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9
WeldTech - Hot Air Welder Instruction Manual
1.092909
3. BEAD FORMS
There are a great number of bead forms that can be specified; most common ones are shown on
the following page. Certain aspects must be taken into consideration before starting to weld. A
smaller diameter welding rod is preferable to a large one since it is easier and faster to control
the plasticizing process. The disadvantage is that multiple beads are required for thicker gauged
sheet. This could cause excessive heat stress due to the alternating heating and cooling cycles.
Therefore, it is sometimes advisable to choose a different method of welding, e.g., extrusion or
butt welding. Heat stress should also be taken into consideration when bead forms are selected.
For example, a butt joint performed with a double V design will generate less stress than a single
V design. Here both sides of the base material are heated up an even amount of times that
equalizes the expansion and contraction cycles.
Another consideration is the human factor. Hot air welding depends heavily upon the skill of the
operator, and sufficient margins should be designed into the finished product, A little extra time
spent on proper design and fabrication will help you to avoid the headaches caused by improper
design and hastily performed welds.
4. HEAT STRESS PROBLEMS
We will discuss heat stress in a little more detail in this section. Physical law dictates that
when materials are heated they expand, and they contract when cooled. This is known as the
coefficient of linear thermal expansion. Compared to metals, commercially used thermoplastics
have a much higher expansion factor under identical temperature conditions.
During hot air welding, the material will expand while it is
forced into a given position by the welding rod. When cooling
down, it will shrink back to its original volume but since it is
fixed , it will not shrink to its original shape. A sheet that was
straight may now be bowed. Due to their flexibility, thin gauge
materials may be bent back into their original shape Thicker
gauge materials create more difficulties. An experienced
operator will “pre-bend” the pads prior to the welding operation.
An additional factor to take into consideration is the eventual
service operating temperature of the finished product. A prime
example of this phenomenon experienced by many fabricators
is a PP tank with steel reinforcement. At a high operating
temperature, the tank walls will expand considerably. If the steel
bracing was fit too tightly around the tank, the tank will crack. To avoid this, service temperatures
must be taken into consideration during the design to allow for adequate tolerances.
Sheet Warpage caused by shrinkage
during the colling process
“Pre-Bending” prior to welding
After welding, the sheet pulls
itself straight
