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Spring mats: complex structures manufactured under one roof

29 December 2015

Fully accredited to the TS16949 automotive quality management standard, William Hughes began supplying the automotive industry nearly 30 years ago, and this important sector now forms the largest part of its business.

The modern spring mat is a ‘composite structure’, comprising the best attributes of two important engineering materials: the spring-like quality of steel and the added functionality delivered by moulded plastic features
The modern spring mat is a ‘composite structure’, comprising the best attributes of two important engineering materials: the spring-like quality of steel and the added functionality delivered by moulded plastic features

Among the company’s automotive market activities is the production of spring mats, or car seat frames. These complex assemblies unusually require a mixture of technologies not normally found in the same factory. Seat mats require expertise in wire bending and injection moulding, the moulding tools requiring such high levels of accuracy that the wire parts have to be made in close proximity in order to ensure that they will fit the mould tool.

These products are in demand from a number of tier one automotive suppliers such as Johnson Controls and Lear Corporation who, in turn, supply global OEMs, including Volkswagen, Kia and Mercedes. Some 500,000 seat mats made by William Hughes end up in Volkswagen cars every year, along with 120,000 for Kia.

“In the same way that coil springs are located under the surfaces of bed mattresses, so spring mats sit beneath car seat covers,” explains Emma Burgon, engineering director at William Hughes. “They are manufactured from bent wire forms known as zig-zag wires. These high tensile/spring steel wires are currently produced in three sizes: 3.0, 3.5 and 4.0mm diameter, although other sizes can be accommodated upon request.”

The wires are formed on seven Wafios BT3.2 CNC multiple-head wire bending machines, located at William Hughes’ facility in Plovdiv, Bulgaria. However, the same process can easily be replicated at the company’s UK plant in Stalbridge, Dorset. Zig-zag wires, which can typically comprise 50 bends or more, are produced at rates of around 225 per hour. Each component is stress relieved at the same rate using in-line ovens at a temperature of 320°C. The company is currently exploring ways to stress relieve the parts via induction heating, and it is anticipated that the project will be completed by April 2016.

Traditionally, the finished spring mats would sit freely within the seat structure or be secured together using steel clips. However, in the past ten years a distinct industry trend has seen the emergence of over-moulded spring mats. This new process dispenses with the need for metal retaining clips, providing an assembly that is both strong and quicker to manufacture. Emma Burgon again:

“The over-moulding process allows for variations in the finished assembly. In car seat assemblies, retaining bars for the seat cloth can be included or, where heated seats are required, connectors and cabling can be incorporated so that heating elements can be simply plugged into encapsulated boots.”

Essentially the modern day spring mat has become what is known as a ‘composite structure’, comprising the best attributes of two important engineering materials: the spring-like quality of steel (after all, comfort is vital); and the added functionality delivered by moulded plastic features. 

Once cooled, the zig-zag wires are loaded into two special vertical injection moulding machines providing 250 tonnes of force and featuring 1,800mm diameter rotary ram tables. The injection moulding machines (supplied by Battenfeld and Multiplas) need to be vertical as conventional horizontal models would fail to support the wires. The machines are configured with two bases located on a rotary table with a single top. The bare zig zag wires are loaded into one of the bases, which then rotates to meet the descending top so that molten plastic can be injected into the mould tool. 

“At present, the plastics used are either polypropylene or POM [polyoxymethylene], while the colour options are black or white, although again we can happily accommodate specific customer requests,” says Emma. “While one zig zag wire is being over-moulded, the operator is busy loading another wire in the opposite base. The cycle time to over-mould each component is typically between 25 and 45 seconds, depending on the part.”

Any 2D or 3D wire form that involves twists, turns, spirals, protrusions, indentations – or just plain bends – can be accommodated. William Hughes is now looking to establish cellular type manufacturing for over-moulded spring mats that will see bending, stress relieving, cooling and over-moulding take place in consecutive, co-located processes. Emma Burgon concludes:

“Importantly, the precision of the tooling is vital as the wire needs to sit perfectly centrally in the tool to allow uniform plastic flow and ensure no potential for squeaking. Here, the company works with precision toolmakers who have proven their ability to deliver high quality, sophisticated mould tools time and time again. We can be up and running on a new set of tools within four to five months of initial concept.”

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