Background and basics of the Lost Foam Casting Process

Transcription

1 Lost Foam Background and basics of the Lost Foam Casting Process Casting techniques using patterns made from foam have been referred to by a variety of names. Among these are lost foam, evaporative pattern casting, cavityless casting, evaporative foam casting, full mold, Styrocast, Foamcast, Styrocast, Policast, and foam vaporisation casting. The basics of the process are explained here. The origins of the lost foam casting process go back to the 1950s when the use of foam patterns for metal casting was patented by H F Shroyer on 15 April In Shroyer's patent, a pattern was machined from a block of expanded polystyrene (EPS) and supported by greensand during pouring. The foam pattern remained in-situ during casting and this process is known as the full mould process. In the process the pattern is usually now machined from an expanded polystyrene (EPS) block and is primarily used to make large, one-of-a kind castings. In 1964, M C Flemmings used un-bonded or loose sand with the process. This is known today as lost foam casting (LFC). With LFC, the foam pattern is moulded from polystyrene beads. The LFC Process The basic steps of the lost foam casting process are: 1. Making the pattern (bead pre-expansion and conditioning, tool preheat, pattern moulding, pattern aging) 2. Pattern/cluster assembly 3. Pattern coating and drying 4. Sand fill and compaction, usually with vibration 5. Metal casting and cooling 6. Shakeout, clean-up, and finishing The EPS or foam patterns are created from polystyrene beads. The beads start as hard granules which are then expanded by heating, usually with steam, allowed to stabilize and then moulded into the desired shape. This foam pattern is used to form the shape of the casting, coated with a refractory, and is then invested into the loose sand. Sand is introduced into all the voids and supports the foam patterns external form. As the metal is poured in the foam is evaporated as the metal is poured in. The sand needs to allow the escape of the gases that are evolved into the sand. As the sand is not bound, the separation of the sand from the casting, and subsequent reuse of the sand, is easy. More complex shapes can be made using multiple patterns glued together and gating and running systems can be added in the same way. Multiple patterns can also be assembled, attached to a generic, central foam piece called a tree, in the same way that investment castings can be assembled for casting. Metal pouring needs to be carefully controlled to ensure that there is no interruption or hesitation as this can lead to mould collapse and automatic pouring is commonly used but most metals can be cast using this process (steels and other low carbon content ferrous alloys may require some special processing to avoid pick-up of carbon from the EPS). Lost Foam castings today are used for many critical applications, including engine heads, marine motors, high-pressure pumps and valves and well as for large castings such as wind turbines. Process advantages and disadvantages Advantages of the process include the fact that complex shapes can be produced without the need for cores, no draft taper is required as the pattern is not removed prior to casting and cleaning is easier and requires fewer operations since there are no fins or parting lines to remove. Since foam easy to manipulate, carve, machine and glue, it means that there is a lot of design flexibility. The process also enables complex components to be produced as one part; other manufacturing processes would require the production of one or more parts that then have to be assembled. The ability to re-use the sand and the absence of any binders means that it is also considered to be environmentally friendly and it also easily allows for cast-in inserts and other cast in features. The two main disadvantages of the process are that pattern costs can be high for low volume applications and the patterns are easily damaged or distorted due to their low strength. Dies can be used to create the patterns and in this case there would be a large initial cost. Low carbon steel castings require special processing to avoid carbon pick-up as mentioned above. Variant of the Lost Foam Process Replicast To overcome the problem of carbon pick-up for steel castings with the lost foam casting process, Castings Technology International developed the Replicast process in which the polystyrene is fully burnt out before casting. This allows a wide range of alloys to be cast in the mould - from ultra low carbon stainless steel to nickel based alloys as the pick-up of carbon from the EPS pattern is eliminated whilst the advantages of the lost foam process are retained. Replicast compared with other processes In the Replicast process a ceramic shell is built up on the polystyrene pattern. On firing the ceramic, all traces of the polystyrene are removed before the mould is embedded in the supporting un-bonded sand. In many respects, the process is very similar to the Lost Wax (or investment casting) process but with certain key advantages. For example, polystyrene is lighter and relatively insensitive to temperature; patterns can be produced in very thin or thick walls (2-200mm); complex shapes can be formed by gluing sections, affording the design freedom of the Lost Foam process; the physical properties of polystyrene allow a much thinner ceramic shell to be produced; because the polystyrene is removed from the mould, a higher density pattern can be produced. Cti report that superior dimensional accuracy and surface finish can be achieved, equivalent to those from Lost Wax casting and the lighter patterns and ceramic moulds allows much larger castings to be produced than would realistically possible in any Lost Wax operation. Castings weighing in excess of 500kg, with a poured weight in excess of 1 tonne are possible. Compared with sand casting, the process can achieve a weight reduction commonly as high as 25%; a 50% reduction of feed metal is typically obtained; 40% more castings per melt can be expected; fettling and finishing costs can be reduced by more than 30%. Sources of information: ASM Handbook, Castings, Vol 15, pub ASM, Lost Foam Casting Made Simple, Ed F Sonnenberg, pub AFS, 2008 Manual of Foundry Technology, pub IBF (now ICME) Castings Technology International, More information about the books mentioned is available from the Institute of Cast Metals Engineers website, 12 FTJ January/February 2010

2 TheWorld Leader in Lost Foam From Raw Beads to Finished Castings... Pressurized Lost Foam System Bead Expansion & Storage Pattern Molding & Assembly Stabilizing & Drying Ovens Cluster Assembly & Coating Patented Compaction & Fill Pattern Casting & Finishing...We Know Lost Foam LostFoamExcellence... Systems Designed and Built to Patented* Compaction System * Existing and PendingP U.S. and Foreign PatentsP Target Your Process and Meet Your Needs for World Class Competitive Castings Vulcan works as a partner with our customers to enhance their production and competitive edge. The success of these partnerships is well known by our numerous repeat customers. Options Vertical Horizontal VECTOR-FL -FLO Compaction Table To Suit Custom Project Vulcan Engineering Co. One Vulcan Drive P.O. Box 307 Helena, Alabama Phone: 205/ Fax: 205/ sales@vulcangroup.com Vulcan Europe Inc. Unit 2, Ash Court Hilltop Industrial Estate Walker Road Bardon, Coalville Leicestershire, England LE67 1UD Ph: +44 (0) Fx: +44 (0) sales@vulcaneurope.com FTJ January/February

3 Lost foam casting: process of the future 35FPH aluminium line to produce truck parts w/1 hour cooling note the casting extraction robot The Lost Foam Casting Process has come a long way since its first introduction in 1953, but the first real production line was started in 1980, says Bryan Baker of Vulcan Engineering Co. Here he gives an overview of the process and the considerable improvements, both in basic materials and design of the equipment, over the past 30 years. The process is rather unique and for production of the right selection of casting nothing comes close to it. The benefits are such that it is worth considering redesigning the casting or adding different items to it such as brackets or combining multiple parts into one casting. The process allows maximum flexibility in design, some of which are not possible with other casting processes. There are various benefits that must be considered in using this process: Additives not required Binders not required Flexibility in design Cores not required Minimum scrap Less machining Sand fill hopper Reduced finishing Environmentally friendly Reduced energy required Reduced insurance premiums Smaller footprint Reduced manpower In recent years, we have seen changes not only in raw materials, but also various aspects of equipment improvements and these are now considered. Raw materials have changed in a couple of areas: Polystyrene The introduction of modified raw beads for the production of patterns and the various co-polymer beads to assist the use of the process for the production of iron castings and reduction of lustrous carbon defects. Robotic cluster coating unit Sand or media The introduction of synthetic sand produced in a ceramic form has several different advantages, more fluidity and therefore, better compaction and longer lasting with minimum fines. Coatings have been improved to give better control of coverage to the foam pattern and assist in removing the gases from the polystyrene foam pattern. The use of a robot for dipping the foam pattern cluster into the coating is preferred as it gives a more 14 FTJ January/February 2010

4 Lost Foam Sand rain gate consistent coating application. In terms of equipment the changes are more numerous: Sand fill hopper and rain gate The sand fill hopper has been developed for assuring consistency of the use of the media, in the past the correct amount of media was either by weight or time. The new patented system precisely controls the media by volume, which guarantees consistency in the correct amount of media to be delivered to the flask. Sand fill and compaction Equipment for raining the media around the foam cluster must be precisely controlled and must be interlocked with the compaction system. The patented rain gate operates in three modes: fast, slow or stopped. Vector-Flo compaction system The new patented Vector-Flo Compaction Table System provides advanced control of media flow and compaction which can be altered to fit each part and with precise repeatability. The compaction system uses four motors with a clamped flask, allowing the vibratory motor vectors to be adjusted MD-654 sand cooler to accommodate various programs of differing recipes while filling the flask with media. In fact, it is now possible to ensure that sand can flow up hill to completely compact blind holes, even at the base of the foam pattern. Test units are now available to try the process for a product which will allow customers to carry out detailed tests. Sand cooling A redesigned sand cooler allows the sand to cascade through various sections of the cooler passing over water cooled tubes. The tube bundles are designed to easily be replaced when required with a minimum of down time. Automation Automation is playing a more important part in the Lost Foam Casting Process, as robots are used in several areas, i.e. cluster insertion, molten metal pouring and casting extraction; to ensure consistency, reduce manpower and increase safety. Degating is also being automated, which makes it more environmentally friendly, reduces weight and reach by operators and is a lot safer than a manual approach for this task. For the right products, such as cylinder heads, cylinder blocks, pipe fittings, valves, fire hydrants, motor starters, and many more, the Lost Foam Casting Process is the perfect match. Lost Foam is the way of the future as foundries are faced with reductions in manpower, the need to provide a safer working environment, together with the many other savings which can help the foundry increase production and increase profits. or Automatic insertion of foam clusters into flasks Patented VECTOR-FLO compaction system Right: Robotic degating FTJ January/February

5 Innovative company - a full mould casting success The Kimura Group (Japan) has exploited the full mould casting process to enhance its productivity. Here Juan Leceta, past president of the World Foundry Organization, gives an insight into how the foundry has taken advantage of the benefits the process offers. Foundry technology has changed more along the last fifty years than in the previous five millennia a statement from the III International Foundry Forum in Bilbao, Spain, Sept It was in the 1960s when Prof A Wittmoser invented the Full Mould Casting Process, FMCP, inspired by the oldest foundry moulding process in the world, the lost wax casting process. The process uses expanded polystyrene to produce the pattern, which is positioned into a flask, and then bonded moulding sand is rammed both inside (to form the cores) and outside. This pattern is left in the mould and is vaporised and replaced by the liquid metal during casting. The FMCP is still used in the same way it was as years ago, but some foundries have made a number of innovations over the years resulting in the process now offering potential that was unthinkable a few years ago. Some components can now be produced in a large number, 3000 off, which means that 3000 EPS patterns have been produced, demonstrating that the process is applicable for mass production as well as one-off units. The Kimura Group, which has a history of persistency and success in R&D and in innovation, was founded in 1927 at Shizuoka prefecture, Japan; it now has three foundry plants and four pattern shops. The evolution of Full Mould Casting Process at KIMURA: 1966: FMCP was introduced 1967: Patternshop was established and started manufacturing EPS patterns 1970 to 1975: Quality improvement of FMCP commenced 1975: FMCP was advanced to machine tool castings. 1976: The end of the use of the cavity mould process and the foundry now specialises in FMCP 1982: 3D numeric control processing machine introduced in patternshop 1987: CAD/CAM system introduced in patternshop 1989: Mass production of medium volume quantities commenced Composite castings by FMC process Details of process plant at Kimura Pattern manufacturing The Kimura Group employs about sixty technicians who are engaged in processing CNC programs in 2D into 3D and CAM and operating more than 48 CNC milling machines producing polystyrene patterns. The company also employs specially developed machines (developed in partnership with the machine manufacturer) to paint the patterns (with paint that was also developed in-house). Moulding media One of Kimura s three plants uses artificial sand that has a round shape and flat surface; this allows for the use of very low binder contents, 0.3% to 0.5 % which means the sand acts as liquid sand with all consequent advantages of good mould compaction and good filling of the internal cavities in the pattern. At the other two foundry plants, silica sand with similar properties as described above is used. Casting surfaces are very smooth, as there are no parting lines or core assembling joints and consequently fettling operations are almost limited to in-gate removal. Note that the process is very environmentally friendly as over 99% of sand is recycled and the foundry is much cleaner. Moulds are moved on after closing to the pending area for melting by rolls leaving space for the next flask for moulding. Melting and pouring At the three foundries, melting is done with medium frequency induction furnaces automatically charged with raw materials by computerised systems. In most cases pouring is done through a basin with stopper, particularly when pouring spheroidal graphite cast iron. In some instances a vacuum is applied to collect the pattern vapours evolved during casting. 16 FTJ January/February 2010

6 Lost Foam Flasks for pouring are positioned in a closed tunnel adjacent to the melting furnaces, with a sliding door which is closed after pouring so all the fumes are collected and filtered before discharged outside to the atmosphere. Flasks that have been cast are moved into the cooling area. Cooling area As it is well known by foundrymen one of the bottle necks for a foundry producing heavy castings is the cooling process, because all castings have to be left to cool down in the flask for several days until the average casting temperature is below 300ºC to avoid stresses resulting in the castings. This takes up considerable moulding space. KIMURA use a continuous flask circuit system whereby once a mould is closed, it is moved through rolls to a curing area, pouring tunnel and cooling area. The time in the cooling area is shortened to less than half by using a sophisticated water showering cooling system that was the subject of a four year development programme and has since been patented. Finishing Silica sand does not generally stick to metal, except in exceptional cases and since there are no mould or core joints, fettling and cleaning operations are minimal, limited only to in-gate removal and shot blasting and painting. Inspection Though patterns are dimensionally checked prior to moulding, all castings are dimensionally checked and reported to the customer, either by CNC or optic methods. Machining The foundry currently delivers about 10-15% of castings in the proved or finished machined condition, and has a target to increase gradually the proportion supplied in this condition to the point where almost all castings are to be supplied as fully machined, painted components, ready for assembly. R&D and innovation The group has an R&D team, consisting of 18 fulltime staff among which four hold PhDs. There are facilities for carrying out all development programmes, including pilot foundry work, a ceramics laboratory and stamping presses to test material developments in cast stamping dies, etc. A positive attitude towards technological development is embedded within all levels in the company resulting in constant innovation in all areas of the process. Production Just prior to the present world-wide recession that we are experiencing, in 2007, KIMURA produced just under 100,000 tons of ferrous castings of which 65-70% were grey and alloyed cast iron and 30-35% were spheroidal graphite cast iron. The foundry has a range of flask sizes from around Water cooling of moulds 4500 x 2800 x 3400mm to around 8600 x 3000 x 1300mm with a maximum iron casting weight of 30t. Products Traditionally, using the FMCP, the group has produced stamping dies and machine tool beds and columns, although along the last few years quite a wide variety of cast components are being produced including gears, compressors, pump housings, engine cylinder blocks and wind power generation structural components. Many other new products are currently under development. The ability to include other material inserts (graphite, steel sliding rods and frames, pipes, etc.) within the castings using the FMCP offers an advantage which the group is now able to exploit. Conclusion KIMURA is a good example of company that embraces R&D and continuous innovation, to transform a process traditionally considered suitable only for one-off production into a process which is competitive for the mass production of large heavy castings. This has been achieved by the development of new solutions for the auxiliary processes, such as pattern materials, made using their own specifications, patternmaking and developments in painting, as well as offering customers fully machined and finished parts with ready-for-assembly components. The company s use of FMCP means that it is capable of transforming a design idea into a real casting in a matter of a few weeks, if necessary as few as two weeks, as well as being able to transform a fabrication design into a casting. This is quite an achievement when you consider that the castings weigh 30 tons. Also noteworthy is the fact that there is an insignificant investment required in the tooling compared with wood or other pattern equipment materials, and an advantage is that if any design alteration is required it can be done without cost within a few days FMCP provides designers significant design freedom: undercuts are possible, no draft is required and modifications are easy to make. In some specific grades of spheroidal graphite cast iron and steels, problems related to the pattern media (carbon pick-up and alike) have still not been solved for some components, but the foundry continues to work to overcome such inconveniences and in the author s opinion the KIMURA FMCP will be the process for the production of large ferrous castings, it is already one of the best. I commend the foundry for being one of the cleanest and safest foundry units I have ever seen. Acknowledgment I would like to thank the KIMURA GROUP for its support and for allowing me to disclose the information within this article, sharing their experience with the worldwide foundry community. FTJ January/February