Issue Features

Blown/cast film
This article by Oliver Hissmann, Sales Manager, Optical Control Systems (OCS), takes a look at the technology, benefits and costs of having web inspection systems. Established in 1989, Germany-based OCS specialises in optical quality control and to date has installed more than 1,000 inspection systems worldwide.

In today’s rough economic environment, value-added film extrusion needs a 100% on-line production control. Modern web inspection systems alert the operators immediately so that they can react and avoid more damage to the final product. Furthermore, the quality of the raw materials and also the process parameters are fully controlled.

Today’s state-of-the-art web inspection systems consist of fast, standard CCD (charged coupled device) sensors, sophisticated lighting technology and self-learning software to control and optimise the production process. Critical defects are marked and reworked on a roll doctor.

The web inspection systems are in some cases directly integrated with the IPC of the cast or blown film lines. Also, this integration enables a roll data handling in combination with all the other production parameters. These systems also recognise the position in machine and cross direction of the defect and take a picture of the defect. The defect data is stored in a network environment (Windows XP client-server architecture). The system delivers quality print protocols and sets alarms to activate a horn, lamp or marker in real time.

Benefits of web inspection
The benefits of web inspection can be realised in three parts of film extrusion: the process itself, the raw material or the final product. In process control, a web inspection system helps to optimise the production by eliminating waste production directly because of warnings. By anticipating rinsing intervals, the machine downtimes for die cleaning will be reduced and capacity enhanced.

It also helps to optimise the raw material and to find the best material recipe. The offline software can add the produced rolls of a whole raw material lot to one roll and analyse, for example, the total gel level. Thus, the 100% inspection gives a good control of the raw material itself.

Defect types/origin of defects
Typical classical defects in film and coating are gels, black specs, fish eyes, holes and foreign particles/insects. other important defects are die/flow lines, oil stains, laminating defects, cracked coatings, air bubbles/inclusions, streaks, wrinkles, lack of adhesive or craters. The defects depend on the type of production process and a system should be open to teach new defects that will occur in the future. It is getting more and more crucial for the processor to identify and classify the defect type immediately to have an idea about the origin of the defect to avoid further waste and improve the production process.

The origin of the defect can be based on the extrusion line, the raw material or the process itself. An example for the process is a screen change or a change in the extruder temperatures with the result of gel/black spec showers. Regarding the extrusion line, the screw geometry can be sub-optimal and produce some cracked material over time because of dead zones. Also, the raw material can be contaminated by the cracking process or by the transportation of the pellets.

Embedded solution/technology
The embedded solution concept combines the advantages of smart cameras with those of classical client/server systems. An embedded PC with a high-speed frame grabber and disc on chip technology achieves data rates up to 160 MHz. long distances of more than 100 m between the inspection system and the server can be bridged via an Ethernet 100 MB connection while maintaining very high calculation capability in real time (buffering up to 1,800 defect photos per second camera). The server (Windows XP) visualises the defects, documents them in a data base and alerts with alarms (horn, lamp or marker).

Because of this concept, digital CCD line scan cameras, with high-speed data rates up to 160 MHz, can be combined with sensors of 2048, 4096, 6144 or 8192 pixel. For slow web speeds (100 m/minute), long CCDs with 8192 pixel are suitable to work with only a few cameras. For high-speed applications (up to 1,000 m/minute), with a desired resolution of 250 microns, 160 MHz CCD line scan cameras with 2048 pixel and a scan rate of 72,000 scans/second are needed.