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Process Monitoring in Production
Schema der Prozesskontrolle im Produktionsprozess

Process monitoring is a major issue in production with the target to ensure functionality and quality of the product. In order to achieve this target, quality control methods are defined before, during or after each process step. Such methods are evaluated during process development before implementation into production.

Monitoring methods are versatile and should be adapted to the overall production process. Here both can be relevant, the controlling of active parameter or the passive monitoring of single process steps. For an early detection of bad parts a cross-production monitoring is recommended. Indicators of weldability may be identified with simple monitoring methods. Components can be verified prior welding on dimensions, evenness and fit.

Depending on the welding technique either monitoring or controlling methods are applicable during welding. Downstream quality control may also be appropriate, as long as the components are not damaged. The condition of the components as well as production environment determines which monitoring methods are most suitable.

Quality control for Laser plastic welding can be divided in three phases:

  1. Pre-pprocess, monitoring prior welding
  2. In-process, an in-process monitoring or controlling
  3. Post-process, a downstream monitoring

In all three phases welding technology related quality control methods are available. As an example the pictures below shows a choice of methods for laser welding plastics.

Welding relevant data like optical properties, dimensions or shape of the welding zone are in general identified in pre-process and based on materials and design. Path measurement and pyrometer are the most commonly used methods during welding. But a statement on the welding quality is only possible in post-process. Since most of the methods used in this phase damage the components, they are only tested on randomly selected samples.

In the following a few common test methods are described in more detail.

In-Process

In-Process
  • Laser power

    A continuous measurement of laser power during the welding process is possible at different locations. Measurements directly behind the laser source and inside the optic are applicable to qualify power losses or deviations within the optical system. In which tolerances the emissions may vary can be set and monitored in the quality control tab.

  • Welding time/welding speed

    In laser welding systems equipped with servomotors the characteristics of the axis are internally monitored. The Novolas WS-AT for example displays an alarm as soon as the axes deviate from defined values.

    In simultaneous welding processes the welding time is relevant and can be monitored by the system.

  • Clamping force

    The clamping force can either be directly measured by using a force sensor or indirectly by verifying the set pressure.

  • Welding path

    Components designed with a welding rib are generally welded in simultaneous or quasi-simultaneous technique. Simultaneous heating of the welding contour and applying of clamping pressure forces the parts to collapse along the welding rib. The path of the collapse can be detected with path measurement sensors.

    Depending on the design of the components several different control options are available:

    • Welding path

      Termination after reaching a set path

    • Welding on time

      Termination after reaching a set time

    • Welding on a mechanical stop

      termination after reaching a set collapsing speed

    A typical curve of a welding path describes a short-term negative path. In this heating time the volume of the plastic material is already expanded before it passes into the fluid state.


  • Pyrometry

    Pyrometry is a radiation thermometry that measures surface temperatures in a certain wavelength range contact-free. The pyrometer focuses on the welding area and detects the heat penetrating through all the media within the beam path. However, in laser penetration welding the transparent welding partner is at least within the beam pass and attenuates the signal. The signal intensity that penetrates through the media depends on both the media properties and the penetration path through the media, so in general from thickness and scattering. Therefore the use of a pyrometer in production should be evaluated in preliminary tests. In general the pyrometer can be operated in contour or quasi-simultaneous welding method. A typical temperature curve along the welding is used as reference for qualification of good and bad parts.

Post-Process

 

Post-Process - Non Destructive Tests

  • Optical qualification

    The appearance of a visible welding seam may also be qualified by trained employees or camera systems. The welding seam is inspected of defects such as constrictions, burnings or foreign particles and then individually qualified. With that method, however, the welding strength cannot be distinguished.

  • Leak testing

    The tightness of components carrying fluids may be proofed in leak tests. In this test a predetermined pressure is applied over a certain time while the pressure drop is measured. The decrease of the pressure is an indicator for the tightness of the product.

Kamera für Prozesskontrolle beim Laserschweissen

 

 

Post-Process - Destructive Tests


Post-Process

 

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