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The situation is well-known in the plastic packaging industry: We want to produce flawless multi-layer preforms. More than ever, especially in the food and beverage packaging industry, brand owners are using barrier layers to protect their products from harmful external influences. Also, nothing of the contents – like carbon dioxide – should escape from inside. Fulfillment of these multiple requirements is a challenging task.
An even greater challenge is to maintain a continuously high product quality. Until now, there has only been one major way to control quality of preform layers: Preforms are cut into slices in the lab and the invisible layers are inspected at the according positions. While it is a safe way to detect certain defects or missing layers, there are obviously major disadvantages to this approach. First of all, the preform has to be destroyed, creating scrap. Second, the cutting and inspection are time-consuming and require expensive laboratory devices, with the inspection results often being of minor quality due to possible inaccuracies.
Fig. 1: The preform layers are measured all around by coherentinfrared light within 25 seconds
Innovative measuring technology
INTRAVIS GmbH together with Husky Injection Molding Systems Ltd. has developed an imaging device to inspect preform layers in a non-destructive way. The offline inspection system measures and reports the presence, position and thickness of different layers. It works for all transparencies, translucencies, as well as for a wide variety of opaque preforms. The LayerWatcher is intended for laboratory use and inspects individual preforms. Instead of determining the layer thickness at one single point, a high data rate provides an entire network of points to allow a detailed description of the layers in the entire preform. Measuring results incorporate thousands of point positions within the layers. The positions have a minimum grid step of 0.2 mm and thus create a dense mesh that indicates the layer thicknesses. The resolution of the thickness measurement is approximately 10?m (micron) or better. The measured data is processed and compressed into a few clear parameters defining the characteristics of the layers: Where does the layer start and end? How robust is the barrier layer at the leading and trailing edge? Are there any voids? According to these characteristics, the user can re-adjust the production process and refine the production parameters accordingly. Since the measuring process just takes about 25 seconds, it is much faster and more comprehensive than the traditional destructive method. Therefore you get an immediate answer to what modifications in your process mean for your products. The fact that you don’t have to enter any information prior to measuring, additionally allows for a faster set up process of a product.
Fig. 2: The LayerWatcher requires little floorspace and is suited for a laboratory use
Images instead of numbers
During the measurement process, it is possible to set up a live view of gray scale images on the screen. In addition to displaying these images, the system also provides the option to evaluate the quality of the layers directly and turn them into an exact 3D image. This is different from other technologies which typically present their results exclusively numerically. The measured data is immediately displayed on the screen, with the system identifying the layers by their different characteristics. The LayerWatcher inspection technology is designed to inspect preforms made of PET material, either monolayer or multi-layer. PET must make up the outer and inner dimensions of the preform and at least one middle layer of a barrier material. PET may be any of the commercially available grades, high or low IV, homopolymer or copolymer. Inspected common barrier materials include nylon MXD6, Aegis BarrierPro2, polyprotect, valOR and amosorb. This new development already has found its first customers: Since entering the market about a year ago, the LayerWatcher is in use at several different sites all around the world.
Fig. 3: Not completely closed barrier layers can easily be detected in the 3D false color as well as in the grey scale images
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