Drying system improves safety air bag manufacture

A carbon infra-red drying system, from Heraeus Noblelight, is helping Airbags International to improve the manufacture of side impact airbags.

A carbon infra-red drying system, from Heraeus Noblelight, is helping Airbags International to improve the manufacture of side impact airbags.

The system has also significantly reduced scrap levels on a coating line at the company's Congleton factory.

Airbags International is a company within the Autoliv organisation, an automotive safety products provider, both in seat belts and airbags, whose list of innovations includes the side-impact airbag for chest protection, the inflatable curtain for head protection in side impacts and the anti-whiplash seat.

At its Congleton factory, Airbags International manufactures both frontal airbags and inflatable curtains.

Both of these products feature a cushion which is folded in a specific way to make it unfold fast and safely in the event of a vehicle collision.

The cushions, for both frontal airbags and inflatable cushions are produced from Nylon 66, which is woven on-site at Congleton.

An important stage in the finishing process is the silicon coating of the nylon web, after it has been washed and dried and then fixed in a Stenter.

The smooth coating ensures that the cushion will unfold without sticking when deployed, at inflation times as fast as 50 thousandths of a second.

Its smoothness also prevents any possible damage to facial skin.

However, in common with all textile factories, the environmental humidity is quite high and, as the Nylon 66 fabric is very hydroscopic, its moisture content can be as much as three per cent to five per cent when the relative humidity rises above 38 per cent.

That was causing a problem at the coating stage for the heavier cloth used for inflatable curtains.

Consequently, Airbags International decided to investigate ways to reduce the cloth's moisture content down to two per cent to ensure the correct adhesion, drying and surface finish of the silicon coating and to reduce scrap levels.

After rejecting overall humidity control in the coating area because of the inordinate cost, on-site trials were carried out using carbon infra-red emitters from Heraeus Noblelight.

Using the data obtained in tests to establish the correlation between the relative humidity, the moisture content of the cloth and the temperature of the cloth, the infra-red heaters were operated to determine the heating profiles necessary to achieve the required moisture reductions for the various cloth recipes.

As a result, a 120kW carbon, medium wave infra-red system was installed and it proved so successful that it achieved pay-back in just 10 rolls of fabric said Heraeus.

In operation, the infra-red system is controlled by an optical pyrometer, which measures the surface temperature of the fabric and then transmits a signal to regulate the power of the 64, 2kW emitters within the infra-red module, which is mounted above the fabric web.

The cloth is coated with silicone on both sides but the critical heating operation takes place just before the coating is applied to the bottom side.

It is essential that the moisture is removed before this bottom coating is applied, otherwise it will be sealed in, potentially causing damage to the final surface finish.

The new system has been designed to heat fabric from ambient temperature to around 120 degrees C, at line speeds of up to 30 metres per minute, to suit web widths of two metres and 2.4 metres.

It has proved ideal for this application, as Mark Smith, project engineer at Airbags explained: "The infra-red module has been retrofitted very easily in to the coating line, with very little space requirement.

Operationally, because medium wave infra-red targets the water rather than the fabric, there has been no burning or scorching, as there could have been with alternative drying systems.

And the fast response of the carbon emitters also means that there is no damage to the coated fabric in the event of unexpected line stoppage.

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