Why metal bonding?
No matter what you design, from computers to cars, assembly is always a challenge. What is the lowest cost, highest performance technique for joining parts and subassemblies? When it comes to metal-to-metal bonding, options include thermal joining techniques such as welding, brazing or soldering; mechanical fastening with hardware; and bonding using a metal bonding adhesive or tape.
Each method comes with its own advantages and disadvantages. For example, when mounting metal panels to the metal frame of a machine housing, solutions include metal-bonding tapes, metal bonding adhesive, welding, or mechanical fasteners, such as rivets.
So, how do you decide which method to use for metal assemblies?
Metal-Bonding Adhesive vs Welding
Welding is a very common technique for metal-to-metal assembly operations, and in some ways using adhesive for metal is very similar. For example, the lack of protruding fastener heads gives both welded parts and glued metal parts a more streamlined appearance.
However, welding also has a number of disadvantages that would not apply to a metal-to-metal adhesive bonds.
Welding introduces many complications and problems which often aren’t immediately visible.
- Thermal distortion – Every weld pulls and distorts the final product and can cause cracking.
- Welder skill – Anybody can bolt two pieces of metal together but even the simplest welding requires skilled labour or automation
- Variables which change with material and joint type – Welding mild steel is fairly straightforward but if you are welding a high alloy steel or, other metals you will need someone who knows what they are doing. These variables include:
- Voltage, current, wire feed speed, travel speed
- Heat input
- Filler metal
- Preheat, post heat, inter-pass temperature
- Shielding gas, flux
- Hydrogen control
- Mechanical weld adjustments – Grinding, peening
- Ability to join dissimilar metals is limited at best
- Many problems are not visible – Testing methods are used to find these discontinuities. The main four are liquid penetrant and magnetic particle for finding surface or near surface problems and ultrasound and radiography to find subsurface problems. All these methods require skilled labour.
In contrast, most adhesive products can be correctly applied with minimal training. This translates to more consistent quality and lower overall production cost.
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Metal-Bonding Adhesives vs Fasteners
Fasteners such as bolts and screws are very common methods of bonding for temporary joints. Fasteners are also suitable for semi temporary joints such as those in automotive assemblies. Nevertheless, problems may occur as fasteners require carefully placed holes, are prone to corrosion and may loosen over time.
This is where metal bonding adhesive come into picture: removable adhesives and tapes are suitable for temporary bonding, and they do not leave traces behind after removal. Adhesives are also suitable for semi temporary joints as they seal from corrosion, are generally lighter and cheaper than mechanical fasteners, and distribute the stress throughout the bond making the assembly stronger. Some metal bonding adhesives come with special properties such as chemical resistance and heat resistance. Heat resistant glue for metal is common in public transport vehicles, automotive industry and general industrial applications. Metal glue can also be used for supporting fasteners: threadlocking and thread sealing are common applications where a metal glue makes sure the fastener does not loosen over time.
If your design uses fasteners, click hereto receive more information about other adhesive options.
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Types of metal adhesives
- Epoxy adhesive is one of the strongest adhesives for metal. It exists in different types which have unique properties such as chemical resistance and heat resistance. Epoxies come as one or two component systems. Single component epoxy glue for metal cures as a result of additional heat. A 2K system is a mixture of two parts with react with each other starting the curing process. When looking for the strongest glue for metal, a 2K epoxy for metal should be considered.
Limitations of epoxy adhesiives:
- 2 part mixing is messy, time-consuming, not suitable for continuous production.
- Epoxy is brittle by nature, can’t take vibrations or flexing
- labour intensive, application cannot be automated
- 2 part cartridge requires special gun applicator and still not user friendly.
- it has a typical smell
2. Acrylic adhesive metal glue exists as two types: surface activated and and bead on bead acrylics. The latter refers to applying a bead of adhesive on both substrates before connecting them. The bonding starts when the parts come in contact with each other and become subject to pressure. Surface activated acrylics, in turn, require a water thin initiator applied to one substrate and the resin to the other.
- 2part system- time consuming, not friendly for high speed production lines.
- limited flexibility, joints cannot be subjected to flexing
- adhesive cost is high
3. Cyanoacrylate, also known as instant adhesive, is the super glue for metal to metal bonding. Cyanoacrylate adhesives is suitable for most metals as long as they are reactive. Therefore, the super glue for metal works better on brass and copper than on steel. Due to the excellent performance as metal glue, commercial cyanoacrylate adhesives are very popular among miniature and modeling hobbyists.
- brittle bondline after curing, cannot be subjected to joint movement and flexing
- debonds or bond fails if in contact with water continuously
- health hazard- bonds to skin instantly, while curing it emits vapours with a strong smell, eye irritant
- not suitable for large bonding areas, cannot fill high gaps ( > 1 mm)
4. Anaerobic metal adhesives are only used for glueing metal to metal as they require the presence of metal and absence of oxygen to be able to cure. Anaerobic adhesives are ideal for securing fasteners in terms of thread-locking and thread sealing as well as gasketing and retaining.
- cannot be used for high gap fill applications eg: sheet metal bonding
- cured adhesive is not flexible so cannot withstand flexing and joint movement.
- Suitable only for specific engineering application only. Not suitable for typical large surface area bonding
- Doesn’t work with non-metal substrates
5. Polyurethane sealant and adhesive : PU sealants are excellent for bonding / sealing metal to metal joints. In fact, in India, it is a popular adhesive used as a bodysealant for busbody manufacturing. It has high elongation, flexibility, bonds well on metals, paintable and at affordable cost too.
- Need to protect from UV light.
- contains solvents, not VOC free.
Supex PU 25: Moisture cure sealant suitable for metal bonding.
- Hardness (ASTM D 53505): 40±5 Shore A
- Elongation at Break(DIN 53504): ≥ 500%
- Modulus 100 % (DIN 53504): > 0,4 Mpa
- Tensile Strength: 10 N/mm2
6. MS polymer adhesive : This chemistry was developed to combine the weathering/ environment resistance of Silicones and high gap fill, flexibility, paintability of Polyurethanes. Also it is solvent and Isocyanate free, high strength with high elongation, paintable, UV resistant. It is a verstile bonder and bonds to most building material and engineering substrates except telfon, PP, HDPE, LDPE. This technology is already a dominant player in Japan, gathering pace in USA, Europe. It is widely used for automotive, Industrial and construction applications.
- Product cost is higher than Silicone or polyurethane sealants.
- Temperature resistance, works up to 100 C. is lower than RTV Silicones.
Supex Bond All HT – One part, moisture cure, modified silane, flexible, high bond strength, UV resistant, VOC free weather resistant, works underwater, high tack, verstile adhesive.
Bonding Metal to Metal
Structural adhesives can also be applied quickly and easily, such as with a dispensing gun, though they require curing time before the bond reaches full strength. Using faster, simpler assembly techniques can reduce labor and training costs, consequently reducing physical demands on human workers.
Another advantage of using adhesives in metal-to-metal applications is that they seal, potentially eliminating unnecessary steps in the assembly process. This seal can even be flexible, allowing for relative movement of parts without weakening the bonds. This makes adhesives suitable for applications involving temperature changes, weather and vibration, such as with many automotive and consumer goods.
The number of instances where adhesively bonding metal to metal is the most cost-effective solution may surprise you.
Surface Preparation for Metal Adhesive Bonding process
Selecting the best adhesive for your metal-to-metal application can be difficult. Different metals subjected to different environmental conditions will consequently behave very differently. As a result, there is no such thing as the strongest adhesive for bonding metal. That’s why it’s important to choose the right adhesive based on a number of design considerations.
Most metals have relatively high surface energy, allowing adhesives to wet the surface and thereby improving bond strength. However, painted and powder-coated metals present a different surface.
On a painted surface, the adhesive is not bonding to the metal but instead to the paint coating. Nevertheless, it’s possible to achieve a successful bond to the metal or its surface coating, depending on the bond strength you need. One option is to mask the mating surfaces before coating, preserving the metal surface. Another is to choose an adhesive solution appropriate for bonding directly to the painted metal.
A “powder coat” is an electrostatically applied resin to a metal surface, heat-cured to form a solid layer. Powder coats are significantly more difficult to bond than paint because they often present a lower-surface-energy substrate. This will affect the adhesive you choose.
Adhesives form the best bond to clean, dry, oil-free surfaces, including oils from workers’ hands touching the surface.
It’s also important to consider the surface condition of your substrates. Rough surfaces can make for stronger mechanical bonds by increasing the surface area of the joint or removing residual contaminants much better than cleaning alone.
However, this benefit is only realized if the adhesive can penetrate the small cracks and fissures and wet the entire surface. A bonding tape, for example, is not able to wet deep inside cracks or around raised scratches of aggressively abraded surfaces. Therefore, a liquid adhesive is best for highly abraded surfaces. Metal Bonding – Which Process?
When selecting a method for metal-to-metal bonding, the most important thing to keep in mind is how your chosen method will fit into your assembly process.
Are you looking for a solution that will fit into your current workflow, or can the workflow change to accommodate the new solution?
In addition to assembly considerations, you should also think about how the final product will be used.
All of these considerations—the assembly process, end use and overall cost—need to be taken into account when selecting an adhesive for metal bonding.
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