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Oct 18, 2019

The 4 Pillars of Laser Polymer Welding

Dax Hamilton
Director and Global Product Manager - Laser Plastic Welding
Dukane

Mastering the 4 pillars of Laser Polymer Welding using 1 micron Laser will provide an excellent foundation for you to design-in the technology.

Pillar #1 – IR (Infrared) Transparent Upper Layer

The first layer of material that the laser beam encounters (the IR Transparent Layer) must allow a certain portion of the laser energy to pass through. It is widely recommended that at the very least, about 5% of this energy passes through to be able to effectively heat the lower absorbing layer before degradation or burning occurs in the IR Transparent Layer. I have personally seen some applications where less than 1% of the energy passes through but special welding parameters need to be used in order to create a stable process and avoid any surface degradation or burning. Of course, there are various misconceptions about the term “transmissive and/or transparent” when it concerns laser plastic welding. The first is that the material does not have to be clear or transparent to the human eye, it just needs to pass the respective wavelength of the laser. In most common laser polymer welding applications, the wavelength used is around 980nm, (1 micron) which is supplied by a solid-state semiconductor diode laser source.

Pillar #2 – IR Absorbing Lower Layer

The absorbing layer of plastic serves as the most important of the two. This is because the inception of the welding process happens at this layer. As the laser contacts this layer, the plastic heats up to the point where this heat is transferred to the upper layer and both plastics melt and mix as long as there is tight mechanical contact between the two. Part of this is due to the thermal expansion of both plastics as they heat and expand into each other, ensuring good mixing of the materials at the joint interface. In order for the process to work, the absorbing layer needs to be doped with a compound that absorbs the IR laser energy. The most common is carbon black and is doped in at 0.2% – 0.5% by volume. It is cheap and easy to come by but the resulting plastic in most cases is either grey or black. Titanium dioxide is also used in conjunction with carbon black when colors other than black or grey are desired. Certain pigments can be added with the right mixture of TIO2 and/or carbon black to be able to get virtually any color desired. If a translucent absorbing layer is required, there are a couple of different suppliers of a compound that can be doped into the plastic that absorbs the IR energy but still allows the plastic to be somewhat translucent. BASF has a product called Lumogen and Crystalyn has a product called Clearweld, both of which have a greenish hue. There is also a new product that is virtually clear when doped into the base resin, it is from a supplier called Brilliance Laser Inks.

Pillar # 3 – Tight Mechanical Contact Between Both Layers

Tight mechanical contact between the parts slated for welding is probably the most critical factor when it concerns getting a good and stable process. I can’t stress enough how important this is. It is also important to select the correct clamping style for the design of the part, whether it be a glass plate, inner or outer metal clamping masks, etc. The part itself needs to be able to handle the resulting forces applied while clamping to avoid issues with a deflection that may result in sporadic un-welded areas along the weld seam. In some cases, a thin transparent silicone rubber sheet in between the part and the clamping fixture will mitigate issues with non-planar surfaces and allow for full contact between the two pieces. The fixture or part nest is also a key item in the package as the fixture needs to provide rigidity under the part to allow proper mechanical contact to occur. The fixture needs to contact the underside of the part in all areas where there is a weld seam, no questions asked. (I will provide a detailed look at clamping technology in a future article.)

Pillar #4 – Material Compatibility

In general, like thermoplastics weld very well to themselves and there shouldn’t be any issues with weldability in this case. Most laser plastic welding applications are able to use as materials. However, there are certain cases that call for two different materials to be welded together. Whether it be for mechanical reasons, biocompatibility, structural, etc.

When welding dissimilar materials to each other, two factors need to be considered:

  1. The first factor is that the two plastics need to have similar chemical compatibility. For example, you would not be able to weld silicone to polycarbonate as their chemical compatibility is just too different. I am not a materials scientist so I will leave this to the experts in that field, however, there is some helpful information in a book titled: Laser Welding of Plastics by Dr. Rolf Klein which details this.
  2. The second factor is what I call the “process window” of the plastic. The process window needs to have at least a 50 °C overlap with the process window of the other plastic you intend to weld. To clarify, once heating commences, there is a point at which the plastic begins to soften called the glass transition temperature, and then as heating progresses, a point at which the plastic degrades or decomposes and turns to carbon. Most resin suppliers will provide the glass transition temperature in the specification sheet, however, the degradation or decomposition temperature is not always stated. With that in mind, the goal is that during material selection, the two plastics have a minimum of a 50 °C process window overlap in order to sustain a good and stable process. The greater the process window overlap the more stable the process will be.

In Summary

Laser Polymer Welding can seem like a bit of a black art, however, it actually makes sense if you break it down per the above 4 pillars. Anomalies may arise that will fall outside the techniques listed above, however, these can be taken on a case by case basis with the help of expert advice.

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