Surface Inspection

Functionality and aesthetics are essential quality criteria. Surface defects sometimes lead to impairment of the functionality of components, while in other products, they tarnish the appearance. It is not uncommon for complaints to arise from cracks, scratches, etc. During the surface inspection, shells of objects are visually checked. As part of quality assurance, the procedure helps to optimize manufacturing processes.

When it comes to surface inspections, a distinction is made between tactile and visual methods. The first variant is one of the profile methods. Control devices with diamond tips measure the roughness of surfaces. The method delivers precise values that are in the nano range. Visual surface inspection, on the other hand, is a non-contact test method. The simplest method is visual inspection by the human eye. This is a good and inexpensive way of roughly inspecting objects in the context of single and small series production. Tools such as magnifying glasses, microscopes and UV light are sometimes used. Automatic systems work more accurately and reliably. However, damage to the material cannot be determined by the surface test. Other methods that work with the help of ultrasound waves, X-rays or gamma rays show their advantages here.

The surface inspection is largely carried out by automatic inspection systems. Cameras and sensors form the core. Powerful lenses pick up external surfaces and pass the images on to a PC, which processes them digitally. One approach has proven itself in practice: Based on predefined criteria, the software used recognizes minimal and gross deviations. The data are compared with reference images or target values. If irregularities occur, it is very likely that it is scrap. Surfaces can also be scanned using laser technology, and three-dimensional images are created in the computer. Test systems decide within a few seconds whether products are defective or not.

With enormous computer capacities, efficient microchips and high-resolution cameras, the visual surface inspection can be carried out even with fast production processes. The process comes into its own with short test times, provided the equipment used meets the requirements. The recordings are often made by several cameras and from different perspectives, so that products can be almost completely displayed in one operation. The surfaces do not have to be smooth and even; goods by the meter such as latex, fabrics, plastic films and cables can also be tested.

A technical photographic selection is difficult without contrast. Fingerprints, scratches, cracks, burrs and pressure points can be recognized by a system if the points stand out strongly from the environment. The intensive illumination of the recording areas plays a central role. Very bright light sources and UV light are often used, but the actual contrast generator is in the computer. A simplified image processing continuously optimizes the transmitted images. Similar to photo editing software, it adjusts gradation curves and sets exposure thresholds to minimal tolerance ranges. The black and white images show product surfaces extremely light or dark. Even the smallest dirt and grooves stand out due to the strong contrast.

How high the error detection turns out depends on numerous factors. Test systems do not recognize the actual condition of surfaces. They take pictures and falsify them in order to deliver measurable results through contrast. It can happen that a hydraulic hose no longer remains in production because the wall has minimal contamination. However, the article is in a technically perfect condition. It is important to set the parameters exactly to the requirements. As a rule, inspection systems for visual surface inspection are excessively cautious.

Visual surface inspection is part of every branch of industry. Especially companies that produce products in large numbers are dependent on the process. The manufacture of safety-relevant components in aircraft construction cannot do without surface testing. Whether at automobile manufacturers, in metal processing or in the field of electrical engineering, the visual surface inspection ensures high quality standards during production. In principle, modern pipe and sewer cleaning also works with this method. Probes and robots equipped with cameras scan lines and transmit their recordings directly to a monitor. Assessment of masonry and interior walls is mostly done by humans. Visual surface testing has even established itself in the food industry. If the dimensions of high-quality pralines and chocolate bars do not meet the specifications, they will not be sold. The quality of packaging can also be checked. Almost all soda, beer and juice bottles are made of transparent materials such as plastic or glass - an advantage that pays off when inspecting large batches. Contrast-generating flashes of light penetrate the materials so that cameras and sensors detect the tiniest hairline cracks, liquid residues and contamination. The processes take place in shielded sections and can hardly be seen with the naked eye.