Signal conditioner aids seal strength testing

Status Instruments is helping ensure manufacturers minimise the problems associated with lid failure on heat-sealed packs, tubs and containers.

Status Instruments is helping ensure manufacturers minimise the problems associated with lid failure on heat-sealed packs, tubs and containers.

Its Medacs signal conditioner and HMI constitute a vital part of a new seal strength measurement system developed by leading exponent of this technology, Insteng Automation.

A rapidly increasing range of food and non-food products ranging from yoghurts, soups and ready meals through to domestic cleaners, personal care and industrial products are now packed in semirigid containers, closed using heat-sealed flexible plastic lids.

The requirement to demonstrate due diligence and the need to exercise a duty of care means that formalised seal strength tests are now being performed by a wider range of processors and packers than ever before.

Seal strength has become a critical control point and seal failure at any part of the manufacturing or supply chain, can have expensive and extensive consequences.

Sealed containers that fail during production line handling, subsequent processing or during transport to the retailer can cause the rejection of complete batches due to contamination and such failures also bring into question the strength of other containers in the batch especially if there are no documented measurements of seal strength.

Seal testing is not new, and today's test protocols are based on those developed by the military to test the strength of seals on army ration pouches.

The protocols are used in production lines to ensure that not only are the packaging materials sealing as expected and the sealing machine control systems achieving the optimum conditions required for successful sealing but also to ensure that seal strength is sufficient to allow the closed container to survive subsequent handling and processing so that it protects its contents right through to the consumer.

Until recently, test equipment has been relatively expensive, generally using a PLC to program the specific pressure profile ramps.

While this has tended to limit their use, Insteng believes its new lower cost seal strength measurement system will allow increased testing in manufacturing plants and laboratories.

A typical test involves applying a small pressure of say 2lb/in2 to inflate the container.

This pressure is held for a fixed time of a few seconds and then increased at a known controlled rate until the seal ruptures or the maximum test pressure is reached.

The Insteng unit consists essentially of a pressure generator, Medacs dual channel signal conditioning unit, HMI unit, pressure sensor and probe for introducing the air under the seal.

The Medacs dual-channel unit in conjunction with the HMI, is used to control and monitor the whole test procedure.

Unlike conventional signal conditioners, Medacs can be configured to monitor the relationship between different inputs, carry out calculations and comparisons on the variables and then provide outputs based on the result.

The secret of the flexibility lies in a simple yet imaginative piece of software developed by Status called TFML which, once downloaded into the unit, provides custom functionality from an otherwise standard unit.

A specific TFML program reads Medacs inputs, applies a transfer function, and produces an output, hence the acronym TFML - Transfer Function Module Library.

In other words, TFML offers the capability of inserting a transfer function between the input and output to provide a special control function.

Channel one takes the 4-20mA output (proportional to 0-4000mbar) from a pressure sensor and uses this to control the pressure generator.

The pressure is input to the sealed package which may burst at some unknown higher pressure.

Channel two controls two relays, one which opens while the pressure is being ramped up and one which opens if the maximum test pressure is reached without seal failure.

By comparing the generated pressure with the measured one, the unit can sense if the package bursts and the generator can be switched off.

Likewise if the maximum test pressure is reached the generator is switched off and the peak pressure recorded.

All pressure results are recorded in the HMI unit.

Seal strength is a complex function of a range of factors including container and lid types and materials of construction, sealing machine style and seal tool profile as well as the interaction between the seal, the product and any subsequent processing.

The power of the TFML and the ability to program the tester via the HMI screen allows the tester to be configured to measure seal strength in virtually any situation.

As Gareth Burris from Insteng concludes: "We selected Medacs because of its flexibility together with its low cost and its ability to accommodate both high accuracy analogue and digital signals.

This combination has allowed us to break new boundaries with our latest test equipment.

The HMI provides the user with a simple friendly interface and is easily configured for a wide range of programmable test procedures.

The new unit compares very favourably in technical terms with the PLC based units and we are confident that the price reductions we are able to pass on to the customer will promote increased testing and due diligence".

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