowed to do its work and let the molten metal flow from the gating system and the mould cavity to produce solidified casting.
But in the case of die casting low to high external pressure is induced on the molten metal to force the metal into every section of the mould cavity.
Die castings are made from low-pressure die-casting processes and high-pressure die-casting processes where force is an important element in producing casting.
In the die process, molten metal enters into every section of the mould cavity producing complex casting products.
However, the complex shapes in the permanent moulding casting (PMC) process are difficult to produce as metal enters the mould cavities through gravitational force as compared to die-casting.
Die casting is a very versatile, automated, less manpower-consuming process where all casting operation steps are automated from pouring molten metal to taking out casting from the mould.
The step of die closing and opening is also automated where at the end a robotic arm takes out solidified casting which solidified uniformly in all directions resulting in a casting with a uniform grain growth structure.
The permanent mould casting process consists of only a metal block with an accurate internal mould cavity, it does not have an automated cooling system, sophisticated high-end calibration setting for the machine, data collection system, or feedback close loop system.
The casting result is mostly dependent on the skill of the operator in the foundry whereas in the die-casting process dependency on the operator is reduced as the machine is programmed to do all the difficult jobs.
The downside of this process is a complex shape with very thin casting wall thickness casting cannot be produced using this process as compared to die casting.
Simple casting cavities can be made using sand cores as shown below in the gravity die casting (GDC) process.
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| (g) Gravity Die Casting |
As shown above in diagram (g) cores rest in the mould cavity and are utilized to produce cavities in the final casting.
Semi-Permanent Mould Casting Process
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| (f) Semi-Permanent Casting |
This moulding process tries to strike a balance between the permanent mould casting process where dies are made from metal and the sand mould casting process (sand casting) in terms of the cost of production of casting
Semi-permanent mould casting process consists of manufacturing casting with mould that has finite life. The mould material used here is not made durable for a longer time as we see in the permanent moulding process.
Mould materials selected here are made from plaster, metal alloys and a combination of metal and sand cores to strike a balance between the utilization and cost of manufacturing casting.
This mould is made with the intent of a few production cycles and then discarded it. This process can produce high-quality cavities and surface finish in casting due to the use of shell cores.
The production rate of semi-permanent moulds is lower as compared to permanent moulds as strong durable materials are not used to make this mould.
This mould has high maintenance, high service, breakdown, repair and less reliability.
The cost of production of the casting of this process is higher than the permanent mould casting process but the cost of production of mould is lower than permanent mould casting.
Permanent Mould Casting Application
This process is used to produce the following casting products using permanent mould gravity die casting and semi-permanent moulds using brass, steel, iron, cast iron, aluminium, aluminium alloys, copper alloys, lead, tin, zinc, graphite and magnesium.
This process is used in the automobile industry, manufacturing sector, aerospace industry, farming equipment, machine industry, hydraulic and pneumatic industry and electrical industry.
- Motor parts and stators.
- Agriculture plow jaws.
- Actuator components and robotic arms.
- Bevel gears, rack and pinion and worm gears.
- Casting blocks, cylinder blocks, brass valves and engine covers.
- Cast iron hand lever, clamps, holders and stands.
- Components of the windmill.
- Auto, e-bike components and railway parts.
- Aircraft engine components and fittings.
- Hubs and cases of two wheels.
- Heat exchanger and HVAC components.
- Aluminium wheels, pins and housing.
- Steel brackets and cast iron cylinders.
- Suspension and brake system parts.
- Electrical and electronic units.
- Fuel injection housing system.
- Silver and gold jewellery and ornaments.
- Ship engine and IC engine parts.
- Machine tool, hydraulic and pneumatic press.
- Parts of engines such as pistons and aluminium manifolds.
Permanent Mould Casting Or Gravity Die Casting Advantages and Disadvantages
Permanent Mould Casting Advantages
- This process is suitable for mass production because of expandable metal dies as compared to sand casting.
- Mass production of casting is possible as mould is made from metal, mould is reusable and multiple casting cycles are possible.
- The life of mould dies is longer producing highly accurate casting than the traditional sand moulding process.
- Sand cores can be easily used in this method reducing the cost of manufacturing the die.
- Blow holes, porosity, gas defects and sand inclusions are less in this casting process as molten metal comes in contact with the metal surface directly reducing casting surface defects.
- The flow of metal in the gating system is smooth reducing turbulence as metal flows on the smooth metal mould surface.
- Permanent moulds have higher flexibility than sand moulds as advanced manufacturing processes such as EDM, ECM, AJM, USM and LBM are used to cut the metal casting block and produce complex die shapes.
- Environmental factors, shelf life and storage facilities do not have an impact on the function of the die moulds, mould can be used whenever possible.
- The productivity of this process is high because molten metal is cooled rapidly and the directional solidification rate of this process is better than sand casting resulting in a better consistence casting structure eliminating casting defects.
- The process can be repeated many times to achieve the production target.
- This mould is heavy but does not require high inspection during transportation as permanent metal moulds are stronger than sand moulds.
- Scarp produced in this process is low due to thinner gating system elements such as sprue, runner, riser and in-gates used.
- Casting from 20kg to 400kg can be produced in this process without any difficulty.
Permanent Mould Casting Disadvantage
- The cost of producing these moulds is higher than sand moulds due to the cost of the metal die, tooling cost, material handling cost, time required, precision, casting large casting blocks and using an advanced production process to carve perfectly high tolerance accurate die.
- Ferrous high-temperature metals such as cast iron are difficult to mould in permanent mould dies. This process is suitable for low melting temperature metals.
- These moulding moulds can be manufactured only after there are high orders from customers where the customer's orders match with the profit gain.
- There are limitations on the size of the casting to be produced as there are limitations on the size of the mould that can be produced. Large-size castings are manufactured separately and assembled together by joining processes such as welding.
- Handing permanent mould gravity die becomes a difficult task in the foundry due to its weight which requires sophisticated costly equipment.
- Small batch production is difficult as the cost of manufacturing mould is higher and requires a highly skilled workforce and types of equipment.
Permanent Mould Vs Sand Mould
| Permanent Mould | Sand Mould |
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| 1) The permanent mould cavity is made from metal and not sand. | 1) Sand is an important mould element which is bonded with clay and water. |
| 2) This is a non-expandable mould, which means mould need not be broken and can be used multiple times. | 2) This is an expandable mould which means mould needs to be broken every time for producing new casting similar to shell moulding. |
| 3) The mould cavity is produced using advanced manufacturing machines such as USM, AJM, ECM, EDM, WJM, Wire-EDM and heat treatment process. | 3) The mould cavity is produced using simple tools and pattern in the foundry. |
| 4) The surface quality of the mould is higher as compared to sand mould giving fine-grain casting. | 4) The surface quality of the mould is rough as compared to permanent metal mould giving course grain casting. |
| 5) Manufacturing cost, resources, time, material consumption, material handling, and energy consumption should justify the production order and profit of the foundry when it comes to the production of gravity die-casting mould. | 5) Manufacturing cost, resources, time, material consumption, material handling, and energy consumption requirement are low for the production of and profit of sand mould. |
| 6) Permanent metal mould gives better directional solidification and uniform cooling. | 6) Sand mould does not give better directional solidification and uniform cooling as compared to die casting. |
Permanent Moulding Process Vs Investment Casting Process
| Permanent Mould Casting | Investment Casting Process |
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| 1) This is a type of non-expandable moulding process where mould is not broken to take out casting. | 1) This is a type of expandable moulding process where shell mould is broken to take out solidified casting. |
| 2) Multiple casting can be mass-produced using a single metal die mould. | 2) Only one casting can be made at a time using investment casting shell mould. |
| 3) The process is simple pouring molten metal into the die mould and removing the solidified casting out. | 3) The process is complicated and involves producing patterns and then the shell mould-making process is more complicated. |
| 4) Casting products manufactured from this process have lower dimensional tolerance, accuracy and surface finish than the investment casting process. | 4) Casting products manufactured from this process have higher dimensional tolerance, accuracy and surface finish than the permanent gravity die mould casting process. |
| 5) High-temperature metal is not suitable for this process as the surface quality of casting and die life reduces over time. | 5) All metals can be used in this process including alloys, pure metal, high temperature, low temperature, ferrous and non-ferrous metals |
Permanent Mould Casting Process Summary And Conclusion
This process produces the second most aluminium lightweight corrosion resistance casting products after the die casting process compared to other casting processes such as sand casting, investment casting and shell casting.
For permanent mould casting the mould is made from metal while for semi-permanent mould casting mould is made from a combination of metal, sand, ceramic, plaster and alloys.
No pressure is used in permanent mould casting to transfer molten metal to all sections of the casting, which is why this process is also called as gravity die-casting moulding process.
This process is a non-expandable moulding process used to produce high surface finish casting using reusable moulds used multiple times without breaking the die mould at a faster production rate as the solidification process is faster.
This special casting process has become very popular for producing aluminium casting and brass gravity die casting products as porosity in this process is less as compared to sand casting products.
Not only metal but also wax and plastic material can be used to mould complex casting using permanent or gravity die casting moulds.