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Green Sand Metalcasting Foundry News

Review of American Foundry Society's "Core Processes"

Posted by Hill and Griffith Company on Feb 5, 2019 3:45:49 PM

An editorial review of AFS's "Core Processes" Chapter from the "Casting Buyer's Guide"

Hill and Griffith specializes in compounds and coatings for Green Sand Foundry Process patterns and cores. This post highlights a few parts of the "Casting Buyer's Guide" chapter two on "Core Processes." With a better understanding of cores, you will understand our diverse product selection.
 
"One of the reasons castings are so widely used in all types of applications is due to the great design flexibility to form the internal shape of a casting which is inherent in a variety of processes available to produce a core.
 
The core, which is usually a pre-formed sand aggregate (metal for die casting), is inserted into the mold to shape the interior of a casting or that part of a casting which cannot be shaped by the pattern. The core is located and held in the mold by core pockets, formed by the pattern. In areas where the core may be thin in cross-section or require additional support to maintain metal wall thickness, chaplets are used.
 
Foundry-Additives-Glossary-A-1
 
In the core processes described below it is not possible to describe every known core production method, but the presentation of the processes listed is directed toward the major core binder systems which are presently used in casting production.
 
2.1 GREEN SAND CORE PROCESS 

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.  

Advantages
Cores and molds produced from the same sand system. No baking or curing of the core required. 

Disadvantages
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.

Advantages
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. 

Disadvantages
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. 

Chemical Binders 

ORGANIC

Thermosetting processes 
Shell:
Hot box (furan/phenolic):
Warm box:
Core oil:

Self-setting systems 
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 

Gas-cured processes 
Free radical
Phenolic urethane-amine Furan/peroxides-SO2
Epoxy/SO2
Phenolic-ester cured

INORGANIC
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:

Advantages
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. 

Disadvantages
Costs of the various binders are higher; process control must be consistent.


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