Introduction to Sand Casting Terminology
Sand casting is a metal forming process in which a mold is first formed from a three-dimensional pattern of sand, and molten metal is poured into the mold cavity for solidification. The sand shell is subsequently removed after the metal components cooled and formed. Some components require a secondary machining process after casting.
Typical products made through the process of sand casting include engine blocks, cylinder heads, cylinder pistons, frames, brackets, manifolds for high-horsepower engines, impellers, housings, gearbox parts of transmission systems for construction, and agriculture equipment. This article looks at some of the basic terminology associated with the sand casting process.
Key Terms Used in Sand Casting
Sand casting operations are performed in a foundry. In order to better discuss your needs with a foundry representative, it is important to know some of the key terms used in the sand casting process:
- Sand: Types of sand used in sand casting include resin-sand, a type of quartz sand that is mixed with a resin material. With a green-sand process, no chemical additives are used.
- Pattern: A pattern is a shape used to form a cavity in the sand. It is usually slightly larger than the finished casting to allow for shrinkage.
- Mold: Molten metal is poured into the cavity to create a casting. There are two parts to the mold – the top is referred to as the cope, while the bottom is called the drag. A parting line shows the separation of the two mold halves.
- Core: A core is a preformed insert that is placed into the outside mold to shape the internal features of the casting. It keeps the material from filling the entire mold, to achieve the desired shape. Two processes can be utilized to create the core – hot-box and cold-box.
- Flask: Usually made of aluminum, steel, or wood, this container box holds the molding sand together. A sand rammer packs the sand into the flask. An aligning system helps to bring the cope and drag into the same position around the parting line.
- Furnace: The source material is turned into molten form here, at a very high temperature. Charge is the weight of the metal that is introduced into the furnace.
- Ladle: A large-size ladle transfers molten metal from a furnace to a pouring basin to separate impurities, and then into the mold.
- Sprue (or downsprue): This is the connecting passage between the pouring basin and the gating system. It is vertical, with a straight tapered circular cross-section. Its height helps to control the speed of the molten material.
- Gating (or running) System: As material flows from the melting furnaces to be delivered to the mold, it is managed by a carefully controlled gating system. Runners are in a horizontal position.
- Risers: This part of the running system acts as a sort of reservoir and is used to compensate for shrinkage as the molten metal solidifies.
- Casting Cavity: This is where solidification takes place. Any remaining molten metal comes out through the riser.
- Vents: These are small openings in the mold, which allow gas to escape the gases during solidification.
- Chill: A chill is a metal insert which may be used in the mold to equalize solidification across the casting.
- Shakeout: Shakeout is the process of separating the solidified casting from the mold material.
- Acceptable Quality Level (AQL): This quality level relies on a system of inspection using samples selected at random.
- As-Cast: This means using the casting without secondary machining.
- Binder: This agent is used to add strength to the sand mixture.
- Cure: Cure means to harden.
- Finish Allowance: This is the amount of material left on the surface of a casting, to allow for further machining processes.
- Rejection Rate: This figure represents the ratio of the number of parts scrapped to the total number of parts manufactured, and is expressed as a percentage.
Defects Caused By Part Design
- Hot Tears: A type of crack that occurs when shrinkage pulls the metal apart in the cavity.
- Shrinkage: This occurs when the design leads to solidification that prevents liquid metal being drawn back.
- Dips: Another type of shrinkage defect that results from metal pulling away from the sand walls.
- Non-Clean Up (NCU): This type of defect only becomes apparent when the casting is machined, because there is not enough metal for machining to leave smooth, flat, or cylindrical surfaces and the as-cast surface remains.
Defects Caused By Errors in Mold Preparation
- Sand Inclusions: Foreign bodies in the casting, which reduce integrity, and create problems during machining.
- Cuts and Washes: Areas where flowing metal has eroded part of the sand mold.
- Swells: These look like bulges in the surfaces of the cast metal part, and are caused by poor sand packing.
- Shift: Shift can show up at the parting line where cope and drag meet, resulting in an offset along the parting line in the cast part. In addition, cores may shift within the mold if they are not held in place securely.
Problems Resulting From the Pouring And Solidification Process
- Porosity: Air can enter the metal and rise to the highest points of the cavity or become trapped in the bulk of the metal.
- Oxide Defects: These result when the metal is exposed to air, and can cause regions of weakness.
- Cold Shuts: If flow is such that two metal fronts meet, they will touch without merging together, creating a weak area in the cast part.
- Non-Fills: Metal solidifies before the cavity is completely full, resulting in areas where metal is missing.
Learn More About Sand Casting
Impro is a global, integrated manufacturer of high-precision, high-complexity and mission-critical components for diversified markets. We use state-of-the-art process technologies and equipment. If you are interested in finding more information about sand casting and our process capabilities, please don’t hesitate to contact our team of experts today.