Understanding Isostatic Pressing: Processes, Materials, and Design

Principle of Isostatic Pressing

Wet Bag Technology

In the wet bag technology of isostatic pressing, the powder is filled into a mold and tightly sealed outside the pressure vessel. Once the mold is filled with powder, it is submerged in a pressure fluid within the pressure vessel. Isostatic pressure is then applied to the external surface of the mold, compressing the powder into a solid mass.

This process is not as common as the other type of cold isostatic pressing. There are over 3000 wet bag presses used all over the world today. These wet bags have sizes ranging from as small as 50mm to as big as 2000mm in diameter.

The wet bag technology is relatively slow, taking from 5 to 30 minutes to process material. However, high-volume pumps and improved loading mechanisms can help speed up the process.

Working of Isostatic Press

Isostatic pressing is a process that reduces the porosity of a powder mixture by compacting and encapsulating it using equal pressure from all directions. The metal powder is confined within a flexible membrane or hermetic container, which acts as a pressure barrier between the powder and the pressurizing medium (liquid or gas) that surrounds it.

Isostatic forming technology involves placing the powder specimen to be pressed in a high-pressure container. The incompressible nature of the liquid or gas medium allows for uniform transfer of pressure from all directions, uniformly pressurizing the specimen. This process, known as isostatic pressing, can provide isotropic, ultra-high molding pressure for products. It can be categorized into three types based on the molding temperature: hot isostatic pressing (HIP), warm isostatic pressing (WIP), and cold isostatic pressing (CIP).

Principle of Isostatic Pressing

The principle of isostatic pressing involves placing and sealing the powder in a flexible mold, such as polyurethane, and subjecting it to a uniform hydrostatic pressure. There are two techniques used in isostatic pressing: wet-bag technique and dry-bag technique.

The wet-bag technique involves submerging a flexible bag containing the powder in the pressure vessel filled with a pressurizing fluid, such as soluble oil. On the other hand, the dry-bag technique involves fixing the bag in the pressure vessel and loading the powder without removing the bag from the vessel.

Alternative Processes

Isostatic pressing is a powder processing technique that uses fluid pressure to compact the part. Metal powders are placed in a flexible container, which serves as the mold for the part. Fluid pressure is exerted over the entire outside surface of the container, causing the container to press and form the powder into the correct geometry. Isostatic pressing is unique in its use of all-around pressure.

There are two general types of isostatic pressing operations: wet bag and dry bag. In the wet bag variation, an elastomeric mold is loaded outside of the press and then submerged in the pressure vessel. The dry bag variation circumvents the immersion step by creating a mold integrated into the pressure vessel.

Isostatic pressing has been widely used in various industries for consolidating powders or defect healing of castings. It is suitable for materials like ceramics, metals, composites, plastics, and carbon.

Dry Bag Technology

In the dry bag technology of isostatic pressing, the mold is fixed in the pressure vessel. The powder is then filled into the mold while it is still in the pressure vessel. Isostatic pressure is applied to the external surface of the mold from the pressure liquid, compressing the powder into a solid mass with a compact microstructure.

Dry bag technology is ideal for mass production of materials and is much faster than wet bag technology, taking as little as 1 minute.

Isostatic Pressing Process

In the isostatic pressing process, products are placed in a closed container filled with liquid and subjected to equal pressure on each surface. This high-pressure environment increases the density of the products, allowing them to take on the desired shapes. Isostatic presses are widely used in the forming of high-temperature refractory, ceramics, cemented carbide, lanthanum permanent magnet, carbon material, and rare metal powder.

Isostatic pressing offers unique benefits for ceramic and refractory applications, as it applies a uniform, equal force over the entire product, regardless of shape or size. This process helps in achieving precise tolerances, reducing the need for costly machining.

Manufacturing Process

Cold Isostatic Pressing (CIP)

Cold isostatic pressing (CIP) is a manufacturing process used to compact powders into components of varying sizes and shapes. It offers several advantages compared to other methods, such as high green strength and the ability to press difficult materials without the need for lubricants or binders.

Wet-Bag Technique

In the wet-bag technique, the powder is filled in a mold and sealed tightly outside the pressure vessel. The mold is then submerged in a pressure fluid within the vessel, and isostatic pressure is applied to compress the powder into a solid mass. This method allows for the production of mixed shapes and is commonly used for batch processing.

Dry-Bag Technique

The dry-bag technique involves using a flexible membrane inside the pressure vessel to isolate the pressure fluid from the mold. The powder is loaded into the bag without the need for the bag to leave the vessel. This process allows for more automation and higher rates of production, making it suitable for mass production of powder products.

Advantages of CIP

CIP offers several advantages in the manufacturing process:

• High compaction and uniform density, ensuring predictable compression during subsequent processes.
• Ability to process large and complex shapes, saving time and cost in post-treatment.
• Generation of large aspect ratio parts with uniform densities.
• Green strength allows for in-process handling and treatment, reducing production costs.

CIP Equipment Market Scenario

According to a report by DataIntelo, the Global Cold Isostatic Pressing (CIP) Equipment Market is expected to grow with a substantial CAGR during the forecast period. The surge in demand for CIP-processed products in industries like precision manufacturing, aerospace & defense, and automotive is driving the market growth.

Dry-Bag CIP

The dry-bag method is suitable for producing same-shaped parts. It involves using automated dry-bag isostatic pressing equipment, which was developed in the 1930s for compacting spark plug insulators. This process offers rapid cycles and is ideal for automated mass production of powder products.

Overall, CIP is a versatile manufacturing process that offers high green strength, the ability to process difficult materials, and the production of complex shapes. It is widely used in various industries and continues to grow in demand.

Materials Suitable for Isostatic Pressing

All Powders, Including Ceramics

• All powders can be cold isostatically compacted, including ceramics such as alumina spark-plug shells and silicon nitride components.
• Cold isostatic pressing (CIP) compacts powders to achieve a uniform density in the compact.
• Cold isostatic pressing ensures that materials have a uniform density and uniform strength.
• CIP can be used to produce difficult shapes and large-sized materials.
• Cold isostatic pressing is used in powder metallurgy to produce complex shapes and dimensions.

Tungsten Powders

• Tungsten powders are often used in cold isostatic pressing.
• Tungsten ingots of up to 1 ton can be pressed using the wetbag technique.
• Tungsten carbide rods can be produced by dry-bag isostatic pressing.

High Alloy Ferrous Billets

• High alloy ferrous billets are subjected to cold isostatic pressing before hot isostatic pressing (HIP).
• Cold isostatic pressing is used as a forming technique to guarantee pressing uniformity and achieve optimized mechanical performances.

A Wide Range of Ceramics

• A wide range of ceramics including alumina, silicon nitride, silicon carbide, and sialons are suitable for cold isostatic pressing.
• The range of ceramic products produced by the isostatic process is large and includes balls, tubes, rods, nozzles, fuse tubes, dinnerware, crucibles, and more.
• Cold isostatic pressing allows the production of simple-shaped small or large powder compacts with a uniform green density.

Isostatic pressing, both cold and hot, offers numerous advantages in terms of material density, strength, versatility, corrosion resistance, and improved mechanical properties. It is widely used in various industries, including powder metallurgy, ceramics, refractory metals, and more. The choice of materials suitable for isostatic pressing depends on the specific application and desired properties.

Design and Outcomes

Cold Isostatic Pressing

Cold isostatic pressing (CIP) is a technique used to produce components and preforms with greater size and complexity compared to uniaxial die compaction. It applies pressure uniformly over the entire surface of the mold, resulting in components with uniform density. This eliminates die-wall friction and allows for higher pressed densities and the production of complex shapes. Cold isostatic pressing can also produce components with longer length-to-diameter ratios and higher green strengths compared with die compacted counterparts.

Ceramic Billets

In the cold isostatic pressing process, ceramic billets are machined before final hot pressing. This machining step ensures the desired shape and dimensions of the components before they undergo the final hot pressing stage.

Technical Considerations

Compared to cold pressing, cold isostatic pressing offers several advantages. It provides more uniform densities, eliminates die-wall lubricants, and allows for the compaction of complex shapes. It also allows for the evacuation of air from the loose powder before compaction, resulting in increased and more uniform density. Cold isostatic pressing is particularly suitable for compacting brittle or fine powders and can be used to produce components with a wide range of sizes.

Benefits of Cold Isostatic Pressing

1. Uniform density: Cold isostatic pressing produces compacted pieces with uniform shrinkage during sintering or hot isostatic pressing. This results in little or no warpage and ensures consistent product quality.

2. Shape flexibility: Isostatic pressing allows for the production of shapes and dimensions that are difficult or impossible to achieve through other methods. This flexibility enables the manufacturing of complex components with intricate geometries.

3. Component size: Cold isostatic pressing can produce components of various sizes, limited only by the size of the isostatic pressure chamber. This capability makes it suitable for manufacturing both small and large parts.

4. Low tooling cost: Compared to other manufacturing methods, cold isostatic pressing has low tooling costs for short production runs. This makes it a cost-effective option for producing components in smaller quantities.

5. Enhanced alloying possibilities: Cold isostatic pressing allows for the enhancement of alloying elements without inducing segregation in the material. This opens up possibilities for creating materials with improved properties.

6. Reduced lead times: With cold isostatic pressing, complex shapes can be economically produced from prototype to production quantities. This significantly reduces lead times compared to other manufacturing methods such as forgings or machined components.

Hot Pressing

Hot pressing is another technique used in the powder metallurgy process. It is conducted at high temperatures, which allows for better compaction, higher green compact density, and higher strength of the parts compared to cold pressing. Hot pressing is particularly suitable for materials that do not sinter to high densities at low temperatures or when a pore-free state is required.

Conclusion

Cold isostatic pressing offers several benefits in the manufacturing of components and preforms. It allows for the production of larger and more complex shapes, provides uniform densities, and eliminates die-wall friction. Cold isostatic pressing is a cost-effective option with low tooling costs and reduced lead times. It is a versatile technique that can be used for a wide range of materials and shapes, making it a valuable process in the powder metallurgy industry.