d. Heat too close or too far away from work
e. Heat source not centered on weld bed
7. Incorrect gas used for heat source
8. Improper or no bevel preparation
As stated before, a good plastic weld requires the following:
1. Thorough root-penetration
2. Proper balance between the heat used on the weld and the pressure exerted on the welding
rod
3. Correct handling of welder
4. Correct sheet preparation
To insure complete root-penetration, the welder must make certain that the proper gap is maintained at
the weld root, on both single ‘V’ and double “V’ butt joints.
Overheating of the weld is a common fault with the novice plastic welder. By paying close attention to the
color of the weld, the welder can tell whether overheating is occurring. When PVC is overheated, it will
begin to discolor from a yellowish tinge to a brown, and eventually will char. When PP or PE is
overheated, it will become transparent and eventually flow away like hot wax from a candle. ABS will
show decomposition craters.
Underheating of the welding rod, the base material, or both, will also result in a poor weld.
Whatever the cause, a poor weld bead should always be removed from the weld bed and a new weld
made. Under no circumstances should a new pass be laid over a burnt weld or one not properly bonded.
The novice welder has a tendency to push the welding rod rapidly into the weld before the heat from the
welding gun has brought it up to the proper temperature. This results in a stretched-rod pass, which
creates locked-up stresses in the weld. The release of these stresses by subsequent passes or by
service-impact loads, will result in weld cracks or separation.
Stretching of the welding rod can be detected by measuring the rod before welding and measuring the