ISOSTATIC PRESSING

The isostatic pressing process was pioneered in the mid-1950s and has steadily grown from a research curiosity to a viable production tool. Many industries apply this technique for consolidation of powders or defect healing of castings. The process is used for a range of materials, including ceramics, metals, composites, plastics, and carbon.

Isostatic pressing applies a uniform, equal force over the entire product, regardless of shape or size. It thus offers unique benefits for ceramic and refractory applications. The ability to form product shapes to precise tolerances (reducing costly machining) has been a major driving force for its commercial development.

There are three basic types of isostatic pressing (Table I.2). Cold isostatic pressing (CIP) is applied to consolidate ceramic or refractory powders loaded into elastomeric bags. Warm isostatic pressing (WIP) differs from CIP only in that shapes are pressed at warm temperature to about 100°C. Hot isostatic pressing (HIP) involves both temperature and pressure applied simultaneously to obtain fully dense parts (to 100% theoretical density), and is used mainly for engineered ceramics requiring optimum properties for high-performance applications.

Cold Isostatic Pressing (CIP)

CIP is mainly a powder-compacting process for obtaining 60 to 80% theoretically dense parts ready for sintering. Because of the good green strength obtained with this forming method, premachining before sintering is feasible without causing breakage.

When comparing uniaxial pressing to isos-tatic pressing, one can say that uniaxial pressing is more suitable for small shapes at high production rates. Die wall friction may result in non-uniform densities, especially for large aspect ratios (greater than 3:1).

CIP is slower than uniaxial pressing but can be used for small or large, simple or complex shapes. The uniform green density offers more even shrinkage during sintering, which is very important for good shape control and uniform properties. In addition, CIP does not require a wax binder as does uniaxial pressing, thus eliminating dewaxing operations.

Low-cost elastomer tooling is used for iso-static pressing, but close tolerances can only be obtained for surfaces that are pressed against a highly accurate steel mandrel. Surfaces in contact with the elastomer tooling may require post machining when tight tolerances and good surface finishes are specified.

TABLE I.2

Comparison of Isostatic Pressing Methods

Wet-Bag CIP

Two types of CIP methods have evolved over the years: wet-bag and dry-bag. The so-called wet-bag method (Figure I.4) is used for producing mixed shapes. It is estimated that there are more than 3000 wet-bag presses in use worldwide today, ranging in size from 50 to 2000 mm in diameter.

FIGURE I.4 Schematic of wet-bag CIP.

FIGURE I.3 Typical intercalation compounds. (a) Potassium in graphite, KC8, a prototype layer intercalate, (b) sodium in polyacetylene, [Na0.13(CH)]x, where x denotes infinitely repeating polymer chains.

A typical cycle time for a production press ranges from 5 to 30 min, depending mainly on size, powder volumetric compaction ratio, and pump selected. This speed is rather slow but can be improved by higher-volume pumps, better vessel use, and improved loading mechanisms. A 5-min cycle calculates to 24,000 annual cycles based on a 1-shift, 8-h operation, or 240,000 cycles in 10 years. That should be the minimum design value of a vessel. Therefore, it is very important to select the proper vessel design to meet the end user’s specified fatigue requirements as proven by the supplier’s theoretical analysis and past performance.

Dry-Bag CIP

The dry-bag process (Figure I.5) is more applicable to producing same-shaped parts. Automated dry-bag isostatic pressing equipment was developed in the 1930s for compacting spark plug insulators, which are exclusively produced that way today for worldwide distribution.

The dry-bag method involves the same flex-bag technology for powder containment as the wet bag except that a stationary polyurethane "master bag," called the membrane, is inserted inside the pressure vessel. The pressure force (water) is transmitted through the membrane to the mold and then to the powder, thus keeping the mold dry. Generally one part is pressed at a time, and loading occurs from the bottom.

A minimum of three cassettes (each cassette typically consisting of the mandrel, mold, and powder) are in motion simultaneously; one is pressed, one is filled with powder, and one is de-bagged. The cycle time is 1 min or less, which calculates to 120,000 cycles/year on a 1-shift, 8-h basis. This rate of cycling places a much higher demand on the pressure vessel fatigue, and proper design is critical to withstand the higher pressures. The dry-bag method is preferred over the wet-bag for automated production of same-size or same-shape parts in lots of about 50 parts per hour or above.

FIGURE I.5 Schematic of dry-bag CIP.

Warm Isostatic Pressing (WIP)

WIP follows the same path as CIP except the parts are compacted both at pressure and low temperature to 100°C. The pressing fluid water may be substituted with oil. To date, there are a few applications for manufacturers in the electronics industry as a cost-effective means of compacting different shaped parts.

Traditionally, a heated platen press has been applied in these applications. The problem associated with this method is the lack of uniform pressure all around the parts, resulting in dimensional variations from one side to the other. WIP, on the other hand, is a well-suited alternative for applying equal and uniform pressure on all surfaces.

Hot Isostatic Pressing (HIP)

HIP (Figure I.6) is a densification method for powders, compacts, or castings. It applies a gas pressure of 100 to 200 MPa and temperatures to 2200°C. An inert gas, most commonly argon, is used as the pressing fluid. The goal is to improve the performance of critical parts by eliminating defects and porosity resulting in fully dense compacts. Improvements in mechanical and physical properties, fatigue, surface finish, reliability, and/or rejection rate are possible using HIP.

FIGURE I.6 Schematic of conventional CIP.

Two HIP methods are used today for compacting parts: direct HIP, which applies to encapsulated powders, and post-HIP, which applies to pre-sintered compacts without interconnected porosity. (The HIP and CIP processes can also be combined, sometimes called CHIP. In CHIP, loose powder is cold-compacted, then sintered, then post-HIPed to achieve fully dense parts.)

Direct HIP involves a ceramic powder enclosed in a container usually made from an impermeable barrier, such as glass, ceramic, or refractory metal. Glass is the most common barrier today for direct compacting of ceramic powders and serves in two ways: (1) it acts as a barrier for consolidation and (2) it isolates and protects the powder from the processing gas.

Several precompacting methods are available for direct HIP, including injection molding, slip casting, CIP, or dry pressing. After preforming, the parts are glass encapsulated prior to HIP. A major goal is to form parts to near-net configurations to minimize final machining, which usually requires expensive diamond tooling.

Post-HIP is an alternative method and is a widely practiced way to HIP products such as oxide ceramics, tool bits, and ferrites. This method is only valid for materials that are able to sinter without pressure to close surface-connected porosity, equivalent to about 92 to 95% of theoretical density. The powder is shape-formed by either casting or CIP, is then pres-sureless-sintered, and finally post-HIP is employed for full densification. Post-HIP is also chosen when material decomposition is to be avoided; the post-HIP process allows the pressure fluid to contact the partially dense parts for reactive processing. For example, a mixture of 95% argon and 5% oxygen is preferred for oxide ceramics, whereas nitrogen gas is preferred for nitride ceramics.

The HIP process is rather slow, and a cycle may take 10 to 15 h, depending on part size, material, and furnace design. One of the reasons for the long cycle is that most installations use conventional furnaces that cool not only slowly but also nonuniformly. Advanced furnaces are available with uniform rapid cooling (URC) provisions that have been used for metals since early 1990. Shorter cycle times improve throughput, thus reducing processing costs.

The rate of cooling is programmable from 1 to 50°C/min. A variable-speed fan is used to select the appropriate rate to avoid cracking when processing thermally sensitive ceramic parts. This feature can reduce HIP cycles to less than 8 h but adds only 10 to 15% to the equipment price.

Currently, the largest HIP unit for processing ceramics is 0.64 m in diameter and is rated at 1850°C.

Versatile Process

Isostatic processing has found wide use for many different materials used in a variety of applications. CIP is mainly a powder consolidation process using inexpensive molds as barriers for compacting simple to complex shapes to 60 to 80% densities. The selection of a "wet bag" vs. "dry bag" method depends on the type, mix, and production lots of parts produced.

Warm isostatic pressing has found a niche in certain industries where combined pressure and low temperatures to 100°C are specified.

The HIP process is gaining momentum in the engineered ceramics field for obtaining near-net-shape and fully dense ceramics for high-performance applications. Either direct HIP or post-HIP may be selected, depending on the material or process specified.