Volumes of production

Due to the length of time it takes to load molds into the tank and produce the buildup of metal, this is not a process for high-volume or rapid production.

Unit price vs. capital investment

This is an economical way of reproducing designs that are intricately patterned without needing a large investment in tooling. The cost of electroforming is partly determined by the amount of metal used, so the final unit price will depend on the surface area of the mold and the thickness of the deposited metal.

Speed

Slow, but depends on the amount of metal to be deposited.

Surface

Due to the nature of this process (the fact that it uses a mold and parts are built up gradually from tiny ions), the surface pattern can be highly intricate.

Types/complexity of shape

An ideal process for making multiple units of complex, highly decorative shapes. Making the mold from materials such as wax, which can be melted out after electroforming, means undercuts are possible.

Scale

The only limitation is the size of the electrolyte bath that holds the mold.

Tolerances

Unlike other metal-forming techniques, electroforming can produce extremely high tolerances, where the buildup of material is exactly the same anywhere on the part. This is unlike when a piece of metal is bent, a process that creates thick areas of material in corners.

Relevant materials

Nickel, gold, copper, alloys such as nickel-cobalt, and other electroplateable alloys.

Typical products

A great deal of highly decorated, hollow Victorian silver tableware was produced using the technique. Today, it is still used for highly detailed silverware, but it is also used for technical laboratory apparatus and in musical instruments—a French horn, for example.

Similar methods

Simple electroplating, and as part of the micro-molding with electroforming process (see p.250).

Sustainability issues

One of the major concerns with electroforming lies in the use of toxic substances in the electrolyte solution. However, systems have been introduced in which a special cleaning process removes any chemicals and metals from the water, which enables it to be recycled back into the process to reduce waste. Despite this, electroforming is still energy intensive as it based upon the use of a continuous electrical charge and so has relatively slow production rates.

Further information

www.aesf.org

www.drc.com

www.ajtuckco.com

www.finishing.com

www.precisionmicro.com

5: Into Solid

Sintering

Hot Isostatic Pressing (HIP)

Cold Isostatic Pressing (CIP)

Compression Molding

Transfer Molding

Foam Molding

Foam Molding into Plywood Shell

Inflating Wood

Forging

Powder Forging

Precise-Cast Prototyping (pcPRO^®)

The transformation of a material into a solid state

This chapter deals principally with a group of processes that fall within the realm of “powder metallurgy.” This term no longer adequately describes the advanced and wide-ranging technology that exists, nor the materials. The advanced materials that are used are not always in powder form, and they include ceramics and plastics as well as metals. Processes that in the simplest terms were based on compacting metal powders into shape and then sintering the “green” component to fuse the tiny particles together, can now be applied to many different (though mostly particulate) materials. The one exception to the powder metallurgy classification is forging, which involves transformation of an object from one solid state to another.

Sintering

including pressureless, pressure, and spark sintering, die-pressing and sintering

Sintering (a derivative of the word “cinder”) was traditionally associated with the manufacture of ceramic objects. The term is now, however, also widely used in the much larger manufacturing area of powder metallurgy. Essentially, sintering involves heating a particulate material to just below its melting point until the particles fuse together.

Various forms of sintering exist in the metals, plastics, glass, and ceramics industries. Pressureless sintering involves a powder being placed in a mold that is heated and vibrated, and then sintered. Pressure sintering involves powder being placed in a mold, vibrated, and then heated, with pressure applied either mechanically or hydraulically. In spark sintering, a pulsed current passes through the mold into the powder, generating heat internally (in contrast to the above methods, where heat is applied). Die-pressing and sintering are used predominantly for ceramic or metal powders. In this process, the powder is first die-pressed into a “green” state of the required form. This is then heated so that the particles sinter, or, in other words, fuse together.

Sintering is used to achieve high density in parts made from materials with high melting points, such as tungsten and Teflon where low porosity is needed. One of the characteristics of sintered parts, however, is that the porosity of the final component can be controlled, especially with certain materials. The porosity of some materials even after sintering can have its advantages: bronze, for example, is often used as a material for bearings, since its porosity allows lubricants to flow through. An alternative method, which eliminates porosity, is hot isostatic pressing (HIP) (see p.170).

There is also an advanced form, selective laser sintering (SLS) (see p.252), in which the application of heat is highly controlled. This method is used for rapid prototyping.

–  Suited to components with varying wall thicknesses.

–  Efficient use of materials.

–  Capable of forming materials that are difficult to deal with in other ways, especially very hard or brittle materials.

–  Parts have good nondirectional properties.

–  Can produce complex forms.

–  Requires a number of different stages.

–  Difficult to achieve high tolerances due to the decrease in overall volume in sintered parts.