Chapter 14 Part 3

Casting and Molding Foundry industry is mostly considered primary manufacturers Make stock for secondary manufacturers Secondary manufacturers finish the parts

Cast Iron Specific to the casting process Types: Sold in ingot form Ductile Gray High-alloy Malleable White (others) Sold in ingot form

Casting Processes Some processes use liquid metal, some use powders Processes classified by type of mold used Permanent (reusable) Consumable (destroyed)

Powder Metallurgy Compacting metal powders in a permanent mold Ferrous and non-ferrous metals 97% of the initially used raw material is in finished part Near-net-shape process Close to desired shape, no additional machining for part shape May still need machining of threads, holes, or other unusual design features Little shrinkage compared to other casting High volume process (thousands of parts per hour) Cost of tooling is too high to use in low volume applications

Steps involved in powder metallurgy Powder mixing Compacting Sintering - holding the part at a temp just below it’s melting temp for a short time. Creates a bond between the powder particles. Part is fragile before sintering (green compact) Can be broken if handled roughly Secondary heat treating may be applied to strengthen

Immersing porous P/M parts into oil or resin after sintering Impregnation Immersing porous P/M parts into oil or resin after sintering Provides lubrication, less friction Used on moving parts (bearings) Care must be taken to make sure oil isn’t removed Cardboard, paper wick oil away

Drawing another metal into the P/M part Improves mechanical properties Infiltration Drawing another metal into the P/M part Improves mechanical properties Also seals pores of the part Helpful if part is to be plated

Used to make a wide variety of parts P/M Injection Molding Finely ground powders coated with resin, used in a conventional injection molding press. Used to make a wide variety of parts Gears Cams Appliance parts

Die Casting Molten metal forced under pressure into a permanent mold Low melting point metals Zinc Lead Tin One of the fastest casting methods 2 piece dies

Permanent Mold Casting Molten metal poured into a mold through a gating system Normally an automated process Usually a turntable with 10-12 stations Each mold is cleaned out with air then filled with molten metal Machine cools each part, opens the molds and ejects the parts

Sand Casting Most used process Expendable sand mold destroyed to remove the part Sand can be reused A pattern is used to make the mold Pattern must be slightly larger than the finished part (shrinkage) Sand is packed around the pattern inside a box called a flask Used to make engine blocks and cylinder heads

Investment Casting Similar to sand casting, uses a consumable mold Precision process for materials that are hard to work with or difficult to machine Pattern made from wax or expanded polystyrene Pattern is dipped in ceramic refractory slurry, dried then repeated until desired thickness of mold is achieved Mold is placed in an oven to melt the pattern Molten metal is poured in the mold Mold is broken off of finished part

Centrifugal Casting Mold cylindrical parts from plastics and metals Same process for either material Relies on rotating forces to form material against a mold Popular for making: Pipes Tanks Bearing rings