Laser Spot-Welding of Plastics
Introduction
The use of high-power near-infrared di-
ode lasers for joining plastics is growing.
More development work is being per-
formed in institutions and in R&D labs,
and applications in industry are slowly
increasing. Several different approaches
are being developed for laser welding of
plastics. The main principle now used to
laser-weld plastics is known as “transmis-
sion welding.” Transmission welding has
demonstrated that precise, controllable
heating and melting of low melting point
thermoplastics can be produced at the in-
terface between a transmissive and an
absorptive plastic. The principles of trans-
mission welding were explained in a pre-
vious Application Note in this series.
Welding trials
Although it is often required to produce a
welded seam by relative motion between
a high-power diode laser beam and the
target, there are some situations in which
a single spot-weld can suffice. In addi-
tion, taking the relative motion out of the
welding process can lead to a simpler
analysis of that process and can help in
comparing the weld performance of dif-
ferent materials. Clear and colored acrylic
sheets (often referred to as Perspex
1
) are
widely used for signage and other appli-
cations where their optical clarity is re-
quired. It is also readily available in a wide
range of colors. Because of their ready
availability and high infra-red transmis-
sion, acrylic sheets have been widely used
for laser welding experiments. Similarly,
polycarbonate (PC), tradename Lexan
2
,
has also been used. As this material has
better mechanical properties, it is used in
more demanding applications, where, for
example, toughness is required. Therefore,
these materials were used for the first stage
of these trials.
A series of experiments was designed to
identify the laser parameters required to
produce high-strength spot-welds be-
tween these two widely used materials.
A large diameter laser spot was used to
reduce power density to an appropriate
level for laser-welding of plastics. This
spot was produced using a Coherent
FAP™ System, an 800
µ
m diameter fiber
and a 1:1 Optical Imaging Accessory to
Figure 4. Acrylic to acrylic weld
100
90
80
70
60
50
20
40
60
80
100
120
Spot Area (mm
2
)
Pulse Energy (J)
10 W
30 W
40 W
Effect of Energy Input, Acrylic to Acrylic
Max Spot Size
Application Tech Note
Figure 1. Spot welding data for acrylic
140
120
100
80
60
40
20
0
0
2
4
6
8
10
12
Spot Area (mm
2
)
Weld Time (sec)
10 W
20 W
30 W
40 W
Spot Area vs. Weld Time, Acrylic to Acrylic
Weld Damage Threshold
Figure 3. Comparing spot welding of pc and acrylic materials
140
120
100
80
60
40
20
0
0
2
3
1
4
6
5
8
7
9
10
11
Spot Area (mm
2
)
Weld Time (sec)
pc/pc, 10 W
ac/ac, 10 W
pc/pc, 40 W
ac/ac, 40 W
Spot Area vs. Weld Time
Weld Damage Threshold
Figure 2. Spot welding data for polycarbonate
140
120
100
80
60
40
20
0
0
2
4
6
8
10
12
Spot Area (mm
2
)
Weld Time (sec)
10 W
20 W
30 W
40 W
Spot Area vs. Weld Time, PC to PC
Weld Damage Threshold
provide a collimated beam of approxi-
mately 12 mm diameter. These parameters
give a power density ranging from
9–35 W/cm
2
. Results are given in Figures
1 and 2.
The weld damage threshold identifies the
point at which thermal damage was first
noted in the melt spot. This was usually in
the form of bubbles generated in the melt
pool. These were noted after the end of
the laser pulse, and were not typical of
shrinkage porosity. It was concluded that
these defects were probably water vapor
generated within the material by the laser
heating process.
Further important information can be ex-
tracted from Figure 3 that compares the
polycarbonate and acrylic materials. To
achieve a particular weld diameter with the
acrylic material requires less energy than
to achieve the same weld diameter with
the polycarbonate material. This is most
likely due to the higher thermal capabili-
ties of the polycarbonate material.
An alternative plot of this data for one
material combination, acrylic to acrylic, is
given in Figure 4. This plot emphasizes
the role of energy input and shows that
welding at higher average power and
higher average power density is more effi-
cient – less total energy is required to
achieve maximum weld-spot size. It
should be noted that average power, mea-
sured in watts, is simply the rate of input
of laser energy, measured in joules. Hence,
these results are readily explained by lower
conduction losses at shorter welding times.
Keeping the pulse duration to a minimum,
therefore, reduces heat loss through con-
duction to the component, which is always
a prime objective for a precision welding
process such as this.
To expand the scope of these trials, the la-
ser weldability of a completely different
type of polymer, polypropylene (PP), was
examined. Polypropylene is very widely
used in industry because of its very low
surface energy. This makes it a very diffi-
cult material to bond, either to itself or to
other substrates. Polypropylene is also at-
tractive because of its extremely low cost
and its recyclability. A standard polypro-
pylene homopolymer was therefore used
