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BRASS & BRONZE VALVE PROCESSES

Brass vs. bronze

	BRASS	BRONZE
Composition	Brass is any alloy of copper and zinc	Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive, but sometimes with other elements such as phosphorus, manganese, aluminum, or silicon
Properties	Higher malleability than zinc or copper. Low melting point (900°C); flows when melted. Combinations of iron, aluminum, silicon and manganese make brass corrosion resistant. Susceptible to stress cracking when exposed to ammonia. Not as hard as steel.	Hard and brittle. Melts at 950°C but depends on amount of tin present. Bronze resists corrosion (especially sea- water corrosion) and metal fatigue more than steel and is a better conductor of heat and electricity than most steels.
Uses	Decorative; low-friction applications (i.e. locks, gears, doorknobs, ammunition, valves); Plumbing/ electricals; musical instruments for acoustic prop- erties; Zippers and uses where its important sparks aren’t struck (fittings and tools around explosive gases)	Used in boat and ship fittings, propellers and sub- merged bearings because of resistance to salt water corrosion. Widely used for cast bronze sculpture; Bearings, clips, electrical connectors and springs For top-quality bells and cymbals.
Color	Muted yellow, somewhat similar to gold, but duller	Reddish brown
History	Brass was first known to exist in about 500 B.C.E.	Bronze dates to about 3500 B.C.E.

The sand casting process

The process cycle for sand casting consists of six main stages, which are explained below.

1. MOLD MAKING

The first step in the sand casting process is to create the mold for the casting. In an expendable mold process, this step must be performed for each casting. A sand mold is formed by packing sand into each half of the mold. The sand is packed around the pattern, which is a replica of the external shape of the casting. When the pattern is removed, the cavity that will form the casting remains. Any internal features of the casting that cannot be formed by the pattern are formed by separate cores which are made of sand prior to the formation of the mold. Further details on mold-making will be described in the next section. The mold-making time includes positioning the pattern, packing the sand, and removing the pattern. The mold-making time is affected by the size of the part, the number of cores, and the type of sand mold. If the mold type requires heating or baking time, the mold-making time is substantially increased. Also, lubrication is often applied to the surfaces of the mold cavity in order to facilitate removal of the casting. The use of a lubricant also improves the flow the metal and can improve the surface finish of the casting. The lubricant that is used is chosen based upon the sand and molten metal temperature.

2. CLAMPING

Once the mold has been made, it must be prepared for the molten metal to be poured. The surface of the mold cavity is first lubricated to facilitate the removal of the casting. Then, the cores are positioned and the mold halves are closed and securely clamped together. It is essential that the mold halves remain securely closed to prevent the loss of any material.

3. POURING

The molten metal is maintained at a set temperature in a furnace. After the mold has been clamped, the molten metal can be ladled from its holding container in the furnace and poured into the mold. The pouring can be performed manually or by an automated machine. Enough molten metal must be poured to fill the entire cavity and all channels in the mold. The filling time is very short in order to prevent early solidification of any one part of the metal.

4. COOLING

The molten metal that is poured into the mold will begin to cool and solidify once it enters the cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. The mold can not be opened until the cooling time has elapsed. The desired cooling time can be estimated based upon the wall thickness of the casting and the temperature of the metal. Most of the possible defects that can occur are a result of the solidification process. If some of the molten metal cools too quickly, the part may exhibit shrinkage, cracks, or incomplete sections. Preventative measures can be taken in designing both the part and the mold and will be explored in later sections.

5. REMOVAL

After the predetermined solidification time has passed, the sand mold can simply be broken, and the casting removed. This step, sometimes called shakeout, is typically performed by a vibrating machine that shakes the sand and casting out of the flask. Once removed, the casting will likely have some sand and oxide layers adhered to the surface. Shot blasting is sometimes used to remove any remaining sand, especially from internal surfaces, and reduce the surface roughness.

6. TRIMMING

During cooling, the material from the channels in the mold solidifies attached to the part. This excess material must be trimmed from the casting either manually via cutting or sawing, or using a trimming press. The time required to trim the excess material can be estimated from the size of the casting’s envelope. A larger casting will require a longer trimming time. The scrap material that results from this trimming is either discarded or reused in the sand casting process. However, the scrap material may need to be reconditioned to the proper chemical composition before it can be combined with non-recycled metal and reused.

From the design, provided by an engineer or designer, a skilled pattern maker builds a pattern of the object to be produced, using wood, metal, or a plastic such as expanded polystyrene. Sand can be ground, swept or 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. Pattern makers are able to produce suitable patterns using “Contraction rules” (these are sometimes called “shrink allowance rulers” where the ruled markings are deliberately made to a larger spacing according to the percentage of extra length needed). Different scaled rules are used for different metals, because each metal and alloy contracts by an amount distinct from all others. Patterns also have core prints that create registers within the molds into which are placed sand cores. Such cores, sometimes reinforced by wires, are used to create under-cut profiles and cavities which cannot be molded with the cope and drag, such as the interior passages of valves or cooling passages in engine blocks.

Paths for the entrance of metal into the mold cavity constitute the runner system and include the sprue, various feeder which maintain a good metal “feed,” and in-gates which attach the runner system to the casting cavity. Gas and steam generated during casting exit through the permeable sand or via risers, which are added either in the pattern itself, or as separate pieces.

MOLDING 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 onone side - the lower portion of the box, closed at the bottom, will be filled with a molding sand. 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 un-latched, 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 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 furnace resins or amine-hardened resins.

CHILLS

To control the solidification structure of the metal, it is possible to place metal plates, chills, in the mold. The associated rapid local cooling will form a finer-grained structure and may form a somewhat harder metal at these locations. In ferrous castings, the effect is similar to quenching metal in forge work. The inner diameter of an engine cylinder is made hard by a chilling core. In other metals, chills may be used to promote directional solidification of the casting. In controlling the way a casting freezes, it is possible to prevent internal voids or porosity inside castings.

CORES

The produce cavities within the casting - such as 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 the 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.

With a completed mold at the appropriate moisture content, the box containing the sand mold is the positioned for filling with molten metal - typically iron, steel, bronze, brass aluminum, magnesium alloys, 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 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.

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 and 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.

The forging process

Investment casting/Brass shell casting

THE BENEFITS OF FORGING OVER CASTING

There currently is a lot of debate over which is best, forging or casting. Much of the reason for the debate is economic. For centuries “forged” has been specified for tools and hardware that had to be the most durable and of the best quality. Manu facturers of less expensive cast goods try to convince their customers that cast is as good as forged.
The best, perfect, casting may theoretically approach a forging in performance but can never achieve the exact same properties. And castings are rarely perfect. Castings often hide hidden defects below the surface (cracks, porosity and sand inclusions) that would be exposed as defects in a forged billet. When a steel billet is created it is rolled or forged, improving its structure. This also closes porosity, welds cracks and there are no sand inclusions from molding. When an item is forged from this billet its integrity is proven again under the hammer and its structure is further refined.

COMPARING FORGING TO CASTING

Forging vs. brass/bronze casting

FORGING	CASTING
Stronger	Casting cannot obtain the strengthening effects of hot and cold working. Whether open or closed die forging is used, the forging process surpasses casting in predictable strength properties - producing superior strength that is assured.
Defects refined in preworking	A casting has neither grain flow nor directional strength and the process cannot prevent formation or certain metallurgical defects. Preworking forge stock produces a grain flow oriented in directions requiring maximum strength. Dendritic structures, alloy segregations and like imperfections are refined in forging.
More reliable, less costly	More reliable, less costly - Casting defects occur in a variety of forms. Because hot forging refines grain pattern and imparts high-strength, ductility and resistance properties, forged products are more reliable. And they are manufactured without the added costs for tighter process controls and inspection that are required for casting versus forging.
Better response to heat treatment	Castings require close control of melting and cooling processes because alloy segregation may occur. This results in non-uniform heat treatment response that can affect straightness of finished parts. Forgings respond more predictably to heat treatment and offer better dimensional stability
Flexible, cost effective production adapts to demand	Some castings, such as special performance castings, require expensive materials and process controls, and longer lead times. Open die forgings are examples of forging processes that adapt to various production run lengths and enable shortened lead times.

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