Metal spraying is used for the production of soft tooling. It involves spraying a thin shell of about 0.080 inch (2 mm) in thickness over a pattern and backing this with epoxy resin to give it rigidity.

Several metal spraying techniques are available. With most RP techniques, the models produced have a low glass transition temperature (i.e., the temperature where the material starts to change to a soft amorphous structure). Therefore, it is important to keep the pattern temperature as low as possible when spraying. If the temperature of the model increases sufficiently it will start to relax and distort, which results in an inaccurate tool.

The most popular techniques for use with RP models are spraying low melting point alloys (lead- /tin-based) with a gun similar to a paint sprayer and metal deposition with an arc system. The arc system feeds two wires into a gun and an electric arc is struck between them. This causes the wire material to melt and then a compressed gas atomizes and sprays it onto the pattern. The higher the melting point of the wire material, the more difficult it is to keep the pattern cool. Therefore, it is common to spray zinc or aluminum based alloys directly onto RP models. It also is possible to spray higher melting point materials onto RP models, but it is necessary to be a little devious. One technique is to apply a metallic coating by using electroless plating or physical vapor deposition. Once there is a metallic coating on the model, heat will be transmitted more readily across its surface.

One problem associated with metal spraying is that it produces shells with high internal stresses. It is possible to counteract these by simultaneously shot-peening the sprayed shell. Steel shot fired at the shell during spraying induces compressive stresses that counteract the tensile stresses.

Metal spraying is typically used on models that have large gently curved surfaces and is indeed most suited to this type of geometry. It is very difficult to spray into narrow slots or small diameter holes. When these types of features are included on the model, it is common to make brass inserts, locate them in the model and spray around them. When the model is removed from the shell, the inserts are permanently fixed into the shell. These inserts also are stronger than the shell material, which is weak and breaks easily if formed as a tall, thin feature.

Spray metal tools can produce more than 1,000 parts depending on the process, material being formed and the amount of tender loving care given to the tool. Clamping and injection pressures for metal-sprayed injection tools are usually less than those for steel or aluminum tools and may affect the mechanical properties of the injection-molded part. And because the shell is very thin and generally backed up with an epoxy-based resin, the thermal conductivity of a metal-sprayed tool is less than that of an aluminum or steel tool. This also will affect the mechanical properties of the injection-molded components and will increase cycle time. Some plastics are much more corrosive and abrasive on tool faces. This can be partially overcome by a variety of techniques, such as plating the tool surface with nickel or chrome, or using aluminum or steel inserts.

Spray metal tools have been used in many applications including sheet metal forming, injection molding, compression molding, blow molding and pre-preg sheet lay up. Various plastics have been molded including polypropylene, ABS, polystyrene and difficult process materials such as reinforced nylon and polycarbonate.

The main advantage of spray metal tooling is that you can produce large tools quickly. The main disadvantage is that it may be difficult or impossible to spray into narrow slots or deep holds – meaning that the part geometry must be relatively simple. Molds are not particularly strong and the process requires special equipment and special operating environment.