page 249
Sandslingers - A high speed stream of sand into the flask tends to pack the sand effectively.
Impact molding - an explosive impulse is used to compact the sand. The mold quality with this technique is quite good.
Vacuum molding - an envelope of plastic is created about the sand using plastic sheeting. Air is drawn from the sand, and the vacuum leads to compaction.
45.1.2 Sands
•The sands used tend to fall into the following categories, naturally bonded (bank) - less expensive
synthetic (lake) - this sand can have a variety of controlled compositions.
•Types of sand include,
-Zircon (ZrSiO4) - low thermal expansion
-Olivine (Mg2SiO4) - low thermal expansion
-Iron Silicate (Fe2SiO4) - low thermal expansion
-Chromite (FeCr2O4) - high heat transfer
•The sand effects the following aspects of the casting,
granule shape - smaller and rounder grains produce a better casting surface.
granule size - a coarse grained sand will be porous and allow gases to escape during casting. a fine grained sand leads to a stronger mold.
collapsibility - if the sand can shift during cooling of the part it will reduce stress tears and cracks
• Green sand molding refers to a slightly wet condition of the sand (much like ‘green wood’). At the right level of humidity the moisture will increase sand binding. But in excess this moisture expand when heated during pouring and blow metal back out of the mold (i.e. explosion). This is one of the least expensive molding techniques).
45.2 SINGLE USE MOLD TECHNIQUES
45.2.1 Shell Mold Casting
•The basic process for these molds is,
1.Create two mating patterns of desired shape.
2.Coat the molds with a shell (sand and binders, such as a resin) until desired thickness and other properties are obtained.
page 250
3.Cure the molds and remove the patterns.
4.The mold halves are mated and held firm while metal is poured.
5.The final part(s) is removed.
Start with matching patterns (the pattern is shown with diagonal lines)
A sand is used to coat the molds, and it is bonded to make shells
The pattern halves are mated, and then backed up to complete the mold
•This technique can be very economical.
•Special care must be taken to assure venting for gasses, as the mold media is less porous.
•This method can easily use cores and chills to make complex molds.
•Graphite molds can be used for materials that would normally react with other materials used for the molds.
45.2.2 Lost Foam Casting (Expandable Pattern)
•This process has a number of basic steps,
1.Make a mold for producing styrofoam patterns.
page 251
2.Make styrofoam patterns using inject molding of expanded polystyrene foam beads (or another low density monomer foam) This process can be automated.
3.Glue the parts foam patterns together, and glue to sprue/runner/gate systems as required.
4.For high quality surface finish the parts may be coated with a ceramic slurry and hardened in a drying oven.
5.Place the pattern in sand, taking care to compact the sand about the pattern.
6.Cast metal into the pattern. The foam will evaporate, and escape through the normal routes gas evacuates through.
7.Wait until the part is hard, and remove from the sand.
8.If a ceramic coating was used this can be removed using impact, vibration, or abrasive techniques as appropriate.
A styrofoam pattern |
The pattern is coated |
is made |
with a refractory |
|
coating and dried |
Molten metal is cast in, and the styrofoam evaporates. The metal takes the shape of the refractory coating
The foam pattern is packed in sand
•This process can be automated, and can be very inexpensive in quantities.
•Complex parts can be made with relative ease by gluing together foam pieces.
45.2.3 Plaster Mold Casting
•This technique is basically,
1.Create a two part pattern.
page 252
2.A mold material is used that is a plaster of paris type mixture (fast setting) to make two cavities. This may have some additives to improve properties. Foamed plaster may be used to increase permeability.
3.After setting these cavities will be dried in an oven to remove moisture.
4.The Antioch process is optional and increases mold permeability by dehydrating in an autoclave, and rehydrating for a number of hours.
4.The mold halves are then mated and heated.
5.After reaching adequate heat levels the molten metal is poured. Mold porosity is low so pressure or vacuum must be used to encourage complete filling of the mold.
6.The final part is removed and cleaned
•This technique is known for its high level of dimensional accuracy.
45.2.4 Ceramic Mold Casting
•Also known as ‘cope and drag investment casting’.
•The basic process is,
1.A wood or metal pattern is placed in a flask and coated with a slurry of zircon and fused silica combined with bonding agents.
2.The mold is removed, cleaned and baked. The shells may be used as given, or they may have other materials, such as clay put on as backing materials.
3.The molds are then used as normal.
•This can make high temperature material parts.
45.2.5 Investment Casting
•The basic steps are,
1.An expendable mold of a part is made in wax, plastic, etc.
2.The part has a gate and runner attached to it, and all are dipped in a ceramic slurry.
3.The slurry is hardened, and the core is melted and/or burned out.
4.The core is burned out and the mold is preheated to the temperature of the molten metal
-644°C for aluminum
-1040°C for ferrous alloys
-etc.
5.Molds are filled by pressure, vacuum or centrifugal force.
6.After cooling, the mold is broken off, the sprues are cut off, and stubs are ground off.
page 253
WAX PATTERN MOLDED
WAX PATTERNS JOINED TO RUNNER
The waxcore is molded in some other place, and the patterns are created XXXXXX
WAX PATTERNS COATED IN CERAMIC, WAX IS MELTED OUT, THE SHELL IS DRIED, AND PLACED IN SAND
METAL IS CAST INTO THE SHELL AND ONCE SOLID REMOVED FROM THE SAND
PARTS ARE FINISHED
PARTS ARE REMOVED FROM
THE RUNNER SYSTEM
page 254
•Many parts can be made at the same time by attaching them to a common gating system.
•Parts can be glued together to make shapes that would normally be too complex to mold.
•Typical methods used are,
-cast iron
-steel
-aluminum alloys
-brass
-bronze
-magnesium
-zinc
•The die used to make the mold cores can be used for thousands of parts.
•Typical large applications are,
-large propellers
-large frames
-nozzles
-cams
-valve parts
•Typical small applications are,
-dental
-jewelry
-orthopedic surgical implants
-camera components
•Advantages,
-fine details can be made
-thin sections are possible
-high accuracy
-weights from <1 ounce to > 100 lb.
-any castable metal can be used
-no parting lines
-good surface finish (60-220 in.)
-can be automated
-many parts can be made at once providing lower per piece cost
-high melting point metals can be used
•Disadvantages,
-less strength than die cast parts
-process is slow
-changes to the die are costly
-more steps are involved in production