Mention modernizing coremaking operations to foundry operators and everyone will picture something different: There is no such thing as a standard core room, because each foundry has it's own specific requirement for cores. Size, shape, quantity, type of process, type of sand, and other factors give each core room an individual identity. But, there are two primary issues that every core room must address: 1) proper sand handling; and 2) on-demand batch sand mixers. Foundries that want to remain competitive casting suppliers must adapt to changing developments in the industry, and react to domestic and international challenges deriving from increased automation and lower labor costs.
“Productivity” is a crucial factor for every business, and it must become one of the most important objectives for foundry operators. Effective productivity can be achieved by a) lowering the break-even point; b) using labor more efficiently; c) maximizing automation; and d) adapting and using available new technology.
Modernizing coremaking operations includes every stage of the production process, from raw sand handling and selection of the binder system and sand mixers to shakeout and sand reclamation.
Distribution of the Sand — How can core-room sand handling be improved? Depending on how much sand has to be moved and how far, in other words – tonnage and distance - will determine if belts or pneumatic conveyors should be used. Contrary to conventional beliefs, there have been some considerable advances in moving sand by pneumatic conveying, so that today we no longer have to be concerned with excessive pipe wear or sand degradation. In fact, systems are available today that you can install and practically forget.
There are three different types of pneumatic sand conveying systems: the Dilute Phase System; the Dense Phase System; and the High-Density System.
The dilute phase system is the oldest of these, and it is the one that has gives pneumatic conveying of foundry sand a bad name. Like so many other machines used in the foundry that were originally developed for another industry, dilute phase pneumatic conveying was adapted from the chemical industry with no accounting for the fact that sand behaves differently than some chemical compounds the system was designed to handle.
If you have sand delivered from a bulk truck into a silo the unloading is done with this type of system. It differs by requiring only low-pressure air, however the air-to-material ratio is quite high and sand velocities required in the piping to maintain fluidized sand grains in motion can be up to 4,000 fpm and more. Of course, the level of wear of piping and sand grain degradation is high, and causes constant problems with equipment maintenance and sand quality.
The dense-phase system, which is similar, also requires sand to be fluidized in the blow tank but air-to-material ratios are lower and the sand velocity in the piping is somewhat less than that required for the dilute phase system – ranging from about 2,800 to 5,000 fpm.
Because the sand travelling along the pipe has to be maintained in an agitated, fluidized state, boosters must be installed along the pipe length to inject additional compressed air into the sand/air mixture.
Due to the still very high velocities of the sand in the dense phase system, pipe wear is quite common, as is sand degradation through abrasion of the grains. Normally Schedule 80 pipe is specified for these systems.
Since dilute and dense phase pneumatic conveying systems have been installed in many foundries and the detrimental side effects are well known, system engineers and sand suppliers generally discourage their application.
The high-density system has been introduced successfully to North American foundries, after excellent results for European foundries over many years. The high-density system is different from dilute and dense phase systems because it uses higher pressure air, up to 85 psi, and moves the sand in the form of slow moving slugs at approx. 350-450 fpm through the pipe run with no need of boosters. Having an even lower air-to-material ratio than either the dilute or dense phase systems, the high density system results in only minimum pipe wear and negligible sand degradation. In the high density system the sand is never fluidized.
Comparing a pressure phase diagrams of dense phase and high density systems we note that starting out at a given pressure, the dense phase system (because fluidization has to be maintained) requires boosters approximately every 20 ft., which adds more air to the fixed-diameter pipe, increasing the velocity of the sand.
The high-density system, because fluidization is not required, uses the full energy of the expanding air to push the slugs of sand toward the receiving bin. This system is able to push sand approximately 350-400 ft. in one shot, and if longer distances have to be covered sand is dropped into an intermediate hopper and a second blow tank is installed in series.
Typically, maintaining the integrity of the sand grains and the low maintenance required in this system far outweigh the increased cost of staging.
A high-density system can deliver raw sand just like water flowing in a piping network, with each use-point being equipped with a valve to refill a hopper on demand. The high-density system is ideally suited to deliver sand to multiple discharge points, and is being applied frequently to “just in time” sand delivery to individual core sand mixers.
Mixing the Sand — The next step in making cores is mixing the sand. Sand mixers used in core rooms have mainly been adapted from similar machines used in other industries. Continuous mixers, because of their lower cost, are the most popular models in foundries and they are ideal when mixed sand is required on a continuous basis.
For jobs using only smaller amounts of sand, such as short-run or higher production core machines, smaller batch type mixers have gained popularity. Capable of small batches (up to 100 lbs.) in just 45-50 seconds, these smaller mixers deliver just enough sand to keep a core machine operating without allowing excessive aging of the mixed sand before use. In most applications these individual mixers operate completely automatically and are equipped with sand and binder metering devices.
There is a growing awareness that smaller sand volumes (just enough to make one blow) not only saves wasted sand but enhances core quality and binder requirements. This applies especially to larger-volume, cold box core production, where using sand directly from the mixer prevents excessive evaporation of volatiles in the sand mixture, which also benefits the properties of cold box cores.
Various medium- to high-production foundries have switched from the large volume batch or continuous mixers to these smaller batch-type mixers that allow individual setting of binder volumes for different jobs on specific core machines. Typically this type of small automatic batch mixer is equipped with a mixing bowl that holds the sand, with rotating blades driven by a vertical or horizontal shaft (which also may be located above a core machine), with mixed sand dumping directly into the core machine sand hopper.
There are several ways to distribute the mixed sand. Single continuous mixers filling large molds in no-bake operations, directly, and batch mixers with moving trolleys filling hoppers above rows of core machines.
But, to benefit from small sand charges for several core machines with lower production, smaller batch mixers can be mounted stationary, discharging into a trolley or clamshell bucket that delivers sand to individual core machine sand hoppers. Other applications are possible where the entire mixer is mounted on a mobile frame on wheels, to deliver sand directly to the core machines. Mixing starts upon receipt of a signal from the core machine needing sand.
What about sand temperature and what about sand type and sand quality? Should you be concerned about these topics? Of course you should.
Core quality can be achieved by maintaining consistent sand temperature, perhaps cooling the sand in summer and heating in winter, and paying attention to sand type and quality is just as important.