January 10, 2002
The ability to efficiently remove scrap from production areas affects the production up time and cost of production. Therefore, scrap removal should be part of the production planning process.
It happens all too often. A stamping facility is modernized, expanded, or a new facility is built, and during the planning process, minor attention is given to the handling and removal of scrap and trimmed pieces. The stamping process, level of automation, and scrap handling capabilities all must be in sync to maximize efficiency.
Contracts are won and kept by producing quality parts quickly, in sufficient quantities, and profitably. To that end, stampers need to analyze production equipment thoroughly to ensure strokes per minute, reliability, and productivity.
Budgeted amounts apportioned for these profit centers are justifiable, because they guarantee production levels are met — they are the heart of the stamping operation. The "veins" of the system, however, are given little or no attention until the planning of the project is well on its way.
The arteries, or the automated material feed equipment and automation, support the heart of the facility — the presses. However, the veins, the scrap handling system, keep production running smoothly. The ability to remove scrap efficiently from production areas affects uptime and cost of production.
When sufficient means and capacity to remove scrap materials from the presses and the facility are not provided, they have the same effect clogged veins have on the heart and body, which suffer and shut down. Therefore, scrap handling systems are the life line of stamping operations.
Consider, for example, the just-in-time (JIT) manufacturing environment in the automotive industry. If the body panels stamping line were to shut down during the morning shift because of scrap backup, within 24 hours the welding lines would grind to a halt for lack of parts. And within 48 hours the paint lines would be down; then final assembly would cease. Inventory levels are monitored closely to make sure this does not happen.
However, scrap overload also causes the line to shut down. Too much scrap affects operator safety and puts tools, dies, and other equipment at risk.
Efficient handling of scrap does not require expensive or sophisticated equipment. To start, each application should have a set of parameters, both objective and subjective estimates that can be guidelines to determine if automatic or manual scrap handling is needed.
The type of press and operation will determine the amount of scrap generated. A press producing square or rectangular blanks will generate little, if any, scrap, while those forming near-net-shape blanks, or transfer presses performing multiple operations, may yield larger amounts.
From the size and shape of expected scrap pieces, you can determine how to handle awkward weights and characteristics and identify the potential for injury in manual operations. The known shape, along with the type of material used, helps you to calculate the anticipated volume of scrap.
Today volume is critical, perhaps more so than weight. For example, a ton of lightweight, 3-D aluminum scrap requires more conveying capacity than a ton of flat steel pieces. Other considerations include plant layout and press automation; presses in pits or with a basement area; and the availability of scrap hauling equipment, such as containers, trucks, or rail cars.
Depending on the amount of scrap generated, scrap removal and hauling efficiency will affect the operation of the facility. The size of available scrap containers or trucks and the time required to change them, plus the hauling distance to the scrap-processing center, will affect the operation and design of the discharge system.
For example, when a facility generates a truckload of scrap every 30 minutes, but the cycle time to change trucks is more than 45 minutes, a backlog will occur. Eventually the production line will shut down when the scrap container is full and cannot resume until an empty truck is available.
While some parameters can be measured, others have to be predicted, based on experience and history. These parameters include the labor market and wage levels in a geographic region.
Research whether workers are available to do the work (consider the work environment) and how costs for manual operations (wages, hiring costs, and benefits) compare to the initial investment and projected maintenance of automated equipment. How long are production contracts expected to last? And what levels of flexibility may be needed for different parts and facility expansion?
After these questions are answered, and if an automated conveyor system appears to be the most efficient solution, develop recommendations and final design criteria. Specific details will vary by application, but here are three general principles of scrap handling:
1. Use equipment of sufficient strength and sturdy construction to provide maximum uptime. This ensures the conveyor system won't collapse under the weight of the material being moved.
2. Incorporate reliable and durable components to ensure service longevity. This minimizes major repairs and replacement needs and facilitates preventive maintenance.
3. Offer a reasonable initial investment.
For optimum scrap handling efficiency, make sure the conveyor provides a buffer or surge capacity that allows scrap to accumulate in case of problems down the line, as well as ample width to prevent spillage and jamming. Conveyor component selection should be based on a worst-case scenario — when the conveyor is fully loaded at the maximum surge rate. Heavy-duty drive system components should be selected as if they were going to run 24 hours a day, seven days a week at full-load capacity.
These other fundamental design criteria should be considered:
• A conveyor belt must be wider than the largest possible scrap dimension. For example, 24- by 24-inch scrap requires a much wider conveyor than 24- by 3-in. scrap.
• A branch conveyor directly under each press requires less volume capacity and freeboard than a main conveyor collecting from numerous presses.
• Larger facilities may need to increase conveyor belt speed automatically as scrap rates change. However, slower is better. For example, when several presses feed scrap into a conveyor system, the burden depth increases, and the depth of scrap increases weight. The calculated belt pull increases, and the ability of the conveyor to move material up inclines decreases. This is why it’s important to have a balance between speed load and belt capacity.
• Main conveyors in large facilities may need to have variable-speed drives to allow operators to control the speed of the conveyor in varying production conditions. Variable-speed drives and interlocks should be used in facilities incorporating balers, compactors, or other downstream equipment.
• Large facilities need the ability to bypass final processing equipment to maintain a continuous scrap flow, because sometimes a shutdown of downstream equipment is required for maintenance or equipment failure.
• Control systems should be selected based on a facility's need for flexibility, operator attendance, lack of operator attendance, and planned production schedule. Is the system to be 100 percent automated, or is a full-time operator required because of material volume, facility size, or lack of scrap hauling capacity?
These are just some of the design elements that should be taken into account, right down to the size of pan surface dimples in relation to the size of scrap pieces. No matter what the application is, however, a scrap conveyor chosen with the proper forethought and planning will contribute to a stamping operation's productivity and profitability.
Bernard N. Goldstein, P.E., is chief engineer with Mayfran International, P.O. Box 43038, 6650 Beta Drive, Cleveland, OH 44143-0038, phone 440-461-4100, fax 440-461-0147, e-mail email@example.com, Web site www.mayfran.com. Mayfran Industries manufactures conveying systems, filtration equipment, separators, briquetting, and chip processing equipment to facilitate material handling and coolant recovery in various industries.
STAMPING Journal® is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping. Print subscriptions are free to qualified stamping professionals in North America.