The plastics and coloring materials used in laser transmission welding
(LTW) greatly vary depending on whether the part is a light-transmitting
resin part or a light-absorbing resin part.
The laser beam must pass through a light-transmitting resin part as much
as possible, and must be absorbed by a light-absorbing resin part, and
generate heat. The plastics and coloring materials need to be selected
according to those parameters. |
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On this
page, an explanation is given with a focus placed on the selection of plastics
and coloring materials only.
In general, coloring materials are divided into pigment-based
materials and dye-based materials. Pigment-based materials are divided into
non-organic pigments and organic pigments.
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| - Light-transmitting resin part (material) |
| Necessary
performance: Must transmit a laser beam (with part thickness of 3 mm, 15% or
more of laser beam transmittance is required). Preferably, 20% or more of infrared
ray transmittance. |
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| - Plastic and filler |
As a light-transmitting
resin part, it is advantageous to use an amorphous resin, but a crystalline
resin can also be used.
Glass
fiber scatters a laser beam, but it can be used according to the thickness of
the welded portion of the resin material.
Talc
reflects a laser beam and lowers laser transmittance.
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| - Pigment |
Pigments
do not dissolve in plastic but get dispersed as particles (agglomerates). For
this reason, pigments absorb or reflect (or scatter) a laser beam and hardly
transmit infrared rays (laser beam).
Therefore,
pigments are unsuitable coloring materials for the laser transmission welding
(LTW) method. |
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| - Dyes |
As dyes
dissolve in plastic at the molecular level, their coloring is uniform and their
hue is stable.
Dyes can
have a high infrared ray (laser) transmittance, although it depends on the
plastic. |
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| For
details, refer to the next page. |
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| - Light-absorbing resin part (material) |
| Necessary
performance: Must absorb a laser beam and generate heat. |
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| - Plastic |
| Contrary
to light-transmitting resin parts, when a laser absorbing coloring matter is
used, both amorphous resin and crystalline resins can be used. |
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| - Pigment-based absorbing coloring matter (example: carbon black) |
Generally,
pigments absorb, reflect, or scatter a laser beam.
In actual
use, attention needs to be paid because a pigment may affect the crystalline
nature of the plastic. |
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As a
common pigment-based absorbing coloring matter, carbon black is used.
However,
as carbon black strongly absorbs a laser beam, an abnormal heat-generation
reaction may be caused depending on its added amount or the scattering condition,
possibly leading to the temperature rising above the composition point of the plastic.
Consequently,
welding defects, gas generation during welding, generation of void (air
bubbles) in a molten pool or deterioration of the plastic may be caused. Therefore,
attention needs to be paid to the composition of the materials of light-absorbing
resin parts. |
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| <Mechanism of light absorbing resin part (schematic diagram)> |
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| - Dye-based
absorbing coloring matter (example: eBIND LAW) |
When only
pigment-based absorbing coloring matter is used, a laser beam may be
excessively absorbed and abnormal local heat generation may occur. On the other
hand, when only a dye-based absorbing coloring matter is used, absorption is
weak and it becomes difficult to sufficiently absorb a laser beam.
To address this issue, pigment-based absorbing coloring matter and dye-based absorbing coloring matter are used in combination to control the heat generation amount. In this manner, suitable welding conditions can be achieved and higher quality welded parts can be provided. |
