There are many factors involved in boosting productivity in sand casting operations, which typically have hundreds of process details to track and just as many variables to manage. In order to achieve highest throughput, these foundries must increase their rate of mold and core production, which means that the refractory coating — also known as a "mold wash" or "core wash" — on those products should be force-dried using a convection oven. These "sand core dryers," "mold dryers," or "core drying ovens" operate at 200°-400°F and dry molds or cores in 10 to 30 minutes, as opposed to 10- 20 hours needed for passive drying at ambient temperature.
The wash is a water or solvent-based liquid or slurry suspension of zircon, magnesite talc, or graphite solids. It is applied by spraying or by dunking the mold or core, and it promotes better surface finishes on the cast products, as well as protects sand in the mold from the heat and erosion of molten metal as it enters the mold cavity.
After the wash is applied, the oven temperature and drying time depend on the mold size, sand porosity, and the efficiency of the oven delivering the heat. By designing the oven for tight temperature uniformity, high air velocity (velocities above 5,000 fpm are common) and good end seals to prevent hot air loss, a lower oven temperature can be used. This reduces energy use and lowers operating costs.
Sand molds and cores are destroyed in the process of removing the molded part, so a typical foundry will produce and use a large number of these, which requires significant drying capacity. Therefore, most of the core dryers used in foundries use a conveyor to transport molds and cores through the oven. It is common for a cooler to be located after the oven, to cool the sand molds for handling.
Conveyor for gear manufacturing —A Milwaukee plant needed to dry a zircon-based core wash on sand molds used to cast large gears. Each molds is 70x50x40 in. (WxLxH) and weighs 3,000 lb. They had to be dried at the rate of one every 5 minutes in order to maintain a production rate of 36,000 lb/hr. Conventional conveyor designs were not up to the task, relying on a flat wire belt conveyor riding on a friction style slide bed, which is simply a structural steel frame to support the moving conveyor belt. With the combination of mold weight and conveyor speed (1 ft/min.), a conventional design would wear out quickly due to the heavy load on the conveyor and the resulting friction between the belt and slide bed.
Wisconsin Oven developed a roller belt support system whereby the conveyor belt rides atop specially designed high temperature rollers. The rollers use a high-temp lubricant that reduces friction by a factor of eight, compared to a conventional, friction-style slide bed arrangement. In addition to significantly extending the life of the conveyor belt, this reduced design allowed the use of a smaller conveyor drive motor. The result is a long-lasting conveyor design that is also rugged and energy-efficient.
The oven has an operating temperature of 575°F (302° C), and a heated length of 21 ft. (6,400 mm), which required 42 support rollers on 6-in. centers.
Dual conveyor oven for reduced footprint, heating flexibility — A Midwestern manufacturer of sensors needed to expand their production capacity, and wanted a conveyorized oven to dry 3,000 lb./hr. of molds. The company processed two different molds with different heating times. Mold A required a 45-minute drying time; mold B required 22.5 minutes. Based on the part size and production rate, an oven-heated length of 24 feet was necessary to achieve 45 minutes of heating, and a 12 feet length was sufficient for 22.5 minutes of heating. Traditional conveyor ovens have a belt running through the oven chamber, whereby all the molds running through the oven at once undergo the same heating cycle duration. Since they all ride the same conveyor belt, traditional oven designs prohibit different heating times for different molds being run through the oven at the same time.
In order to provide different heating times for the two different molds types, Wisconsin Oven provided a unique bi-directional dual conveyor oven. The oven uses two independently powered belt conveyors, each with its own drive system and speed control. Additionally, they run in opposite directions. After mold A exits conveyor No.1, the operator places them on conveyor No.2 to travel back through the oven to achieve the entire 45 minute drying time. When the B molds exit the oven, they are removed from the conveyor, having completed their 22.5-minute drying time.
Thermal consistency, higher throughput
Eck Industries in Manitowoc, WI, supplies premium aluminum castings to customers in military, hybrid vehicles, commercial trucking, aerospace, medical, industrial, and energy markets. Its castings include airfoils, engine blocks, water and air-cooled cylinder heads for all kinds of engines. Many of these engine components include thin walls and finned features. The ability to produce intricate cored elements and details for these difficult castings requires extreme control over coremaking, baking, and heating operations.
A variety of coremaking processes are used (SO2, PUNB and shell molding) to produce small- to medium-sized parts that require high dimensional precision. For some complex castings, in addition to individual intricate cores, assemblies and sub-assemblies of cores are produced.
When customers’ demand for complex cored castings increased significantly, Eck Industries needed a new core oven to more than double its existing drying capacity, and it needed an oven that offered better temperature control, with faster, more uniform core heating.
"A few years ago, we started looking for a new oven," said John Herrenbruck, Eck Industries' v.p., Technical Services. "It was decided that a dual-lane oven would suit our purposes best … ."
According to Wisconsin Oven’s application engineer Doug Christiansen, "This oven was designed for dual use, per Eck's request. They can also use this two-lane indexing conveyor oven as a batch aging oven for aluminum aerospace parts. The system was guaranteed to a thermal tolerance of ±10°F but actually proved out at ±5° at 550°F."
The new oven features a 500,000 Btu/hr. burner system and 30,600 CFM of recirculation air. It is designed with top-down airflow through the work chamber, to deliver air over the full width and length of the heating chamber. The ductwork is adjustable, so it can be set for optimal air-flow efficiency.
Additionally, the furnace uses PLC/HMI controls for indexing conveyance, door actuation, alarms and alarm management, index timing of each lane and I/O status. Variable index rates are available for heating time, and each lane can operate independently of the other, improving the flexibility of the unit to customer production needs. Eck Industries’ typical core now can be heated from 150°F to over 475°F in less time than previously possible.
Cores can be processed on each conveyor and oven capacity is no longer a problem. As each core comes out of the oven (generally on alternating tracks), an operator uses a hoist to offload the hot core.
"The oven basically ran great right out of the box,” according to Herrenbruck, “and has been running extremely smoothly about 20 hours/day ever since. To me, the best part is the faster heat-up and temperature uniformity of core we are now getting."
Mike Grande is the sales manager and senior applications engineer for Wisconsin Oven, which designs and manufactures industrial ovens and heating equipment. Contact him at LinkedIn, or learn more at www.wisoven.com