Why use adhesives over mechanical fasteners?

Selecting an Adhesive or Mechanical joining method?

Some people are more inclined to trust a mechanical joining method, such as 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. However, listed below are some common limitations of processes that adhesives can help solve.

Adhesive or Mechanical Joining Method for Plastics?

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 join plastics/rubber to other substrates such as metal or glass
Cannot join 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
Have 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

Adhesive or Mechanical Joining Method for Metals?

As with plastics, methods of joining metals include mechanical fasteners and thermal joining methods. So, to determine 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
Join similar materials only

Common Reasons to Choose Adhesives over Mechanical methods

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.