A dynamic checkweigher, also known as a checkweigher, automatic scale or erroneously called a "belt weigher" (bulk material weighing), is used in the production of piece goods to monitor, sort (classify) or even influence (weight-wise) them according to weight criteria, e.g. according to FPV (local finished packaging regulation), by direct corrective feedback control (trend controller, trend control) of filling machines. This is a fully automatic weighing of all produced products (100% control), where the products are weighed over the weighing conveyor belt, without stopping, i.e. in dynamic condition. In this case, it is not the scale, but the production speed that determines the period of time when the scale must output a weight value. Dynamic checkweighers are permanently integrated into a production line and check the products in production at throughput rates of usually 20 to 200/min, maximum even 600/min). There are different types of checkweigher: depending on where they are used, they are classified as in-line or end-of-line checkweighers. There are also multi-track checkweighers if the filling and packaging machines deliver the products in parallel on several tracks due to very high capacity requirements. Combination units that combine a checkweigher with a metal detector (critical control point, CCP) offer a space-saving solution, as they simultaneously use the required weighing infeed belt for metal detection.
How does a checkweigher work?
In principle, a distinction must be made between dynamic and static checkweighers: Static scales are table or floor scales on which products have to be placed manually and the scale determines the period of time when it has reached a weight estimation. This leads to highly accurate weighing results, however, this procedure can only be carried out on a random basis.
A typical dynamic checkweigher usually consists of a system of three conveyor belts arranged in a row, whereby the important infeed belt takes over the task of taking the product from the customer's conveyor belt, correctly separating it by product acceleration and transferring it as perfectly as possible to the following belt, the weighing belt. Minebea Intec offers strain gauge technology as well as high-precision and extremely fast EMF (electromagnetic force compensation) technology. The outfeed or discharge conveyor removes the product from the weigh belt and using any of our suplimentry products such as reject systems seperates the products however the customer requires, normally incorrect weights are removed from the production line alowing good products to go to the next processing stage. This can all be recorded either locally at the checkweigher or using suitable software tools such as SPC@Enterprise. Dynamic checkweighers (with or without integrated metal detectors) provide an enormous amount of important data about the actual performance (and thus efficiency) of production lines due to their complete recording of all measurement results. In order to be able to evaluate and assess this data more easily, Minebea-Intec offers a variety of data interfaces, as well as ready-to-use, bi-directionally communicating evaluation tools such as SPC@Enterprise.
What are typical applications for dynamic checkweighers?
Compliance with the pre-packaging regulation
Automatic checkweighers have been in use since about 1965. Many applications of these scales are subject to national legislation regarding minimum and average weights Manufacturers of pre-packaged products with a uniform nominal weight displayed (and thus sold) to the end user must ensure that the products put into circulation do not fall below certain nominal values. Compliance with this rule is ultimately the responsibility of the manufacturers of such pre-packaging. In order to comply with this prepackage regulation, many countries in the world additionally require an (independent) periodic inspection (in Europe "verification") of the measuring equipment (checkweighers) used in these production facilities. In the EU, Russia and Japan, in addition to initial placing on the market and periodic on-site inspections (verification, recalibration), a type approval of these measuring instruments (checkweighers) is required by law in these regions, which proves the measuring accuracy of the instruments used based on previous tests by nationally accredited institutes (e.g. PTB, NMI etc.). (EU:MID regulation). The type approval is a prerequisite for putting a checkweigher into operation or recalibrating it on site under real production conditions.
Products to be weighed can contain liquids, powders but also solid components, which can have an infinite number of different weight values. However, certain products contain a finite nominal number of components whose individual weights are more or less known. Such products can therefore (theoretically) only assume a finite number of weight values. In particular, one might think of only one or a few missing component(s) that make a product (final packaging) incomplete. The task of a dynamic checkweigher is to check products for completeness and to eject incomplete products from the production process with sufficient measurement reliability. Completeness control is used both for individual packaging and for end-of-line packaging such as cartons.
A further task is the purely internal quality control, where a deviation of the target weight (both over- and underweight) indicates faulty production, e.g. the weight control of a casting, from the weight of which it can be seen whether there is, for example, a shrinkage (air bubble inside), which requires the rejection of the controlled part.
In the applications mentioned above, classification is also used, but only in the classification of good and bad, and certain weight values are therefore accepted or rejected. Another application, however, is classification in the sense of class assignment. Often several successive discharge mechanisms are used to feed products of different weight classes into different further lanes or collection containers, which then only contain products with weights within the previously defined weight ranges. For example, a collection container no. 1 contains products with weights between 100g and 120g, collection container no. 2 contains only products with weights between 120g and 140g, etc.
Optimization of the filling and dosing processes (trend controller)
Another task of a checkweigher can be the active control of a filling process. The so-called trend controller of a checkweigher has the task of influencing the filling, portioning or slicing machines upstream of the checkweigher by means of corresponding control signals in such a way that the weight value defined as the target weight is reached and kept as constant as possible. For this purpose the weighing results are evaluated accordingly. As a result of the evaluation, a control signal (manipulated variable) is sent, which is used to adjust the filling unit. The control parameters entered determine the type of evaluation and the control characteristics. These must be adapted to the characteristics of the filling unit to be controlled. The parameter "target value" defines the desired target weight to be maintained.
Fields of application of checkweighers in various industrial sectors
In order to give a better impression of what checkweighers are used for, please find below examples of the tasks performed by the products in the various industrial sectors.
FPV control (e.g. according to international OIML R 87 recommendations)
- Food: ready meals, containers of baking mixes, chewing gum packs, confectionery
- Building materials industry: cement, lime, plaster, mortar
Completeness check (check number per packaging unit)
- Plastic: toy containers, injection moulded parts
- Metalworking: screw assortments, ball bearings, DIY articles, furniture and construction fittings
- Food: ice cream, confectionery, bakery products, frozen food
- Cosmetics: Hygiene products
- Publishers: Books
- Pharma: Cartons with blister pack and package insert
Quality control (testing of quality characteristics by weight)
- Plastic: wall thickness, air bubbles
- Metal processing: Check castings for blowholes, check punched and moulded parts
- Injection molded parts: completeness, air inclusions
- Baked goods: Dough pieces
- Food: Ready-to-eat meals, containers of baking mixes, chewing gum packs, confectionery and other
- Plastic container: stability, wall thickness
Classification (distribution in weight classes)
- Food: Non-balanced products such as fruit, vegetables, meat, fish
- Natural products: Plant cuttings, flower bulbs
- Food: dairy products, ice cream, confectionery, bakery products, spices, baking powder, pasta
- Building materials industry: cement, lime, plaster, mortar
Peripherals (accessories) for checkweighers
A variety of application-specific peripherals are available, such as suitable reject systems. As part of our checkweigher solutions we offer a wide range of high-speed reject systems - for reliable and effective rejection of products with weight deviations beyond the defined tolerance. The rejection systems include pusher, blower, swivel arms and multisegment rejection. In addition, there is a wide range of sorting devices, (monitoring) sensors, hoods, collection containers, special software and interfaces, through which our sales engineers can provide advice.
Which features of checkweighers are important?
Dynamic checkweighers must meet both current and future industry requirements to guarantee a safe production chain. Product changes are not uncommon, which requires a high degree of flexibility of the scales. Minebea Intec offers a wide variety of dynamic checkweighers, all of which can be configured to individual requirements. The weighing belt can also be dismantled later without tools to allow adjustments and minimize downtime.
Important features are high measuring precision and throughput. For example, the Flexus® in-line checkweigher can achieve a product throughput of up to 600 pieces per minute. This product is also available as a high-speed version, offering extreme process speeds of up to 2.6 m/s. The necessary precision is ensured by the use of state-of-the-art electromagnetic force compensation (EMFC) technology. For very special applications, the company provides tailor-made solutions. For example, the frame construction of Flexus® allows the assembly of many different additional modules to offer suitable solutions for special applications. Thanks to its modular and configurable design, the Synus modular checkweigher covers a wide range of applications, including those requiring legal metrology. It is possible to integrate the Vistus metal detection technology even after the initial installation. The checkweigher can thus be extended to a combination instrument. For moderate requirements and limited budgets: the models in the Essentus® range are tailored to the needs of individual user groups: The in-line checkweigher is available in two versions: The Essentus efficiency offers basic features, while the Essentus performance offers advanced functions. Both devices are available for different weight classes: Firstly as an L version, which is suitable for light products up to 6 kilos - and then as an H version, which can weigh heavy products of up to 60 kilos.
Hygienic design also plays an important role when using a checkweigher for the food and pharma industry: The guidelines of hygienic design form the basis for open processes in food production. In food production, product contamination by microorganisms such as bacteria or fungi is an omnipresent risk. Hygienic solutions means the hygienic design of parts, components and production facilities to guarantee complete cleanability. The multi-track systems from Minebea Intec are suitable for systems with several tracks: These can carry out check weighing in up to six tracks simultaneously and can be managed simultaneously. The tracks can be individually controlled. Depending on the product, different variants are available, such as pushers, which push the product sideways off the belt, blowers, which remove the product from the belt using compressed air, swivel arms, which lift defective products off the belt, or multi-segment separators, which minimize material loss through a multi-part separation system.
Another possibility is the use of combination devices, like the Flexus® Combi or the Cosynus, which are a combination of checkweigher and metal detector. In addition to checkweigher functionality, these offer reliable detection of foreign objects through the integrated Vistus metal detection technology. This solution considerably reduces the space required on the line and optimizes user control. Both the checkweigher and the metal detector are only controlled via a common user interface. Some companies fit 3rd party metal detection devices or checkweighers to their own units. Minebea Intec is one of the few companies in the world that design and build checkweighers and metal detectors in one factory.
If you have further questions on this topic or are looking for an individual solution for your application, please use the contact form below or send an e-mail to our sales team.
Trend Control for Checkweighers
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