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Welding Parameters
Five key parameters that affect weld quality are:
•
Frequency
•
Time
•
Melt Penetration
•
Amplitude
•
Force
Frequency is adjusted automatically by the system during
the Autotune process. Weld time is a trade off between
fast cycle times and deep melt penetration. A deeper melt
penetration increases weld strength.
Weld Phases - See the explanation at right. The four phases
are depicted graphically in Figure 9-1.
The amplitude and force determine how quickly the weld
phases progress. These in turn are determined by the type
of plastic and the part geometry. With welding parameters
held constant, cycle time increases with wall thickness.
Weld Time
The strongest welds are produced using the longest weld
time because this allows the melt flow to penetrate deeper
into the parts and eliminates any voids.
Weld Distance
Depending on the part configuration, the material displaced
will generally range from 0.75 mm to 1.25 mm (0.019
to 0.032 inch). This is only a broad estimate, and your
application may require a different value. The amount of
displacement required is affected by the flatness of the
welding interface.
With greater warping, more material needs to be displaced,
and the weld cycle will be longer. The strength of the
weld can be a function of the weld distance for some
thermoplastics and relatively insignificant for others as
shown in Figure 9-3. Do not confuse weld distance which
is the collapse distance with weld penetration which is how
deep the melt flow penetrates.
Dukane Manual Part No. 403-619-00
Section 9 - Optimizing Performance
WELDING PHASES
There are four phases in the vibration weld-
ing cycle.
PHASE 1: Vibration creates kinetic friction,
which generates heat at the joint interface.
No penetration takes place in Phase 1.
PHASE 2: The glass transition (or crystal-
lization) temperature is reached, and viscous
flow occurs. Viscous dissipation in the mol-
ten polymer generates heat. Lateral flow in
the polymer allows the penetration to occur.
PHASE 3: Both melt and flow reach a steady
state in which heat loss through the wall due
to flash equals heat being generated. The
melt flows laterally, and weld penetration
increases linearly with time. The penetration
required to reach a steady state condition
increases with wall thickness, but decreases
with welding pressure.
PHASE 4: Vibrations are halted, but weld
penetration continues because the clamping
pressure causes the molten polymer to flow
until it solidifies. The parts are held clamped
in the final position while they cool enough
to be handled.
Phase
I
II
Heat Loss
Equals Heat
Generated
Glass
Transition
Temperature
Time
Figure 9 - 1
Relationship of Melt Penetration to Melt
Temperature Through the Four Phases
III
IV
Melt
Penetration
Melt
Penetration
Temperature
Increasing
Linearly
With Time
Page 119
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