Edited by Martha K. Raymond
Michael J. Muir
Sales and Applications Engineer
Div. of Fisher Gauge Ltd.
Forming strong joints in assemblies is a never-ending requirement that follows engineers from design to design. Not only does the joint have to look good and be reliable, it also must be economical. Using an ordinary mix of mechanical fasteners or adhesives may work, but there’s a method that produces a joint while streamlining assembly and reducing hardware.
Injection Metal Assembly (IMA) forms a strong permanent bond between metal parts while eliminating fasteners. The process is well suited for assembling complex configurations and delicate material. By following basic guidelines for designing assemblies, manufacturers can easily assemble discs and shafts, multiple components, gears and shafts, and form terminations directly onto cable or wire.
Assemblies that are good candidates for the IMA process typically have characteristics, such as whether the part can be replaced by a shape which is formed as part of the joint, if the ID of a stamping or the OD of a shaft has tight tolerances for concentricity, or if several parts must be positioned in relation to each other — particularly if they are otherwise joined using multiple operations.
Besides using these three criteria, many design engineers choose the IMA process to cut costs and increase production rates. Depending on the number of components to be assembled, part complexity, and the degree of automation, production rates vary from 500 to 700 assemblies/hr. In addition, consolidating parts helps reduce inventories as well as reduce labor costs and operator training.
IMA also can reduce material costs. The process uses low-cost zinc alloy in place of expensive materials without sacrificing the quality of the finished assembly. A basic shaft stock diameter, for example, can be reduced from a hub or pinion diameter to a journal diameter, with the injected metal forming the pinion or hub.
Furthermore, when compared to traditional assembly methods, the IMA process also overcomes typical production and quality problems. The accuracy, part-to-part consistency, and dimensional stability attainable with IMA systems for large production runs gives engineers a few more reasons to consider the process.
How IMA works
IMA forms multiple-part components by combining die-casting technology with assembly techniques. In general, an IMA set-up consists of an assembly tool mounted in an operating head which is connected to a machine containing the melting pot for the zinc alloy. A typical assembly operation consists of the following steps. The parts are aligned into their final configuration and held firmly by a tool mounted in the operating head of the IMA system. The machine injects molten metal into a cavity where the parts meet. The metal flows in and around physical features such as grooves, knurls, undercuts, splines, keys, lugs, ridges, or holes. Taking only a millisecond to cool, the metal quickly solidifies and shrinks to create a strong and permanent lock. Finally, the machine ejects the complete assembly, which is cool enough to be handled safely without gloves and is ready to use.
Both fully and semiautomatic IMA systems can be integrated directly into an automated production line. Semiautomatic systems can easily be changed to handle variations in the components or to switch from one type of assembly to another. Fully automatic systems, on the other hand, are designed to produce large volumes of identical assemblies without requiring much operator input. In all systems, the cycle is controlled by a programmable logic controller. Systems can be dedicated to a single assembly operation, or interchangeable inserts can be used for multiple operations on one component or various assemblies. The components can be automatically fed from vibratory feeders, hoppers, or magazines. The systems are easy to operate, quiet, clean, and environmentally friendly, and they don’t require special installation procedures or services.
Besides being used strictly for assembly, IMA can be used to form attributes such as pinions or bosses on assemblies without additional operations. Engineers can also reduce secondary operations by using IMA to form the assemblies, including external threads in an injected-metal joint, identification marks such as logos or part numbers, and keys, lugs, or ridges can be added to center holes of gears and rotors.
A common application for the system is replacing rivets in assemblies because IMA eliminates having to form holes to tight tolerances. Die cast rivets of any size or shape can be automatically produced with better dimensional accuracy at a lower cost and at higher production rates. For components susceptible to stress concentrations, such as gears and pinions, components can be peened and staked, and designed to eliminate stress at critical points.
Components with any shape made from similar or dissimilar material can be joined using IMA. Assemblies can range from a shaft as small as 0.02 in. in diameter and a disc as thin as 0.02 in. to components up to 6 in. in the largest dimension. Larger parts can be joined by modifying the operating head equipment. The maximum mass of zinc alloy which can be injected to form a hub or termination is 2.7 oz.
Eliminating multiple operations or components
Multiple-step assembly activities can include one or more of the following processes: crimping, press fitting, shrink fitting, swaging, riveting, adhesive bonding, soldering, brazing, or welding, but many times these can be replaced by a single IMA operation. In assemblies with multiple components, one or more of the components can often be eliminated, by forming them in zinc alloy during the assembly process. Production rates are frequently doubled, while reducing costs.
An example of part consolidation using the IMA process is an assembly originally designed as four separate components: a plastic cam, steel pinion, steel shaft, and fineblanked steel gear which required one staking and two press-fitting operations. The assembly was expensive to produce and manufacturers continually had difficulty with shaft-to-gear and the shaft-to-cam runout. With the IMA process, production of the assembly becomes a single operation, and separate cam and pinion parts are replaced with a single component by casting them in zinc alloy as the gear and shaft are assembled.
To complete the assembly, manufacturers load the gear and shaft, either manually or automatically, into a specially designed assembly tool. The tool positions the components, and then molten zinc alloy is injected under pressure into a cavity at the intersection. As molten metal solidifies, it shrinks towards the center, creating a strong lock between the components. By using IMA, engineers were able to increase production rates to 500 units/hr. Precision dies prevent flash, so the assembly can move to the next production step without machining or finishing operations. IMA also lets manufacturers form diamond knurl on the shaft which boosts strength. In fact, strength tests show that the components fail before the zinc alloy hub.
Using zinc to form the assembly cut costs by replacing plastic and steel. Product quality is also improved, by holding shaft-to-gear runout to 0.008 in. and shaft-to-cast hub runout to a maximum of 0.003 in.
A variety of materials
The IMA process adapts to a wide variety of materials, including metals, plastics, ceramics, glass, paper, fibers, and elastomers. It is particularly well suited for fragile and heat-sensitive materials. For example, a paper label can be assembled to a disc, a glass disc joined to a shaft, or an end fitting formed on a fiber rope. Other examples include joining a ceramic magnet to a shaft without damaging the delicate magnet, and forming a metal cage around a magnet and the end of a shaft to permanently lock them together.
An appliance manufacturer took advantage of IMA to join dissimilar materials while replacing a metal gear in a motor drive assembly with an acetal gear. The molded gear, which is 25⁄8 in. in diameter and 7⁄8-in. thick, was to be joined to a 21⁄2-in.-long milled steel shaft. As originally designed, the gear had a square center bore and the shaft was manufactured from square stock, with the two ends turned to form the journals. The two components were then press fit, drilled, and pinned. The process was uneconomical because of the material wastage in machining the shaft.
Although the materials are dissimilar, the IMA process was evaluated to test joint strength. With only minor design modifications, the acetal gear was joined to a 5⁄8-in.-diameter shaft by a zinc alloy hub, which meets torque requirements of 100 lb-ft. No pre or postprocessing of the components is required, and the new design cut costs and runs more quietly than metal versions.
Accuracy, part-to-part consistency, and dimensional stability are easier to control over long production runs using the IMA process compared to other joining methods. First, the components are always positioned in correct relationship by the assembly tooling. Secondly, it is easy to accurately predict the fluidity, strength, ductility, and shrinkage of the zinc alloys. Dimensional variations in the injected-metal joint are virtually nonexistent. The cavity design allows for a consistent shrinkage of 0.7% for the zinc alloys used.
The zinc alloys in the IMA process provide excellent metallurgical properties. The strength of the injected-metal joint is often greater than that of the components joined. For example, on applying torque to a heavy steel shaft, it is the steel which shears, not the zinc alloy joint. Average tensile strength is 41,000 psi, compression strength is 60,000 psi, Brinell hardness is 82, and Charpy impact strength is 43 ft-lb.
An example of an assembly that increased strength and part-to-part consistency using IMA is two stainless-steel studs that form a crank shaft for a postage meter. Casting a zinc alloy connecting plate to the ends of the studs was more efficient than press fitting them to a steel plate. One stud measures 0.25 in. in diameter and 0.61-in. long, while the second is 0.20 in. in diameter with two opposite flats of 0.02 in. and is 0.85-in. long. To provide additional strength to the joint, knurls are formed on the ends of the studs to enhance the locking action of the injected-zinc alloy as it solidifies and shrinks onto them. The precision of the assembly tool ensures the correct positioning of the studs to the plate being cast to ensure for part-to-part consistency. The studs’ outside diameters are perpendicular to the connecting plate face within 0.013 in., and the flats on the 0.20-in.-diameter stud are perpendicular to the assembly centerline within 0.0014 in.
One reason for such precision is that the precision of the die cavity helps cast features to very close tolerances, typically 0.001 in. or less. Fit across the cavity’s parting line is within 0.0003 in. to prevent flash of molten alloy during injection. The cavity can also accommodate inserts which lets the manufacturer produce different variations of an assembly using one tool and reduces overall tooling costs.
A Test Of Strength
Zinc alloy is used for most joints. One of the methods used to test joint strength are destructive tests in which a gear-to-shaft assembly with a hub is loaded. Data shows that even with a high tensile steel shaft, the steel shears and the zinc alloy remains intact.