Cleaning and finishing: getting the casting ready for shipping.
By the time a casting reaches the cleaning room, it will have achieved its final dimensional shape and its initial surface finish. Unless it requires some secondary processing, its chemistry and physical properties are intact as well. But in nearly all cases with all molding and casting processes, some degree of cleaning and finishing is required to make a metal casting presentable for shipping to the customer. In some instances, cleaning and finishing operations are used to merely improve the appearance of the casting. In others, they are utilized to correct deficiencies created by poor molding, coremaking and melting practices.
Because of the amount of handwork involved, particularly chipping, grinding, and general casting handling, this step in the metalcasting process can account for as much as 40% of a foundry's direct labor costs. Minimizing the time and effort required to get a casting ready for shipping is important in a foundry's bottom line performance.
The basic operations required to clean and finish a casting generally include the removal of core and molding sand, as well the gates, risers, runners and fins not removed during shakeout. Scale, caused by carbonaceous materials such as seacoal or mold and core washes, as well as scale caused by heat treating and stress relieving operations, must be removed. Final quality checks, painting, packaging and shipping are often carried out in the cleaning room as well.
The most efficient cleaning and finishing operations are those that utilize straight-through processing in which each operation follows in a logical, productive manner with a minimum of casting handling. Unfortunately, casting flow patterns are often influenced by poor foundry practices in departments preceding cleaning and finishing. But, where it is possible, a straight-line operation is the most effective method for moving and handling castings in the cleaning room. In general, castings should be processed through shakeout, shot blast and automatic grinding before performing any manual operations.
Shakeout & Casting Cleaning
While casting shakeout and cooling are not cleaning room functions, they can be important to overall cleaning room operations because the length of cooling time and method of shakeout can play an integral part in overall cleaning cost and efficiency.
Casting cooling can be accomplished in three ways: in or out of the mold or a combination of both. For metallurgical reasons, some castings require a longer cooling time in the mold. Unless controlled cooling in the mold is needed, it is to the advantage of the foundry to remove the casting from the mold as quickly as possible. The cooler the casting becomes, the more difficult it is to remove core and mold sand. Also, the more sand that is removed in the cleaning room increases the amount of handling and environmental control equipment needed.
In addition to the benefits provided by an efficient shakeout operation (see Part 10 of this series, modern casting, Oct 1989, p 45-47), other benefits are afforded in the finishing room. By removing as much sand as possible in the shakeout, wear on shot wheel vanes and cups, shot blast liners and dust collection systems are reduced. Contamination of shot and grit by core and mold sands is reduced and worker environment is improved due to the decrease in the amount of sand carried with castings during processing.
Shakeout to Cleaning
Getting castings from shakeout to the cleaning room can be done several ways, the proper method depending on production rate, casting shape and size and the justifiable foundry cost. Increasingly, foundries are opting for conveyors, either power or free-rolling, overhead trollies or floor-mounted belt or roller designs. Each design has its advantages and offers the flexibility of variable casting cooling rates, distribution to a variety or work locations, providing storage, and, in the case of overhead units, requiring no floor space.
Distance from shakeout to cleaning, casting sizes, cooling requirements and cost are the operative selection parameters. One type of cooling and transfer conveyor uses a vibrating pan system to provide a short vibration cycle for sand removal and despruing and a dwell period for additional cooling. Castings also can be transferred in conveyorized tubs, by forklift trucks or free-roller conveyors.
A low-cost method of getting castings from shakeout to the cleaning room uses containers, or tubs, transported by forklift or roller conveyors. This method is used in limited production shops where small or medium-sized castings are produced. The shape and size of containers may vary. Usually, the castings remain with their containers throughout the finishing process. Drop-bottom containers, four-way fork entry and conveyable-type runners are important considerations and are determined by the work flow and material handling methods selected.
Casting flow through the cleaning operations will also vary according to casting size, weight and operations required. If overhead basket or bucket conveyors are used to deliver castings from shakeout, they can be emptied directly into a tumblast loader. When pan or tub conveyors are used, the castings can be fed onto the blast cabinet work conveyor.
Larger castings arriving for cleaning are usually hung on monorail conveyors travelling through a blast cabinet, or are off-loaded onto table-type blast units for batch cleaning. Whichever transfer system is used, ample conveyance to and away from the blast is key to avoiding work flow delays.
Whenever possible, it is good practice to process castings through the shakeout, shot blast and automatic grinding processes before performing any manual operations.
Ideally, all risers, gates, runners and other excess metal, such as fins and flashing, will be removed during shakeout. This is often not the case, though, and it becomes the job of the cleaning room to remove any metal that is not part of the final cast shape. It is usually best to remove excess metal before further processing of the casting through cleaning and finishing.
The type of alloy involved will determine how risers and gating system components are removed. Steel castings, generally, have large risers, shorter production runs, wider pattern varieties per number of pieces, have more fins, require heat treating, are more easily repaired and often require straightening. They commonly require the cutting off of risers and sprues.
Gray and ductile iron castings, conversely, are usually made in larger quantities on a wide range of molding machines, and show up in the finishing room in much larger batches of the same casting. In the case of gray iron castings, many times the gates and in some cases the risers will snap off in the shakeout. Ductile iron castings require a more severe break-off procedure for gates, risers and sprues than gray iron. Nonferrous castings usually require shearing or cut-off of risers and more careful handling through blast cleaning and grinding.
Machines for removing risers are gradually becoming available to aid in reducing operator fatigue and improving the work area environment. One such breaking system locates a casting in a simple fixture mounted on a rotating table. During rotation, the riser is forced against a pressure roller, breaking the riser from the casting. Another uses a wedge-shaped blade that is forced between the riser and the casting to break the riser free. Newer designs of these types of machines are lighter and provide the flexibility for performing the derisering operation without physically handling the casting as it moves along the conveyor.
Usually the most expensive piece of cleaning equipment for a foundry to buy and operate, the shot blast machine provides a final finish to the surface of the casting and cleans internal passages. It removes all residual sand, but is improperly and expensively misused as a shakeout substitute in some foundries, creating high maintenance costs, work-flow bottlenecks and often adversely affecting finished casting quality.
The shot blast machine uses abrasives directed at the casting in a concentrated area at a very high velocity to clean the surface and reachable internal cavities.
The primary method of projecting the abrasive is through the centrifugal action of spinning wheels. Abrasive is fed into the center of the rotating blades, or vanes, which are turning at 1800 to 3400 rmps, depending on the diameter of the wheel. The abrasive is flung at the castings at high speed. Compressed air can replace the centrifugal system for delivering the abrasive stream. The shot blast system is composed of the wheel assembly contained within a cabinet to contain the abrasive and a collector system to control dust and metal debris. Shot blast systems may contain as many as 12-15 abrasive wheels, depending on the casting configurations or the cleaning rate required. Parts may move through the abrasive stream on a table, a conveyor, on a rotating fixture or simply tumbled within the cabinet.
Grinding and Chipping
Grinding generally succeeds shot blast as the final finishing operation, but sometimes it is completed prior to it. Grinding off fins and gate and riser contacts improves shot blast efficiency and in some cases the shot blast operation effectively removes grinding marks.
Grinding castings can be done manually or automatically, and should be done as soon as practical to make castings safer to handle. Manual grinding is usually done on larger castings or castings moving along a conveyor line in a holding fixture. Stand grinders use larger grinding wheels to remove metal from hand-held castings.
Automatic grinders require a method of loading and unloading a holding fixture, and they may use several grinding wheels at the same time to grind several surfaces of the same casting. The quality, consistency and productivity of automatic grinding machines can be very high when fixtures and work flow in a finishing operation are properly designed, though their initial costs are high. A good tooling program, including the use of robotics, can reduce the grinding requirement in the casting finishing area.
In some cases of steel, cast iron and come copper-based castings, the use of pneumatic chipping tools to remove fins and flash is still prominent. Chippers are also used to remove core sand which failed to be removed at shakeout or in shot blasting.
Planned process flow provides for the most effective and efficient movement of castings into, through and out of the cleaning and finishing operations. It takes into account product mix, casting finishing requirements and the physical features of the cleaning facility itself. Once the cast metal type is known, preparations for operational sequences are facilitated by the orderly sequencing of people, products and processes. Sequences are determined by operating and engineering personnel to reflect the required finishing operations, foundry equipment and practices.
Foundry practices will establish, for exam le whether or not a core system will present core sand breakdown difficulties, necessitating a separate core knockout operation. They also will determine if the castings enter the cleaning room in significant quantities of the same castings or as a mixture of castings which may require sorting before blasting. Process flow studies should consider such items as arc/air versus torch burning for riser removal, arc/air versus chipping in fin removal, shot versus sand blast, and shearing versus the use of cut-off saws. Each of these and other factors will affect casting flow in the cleaning room.
Considerations in the process flow that can affect material movement within the cleaning department in terms of time and labor per casting include the selection of processes that combine the work content of more than one operation. This reduces excessive movement and handling of castings in the flow of the total work in process.
Factors influencing the number of cleaning and finishing work stations and manpower requirements are related to the anticipated product mix, complexity of the product, condition of patterns and coreboxes, gating and risering practices and location in relation to molding and melting facilities. For optimum work processing efficiency, work standards for all operations are important, as is the inclusion of equipment sufficient to handle physically any given casting quantity and product mix.
A set of work standards should be used to analyze how work arrives, is processed and disposed of at each cleaning and finishing department operation. An analysis of the methods and equipment technology applied to transferring castings between workstations also should be an integral part of the evaluation and design of work stations plant layout. Considerations should:
* review the amount of casting handling to actual work functions performed
* seek to combine similar operations, such as chipping and grinding, machining off gate and riser connections, quality checks, sorting and positioning castings for subsequent operations,
* orient the castings for each subsequent operation by correctly positioning for handling. Use simple mechanical devices to preposition or relocate the castings for subsequent operations, where possible
* use a common conveyor or mechanized carrier system for moving castings between operations, where possible;
* eliminate intermediate movements of castings into and from storage areas or casting tubs by providing a system that transfers castings directly from one operation to the next;
* buffer work stations with a supply of castings to assure sufficient work flow during interrupted periods, such as no castings arriving in finishing areas or during periods of equipment or plant downtimes,
* review casting repair operations and determine how and where they would best be performed. It may not be economical to remove simple repairs during the process flow.
Some foundries have found it economical to automate the handling of individual castings of the same type, from cooling through all finishing operations, totally eliminating operator handling.
The selection of a process that combines the work content of more than one operation obviously reduces the movement and handling of castings. Handling time at a work station is a critical job element, and a small savings here, particularly in small castings needing little work, can significantly increase productivity. Benefits of work handling reduction include:
* decrease in tool handling,
* when flash removal requires both a hand grinder and a pneumatic chipping hammer, the operations can be performed at adjacent work stations,
* consolidation of grinder usage,
* automation of grinding to reduce tool and casting handling,
* where two or more tools are used at a single work station, use of overhead, counter-balanced hoist or manipulator allows operator to get and release tools without eye contact.
Careful consideration of the methods used to "present" individual castings at each workstation often can justify relatively high machinery expenditures. Each casting ideally should come and leave at a convenient work level and remain within the shortest and easiest possible reach of the operator, Parts should arrive at each work station in the same position in the same work space without variation. Identical casting placement and specific allotted work time per operation will allow the operator to do his job of work with little visual contact and with maximum efficiency.
Methods of locating castings at a suitable level for grasping include placing castings between successive workstations for easy "pick and place," using a trough or on a skid plate, feeding the castings using a chute-type magazine or via a conveyor fed from a hopper or continuously fed by an automatic dumping conveyor.
In contrast, when little consideration is given to the proper location of the casting for the operator, productive work time is lost by increasing the reaching distance as the casting container is emptied. Improperly timed conveyors running too fast or too slow affect worker productivity.
Combining Casting Movement Handling Operations
The use of conveyorized fixtures or holding devices for castings which also serve as carriers between work stations, reduce casting handling time. Examples might include: o transfer and handling operation that uses a monorail conveyor to hold the casting as it moves through the cooling, knock-out, shot blast and some grinding and chipping operations before being automatically deposited onto a vibrating conveyor or belt without being handled, o the use of a vibrating oscillator for core knock-out, flash removal and to move casting to subsequent operations, o for small batches or large castings, using a stackable container that serves as a holder for the workpiece and to move and hold the casting through knock-out, shot blast and some grinding and painting operations; o a continuously moving processing belt requiring that the work task for each operation have similar time demands, eliminating the need to handle the casting.
The storage and movement of castings using tubs or baskets provides a flexibilesystem and includes the ability to provide the necessary buffer of casting between operations. The storage may be located at the work stationor a distance away, and, except for excessive handling, is ideal for small batches of castings.
A powered roller conveyor systems, either operator-controlled or continuously-moving, may be used to move castings to, through and away from a work station, both providing a given casting delivery rate, and assuring a consistent work flow and high productivity.
Material and process flow contains many variables in the planning of casting cleaning and finishing. The allowable variables established to assist in the efficient control of the manufacturing processes for molding, melting and coremaking operations usually do not provide a consistent casting surface quality for cleaning. Therefore, additional operations must be given consideration when establishing the space requirements of castings necessary to maintain an uninterrupted flow of work to each workstation.
In principle, establishing zones to accumulate castings may be necessary to provide for a uniform workflow. The implementation of such a process must provide a direct and uniform flow of castings through all clearning room operations in the shortest possible time, and require as little handling and floor space as is economically feasible.
After progressing through the operations and completing the selection of methods and processes, the next step is to choose the proper equipment. This selection is based on specific characteristics of the castings, and can be separated into two categories: production and auxiliary equipment.
Production equipment should be selected on the basis of:
* production rate--for high volume, short run or indeterminate quantities;
* casting mix--known manufacturing requirements by sizes, degree of fragility, types of coring, etc;
* metal type--metal pecularities needing special consideration;
* labor--considering such factors as experience required, trainability, wage rates and factors that preclude consideration of sophisticated, automated or mechanized machinery.
Auxiliary equipment includes cranes, hoists, conveyors, lift trucks, work assiss and skids, pallets or boxes. These are selected for their ability to handle materials between operations and work stations.
A brief summary of equipment innovations that recently have been applied successfully to casting cleaning and finishing follows.
The use of high frequency shakeouts should be considered for the following reasons:
* a low stroke, high frequency vibration;
* a lower, less violent velocity; o less inter-casting impact;
* reduction of casting damage; o offers fewest moving parts for easier maintenance;
* less horsepower requirement; o more isolated vibration.
With proper deck design, it is possible to rotate the casting to present several surfaces as it travels along the shakeout deck, aiding sand, flash and fin removal.
The casting process usually provides a shakeout operation soon after pouring to remove molding sand, but core sand in the casting generally has not collapsed enough to allow the sand to flow out of the internal passageways during initial shakeout. A shakeout located at the start of the cleaning room operations provides the opportunity to remove the majority of the core sand, some residual sand remaining on outside surfaces, flash and fins.
Trying to remove sand by shot blast is expensive compared to shakeout. New shakeout table designs are more effective than older styles, resulting in less casting damage caused by the length of stroke and the erratic casting travel which caused castings to hit each other as they moved over the shakeout deck.
Shot Blast Equipment Technology
Three new techniques for handling a casting during the shot blast operation include robot holding and transfer, the oscillating conveyor cage with roll feature and the vibrating work conveyor.
In the robot system, the casting is clamped by a robot or similar manipulating device used to transfer the casting. It travels on a track, stopping at each shot wheel position where it rotates the casting through a prorammed abrasive cycle.
In considering robotics in cleaning and finishing, one must determine if a robot is to handle the cating (placing it into a fixture) or if it is to be used to handle tools or the casting to perform a cleaning o eration. Both require careful consideration prior to robot selection. Also to be considered are the size, shape and weight of the castings to be handled, the surface area and location to be cleaned and the production rate expected.
A successful robot application takes into account work station design and layout, the delivery and disposition of product, effective tooling and an on-going preventive maintenance program for the robot and its tooling. A carefully-selected, programmed and tooled robot, typically, can do finishing operations in cycles averaging 40% less time than those done manually. Robots may not miss work or tire, but they need care, and lots of it.
They have the ability to be programmed to oscillate fast or slow in a pattern which concentrates the abrasive on surfaces that are difficult to clean, and providing production flexibility by allowing the scheduling of different castings simultaneously. Their disadvantages are low volume production rates and machine complexity.
In the oscillating conveyor design, the casting slides along the bars of a rocker barrel cage which is rotating 65 [degrees] off the vertical center in both directions for effective and safe tumbling. It provides control of the casting through the cleaning compartments, allowing maximum casting exposure to the abrasive pattern, and removes accumulated shot located in pockets and internal passages.
Advantages include good casting shot pattern exposure and the ability to handle a variety of castings to be cleaned simultaneously. A drawback is that it makes handling castings difficult, especially when loading and unloading the cage.
Vibrating work conveyors use a high frequency vibrating conveyor to transfer the castings through the shot patterns of each abrasive wheel, and are designed to handle a variety of shapes and sizes. Because of the location of the shot wheels, excellent cleaning action of internal casting surfaces is possible.
Advantages include the ease of loading and unloading the fixed-position castings, plus the fact that changes in feed rates can be made during operations to accommodate changes in the condition of castings entering the blast. A high frequency shot shakeout also can be installed to remove shot from internal passages of castings after they exit the shot blast work conveyor. This provides for the return of the shot to the abrasive handling system of the blast and additional operations saved to remove the shot from the internal passages.
The installation of the sand shakeout, shot blast and shot shakeout provides more casting travel time over vibrating conveyors and shakeouts, which reduces oveall casting sand retention.
Internal Passageway Cleaning
Cleaning internal passages of castings has been a difficult problem to solve. Recent advances have reduced the amount of retained material to a specific amount, particularly in intricate automotie head and block castings.
Vibratory media cleaning has been successful, in reducing retained internal materials. Its equipment consists of a eccentrically oscillating bowl which contains a media made of steel shot in a water-based mixture of a soap compound and a rust inhibitor. The casting is submerged in the media, and the vibrating motion of the bowl drives the media into the internal passages of the casting with a srubbing motion on all casting surfaces.
A secondary benefit is the improved finish apparent on the outside of the casting and the removal of up to 75% of grind wheel burn. Sand can be filtered out of the media for recycling. Media cleaning is being used on lost foam process castings where its burnishing action is also useful in removing refractory coating and rounding sharp edges and corners.
Water Jet Cutting
New ultra-high pressure water jet metal cutting technology can be used effectively to clean hard to reach csting sections. The process produces a very fine water jet traveling at the speed of a bullet. Using a small orifice (0.004-0.018 in.) and a small volume of water (one to two gal/hr) at pressures up to 55,000 psi, it produces a water stream travelling at 2855 feet per second, or roughly 775 mph.
Abrasive materials are added to the jet stream to produce a metal cutting stream tht cuts as effectively as a flame cutter. Test indications have the water jet cutting 1/16 inch steel at 35 in./min.
Though slow, the water jet has the advantage of producing an excellent finish on the cut surface. Cleaning internal passages and restricted areas, inaccessible to chipping hammers and other clearning tools, and burn-in sand are promising applications for this technology. Disadvantages are its high operating cost, induced rust problems and a 80-100 dBA noise level.
Routing work into and out of the cleaning and finishing department can be critical to foundry operations. Poorly-defined casting flow or equipment selection and/or placement can cause major operational disruptions, usually when they can least be accepted. A good finishing operation layout should take into account its relationship to the rest of the foundry and how it can optimize its own internal work flow.
Since the equipment is subjected to an intensely abrasive environment and the profitability of the finishing room is contingent on keeping the equipment operable, quick and easy access for maintenance is of prime consideration. The finishing operation also should be easily accessible to rail and highway transportation, and it must have easy routes for the return of sand, tramp and scrap metal, returns and core rods the to the appropriate storage areas. As mentioned above, it should be close by the main shakeout ares, as well as having its own shakeout section.
The department should have sufficient flexibility designed into it to permit some deviation from the normal product mix and allow for both near- and long-term expansion planning. And, amid all this, it must meet current foundry finishing needs while keeping a good, orderly house.
Contingency plans should compensate for sudden downtime, abrupt changes in product, maintaining adequate casting storage areas and making certain that personnel and environmental provisions keep pace with current production and expansion requirements. That is all in addition to maintaining tooling, consumables like oxygen and acetylene supplies, compressed air, storing and getting rid of returnables, providing for exhaust and dust collection, ductwork, plus heating, ventilation, and lighting.
"Plan ahead" are the key words. Know equipment production capacities, operational sequence for the foundry product mix and make the layout of the cleaning room conform to current work load, yet sufficient to meet expansion needs.
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|Title Annotation:||The Metalcasting Process: Part 11 of 12|
|Date:||Nov 1, 1989|
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