Modular tooling helps drive innovation
4 min read
Dean Palmer reports on a redesigned AC inverter drive that replaces a sheet metal cabinet with a composite plastic
In 2003, when Control Techniques was looking to re-design the larger models in its Unidrive SP drives range, it took a fresh look at every aspect of drive construction - including the materials used. "The concept of replacing a conventional sheet steel chassis with a plastic alternative offered some worthwhile technical benefits," explained Tony Fleming, mechanical engineering design manager at the company. "Not only was there the potential to reduce manufacturing costs, but we could reduce 'creepage' and clearance within the product." This, he said, could dramatically reduce the product footprint - realising the goal of an increased power-to-size ratio. The four models in its high power Unidrive SP AC range of drives now use a one-piece chassis constructed entirely from glass-reinforced polyester sheet moulding compound (SMC). The drives, which were launched around nine months ago, will sell in volumes of between 500 and 4,000 per year for each of the SP sizes, so this is not mass production. Fleming told Eureka: "The key to making this project cost effective was to make the chassis tooling modular. We achieved this by looking for a 'telescopic' tooling solution in a common bolster. We decided to constrain the width of the drives, retain a constant depth and design constant-geometry chassis end-features." This yielded a workable tooling solution - with length variation coming only from growing the centre insert of the tool, he said. According to Fleming, the part count fell from 10 or 15 parts on the original sheet metal chassis, to a one-piece SMC moulding. "We reduced the footprint of the drives by 10-15%, which saves customers valuable space in control cubicles on their factory floors," he said. "Weight reduction is around 10% and the tooling pays for itself within six months. The manufacturing cost of the sheet metal chassis was £178 compared to just £27 for SMC." Manufacturers of large AC variable speed drives traditionally use folded and painted sheet metal fabrications for large drives. But these are costly to manufacture and involve many auxiliary supports for electrical insulation. Increased creepage and clearance allowance also leads to a larger footprint. The aim for all manufacturers is to increase the power-to-size ratio of the drives, in order to minimise the size of control cubicles in the customer's factory. At the start of the project, it was decided that the most suitable materials for the chassis would be gas injected thermoplastics, injection moulded thermoset BMC (bulk moulding compound) and thermoset glass-reinforced SMC. Initial tests showed that injection moulded BMC did not offer the strength requirements - the main issues being its non-uniformity of glass length and glass-distribution. Controlling the uniformity of the glass fibres during injection moulding is difficult. Gas injected thermoplastic were also discounted for similar reasons, although many structural versions are on the market. So the focus moved to SMC, which is used successfully in the automotive business, for car bonnets and roofs. However, it has made little impact in the complex shapes required for large drives chassis. SMC is manufactured in a continuous, in-line process, with the material being sheathed top and bottom with a plastic film. A paste is prepared comprising resin, styrene, heat-activating catalysts, inert fillers, release agents and thickeners. This is spread onto the bottom film and chopped glass fibres are then carefully distributed onto the paste. The top film is then introduced and the sandwich is rolled into a pre-determined thickness. The main way of processing SMC is by compression moulding. The film is stripped off and the material is cut up into suitable pieces, comprising the 'charge' for the mould tool. Heated moulds are used and pressure is applied. The base resin, a thermosetting material, cures and hardens. SMC components are hard, rigid mouldings with excellent electrical resistance properties. "Compression moulding with SMC means we didn't need hot runners or gates on the mould tool as you would for thermoplastics," said Fleming. SMC has excellent dimensional tolerance, with virtually no shrinkage. It met all the criteria set down by Control Techniques' own design engineers, and processes relatively simply as it uses compression moulding. Fleming said these modular, telescopic tooling techniques could not have been replicated easily using thermoplastics. Fleming's first steps on the project involved feasibility studies, where he sketched his vision of what the support chassis might look like when made from SMC. This gave the estimated weights and the data to make a sensible approach to a host of potential suppliers - including toolmakers in the UK, Spain, Germany, Czech Republic, Hungary, Italy and France. The company chose to work with an Italian tool maker, Terzistampi. "The tooling costs were 20% less than the quote from our UK supplier and the firm had more experience in working with modular tooling," said Fleming. The power terminals are fixed within the moulding during manufacturing, to form an integral part of the chassis. The strength of the material allows a complete 75kg drive to be lifted and positioned using a bracket fixed only to the power terminals. Stress-strain behaviour, with time and impact resistance of plastics, was a key consideration. These were important for environments in which shock and vibration in the region of 10-18g can be experienced, such as on a crane. Other factors had to be considered, including a need for excellent high-voltage, electrical tracking resistance and a desire for halogen-free flame retardant polymers, to satisfy forthcoming RoHS legislation and the ability to pass stringent UL tests. Electromagnetic compatibility (EMC) of the plastic frame might also be considered by some engineers to be a problem. At key stages of the 3D design (using Pro/Engineer), the structure was analysed using Finite Element Analysis. Two to three prototypes were produced for each size of SP drive. Certain parameters, established during the part qualification stages, are monitored and controlled at the production press. The problem, if anything, with this process is consistency of glass flow with the carrier resin. Part strength here is the concern. Prime factors for consistent mould quality are: charge pattern; tool temperatures; vacuum level; cycle time; and material quality. "SMC proved to be the solution in every aspect," said Fleming. "It has proved to be exceptionally strong - better in many ways than the steel equivalent - with a high dielectric strength. What's more, it cuts the cost of manufacturing so the end product becomes very competitive." The drives are now competitively priced as some of the cost reductions have been passed on to the customer. The UK trade moulder for the chassis is Mitras, although the drives are assembled by Control Techniques at its factory in Newtown, Wales. "Our objectives, as design engineers, are to reduce the number of parts, simplify manufacture and produce a product that's better than the competition," said Fleming. "The use of SMC, modular tooling and flame retardant resins have been the keys to unlocking these objectives."