the pressures of the molten metal. T e green sand mixture is compacted
by hand or through mechanical force around a pattern to create a mold. T e mechanical force can be induced by slinging, jolting, squeezing or impact/ impulse. T e following points should be taken into account when considering the green
sand molding process: • Often, green sand processes are the most cost-eff ective of all metal forming operations.
• T ese processes readily lend themselves to automated systems for high-volume work, as well as short runs and proto- type work.
CONSIDERATIONS FOR SELECTING A CASTING PROCESS
• Required quality of the casting surface. • Required dimensional accuracy of the casting. • Number of castings required per order. • Type of pattern and corebox equipment needed. • Cost of making the mold(s). • How the selected process will affect casting design.
• Excellent casting surface fi nish. • Excellent characteristics for high-
• In the case of slinging, manual jolt or squeeze molding to form the mold, wood or plastic pattern materials can be used, whereas high-pressure, high- density molding methods almost always require metal pattern equipment.
• Excellent core and mold shelf life. Shell Process—In the shell process,
production runs since production cycles are short.
• High-pressure, high-density molding normally produces a well-compacted mold, which yields better surface fi n- ishes, casting dimensions and tolerances.
• T e properties of green sand are adjust- able within a wide range, making it pos- sible to use this process with all types of molding equipment and for a majority of alloys poured.
Chemically Bonded Molding Systems
Chemically bonded sand casting
processes are used widely throughout the metalcasting industry because of their economics and high productivity. Each chemically-bonded molding process uses a unique chemical binder and catalyst to cure and harden the mold and/or core. Some processes require heat to facilitate
the curing mechanism. Gas Catalyzed or Coldbox Sys-
tems—Coldbox systems utilize a family of binders where the catalyst is not added to the sand mixture. T e sand- resin mixture is blown into a corebox to compact the sand, and a catalytic gas or vapor is permeated through the sand mixture, where the catalyst reacts with the resin component, hardening the sand mixture almost instantly. Any sand mixture that has not come into contact with the catalyst is still capable of being cured, so many small cores can be pro- duced from a large batch of mixed sand. Several coldbox processes exist, includ-
ing phenolic urethane/amine vapor, furan/ SO2
In general, coldbox processes off er: • Good dimensional accuracy because the cores are cured without the use of heat.
, acrylic/SO2 and sodium silicate/CO2 2016 CASTING SOURCE DIRECTORY .
After curing, nobake molds are covered with a refractory wash or coating that provides a better surface fi nish on the casting and protects the mold sand from heat and erosion of the molten metal as it enters the mold cavity.
METAL CASTING DESIGN & PURCHASING 7
sand is pre-coated with a phenolic no- valac resin containing a hexamethylene- tetramine catalyst. T e resin-coated sand is dumped, blown or shot into a metal corebox or over a metal pattern that has been heated to 450-650F (232-343C). Shell molds are made in halves that are glued or clamped together before pour- ing. Cores, on the other hand, can be made whole, or, in the case of compli- cated applications, as multiple pieces glued together. Benefi ts of the shell process include:
• An excellent core or mold surface, resulting in good casting fi nish.
• Good dimensional accuracy in the casting because of mold rigidity.
order to improve productivity and eliminate the need for heat or gassing to cure mold and core binders, a series of resin systems referred to as nobake or airset binders was developed. In these systems, sand is mixed with
one or two liquid resin components and a liquid catalyst component. When the resin(s) and catalyst combine, a chemi- cal reaction begins to harden (cure) the binder. T e curing time can be length- ened or shortened based on the amount of catalyst used and the temperature of the refractory sand. T e mixed sand is placed against the
• Storage for indefi nite periods of time, which improves just-in-time delivery.
• High-volume production. • Selection of refractory material other than silica for specialty applications. • Savings in material usage through
hollow cores and thin shell molds. Nobake or Airset Systems—In
pattern or into the corebox. Although the sand mixtures have good fl owability, some form of compaction (usually vibration) is used to provide densifi cation of the sand in the mold/core. After a period of time, the core/mold will have cured suffi ciently to allow stripping from the corebox or pattern without distortion. T e cores/molds are allowed then to sit and thoroughly cure. After curing, they are covered with a refractory wash or coating that provides a better surface fi nish on the casting and protects the sand in the mold from the heat and erosive action of the molten metal as it enters the mold cavity. T e nobake process provides the fol-
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