Manufacturing: Joining Plastic Components

Study Notes

Many products are too complex to model as a single part, making assembly of sub-components critical for manufacturing.
There are three major categories for joining plastic components: Mechanical joining, Adhesive bonding, and Welding.

Mechanical fasteners can quickly join two components using metallic or polymeric screws, clips, fasteners, snap-fit, and press-fit.

Adhesive is placed between the parts, serving as the material that joins the part and transmits load.
Adhesives provide good properties and fully sound joints, but they are difficult to handle and slow to cure.
Types of adhesive bonding include Cross-linking, Solvent bonding (welding), and Hot melt.

Welding involves melting or softening the polymer at the interface to be welded.
Welding can produce bonded joints with mechanical properties that approach those of the parent material.

This method uses mechanical vibrations to form the joint, generating heat at the joint interface to produce a good quality weld.
The parts to be assembled are held together under pressure between the oscillating horn and an immobile anvil and subjected to ultrasonic vibrations of frequency 20 to 40 KHz at right angles to the contact area.
Ultrasonic welding is divided into two major groups: Near-field and Far-field.
Near-field: The distance between the horn and the weld interface is less than 6mm.
Far-field: The distance between the horn and the weld interface is greater than 6mm.
Advantages: Used in mass production due to relatively short welding times, applicable for both amorphous and semi-crystalline thermoplastics, and can bond dissimilar materials.
Limitations: Horn size is limited by design constraints, and the process and tools must be adapted to the materials to be welded.
Applications: Food packaging, computer components, and standard disposable butane lighters.

Friction welding of thermoplastic (also called spin welding) is based on the same principle as welding metals.
The process involves rotating one of the substrates with a controlled angular velocity while the other is fixed.
Frictional heat causes the polymer to melt, and a weld is created on cooling.
Major welding parameters include rotational speed, friction pressure, forge pressure, weld time, and burn-off length.
There are four main variations: Linear welding, Orbital welding, Spin welding, and Angular welding.