QUASI-SIMULTANEOUS LASER WELDING OF PLASTICS

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					QUASI-SIMULTANEOUS LASER WELDING OF
PLASTICS – COMPARISON OF DIODE LASER
WELDING AND FIBER LASER WELDING
S. Ruotsalainen1, P. Laakso1
1
    Lappeenranta University of Technology, Lappeenranta, Finland
2
    VTT Industrial Systems, Lappeenranta, Finland


Abstract
Diode lasers have been commonly used for welding of plastic parts. Single mode fiber lasers
offer a competitive solution for quasi-simultaneous laser welding. Both lasers are possible
alternative for quasi-simultaneous laser welding of plastics in mass-production where welding
time is often the most critical parameter. The required welding time can be reached with
several different parameter combinations with the quasi-simultaneous laser welding
technique. The parameters that effect the most to the welding time are welding speed and the
number of scans around the welding path. Both of these parameters effect to the laser power
needed to achieve adequate weld strength and joint quality.
      To investigate the difference of used laser in quasi-simultaneous laser welding of
plastics, polycarbonate samples have been welded with diode laser and fiber laser. The
parameters varied were welding speed, number of scans and laser power. Welding time
determined used parameters. Three different welding times were used and welding speed was
chosen according to the number of scans and needed welding time. Laser power was chosen
according to used number of scans and welding speed. The width and the strength of the weld
were measured and the visual appearance of the weld was determined

Keywords: plastic welding, quasi-simultaneous welding, diode laser, fiber laser


1       Introduction
Through transmission laser welding of plastics with quasi-simultaneous technique has become
an interesting alternative for mass-production applications. This technique has several
advantages compared to conventional welding techniques like good visual properties of the
weld, minimal thermal, mechanical and electrical effect to the product, possibility to weld
hermetic 3D welds and thermoplastic elastomers [1].
      Several different laser transmission welding techniques have been demonstrated in the
past few years. These techniques are contour welding, simultaneous welding and quasi-
simultaneous welding. Quasi-simultaneous technique is very suitable for mass-production
because with this technique it is possible to achieve short welding times needed in the mass-
production applications. Short welding times can be achieved with several different parameter
combinations. Parameters which effect to the welding time are the number of scans and the
welding speed. Both of these parameters effect to the laser power needed to achieve adequate
weld strength and joint quality.
      Diode lasers have been used widely to weld plastic parts with laser transmission
welding technique. Single mode fiber lasers offer a competitive solution for diode lasers. Both
lasers are possible alternative for welding plastics with quasi-simultaneous laser welding
technique. Diode laser has a top hat beam shape which is more favourable in laser welding of
plastics than the highly gaussian beam of the fiber laser. To achieve a top hat beam shape with
fiber laser, beam shapers can be used which convert the characteristic spatial profile of the
beam into a more rectangular top hat shape. A top hat beam shape should in principle allow
higher energy input into the material than gaussian beam shape.
      This paper will present the results gained with the quasi-simultaneous laser welding
technique by using of fiber laser and diode laser. Welds were done with both lasers with top
hat beam profiles and difference between these two lasers is discussed based on these results.


2        Experimental procedure
The welding experiments were performed at Lappeenranta Laser Processing Centre (LLPC),
which is a research centre formed by Lappeenranta University of Technology and VTT Laser
Processing Group. Two different laser systems were used in the experiments, Fig. 1. The first
laser system used in the experiments was a Laserline LDF400-200 fiber coupled diode laser,
where the laser beam was guided via an Ø400 µm optical fiber to a scanner head. The diode
laser is operated at 940 ±10 nm wavelength and the focal length used were 163 mm resulting
an Ø1.0 mm focal spot on the work piece. The second laser system used was a SPI 150 W
continuous wave fiber laser. Laser beam was guided via a scanner head to the work piece. The
fiber laser is operated at 1090 nm wavelength and the focal length used was 300 mm resulting
an Ø 0.15 mm focal spot on the work piece. To achieve a top hat beam mode the beam was
formed using a refractive optic module to a Galilean design (piShaper from Moltech GmbH).
Because the diameter of the focal spot for the used optical configuration with fiber laser is
only 0.15 mm the beam was defocused to create wider welds. The defocus used with fiber
laser was + 48 mm.

    a)                                          b)




Fig. 1: Setup of the quasi-simultaneous welding equipments used in the welding tests, a) the
diode laser equipment, b) the fiber laser equipment.
      The material used in the experiments was an amorphous polycarbonate. The transparent
material was natural polycarbonate and the absorbing material was commercial black
polycarbonate. The welding tests were preformed on flat (60 mm x 60 mm x 1 mm) samples
using an overlap joint configuration and applying a constant clamping pressure.
      Limiting values for input energies were established by varying the laser power, the
number of scans and the welding speed. Welding experiments were performed using a
welding time of 0.5, 1.0 or 2.0 sec. Numbers of scans used were between 5 and 50 scans and
the welding speed was adjusted according to welding time (time = (scans x lenth) / speed).
The length of the welding path was a 150 mm. With all different parameter combinations the
laser power was increased until the material started visually to decompose. The aim of the
study was to establish the effect of the used laser type on the strength and quality of the
welds. Parameters and welding times were selected to fit the productivity requirements of
mass production.
      The strength of the weld was measured with a tensile strength test. Two tensile test
pieces were cut from each welded specimen and the strength was measured from the both
pieces. The value used in comparison was the average strength of these two samples from one
welded specimen. The width of the weld was measured from the pictures taken with a
microscope at the top of the weld. Also the destroying of the weld was investigated visually.
In Fig. 2 is presented the tensile test setup and the measuring of the width of the weld.


   a)                                           b)


                              Weld


                                                               Weld width




                  Tensile test piece


Fig. 2: a) Tensile test bars, b) weld width.
3     Results
3.1   Weld strength

The welding results with 0.5 sec, 1 sec and 2 sec welding times showed that a slightly higher
weld strengths (N/mm2) could be achieved using the fiber laser than diode laser. The
difference in the weld strengths between the diode laser and fiber laser became smaller as the
number of scans was increased and the welding time is increased. The used laser power was
adjusted such that the maximum power was used so that the weld was still visually
acceptable.
      The effect of the number of scans and the effect of used laser on the strength of the
welds at 0.5 sec welding time is presented in Fig. 3. With diode laser there where a slight
effect of the number of scans to the weld strength when the welding time was 0.5 sec. With
higher number of scans a little bit higher weld strengths were achieved. With diode laser the
weld strengths were between 32.3 N/mm2 – 33.4 N/mm2. With fiber laser the weld strengths
were nearly the same with all used number of scans. Weld strengths were between
34,9 N/mm2 – 35.1 N/mm2.
                                                   0.5 sec w elding tim e
                                     40

                                     35

                                     30
             Weld strength [N/mm ]
             2




                                     25

                                     20

                                     15

                                     10
                                                                                 Fiber laser
                                      5                                          Diode laser

                                      0
                                          0   10    20         30           40      50         60
                                                         Number of scans

Fig. 3: The effect of scans to the weld strength with diode laser and fiber laser. Welding time
was 0.5 sec.
      With 1 sec welding time the effect of used laser and number of scans was same as it was
with 0.5 sec welding time. Higher strengths were established when the fiber laser was used.
The effect of the number of scans and the effect of used laser on the strength of the welds at
1 sec welding time is presented in Fig. 4. With both used lasers the welds strengths seems to
be nearly the same with all used number of scans. With diode lasers the weld strengths were
between 31.2 N/mm2 – 32.0 N/mm2. With fiber laser the weld strengths were between
33.6 N/mm2 and 34.6 N/mm2.
                                                                              1 sec welding time
                                                        40

                                                        35


                                  ]
                                  2                     30
                                  Weld strength [N/mm
                                                        25

                                                        20

                                                        15

                                                        10
                                                                                                            Fiber laser
                                                            5                                               Diode laser

                                                            0
                                                                0        10    20          30          40        50             60
                                                                                    Num ber of scans
Fig. 4: The effect of scans to the weld strength with diode laser and fiber laser. Welding time
was 1 sec.
      When 2 sec welding time was used there were not any difference in the strengths of the
welds between the diode laser and fiber laser, Fig. 5. Weld strengths were nearly the same
despite of used laser and the used number of scans. The maximum weld strengths with 2 sec
welding times were between 33.1 N/mm2 and 34.0 N/mm2.
                                                                              2 sec welding time
                                        40

                                        35

                                        30
          Weld strength [N/mm ]
          2




                                        25

                                        20

                                        15

                                        10
                                                                                                                  Fiber laser
                                                  5
                                                                                                                  Diode laser

                                                  0
                                                        0           10        20           30          40          50                60
                                                                                    Number of scans

Fig. 5: The effect of scans to the weld strength with diode laser and fiber laser. Welding time
was 2 sec.
     Welding time found to have almost no effect to the achieved maximum weld strength.
Nearly the same strengths were achieved despite the welding time used. Joint strengths
welded with diode laser and 0.5 sec and 2 sec welding times were nearly the same. Little bit
lower strengths were achieved with 1 sec welding time, but this difference is not so significant
that we have to take it into account. Same effect can be seen with 10 scans (Fig. 6) and
30 scans (Fig. 7). Weld strengths with 10 scans and with fiber laser were between
33.9 N/mm2 and 35.0 N/mm2 and with diode laser between 31.6 N/mm2 and 33.6 N/mm2. The
maximum weld strengths with 30 scans and with fiber laser were between 33.6 N/mm2 and
34.9 N/mm2 and with diode laser between 32.0 N/mm2 and 33.4 N/mm2.
                                                     10 scans
                                  40

                                  35

                                  30
          Weld strength [N/mm ]
          2




                                  25

                                  20

                                  15

                                  10
                                                                          Fiber laser
                                   5
                                                                          Diode laser

                                   0
                                       0.0   0.5   1.0             1.5   2.0            2.5
                                                   W elding time [s]

Fig. 6: The effect of welding time to the weld strength with diode laser and fiber laser.
Number of scans was 10.


                                                     30 scans
                                  40

                                  35

                                  30
          Weld strength [N/mm ]
          2




                                  25

                                  20

                                  15

                                  10
                                                                          Fiber laser
                                   5
                                                                          Diode laser

                                   0
                                       0.0   0.5   1.0             1.5   2.0            2.5
                                                   W elding time [s]

Fig. 7: The effect of welding time to the weld strength with diode laser and fiber laser.
Number of scans was 30.
3.2   Weld width

      The load that the weld can carry (N/mm) can be optimized by controlling the width of
the weld. If we have wider welds the load that the weld can carry is higher. The width of the
weld can be increased by increasing the number of scans or increasing the laser power, if the
size of the laser beam at the work piece is kept constant.
      The weld width was found to be dependent on the number of times the weld seam was
scanned during the welding and the laser power used. With higher number of scans wider
welds can be achieved before the material in weld started to degrade. The welds made with
diode laser were wider than welds made with fiber laser because the size of the diode lasers
beam on the work piece was bigger than the size of the fiber lasers beam. The weld width as a
function of the number of scans at 0.5 sec welding time is presented in Fig. 8 and with 1 sec
welding time is presented in Fig. 9. In these pictures points which have been circled were still
visually ok; no degradation appears in the weld. After this point, if the laser power is
increased the weld starts to degrade. In these figures we can see also the effect of number of
scans and laser power used to the strength of the weld (N/mm). Because the width of the weld
is increased the weld can carry a higher load and higher strengths (N/mm) can be achieved
with diode laser than with fiber laser. But if we compare these results to the strength of the
weld (N/mm2) where the width of the weld is taken into account we can see that the strength
(N/mm2) is nearly the same with both used lasers. When the laser power was increased and
the weld started to disintegrate, the strength (N/mm) was still increased, because the width of
the weld is also increased. Similar results could be seen regardless of the number of scans.
                                                                0.5 sec welding time
                                      40                                                                            2.0


                                      35
           Load/ weld lenght [N/mm]




                                      30                                                                            1.5


                                      25                                                                                  Weld width [mm]

                                      20                                                                            1.0


                                      15

                                      10                                                                            0.5


                                       5

                                       0                                                                            0.0
                                           0               50               100              150                  200

                                                                      Laser power [W ]

                                       Load/lenght: diode laser, 10 scans          Load/length: fiber laser, 10 scans
                                       Load/lenght: diode laser, 30 scans          Load/lenght: fiber laser, 30 scans
                                       Width: diode laser, 10 scans                Width: fiber laser, 10 scans
                                       Width: diode laser, 30 scans                Width: fiber laser, 30 scans


Fig. 8: The effect of number of scans and laser power to the weld width and weld strength
(N/mm) with diode laser and fiber laser. Welding time was 0.5 sec.
                                                        1 sec welding time
                             40                                                                          2.0

                             35

                             30                                                                          1.5
         Load/ weld lenght




                                                                                                               Weld width [mm]
                             25
              [N/mm]




                             20                                                                          1.0

                             15

                             10                                                                          0.5

                              5

                              0                                                                          0.0
                                  0              50               100             150                200

                                                           Laser power [W ]

                             Load/lenght: diode laser, 10 scans         Load/length: fiber laser, 10 scans
                             Load/lenght: diode laser, 30 scans         Load/lenght: fiber laser, 30 scans
                             Load/lenght: diode laser, 50 scans         Load/lenght: fiber laser, 50 scans
                             Width: diode laser, 10 scans               Width: fiber laser, 10 scans
                             Width: diode laser, 30 scans               Width: fiber laser, 30 scans
                             Width: diode laser, 50 scans               Width: fiber laser, 50 scans

Fig. 9: The effect of number of scans and laser power to the weld width and weld strength
(N/mm) with diode laser and fiber laser. Welding time was 1 sec.



3    Discussion
Quasi-simultaneous transmission welding of polymers has a high applicability potential in
mass production. In these applications productivity is one of the most important factors.
Welding times around 1 sec have to be reached for the welding process to match the rate of
the production line. These welding times can be achieved with various different parameter
combinations. Usually most of the parameters are determined when designing the product and
by the laser system that is available, but some parameters like laser power, number of scans
and welding speed can be varied in order to achieve the best possible weld strength for the
application.
       The experiments described in this paper have been designed to meet the requirements of
mass-production and therefore the welding time was kept constant, at 0.5, 1 and 2 sec per
weld. Other parameters; the scanning velocity, number of scans and the laser power were
varied to establish the parameter window in which a good weld quality could be achieved.
       The weld started to form at the center line of the scanning path. The input energy was
the highest at the centreline due to the circular shape of the beam and the material began to
melt first in this region and that’s why material also began to decompose first in this region.
As the laser power or the number of scans was increased or the welding speed is decreased
i.e. the line energy increased, the temperature of the material reached higher values further
from the centreline and the weld became wider. In some certain temperature point, the weld
started to decompose and the weld was not anymore visually acceptable. Depending on the
applications there could be some limitations for the weld width and we can not increase the
width of the weld for an indefinite dimension. If we have wider welds the load that the weld
can carry (N/mm) is of course higher. But because we need different widths of the weld it is
also important to compare the weld strengths (N/mm2).
      The welding results showed that a slightly higher weld strengths (N/mm2) can be
achieved using the fiber laser than a diode laser. The difference between the strengths is so
small that in generally can be said that same strengths can be achieved with both used laser
types with the top hat beam mode. The effect of number of scans and welding time to the
weld strengths wasn’t so significant with used parameters that any conclusions for the best
parameter combinations can be made. Can be said that with all tested number of scans and
welding times the maximal weld strengths (N/mm2) can be achieved.
      A difference between different lasers and parameters comes significant when the weld
width and the load that the weld can carry are compared. The width of the weld can be
increased by increasing the number of scans or increasing the laser power, if the size of the
laser beam at the work piece is kept constant. With higher number of scans wider welds can
be achieved before the material in weld started to degrade. The welds made in this study with
diode laser were wider than welds made with fiber laser because the size of the diode laser
beam on the work piece was bigger than the size of the fiber laser beam. When the width of
the weld is increased weld can carry a higher load and higher strengths (N/mm) can be
achieved. Increasing the power or the number of scans eventually led to disintegration of the
material at the centreline. The strength of the weld (N/mm) still continued to increase at this
point, because the width of the weld is increased, but the visual quality became non-
acceptable.
      If we don’t have any limitations to the weld width we can use the beneficial properties
of quasi-simultaneous welding. By using the higher number of scans we can increase the weld
widths and attain a wider parameter window, especially with fiber laser. As can be seen from
Fig. 8, when the number of scans was 10 the weld destroyed earlier than with number of scans
30. With fiber laser the difference between different number of scans is bigger than with
diode laser. Use of fiber laser and 10 scans destroyed the weld when the laser power exceeded
98 W and with 30 scans the weld destroyed when the laser power exceeded 113 W.
Consequently the width of the weld was increased approximately 5 % and the strength of the
weld (N/mm) was improved approximately 5%.
      When the weld was scanned only few times the thermal cycle was strongly gradual and
the weld was easily destroyed. As the number of scans was increased, the ascending part of
the thermal cycle became smoother and the higher energies at the center of the beam did not
cause as severe temperature gradients in the material. This effect can be seen in the Table 1
where the processing parameters used for samples welded with fiber laser and 0.5 sec welding
time are compared. Even if we bring more energy to the weld with higher number of scans the
energy for one scan is still lower with higher number of scans.
Tab. 1: Used parameters with 0.5 second welding time with top hat beam profile.
 Number of     Welding       Laser       Total line   Line energy
   scans        speed        power        energy        (1 scan)
                [m/s]         [W]          [J/m]         [J/m]
      10          3            98           325           32.5
      30           9           113          375           12.5
      50          15           150          500           10.0


4     Conclusions
Quasi-simultaneous laser welding of plastics is a fast and flexible welding method to join
plastics. It can be also used in mass-production applications where short welding times below
1 second are required. The parameters have the greatest effect on the welding time are
welding speed and the number of times the weld seam is scanned.
      The experiments show that the strength of the weld (N/mm2) is nearly the same
regardless the welding time or number of scans used. Instead the load that the weld can carry
(N/mm) is dependent of the width of the weld. When the number of scans is increased wider
weld widths and that’s why higher strengths (N/mm) can be achieved. Results show that by
using of diode laser or fiber laser nearly the same weld strengths (N/mm2) can be attained. By
choosing of different laser and optic used we can effect to the weld widths and the load that
the weld can carry.


5     References
[1]   Polyweld:
      New Advances in Polymer Laser Welding, Brochure of the BriteEuram project –
      Polyweld, 22 pp (2001).