The usual spark-coil tester used in plastic-weld inspection is in the 10,000- to 15,000-volt range, the
average being 8,000 volts. Care should be used not to apply voltage that could create porosity by
inducing decomposition.
In using a spark-coil tester, the pencil tip of the instrument is run along the weld area. If sparks appear,
there is porosity in the weld. In testing welds on units with no metallic parts, a ground must be provided
for the spark-coil tester. This is accomplished by use of metal foil, or electrical conducting paint to create
the ground connection.
Radiography
Radiography is perhaps the most efficient method of plastic weld inspection because it shows defects in a
plastic weld that are not readily discernible by any other inspection method. It gives a complete, detailed
picture of the weld joint, and a permanent record of the weld. The one disadvantage of radiographic weld
inspection is cost. Inspection of welds on a large unit or assembly by radiography would be expensive.
However, this expense can be justified when the unit is to handle dangerous chemical materials where
weld failure would be disastrous.
Chemical Test
Immersion of a PVC-weld test-specimen in acetone for two to four hours is a chemical method of plasticweld
inspection. Faulty welds will separate from the base material, and strains and stresses in the weld
will be indicated by swelling of the material. Another chemical inspection of plastic welds is by the dyepenetrant
method. A dye penetrant is painted or sprayed on a weld. A poor weld will be disclosed by the
penetrant seeping through and discoloring the weld.
In cases where it is known that the temperature and chemical action on a plastic will approach the
chemical limits of the material, weld specimens should always be tested chemically. Butt welds and
corner welds should be immersed in the chemical intended to be used, put under stress, and the
proposed temperatures maintained. If stress cracking occurs, or a weld separation is evident, the material
is unsatisfactory for this service.