Induction-heated moulds slash cycle times and costs

Induction heating of mould surfaces is key to a process that slashes moulding costs — especially for complex parts made from advanced composites. The first volume commercial products made using RocTool’s Cage System are likely to be car parts. These will be made of polyurethane and other thermoplastics – reinforced by glass fibre, foam cores and other additions that would ordinarily be difficult and expensive to fabricate. At least two major end users – one a machinery maker, the other a fabricator – report significant reductions in cycle time when making composite parts. The basic process applies magnetic fields at frequencies of around 50kHz from using copper conductors around the outside of the tool. These form a cage round the mould when it closes, to give the process its name. Only the mould surface is heated, because the induced currents are mainly restricted to the outer layers of the mould metal, because of the high working frequency. RocTool CEO Alex Guichard told Eureka that the process was developed as a faster way of making carbon fibre-framed bicycles. Guichard realised that this could be achieved by heating the plastic rather than the whole mould. Bike building “I got the idea that to accelerate the cycle time, you have to avoid heating the whole mould,” he said. “I did some trials in a garage with a partner. We started to heat just the carbon fibres but it did not work industrially so we switched to just heating the mould surface. Our first idea was to put the inductors inside the mould, then we put the inductors around the outside of the tool.” The inductor bars only form a complete cage when teeth engage on the front and rear of the moulds – joining the coil halves and completing the circuit. Applying 20-500kW at 50kHz induces current in only the surface of the mould because of the skin effect. The mould surface and the plastic then heats very rapidly. Once the power is turned off, water passing through cooling tubes within the mould cools it back down, after which the finished part is removed. The bulk of the mould is cold at all times, which accelerates cooling. The technique has begun to spread across industry. Krauss-Maffei, a leading manufacturer of injection moulding machines, developed a method called long fibre injection (LFI), which adds long fibres to moulded parts. Walter Tesche, managing director of its global polyurethane division, said it had experienced “problems with price and cycle time”. LFI parts involve mixing polyurethane with glass rovings for added strength. Tesche said that LFI is being combined with honeycomb to produce “extremely stiff parts with very low weight”. Applications include BMW instrument panels, a car roof and a tractor bonnet. In the case of the roof, the polyurethane is poured and glass fibres pressed into it. The mixture is placed in an initially cold mould so that the reaction between chemical ingredients does not start too early. Conventional moulding technology requires use of a heated upper mould to save time, but this can result in warpage and a difference between upper and lower surfaces. Tesche says of the Cage process: “There is an opportunity to make our process more economical. It could reduce cost-per-part in our industry by 15 to 25%.” Cycle time, he says, is typically reduced from 330s to 120s. The company first presented its combination of Cage and LFI technologies to customers at an in-house event in May. Another industry user is Azdel, which is part-owned by GE Plastics. Melanie Cook, president and general manager, says the company – which supplies glass-mat thermoplastics – is using the Cage Process to streamline the production of automotive bonnets. Lighter than SMC She claimed that a steel car bonnet might weigh 9.3kg and one made from SMC would weigh 10kg – but a high performance plastic composite part should only weigh 4.5kg. “This looks like Nirvana but we think we can get there,” she said. Azdel’s material combines a low density thermoplastic PBT core with glass-reinforced PC/PBT skins. The finished part has a modulus of 15GPa. In 56km/h crash tests, it offers improved damage resistance because it tends to spring back, improves energy resistance, and deforms in a way likely to cause less injury to pedestrians. In a demonstration using a 315-tonne press – equipped with the Cage system, using 300kW power from the induction generator – it produced quarter-sized car bonnets suitable for online painting in 250 seconds. Azdel intends to sell the sheet and licence the processing technology. Offline paintable automotive body parts will be available early next year and online paintable parts by the end of 2007. Cook said that the online technology requires that the panels go through painting as part of the body in white and have to be electrically conducting and able to withstand 210 deg C in the painting and subsequent heat curing process. Other end users are also interested in applying the technology to their own products. Ahmet Muderris, of Swiss company Composites-Busch, told Eureka: “We know this technology. We are very happy to see the path it is following.” The company uses carbon fibre, Kevlar and glass fibre-based composites in a number of applications, including luggage and both sports and medical equipment. “We were the first producers of carbon-fibre ice hockey sticks and the only company to make them for ten years,” he said. “Using carbon fibre inserts in watches allows them to be sold for a higher price. What we like about this process is the speed,” he added. Pointers * A high frequency magnetic field applies heat to the surface of mould tools * The process can halve production time in the moulding of complex composite parts, including those based on polyurethane * Composite polyurethane part costs are reduced by 15-25%