Laser transmission welding

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The laser transmission welding is compared to other plastic welding methods such as ultrasonic welding or hot plate welding, one only since the mid-1990s industrially established process. All of these plastic welding processes involve a cohesive joining process in which the plastic is plasticized by applying energy.

Functional principle of laser transmission welding

Laser transmission welding is a one-step process in which the heating of the plastic and the joining process take place almost simultaneously. One of the joining partners must have a high degree of transmission in the laser wavelength range and the other has a high degree of absorption . Before the welding process, both components are positioned in the desired end position and the joining pressure is applied.

Principle of laser transmission welding

The transparent joining partner is penetrated by the laser beam without any significant heating. Only in the second joining partner is the laser beam completely absorbed in a layer near the surface, the laser energy being converted into thermal energy and the plastic being melted. Due to heat conduction processes, the transparent component is also plasticized in the area of ​​the joining zone. The internal joining pressure applied from the outside and the internal joining pressure resulting from the expansion of the plastic melt results in a materially bonded connection between the components. Common laser sources used in this joining process are high-power diode lasers (HDL, λ = 900–1100 nm) and solid-state lasers (fiber lasers, Nd: YAG lasers, λ = 1060–1090 nm), since almost all natural-colored and unreinforced thermoplastics have a high level in this wavelength range Have transmittance. The main requirement for the optical properties of the transparent joining partner is thus met. Absorbent pigments are added to the absorbent joining partner, which are mostly carbon black pigmentation, which results in the black color of these components for the human eye. However, so-called infrared absorbers also exist, which in the visible wavelength range can have a non-black color. In addition, various approaches to welding transparent components using laser radiation are currently being intensively investigated.

Process variants

Basically, the four different process variants contour welding, mask welding, simultaneous welding and quasi-simultaneous welding can be distinguished. The most important distinguishing features are the type of energy input and the beam formation. There are also variations of these processes, such as the TWIST process or GLOBO welding.

The selection of a variant depends on criteria such as B. the complexity of the joint geometry (2D or 3D), the number of pieces to be produced, the available investment costs and the requirements for the weld properties.

Contour welding

With contour welding, a point-like beam works off the geometry. This can be done either by moving the laser beam or the component. The main feature of the process is that each point of the weld seam only comes into contact with the laser radiation once. The size of the interaction time depends on two factors: the beam diameter in the focus and the feed. By following the entire seam contour, the seam length - in addition to the selected feed rate - influences the achievable welding times.

Mask welding

With mask welding, the weld seam geometry is created with the help of a mask, which is either followed sequentially in the contour process or fully illuminated in the simultaneous process. Mask welding is particularly suitable for complex seam geometries, where minimum seam widths of 100 μm are possible. The mask consists of thin sheet metal or metalized glasses.

Simultaneous welding

With simultaneous welding, several beams, if necessary shaped to match the seam, are used to irradiate the entire seam contour at the same time. This ensures an extreme reduction in process times (to <1 s) and enables the gap dimensions to be bridged by melting. In addition, the weld seam is stronger than, for example, with contour welding, since simultaneous welding has a longer interaction time. The more complex the seam contour, the more complex it is to create the appropriate jet geometry and, in particular, to set a homogeneous power density distribution.

Quasi-simultaneous welding

With quasi-simultaneous welding, a laser beam is deflected back and forth between the welding locations so quickly that heat is applied to all locations at the same time. It can thus be understood as a combination of contour welding with simultaneous welding. As with simultaneous welding, the high feed rate of the process causes the entire seam surface to be plastified.

TWIST process

The TWIST method ( T ransmission W elding by on I ncremental S canning T echnique, dt. About Transmitting welding by an incremental scanning technology ) was founded in 2009 by the Fraunhofer Institute for Laser Technology presented and combines the characteristics of contour welding with those of the quasi simultaneous welding. The feed movement of the laser is superimposed with a movement perpendicular to the direction of travel. The laser radiation is guided along a circular path along the feed movement and passes each contour increment several times. The coupling of the two directions of movement also enables the use of the high intensities of the focus diameter, which is just a few micrometers in size. Ultimately, this means that the smallest weld seams with widths <100 μm can be achieved. Since welding takes place at a very high path speed, a homogeneous energy input takes place over the weld seam within the contour increments. This in turn ensures a minimal depth of the heat-affected zone.

The advantages of the TWIST process are the high process speed and flexibility in the design of welding contours. It is particularly suitable for small and medium-sized series that require quick conversion.

GLOBO welding

GLOBO welding is a variant of the contour welding process. In contrast to this method, however, the joining pressure is only applied to the joint and not over the entire area of ​​the weld seam. The laser radiation reaches the joining plane via a ball which provides the necessary joining pressure.

GLOBO welding enables the welding of components with three-dimensional seam contours (e.g. car rear lights).

application areas

The main areas of application for laser transmission welding are:

  • Housing, containers and lights (automotive sector)
  • Sensors
  • Electronics (e.g. lamination of conductor tracks)
  • Medical technology

Advantages and disadvantages

Compared to other welding processes, laser transmission welding offers a number of advantages:

  • contactless energy input
  • no mechanical stress on the joining partners due to the energy input
  • no vibrating load on the joining partners
  • Can be used for both micro and macro areas
  • low heat affected zone due to locally limited energy input
  • no thermal stress on sensitive component areas
  • no surface markings from the welding process
  • great design freedom of the components to be welded
  • good automation and integration in series production
  • Welding of pre-assembled components possible
  • good external appearance for seams in visible areas

However, there are also some limitations:

  • Joining partners must have different optical properties
  • Laser absorbing pigmentation must be used
  • It is necessary to touch the joint partners with as little gap as possible
  • The weld seam must be accessible by the laser beam

See also

literature

Regulations / standards

Germany has a pioneering role in the industrial use of laser transmission welding. This is due not least to the industrial and academic structure in Germany as well as to cross-disciplinary and cross-company committee activities, such as As the DVS - German Welding and Allied Processes A set of rules for joining plastics is in Germany by the section "Joining of Plastics" in the Technical Committee (AfT) of DVS - German Welding and allied processes developed and in Published in the form of DVS leaflets and guidelines as well as collected in book form. The DVS guideline 2243 and its supplementary sheets contain specific information and guide values ​​for laser transmission welding of plastics.

Books

  • Paperback DVS data sheets and guidelines for joining plastics DVS Media, Düsseldorf 2010, ISBN 978-3-87155-224-3 .
  • UA Russek: Laser welding of plastics. Süddeutscher Verlag onpact, Munich 2009, ISBN 978-3-937889-90-0 .

Magazines and articles

  • The AG W 4.12 "Laser beam welding of plastics" introduces itself. 05 (2011) 02 Joining of plastics, pp. 82–84. (Joining of plastics)

Institutes

Sources and web links

  1. ↑ Archived copy ( memento of the original from July 8, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Research projects at the Rheinische Fachhochschule Cologne on laser transmission welding @1@ 2Template: Webachiv / IABot / www.rfh-koeln.de
  2. ↑ Laser transmission welding of thermoplastics (PDF) on wiley.com, accessed on November 14, 2016.
  3. ↑ Patent DE10350568A1 from February 10, 2005