An editorial review of AFS's "Core Processes" Chapter from the "Casting Buyer's Guide"
One of the oldest methods of producing a core is by using the same "green sand" used to produce the mold. The core may be produced by hand ramming, jolting, squeezing, or by blowing the core in a core blowing machine. Cores produced using green sand are simple in design and must be handled with great care. An advantage of green sand cores is that they do not require further processing (e.g., baking) once removed from the core box.
Cores and molds produced from the same sand system. No baking or curing of the core required.
Difficult to produce complex cores. Core must be handled carefully and requires constant care in handling.
2.2 OIL SAND CORE PROCESS
One of the oldest materials used to bond grains of core sand together is core oil. The composition of oil sand cores is generally 1 % cereal binder and 1 % core oil (by weight of sand) mixed with a clean (washed and dried) silica sand. The sand is mixed and blown on a core machine into a core box, or by hand ramming the sand mixture in the core box. The core is then stripped onto a metal form (driers) or plate, which supports the core form during the baking cycle. The core should be baked for approximately one hour per inch of cross-section at a temperature from 375F (190C) to 450F (232C), depending upon the core oil manufacturer's recommendations. At the completion of the baking cycle, the core is strong enough to be handled for further processing. Its composition depends largely on the size and type of the castings which are to be produced, the production baking cycle, and the core oven facilities. The oil sand core process is rapidly being replaced in the foundry industry by the chemically bonded coremaking processes which produce a core with better dimensional accuracy, improved physical properties, and higher productivity; however, there are many foundries still using oil sand cores in their casting process.
Low material cost. Improved core dimensional accuracy over green sand cores. Cores can be handled and stored for a limited time allowing flexibility in production scheduling which is not possible when using green sand cores.
Dimensional accuracy is not consistent. Low productivity.
2.3 CHEMICALLY BONDED CORE PROCESSES
This section provides a brief description of a variety of chemically bonded coremaking processes. The two major methods used to transfer sand to a core box are by blowing the sand mixture into a closed core box, or by manually, mechanically, or pneumatically feeding the sand mixture into an open core box. The methods and materials used to fill the core box cavity and produce the chemical reaction necessary to complete the curing of the core binder system vary significantly. In the first section high-volume production systems (blowing into a closed core box) are described, followed by medium- to low-volume production systems (feeding the sand into an open core box), and a listing of the various chemical binder systems.
2.3.1 High-Volume Production Systems
The majority of high-volume production systems produce a core by blowing the sand into a closed core box. The sand has been mixed prior to transferring to the core blowing machine. The completion of the curing cycle depends upon the core binder requirement for heat, gas, or a chemical reaction of the resin and catalyst to complete the cure (hardening) of the core before the core box can be removed. A gas-purge system requires more equipment to complete the gas and purge cycle to producing a core on the cold box system than a heating system required to produce a core on the hot box system. The application of heating or gas-purge systems vary in cost when using a three-part binder system. Some of the three-part binder systems can produce a core without the application of heat or gas. This machine has three small sand mixers located on the top of the machine which feed the mixed sand into the sand magazine for flowing into the core box. After blending of the total resins and catalyst with sand a very fast cure occurs. This system requires very close process control during the blending and mixing cycle, but does not require the added equipment required in the other two methods described above. The materials used for core boxes for the above production systems are usually metal or plastics due to the high-volume requirements. Lower volume requirements make it possible to use wood or plastic patterns for the gas-purge system or the three-part fast cure systems.
2.3.2 Medium- to Low-Volume Production Systems
The equipment required to service a medium- to low-volume core production system has less equipment and usually the equipment is not as complex as a high-volume production system. The core boxes are usually designed as a "split" core box which is open on the top to receive the sand from a screw type tube mixer. The sand flows out of the mixer into the core box which is usually mounted on a compaction table required to produce a dense core. After filling the core box, the overflow sand is "struck-off" to the top of the core box. The core box is then moved to an area provided for curing. Curing time is dependent upon the binder system and the cross-section of the core. This process is used extensively to produce large cores. One of the disadvantages of this process is the pasting of cores together since most cores must be produced in sections.
2.3.3 Listing of Various Chemical Binder Systems
Chemical binder systems can be further categorized as organic or inorganic, and thermosetting, self-setting, or gas-cured. A list of the major core binder systems which are presently used in the foundry industry in the production of castings will be found on the next page.
Hot box (furan/phenolic):
Furan: High-nitrogen furan-acid, Medium-nitrogen furan-acid, Low-nitrogen furan-acid
Phenolic: Phenolic-ester cured, Phenolic-acid
Urethanes: Alkyd-organometallic (alkyd-oil), Phenolic-amine, Polyol-amine
Phenolic urethane-amine Furan/peroxides-SO2
Self-setting processes: Silicates, Sodium silicate-ester cured
Gas-cured processes: Sodium silicate-CO2
For further information regarding various binder systems contact the American Foundrymen's Society. The advantages and disadvantages of chemically bonded systems when compared with oil sand cures are noted as follows:
Capital expenditures are reduced; higher cured strength at room temperature; reduced skill required; less damage in handling and storage; core storage for longer periods of time; better dimensional accuracy; higher productivity; chemically bonded systems usually makes quality control less complex and results more consistent.
Costs of the various binders are higher; process control must be consistent.
Metal Foundry Related News from The American Foundrymen's Society
Hill and Griffith Customer Service
The Hill and Griffith Company's green sand metalcasting foundry supplies help achieve the EPA's M.A.C.T. standards and reduce Benzene emissions. Our variety of environmentally sound release agents, coatings, partings, lubricants, core oils and specialty products will help you meet your metal casting's needs. We're known for our hands-on approach. Let us visit your plant and recommend products that suit your needs.
We are pleased to provide samples in quantities large enough to allow you to "try before you buy."
Contact Us »
Technical Services & Support
On-site casting defect investigations, product testing, machine start-ups and much more. Also, lab facilities are available to provide testing upon request.
Contact Us »