Process Monitoring in Production

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.

Overview
Pre-Process
  • Measurement of optical properties

    A simple method to test the transparency of materials is to measure laser power of the operating laser with and without a sample. The detected difference in power is a rate of the transparency and should stay in evaluated limits.

  • Components and their positions

    In laser welding of plastics a contact in the joining areas of the welding partners is necessary. Therefore part dimensions as well as the location of the components in the fixtures should stay in tolerance.

    For qualification simple senor systems are often sufficient. For more complex tasks camera systems with good resolutions can analyze weld ability even in separated areas.

  • Coding

    • Fixtures

      If multiple components are welded on a single system, the welding of mating components can be ensured by coding the fixtures and clamping glasses.

    • Components

      A full traceability of process steps in production is in some applications provided by a coding of the component carriers or the component itself. In general the coding is read by a scanner and associated to the process.

In-Process
Post-Process - Non Destructive Tests
Kamera für Prozesskontrolle beim Laserschweissen
User interface for optical qualification
  • 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.

Post-Process - Destructive Tests