Ceramic Industry

Uniform Powder Compaction

June 1, 2011
Cold isostatic pressing provides significant processing benefits for a growing number of new applications.

Millions of strong, durable products and components start out as powder, including advanced ceramics, metal alloys, cemented carbides, refractory materials, graphite, composites, and polymer compounds. Powders are typically mechanically compressed into green bodies, sintered and machined to final form.

Mechanical pressing is relatively fast and inexpensive, but it has some disadvantages. Often, only one compact can be processed per cycle. Tooling, especially for complex shapes in lower volumes, can be costly. In addition, friction from the forming die can cause uneven density, especially if the diameter-to-length ratio is greater than 1:2. This will result in distorted part dimensions after sintering.

Giant "giga-CIPs" are engineered for volume production of large parts and/or large batches. Their capacity per cycle is measured in tons rather than pounds.

The Isostatic Difference

Unlike mechanical force, which compresses a work piece from one or two sides, isostatic pressure is applied uniformly on all sides of an object. Pressures of up to 60,000 psi result in high, uniform density, particularly at the core of the object, which results in more predictable shrinkage during the subsequent sintering operation.

Cold isostatic pressing (CIP) consolidates powders in elastomer molds that are pressurized in a chamber through a liquid medium. The process typically compresses the loose powder from about 30-65% tap density to 60-80% green density. Depending on the size of the chamber and the molded item, dozens or even hundreds of parts can be compacted in a single 5-45 minute cycle.

CIP has the unique ability to form near-net shape objects with higher green strengths and useful isotropic properties (i.e., equal values can be measured along any axis). This saves time and cost in machining and other after-treatments while significantly reducing material loss. CIP can also produce large, complex geometries with aspect ratios greater than 2:1-all with extremely uniform density.

Emerging Applications

CIP has been used for decades in the production of ceramic and powder metal products, but a number of innovative new applications are now-or soon will be-on the scene. In the field of cosmetic dentistry, zirconia powders are being isostatically pressed into blocks and discs from which substructures are fabricated to give crowns and bridges a whiter, more translucent quality. CIPped zirconia has proven to be up to five times stronger than all-porcelain restorations.

Added strength can also be given to gypsum dental plasters. Tests have shown that the compressive strength of CIP-processed gypsum is more than triple that of conventionally processed materials.

Zirconia and certain other ceramics are 100% biocompatible, which means that they can be used in medical implants. CIP transforms the powders into uniformly dense structures with controlled porosity that can be fabricated into artificial joints and a variety of other prosthetic devices.

Another novel technique that is now under experimentation is a process that seeds shaped living bone composed of patient stem cells into porous bioceramic nanocrystals. The use of CIP to consolidate this compound will produce bodies with larger, more biologically active surfaces and will permit the loading of bone-promoting chemicals and drugs onto the nanocrystals during preparation.

CIP is also increasingly used to form the green bodies of a broad range of electronic components. In photonics, CIPped semiconductor nanoparticles add reliability to solar cells. For lead zirconate titanate (PZT) thick film, a piezoelectric ceramic used in microelectromechanical systems (MEMS), CIP is employed to reduce porosity and thus improve the film's bending and forming capabilities.

CIP has long been a standard operation in the production of thousands of fine-grain, high-purity graphite products. Recently, extremely large presses have been designed with work zones exceeding 90 in. (2.3 m) in diameter and 15 ft. (4.5 m) in height. This giant capacity permits the cost-efficient production of large rolls, bars and blocks that serve as economical raw material for graphite product manufacturers.

Mid-range CIPs are designed for pilot plants and production operations requiring automated cycle control.

Press Selection

Virtually all cold isostatic presses are available with working pressures from 5000-60,000 psi. Most manufacturers can custom-design a CIP to meet specific customer requirements, even up to ultra-high pressures of around 100,000 psi. Press sizes are generally based on pressure vessel volume and fall into three basic categories: laboratory/research scale, mid-sized production and high-volume production.

Laboratory and research scale CIPs are compact, self-contained, manually operated units designed for testing, feasibility studies and prototyping, as well as small batch production. Vessel diameters generally range from 2-6 in., with heights up to about 2 ft.

Mid-sized production CIPs offer more automated operation for pilot plant and higher volume production applications. The entire cycle, from vessel closing to depressurization, can be electronically controlled. Maximum work zone sizes are in the area of 16 in. diameter and 3 ft in height.

High-volume production CIPs are specifically designed for continuous operation and maximum cycle life. Most production CIPs are custom-engineered for the application with sophisticated digital process control and monitoring systems. High-capacity designs can feature vessel diameters of more than 100 in. and heights exceeding 18 ft.

For additional information, contact Avure Technologies Inc. at 3721 Corporate Dr., Columbus, OH 43231; call (614) 891-2732; fax (614) 891-4568; or visit www.avure.com.