Plastics and coloring materials
used in laser transmission welding

拡大する 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.

 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.
 - 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.
 - 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.
 - 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.
 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.
 - 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.
 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.

 <Mechanism of light absorbing resin part (schematic diagram)>
 - 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.

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