Laser transmission welding becomes the technology of choice

According to LPKF laser plastic welding offers three main advantages: it is economical, it delivers assured quality and it opens up novel product layouts

Medical microfluidics demand high specifications, precision, cleanliness and a high quality joining method. The laser welding of thermoplastics is well established and goes back to approximately 2000.The technology joins together a laser-transparent and a laser-absorbent material. The laser beam passes right through the upper joining partner to hit the welding zone where it is absorbed by the lower joining partner. The lower part of the weld seam is heated up until it melts. 

In principle, it is possible to safely weld almost all thermoplastics as long as the two joining partners melt in the same temperature range.    

Thermal conduction passes the energy onto the upper joining partner which warms up to such an extent that the molecular chains diffuse in the affected areas. An adhesive bond is created when the parts have cooled down. The strength of a laser weld seam is similar to that of the solid material and almost achieves a weld factor of 1. This technology is also known as laser transmission welding.

In principle, it is possible to safely weld almost all thermoplastics as long as the two joining partners melt in the same temperature range. This is rarely a problem when the plastics are made of the same material. Common plastics are usually adequately transparent to laser beams in their natural form and therefore suitable for use as the upper welding partner. Pigment particles or soot particles are added to the plastic forming the lower joining partner to increase its absorption rates.    

The laser sources used in this technology usually have laser wavelengths in the near infrared range (NIR) – between 800 to 1100 µm – and power outputs of between 30 and 600 watts. Modern, low-maintenance diode or fibre lasers have replaced the laser sources used when this technology was first developed and which required a great deal of servicing. 

Because the laser beams are in the non-visible range, it is also possible to weld plastics which appear optically opaque. This increases flexibility: in a large number of detailed test series, LPKF has determined the optimal laser parameters for different combinations of plastics and colours.

Comparison between different welding methods

All of the methods for joining together plastics have strengths and weaknesses. The cost of modern laser welding systems is similar to those of other methods. The laser technology scores when it comes to tool costs, consumables costs and expenses associated with wear and tear.

Because there are no significant mechanical, dynamic or thermal stresses on the welding system and components, it is usually possible to use simple clamping tools and component holders. This and the software-controlled laser welding paths, enable economic production flows to be achieved, even with a range of different components.

Laser welding systems are almost maintenance-free, and can also undertake process monitoring functions. The laser system can identify irregularities during the ongoing process and guarantee production quality by separating good parts from bad parts. Moreover, all of the parameters can be recorded for end-to-end tracking and tracing. 

The properties of the products demanded by manufacturers are another means of selecting the best joining method. A separate tightness test can be dispensed with during laser welding thanks to the inline process control of the laser-welded components. For sophisticated components the microstructural quality of the weld zone is an important aspect. Unlike vibration welding methods, no particles are created during laser transmission welding, no additives are added to the components, and the weld seams satisfy even the highest optical specifications. 

Laser transmission welding has become the technology of choice when applications demand higher quality welds and when they involve sensitive components. The important arguments for laser welding technology are the project-specific costs – which can be up to one third lower – the high degree of flexibility of the systems, the integrated process control and the high yields of good parts – even if the quality of the upstream products fluctuates. 

Laser welding in microfluidics applications

Patients used to wait several days for laboratory results after visiting their doctor – analysis of blood is now done by mini laboratories within a few minutes.

This is one of the areas of application of microfluidics. Fine, highly precise channels are specified for microfluidic systems. These channels have to be sealed. Transparent material is required for many applications because the reactions are evaluated optically. A special laser welding method also provides the necessary technology for this as well.

LPKF ClearJoining technology uses a laser with a wavelength of 2 µm to penetrate both joining partners and focus the laser energy precisely along the welding horizon. The slight absorption value of the plastic is enough to create a reliable weld.