General Design Principles for DuPont Engineering Polymers - Module

For this reason, if the strength of the joint weld is
required to equal that of the reinforced resin, the joint
area must be increased in relation to the wall thickness.
This can be easily done with the shear joint.
Minlon ® Engineering Thermoplastic Resin
The comments made before about glass-reinforced
Zytel ® are valid for Minlon ® , matrix of the resin being
the same. Minlon ® contains 40% mineral filler which
allows an outstanding welding speed (30–50% faster
than Delrin ® 500). However, we have noticed a certain
sensitivity of molded parts to sharp angles, badly cut
gates or any other weak areas which can break under
ultrasound and particular attention must be paid to the
design of the part, more especially for Minlon ® 10B40.
Rynite ® PET Thermoplastic Polyester Resin
Because of its high stiffness this glass-reinforced
polyester resin is easy to weld. It is preferable to
always use a step-joint for such a resin which is often
used in very demanding applications (sometimes even
at high temperatures). An over-welding time may
generate burned material in the sonotrode area.
b) Effect of Moisture on Zytel ®
Nylon resins absorb somewhat more moisture from
the air after molding than most other plastics. When
released from joint surfaces during welding, moisture
causes poor weld quality. For best results, parts of
Zytel ® should either be ultrasonically welded immediately
after molding or kept in a dry-as-molded condition
prior to welding. Exposure of 1 or 2 days to 50%
relative humidity at 23°C is sufficient to degrade weld
quality by 50% or more as shown in Figure 11.60.
Welding parts at longer than normal weld times may
offset this loss of weld quality, but often at the expense
of heavy weld flash and marring under the
welding horn. As was shown in Figure 11.42, the part
temperature near the horn approaches that at the joint
during welding, and therefore lengthening weld cycles
may cause severe problems.
Parts may be kept dry for periods up to several weeks
by sealing them in polyethylene bags immediately
after molding. For longer periods, greater protective
measures must be taken such as the use of jars, cans,
or heat sealable moisture barrier bags. Parts which
have absorbed moisture may be dried prior to welding
in a drying oven. Procedures for this are described in
Zytel ® design and molding manuals.
c) Pigments, Lubricants, Mold Release Agents
The influence of pigment systems on ultrasonic
welding can be considerable. Most pigments are
inorganic compounds and are typically used in
concentrations ranging from 0.5 to 2%. With welding
equipment set at conditions which produce quality
welds in unpigmented parts, the quality of welds in
pigmented parts may be markedly lower. Poor quality
107
Figure 11.60 Effect on weld strength vs. time of exposure
(prior to welding) to air at 23°C, 50%
R.H. for Zytel ® 101 NC10 nylon
Weld strength, MPa
90
80
70
60
50
40
30
20
10
0
0 1 10 100
Exposure time, h
is reflected in welds of low strength and greater
brittleness.
The mechanism by which pigments influence welding
has not been identified to date. The presence of
pigments appears to influence the means of heat
generation at the joint. Often lower weld quality may
be offset by welding pigmented parts at longer weld
times than anticipated for unpigmented parts. Weld
times may have to be increased by 50% or more.
However, these longer weld times may produce
undesirable effects such as the formation of excess
weld flash and marring under the welding horns.
When ultrasonic welding is contemplated for assembling
parts which must be molded in pigmented
material, test welding of molding prototypes is
recommended to establish the feasibility of the
application. In many commercial applications, weld
strength and toughness are not critical requirements.
Use of dye coloring systems, which do not significantly
effect ultrasonic welding, may offer an alternative
solution.
The above comments apply also to the welding of
materials with externally or internally compounded
lubricants and mold release agents. Relatively small
quantities of such materials appear to adversely affect
the means of heat generation in the joint during
welding. While the increase in weld time may offset
some of this influence, the consequent problems
mentioned above may occur. If spray-on mold release
agents are used in molding of otherwise unlubricated
molding material, these parts should be thoroughly
cleaned prior to welding.