Text c sand casting mold

The set of channels through which a molten metal flows to the mold cavity is called gating system. Typical gating system consists of a pouring cup and a sprue receiving the poured melt, runner – a channel through which the melt is supplied to the gates through which the molten metal enters the mold cavity. A gating system may include a riser (feed head) – a cavity connected to the gating system feeding the casting when it is shrinking.

Air within the mold cavity and gases formed when a molten metal contacts the mold surface are removed through the vents.

The interior cavities of a casting are formed by a separate inserts called cores. Cores are usually made of sand and backed.

A mold frame (flask) consists of two parts: cope (the upper part) and drag (the lower part).

A mold cavity is formed in the process of pattern molding, when the pattern (commonly wooden) is embedded in sand in the flask forming an impression of the casting. After the sand packing the pattern is removed from the flask, the cores and the gating system are arranged.

Cores, runner and gates are arranged in the drag; pouring cap and sprue are placed in the cope. Then the two parts of the mold are assembled and poured.

After the metal has solidified and cooled to a desired temperature, the casting is removed from the mold by the process called shakeout.

TEXT D

Investment casting process

The investment casting process uses expendable patterns made of investment casting wax. The wax patterns are commonly prepared by injection molding technology which involves injection of wax into a prefabricated die having the same geometry of the cavity as the desired cast part. The wax patterns are then attached to a gating system (a set of channels through which a molten metal flows to the mold cavity).

The next stage is the shell building - the wax assembly is immersed into refractory ceramic slurry of hardening mixtures followed by drying. This operation is repeatedly carried out resulting in formation of a solid ceramic shell of 1/4” -3/8” (6mm – 9mm) thick.

The next stage is dewax. At this stage the assembly is heated in an autoclave where the most of the wax is melted out. This operation is followed by burning out the residual wax in a furnace. The mold is then preheated to 1830°F (1000°C). Now the mold is ready for filling with a molten metal.

Casting stage is a conventional operation involving pouring a molten metal into the shell through the gating system. After the metal has solidified and cooled to a desired temperature, the shell is broken and the castings are cut away from the gates and sprue.

The last stage is finishing carried out by sandblasting or machining.

Text e graphite mold casting

Foundrymen have for a long time recognized the benefits of casting metal into graphite permanent molds. Graphite’s superior thermal conductivity promotes rapid solidification which results in castings with improved surface finish and superior mechanical properties. Graphite is also known for its excellent thermal stability and it will not warp or distort when used as a die. But graphite tends to oxidize rapidly at elevated temperatures and relatively short mold life results when conventional ferrous and nonferrous materials are cast in graphite molds.

For the past ten years, however, a new process has evolved using graphite dies with new zinc gravity casting alloys. The zinc materials do not affect graphite because of their low casting temperatures. For the first time graphite has become a viable high production tool material, which is capable of making in excess of 20,000 zinc castings from a single mold. In addition, the low economics and superior casting tolerances offered by the process, and the high strengths and wear resistant characteristics of the new zinc alloys combine to provide designers and casting specifiers with a preferred manufacturing method for many precision casting applications.

The zinc alloys used with graphite permanent molds are relatively new products developed specifically for sand and permanent mold nonferrous foundries. The new materials are radically different from zinc diecasting materials, since they contain higher aluminum content (up to 27%).

These new alloys are changing the old image of zinc as being solely a diecast material. These foundry grades are gaining popularity with nonferrous foundries because they are easy-to-cast, low temperature materials with clean, energy-saving melting characteristics.

Casting buyers are beginning to specify zinc gravity castings because they offer easy machining with good general purpose engineering properties, which permit direct substitution for cast iron, malleable iron, bronzes and heat treated aluminums. Zinc alloy costs are marginally higher than aluminum, but about one-third the cost of bronze. Existing sand and iron permanent mold tools are used during substitution with only minor modifications.

The zinc/graphite process incorporates more refined methods than conventional permanent mold casting with ferrous dies. Iron dies are often mounted on home-built, hand-operated mechanisms individually fabricated for each tool. The shops that have adopted the zinc/graphite process, on the other hand, incorporate standardized production equipment which use semiautomatic casting machine principles. These principles include mounting dies (usually vertically) on electro-pneumatically controlled platens with built-in ejector systems. The machines operate much like a simplified diecasting machine. The operator starts the casting cycle by pushing the start button on his control panel after molten metal has been hand ladled into the die.

A thermocouple in the mold provides a visual temperature display for the operator to check die performance. Ejector pins push the castings off the die during the open cycle, and the operator removes the parts by hand. Tongs are not required.

After part removal the die automatically closes and is ready for the next cycle. Careful control of the casting parameters sometimes allows for immediate trimming by hand while gates are still soft. In most cases, however, gates are sawed off and ingate areas are ground to blend smoothly with the casting profile. Bench hand molds are common; however, most castings are made using the semiautomatic machines.

There are good reasons why the process has evolved using these practical casting machines. The rapid chilling effect of graphite makes the process sensitive to metal and die temperature variations which can affect castability. Once optimized, the casting cycle variable is fixed by dialing the parameters into the control panel. The operator, therefore, is freed to concentrate on part removal and improving productivity. The ejector system is also important. Graphite is brittle and parts must be ejected accurately off the die, otherwise mold chipping and breakage will result. The typical manhandling which occurs with iron molds cannot be tolerated with graphite. The machine reduces the possibility of casting personnel causing damage to the graphite dies.