Inspection systems in the filling plant
Inspection systems fulfil many and diverse tasks in a modern drinks filling plant. Today´s customary quality standards demand that a container is continuously checked along all areas of a filling line. The following serves as a brief insight into the different areas of use and the inspection techniques used in each case. A line for processing returnable bottles has been taken as an example, whereby individual elements are used exclusively for glass or plastic containers.
1. Check for crates with empties
Checking crates with empties is a decisive factor for the efficiency of a filling line for returnables. The wide variety of containers available on the market today and the assortment of returns necessitates an exact inspection of the incoming crates in order to avoid interference to the unpacking process and a high quota of foreign bottles on the bottle conveyors. The check for crates with empties examines the crate on the basis of such criteria as how it lends itself to being unpacked, the amount of foreign bottles and the glass/plastic container ratio. Ultrasonic measuring procedures are used to determine the height of the containers and to detect foreign objects in the crates. Differentiating various containers with the same height is possible using modern image processing systems. For example in this way PET bottles with differently shaped neck rings can be identified. If the bottles have the same shape but are not identical in colour they can be distinguished using a colour camera.
2. Empty crate check
After the crate has been unpacked it is examined to establish whether or not it can be used for the actual production. It must not be damaged or contain foreign objects and its colour and logo must correspond to the filled product. Nowadays all three criteria are generally checked using camera systems. A new trend is beginning to emerge as regards the detection of foreign objects: the more robust ultrasonic measuring method is being increasingly used for detecting plastic bottles which have become wedged in the compartments. As a rule foreign objects tend to fall out in the crate turner and therefore this is an interesting alternative requiring less maintenance.
3. Empty bottle sorting
On the basis of the large assortment of empty bottles usually present in a crate today not all the foreign containers can be rejected in the crate area. Crates with a small share of foreign bottles are unpacked. As a result the foreign bottles must be sorted during bottle transport. As a rule the most important element of the bottle sorting unit installed there is a camera system using an image processing technique which measures the shape of the bottle and compares it to the sample shape of the production bottle. The examination of the bottle can be augmented by a colour test and a closure presence check if required. Systems using fuzzy logic for evaluation purposes have proved their worth for assessing the measuring results. This technique evaluates the results of the individual detections together as a result of which the identification of the bottle types is much more reliable.
4. Empty bottle inspection
The size of each incoming bottle (washed and the correct type) is checked again before the empty bottle inspector. An infeed sorting unit can for example detect a bottle where the neck has broken off in the washing machine and rejects it before the inspection device in order to avoid interference and/or damage to the machine.
The containers are subjected to various inspection procedures in the empty bottle inspector. There are two methods of installation and transport within the machine: in the past there were almost only so-called rotary conveyors but since then linear transport has established itself in many areas.
The inspection technique used by the empty bottle inspector consists for the most part of cameras. The different areas of the bottles are generally photographed with CCD cameras. For a base and finish check one picture of each usually suffices to evaluate a bottle. Transparent foreign objects, such as transparent foil, can often only be reliably identified using a specially designed lens during a second base check.
A sidewall check is more complex due to the size of the surface to be inspected. The bottle has to be photographed from different angles in order to examine it completely. This occurs either by taking several pictures of the bottle whilst it is being rotated or by using mirror systems to increase the viewing angle of the camera. Both these techniques are combined in the most efficient empty bottle inspectors.
A further important test procedure is the residual liquid check. High-frequency and infrared measuring techniques are used to detect residual caustic solution or oil and varnish on the base of the bottle.
5. Residual caustic solution check
Residual caustic solution in a bottle can present a serious danger to the consumer and therefore an additional residual liquid check is often installed directly before the filler, in accordance with an HACCP concept, as an extra safety precaution. A high-frequency measuring technique is also used here, a reliable method of identifying even the smallest amount of residual caustic solution in a bottle.
6. Filler and closer management
Different factors are checked during and directly after the filling process and samples taken from the production flow.
A bottle is checked during filling in the filler to establish whether or not the pressure applied in the prestress or filling stage will cause it to burst. If a burst container is detected the adjacent and following containers, which could contain broken glass, are rejected at the downstream fill level check. In order to avoid such bottles being returned to the bottle flow manually (because the operator has not identified the fault and considers them to be good) these containers can be purposely underfilled. A shower device which washes the affected valves down can also be activated from the check unit. The following inspections are carried out in the outfeed area of the filler block: fill level, closure presence and if necessary its position, residual air (especially for beer) and tightness.
The technique used for the fill level measurement depends very much on the product and container. In the simplest case a photocell suffices. The measurement of the radiation absorption from X-rays or gamma rays is frequently used. The outstanding feature of this technique is the fact that it requires minimum maintenance. However it does require a special licence and specially trained staff to observe the radiation protection regulations. This has contributed to the widespread use of the high-frequency measuring technique. During this process the container passes through a high-frequency electric field. The volume of liquid in the bottle neck influences and changes this field. This effect can be recorded and evaluated. It was first possible to measure the filling volume of liquids which had been foamed-up with this technique. In addition several parameters of the electric field are measured using special software and compared to statistically acquired reference values.
Modern inspection devices do not just reject underfilled and overfilled bottles but they also produce statistics over how many times a particular filler valve has produced an underfilled container. This data facilitates purposeful maintenance and repair work and helps maintenance staff to keep a high standard with regards to the efficiency of the filler by detecting possible faults at an early stage.
The possibilities for checking a closure range from a simple presence check using a light scanner or metal detector to checking its position and correct fitting with a camera system. Such a system can even be significant in the case of a crown cork closure in order to identify protruding metal which was sheared off during the closing process and could now cause an injury. Camera systems are more frequently used for screw closures in order to check they have been correctly applied and for example to check that the tamper evidence ring is intact. This subject has increased in importance especially in view of the recent safety discussions regarding the extent to which products can be manipulated.
The residual air detection was developed especially for bottling beer. The crown cork is stimulated with an electromagnet pulse because even small air bubbles in the headspace of a beer bottle can cause the product to go off quickly. The acoustic echo produced is analysed as a result of which even the smallest air bubbles can be identified.
The closure of a glass bottle is examined in order to check the tightness of the bottle. In the case of plastic bottles the complete bottle has to be checked for leakages. There are now systems available on the market which apply pressure to the bottle sidewall and at the same time measure how the bottle reacts to this. The bottle has a leak if for example the fill level rises considerably whilst pressure is being applied.
A well-equipped filler management system also has an integrated sampling program so that the quality of the filled product can also be checked. It is also possible, depending on the equipment, to single out and reject a container filled at a particular valve or reject a complete filler circuit in order beginning with valve 1.
7. The bottle is given a final look-over after the labelling machine before it is packed in the crate. First the labels must be checked to ensure they are all present. This is generally carried out with a light scanner either in or after the labelling machine. Product liability and the increasing demand for quality have also resulted in a need to upgrade inspection devices. Camera systems are used increasingly not only to examine the presence of labels but also to check that a date imprint is present and correctly printed. In addition to these factors, which are directed more towards product safety, the quality of the label application is also examined closely. The label alignment of the individual labels is examined as well as the alignment of several labels in relation to each other.
A closure inspection can be carried out at this point instead of after the filler. In this way modular designed systems can incorporate several camera inspections in a check device thus simplifying maintenance.
A second fill level check is also often carried out at this point. On the one hand the abundance of checks ensures increased safety but the measuring accuracy at this point is also greater due to the fact that the product has settled.
8. After the crate has been packed another check is carried out before palletising. The full crate check verifies that the crate is complete and that the bottles are in the correct position. The simplest version of the sensor technology used consists of light scanners or metal detectors. X-ray technology is becoming more widespread. Even underfilled containers in a crate can be detected using this method. In addition the dimensional accuracy of a crate can be examined with photocells in order to avoid malfunctions to the palletising process.
Which inspection equipment is right depends on the individual case. However in view of the trend regarding product liability and consumer protection laws the future extensibility of the individual devices should be considered when equipping a filling line.
Translation from a report published in the "Getränkeindustrie" journal - June 2002 edition