Joining of plastics

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The joining processes play an important role in plastics processing .

These processes can be roughly divided into mechanical, thermal ( welding ) and adhesive connections.

Procedure

The following thermal bonding processes are mainly used for plastics:

Basically only thermoplastics can be welded.

The main methods for mechanical joining of plastics are:

Process description

Circular welding

With circular welding, the required melting energy is introduced by a relative movement between the circularly moving upper part and the stationary lower part.

During welding, one part is vibrated in a circular motion in two dimensions, the other part remains clamped immovably. The frequency used here is adapted to the material used.

The circular kinematics described above corresponds to that of orbital sander when vibrating . In contrast to these, the starting and end positions of the parts to be joined are always the center of the machine. The kinematics are absolutely circular and their size can be adjusted, which enables precision welding. Due to a spindle servo drive system, the welding frequencies are freely programmable, in contrast to spring-mass systems that work with a fixed frequency.

Laser transmission welding

There are two types of plastics for laser beams: transparent and absorbent . Most thermoplastics can easily be irradiated with typical laser wavelengths. Additions to the plastic increase the absorption. When the laser beam hits an absorbing substance, its energy is converted into heat.

In laser transmission welding, a material that is transparent to the laser wavelength lies over an absorbing material. A clamping tool presses the two joining partners together. The laser shines through the transparent material and melts the surface of the absorbing material. The contact surface of the transparent material also plasticizes through heat conduction.

With plastic welding, this process can be precisely controlled and checked in real-time using laser radiation. After the plastics have re-solidified, the contact zone is reliably and permanently welded. Great strengths and tightnesses can be achieved.

Ultrasonic welding

In ultrasonic welding, a generator converts mains voltage into high-frequency alternating voltage, which is converted into mechanical oscillation in the converter. The vibrations are introduced into the components to be joined via the sonotrode (welding tool), which causes the material to be heated and plasticized by internal molecular and external surface friction. After the sound introduction has ended, a short cooling phase under pressure ensures a homogeneous welded joint between the two parts to be joined. The process parameters frequency, amplitude, period of oscillation and pressure depend on the material properties and the component shape and size.

Further forms of ultrasonic welding are ultrasonic riveting, clawing and flanging. This can also be used to join materials of a different nature, whereby at least one of the two joining partners must be a thermoplastic.

application areas

The main areas of application for joining plastics are:

  • Medical technology
  • mechanical engineering
  • Automobile manufacturing
  • Pipeline and container construction

literature

Regulations / standards

A set of rules for joining plastics is being developed in Germany by the "Joining of Plastics" specialist group in the Technical Committee of the DVS - German Association for Welding and Allied Processes and published in the form of DVS leaflets and guidelines as well as in book form.

Books

  • AG W 4 of the DVS: Technical book Leaflets and guidelines - Joining of plastics . DVS Media, Düsseldorf 2010 (13th edition). ISBN 978-3-87155-224-3

Magazines and articles

Institutes

Web links