Hollow Core Sand Casting
A core in sand casting is used to form the inner contours and cavities of the final sand casted product. Cores are normally disposal items and destroyed in order to remove it from the finished cast.
Used in both sand casting and metal injection molding, a stunning example of the use of cores can be seen in the casting of automotive engine blocks, such as the GM V-8 engine, which requires the use of a total of five dry-sand cores.
On this page, you can learn more about:
- Types of Cores Used in Sand Casting
- The Use of Binders in Core Sands
- Important Considerations, Terms and Aspects of a Core in Sand Casting
- Characteristics of a Core in Casting
Types of Cores Used in Sand Casting
The following types of cores are used in sand casting:
A part of the cope and drag, green-sand cores are not a typical type used in core sand casting. The disadvantages associated with this type of core is that they are not very strong, which results in casting narrow and long features extremely difficult, if not impossible.
Typically applied through a hole in a casting, these type of cores, in the cases where they are used to cast long features, results in a need for machining after the casting process.
Overcoming some of the disadvantages associated with green-sand core sand casting, dry-sand cores are formed independently inside the mold, after which it is inserted into the core prints, responsible for holding the core in position, in the mold.
These types of cores are produced by mixing sand with a binder in either a metal or wooden core box. This core box is made to contain the exact cavity in the shape that is required for the cast.
The most simple way of producing these types of cores is by making use of a dump core box. Sand is packed into the dump core box and scraped level with the top of the box. A metal or wood plate is placed over the box and flipped over, resulting in the core segment falling out.
This core segment produced is now baked or hardened, after which multiple core segments are hot glued together or attached to one another by making use of various means and techniques.
Rough spots found on the core are filled and sanded down. These cores are then very lightly coated with graphite, mica or silica, resulting an increased resistance to heat and smoother surface finish.
In cases of single piece cores, no assembly is needed as they are produced by making use of a split core box, made of two halves and featuring at least one hole through which sand can be introduced.
Special core producing extruders are applied in cases where a simple core with constant cross-sections are required. These extrusion are then cut to the desired length and hardened.
There are various types of core boxes, which includes:
The Use of Binders in Core Sands
For added strength, special binders are introduced to the types of cores used in sand casting. Today, synthetic oils, in conjunction with clay and cereal, is used as a binder.
Cores are baked in a convection oven between 392°F (200°C) and 482°F (250°C), which causes the binder used to polymerize or cross-link. In essence a simple process, the dimensional accuracy achieved is however low.
The Hot-box process is another type of binder process used, which applies a catalyst or thermoset for a binder. This process involves the sand and binder being packed into a core box, which is then heated to ± 446°F (230°C).
The binder, which is in contact with the heated surface of the core box, now begins to cure - this takes approximately 10 and 30 seconds. Depending on the type of binder used in the process, the core might require further baking processes to allow it to fully cure.
These types of cores used in sand casting is also sometimes called a shell-core, as only the exterior layer of the core is hardened. When the core box is opened and the core removed, the uncured sand inside the hardened, exterior layer is dumped out and can be reused.
This process can also be seen in some cold-box practices, though it is a much less common practice to produce shell cores using cold-box practices.
In smaller veins, the cold-box process makes use of a binder which is hardened by making use of special gasses. The binder coated sand is packed into a core box and tightly sealed to allow a curing gas to be introduced into the core box.
Often odorous and toxic, these gasses require a special handling system, but on the flip side, because no high temperatures are required for curing, the core box can be made of plastic, metal or wood.
Hollow cores can be formed by introducing gas (via holes in the core's surface) - this results in only the exterior layer of the core hardening, the interior sand to be dumped out and reused. Sodium silicate is one of the cold-box sand casting binders used and, when exposed to carbon dioxide, causes the exterior layer to harden.
A special set of binders is also used in air-set sands, allowing for the production of cores at room temperatures. Because organic binders as well as a curing catalyst is mixed together, the sand used to produce the core does not require the introduction of a gas.
The binder and sand mixed together initializes the curing process, but once this mixing occurs, there is a short time frame in which the sand can be used.
Important Considerations, Terms and Aspects of a Core in Sand Casting
Internal wires and rods can be used in order to increase the strength of cores, while straw can be added to the middle of the core in order to enhance its collapsibility.
A hollow core can also be used in order to enhance collapsibility, which is an important aspects in steel castings, as a large amount of shrinkage is present when sand casting steel.
With the exception of very small cores, all cores do require the presence of vent holes to enable the proper release of gasses formed during the pouring process. This is usually achieved by making use of very small wires, which serves to create holes from the surface of the mold to the core itself.
In cases where wires cannot be applied for creating venting holes, coke and cinder can be added to the core - this will result in an increased permeability.
In cases where a core in sand casting processes only makes use of one core print, chaplets are used to deliver the required core support necessary. Chaplets are small, metal supports used to bridge the gap between the core and the surface of the mold. This technique results in the supports (or chaplets) becoming a part of the final casting. Because of this, the chaplet material must either be made of the same or similar material of that being poured in the sand casting process.
Optimized designs are required when making use of chaplets. Too small, chaplets will melt completely, resulting in the core moving from its required position. Too big, these chaplets' surfaces cannot fuse or melt properly with the material being poured.
The use of chaplets are capable of causing defects and creating weak spots within the final casting.
In cases where a reentrant angle is required, cheeks are used instead of cores. A third segment in the flask (an addition to the drag and cope), cheeks allow the entire sand casting mold to be produced from green sand and removable patterns.
A disadvantage associated with cheeks is that it requires an increased mold making process. On the flip side however, this can be a beneficial aspect when low quantities of castings are required.
In cases of a high quantity of casts required, it is a more economically sound choice to make use of cores.
Characteristics of a Core in Casting
A core used in sand casting must:
- Have a smooth surface finish.
- A minimum production of gasses released during the pouring of the molten metal.
- A high permeability to allow the gasses produced during pouring, to sufficiently escape the cast.
- The core in sand casting is required to be weak enough to break down as the material starts to shrink.
- Because the core itself is surrounded by molten metal during the sand casting process, a good refractoriness is essential.
- Cores must be easy to remove during the shake-out step in the process.
- The core, in its hardened state, is required to handle to forces caused by casting. Its compression strength is required to be between 100 psi (0.69 MPa) and 300 psi (2.07MPa).
- A sufficient amount of strength is required for hardening (in the green condition).
Back to Main Page: Sand Casting
Further Suggested Reading:
- Metals that can be Sand Cast
- Resin Cores for Sand Casting
- Sand Casting Tolerances
- Ultra Large Sand Casting
- Surface Finish and sand types used to achieve fine finishes
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