Laser Welding of Thermoplastics

 

Laser Welding of thermoplastics and thermoplastic elastomers has a number of advantages compared to the classic techniques like bond and bolt.

  • It is a nonimpact process and therefore virtually wear-free.

  • The energy insertion is local thus only on the welding location
  • It is possible to produce air-, gas- and waterproof and invisible welding lines

  • Usually there is no pre- or postprocessing required

  • 3D welding geometries are procurable

  • schmelzeaustrieb is low or non-existent

  • high reproducibility

 

 This technique has also some disadvantages.

  • Very high investment costs

  • Materials with various optical properties are needed

  • Extensive clamping fixture for complex and/or big component parts

 

 

Laser Transmission Welding of Thermoplastics

The contribution of energy into materials in most joining technologies plays a prominent role. Whether through thermal conduction (e.g. heated tool welding), convection (e.g. hot gas welding), friction (e.g. ultrasound- or vibration welding), induction (e.g. electro-magnetic welding), thermal radiation or through light/laser radiation. The laser transmission welding takes a very important part in these joining technologies. A laser transmitting and laser absorbant material is need for this kind of process. During welding the laser beam hits the laser transmitting join partner. The remaining radiation passes through the transmitting material and the laser energy is transformed into heat in the laser absorbant part. The absorbant thermoplastic is converted in its' plastic state. The laser transmitting partner also plastifys through joining pressure and heat transfer. Both smelter join and develop a weld seam after cooling.

 

 

 

 

 

Is this process safe?

 

The welding process can be monitored trough multiple techniques. The heat, which develops during the process on the weld seam and surfaces, can be measured by an infrared camera. If the indicated value differs from the target value, the actual value can be optimized or the component part discarded. Another option is to construct a weld seam ridge on the part. The laser beam melts this ridge and through the joining pressure both component parts move towards each other. Thus a measurable decrease of the component part heighth occurs.

 

 

 

 

Which thermoplastics can be used being laser welded

 

The materials for laser welding of thermoplatics have to fulfill basic requirements. Besides the necessary re-meltability the polymers should have overlapping softening ranges or melting points, so one thermoplastic will not be destroyed thermically while the other one hasn't started the fusible state. Another requirement is the compatibility of the molten masses, so they can form a bond. Furthermore the absorbant thermoplastic has to be laser wave compatible (e.g. 800 nm, 900 nm, 1064 nm). Two equal material can be welded the easiest like. ABS-ABS, or PP-PP. If the thermoplastics get along, there is also the possibility of unequal materials being welded like ABS-PMMA, PP-TPE, ABS-TPE,...

 

 

 

Transmission welding - influencing variables

 

Colorants (dyes or pigments) have a tremendous impact on the optical properties and therefore on the laser welding of thermoplastics. Dyes are soluble in the polymer matrix.They exist as atoms and moleculs. Pigments, however, have an kernel like structure and absorb the laser beam easier. The chemical compounding plays a very important role. The optical impression of color pigments is based on their ability to absorb a defined wave length range of the visible light. If the wavelength of the laser and the selective light absobtion match, the pigment absorbs the laser beam.

Each application should be tested separately, since it is very difficult to predict the influence of the colorant.

 

  

 

Glas fibers and glas balls do not absorb laser beams. The laser beam gets distracted through the deflection- and reflexion effects and the passing through the thermoplastic becomes longer. The absorbtion of the material increases.