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PLASTIC WELDING TECHNIQUES

Many Methods are available for Joining and assembly of plastic parts through
application of either Direct or Indirect heat to the components to be joined. each
providing advantages with in its scope of applications:
Methods of Joining by Direct Heat Application: Joining by direct application of heat
provides an advantageous method of joining many thermoplastic materials that do
not degrade rapidly at their melt temperatures.
Hotplate welding: Hot plate welding works best with like materials and planar
joints. The parts are bought into contact with the hot plate until the joint surfaces
melt, then the hot plate is withdrawn and the melted surfaces are pressed together
to create a weld that is held under pressure until it is sufficiently cooled to remain
stable. The heating platen is coated with a fluorocarbon such as PTFE for non
sticking. Electric-strip heaters, soldering irons, and resistance blades are other
common methods of providing heat locally.
Heat sealing: If a product is made from thermoplastic film, that is, a material thinner
than 0.5mm, then pressing a hot bar against two layers will join them together; the
transmitted heat fuses the material along the line of the bar. For a 100-um thick film,
a weld can be formed in 1-3 s.
Hot air welding: uses a welding gun that has electric heating elements within in it
that is capable of heating either compressed air or an inert gas to 425-700°F and
forcing the heated air onto the substrate surface. A welding rod made of the same
plastic that is being bonded is laid in the joint with a steady pressure. The heat from
the gun is directed to the tip of the rod, where it fills the gap. Small Cracks, splits,
warping and even the loss of material can be remedied with the aid of hot-air
welding gun providing and convenient way to repair /weld plastic part saving time
and expense. Hot gas welding is usually a manual operation where the quality of the
joint corresponds to the skill and experience of the operator
Butt fusion welding : perhaps the single most important application of hot plate
welding, used extensively to join thermoplastic pipes for gas and water distribution,
effluent and sea outfalls.
Socket fusion welding - a shaped heated tool is used to heat the outside of the pipe
and the inside of an injection moulded fitting before one is inserted into the other.
Saddle fusion welding - to attach a saddle fitting to a pipe, a shaped heated tool is used
to heat the top outside surface of the pipe and the under-side of the saddle fitting.
Applications: Automotive- Hydraulic fluid reservoirs and battery cases, Building -
Unplasticised PVC door and window frames, Pipe work - Thermoplastic pipes for
gas, water and effluent
Laser welding: Laser welders produce small beams of photons and electrons,
respectively is used to raise the temperature above the melting point at the interface
of two parts to be joined. Thermal radiation absorbed by the work piece, forms the
weld. The plastics material between the joint interface and the laser source must be
transparent to the laser energy whilst the material below the joint interface must
absorb the energy. The ideal is that almost all the energy should be absorbed at or
near the surface of the energy-absorbent material Power density varies from a few
to several thousand W/mm2 but low-power lasers (less than 50 W/mm2) generally

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