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Watermarks, shrinkage and surface defects: the “invisible” faults on PET preforms

The advantages of “grid inspection”, the innovative patented SACMI solution that can be installed on the entire range of preform inspection systems, both on line and in test labs

A formula plus patented technology to identify, via a simple algorithm, preform defects that would otherwise be invisible to the human eye, detecting them easily, economically and – above all – without any risk of encountering the errors induced by the image ”interpretation” that is necessary on traditional inspection systems which use polarised light. This innovative Sacmi-developed “grid-type” inspection system can be installed on the entire PET preform vision system range, on both production lines (PVS 003 and PVS 024) and in test labs (PVS 2).

Traditional inspection techniques that use backlighting with a polarizer placed immediately behind the product make it difficult to execute easy, economic detection of major defects (caused by, for example, on-resin stress during the cooling process and/or water in the moulds – the so-called watermark). These defects, in fact, generate patterns that then need to be interpreted by the operators; this requires skill and experience as the faults are tricky to translate into inspection algorithms able to be used by a vision system.

This is where the new Sacmi system comes into play: the polarised light has been replaced with a special patented backlighting unit that projects gridlines behind the preform. On flawless preforms the result is a series of straight parallel lines; on faulty areas, instead, these become wavy (photo 1).  
  Using the grid, even small defects (photo 2) are immediately identifiable.
The new patented Sacmi methodology is available as an optional on the entire PVS range, thus allowing completion of the standard checks used to analyse defects on the entire preform, body finish and bottom; this gives the preform quality control department a significant boost as it permits an extremely economic, perfect preform “X-ray” to be made that is error-free (some defects analysed using the polarised light method can escape even the most expert eye).

Sacmi on 15th September during the “Food & Beverage Plastic Day”, a special event organised to show customers and partners our latest technological innovations in plastic containers and closures for the Food and Beverage markets.


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