A robust joining process using zinc can be an alternative to instant-cure adhesives.
Cto and General Manager
Fishertech, FisherCast Global Corp.
Peterborough, ont., Canada
When engineers evaluate ways of bonding small components, they seldom think of molten zinc alloy as an option. But an injected-metal assembly (IMA) process uses a molten zinc alloy in much the same way adhesives join components. IMA is not — in the strict sense — an adhesive surface bond. But it can replace adhesives in many cases. And it does so without the problems of peeling and thermal degradation inherent with conventional adhesives.
IMA closely resembles injection molding around inserts. A screwdriver with a plastic handle is an example. Zinc alloy can be likened to an instant curing "glue." And it can join a wide range of materials. IMA is an option for joining plastics, ceramics, glass, paper, engineered synthetics, textile fibers, and elastomers, as well as metals.
It also shares many advantages of adhesive bonding including excellent stress distribution, joining of dissimilar materials, and joining those of a different thickness. But the molten-alloy bond needs little or no surface preparation and solidifies (cures) in milliseconds. Zinc-alloy joints also perform well in harsh environments where only specialty adhesives and few, if any, injectionmolding resins would maintain their integrity.
The typical IMA process automatically positions components in a precision fixturing tool. It takes less than 20 msec for the molten alloy to inject into the die cavity between the components being bonded. The alloy solidifies in a fraction of a second and the component ejects from the tool ready for use.
The alloy has a predictable 0.7% shrinkage for which the tool design can easily compensate. And although the alloy injects at temperatures up to 815°F (430°C), bonding and solidification speeds prevent component materials from thermally degrading.
Zinc's extremely high thermal diffusivity (over 100° higher than that of plastic) lets solidification complete before the thermal influence zone of the heat-sensitive substrate material has progressed more than a few thousands of an inch. Any heat-distortion stresses are short term because solidification behavior of the zinc mitigates them within seconds.
IMA solidification speeds allow high volume production rates of up to 1,000 component assemblies/hr, depending on the complexity of the components joined.
Adhesives need a prepared surface to bond properly. In contrast, zinc alloy needs few, if any, preparations. Molten zinc alloy adhesion properties are more forgiving of substrate impurities and thus only need an industrially clean surface. Surface roughness won't trap air pockets either. The high fluidity of the molten alloy fills any voids and improves mechanical bonds because there is greater surface area for adherency.
High fluidity also provides good bonding strength in the presence of release oils as well as most coatings and paints that don't outgas at low temperatures. Zinc alloys also bond Teflon and don't need any primers.
Stress distributes uniformly throughout the joint because the molten alloy's high fluidity allows it to fill the space between components. The alloy's 0.7% shrinkage provides a shrink-fit adhesion with high stiffness and resistance to pull-off forces. In strength tests, components fail before the joining alloy bond breaks. Nondestructive testing with X-rays or eddy-current methods can verify joint integrity.
For most bonding applications, Zamak 3 zinc alloy is the top choice. Zamak (acronym for zinc, aluminum, magnesium, and copper) alloy contains by weight 4% aluminum and a small amount of magnesium, while Zamak 5 also contains copper for strength, hardness, and to protect against corrosion. These Zamak alloys have a hardness of up to 82 BHN (Brinell) and sheer and tensile strengths of 31 and 41 kpsi, respectively. Zamak 5 is 15% stronger and is a candidate for more-demanding applications.
Zinc-alloy bonds withstand long-term service in harsh environments. The alloy takes operating temperatures up to 230°F. Bonds between most materials maintain integrity even if they have different coefficients of expansion. If the shrinkage of a component material differs from the zinc alloy, shrink-to features may be designed to compensate. At temperatures down to –40°F, the metal bond won't become brittle or show undue stress.
Zinc alloy has excellent corrosion resistance under normal atmospheric conditions, and in many aqueous, industrial, and petroleum environments. It also resists gases and most solvents, with the exception of strong acids and caustic solutions.
The IMA bonds most small components that can be joined by adhesives, soldering, welding, brazing, and mechanical processes such as staking, press fitting, and crimping. Brittle and delicate materials are well suited for zinc alloy bonding, and the component assembly is immediately ready to use in the next production step.
An aluminum shaft, for example, can be joined to a glass disc in seconds, with no deformation or cracking of the glass. In a ceramic magnet and shaft assembly, an automatically fed shaft can be held within unparalleled concentricity, or the prefabricated shaft can be eliminated altogether by using a single operation to cast the shaft in zinc alloy and adhere it to the magnet.
Fishertech, FisherCast Global Corp., (705) 748-9522, fishercast.com