A recent project involved development of a polyurethane system for the manufacture of refrigerators. To achieve the process flexibility they sought, they found an ideal solution in power management company Eaton’s products and technical support.
Domestic and commercial refrigerators are manufactured using a polyurethane injection process where mixed chemicals are injected into a cavity for insulation and reinforcement purposes. After injection, the polyurethane expands within the cavity, generating a pressure as it does so. Accordingly, the products must be moulded using fixtures to avoid visual and/or structural problems.
Traditionally, most systems are designed to handle just a single product but this means that a new version is needed for every product change. This creates huge investment costs and the low production volumes make each system prohibitively expensive. Additionally, they create problems due to inefficiency as well as inflexibility. They have hinged walls and hydraulic upper tables, moved by positioning valves with no position feedback. Hydraulic cylinders are driven from bottom stroke to top end, with no ability to be stopped at an intermediate position or otherwise positioned numerically.
The inefficiency in this traditional design impacts productivity because in serial production, small adjustments are needed to compensate for deformation of moulds, fixture guides, product carriage and other parts. Most operators use tapes and bands to cover the difference between fixture and product, however, this process is labour-intensive, time-consuming and expensive.
MET’s idea was to develop a universal system that could be adjusted easily for different-sized products, giving flexibility, and therefore enabling customers to increase productivity and yield. The challenge was to develop a system with numerically controlled walls that could be moved in seconds to accommodate different product sizes, while withstanding forces generated by the injection pressure.
MET turned to Eaton for support in designing a hydraulically-activated press table to maintain the necessary force in the limited space required. It has four independent servo hydraulic axes positioned around the side and front walls, and allows the upper table to be positioned parallel to the product table. Control is achieved using closed loop feedback obtained from linear scales, with all four axes synchronised to a virtual axis. This motion control system allows speeds of one metre per second with 0.05 mm positioning accuracy for anywhere within the stroke. The table achieves this performance while working up to 30 tons pressure and handling a mass inertia of up to three tons.
Key to this hydraulic system are the Eaton Vickers K(B)FD/TG4V proportional valves, which provide a controlled hydraulic fluid flow in direct proportion to a command signal. The valves feature integral control electronics, with factory-set adjustments of gain, spool deadband compensation and offset to ensure consistent valve-to-valve repeatability.
MET’s universal solution improves production efficiency and throughput dramatically. Operators can adjust the length, width and depth of the system in seconds, allowing production change to a different model within a couple of minutes. This flexible system can also compensate for machining errors in producing the mould. Refrigerator bodies are manufactured from sheet steel using a combined blanking, punching and bending process. However, there could be discrepancies of tenths of mm between the refrigerator body sizes, so that in some cases they are slightly deeper than the fixture by for example 0.2 mm. MET system’s speed and precision of adjustment allows users to easily overcome problems such as these – in this case, by easily positioning the upper table 0.5 mm higher.
MET expects their customers to realise a return on investment (ROI) break-even point within one to two years if they have low production volumes and a large variety of products, or three to four years if their product variety is lower.
“Eaton’s advice and support as well as their products have contributed significantly to the project’s success, they provided us with a complete solution rather than just a kit of parts,” says to Murat Merdin, R&D Manager at MET. “We originally selected Eaton because their price/performance ratio was satisfactory, while allowing us to achieve the positioning accuracy and maximum travel speed we were looking for. As we worked together, we found Eaton’s local support to be invaluable. This was enhanced by effort from their local distributor Entek, particularly their engineer, Ulas Basa.”