General Design Principles for DuPont Engineering Polymers - Module

Figure 11.54 Tongue-in-groove
0.5
B
0.5
B
A
B
A
0.6 B
0.4 B 0.6 B
Dimension A 0.4 mm for B dimensions from 1.5 to 3 mm
and proportionally larger or smaller for
other wall thicknesses
Dimension B General wall thickness
Dimension C Optional recess to ensure good contact
with welding horn
Figure 11.55 Butt joint—variations
Strong joints can be achieved with amorphous resins,
however, it may be difficult to obtain hermetic seals in
complex parts.
The main feature of the butt joints is a “V” shaped
bead or “energy director” on one of the two mating
surfaces which concentrates the energy and limits
initial contact to a very small area for rapid heating
and melting. Once the narrow area begins to soften
and melt, impedance drops and further melting occurs
at a faster rate. The plastic in the energy director melts
first and flows across the surfaces to be joined.
Amorphous plastics have a wide, poorly defined
softening temperature range rather than a sharp
melting point. When the plastic flows, sufficient heat
is retained in the melt to produce good fusion over the
entire width of the joint.
Delrin ® , Zytel ® , Minlon ® and Rynite ® PET are
crystalline resins with no softening before melting and
a sharp melting point and behave different than
amorphous resins. When the energy director melts and
flows across the surfaces, the melt being exposed to
C
B
1.5
B
10°
10°
60°
102
air can crystallize before sufficient heat is generated to
weld the full width of the joint. It is necessary, therefore,
to melt the entire joint surface before significant
strength can be obtained. (In the case of Zytel ® ,
exposure of the high temperature melt to air can cause
oxidative degradation which results in brittle welds).
This phase of the weld cycle is very long as can be
seen in Figure 11.56 and 11.57, which are graphs
showing typical weld sequences for parts of Delrin ®
and Zytel ® using the basic butt joint.
The dotted lines indicate the weld time at which an
objectionable amount of flash occurs. This flash is a
limiting factor in most applications. Beyond this time,
results are extremely variable, especially with Zytel ® .
Figure 11.56 Butt joint—typical performance, burst
pressure vs. weld time
Burst pressure, MPa
15
12.5
10
7.5
5
2.5
Delrin ® 500 and 900 F
Delrin ® 100
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Weld time, sec
Figure 11.57 Butt joint—typical performance, burst
pressure vs. weld time
Burst pressure, MPa
12.5
10
7.5
5
2.5
Zytel ® 101 (DAM)
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Weld time, sec