Effective material handling: guidelines for planning cleaning operations.
He was expertly handling a cutoff machine at a riser and sprue cutoff station in a typical, busy foundry. Without looking away from his work, he said, Sure, casting's important. Just ask the guys on the pouring line. Need a good casting to make a good part. But the real work is right here. We make the difference. You can't ship it and bill it 'til we're finished with it.'
He was right on both counts. Good castings are important; how well a foundry does its melting and molding is a mark of a successful company. An often unstated part of the success equation, however, is how efficiently a foundry handles the materials it must reclaim or get ready for shipping.
Often an orphan in the scheme of product flow planning, too many foundries pay scant attention to routing a casting through shakeout, cleaning and finishing. Moving molten metal around involves incisive planning lest expensive heat be lost and a melt compromised. Automation of molten metal handling is an accepted and advancing science. Handling castings can be, but too often is not.
Effective Material Handling
Moving materials economically involves a system that provides the most suitable flow for a particular casting mix and it does this by factoring in casting finishing requirements and the physical features of the cleaning facility. Once the category of metal type is defined, determining cleaning operational sequences is possible. They are determined by operating and engineering personnel who define the required finishing operations, material conservation demands and related material handling.
Foundry practices determine, for example, whether or not oil sand, hotbox, coldbox cores with core rods or CO2 cores with their inherent breakdown difficulties will require a separate core knockout operation necessitating additional handling.
They also will determine if the castings enter the cleaning room as significant quantities of the same lot or as a mixture of castings that may require sorting before blasting. Each operational step requires consideration for extra material handling and determines the most practical processes and techniques for moving materials. These might include arc-air versus torch burning for riser removal, arc-air versus chipping for fin removal, shot versus sandblast and shearing versus cutoff saws. Each casting brings with it individual processing and handling needs.
Before determining equipment needs, it is necessary to complete a cleaning operations evaluation and weigh material movement options for each cleaning operation. The next step is to select the proper material handling equipment based on the specific characteristics of the castings. These can be separated into two categories-material handling and auxiliary.
Material handling equipment should be selected on the basis of production rate and casting mix. Knowing whether production runs will be short or long, the casting size and complexity, whether heavily cored or fragile all are factors that influence material handling methods and machinery.
Peculiarities of the metal poured and any factors relative to the experience, trainability or wage rates of the labor force also may affect consideration of highly sophisticated, automated or auxiliary material handling equipment.
Auxiliary handling equipment includes cranes, hoists, conveyors, lift trucks, work assists and skids, pallets or boxes. They are selected to move materials between operations and work-stations.
New auxiliary processes that apply material handling equipment functions might include high frequency shakeouts, automatic casting inspection, coordinate measuring systems, machine vision inspection systems, robots and computer vision systems for sorting.
Removal of core and/or molding sand is one of the first requirements when the casting arrives in the finishing area. In the majority of foundries where casting shakeout occurs as soon as castings are removed from the mold, sand removal provides an opportunity to improve and shorten casting cooling rates and maintain consistent molding sand quantity and quality.
The necessity of a shakeout immediately at the start of processing in the cleaning area is important because it removes the majority of core and molding sand from the casting when removal is easiest. It also reduces the amount of sand on castings carried to other parts of the cleaning area that must be collected and returned to the molding sand system. If not done as soon as practical after casting, the complexity and difficulty of the initial cleaning operation increases because molding sand on a cold casting is more difficult to remove.
It is evident that handling sand dispersed to locations away from cleaning operations increases the need for additional sand handling equipment and environmental controls.
There are other benefits of the initial shakeout. It removes some fins, sprues and runners that break off and it also reduces contamination of sand with shot in the shotblast equipment, cutting wear on the shotblast wheel vanes, cups, blast liners and blast dust collection systems.
Castings using other types of molding processes not requiring removal of core or molding sand normally will be processed in a different flow pattern, eliminating the need for a shakeout operation and usually starting with fin and riser removal.
Core knockout for many nonferrous castings uses casting clamps or grips that apply a strong vibrating action to remove the core sand. Again, it is not unusual for some of the runners and fins to drop off the casting during this process.
The removal of sand and excessive metal should be the objective of the first two major cleaning operations. Prior to entering the shotblast operation, it is important that all sand is removed from the casting to reduce sand carryover and to improve cleaning room environment. it is desirable to process castings through a shakeout before defining.
The type and sequence of finishing operations are greatly influenced by the metal being poured. Steel castings generally feature large risers, shorter production runs and a larger variety of patterns per number of pieces. This influences the design of the handling system.
For instance, steel castings require repeated handling for heat treating, salvage operations, welding repairs and straightening.
Gray and ductile iron castings are usually made in larger quantities on a wide range of molding machines, resulting in a larger number of the same castings being handled in the cleaning room.
Whatever the metal being cast or the casting size, a balance between molding and cleaning can be achieved. The shakeout, cooling, cleaning and secondary shakeout before shotblast and definning operations remain essentially the same.
A low-cost method of conveying castings from the molding shakeout to the cleaning room uses metal container's or tubs transported by forklift trucks or roller conveyors. It is used in low production shops producing small to medium sized castings. The shape and size of containers may vary. Usually, the castings remain in this container, or are transferred to tubs of the same size, throughout the finishing process. Drop bottom containers, four-way fork entry types and containers with conveyable type runners are variations and their use is determined by product flow and the material handling methods-selected.
Another method of conveyance from the shakeout is by truck alone but is practical only when the casting size is such that other methods are insufficient.
Handling through the cleaning room will vary due to casting size, weight, and number and type of operations required.
If trolley conveyors, using hanging baskets or buckets, have been used to transport small to medium sized castings from the shakeout, the basket or bucket can be emptied into a tumble blast loader or transferred directly through a cabinet blast with a minimum of manual handling. When pan conveyors are used, the castings can be fed directly onto the cabinet blast work conveyor. Tubs also may be emptied into the skip hoist of the tumble blast unit.
Larger castings may be handled using hooks-on monorail conveyors which travel through a blast cabinet or may be loaded onto table-type blast units for batch cleaning. Whatever the method employed, handling and flow into and away from the blast unit is an important consideration. The blast unit should be located so casting loading and discharge do not cause bottlenecks or congestion.
As castings are handled through the various finishing operations, they may be loaded back onto trolley conveyors, tubs, pan conveyors or belts. The method, again, is dictated by casting size, weight and down-line processes.
A workstation analysis should be done to determine how casting handling and positioning can be coordinated with tool handling. The objective is to eliminate or reduce the physical handling individual castings. Material handling of product flow and the latest equipment technology should be evaluated with consideration given to:
* a review of the amount of casting handling required to perform the actual work functions;
* evaluating each casting orientation for successive operations by correctly positioning the part;
* simple mechanical devices to locate each casting for subsequent operations where possible;
* where it is possible to use a common conveyor or mechanized carrier system for moving castings between operations;
* eliminating intermediate movements of castings to and from storage areas or tubs by providing a system that transfers the casting directly from one operation to the next;
* buffer areas to maintain a supply of castings to a workstation during interrupted periods.
Some foundries have found it economical to automate the handling of individual castings of the same type from cooling through all finishing operations. Castings are automatically conveyed between shotblast, heat treat, definning, degating and risering operations on belt-powered roller conveyors, walking beams or indexing devices.
To reduce material handling and decrease labor necessitates the selection of a process that combines the work of more than one operation to reduce the movement and handling of castings for the total process. Examples would be:
When flash removal requires both a hand grinder and a pneumatic chipping hammer; one operation should be performed at an adjacent workstation to reduce tool handling; pedestal grinding of easily accessible areas of a casting using a feeding device for the workpiece, or on an automatic grinder prior to small grinding and chipping operations, will usually reduce tool and casting handling time;
* where different tools may be required at one workstation, the provision of tool holders with an overhead counterbalanced hoist or manipulator for each tool allows the operator to grasp and release each subsequent tool without direct eye contact, thereby increasing productivity; the use of a rotating conveyor with fixtures for a variety of castings and providingsimple ejection devices-can result in higher productivity and a minimum of casting handling.
Material handling time at a workstation is a critical job element during which a slight time savings, particularly on small castings, can greatly increase productivity.
Studies of the total work done at an average foundry workstation revealed that up to 50% of the time is used for casting handling. Analyzing movements within the workplace for casting delivery and removal and the condition of the casting as received, are key considerations in evaluating the productivity of each workstation. The elimination of casting handling wherever possible is an essential part of economical material flow.
Careful consideration of the methods used to supply and remove castings can justify high material handling equipment expenditures.
Consistency in presenting work to the operator is important to efficient workstation layout. Castings should arrive at the correct level, within easy reach and in the same position for every job cycle. This allows the operator to pick up the casting without visual contact and perform an operation with maximum efficiency.
Methods of material transfer and locating castings at a suitable level for grasping might include placing a storage bench between two workstations, installing a trough or skid plate with guides or a chute type magazine filled by a dumper or mechanized conveyor.
When little consideration is given to the proper location of the casting for the operator, several things can happen that can slow the work flow. There is an increase in worker fatigue caused by reaching or stretching, empty containers take up conveyor space and castings can build up at one workstation while another may remain empty.
The dual use of a fixture or device that holds the casting at the workstation and also serves as a carrier between workstations can minimize casting handling. An example would be a transfer and handling device that uses a monorail conveyor to hold the casting as it moves through the cooling, knockout, shotblast and some grinding and chipping operations before being automatically deposited onto a vibrating conveyor or belt.
Another example would be the use of a vibrating oscillator for core knockout, flash removal and moving castings for subsequent operations. For small batches or large castings, the use of a stackable container that serves as a holder for the workpiece may be used to move and hold the casting through knockout, shotblast, and some grinding and painting operations.
The use of a continuously moving process belt requires that the task for each operation have similar time requirements. Performing finishing operations while the casting is on the moving belt conveyor eliminates any handling. Variation in casting conditions may increase the actual work performed beyond the time allotted and result in the temporary addition of operators.
Work-in-progress requires the moving of castings between workstations, and an analysis of it may indicate the need for casting storage near several workstations. Containers and equipment that may be used to provide for buffer storage and transfer would include tubs, flats or baskets moved by lift truck; hoppers and chutes moved by lift truck; roller and monorail conveyors; powered rollers; and conveyor belts.
The storing and moving of castings using tubs or baskets provides a flexible system and includes the ability to provide the necessary buffer of castings between operations. The storage may be located at the workstation or a distance away and, except for excessive handling, is ideal for small batches of castings.
Roller convey or systems can be ideal for moving castings of medium size batches, groups of components and small batches of large castings. An efficient layout can provide a good work flow with the necessary buffer of castings to maintain a consistent flow of castings for each operation.
A successful method of moving castings between operations also can be done by providing a step-type sliding chute or plate arrangement connecting several workstations. Consideration must be given to the size of each chute to allow for the different time element required to complete each workstation operation.
An operator-controlled or continuously-powered roller conveyor may provide a method of transferring the castings to and from a workstation.
A continuously moving belt or monorail conveyor provides a consistent work flow of castings to each operation to control productivity. This avoids the inefficiencies inherent in the "move it, stack it and move it again" approach.
The allowable variables for molding, melting and coremaking operations usually do not provide a consistent casting surface to the cleaning room; so, provisions for additional operations must be given consideration when establishing the handling and space requirements.
A process that provides direct and uniform handling and flow of castings through all cleaning room operations in the shortest possible time with a minimum of handling and floor space is economically feasible.
Mr. Luther is also an associate with Lester B. Knight Cast Metals, Chicago.
|Printer friendly Cite/link Email Feedback|
|Date:||Jan 1, 1992|
|Previous Article:||A global look at the metalcasting industry.|
|Next Article:||Mexico's foundries show strengths, larger trade potential.|