How does a multi-head weigher work?

Multi-head weighers (also known as combination scales) are the equipment standard for weighing applications in a variety of industries.

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But for people who are evaluating their first packaging automation project, or even starting with a semi-automatic packaging system, the concept is something which might benefit from some explanation.

At a basic level, a multi-head weigher takes bulk product and weighs it into smaller increments according to the weights programmed into its software.

That bulk product is fed into the scale through the infeed funnel at the top, generally via an incline conveyor or a bucket elevator.

The top cone and feed pans vibrate, and gently move that product out from the center toward the buckets mounted around the edge of the scale. The system has various options and software settings which are adjusted for the product and fill weight.

For some applications, the scale will have dimpled steel contact surfaces, which provide less surface area for sticky products – such as gummies – to stick to. The sizes of the buckets also vary based on the product being weighed and the fill amount. While every application is different, bulkier product and/or larger containers generally leads to larger buckets (while the inverse is also true).

Each of the weigh buckets has its own load cell, which are constantly weighing the amount of product they contain, all while product continues to feed into them.

In order to reach the target weight, the software of the scale chooses the sum of a combination of buckets which add up to the correct amount.

An example of how this might play out is depicted in the figure below. With the target weight of a particular package set at 100 grams, the system evaluates all 14 weigh buckets and chooses to drop buckets 1, 3, 10, and 12. A fraction of a second later, it will reevaluate all of the buckets and choose the next combination.

The two main advantages of combination weighers are speed and accuracy.

Each of the load cells on an nVenia OHLSON Brand multi-head weigher is extremely accurate, with accuracies down to fractions of a gram. With additional checkweighing and other modifications, that accuracy range can be tightened up even further.

In addition, a multi-head weigher does not face the same issues with weighing clumps that might be present in a linear scale.

The other main advantage is speed. Because each of the heads on the scale are constantly replenishing and weighing the amount of product they contain, they can move much faster than an operator using a manual scale.

Standard multi-head weigher models under nVenia's OHLSON Brand range from 10 to 24 heads.  The more weighing heads a scale has, the more options its computer has to create combinations which hit the correct amount; ultimately resulting in higher possible speeds and better accuracy.

Have a weighing need? Request a quote and we’ll help you find the best weigher for your product.

Weighing machine

Multihead weigher (28 heads) Multihead weigher (24 heads) with memory hoppers for quick discharge. Combination of partial weights with multihead weigher (14 heads), target weight 250g.

A multihead weigher is a fast, accurate and reliable weighing machine, used in packing both food and non-food products.[1]

History

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The multihead weigher was invented and developed by Ishida in the 1970s and launched into the food industry across the world.[2][3][4][5]

Today this kind of machine, thanks to its high speed and accuracy, has achieved widespread adoption in the packaging industry and is produced worldwide by a number of manufacturers. Some manufacturers offer complete packaging lines, integrating the multihead weigher with other packaging machinery ranging from bagmakers (including Vertical Form Fill and Seal bagmakers) to traysealers and inspection systems. The latter include checkweighers and X-ray inspection systems.

How multihead weighing works

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A ‘typical target’ weight per pack might be 100 grams of a product. The product is fed[6] to the top of the multihead weigher where it is dispersed to the pool hoppers. Each pool hopper drops the product into a weigh hopper beneath it as soon as the weigh hopper becomes empty.

The weigher’s computer determines the weight of product in each individual weigh hopper and identifies which combination contains the weight closest to the target weight of 100g. The multihead weigher opens all the hoppers of this combination and the product falls, via a discharge chute, into a bagmaker or, alternatively, into a distribution system which places the product, for example, into trays.

Dispersion is normally by gravity, vibration or centrifugal force, while feeding can be driven by vibration, gravity, belts, or screw systems.

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An extra layer of hoppers (‘booster hoppers’) can be added to store product which has been weighed in the weigh hoppers but not used in a weighment, thus increasing the number of suitable combinations available to the computer and so increasing speed and accuracy.

How multihead weighing can help a business

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Multihead weighing can help in the following ways:

Filling bags

The range of bags which can be filled using multihead weighers is immense. At one end of the scale are large catering packs of many kilogrammes. At the other are small bags of crisps which can be handled at high speed and efficiency.

Mix-weighing

Products containing up to eight components can be mixed on a multihead weigher, very accurately at high speeds. The weigher is divided into sections, each with its own infeed. For example, a breakfast cereal containing hazelnuts and dried fruit plus two relatively cheap ingredients, could be weighed on a multihead with say eight heads devoted to each of the more expensive components and four heads to each of the other two. This would ensure high weighing speed while ensuring that overfilling of the expensive ingredients was negligible.

Placing into trays

A well-engineered distribution system enables you to combine the speed and accuracy of multihead weighing with precise, splash-free delivery of product into trays.

Applications

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Multihead weighers were used initially for weighing certain vegetables. Their use expanded exponentially in the 1970s and 1980s when they were applied to the rapid weighing of snacks and confectionery into bags. What cherry tomatoes and crisps had in common was that they flowed easily through the machine and into the pack, with no more encouragement than gravity and a moderate level of vibration of the feeders. Since then, the accuracy and relative speed have been extended to many products which would in the early days of the technology have been seen as difficult to handle.

Sticky products

Fresh meat and fish, whether in a sauce or not, poultry and cheese (including grated cheese) can be moved along by using belts or screw feeders rather than vibration.

Granules and powders

While free-flowing, fine-grained powders can be weighed more cheaply by other means (such as cut-gate or linear weighers, or volumetric feeders), granules such as coffee granules and products such as loose tea can be weighed on today’s multiheads.

Fragile products

Weighers with more shallow angles of descent and various cushioned inserts have made it possible to pack delicate and brittle items such as hand-made chocolates and gourmet biscuits. These are often paired with baggers or other packaging systems designed to handle fragile products.[7]

Complex products

Using mix-weighing combined with a distribution system tailored to deliver separate components into a tray, a ready meal can be assembled with just the right quantities of, say, rice, meat and vegetables in the appropriate compartments.

References

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• Yam, K. L., "Encyclopedia of Packaging Technology", John Wiley & Sons, 2009, ISBN 978-0-470-08704-6

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