practiced in metal welding, except that beveling of thermoplastic edges is essential to obtaining a
satisfactory weld. Flux is not required in the welding of thermoplastics. However, some materials weld
more satisfactorily in an atmosphere of inert gas. Procedures used in evaluation of the strength of the
completed weld are likewise similar to the inspection and testing methods used in metal welding.
Because of the differences in the physical characteristics of thermoplastics and metals, there are
corresponding differences between welding techniques for metals and thermoplastics. In the welding of
metal, the welding rod and the parent material become molten and fuse into the required bond to form the
welded joint. There is a sharply defined melting point in metal welding. This is not the case in thermoplastic
welding. Because they are poor heat-conductors, thermoplastic materials are difficult to heat
uniformly. As a consequence of this poor heat-conductivity, and when heat is incorrectly applied, the
surface of the plastic welding rod and the parent material can char, melt, or decompose before the
material immediately below the surface becomes fully softened. The weldor will, therefore, have to
develop skill in working within temperature ranges that are narrower than those in metal welding. The
welder will have to become accustomed to the welding rod not becoming molten throughout. In fact, the
exposed surface of the rod will seem unchanged except for flow lines on either side. The finished weld
may appear incomplete to the novice welder. The welder must understand that the familiar molten crater
in metal welding is not found in plastic welding; only the lower surface of the welding rod becomes fusible,
while the inner core of the rod merely becomes flexible. Hence, the materials do not flow together as a
liquid. Instead, the welder must apply pressure on the welding rod to force the fusible portion into the
joint, thus creating the permanent bond.
Hot-Gas Welding
This method is the most versatile one and is useful with molded or formed articles. It is widely used with
all materials mentioned in this handbook. The tensile strength can be approximately 90% of the base
material or better.
There are three basic types of welding operations:
a) tack welding
b) hand welding
c) hand high-speed welding