Cosmetic defects are the bane of manufacturers across all industries. Producing a product with the correct quality in terms of measurement, assembly, form, fit, and function is one thing – but cosmetic defects are more easily noticed and flagged by the customer and end-user of the product. Most of the time, they have no detrimental effect on the use or function of the product, component or sub-assembly, they just don’t look good! They are notoriously difficult for an inspection operator to spot and are therefore harder to check using traditional quality control methods. The parts can be checked on a batch or 100% basis for measurement and metrology checks, but you can’t batch-check for a single cosmetic defect in a high-speed production process. So for mass production medical device manufacturing at 300-500 parts per minute, using vision systems for automated cosmetic defect inspection is the only approach.
From medical device manufacturers, cosmetic defects might manifest themselves as inclusions, black spots, scratches, marks, chips, dents or a foreign body on the part. For automotive manufacturers, it could be the wrong position of a stitch on a seat body, defects on a component, a mark on a sub-assembly, or the gap and flush of the panels on a vehicle.
Using vision systems for automated cosmetic inspection is especially important for products going to the Japanese and the Asian markets. These markets are particularly sensitive to cosmetic issues on products, and medical device manufacturers need to use vision systems to 100% inspect every product produced to confirm that no cosmetic defects are present on their products. This could be syringe bodies, vials, needle tips, ampules, injector pen parts, contact lenses, medical components and wound care products. All need checking at speed during the manufacturing and high-speed assembly processes.
How are vision systems applied for automated cosmetic checks?
The key to applying vision systems for cosmetic visual inspection is the combination of camera resolutions, optics (normally telecentric in nature) and filters to bring out the specific defect to provide the ability to segment it from the natural variation and background of the product. Depending on the type of cosmetic defect that is being checked, it may also require a combination of either linescan, areascan or 3D camera acquisition. For example, syringe bodies or vials can be spun in front of a linescan camera to provide an “unrolled” view of the entire surface of the product.
Let’s drill down on some of the specific elements which are used, using the syringe body as a reference (though these methods can be applied to other product groups):
These defects are typically impurities, particulates, fibers and bubble. This needs a lighting technique with linescan to provide a contrasting element for these defects:
These defects are typically “hidden” from view via the linescan method and so using a direct on-axis approach with multiple captures from an areascan sensor will provide the ability to notice surface defects such as white marks (on clear and glass products) and bubbles.
Cracks and scratch defects
A number of approaches can be taken for this, but typically applying an axial illuminiation with a combination of linescan rotation will provide the ability to identify cracks, scratches and clear fragments.
Why do we use telecentric optics in cosmetic defect detection?
Telecentric lenses are optics that only gather collimated light ray bundles (those that are parallel to the optical axis), hence avoiding perspective distortions. Because only rays parallel to the optical axis are admitted, telecentric lens magnification is independent of object position. Due to this distinguishing property, telecentric lenses are ideal for measuring and cosmetic inspection applications where perspective problems and variations in magnification might result in inconsistent measurements. The front element of a telecentric lens must be at least as large as the intended field of view due to its construction, rendering telecentric lenses insufficient for imaging very large components.
Fixed focal length lenses are entocentric, catching rays that diverge from the optical axis. This allows them to span wide fields of view, but because magnification varies with working distance, these lenses are not suitable for determining the real size of an item. Therefore, telecentric optics are well suited for cosmetic and surface defect detection, often combined with collimated lighting to provide the perfect silhouette of a product.
In conclusion, machine vision systems can be deployed effectively for 100% automated cosmetic defect detection. By combining the correct optics, lighting, filters and camera technology, manufacturers can use a robust method for 100% automated visual inspection of cosmetic and surface defects.