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Metal Casting Technologies : September 2008
TECHNICAL FEATURE Figure 4. Lost foam production lines at Group A (left) and at Group B (right) Z-axis horizontally. It is also claimed that the driving motor is separated from the table top so there is no moving of the motor itself, which would eliminate resonance to reduce the deformation on foam patterns. Automatic flask handling and pouring systems can be designed, manufactured and installed by several Chinese equipment suppliers, including sand filling and compaction, pouring stations, sand cooling, sand reclamation, and casting parts cleaning. A mechanical or conveyor transportation system is the predominant operation mode. It offers reliable operation with flexibility of process control at a reasonable cost. Current production lines in operation can pour up to 10,000 tons of casting per year. Figure 4 shows two lost foam production lines, one is at Group A with annual casting capacity of 5,000 ton and another at Group B with annual production output of 10,000 ton. Tooling Traditional work flow of tooling manufacturing is as follows: design mold based on casting and get the 2D drawing of molds; make wood molds; pour mold parts by sand casting, machine; finish and repair mold parts. This process takes a long lead time to get the finished tooling. The precision of tooling can be limited. Several top tooling suppliers have adopted the rapid prototyping method for tooling. A 3D design of tooling is first developed using CAD software. Computer files of the tooling design are transferred to the prototyping machine and molds are made in the prototyping machine in several hours. Prototyped molds can be sand, wax, layered paper, plastic, or sintered metal powder, depending on the prototyping method. Castings for the tooling are then made from the molds and machined. Prototyping method can greatly reduce the lead time for tooling. The lost foam process has been demonstrated as a good method to manufacture tooling for the lost foam process itself! A foam pattern can be machined 70 www.metals.rala.com.au by a digitally controlled machining center based upon the CAD file of tooling design. The foam pattern is then used to pour the mold casting by the lost foam process. This research has shown that tooling is the part of the whole process that has adopted the most advanced technologies in the lost foam casting industry in China. CAD/CAM is widely used for tooling design and manufacturing. Consumable materials The material used to produce foam patterns was expandable polystyrene (EPS) for packaging in the early stages of lost foam process development. Because the lost foam process is mostly used for iron and steel castings in China, carbon related defects became a big concern for lost foam foundries. Development work on foam materials for the lost foam process started in the 1990s. Now several companies can provide foam materials developed exclusively for the lost foam process, including EPS, expanded polymethyl methacrylate (EPMMA), blends of EPS and EPMMA, and certain copolymers such as styrene-methyl methacrylate copolymer (StMMA). A patented copolymer foam material (StMMA) from a Chinese supplier is sold in the Chinese domestic markets and exported to Korea, Europe, and the United States. A wide variety of glues are used to join two or more molded foam sections together to form the foam pattern in China: one- part cold glue, two-part cold glue, one-part hot-melt glue, etc.8 Modified polyvinyl acetate (PVAC) and polyacrylate/epoxy emulsions are the two most popular cold glues for the lost foam process. Hot melt glues include polyethly vinyl acetate (EVA) and polyamide (PA) based formulations. A study in China has shown the possibility to use recycled EPS as the primary material to formulate cold glue and hot melt glue for foam assembly. This has great implications in terms of enhancing glue compatibility with EPS foam and reducing EPS waste. Cold glues are used more often than hot melt glues. Gluing remains a mostly manual process in China, with automatic machines only used in some foundries.