LARGE-SCALE WEIGHT WATCHING

by Roland Bosshard, Mettler Toledo

A CERTAIN DEGREE OF VARIABILITY IS AN UNAVOIDABLE PART OF ANY FILLING PROCESS. BUT TO KEEP FLUCTUATIONS IN, FOR EXAMPLE, THE NET WEIGHT ON A TOLERABLE LEVEL, ADEQUATE CONTROL SYSTEMS MUST BE IN PLACE.

Each day, manufacturers worldwide produce millions of packages for consumer goods, ranging from chocolate bars to shampoos to prescription medicines. The packaging designs are as varied as the products themselves and may consist of paper, foil, or some sort of plastic or glass container. However, there is one characteristic that all these products share: the net weight or volume or number of items that is displayed for the consumer to see. And, for this figure to be accurate, it is crucial to have adequate control systems.

THE FILLING PROCESS
In any production process, no matter how well designed or well maintained, a certain amount of inherent variability will exist. This is the cumulative effect of many small and unavoidable influences. For example, a machine filler may drift. This is a consequence of differences in the physical characteristics of the product itself or of the machine, causing variations in the filler output. Mathematically, this is expressed as the deviation from the mean value of filled weights. Controlling this variation requires balancing the economics of operation and the risks associated with underfilling. The control system, whether manual or automatic, provides adjustments so that the mean of the filled weights rests on the target fill weight.

LEGAL REQUIREMENTS
Legal regulations worldwide generally fix a product’s declared nominal fill quantity as a rigid lower tolerance for the filling process. To be able to comply with this lower limit, a producer must institute a sampling programme. Simply overfilling is insufficient – the amount of necessary overfill must also comply with the law. The most basic legal requirements acknowledge that variation in the filling process exists, but also sets guidelines for process control in order to protect the consumer. The law generally prescribes that:

• A packaging’s actual fill quantity must not, on average, be less than the nominal fill quantity.
• The proportion of filled packagings with a negative deviation greater than the prescribed tolerance limit must not exceed the specified allowed number of defective packages for a particular lot size.
• A filled package with a negative deviation of more than twice the prescribed tolerance level does not meet the legal requirements.

Regulations in the pharmaceutical sector are even stricter and differentiate themselves from standard prepackaging fill regulations by distinguishing between the different forms of drugs – such as capsules or tablets – by relating the tolerance requirements to the mean value of random samples rather than to a nominal fill weight, and by requiring both upper and lower tolerance limits.

The basic tasks of a fill quantity control system are:

• To measure; the balances used must be certified by the country’s weights and measures organisation.
• To evaluate; statistical algorithms are applied to assess process compliance to legal and company requirements.
• To give feedback; there must be trigger warnings, alarms and recommendations for the amount of necessary machine adjustment.
• To monitor; filling process status information must be provided.
• To report; legal documentation requirements with a wide range of informative reports and charts must be fulfilled.

SOLUTIONS THAT PROVIDE HELP
Mettler Toledo makes it easy for a company to cope with all these regulations, requirements and tasks. Its solutions are specifically tailored for statistically controlling the quality of a company’s filling process, and range from compact stand-alone balance systems to real-time computer network applications for an integrated quality control program. In its simplest form, a statistical quality control (SQC) system consists of a precision or analytical balance and a printer. The system is operated using a clearly laid out keyboard and the display of the balance. Printed reports of the results safeguard the data for checking – even many years later. This, together with the built-in control and calibration of inspection, measuring and test equipment, allows working procedures to be organised in accordance with ISO 9000, good laboratory practice and good manufacturing practice. Control charts and histograms enable rapid and permanent checking of the fill quantity directly at the workplace. In combination with the various legal tolerance systems permanently stored in the SQC products, this ensures that the solutions comply with legal regulations while almost entirely avoiding wasteful overfilling. Thus, a company can manage and control a database of products while maintaining two or more independent statistics. It can also model its own production environment with confidence by applying Mettler Toledo weighing process know- how, batch handling and production statistics, and ensure database integrity through secure access procedures. Using Mettler Toledo SQC products, a company can be statistically certain that its fillers are performing to its predefined quality standards.

A CASE STUDY

Frisco-Findus (Nestlé), Switzerland, produces about 200 different frozen food products and as many different types of ice cream. From the development laboratory and the storage areas to production and final checking, a large number of Mettler Toledo scales and weighing systems are used. The ice cream production department uses in-line checkweighing and the SQC software package FreeWeigh to assure legal compliance and avoid underfilling in the packaging area. At the factory in Rorschach, where the summer period begins in February, the wide range of ice cream products is produced virtually round the clock to meet the high demand.

CHECKING OF FILLED WEIGHT
When the flavoured and coloured ice cream has been packed, Garvens^'1 automatic checkweighers, located in the middle of the conveyor belt, record the filled weight of each individual bulk pack. On other production lines, samples are taken and checked at separate weighing stations. Both methods comply with the legal requirement to check filled weight. All the measurements are recorded centrally on the computer in the quality assurance laboratory using the Mettler Toledo software FreeWeigh, and then they are statistically evaluated and saved as production data.

PROBLEM-FREE EQUIPMENT
For Guido Sutter, head of production technology for Frisco-Findus, it is vital that all the equipment in the production process comes up to industrial standards. ‘It is crucial for us that all modules are available at all times, and data communication has to function absolutely perfectly,’ he says. ‘We have never had any problems with these scales since we first started using them.’ This Frisco- Findus case study underlines the importance of having scales, weighing sensors, weighing terminals and automatic checkweighers that are completely integrated into the complex data world of process control.
^{1. A Mettler Toledo company.}

Mettler Toledo is the world’s largest manufacturer of weighing equipment and one of the leading suppliers of analytical instruments. The company has manufacturing facilities in five countries and service organisations throughout the world. It has been supplying innovative products to industry and laboratories for almost a century.

The company’s extensive range of products includes analytical and process instruments for the measurement of pH, thermal properties, dissolved oxygen, moisture content density, refractive index, carbon dioxide and much more. It also includes laboratory balances, industrial scales, in-line checkweighers, weighbridges and complete weighing systems.