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Lightweighting Improvements through Supplemental Electric Heating

Grahame Stuart* discusses how the application of electrode heating can assist thermal homogeneity and gob quality in lightweight container manufacture.

A lot of time and resources have been dedicated to developing forming processes and coatings that allow the container manufacturer to produce lighter, more environmentally friendly packaging to suit today’s market requirements. However, little has been said of the glass quality improvements that are necessary to make lightweighting a success.

It is true that the gob condition will directly influence the quality of the final product.  Small variations in gob weight, temperature, homogeneity and shape can result in loss of productivity through increased defects, particularly in coloured glasses

Problems with gob condition can be amplified as the width of the forehearth channel is increased, and with many large scale container manufacturers operating forehearths well in excess of 36” wide there may be significant scope for operational improvements.

Wide gas fired forehearths can be difficult to heat evenly across their entire depth and width meaning that the hottest glass will travel along the centre-line potentially leading to cooler glass along the sides and bottom of the channel resulting the in problems described above.

Even temperatures can be achieved much more easily in an ell-electric forehearth where special profile heating   elements and immersed electrodes can be designed to heat to coolest areas whilst allowing the hottest area, typically along the center-line, to cool naturally.

However, in plants where investment in gas fired forehearths has already been made it is possible to add supplemental energy to the process by adding in-glass electric heating.

The Electroglass Temptrim is one such system. Initially developed as a means of improving glass conditioning on smaller, more specialist sealed forehearths, the Temptrim has rapidly become popular with large scale container manufacturers.

The system allows accurate application of small amounts of power, typically only a few kilowatts, directly to the glass using special dry electrodes at points in the forehearth where it can correct temperature inhomogeneity of the glass entering the spout, improving gob shape, forming and overall productivity. The system consists of single or multiple heating zones and can be configured in several different ways in order to combat specific problems or to operate in different glass types. Each heating zone will typically consist of two or more electrodes powered from an air-cooled transformer. Automatic power and temperature control is included as standard. Other options include independent heating on each side of the forehearth allowing compensation of non-symmetrical temperature profiles. Typical improvements seen have been the reduction in gob weight differences from 12-15 grams to 1-2 grams and near zero rejects due to gob shape and temperature.

The electrodes employed in the system are a special design developed and manufactured by Electroglass. The dry electrode has many     features designed to prevent contact between dissimilar metals and help prevent the risk of   oxidization of the molybdenum. They require no water cooling, and system design avoids the need of electrode advancing during normal   forehearth operation. The usual service life of these is the full forehearth campaign.

An Electroglass Stepped Dry Electrode

The electrodes can be manufactured to suit different immersions and different forehearth construction and can be mounted horizontally or vertically. They are available with a protective coating upon request allowing them to be installed prior to glass.

A major advantage of the system is its ability to be installed without the need to stop the forehearth. An installation can take place in one or two days by an Electroglass Specialist Hot Drilling Engineer, ensuring a machine downtime of no more than 12 hours.

Once installed the system requires very little maintenance, other than routine recording of operating data and inspection of electrodes and cabling. As with all Electroglass systems, we invite customers to forward operating data to Electroglass allowing the system operation to be monitored and necessary advice given. This a free of charge service and one we have offered to our customers for more than twenty years.

With increased emphasis being placed on environmental protection, and glass manufacturers and their customers requiring higher and more exacting standards, the future for systems such as the Temptrim seems very bright.

Grahame Stuart is Technical Sales Manager at Electroglass Ltd

T: +44 (0)1268 565577

F: +44 (0)1268 565594

4 Brunel Road, Manor Trading Estate,
Benfleet, Essex, SS7 4PS, England