Flange and shoulder inspection for aerosol cans Pros and cons of different technical methods The quality inspection of the flange and shoulder of aerosol cans subsequent to the necking process has become more essential than ever. Increased throughput rates, complex can shapes, the growing usage of recycled aluminum and serious efforts to reduce the wall thickness of the can to a minimum have tightened both quality standards and inspection requirements. More defects are critical now and even the smallest must be detected before the can gets further processed and filled. Different technical approaches to meet this challenge Nobody can deny that automatic visual inspection is inevitable in can production, but there is more than one way to implement vision solutions for top quality control. From an optical physics perspective, the best way to acquire an image of a cylindrical surface like the shoulder of a can or the backside of a flange is to take one rotationally symmetrical picture at the optical/symmetry axis instead of several images from the side. The side view creates a few problems: when looking at a curved surface, the edge areas of the image appear slightly distorted/clinched. The image resolution declines and this effect is worsened since possible defects look smaller compared with those in the center of the image. Stitching all camera images together for better evaluation does not eliminate the optical distortion and the degradation of the image resolution. The only way to minimize this problem is by using a sufficient number of cameras. Multiple-camera systems A multiple-camera system does not have to cover the whole lateral area only. It needs a large overlapping of the camera viewing fields to keep the areas with limited resolution as small as possible. Calculation, as well as experience, shows that at least eight cameras are necessary to cover 360° with almost constant image resolution—with less than 10% 24 Spray May 2016 resolution loss in the overlap area, to be precise. This is the basic prerequisite to spot small surface defects in any position. One top view camera is needed to see the flange from above. The focus of multiplecamera systems is on the can body rather than on the can top. More Ralf Fr eiberger, R&D and Christiane Blasius Marketing, the Mühlbauer Group This paper was originally presented at the Aerosol & Dispensing Forum (ADF) in Paris, France. compact and easy-to-handle equipment seems to be preferable for flange and shoulder inspection. The pericentric lens Another consideration is to use only one top view camera equipped with a pericentric lens to achieve a 360° inspection. Unlike traditional entocentric or telecentric lenses, a pericentric lens has a large numerical aperture and is designed to image the outer sides of cylindrical surfaces. In the resulting image, a part of the can body appears like-grouped around the top view of the inspected object in concentric rings. Top view and side view are combined in one analysis image. To what extent the body of the object can be imaged by this method depends on the version of the pericentric lens. Beside technical drawbacks—such as difficult setting procedures, the necessity of extremely precise can positioning and a declining resolution toward the edge of the image—the pericentric lens is a real heavyweight and an expensive solution. All in all, it is a good attempt but not the best choice for flange and shoulder inspection of aerosol cans. The aspheric optic The logical consideration is either a multiple-lens design or, better yet, an aspheric optic. As the name suggests, the surface profiles of an asphere are not portions of a sphere or a cylinder. Like the pericentric lens, the aspheric optic can be used with a standard matrix camera to enable top view and side view in one image. Additionally, if integrated in the necker tool plate, it can do something no other method is able to perform with such perfection: it shows the backside of the flange, which is almost like looking around the corner. The aspheric optic has many other advantages. Resolution does not decrease towards the edges of the image and the image can be optically (not digitally) magnified in radial direction, which means more pixels are available for the evaluation. The aspherical surface is an excellent light collector and can be designed in different shapes according to the requirements of the inspection task (e.g. see more of the flange, the bottle neck or the shoulder). Another advantage is the compactness and robustness of the optical hardware. Combined with flashed high power LEDs, it can be installed in the necking machine and operate at a typical line speed, e.g. 240 cans/min. Camshift sensors for the camera trigger work precisely and the short exposure time in combination with a high photon flux and a small aperture for a large depth of focus is the perfect basis for very clear images. Analysis images: resolution & contrast Image resolution as an important parameter of optical inspection is often mentioned and just as often misunderstood. Simply stated, resolution is the detail an image provides. The Rayleigh criterion describes how diffraction limits the detail we can obtain in imag- Can top inspection at outlet conveyor
Spray May 2016
To see the actual publication please follow the link above