Multihead weighing: Growth, development and trends

MOST food products are not easy to weigh to a pre-determined weight.  The individual weight of each frozen chicken nugget or meatball in a 200g pack, for example, can vary considerably - meaning a pack can very quickly go from being underweight (i.e.180g) to extremely overweight (i.e. 220g) by adding just one extra piece.

Apart from the legal requirement of producing packs at their declared weight on the pack, product giveaway - effectively giving the consumer free product due to overfill - is wasteful, particularly in today's highly competitive markets.  Just as importantly, unreliable and inconsistent pack weights harm brand reputation.

The solution to this problem, first developed by Ishida in Japan in the 1970s, is the multihead weigher.  The mostly circular-designed weigher (there are also versions of linear design for sticky and fragile products) works in the following way:
(1) Product is fed from a vibratory feeder to the top of the weigher.
 (2) Using vibration, the product is evenly distributed across the top of the weigher to a set of pool hoppers where the product is stabilised.
(3)  The content of each pool hopper is discharged to a corresponding weigh hopper directly underneath, where the product is precisely weighed.
(4)  A built-in computer instantaneously calculates all possible weight combinations and selects the one that comes closest to the target weight.
(5) The selected weigh hoppers discharge their contents into a packaging machine.

The impact of this technology in food markets was particularly significant.  Product giveaway was reduced to approximately 1% from as much as 10% using manual weighing on bench scales. 

At the same time, accurate filling meant packs could be designed smaller, reducing the cost of packaging, storage and transport.  In terms of speed, even the earliest models were able to achieve around 30 packs per minute (ppm) for frozen meat products, and 20ppm for fresh meat.  Today's models can weigh chilled diced products at up to 280ppm.

Even more important to food manufacturers is the increased productivity that accurate weighing can bring.  For example, high-value meat, poultry and fish products previously wasted in overfill can now be packed into additional bags or trays.  Output can therefore be increased without additional input.

Over the years, continuing developments and enhancements have enabled multihead weighers to deliver even faster speeds and greater weighing accuracy.

The original multihead-weighing concept is ideal for easy-to-handle (mainly frozen) meat and poultry or fish, but there are many products that do not conform to these simple characteristics.  Another key factor has therefore been the introduction of features and application-specific multihead weighers that can deal with more difficult products.

Handling fresh, sticky products

Fresh meat and poultry are perfect examples and have contributed to the development of features such as anti-stick contact surfaces, plastic hoppers with scraper gates and belt feeders.  Equally important, the growth in pre-packed and ready-to-eat meat products means the need to handle ‘sticky' applications quickly and accurately has become even more critical, leading to a further adaptation of multihead weighing technology. 

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