What is laser transmission welding?
Laser Transmission Welding has been attracting attention as an alternative method of plastics joining other than hot plate welding, vibration welding and ultrasonic welding.

Laser transmission welding (LTW) is categorized as a heat welding method. Similar to hot plate welding, vibration welding and ultrasonic welding, each of these applications employ heat as a means to join without an additional adhesive.

However, in laser transmission welding (LTW), the heat source is different from that in other methods.
A hot plate (heated metal plate) is used as the heat source in hot plate welding, and heat from friction is used for melting in vibration welding and ultrasonic wave welding. In laser transmission welding, however, a plastic part is made to absorb the laser beam to heat and melt it.

You may have had the childhood experience of focusing sunlight on a point on a piece of black paper using a magnifying lens and starting a fire. In laser transmission welding (LTW), the laser beam serves as the sunlight and the plastic part serves as paper, and light energy is converted into heat energy.

We have learned that plastic is welded by means of a laser beam. How are the plastic parts actually bonded with each other?

The answer is that bonding is performed by placing a plastic part through which a laser beam is transmitted (light-transmitting resin part) to a plastic part that absorbs the laser beam (light-absorbing resin part). First, when a laser beam is radiated while those two plastic parts are pressed against each other at an appropriate pressure, the laser beam passes through the light-transmitting resin part and a portion of the light-absorbing resin part near the border of the light-transmitting resin part is heated and melted. Then, the heat is also transmitted to the light-transmitting resin part by heat conduction, and the light-transmitting resin part is melted and a molten pool is formed. Finally, the light-transmitting resin part and light-absorbing resin part are bonded with each other.

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 - Laser beam
To conduct laser transmission welding (LTW), first of all, the type of laser is important. The types of laser for resin include fiber laser [wavelength: 1070 nm], YAG (Yttrium-Aluminum-Garnet crystal) laser [wavelength: 1064 nm] and LD (laser diode) [wavelength: 808, 840, 940 nm]. Generally, by comprehensively judging the beam quality (heat source), power stability, and cost, an optimum laser is selected. Depending on the type of plastic, some materials absorb lasers of these wavelengths.

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 - Mechanism of laser transmission welding (LTW)

Features of laser transmission welding
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