Volumes of production
HIP is generally suitable only for medium-scale production quantities, typically less than 10,000 pieces.
Unit price vs. capital investment
The process requires large setup costs with expensive components.
Speed
Slow.
Surface
It is possible to achieve very high surface quality with ceramics, but other materials may require subsequent machining and polishing.
Types/complexity of shape
Simple to complex shapes are possible.
Scale
HIP caters for a range of sizes, from components measured in fractions of an inch to large-scale products up to several feet in length.
Tolerances
Low.
Relevant materials
Most materials can be used, including plastics, but the ones that are employed most commonly are advanced ceramics and metal powders such as titanium, various steels, and beryllium.
Typical products
The cost of the operation limits its use to high-spec components that require high physical and mechanical properties, such as turbine-engine components and orthopaedic implants. In advanced ceramics, HIP is used to form zirconia knife blades, silicon nitride ball bearings, and oil-well drilling bits made from tungsten carbide.
Similar methods
Cold isostatic pressing (CIP) (p.172). There is also a sort of injection molding that is suitable for ceramics.
Sustainability issues
Microshrinkage can occur during solidification, which can weaken the part internally and render it faulty, resulting in wastage. However, the defective parts can be salvaged and recycled back into the process to minimize material consumption and the use of raw resources. Moreover, the process causes the materials to strengthen and densify, which means that wall thicknesses can be reduced further to minimize material use.
Further information
www.mpif.org
www.ceramics.org
www.aiphip.com
www.bodycote.com
http://hip.bodycote.com
Cold Isostatic Pressing (CIP)
Product |
spark plug |
Materials |
alumina ceramic |
Manufacturer |
NGK |
The spark plug is a common product but it is made using a little-known process. The white alumina is the part that has been made using CIP.
The best way to sum up this process is to think of squeezing wet sand between your hands so that most of the water is forced out, leaving a fairly hard lump that resembles the inside of your hand. Although this sort of pressing can be done at elevated temperatures, cold isostatic pressing (CIP) is a method of forming ceramics or metal components at ambient temperatures from powders, and it involves the powder being placed in a flexible rubber bag, which squeezes around the mold when equal pressure is applied from all directions, compressing and compacting the powders into a uniform density. The process provides a uniformity of compaction around the entire component, unlike conventional forms of pressing, such as compression molding (see p.174), which require two-part molds.
The process is broken down into two types: wet bag and dry bag. In wet-bag pressing the rubber mold is placed inside a liquid, which, as you would expect, transmits the pressure from all directions. In dry-bag pressing, the pressure is exerted from fluid which is pumped through channels in the tooling.
– The main advantage of CIP over other powder metallurgy methods lies in its ability to produce parts with a uniform density, with predictable shrinkage rates on a larger scale.
– Low production rates.