Advanced Streak Inspection On Coated Board And Paper Production

Introduction

Quality requirements of the coated board and paper have risen significantly, since first web inspection systems were introduced for the mills. Nowadays most of the inspection is done by CCD-camera based systems. The inspection has been focusing on the ever-increasing demands of automatic classification and image analysis. This has been done at the cost of the accuracy. As a result, some of the most serious defect types have been almost neglected, streaks as a good example.

CCD-camera based systems have offered streak inspection as well, but the performance level on detecting this defect type has widely not reached the expectations. This has led to the development of totally new type of inspection system optimized on certain defect types. The latest sensor and illumination technologies have made it possible to develop high accuracy defect inspection system for pinpointed demands like streak inspection, and still performing traditional web inspection for other defect types. As a result, mills are now able to get a warning of the streaks even before they grow critical.

Streak Inspection For Good Printing Quality

Streak inspection in the mill is typically required to ensure good printing quality for the material end-users, but it can also be used as a maintenance tool.

Streaks are long and narrow defects on the coated surface. They can have depth from few micrometers up to more than 100 micrometers depending on the material or appear as long and narrow glossiness on the surface. There is not any color contrast between the streak and the material itself.

Streaks result mostly from the particle trapped on the coating equipment f. ex coating blade. In the beginning it is very small, therefore the streak resulting is very small as well. If the particle is not removed, it will grow and in the end the streak will be clearly visible by eye. By that time, the streak has been already long time on the web and previous rolls. Inspecting the emergence of the streak as early as possible is vital in avoiding greater losses in rejected rolls and possibly lost customers.

Statistical defect database is a helpful tool in improving the process and identifying defect origins. Real time defect database can also be utilized for reducing the lead times in total logistical process. Defect database can also be utilized for production optimization, by analyzing the changes in different production parameters in comparison with the defect database. In the end, threshold levels for each parameter can be set according to the quality required. This will eventually lead to avoidance of over- and under-quality, in other words optimized quality.

Revolutionary New Inspection Method

SR-Instruments Oy has introduced a new type of web inspection concept for demanding applications in the board and paper industry. The system has been especially optimized for streak detection, but it is also full featured for all paper surface quality control.

Fig. 1. Detmaster has integrated light source and detectors in the same frame, making it a compact package.

The system is based on new type inspection frame, which contains a line of smart illumination and detector modules. Each detector module has an own illumination and receiver optics section and a powerful DSP-signal processing electronics. Each module illuminates the surface from several illumination directions by using high frequency modulated LED-light sources. Different illumination directions are modulated with own frequencies. Patent is pending for this technology in all major markets. The signal detection is done by high resistive silicon sensors and the system measures surface reflectance properties for each illumination direction simultaneously. By using the combinations of different measurements the system can determine from each surface point glossiness, reflectivity and slope, which provides a good basis for specific defect detection and classification. As a result, system can collect much more information from the surface than by using conventional CCD-camera technology. The synchronous modulation method and silicon sensors provide also much wider dynamic range and higher signal to noise ratio than CCD-cameras, yielding to accurate and to very important reliability in defect detection.

The compact frame can be installed in confined places in the production line. The modular detector structure and signal processing architecture makes the system easy to maintain.