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Why use adhesives over mechanical fasteners?

Selecting an Adhesive or Mechanical joining method?
Some people are more inclined to trust a rivet or weld over an adhesive to secure two things together. Adhesives, however, overcome the limitations found in traditional methods of joining plastics and metals.

When it comes to joining plastics, mechanical fasteners, ultrasonic welding, and solvent welding all have their place. Listed below are some common limitations of processes that adhesives can help solve;

Mechanical Fasteners for Plastics
Create stresses in the plastic/rubber that could lead to distortion or cracking
Require altering the design to allow for holes
Create appearances that can interfere with the styling of the end product
Concentrate all the holding power at the fastener location causing the applied force to be carried by a small area of the plastic or rubber.

Ultrasonic Welding
Cannot be used on thermosets
Cannot be used to join plastics/rubber to other substrates such as metal or glass
Requires tighter design tolerances (small area to concentrate the ultrasonic energy is desirable)
Limits the capability of joining different thermoplastics in the same assembly
Requires a large investment in machinery

Solvent Welding
Cannot be used to join plastics/rubber to other substrates such as metal or glass
Cannot be used with thermoset plastics/rubber
Has a tendency to cause stress cracking
Requires pre-set and consistent time between the application of the solvent and mating of the parts

Solvent Cements
Have poor resistance to heat and solvents
Emit fumes that may be toxic or flammable
Require ventilation
Require extensive drying time
Require design to avoid trapped solvent which may lead to weakness
Have poor gap filling capability
Require a long time to achieve full strength

As with plastics, methods of joining metals include mechanical fasteners and thermal joining methods. In determining if an adhesive or mechanical system is best for your application, review the limitations in these mechanical methods that adhesive joining can overcome listed below.

Mechanical Fasteners for metals
Create stress concentration points
Are visible additions to the joint or seam
Limited load distribution points

Welding, Brazing & Other Thermal joining methods
Distort components
Require secondary operations (painting, grinding) to achieve an aesthetically pleasing appearance
Are limited to similar materials

Throughout this list there are some common threads, in selecting an adhesive or mechanical joining system including a) engineering adhesives are designed to join dissimilar materials, for example, plastic to metal and b) adhesives distribute the stresses over the desired area instead of concentrating stresses, and c) that adhesives do not destruct or distort the substrates.

For further help and advice, please contact Permabond.