Non-destructive Testing
Visual Inspection
It is essential that the weld be properly inspected and evaluated before the part having the weld joint is
put into actual service.
A plastic weld can be partially evaluated for strength and bond by visual examination of the weld bead.
Incomplete weld penetration can produce a weld that appears sound from visual inspection but can be
actually defective.
A good weld will have flow lines or little wave-like lines along each side of the deposited weld rod and will
show no signs of decomposition. If these flow lines are present, continuous, and uniform, it is visual
evidence of a good weld. The continuity of these flow lines indicates there was sufficient heat on weld rod
to create flow and correct pressure on rod, by welder, to force the hot viscous material out of the weld
bed, bonding the plastic parts together.
It is difficult to detect air inclusion or cracks in localized sections of the weld joint. An ordinary inspection
light held obliquely to the weld angle will show air inclusions farily well, provided, of course, the plastic is
not pigmented. The light-inspection method is limited in scope to clear or translucent plastics.
Spark-Coil Testing
One of the best methods of plastic-weld inspection is the use of a high-frequency, high-voltage spark
tester that will show pores and cracks, in a weld that are not visible by any other inspection method.
The spark-coil tester sends a line of sparks through the cross section if the weld is porous or cracked.
Porosity is indicated by a straight line of sparks passing through the weld to the metal ground.
The voltage rating of the coil may vary from 5,000 to 30,000 volts depending on the thickness of the
plastic ground liner and the magnitude of porosity required by the inspection specifications.