Monday, 29 March 2021

Joining Techniques - Laser Welding of Plastics

 


Hello and welcome to this blog post. Today we discuss laser welding of plastics as an efficient and effective way of joining plastic parts together.

Overview joining of plastics

There are three established ways on how to join plastic parts together: mechanical joining using screws and bosses, adhesive joining using different types of glues, and welding. Within the welding techniques we can further distinguish between vibration, ultrasonic, high frequency, extrusion, hot air, hot plate, and laser using infrared light as energy source.

Let us now focus on the laser welding technique.

Laser welding – 4 key working principles

Important for the understanding of how laser welding works are the four key working principles:

1.    Transmissive upper polymer layer: most thermoplastics transmit near-IR beam and the upper polymer layer needs to be transparent for wavelengths between 808 nm – 1064 nm. Laser welding radiation is outside the visible light spectrum of the human eye and therefore, polymer parts are laser transparent although they are not optically transparent through our human eye view. For example, you can have a black upper layer which is transmissive to a 980 nm laser. A minimum transmission rate of 3% is required.

2.    Absorbing lower polymer layer: the lower polymer layer is responsible for turning the remaining laser energy into heat at the surface of the absorbing layer. Additives, colorants and fillers all play a role for absorption. The most effective and most economical additive for the absorbing layer is carbon black soot. The amount is usually between 0.2 and 0.4% by volume and provides excellent absorbing properties to any thermoplastic.

3.    Contact between upper and lower layer: excellent contact during the welding process to ensure proper heat conduction. Contact is accomplished with various methods of clamping devices or special component designs.

4.    Material compatibility: two polymers which are to be joined can, but are not required to, be the same type of thermoplastic. The most critical material factors are melt-temperature, and the surface energy of the plastics. The most common thermoplastics such as PA 6, PA 66, POM, PBT, PC, ABS, PP, TPE and PE are easily weldable.

Advantages and disadvantages

In the table 1, advantages and disadvantages of plastic laser welding is shown:

 

Advatages and disadvantages of plastic laser welding

4 process types of laser plastic welding

Next we discuss the four most commonly used process types in plastic welding: contour welding, simultaneous welding, quasi-simultaneous welding, and mask welding.

In contour welding, the laser beam is focused into a point which moves relative to the component contour. It is suited for large parts and three dimensional parts.

In simultaneous welding, the entire weld seam is heated at the same time. This is achieved by using specially designed fiber-optics. The laser energy is formed into the pattern of the weld seam and is projected onto the entire seam simultaneously. This process is suitable for high volume runs that require ultra-low cycle times and little flexibility or variation.

Quasi-simultaneous welding is a combination of contour and simultaneous welding. In this process, a single, focused laser beam is guided by galvo-scanning mirrors, which traces the weld path multiple times at very high speeds. The entire joint line is effectively heated simultaneously.

Mask welding is an inflexible process since for each contour, a new mask is needed. Allover, it consumes much more laser power than needed and results in an inefficient process.

Overview joint types and clamping units

There are several possibilities of joints such as the lap joint (overlap of the two polymer parts), butt joint (end-on-end of plastic parts; more difficult to realize), radial joints (in case you weld pipes or cylindrical cases), and T-joints (also called collapsing rib joint since the inner rib will be heated and melted which results in a collapse of the rib).

Training video with application examples

I invite you to check out my training video on plastic laser welding which provides an introduction and includes also several welding examples:

 


Thank you for reading and #findoutaboutplastics

Greetings,

Herwig

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Literature:

[1] https://www.lpkf.com/de/branchen-technologien/laser-kunststoffschweissen/technologie-schweissverfahren

2 comments:

  1. I like this post,And I guess that they having fun to read this post,they shall take a good site to make a information,thanks for sharing it to me.plastic mold

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    1. Hello Tam, thanks for your feedback - glad to hear that you enjoyed the post! best regards, Herwig

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