Chapter 14 Flashcards
Three categories of casting
Single use molds with multiple use patterns. Single use molds with single use patterns. Multiple use molds.
Expendable mold casting processes
Single use molds with either multiple or single use patterns.
Most common metals casted
Cast iron and aluminum are the most common because of their low cost and good fluidity.
Sand casting
The most common and the most versatile of the casting processes. Used for iron and steel casting.
Sprue
An opening is cut from the top of the mold through the sand and fo next to the system of horizontal channels called runners.
Gates
Mold metal is poured down the sprue and enters the savory through here.
Pattern
First step in making sand casting is the design of the pattern.
Material of patterns
Wood is easy to make, but not stable. Metal is expensive, but durable.
One piece of solid patterns
Simplest and least expensive. Used when the shape is relatively simple and the number of duplicate castings is rather small.
Split patterns
Used when moderate quantities of a casting are desired.
Match plate patterns
Can be coupled with modern molding machines to produce large quantities of duplicate molds. Used when moderate quantities of casting are desired
Match plate
Cope and drag of a split pattern and permanently fastened to opposite sides of this.
Sands are compounded with additives to net requirements:
1) refractoriness ability to withstand high temps without melting. 2 cohesiveness ability to retain a given shape when packed into a mold. 3 permeability the ability of mold cavity mold and gases to escape the sands. 4 collapsibility the ability accommodate metal shrinkage after solidification and provide for easy remove of casting through mold disintegration, shakeout.
Green sand mixture
Molding material is often reclaimed and recycled. This ingredients are put through a mullet, a device with kneads on it, to ensure each grain of sand is coated uniformly.
Green sand casting
Most widely used process for casting both ferrous and nonferrous metals. Material is imposed if sand, clay, water, additives. Tools cost are low, very inexpensive. Rough surface finish, and a need for machining.
Cores
Used for holes, also used to improve casting design and permit optimization.
Green sand cores
Have relatively low strength, may be difficult to withdraw pattern without breaking them.
Dry sand cores
Produced separate from the remainder of the mold then inserted into core prints that hold them in position.
How is dry sand cores made
Packed into a box with the desired core shape, called a dump core box. A plate is placed over the box and it is inverted. Baking and gardening and sanding is done on the product. A coating such as graphite silica or. Mica can be sprayed in the surface for greater heat resistance.
Plastic molding
The mold material is plaster of Paris, combines with various additives to improve strength, permeability.
How is plastic molding made
The mold material is mixed, then poured over a metal or plastic pattern and allowed to set. Hydration of the mold is done. It is baked to remove excess water.
With metal patterns and plaster mold material
Surface finish and dimensional accuracy are excellent. Cooling is slower, and is limited to lower melting temperature nonferrous alloys.
Ceramic mold casting
Similar to plaster mold, but made from a ceramic material that can withstand the higher melting temperatures of metals. Greater cost of the mold material, long mold preparation time. Production is limited to small size castings in small to medium quantities.
In large molds, the ceramic can be used
To produce a facing around the patterns, which is back up by a less expensive material such as reusable fire clay.
Investment casting
A very old process, used in China and Egypt, and performed by dentists and jewelers. Offer unlimited freedom in the complexity of shapes and the types of materials that can be cast.
Step 1 of investment casting
Produce a master pattern
Step 2 of investment casting
From the master pattern, produce a master die.
Step 3 of investment
Produce a wax pattern. Patterns are made by pouring molten wax into the master due or injecting it under pressure and allowing it to harden.
Tree
Assemble the wax patterns into a common wax sprue. Using heated tools and melted wax, a number of wax patterns cna be attached to a central sprue and runner system to create a pattern cluster, or tree.
Step 5 on investment
Coast the cluster or tree with a thin layer of investment material. This ensures a smooth surface and good detail in the final product.
Step 6 of investment
Form additional investment around the coated cluster. The cluster can be refilled, and refractory aggregate is rained over the surface In a processes called stuccoing.
Step 7 of investment
Allow the investment to fully harden
Step 8
Remove the wax pattern from the mold by melting or dissolving.
Lost wax process
When molds are placed in the oven, the wax disappeared.
Step 9
Heat the mold in preparation for pouring. Heat to 550-1100 C, this ensures complete removal of the wax, and cured the mold to give extra strength.
Step 10
Pour the molten metal.
Step 11
Remove the solidified casting from the mold.
Pros and cons of investment
Complex and expensive. Complex shapes can be cast as a single piece. No draft required. Excellent dimensional precision.
She’ll cracking
A common problem during wax removal, is this, which destroys the shell and results in loss of the time and expense of reaching that stage of manufacture.
So how use exoendablemold
Green sand min is the most common, with no bake molding a second. Then gas hardened, investment, vertical parted green sand, shell molding, plaster molding, lost foam.
Core production
She’ll and no bake are the two most common methods, followed by green sand, gas hardened, cold box, core oil.
