What Is Squeeze Casting?
Squeeze casting is a casting method that combines die casting and forging. It starts with low pressure casting, followed by the application of very high pressure as the material cools, producing a high-quality casting. This is often carried out using a hydraulic press as part of the casting apparatus.
Squeeze casting was originally created to make stronger metal parts for use in the construction and defense industries.
The metal parts created by this process are more resistant to wear and heat and have historically been very expensive to produce. The market for these parts has grown to include the agricultural and automotive industries.
Types of Squeeze Casting
There are two available squeeze casting methods, direct and indirect. Here’s a quick overview of what each process entails and the benefits they offer.
Direct Squeeze Casting
The direct process, also known as liquid metal forging, is completed with equipment similar to the ones used in other die casting and forging processes.
Usually, this includes a hydraulic forging machine, otherwise known as a forging press.
In this process, liquid metal is poured directly into a lower die segment within the hydraulic forging press, while the upper segment is closed off.
The pressure applied during the direct process is quite high, usually 100 or more MPa.
The benefits of using the direct process are as follows:
- The resulting casting emits no gas, experiences no shrinkage, and has no porosity (empty spaces).
- The high pressure used results in a high cooling rate granting more control over the microstructure of the mold and liquid metal.
- Fine-grain structures can be made without having to resort to other additions or procedures that help create such structures.
- There is no need for any risers or feeders during the process.
Another distinctive feature of the direct process is the controlled way the molten metal is poured into the mold.
The slow rate of the pour allows the die to automatically flow into the metal at a controllable speed which produces the desired shape in a non-turbulent way.
Indirect Squeeze Casting
The indirect casting process uses similar casting equipment to the kind used in the direct process, yet the procedure is quite different.
Here, the liquid metal is first cleaned and then grain-refined before it is inserted into a casting machine.
The casting machine used in the indirect process can either be horizontal or vertical.
After it has been cleaned, the heated metal is “injected”, instead of poured, through a small-diameter piston into the die through large gates in the machine at a very slow rate—approximately 0.5 m/sec.
After this, the melt (i.e., heated/liquified metal) is pressurized (55 MPa-300 MPa) until it becomes solid.
One major difference between this process and the direct one is that the pressure chosen to solidify the melt remains constant throughout the solidification process.
Although the indirect process usually results in lower-quality metal properties than the direct method offers, it is still an ideal process for heavy suspension automotive parts like steering knuckles.
Both the direct and indirect processes share similar advantages over more traditional forging and casting processes.
Some of these include:
- No internal/external defects requiring NDT (non-destructive testing).
- Better quality mechanical properties.
- Shorter production cycle times (up to 66 percent less than other die casting processes).
Both processes can improve a wide range of metal alloys as well (iron, steel, magnesium, zinc, and aluminum), helping to produce better machine parts with stronger and more flexible metallic properties.
Tools Needed to Complete the Squeeze Casting Process
As with all forging and casting processes, the right tools and equipment are necessary to achieve the desired result.
There are four main pieces of equipment (tools) needed for squeeze casting:
- Squeeze Casting Machine
- Oil Die Heating System
- Vacuum System
- Process Monitoring System
A brief explanation of each of them is given below:
Squeeze Casting Machine
This is the main tool used for squeeze casting.
It is where the liquid metal is poured or injected into a mold full of dye and solidified under high pressure.
It can either come in a horizontal or vertical shape and usually weighs anywhere between 50 to 350 metric tons.
Oil Die Heating System
This machine is used to heat the die before it is put into the squeeze casting machine and combined with the liquid metal.
Vacuum System
This machine is used to create the high pressure needed to solidify the die and melt mix into the shape of the mold.
Process Monitoring System
This tool helps to monitor the entire squeeze casting process from start to finish making sure that everything runs smoothly throughout and alerting fabricators when and where something breaks down during the process.
It helps maintain regularities in:
- casting alloy compositions
- pressure levels and duration
- die preheating, pouring and injecting temperatures
- die coating (lubrication)
- melt heating
- punch temperatures
- delay times needed for producing optimal temperatures
At first glance, the tools needed for squeeze casting may seem expensive.
However, in actuality, the machines used for this process and the process itself are less expensive than other forging and casting processes—like HPDC (high-pressure die-casting), for instance.
Squeeze casting Parameters
The main process variables in squeeze casting are melt volume, casting temperatures, tooling temperatures, time delay, pressure levels, pressure duration, and lubrication.
These variables, which depend on the casting alloy and the shape of the product, affect the strength and quality of the casting.
Aside from the tools, every squeeze die-casting enthusiast must know the monitoring parameters indicated below to get quality squeeze-cast parts.
#1. Casting temperature.
This is the temperature of the molten metal when you pour it into the mold. Generally, the temperature is between 60C to 550C higher than the metal melting point. Nevertheless, it depends on the metal alloy and its geometry.
#2. Squeeze pressure.
The amount of pressure applied to the molten metal as it begins to solidify. It also depends on the metal but has a typical range of around 70MPa to 140 MPa.
#3. Tooling temperature.
The tooling temperature is the temperature of the mold during casting. The typical range is between 190 to 315 degrees C.
#4. Lubrication.
Lubrication in squeeze casting depends on the material but is necessary as it helps in the ejection of the final part.
Materials such as copper, aluminum, and magnesium, use any grade of colloidal graphite as spray lubricant applied to the warm dies before the casting process.
#5. Time delay.
Time delay is the time (measured in seconds) between pouring the liquid metal and the beginning of the pressurization process.
#6. Squeeze pressure holding time.
Squeeze pressure holding time is the duration of holding the squeeze pressure. It depends on the weight of the casting. For example, castings that weigh about 9 kg have a duration in the range of 30 to 120s.
Squeeze Casting Process
Squeeze casting, also called liquid forging, is a hybrid metal forming process that combines permanent mold casting with die forging in a single step in which a specific amount of molten metal alloy is poured into a preheated and lubricated die and subsequently forged and solidified under pressure.
Squeeze casting dates back to the 1960s, when it was first introduced to metal fabrication manufacturers in the United States.
Since then, squeeze casting has been simplified and optimized to get the job done in only four steps.
These steps include melting metal, pouring and mixing melt into die, then closing it and applying pressure.
Finally, after the process is complete, the end result is ejected and the entire process repeated.
Each of these four steps is explained below.
Melting Metal
In this step, the metal being used will be preheated before it is poured into the casting machine and mixed with die.
A melting furnace is typically used to liquefy solid metals.
The temperatures needed to heat different metals varies widely and can range anywhere from 660°C (1220°F) to 3400°C (6152°F) depending on the metals being used.
Pouring/Mixing Melt Into Die
The second step involves pouring or injecting the molten metal with the help of a launder into the mold cavity (bottom die cavity), where it is mixed with the bottom half of the pre-heated die.
Closing Die and Applying Pressure
Here the top half of the pre-heated die (punch) is used to close off the melt. Once the melt is closed off, pressure is applied through the use of a ram to form the cast in the shape of the mold.
Pressure levels of 50 to 140 MPa are typically used to shape the metal/die mix.
Ejecting/Repeating Process
After the solidification process is complete, the punch is withdrawn.
The cast component is ejected, the die is cleansed, and the melt stock is charged so that the process can be repeated as soon as it is needed.
As was mentioned earlier, pressure levels of 50 to 40 MPa are used to form the cast, but this must also be accompanied by temperature levels of 6-55 degrees Celsius so that enough cooling can take place to solidify the cast.
Common Materials Used in Squeeze Die Casting
During squeeze casting, there are some procedural differences compared to other types of die casting.
The process allows you to use a variety of materials, including lightweight metal alloys such as zinc, aluminum, and magnesium. However, there are also longer cooling times.
Thus, certain lubricants are also needed to achieve the final result.
This requirement can increase the investment needed to produce these parts, making it advantageous to outsource this process to specialists such as Bunty.
If aluminum castings are used, they should be without iron aluminide needles. These can impede mechanical properties that are necessary for high-quality products.
In addition to raw materials, there are certain tools needed for proper squeeze casting:
- Squeeze Casting Machine. This is the primary tool needed for squeeze casting. Metal is poured (or injected) into the mold. It is filled with dye, and then high pressure solidifies it. There are two shape options for the machines: horizontal or vertical. Casting machines typically weigh somewhere between 50 and 350 metric tons.
- Oil Die Heating System. Die needs to be heated up before entering the squeeze casting machine. The oil die heating system performs this function and combines the die with liquid metal.
- Vacuum System. High pressure is a key component of squeeze casting. The vacuum system creates this pressure to solidify the die, melting the mix later to fit the shape of the mold.
- Process Monitoring System. Quality assurance is of the utmost importance in this process. The process monitoring system ensures that everything is working as intended, alerting fabricators if something goes wrong during the process.
Now that we know what tools are needed for squeeze casting, let’s take a look at its many applications.
Advantages of the Squeeze Casting Process
- High Strength and Density: Due to the thorough filling of the mold cavity under high pressure, squeeze cast products typically exhibit higher strength and better density, suitable for applications requiring high strength and precision.
- Reduced Machining Costs: Compared to traditional casting and forging processes, squeeze casting can often reduce machining costs by at least 40% because it can produce near-net-shape parts, reducing the need for subsequent machining.
- Increased Production Efficiency: Squeeze casting boasts efficient production cycles, enabling rapid production of large quantities of products while maintaining consistency and stability, thus enhancing production efficiency.
- Lightweight Design: Squeeze casting allows for the production of complex shapes and thin-walled designs, reducing the weight of products and improving performance and energy efficiency.
Disadvantages of the Squeeze Casting Process
- High Initial Setup Costs: The initial setup costs for squeeze casting can be high, particularly for the development of molds and the acquisition of specialized equipment. This can be a barrier for small-scale production or companies with limited budgets.
- Limited Material Options: While aluminum is an excellent material for squeeze casting, the process may not be suitable for all types of metals. This can limit the range of materials that can be used, potentially restricting the applications of squeeze casting.
- Complexity of Process Control: Squeeze casting requires precise control of various process parameters, such as pressure, temperature, and cooling rates. This complexity can make the process challenging to manage, requiring skilled operators and advanced monitoring systems.
Applications of Squeeze Casting
Squeeze die casting is applicable in several industries due to its unique process, advantages, and the properties of the casted parts.
Automotive Industry
A common application of the casting process is in the automotive parts industry where it is suitable for making high-quality automotive parts such as brackets, nodes, chassis, and frames. A common example is Porsche which uses the process in making their engine components.
Military
Squeeze die casting is also suitable for making military weapons and heavy machinery. With materials such as magnesium alloys, the squeeze casting process produces high-quality parts such as bombshells, bevel gears, pipes, blades, and discs.
Marine Parts
The casting process is also compatible with materials such as aluminum in making small turbine blades and boat propellers.
Its compatibility with heat treatment methods such as annealing also makes it common here as this can be used to improve the part operation in the marine environment.
Squeeze casting Vs Die Casting
| Properties | Squeeze casting | Die Casting |
| Material Flow | in squeeze casting, better control over material flow is observed as direct pressure is applied to the material. | As molten metal is pushed towards the cavity through considerable pressure in high pressure die casting, uniform material flow and distribution are observed |
| Porosity | Squeeze casting usually produces denser parts but with low porosity as compared to high pressure die casting. It becomes suitable for applications with superior mechanical characteristics. | High pressure die casting provides high porosity due to gas entrapment as well as rapid cooling. |
| Mechanical Properties | Squeeze casting offers improved mechanical properties including higher strength as well as ductility. This is due to reduced porosity combined with optimized grain structure. | High pressure die casting requires additional heat treatment to match the mechanical properties of squeeze casting. |
| Cycle Time | Squeeze casting comes with longer cycle times due to the applied pressure. However, it comes with superior material properties. | High pressure die casting provides exceptionally faster cycle times, making it the best in the cycle time comparison with squeeze casting. It is ideal for high-volume production. |
| Tooling Life | Squeeze casting comes with longer tool life as their tools endure less stress. | High pressure die casting usually faces more wear and tear due to the application of advanced pressure injection. |