TWI Industrial Member Report Summary 826/2005
By I Jones, S Rostami and N Woosman*
The Clearweld® process for laser welding of thermoplastics was invented at TWI in 1998. 
It offers a method of welding similar thermoplastics, in an overlap configuration up to many millimetres thick. Transmission laser welding often uses a carbon black loading in the bottom plastic layer as an absorber for the laser beam. The Clearweld® process uses infrared absorbers with very little visible colour. Plastics can be welded using lasers as long as one of them transmits the infrared (IR) laser radiation. Laser welding offers the opportunity of welding plastics using a rapid, automated process, with no vibration and minimal distortion. Clearweld® offers the additional advantage that it may be used with any colour of component, and two clear parts can be welded with a clear joint region. Clearweld® is being commercialised by Gentex Corporation.
Using laser radiation as the energy source enables precisely controlled welding both in terms of the position and the amount of applied energy. To optimise this, methods are required to verify the correct location of deposited laser absorbing dye, the welding process and to view the weld after completion to ensure the quality and performance of the joints produced. The ideal monitoring method would provide real time measurements of the weld as it forms, allowing continuous control of the process in a feed back loop, but much can also be gained from methods that provide information about the parts and process before and after welding. To some extent the Clearweld® process makes monitoring more complex, because the weld is difficult to see. However, the changes that occur during welding that are specific to Clearweld® can be used as an aid to monitoring whether a weld has been completed successfully.
The rationale behind this project was to consider the tasks required of a monitoring system, and then to test equipment that was currently available to define which of the monitoring requirements could be achieved. Suitable systems were only found for optical methods.
To assess suitability of potential methods for monitoring welds made by the Clearweld® process.