Sand casting

Casting is the process of production of objects by pouring molten material in to a cavity called a mould which is the negative of the object, and allowing it to cool and solidify. Sand casting is a means of producing rough metal castings using a mould usually made from sand formed around a replica of the object to be cast that is removed once the sand has been compacted. Castings made by this process can be further refined by any or all of hammer peening, shot peening, polishing, forging, plating, rough grinding, machine grinding or machining. Sand castings not further worked by polishing or peening are readily recognized by the sand-like texture imparted by the mold. As the accuracy of the casting is limited by imperfections in the mold making process there will be extra material to be removed by grinding or machining, more than is required by other more accurate casting processes.

Patterns

From the design, provided by an engineer or designer, a skilled patternmaker builds a master of the object to be produced, using wood, metal, or plastic, other materials to be used can be polystyrene or even sand strickled into shape. The metal to be cast will contract during solidification, and this may be non-uniform due to uneven cooling. Therefore, the pattern must be slightly larger than the finished product, a difference known as contraction allowance. Patternmakers are able to produce suitable patterns using 'Contraction rules'. Different scaled rules are used for different metals because different metals / alloys contract at different rates. Patterns also have coreprints; these create registers within the moulds, into which are placed Sand cores. Sand cores are used to create under cut profiles and holes which cannot be moulded.

Paths for the entrance of metal, during the pouring (casting) process into the mould cavity constitute the runner system and include the sprue, various feeders which maintain a good metal 'feed' and 'runners', and ingates which attatch the runner system to the casting cavity. Gas and steam generated during casting exit through the permeable sand or via the riser, are added either in the pattern itself, or as separate pieces.

Cope & drag (top and bottom halves of a sand mould), with cores in place on the drag

Moulding box and materials

A multi-part molding box (known as a casting flask, the top and bottom halves of which are known respectively as the cope and drag) is prepared to receive the pattern. Molding boxes are made in segments that may be latched to each other and to end closures. For a simple object—flat on one side—the lower portion of the box, closed at the bottom, will be filled with prepared casting sand or green sand—a slightly moist mixture of sand and clay. The sand is packed in through a vibratory process called ramming and, in this case, periodically screeded level. The surface of the sand may then be stabilized with a sizing compound. The pattern is placed on the sand and another molding box segment is added. Additional sand is rammed over and around the pattern. Finally a cover is placed on the box and it is turned and unlatched, so that the halves of the mold may be parted and the pattern with its sprue and vent patterns removed. Additional sizing may be added and any defects introduced by the removal of the pattern are corrected. The box is closed again. This forms a "green" mold which must be dried to receive the hot metal. If the mold is not sufficiently dried a steam explosion can occur that can throw molten metal about. In some cases, the sand may be oiled instead of moistened, which makes possible casting without waiting for the sand to dry. Sand may also be bonded by chemical binders, such as furane resins or amine-hardened resins.

Chills

If it is desired to have most of the—iron or steel—casting in a tough, ductile, state but with a few surfaces hard, it is possible to place metal plates—chills— in the mold, where the metal is to be hardened. The associated rapid local cooling will form a finer-grained and harder metal at these locations. The effect is similar to quenching metals in forge work. The inner diameter of an engine cylinder is made hard by a chilling core.

Cores

To produce cavities within the casting—such as for liquid cooling in engine blocks and cylinder heads—negative forms are used to produce cores. Usually sand-molded, cores are inserted into the casting box after removal of the pattern. Whenever possible, designs are made that avoid the use of cores, due to the additional set-up time and thus greater cost.

Two sets of castings (bronze and aluminium) from the above sand mold

With a completed mold at the appropriate moisture content, the box containing the sand mold is then positioned for filling with molten metal—typically iron, steel, bronze, brass, aluminum alloy, or various pot metal alloys, which often include lead, tin, and zinc. After filling with liquid metal the box is set aside until the metal is sufficiently cool to be strong. The sand is then removed revealing a rough casting that, in the case of iron or steel, may still be glowing red. When casting with metals like iron or lead, which are significantly heavier than the casting sand, the casting flask is often covered with a heavy plate to prevent a problem known as floating the mold. Floating the mold occurs when the pressure of the metal pushes the sand above the mold cavity out of shape, causing the casting to fail.

Left:- Corebox, with resulting (wire reinforced) cores directly below. Right:- Pattern (used with the core) and the resulting casting below (the wires are from the remains of the core)

After casting, the cores are broken up by rods or shot and removed from the casting. The metal from the sprue and risers is cut from the rough casting. Various heat treatments may be applied to relieve stresses from the initial cooling and to add hardness—in the case of steel or iron, by quenching in water or oil. The casting may be further strengthened by surface compression treatment—like shot peening—that adds resistance to tensile cracking and smooths the rough surface.

Design requirements

The part to be made and its pattern must be designed to accommodate each stage of the process, as it must be possible to remove the pattern without disturbing the molding sand and to have proper locations to receive and position the cores. A slight taper, known as draft, must be used on surfaces perpendicular to the parting line, in order to be able to remove the pattern from the mold. This requirement also applies to cores, as they must be removed from the core box in which they are formed. The sprue and risers must be arranged to allow a proper flow of metal and gasses within the mold in order to avoid an incomplete casting. Should a piece of core or mold become dislodged it may be embedded in the final casting, forming a sand pit, which may render the casting unusable. Gas pockets can cause internal voids. These may be immediately visible or may only be revealed after extensive machining has been performed. For critical applications, or where the cost of wasted effort is a factor, non-destructive testing methods may be applied before further work is performed.

Decorative use of patterns

Old wood-patterns, once used to make molds for casting machine parts, are sought out and collected by some for use as interior decorations.

Alternative casting methods

Sand casting for mass production has largely been superseded by other methods.

• Modern mass production methods can produce thin but accurate molds—of a material superficially resembling paper mache, such as is used in egg cartons, but that is refractory in nature—that are then supported by some means, such as dry sand surrounded by a box, during the casting process. Due to the higher accuracy it is possible to make thinner and hence lighter castings, because extra metal need not be present to allow for variations in the molds. These thin-mold casting methods have been used since the 1960s in the manufacture of cast-iron engine blocks and cylinder heads for automotive applications.

• Various automotive mechanical components are now frequently made of aluminum, which for appropriately shaped components may be made either by sand casting or by die casting, the latter an accurate process that greatly reduces both materials use and machining and finishing costs. While the material and the processing setup is more expensive than the use of iron this is one of the most straightforward ways to reduce weight in a vehicle, important as a contributor to both fuel economy and acceleration performance. For front engine vehicles with rear weel drive the improvement in weight distribution can improve both handling and traction and weight saved in the engine is multipled in that this enables use of lighter suspension components.

• Starting in the early 1980s, some castings such as automotive engine blocks have been made using a sand casting technique conceptually similar to the lost wax process, known as the lost foam process. In this process, the pattern is made of polystyrene foam, which the sand is packed around, leaving the foam in place. When the metal is poured into the mold, the heat of the metal vaporizes the foam a short distance away from the surface of the metal, leaving the molding cavity into which the metal flows. The lost-foam process supports the sand much better than conventional sand casting, allowing greater flexibility in the design of the cast parts, with less need for machining to finish the casting. This technique was developed for the clay mold casting of abstract art pieces and was first adopted for large quantity commercial production by the Saturn company.

External links

• Iron casting in a hobby foundry
• Aluminum and bronze casting in a homemade foundry