Manufacturing titanium is a complex process. Timet’s plant in Witton near Birmingham processes more than 5,000 tonnes of titanium 1 sponge’ (chlorinated titanium-containing rutile ore). The sponge is melted with recycled material and alloying elements, such as vanadium, aluminium, molybdenum, tin and zirconium in a vacuum arc reduction (VAR) furnace to produce cylindrical ingots weighing up to 6.5 tonnes. The VAR furnace comprised of a column of four one-metre diameter cylinders, rising to a height of 30 metres, and operates at temperatures in excess of 2,000°C under a vacuum of 10-3 torr.
The Importance of Furnace Seals
Molten titanium is very sensitive to the presence of oxygen and nitrogen, so effective sealing of the bonds between each column section is essential to preserving the furnace vacuum. ‘Each batch can be worth up to £60K, so seal failures can be expensive,’ explained Timet plant engineer, Bob Allen. Sealing is so important that the furnace is stripped down between melts and the furnace’s top and bottom joints seals are replaced. Timet had been using spliced cord silicone ring seals made from vulcanised extruded silicone, but found the quality of jointed sealing varied owing to the materials used in the joint, which proved less reliable than they required.
‘O-ring performance is critical since failure would not only result in scrapping of products, but also disrupt production. With the jointed seals we have had instances where the seal had failed during the melt, despite pre-melt vacuum tests. Sometimes we find that seals leak during pre-production vacuum tests, which is not so serious, but we are then faced with stripping down the furnace and starting again, wasting valuable production time,’ he added.
Fully Moulded Silicone Rubber Seals
To overcome these problems, Bob Allen began looking for fully moulded silicone rubber seals. ‘We’d reached the point where we had to find a seal with consistent tolerances, offering excellent performance under vacuum and moderate temperatures, to avoid production hold-ups,’ said Allen.
Timet eventually chose silicone rubber seals from Precision Polymer Engineering. ‘Precision Polymer Engineering demonstrated that they could manufacture moulded silicone rubber seals to a consistent level of tolerance, leading to a better sealing performance,’ said Allen.
Improving Manufacturing Tolerances
Having demonstrated the Precision Polymer Engineering seals could deliver the performance Timet was looking for, Precision Polymer Engineering then went a stage further and refined the design of the seal to reduce tolerances. This went some way to addressing a potential source of seal failure which, although rare, could have devastating consequences because of the type of equipment used by Timet in certain areas of production. Precision Polymer Engineering developed a higher specification moulding tool, reducing the standard ISO3302-1 tolerance on a moulded ring by half. ‘This gave a better fit to the seal, reducing the risk of it not fitting correctly,’ explained Allen.
Quality Assurance at The Manufacturer’s End
Precision Polymer Engineering conducts a full dimensional analysis of each seal before it is dispatched to Timet reducing inspection times at the titanium manufacturer.
‘Precision Polymer Engineering has demonstrated how well-designed, moulded silicone seals can reduce plant downtime, improve plant safety, and contribute to the overall quality of our production processes by ensuring more consistent production conditions,’ said Bob Allen.