Despite years of experience I still find that the issues thrown up by bag making machines have a curious fascination. Producers of the supermarket bags we all love to hate are paid in fractions of pence per bag so speed and reliability of throughput are fundamental to their profitability.

As is often the case here again the devil is in the detail. What the producer is looking for is a highest possible continuous cycle rate. The key word here is ‘continuous’ as high speed with significant downtime to clear blockages is not acceptable and anyway defeats the object.

In this instance the bag machine I am thinking of is producing the type of semi clear bag that you find on fruit and vegetable self serve displays in supermarkets. The machine is processing a continuous tube of film that is sliced and sealed by a heated cutter bar in indexed lengths. So the need is for a smooth flow of film through the cutter bar at maximum speed. Film is not a material which likes to flow without support and this it has to do for critical fractions of a second from the final feeder rollers to the heated cutter bar and beyond to the pick up tapes carrying it to the collation point. Air fingers help to support it through this tricky transition but the key is resolving the contradiction of a smooth flow with a stop/start process.

Experience has shown me that the ideal profile for calculating the requirements is Sine. Step profile is too harsh causing bunching and jams, Triangle too slow in acceleration/deceleration to feed enough film in the time allowed and Trapezoidal is better but still results in an unacceptable degree of ruckling and jams. The primacy of a smooth profile is underlined when you consider that the production requirement will be of the order of 300 bags a minute, which means one bag every fifth of a second. As half of this fifth of a second is taken with cutting and sealing and therefore film transport stopped, so a standard 250mm bag length has to be started, fed and stopped in the remaining tenth of a second.

The result is a happy producer who has squeezed the optimum production and margin from his machine while maintaining an uninterrupted flow.

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