What is Sand Casting?

Sand casting is a widely used form of casting. As the name implies, sand is used to create the casting mold. In sand casting, a pattern of the finished part is placed in sand. The sand is then packed around the pattern to form the mold. If the pattern must be removed before pouring the metal, the mold will be made in two or more pieces. The mold must be modified to include a hole through which the metal will be poured. Molten metal is poured into a mold cavity formed out of sand (natural or synthetic). The processes of sand casting include patterns, sprues and runners, design considerations, and casting allowance. The mold remains intact until the metal has solidified. The mold is destroyed when the part is removed, so a new mold has to be made for each casting. Sand molds can be used in iron casting, bronze casting, brass casting, and aluminum casting.

View a typical process flow of sand casting.

Sand is used as a refractory material in sand molding systems. A binder maintains the shape of the mold while pouring molten metal. There are a wide range of sand/binder systems that are used in sand casting systems. Bentonite clay is used in 4-10% of the sand mixture in Green sand systems, the most common sand casting system. Water, which makes up around 2-4% of the sand mixture, activates the binder. Carbonaceous material such as charcoal (2-10% of total volume) is also added to the mixture to provide a reducing environment. This helps in preventing the metal from oxidizing while pouring. The remaining 85-95% of the total mixture contains sand.

A range of chemical binders is used by other sand molding processes: Oil binders are a mixture of animal oils, petrochemicals and vegetables. Some of the popular synthetic resin binders are: ureaformaldehyde, phenolics, phenolformaldehyde, urea-formaldehyde/furfuryl alcohol, alkyl isocyanate and phenolic isocyanate. Chemical resin binders are frequently used for foundry cores and, less extensively, for foundry molds.

How Patterns are Made
The first stage for developing a new casting is pattern making. The pattern is just a replica of the finished product. Generally, it is made of wood, but metal, plastic, and plaster can also be used. These patterns are permanent so can be used to form a number of molds. Pattern making is a highly skilled and precise process that is critical to the quality of the final product. Many modern pattern shops make use of computer-aided design (CAD) to design patterns. These systems can also be integrated with automated cutting tools that are controlled with computer-aided manufacturing (CAM) tools. Cores are produced in conjunction with the pattern to form the interior surfaces of the casting. These are produced in a core box, which is essentially a permanent mold that is developed.

How Molds are Made
The mold is formed in a mold box with two halves that helps in removing the pattern. As sand molds are temporary in nature, a new mold has to be formed each time for individual casting.

View a cross-section of a typical, two-part sand mold.

When the core is inserted on the top of the furnace, its burner starts the melting process immediately.

Drag, the bottom half of the mold, is made on a molding board. Cores require greater strength to hold their form during pouring. Dimensional precision also needs to be greater because interior surfaces are more difficult to machine, making errors costly to fix. One of the chemical binding systems is used in forming the cores. Once the core is inserted, the top half of the mold or the cope is placed on top. The interface between the two mold halves is called a parting line. Sometimes, weights are placed on the cope, which helps in securing the two halves together.

Mold designs include a gating system which is designed to carry molten metal smoothly to all parts of the mold. The gating system typically includes a sprue, gates, runners and risers. The sprue is where the metal is poured. Gates allow the metal to enter the running system. Runners carry the molten metal towards the casting cavity. Risers may have several functions including vents to allow gases to be released, reservoirs prior to the casting cavity to aid progressive solidification, and waste cavities to allow metal to rise from the casting cavity to ensure it is filled and to remove the first poured metal from the casting cavity, thus avoiding solidification problems.

Melting and Pouring
Many ferrous foundries use a high proportion of scrap metal to make up a charge. As such, foundries play an important role in the metal recycling industry. Internally generated scrap from runners and risers, as well as reject product, is also recycled. The charge is weighed and introduced to the furnace. Alloys and other materials are added to the charge to produce the desired melt. In some operations the charge may be preheated, often using waste heat. The furnaces commonly used in the industry are described below. In traditional processes metal is superheated in the furnace. Molten metal is transferred from the furnace to a ladle and held until it reaches the desired pouring temperature. The molten metal is poured into the mold and allowed to solidify.

View a cross-section of a sand mold pour.

Cooling and Shakeout
The mold is transported to a cooling area, immediately after the molten metal has been poured in. The casting needs to cool for a long time, often overnight, before it can be removed from the mold. Castings may be removed manually or using vibratory tables that shake the refractory material away from the casting. For rapid cooling of castings, many foundries also use quenching baths. This speeds up the process and also helps achieve certain metallurgical properties. To prevent oxidation, the quench bath may contain chemical additives.

Sand Reclamation
Foundries recover a significant proportion of the waste sand for internal reuse. It significantly reduces the quantity of sand that must be purchased and disposed of. Generally, the sand is reclaimed mechanically. Cores and large metal lumps are removed by vibrating screens and the binders are removed by attrition in which the sand particles rub together.

Fine sand and binders are removed by extraction and collected in a baghouse. In some systems metals are removed using magnets or other separation techniques. For operations using mechanical reclamation, the recycle rate is often limited to around 70%.

This is due to the need to maintain a minimum sand quality. For large iron foundries, where sand quality requirements are less stringent, over 90% reclamation can be achieved by mechanical means. For many processes, mechanically reclaimed sand is not of sufficiently high quality to be used for core production. Thermal reclamation is becoming more widely used. This process heats the sand to the point where organic materials, including the binders, are driven off. This process can return the sand to an 'as new' state, allowing it to be used for core making. Thermal reclamation is more expensive than mechanical systems.

Sand can also be reclaimed using wet washing and scrubbing techniques. These methods produce sand of a high quality but are not commonly used because they generate a significant liquid waste stream and require additional energy input for sand drying. The amount of internal reuse depends on the type of technology used and the quality requirements of the casting process. Reclamation processes, particularly mechanical ones, break down the sand particles and this can affect the quality of some metals. Also, for mechanical reclamation techniques, impurities may build up in the sand over time, requiring a proportion of the material to be wasted. Large iron foundries do not require a high sand quality to typically achieve the highest rate of reuse in the industry. Often, sand cycles through the operation until it is ground down to a fine dust and removed by baghouses.

Cleaning and Finishing
The gating system is removed, after the casting has cooled, using abrasive cut-off wheels, bandsaws, or electrical cut-off devices. A 'parting line flash' is typically formed on the casting and must be removed by grinding or with chipping hammers. Castings may also need to be repaired by welding, brazing or soldering to eliminate defects.

The casting may undergo additional grinding and polishing to achieve the desired surface quality. The casting may then be coated using either a paint or metal finishing operation such as galvanizing, powder coating or electroplating.

Shot Blasting
Propelling abrasive material at high velocity onto the casting surface — is often used to remove any remaining metal flash, refractory material or oxides. Depending on the type and strength of the metal cast, the grade of shot may vary from steel ball bearings to a fine grit.

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Permanent Mold Casting

A form of permanent mold casting is when the molten metal is poured into the mold, either directly or by tilting the mold into a vertical position. In this process, the mold is made in two halves from cast iron or steel. If cores are to be used, they can be metal inserts, which operate mechanically in the mold, or sand cores, which are placed in the molds before closing (semi-permanent molding).

The mold halves are preheated and the internal surfaces are coated with a refractory. If static pouring is to be used, the molds are closed and set into the vertical position for pouring; thus, the parting line is in the vertical position. In tilt pouring, the mold is closed and placed in the horizontal position at which point molten metal is poured into a cup(s) attached to the mold. The mold then is tilted to the vertical position, allowing the molten metal to flow out of the cup(s) into the mold cavity.

The various permanent mold techniques—static pour and tilt pour—offer a variety of advantages for a variety of metal forming applications. Benefits include:

• Castings with superior mechanical properties because the metal mold acts as a chill
• Castings are uniform in shape and have excellent dimensional tolerances because molds are made of metal
• Excellent surface finishes
• High‐production runs
• Sections of the mold that can be selectively insulated or cooled, which helps control the solidification and improves overall casting properties.

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Piston Engineering

Pistons come in all shapes and sizes. The following is intended to provide a brief overview of piston configurations and terminology. As you will see there are numerous variations and complexities in the design not to mention critical aspects of alloy selection and finishing.

Generally, pistons differ by the following characteristics:

View a diagram of Piston Design Terminology.

Material Considerations

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Design Considerations

General
Sand casting is the most popular casting process employed in industry. Sand casting molds use silica based sands. There are two general types of sand: naturally bonded and synthetic sands Synthetic sand can be controlled better and is the most common used by foundries. In general, sand used in making sand cast molds is fine, round grains that can be closely packed and forms a smooth mold surface. Sand cast molds are designed to have a good collapsibility (the casting shrinks while cooling) to avoid defects in the casting, such as hot tearing and cracking. Clay is used to cohesively bond sand particles, giving the sand strength. Common recognizable parts made by sand casting are: engine blocks, cylinder heads, housings and similar enclosures.

Design
The designer should take into account the following: the parting line, finish, the draft, the presence of ribs, bosses, webs, and recesses, and the machining allowance. In general uniform walls are preferred with a maximum wall thickness not to exceed 5 inches and a minimum wall thickness,25 inches over a 5 inch span max.

Parting Line
The parting line should be identified on the casting drawing. Determining the position of the parting line is a critical step in the casting design. Parting line mismatch, and parting line and seam flash extension allowances should nor exceed .020". Maximum parting line seam flash extension material is about .015".

Finish
As cast surface finish is typically 200 - 500 RMS, post machining operation should be accounted for in material allowances should better surface be required.

Draft
To facilitate the removal of the pattern from the weak, brittle molding sand cast draft should be defined and accounted for. Standard draft for sand casting is 2 degrees with a minimum of about 1 degree for external and internal features.

Section Changes
The design should not contain abrupt section changes. Fillets and tapers are preferred to sharp steps. If a section change of over 2:1 thickness ratio is unavoidable, there are two alternatives: design two separate castings to be assembled together, or use a wedge form between the unequal sections. The taper of the wedged area should not exceed 1:4.

Ribs and Web
Improper rib, web and corner design can lead to sink spots. In general web or wall thickness should as a minimum be 0.10" - Aluminum, .13 for Magnesium and .13 for Steel. See Casting Rib Design for additional information.

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