Plastic Welding

Introduction

Thermoplastics can easily be welded as can regain their shape after heating.

Most commonly welded plastics are PVC, polyethylene, acrylics etc.

They are welded by melting the surface to be joined & allowing them to solidify as they solidify

The methods for joining plastic components can be divided into three major categories:• Mechanical joining• Adhesive bonding• Welding

Categories

Mechanical joining: Mechanical fasteners can join two components quickly. Use of metallic or polymeric screws, snap-fit, press –fit. they do not provide leak tight joint.

Adhesive bonding : An adhesive is placed between the parts it serves as the material

that joins the part & transmits load . provide good properties and fully sound joints, but they are difficult

to handle and slow to cure. Welding:

In welding heat is to melt or soften the polymer at the interface to be welded.

Welding can be used to produce bonded joints with mechanical properties that approach those of parent material.

Classification of Plastic Welding

Welding process are classified by the heating method that is used. Two general methods are:

External heating Internal heating

External heating: It rely on convection & conduction to heat the weld surface. Hot gas, extrusion, implant resistance welding, implant induction.

Internal heating: It rely on conversion of mechanical energy into heat through surface

friction. Further divided into internal mechanical, Internal electromagnetic

heating Ultrasonic , friction , laser.

Hot plate welding:This process applies heat from a hot

plate that is placed between the plastic surfaces to be bonded. The surfaces are pressed and fused together to form a strong joint. Hot plates are made from aluminum alloys.

Contd…….

Applications of hot plate welding: Polyethylene (PE) pipe welding for gas line

installations. Fuel tank assemblies for the automotive industries.

Hot gas welding

This process utilizes the heat of hot gas, usually air that has been heated by electric heating elements in the welding gun.

This is similar to oxy-acetylene welding of metals. The only difference is that the open flame of oxy-acetylene welding is replaced by a stream of hot gas.

Compressed air, nitrogen, hydrogen, oxygen or carbon dioxide is heated by an electric coil as it passes through a welding gun

Applications of hot gas welding: It is used to fabricate pipelines, pond liners, and a

wide variety of vessels. Used to join broken plastics easily.

Advantages : Easy to use. Thermoplastics can be welded easily.

Limitations: Slow manual process cannot be used in mass

production.

Ultrasonic welding

This method uses mechanical vibrations to form the joint. The vibrations are of high frequency.

The parts to be assembled are held together under pressure between the oscillating horn and an immobile anvil and are subjected to ultrasonic vibrations of frequency 20 to 40 KHz at right angles to the contact area.

Alternating high frequency stresses generate heat at joint interface to produce a good quality weld.

ultrasonic welding is divided into two major groups: Near -field Far –field

Near-field : The distance between the horn and the weld interface less than 6mm.

Far-field: The distance between the horn and the weld interface is greater than 6mm.

Contd……

Advantages: Used in mass production because the welding time are relatively

short. Applicable for both amorphous and semi crystalline thermoplastics. Used to bond dissimilar materials.

Limitations: Horn size is limited to design constraints. The process and tools must be adapted for the materials to be

welded.

Applications : Food packaging, computer components.

Friction Welding

The friction welding of thermoplastic (also called spin welding) is based on the same principle as that employed for welding of metals.

In this process one of the substrates is fixed, while the other is rotated with a controlled angular velocity.

When the parts are pressed together, 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 Angular welding

Advantages: The advantages of friction welding are high

weld quality and the simplicity of the process. Limitations:

It is suitable only for applications in which at least one of the components is circular and requires no angular alignment.

Applications: Used to make thermoplastic manifolds for the

automotive industries

Laser welding

There are two modes of laser beam welding: Surface heating Through transmission infrared (TTIr) welding

Surface heating: The surfaces of the components to be joined are heated by direct IR/laser

exposure for a sufficient length of time to produce a molten layer, usually for 2 to 10 s.

Once the surface is fully melted, the IR/laser tool is with drawn from between the parts, the parts are forged together, and the melt is allowed to solidify.

TT Laser welding: It is based on the concept of passing laser radiations through one of the

component to be welded while having the second component absorb the light at the interface.

This absorption results in heating and melting of the interface and allows the parts to be welded.