Active online thermography

The use of active thermography on large flat surfaces requires a heating technique that can heat these objects quickly and uniformly. A pilot plant is available at the Fraunhofer WKI, which can currently be used to examine material up to 100 cm wide at speeds of up to 50 m/min. For this purpose, the material to be examined is passed on a conveyor system at a short distance under an elongated heat emitter and the surface is heated by a few degrees. The heat distribution of the surface is then recorded with a thermographic camera. As with impulse thermography, any adhesion defects are made visible by so-called "hot or cold spots" on the surface.

The individual narrow sheets of veneer, which later form the surface coating of furniture, are joined together in a first operation with zigzag glue threads. During pressing with a chipboard, these threads should melt and spread in the glue joint. If this does not happen, these adhesion defects, which are only visible visually in grazing light, can be detected using thermography.

These defects stand out clearly from their surroundings in the image due to the lighter, i.e. warmer, areas.

Modern office furniture today is often constructed from new combinations of materials. The picture on the left shows a symmetrically constructed cabinet door consisting of a plastic core laminated with aluminum and veneer as a top layer. Adhesion defects between veneer and aluminum or between the aluminum and the plastic are made visible by the thermographic image.

Good exterior house doors have an aluminum insert near the top layer to prevent moisture condensation in the wood-based material layers. However, bonding both decorative papers and wood-based materials to aluminum is problematic.

In this measurement, the material was passed under a heat source on a conveyor belt at a speed of 40 m/min, heating the surface by a few degrees in the process and then observed online with the thermography camera.

In another experiment, a thermographic camera examines the material without heating it beforehand and without moving the material.

In thermographic adhesion failure detection, the elevator and the two wings of a glider are passed under a heat source with the aid of a conveyor belt, and the surface is heated by approx. 2 K. The thermographic images are then recorded. The thermographic camera mounted behind it then recorded the thermographic images of the respective front and rear surfaces.

Noticeably warmer (red and yellow in false color representation) than their surroundings stand out the areas under which the paper honeycomb structure is located, which essentially consists of air (poor heat conductor). The gluing of the plywood surface to the honeycombs is clearly visible. In addition, the honeycombs appear differently warm because the amount of glue underneath, and thus its heat capacity, varies greatly (bottom, left).

Significantly cooler (green and blue in false color representation) than the paper honeycomb surfaces appear the areas directly above the ribs, and where these are well bonded to the rib (good thermal conductivity across the adhesive into the rib). Again, the glue that emerged from the joint during bonding, with its increased heat capacity, can be seen by the frayed edges of the rib.

The areas of the ribs that are no longer sufficiently bonded to the substrate appear warmer in the thermographic image and are indicated by the red colors.