Technical Development: Process Controls and Data Management

April 24, 2006
Improve Green-Sand Cooling and Conditioning

Last year Space Srl, of Milan, Italy, established a manufacturing and marketing agreement with Hunter Automated Machinery Corp., bringing to the North American foundry market a proven and innovative approach to green-sand conditioning and preparation, with equipment to be manufactured by Hunter.

Space is well known in Europe for having the best “know how” for preconditioning and cooling return sand. For many years, from one system supplier to another, there has been very little change in the technique for cooling shakeout sand. Normally, the available equipment is configured around adding water and stirring vigorously, while at the same time passing air through the sand mass and capturing the evaporated steam and fines in a filter system.

Theoretically this concept was sound, but in many instances the theory was flawed because either the supplier or the user was not fully aware of the variables to the sand, or because they had limited control technology to manage the process.

In recent years there have been sand-cooling systems that combine the previous techniques, and add to these sophisticated vacuum systems. These designs are functional, but expensive to install and to operate continuously.

Space, established in 1979, has been offering alternative, workable, cooling solutions to suit metalcasting customers’ needs for 20 years. It was the first company in Europe to offer a computerized moisture-control package. This innovative product evolved into the “PGF” family of moisture controllers for managing precise water additions to mullers — the first time this technology was applied to the addition process.

The PGF device recorded average moisture in the feed sand to the muller, as well as average temperature and precise weight of the sand, either on a batch or a continuous basis. With this data the computer calculated precisely the total water needed by a batch muller to make the output target sand moisture, or the variable water required by a continuous muller to maintain a consistent output target.

With over 600 installations since its introduction, the PGF units are recognized as the best solution for moisture control. However, over 25 years the system has evolved and improved, following closely the developments in programmable-control technology. It was logical that some metalcasting operations began to apply PGF know-how to improve control on existing sand-cooling systems.

The first Space fully-integrated cooling plant was delivered for a 100-tons/hour sand plant feeding two vertical molding lines. Within months, this installation had led to a second order, and then a third. To date over 60 installations have been made, all based on the original idea. The basic components are a continuous pre-mixer homogenizer, feeding into a suitably sized fluid-bed cooler. It is not as simple as it may seem.

A new installation starts with an in-depth study of the existing plant’s general configuration. Although the Space equipment is flexible enough to be retrofitted into many applications, care is taken to avoid compromising the Space solution because of existing plant arrangement or building constraints.

Data from the foundry archives is also important, including casting size and weights across the range of products, sand test figures, and the weight of metal poured in the same period, as well as more confidential data such as scrap figures.

The first stage of cooling takes place in the pre-mixer, where variable amounts of water based on input sand weight, temperature, and residual moisture, are added to produce a continuous flow of sand that is high in moisture but homogenized and ready for the fluid-bed cooler. It is also worth noting that this unit is the best place to add new sand, bentonite, and fines to the process.

The Space fluid-bed cooler is a more advanced unit than generally is seen in foundries. Along its length it is divided into several cooling zones. Air volume in these zones is controlled by control systems that access temperature and moisture data measured at the cooler outlet. These coolers require no additional water spraying, which may lead to various problems. Discharge sand returning to the storage bunkers ranges from 95° to 104°F, with residual moisture of 1.8% to 2.2%. Unlike other fluid bed systems, the Space unit does not produce dry sand or suffer from “mud” blockages.

Recently, at a large jobbing foundry producing commercial diesel engine and gearbox products, the managers had taken an order for truck flywheels as a way to maintain their output tonnage in a slow market. They had a history of sand-cooling problems. Screen temperatures of up to 185°F were normal, but after cooling the returned sand was typically no more than 122°F. However, this sand was more or less dry and the sand plant area was very dirty.

Once the flywheel job was in production, sand temperatures reached 230°F with residual moisture of 0.1 to 0.3% at the screen. Prepared sand temperatures then rocketed —resulting in a rash of defects in the foundry’s casting production.

The foundry decided to retrofit the Space system to its operation, and by the fall of 2002 it began to appreciate the benefits. Regardless of casting campaigns, the prepared sand remained at more or less 98°F. Mulling batch cycle times were down, saving over 12% in Kw/hour used. Casting finishes improved dramatically, too. The sand plant also remained clean, solving an on-going environmental problem. Similar experiences have been noted in many foundries, even in cases where the sand inlet temperature was over 300°F.