Product Inspection and Inline Quality Assurance
Inline quality assurance using dynamic checking and inspection systems
Quality assurance is the sum of all measures for ensuring consistent product quality. When inline quality assurance, frequently integrated in industrial, automated mass production, is based on dynamic product inspections, it is usually associated with fault detection and rejection of products identified as faulty. Inline quality assurance is then often a real-time measurement of process parameters which also includes feedback mechanisms to prevent faults in subsequent processes (for example, weight-accurate cutting of cheese with holes or meat portions). Other examples of inline quality assurance are industrial control processes in OEM machine construction – from level controls for soft drinks to controlling the thickness of bread slices.
Fault detection can relate to product features such as foreign body detection or quantity detection, while fill level or completeness checking (e.g. missing tray, missing blister or patient information leaflet, product underfilling or overfilling) are also conceivable, as well as shape and position checks. X-ray and/or optical inspection systems take over these tasks. With appropriate software support, this also includes coating control (counting control) and surface inspection of products.
The most important quality parameters in product inspection
Mechanical or automated product inspection is very important in the context of high-speed manufacturing processes. A distinction is made in this case between inspection of the product itself and its nature, completeness and weight. The quality parameters specified by the company or due to legal provisions are finally verified in a quality inspection and the product is released or withdrawn. In most cases as part of a line process, products remaining in the product flow after a product inspection must be correctly transferred to follow-on processes or they must be rejected according to the cause of the fault.
In food production, the packaging is usually also part of a final inspection because only at this point can it be assumed that there can be no further influence on the product in its final packaging and that it is in its as-delivered condition. At this point, product inspection systems are integrated into the production process in order to deliver the quality parameters, such as total weight and completeness, freedom from foreign bodies as well as shape and position control, for quality assurance. The product inspection system may include visual sensors, such as sensors that scan the product or detect magnetic fields (X-ray scanners or metal inspection systems) as well as others that determine the product weight (checkweighers). Visual sensors can read and verify codes and plain text.
Weighing technology in production systems
Since there are hardly any production or packaging processes in which weighing technology would not be involved in some way, inline weighing technology using weigh cells, such as those used in systems with very high throughputs, is very important. Here, weigh cells integrated in the filling machines take over the tasks of inline checkweighers. This structurally more complex solution can be implemented even more compactly than the checkweigher solution. It enables higher throughput which is possible due to scalability and equipping the machines with up to 16 weigh cells or more that are fed in parallel.
EMFR weigh cells from WIPOTEC deliver such precise results that they can be used with only a few grams of filling – a frequent application in the pharmaceutical industry. Both methods – the use of checkweighers or inline weighing with the help of weigh cells – enable complete product control. In both cases, measuring the respective weight can be used not only to reject the overweight or underweight products before subsequent processes, but also to adjust the filling quantity.
Checkweighers for dynamic inspection processes
Inspection systems for inline quality assurance tasks that have weight sensors form a separate class in product inspection. Provided that they can measure weights dynamically and are integrated in production or packaging lines, these systems are designed as checkweighers.
In inline processes that manufacture/produce goods, dynamic checkweighers monitor the goods according to weight criteria and reject those whose weight is incorrect. They can equally enable optimisation of the filling quantity in upstream dosing, filling or cutting systems. Dynamic means that belt transport is not halted even during the weighing processes.
The measuring principle is always the same; the products are transported over scales and their weight is measured while moving. Strain gauges or (particularly in the high-speed range or when high precision is required) weigh cells, which operate on the principle of electro-magnetic force restoration (EMFR measuring principle), are used as weight sensors. EMFR weigh cells permit higher throughput rates. This is because the time the weigh cells need to carry out a weight measurement is shorter than when using strain gauges and higher belt transport speeds are therefore possible.
Product handling is an important parameter in connection with dynamic inspection processes. Even the fastest checkweigher is limited in its performance if the products can no longer be transported safely to and from the weighing belt. As a result, inspection systems can be designed for certain packaging types or products, for example top-heavy products (bottles or other small-footprint products with an unfavourable height-to-width ratio) and pouches, cans or boxes that are to be inspected at high Speeds.
Dynamic scales also require that only one product is on the weighing belt at a time. Products therefore have to be separated which means that a predetermined product gap is maintained. Separating belts, screw conveyors and star wheels are used to do this. Complete line integration also includes the synchronised motors of the transport belts of production or packaging machines and checkweighers, coordinated waiting cycles and synchronised restarts.
Inspection systems that can dynamically acquire the weight are frequently in use in the food production but other industries also use these systems. This type of weight acquisition starts in the milligram range (pharmaceutical environment) and finishes with heavy load checkweighers having a maximum load of several hundred kilos.
Food inspection in accordance with the German Food Hygiene Regulations
Food inspection is an important part of food safety. According to the German Food Hygiene Regulations, any company producing, processing or distributing food is obliged to identify, consistently monitor and document critical work stages in the process sequence and it must also define appropriate safety measures. Foreign body detection, including metal detection and glass-in-glass detection, the inspection of packaging in addition to verification and traceability are part of food inspection. Foreign bodies of all kinds can be detected in food with the help of X-ray or metal inspection systems.
Weight checking is carried out using dynamic weighing technology (checkweighers). The background to weight checks is often the need to detect finally packaged products with incorrect weight. The checks are used for quality assurance (ejection of underweight packages as part of consumer protection) but also for optimising production in respect of efficiency (in this case, preventing losses due to overfilling). The legal regulations are stipulated in the Prepackage directive in Germany and in Directive 76/211/EEC throughout the EU. They govern the extent to which the mass or volume of the contents of a package may deviate from the information printed on the package.
Optical product inspection
Optical inspection systems provide the ability to read barcodes or texts, for example the best before date. Appropriately arranged camera systems can also examine labels on the product underside of thermoform packages. Optical camera systems which inspect the products from above with corresponding incident light are used for applications where barcodes or texts are to be identified or read on the cover film. It is also possible to identify miscutting of the cover film on thermoformed packages as well as the position or skewing of labels on the underside.
When using transparent films for sealing, it is also possible to detect product inclusions which impair the seal or render it useless. Image processing specifically tailored to the application carries out both readability checks and also position checks on labels and stickers. In addition to the foreign body check, the outer appearance of the products is checked and any faulty labelling is detected. Sometimes, optical inspection systems are also combined with high-performance X-ray scanners and offered as a compact unit.
Metal detectors in product inspection
Metal detection means detecting ferrous and non-ferrous metals as well as stainless steel in products that are already packaged. These systems for product inspection are also frequently referred to as metal detection systems. Stainless steel is the most difficult metal substance to detect as in most cases it is not magnetic and it is a poor electrical conductor. In terms of inspection technology, metal detection during the inspection of food can be combined with a second function which includes high-precision weighing. Scales and metal detector then form a space-saving unit which can easily be integrated into existing lines and operated via a common user interface. Such combination devices eject contaminated and/or incorrect weight products into separate containers according to the cause of the fault.
Foreign body detection with the help of metal detection systems is increasingly being supplemented (or replaced) by X-ray inspection systems. They reliably detect foreign bodies such as metals and stainless steel, but can also detect contamination with glass, bones or stones. What applies to detecting stainless steel when using metal detection systems to identify foreign bodies, also applies to the glass-in-glass combination of foreign body and packaging material, which presents X-ray inspection systems with the greatest challenges.
X-ray scanners for the inspection of food
Software for product inspection systems
To support quality data management using the results of product inspections, management software (inspection data management) networks inspection systems of various classes (checkweighers, X-ray scanners) and therefore provides access to all inspection data within a company – across all sites and in real time. The same software collects and archives all data arising in the course of inspections and uses it to generate reports, analyses and statistics.
It is possible to compare production data from the past with each other and with current actual performances. Quality assurance as a whole is therefore supported by statements about production figures, rejects, machine utilisation and product quality. Data and events can be tracked in real time and linked to an alarm system. There is no longer any need for time-consuming on-site checks on production lines. Central inspection data management systems therefore provide information for optimised and effective production by detecting production deviations and reporting them in good time.