Volumes of production

Can be used for fairly low production volumes as well as for metal injection-molded parts (see p.216), which require a minimum of 10,000 units.

Unit price vs. capital investment

Tooling costs range from low to high, depending on the specific process. The nature of the process also makes it highly efficient because there is no wasted material.

Speed

This varies considerably depending on the material and the method used. For example, once compacted into shape, in the pressureless method parts are put onto a continuous-belt furnace. Bronze typically needs 5 to 10 minutes at the center of the furnace to sinter, while steel needs a minimum of 30 minutes.

Surface

Although the finished parts can be porous, visually there is no difference in finish compared with, for example, a standard high-pressure die-casting (see p.219) or metal-injection molding. There is also a range of finishes that can be used on sintered parts, including electroplating, oil and chemical blackening, and varnishing.

Types/complexity of shape

Not suited to thin-walled sections. Shapes must not have undercuts.

Scale

Scale is limited to the size of the compacting press up to a maximum of 28 by 23 by 15 inches. Larger presses can produce approximately 2,000 tons of pressure, with parts requiring 50 tons per square inch.

Tolerances

Due to problems with shrinkage (there is a reduction in volume because of the increase of density as material flows into voids), high tolerances are generally hard to obtain unless a part goes through a secondary pressing and compaction.

Relevant materials

A variety of ceramics, glass, metals, and plastic can be sintered.

Typical products

One of the most interesting examples is the production of bearings, where the natural porosity produced by the process allows lubricants to flow through the actual bearings. Other common examples include hand tools, surgical tools, orthodontic brackets, and golf clubs.

Similar methods

Hot isostatic pressing (HIP) (p.170) and cold isostatic pressing (CIP) (p.172).

Sustainability issues

Sintering involves several stages of production, including intensive heating as the materials used have high melting points. This significantly increases energy consumption. However, the process allows for recovered waste materials such as iron to be reprocessed with excellent end results.

Further information

www.mpif.org

www.cisp.psu.edu

Hot Isostatic Pressing (HIP)

Product	knife from the Kyotop range
Designer	Yoshiyuki Matsui
Materials	zirconia ceramic
Manufacturer	Kyocera
Country	Japan
Date	2000

The ceramic blade of this quality knife retains its sharpness well, and has the added benefit that ceramic does not impart any taste to food. The visible pattern, known as the “Sandgarden effect,” is lasered onto the ceramic as a secondary process.

Hot isostatic pressing (HIP) is one of the main processes for forming materials that fall under the umbrella term “powder metallurgy” (a term that now also refers to other particulate materials, including ceramics and plastics). Heat and pressure, typically in the form of argon or nitrogen gas, are applied to powder resulting in parts with no porosity and high density, without the need for sintering (see p.168). The word “isostatic” indicates that pressure is applied equally from all sides.

Essentially, the process involves powdered materials being placed inside a container which is subjected to high temperature and vacuum pressure to remove air and moisture from the powder. The resulting highly compacted component is uniformly and 100 percent dense.

The process can be used either to form components from powder or to consolidate existing components. In the latter case, there is no need for a mold as the shape has already been formed. HIP is often used for castings that need to be made denser by eliminating porosity.

–  Produces parts of high density with no porosity.

–  Because the process produces a uniform pressure, the microstructure of the final components is uniform, without weak areas.

–  Capable of producing larger parts than is possible with other powder-metallurgy processes.

–  Suitable for producing complex shapes.

–  Provides an efficient use of material.

–  Improves toughness and cracking resistance in advanced ceramics.

–  Eliminates sintering (see p.168), which is a secondary process in other powder metallurgy-based methods of production.

–  Costly setup.

–  Shrinkage can be problematic.