The Injected Metal Assembly (IMA) process is a special method devised by Fishertech, a division of Fisher Guage Limited. It provides permanent, reliable assemblies by injecting a small amount of molten zinc or lead alloy into and around components being joined. The molten metal cools and solidifies, accompanied by minute shrinkage that locks the injected metal onto the components. IMA can assemble small parts made of materials including ceramics, glass, plastics, pressed paper, elastomers, fibers, and metals. The technique can be used in place of staking, brazing, soldering, electron-beam welding, riveting, press or shrink fitting, and crimping.
During assembly, components are positioned and held by a custom-designed tool carried in the operating head. Molten metal is injected into the tool cavity containing the mating surfaces of the parts. Grooves, knurls, undercuts, splines, holes, and other features are enveloped by the metal flowing into the cavity. The metal solidifies and shrinks to form a permanent bond. Injection and solidification take about 0.5 to 1.0 sec, after which the assembly is ejected.
Custom-made tooling, essential to the IMA process, serves several functions. It holds the parts in the exact relationship required for final assembly and also rejects oversize components. While the tool helps check tolerances on outer diameters, mating tolerances may be looser, as the injected metal will fill any gaps.
Tool inserts permit variations of a basic assembly, and changing tools can adapt IMA systems for a variety of assemblies. In addition, because the process is derived from die casting, tool cavities can be designed to cast components integrally with the hub. Building a mold into the tool allows users to form parts such as pinions, cams, hubs, sleeves, and rivets during one machine cycle. With appropriate tooling, several components can be joined to a shaft at once.
As assemblies leave the system, they contain no flash, runners, or sprues, and require no secondary finishing operations. Applications which benefit from the use of IMA include those where a component can be replaced by a shape formed as part of the joint, or where components would have otherwise required tight tolerances to ensure concentricity.
Required strength normally dictates the metal used. Lead alloy is common in lightly loaded applications. However, most applications use Zamak 3 zinc alloy. While molten, this alloy easily can be injected into thin cross sections and intricate shapes while maintaining 41,000-psi tensile strength upon solidification. Higher strength requirements are met by Zamak 5, which provides 47,600 psi strength.
IMA systems have size limitations. Typical size for a completed assembly is up to 6 in. in the largest dimension. When several parts are assembled, the number of holding features required may limit the size. On the other hand, components can be assembled to the ends of shafts and cables of unlimited length. Present IMA equipment can inject only up to 1 cu. in. of molten metal. This limits the size of the injected hub.
The permanance of IMA may be an advantage or disadvantage, depending on the application. Assemblies may be difficult to take apart without damaging the components.
Parts may be fed into the assembly tool manually or automatically, but injection and unloading are automatic. Systems can work as stand-alone units or as part of an assembly line. Production rates range from a few hundred to over 1,000 assemblies/hr. Variables include component complexity, number of parts, orientation of functional surfaces, and degree of orientation.