Coremaking adds to metalcasting diversity.

Innovations in the equipment for producing high-quality cores continue to play a significant role in improving casting efficiency and effectiveness.

If the flexibility and variety of moldmaking techniques set metalcasting apart from other metalworking processes, then coremaking adds another significant dimension. The core, together with the mold, allows for the simultaneous formation of both internal and external surfaces of a metal part in one operation--a capability not available with other metal forming processes.

And like molding, a variety of coremaking processes is available to meet the needs of nearly any casting application.

Cores are component parts of a mold that form the internal passages and contours of a casting. They define the inside shape of a casting that cannot be configured by a pattern, just as a mold cavity defines the outside shape of a casting. In most instances, cores are made from various sand mixes by a number of mechanical and chemical processes.

Cores are made in coreboxes, special wood, metal or plastic materials. Cores must be highly refractory because they usually are completely surrounded by molten metal during the casting of a part. They must be sufficiently strong to withstand handling and thermal shock, yet not so rigid as to prevent the metal surrounding them from contracting normally.

Cores must also be formulated to collapse so that they can be removed easily from the casting after solidification.

Core sand is specially mixed so that each sand grain is coated with a specific additive(s) before being placed in a corebox, often using a programmable automatic mixing machine that combines sand and binders, as illustrated in Figs. 1 and 2.

Stationary mixers can be positioned over the receiving sand hopper of a core machine or to feed a transfer car to serve several machines. Sand and resin metering devices operate by a screw-type mechanism measured by the number of revolutions, an orifice-gate type measured by open time and volumetric or batch measuring. Figure 2 is an illustration of a PC-controlled mixer at right that fills a trolley-mounted mixer.

The mixer blends the sand/binder mixture as it moves and discharges the prepared sand through the discharge gate mounted on the left of the cylindrical mixer tank. Automated machines like this one accelerate the core production process, making it faster and more accurate.

Blending of sand and binder must be accomplished smoothly and without delay to maximize the mixture's effect on core density and integrity. Additives and sand grain conformity combine to achieve the proper degree of core density. The coated sand takes the shape of the cavity in a corebox, hardens and is removed to be placed later in the proper position in a casting mold. Core placement is largely handwork, but increasingly it is becoming more automated as new machinery becomes available.

After the mold is made, the flask containing the casting pattern is opened, the pattern is removed and one or more cores, representing the interior of the cast part, are set into the drag cavity just before the mold is closed. Cores are located and supported in the mold in coreprints. When required, cores are also fixed in proper position in the mold with specially shaped metal forms called chaplets that melt and become part of the casting.

When the metal is poured, it fills the mold cavity except in the area occupied by the core. The shape of the solidified casting results from the combined shapes of the mold and its core(s).

Since cores are an added casting expense, foundries strive to eliminate or minimize their use where possible to save labor, energy and material costs. However, a foundry's product mix, the size and shape of cores and the length of production runs are factors that affect the selection of coremaking equipment.

Corebox changes must be made quickly to avoid production bottlenecks, and changeover simplicity is a prime concern for equipment designers. In the face of severe competitive factors, the degree of automation in coremaking machinery assumes greater economic importance.

Coremaking Machinery

Modern coremaking equipment embraces several benefits that have evolved to speed coremaking while improving core accuracy. Machines are based on many operating principles, but the main objective is directed at increasing production efficiency and improving quality. A typical core machine, set up for coldbox cores, is shown in Fig. 3.

Core machines consist of a horizontal clamping mechanism, a vertical clamping attachment and a blow/exhaust system fed by a sand supply hopper into a sand magazine.

Newer machines of this type offer advances that speed core production. Included are such benefits as:

* a level of automation that reduces or eliminates labor per machine cycle;

* faster production (blow and harden) cycles;

* improved core reliability through wider use of computer controls;

* provision for quicker conversion to alternate coremaking processes and tooling;

* better odor and emissions controls;

* sustainable dimensional accuracy.

A recent design allows the job shop as well as the high-production foundry the versatility of running cores and making molds in the same cycle in a single machine. The machine accepts vertical and horizontal tooling made of wood, plastic, aluminum or iron (in configurations up to six parts). It also accepts multiple coremaking processes such as C|O.sub.2~, amine, methylformate and S|O.sub.2~ in any combination.

Using a single process controller, the foundry may manage the entire core production process from sand mixing, core shooting, gas generation, additives and sand supply.

The coreblower is among the oldest forms of coremaking machinery still widely used. It uses compressed air to blow and compact sand into the corebox. Air escapes from the box through finely slotted or screened vents, allowing sand to be compacted onto the core pattern.

Shell core machines use a vertical corebox heated to 450-500F by electricity or gas. The corebox is filled with a special thermal setting resin-coated sand by a compressed air system. After a dwell time of 5-10 seconds, the sand mix against the corebox is heated and the resin softens, becomes sticky and binds to the sand grains, forming a shell of hardened sand.

The corebox then is rotated 180 degrees to dump out sand not affected by the heat, leaving the hollow core in the box. The melted resin bonds the sand to a depth of 1/4-1/2 in. to form the hollow shell. After a 15- to 30-second cure time, the corebox is opened and the finished core is removed automatically by ejection pins.

In some instances, shell cores can be more costly than other coremaking options because of the heat and time requirements, but the tradeoff comes with the resultant cores having exceptional surface quality and inherent strength.

The latest equipment for making coldbox cores is usually equipped with such features as:

* programmable hydraulic operations;

* high closing pressures and short cycle times;

* automatic blow head cleaning;

* quick clamping of corebox;

* short cycle time;

* gassing manifold, purging and scrubber systems.

One such machine features a modular concept that enables foundries to purchase specific equipment to meet current needs, adding additional modules as production requirements increase. It uses a special head to fill a sand magazine via an air regulated sand gate. The amount of sand admitted is controlled by an electronic sand probe. An internal sand chute meters and evenly distributes the flow of sand into the magazine chamber. A shutter regulates air flow into the magazine. External hydraulic cylinders clamp the sand magazine to the corebox.

The air valve opens instantaneously to allow a controlled volume of compressed air to impact the top of the sand column, forcing the sand into the corebox. The machine immediately recycles for another shot.

Another concept introduced in recent years is that of the vibratory mixer, which allows for the rapid and consistent production of sand for coremaking operations. Developments like this and others discussed in this report continue to keep coremaking a vital part of the metalcasting process.

Bibliography

R. Smith, "High Technology Coremaking," AFS Engineering Committee, Div. 1

C. Doerschlag, "Modernizing Coremaking," Alb. Klein Co., Inc.