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Multihead weighers are faster and more accurate than linear weighers, but are they always the better option? Lynda Searby finds out more and rounds up the latest kit

Where I come from, in the north of England, people don’t mince their words. But in the packaging industry it’s never that simple. A bagger is never a bagger, nor a filler just a filler. And a weigher is certainly never a weigher – it’s a linear weigher or a multihead weigher, a radial or a screw feed weigher.

The key to deciding whether a linear or multihead weigher is the best fit for a particular production line is understanding the distinction between these – and indeed other – types of weigher.

“In very basic terms, a linear weigher feeds product onto a weigh pan until target weight is achieved and then discharges,” explains Chris Bolton, sales and operations director at PFM Packaging Machinery.
Or put another way by Ian Hope, managing director of Lingwood Food Services, UK sales agent for Danish multihead manufacturer Bilwinco: “On a linear weigher product is fed into a weighing bucket until the desired amount is in the bucket. When the portion is ready, the product is emptied into the pack. During the time it takes to fill the weighing bucket no packs are filled.”

Linear weighers typically have two channels: a bulk-feed channel which fills the majority of the weight; and a small channel that tops it up more slowly to accurately reach the correct weight at the end of each cycle. This format is also known as a ‘rough and fine’ weigher and is often used where pieceweight is varied.

Multihead weighers – also known as combination weighers – on the other hand, feed a proportion of the target weight simultaneously into a number of weigh buckets or hoppers. The controls then establish which buckets hold the combination closest to target weight and signal these to be discharged.

“A multihead is actually 10 to 28 linear weighers built together. Here we do not fill the exact amount of product into each weighing bucket, but approximately a third of the target weight. Then the controls combine three different weighing buckets  in order to reach the correct portion size and releases these into the pack. Once this has been done, three other buckets are ready to be emptied,” says Hope.

Multihead variations

There are also variations on the combination multihead. Screw feeds are used in multihead systems where goods are particularly viscous and cannot be fed by vibration. This configuration is known as a screw feed weigher. Combination multiheads can also be supplied with the pockets in a linear rather than radial orientation, a format generally reserved for fresh or fragile products requiring very accurate feed control.

The different operating principles of linear and multihead weighers mean that fundamentally, linear weighers are slower and less accurate than multiheads, as James Hosford, European sales manager with Tna Europe, explains: “With multihead weighers, each channel does not aim to hit the target weight, nor does the amount in each bucket need to be precise. This allows the loading, and therefore, bagging, to be much quicker.”

What’s more, he adds, as the software system has thousands of combinations to choose from, the release of product is more accurate.

Ishida Europe marketing manager Torsten Giese provides some examples to illustrate the contrast in speed and accuracy between the two types of weigher. He says that ‘rough and fine’ linear weighers will typically achieve up to 50 products per minute, whereas multiheads can process several hundreds of weighments per minute. In terms of accuracy, he says that on a 1kg washing powder pack, a rough and fine weigher might achieve 5% accuracy, whereas a multihead will usually be within 1% of the target weight.

German firm Multipond has developed a patented ‘dosage mode’ for its multihead machines that can be used to deliver even greater speeds than would be possible using combination technology. UK managing director Geoff Tandy says this comes into its own when feeding a high speed thermoformer, for example, when weighing cereals into a dual compartment yogurt pot.

Given the superior speed and accuracy of multihead weighers, it is hard to see why anyone would opt to invest in a linear weigher.  Historically, the higher price of multiheads gave some buyers grounds for choosing linear weighers, but according to PFM’s Bolton, this is no longer a justification for most.

“The first multihead weighers became available in the late 1970s and since then the price differential between linear and multihead weighers has closed considerably. This means that in the majority of applications encountered by PFM, the speed and accuracy of multihead systems more than compensates for extra capital cost, particularly through reduced product giveaway,” he says.

In addition, Tandy says that the proliferation of cheap Chinese multihead machines has killed a lot of the linear market. His company stopped making conventional linear weighers altogether about three years ago to focus solely on multiheads.

Similarly, PFM says that the advance of small, entry level multiheads has meant that it has been nearly 10 years since it supplied linear weighers for use with its bagging machines.

Even producers of commodity food products such as rice and pasta, historically staunch linear weigher users, are turning to multihead technology, according to Ishida’s Giese.

“Where products were previously not considered high value enough for multihead weighing, greater pressure on margins is prompting a move to multihead weighing due to the lower giveaway,” he says.
Still, there are some pockets where linear weighers are hanging on to their market share.

“There will always be some applications where linear weighers are suitable, for example, in the packaging of smaller product runs where bulk output is not a major requirement,” notes Tna’s Hosford. “However, more and more users are turning to multihead weighers due to their increased speed, accuracy and comparable cost.”

And as innovation continues to forge ahead, the argument for multihead weighers is only going to become more compelling. Tna says it is continually developing new technology to increase speed without compromising on accuracy.

“For example, we are currently developing a close loop control which allows the distribution system, on-head flavouring systems and packaging system to communicate and adjust their processes to cope with changing factors at any point in the production line, enabling maximum efficiency,” says Hosford.

In addition, last year, Tna launched the new Intelli-weigh Omega multihead scale, which is said to increase weighing speeds by 30%.

Ishida’s Giese lists the main areas for advancement as the use of more sophisticated, faster electronics to calculate the best combinations and improvements to the interface between the weigher and packaging machine.

Multipond’s Tandy makes similar observations, saying “feeding product better onto the machine, increasing output and improving integration with packaging machines” are some of the main focus areas.
PFM’s Bolton, meanwhile, says the principal innovations in multihead weighing now lie in the electronic control systems that allow machines to function mechanically much more quickly and accurately. For example, PFM has developed new software and control systems for its MBP C1 and C3 weighers.

“The improvements in accuracy come from a new powerful microprocessor within the industrial PC used to control the weigher,” explains Bolton. “This runs newly designed software to cancel the effects of external vibrations close to the machine and control weight combinations in a fraction of the time compared to the previous system.”

Consequently, the prognosis for the long term survival of linear weighers appears bleak.

“Ultimately, linear weighing will probably die out,” predicts Giese. “As soon as people can switch, they will.”

New filling equipment round-up

Dico & Gravfil has launched a generation of liquid filling and capping equipment for use in the petrochemical, agro-chemical, household product and food industries. The Dico-Delta capper can achieve speeds of up to 400 caps per minute and the Gravfil-Excel liquid filler is capable of filling container volumes from 50ml to five litresl. According to Dico & Gravfil, the lines use dual servo-controlled filling technology for guaranteed fill speed optimisation. In addition, energy-saving Siemens hardware is on hand to reduce operational costs and advanced remote diagnostic capabilities is designed for maximising machine uptime.

Disocont Tersus is a new system from German firm Schenck Process for controlling material feeding machines such as gravimetric loss-in-weight feeders and screw weigh feeders. The new system has a touch screen interface, offers improved modularity and is said to be easier to install and use than previous models.

IMA Life has developed the Sterifill Smart aseptic filler to answer calls from pharmaceutical companies for quicker size changeovers, more compact layout and improved process control. The filling, stoppering and capping machine can handle vials of up to 500ml.

GEA Avapac has extended its Limited intervention (Li) range of 25kg bag filling systems for powders with the RBF-1000Li, a line with a higher capacity but the same small footprint as the RBF-800Li. The company’s Li technology enables companies to fully automate their plants from empty bag handling right through to finished pallets.