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[Table of
Contents] |
- Introduction |
- Effects on welding quality compared through two types of energy distribution |
- About the laser scanning method - robot control/galvano scanning |
- Impact of energy distribution and directivity on welding |
- Diversity of welding method by means of galvano scanning - welding utilizing
thermal behavior |
- Conclusion |
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Introduction |
For laser
resin welding, it is said that a laser with top hat distribution like LD laser
is suitable. However, even with a fiber laser having a beam with Gaussian
distribution shape, welding is possible. Regardless of whether the laser is
converging light or parallel light, welding is possible. The welding method
should be selected according to the shape or size of the actual workpiece,
resin type, and demanded welding quality. It is important to optimize the absorbing
power and set conditions according to the material, demanded quality, and laser
characteristics. Such welding methods that use high directivity and galvano
scanning also exist. This section explains a welding case example in which our LAW
material is used to control the material absorption value (calorific value) by
means of material and galvano-type fiber laser used, and its utilization
method. |
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Effects on welding quality compared through two types of energy
distribution
This
section gives an explanation using semiconductor laser and fiber laser, which
have two different energy distributions. The differences between them in terms
of principle are explained in detail in various materials related to laser and
manufacturers’ websites. Therefore, those differences are omitted here. This
section eyes only the energy distribution that affects laser welding behavior. |
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- Energy distribution of laser beam (Schematic diagram) |
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The
distribution in the left diagram is called top hat distribution, in which
almost-uniform energy is radiated to the irradiation area and causes uniform
heat generation in the spot diameter. For example, LD laser is a representative
laser with such a distribution. On the other hand, the distribution in the
right diagram is called Gaussian distribution, in which energy tends to
increase toward the center of the irradiation area, causing a difference in calorific
value between the spot center and periphery. In other words, the temperature at
the center is higher compared to that at the periphery. For example, many fiber
lasers have an energy distribution like this. |
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Before
the introduction of data related to welding, the method to operate laser
(scanning) is briefly explained here. |
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About the laser scanning method – robot control/galvano scanning |
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<Source
of image: Panasonic Industrial Devices SUNX Co., Ltd.> |
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As shown in
the figure above, laser scanning is roughly divided into two types in general. One
type is the robot-driving or stage-driving scanning method, in which the
workpiece or laser head is moved to operate the laser. This method is subject
to the influence of the workpiece or laser head on the tracking accuracy
depending on the weight or driving speed of the workpiece or laser head. In the
case of the welding of the workpiece (part) corner, for example, accuracy also
depends on the corner shape. This method is commonly employed in LD lasers, and
the upper limit of the normal scanning speed that enables stable welding is
said to be around 100 mm/s. On the other hand, galvano scanning performs scanning
by reflecting a laser by controlling the galvano mirror. Therefore, it exhibits
excellence in tracking accuracy and high-speed scanning as fast as over 5,000
mm/s. This system is employed in lasers with high directivity such as fiber
lasers and is capable of operation for parts with complicated shape, but it has
the disadvantage of difficulty in laser irradiation on shadows when a
protrusion is present. For details, contact a laser equipment manufacturer. In
jig design, it is very important to realize a pressured state in consideration
of the influence of shrinkage and warping of the formed product. When a gap is
formed between parts to weld, heat conduction is not performed in laser transmission
welding and a welding defect is caused. |
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Next, an
actual welding case example is introduced. |
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Impact of energy distribution and directivity on welding |
Differences
between LD and fiber laser and between top hat distribution and Gaussian
distribution have been explained. This section explains how those differences
affect welding. Please take a look at the following data. |
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- Welding
strength and its stability |
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The
figure above shows an example of a comparison between welding behaviors caused
by different laser energy distributions when we used forming plates that were
made of PBT resin GF30% from company A and added with our eBIND LAW-9810 as an absorber,
and whose absorbing powers had been adjusted to respective laser wavelengths
(940 nm/1070 nm) of LD and FAYb lasers. In terms of both strength and
stability, the results showed differences from LD. This indicates that the heat
generation behavior of FAYb laser with Gaussian distribution were different. Due
to the difference in temperature distribution between the center and periphery,
the range where a favorable energy amount is obtained is narrow in both
portions compared to that in laser with top hat distribution and it is
considered that strength is likely to vary. Also, as excessive heating at the
center is likely to cause strength degradation and decomposition of resin, the
welding strength, which is strength per unit area, is also low. Regarding heat
generation behavior, the calorific value at the center tends to be large even
with a laser of top hat distribution, but the figure indicates that the degree
of heat generation also differs depending on the difference in energy
distribution. As there are differences in welding behavior between LD and FAYb
as shown above, it is crucial to select welding methods suitable for respective
lasers. The welding method considered to be suitable for FAYb is multi-scan
welding as explained below. |
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Diversity of welding method by means of galvano
scanning – welding utilizing
thermal behavior |
It has
been explained earlier that by using galvano scanning, high speed scanning is
possible. It has also been explained that the large energy of fiber laser at
the center affects welding behavior. As a method to utilize galvano scanning, this
section introduces the method to alleviate the disadvantageous influence caused
by energy distribution. In this section, its welding method is referred to as
multi-scan. This method requires more time than single scanning, but the
welding quality will be stabilized. |
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The above
figures show examples of the irradiation method. “Single scan” refers to an
irradiation method in which with a single laser irradiation, welding is completed.
On the other hand, “multi-scan” refers to a method in which laser is radiated
multiple times at high speed on the same track to gradually perform heating,
melting, and welding. If the track is a circle, laser is radiated like drawing
many circles with the same track. |
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The
multi-scan and single scan are compared with each other, and their advantages
and tendencies are summarized in the table below. |
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The
effect of limiting bead width shown in the table above is expected to be
particularly remarkable when laser with Gaussian distribution is used.
Utilization of multi-scan can alleviate disadvantageous phenomena related to
energy distribution in a Gaussian distribution shape. However, there are the
following two disadvantages. The first one is bad energy efficiency, which
means it takes more time in processing than a single scan. Another disadvantage
is that the influence of thermal factors (thermal conductivity of resin,
cooling effect of jig and the like, thickness, etc.) is great. Control of
thermal factors is very important for achieving stable welding. |
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In the
next column, we will introduce data related to actual welding behavior
(strength/air tightness evaluation). |
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The
material is PBT resin MBS230H03L (to product website) for automobile use from Automotive
& Industrial Systems Company, Panasonic Corporation. |
(As of
today, as the PBT resin material of the same series that enables black x black
welding, laser transmission black PBT resin MBS230H93L is released from Automotive
& Industrial Systems Company, Panasonic Corporation.) |
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As the laser welder, the VL-W1 series (to product website) from Panasonic
Industrial Devices SUNX Co., Ltd. was used. |
These are data obtained due
to the cooperation from the above two companies. |
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As the
workpieces, we used a box-shaped forming product like this photograph for
evaluation. |
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Example
of coloring of box-shaped forming product by eBIND® - PBT - |
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Conclusion |
In this
column, we have explained that condition setting according to use and
environment including top hat distribution and Gaussian distribution as well as
galvano scanning and multi-scanning is important for stable production using
laser resin welding. In the column we are going to publish next time, we are
planning to introduce data from the viewpoint of “airtightness,” which is a
very important theme for laser resin welding. |
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- eBIND,
LTW, and LAW are registered trademarks of Orient Chemical Industries, Ltd. |