5.1 Foundry

5.1 Foundry

1. Pattern Making

1. Definition:

   • A pattern is a replica of the final casting with dimensional allowances.

   • Used to form the mold cavity in sand casting.

2. Pattern Materials:

   • Wood (most common, cheap, easy to shape).

   • Metal (for long production runs, durable).

   • Plastic/Resin (good surface finish, moisture resistant).

   • Wax (for investment casting).

3. Pattern Allowances:

   • Shrinkage Allowance: Compensates for metal contraction during cooling.

   • Draft Allowance: Taper on vertical surfaces for easy pattern removal.

   • Machining Allowance: Extra material for final machining operations.

   • Distortion Allowance: For castings that warp during cooling.

   • Shake Allowance: Slight pattern enlargement for easy mold removal.

4. Pattern Types:

   • Single-piece: Simple shapes, for small production.

   • Split: For complex shapes, has parting line.

   • Match-plate: Pattern mounted on plate, for mass production.

   • Cope and Drag: Separate patterns for top and bottom mold halves.

   • Gated: Includes gating system channels.

2. Melting and Casting

1. Melting Furnaces:

   • Cupola: Vertical furnace for melting cast iron, uses coke fuel.

   • Electric Arc: High temperature, for steel alloys.

   • Induction: Clean melting, good temperature control.

   • Crucible: Small batches, various metals.

2. Melting Process Steps:

   • Charging (loading raw materials).

   • Melting (heat application).

   • Refining (impurity removal).

   • Alloying (adding elements).

   • Pouring temperature adjustment.

3. Casting Processes:

   • Sand Casting: Most common, uses sand molds.

   • Permanent Mold Casting: Reusable metal molds.

   • Centrifugal Casting: Uses centrifugal force for hollow parts.

   • Continuous Casting: For producing long shapes with constant cross-section.

4. Pouring Considerations:

   • Temperature control.

   • Pouring rate.

   • Turbulence minimization.

   • Slag/skimmings removal.

3. Die and Precision Casting

1. Die Casting:

   • Process: Forces molten metal into metal die under high pressure.

   • Types:

     • Hot-chamber (for low-melting-point alloys like zinc).

     • Cold-chamber (for aluminum, brass, magnesium).

   • Advantages:

     • High production rates.

     • Excellent dimensional accuracy.

     • Good surface finish.

     • Thin sections possible.

   • Limitations:

     • High die cost.

     • Limited to non-ferrous metals.

     • Part size restrictions.

2. Precision Casting Methods:

   • Investment Casting (Lost-wax):

     • Wax pattern surrounded by ceramic slurry.

     • Wax melted out, molten metal poured in.

     • Excellent surface finish and dimensional accuracy.

     • Complex shapes possible.

   • Shell Molding:

     • Uses resin-bonded sand as thin shell.

     • Better finish than sand casting.

     • Good dimensional accuracy.

   • Plaster Mold Casting:

     • Uses plaster of Paris molds.

     • Excellent surface finish.

     • Limited to non-ferrous metals.

   • Ceramic Mold Casting:

     • Similar to plaster but higher temperature resistance.

     • For ferrous and non-ferrous metals.

3. Tolerances Achievable:

   • Investment casting: ±0.1% of dimension.

   • Die casting: ±0.05 mm for small dimensions.

   • Better than sand casting by factor of 2-5.

4. Cleaning and Applications

1. Cleaning Operations:

   • Shakeout: Removing casting from mold.

   • Core Removal: Breaking/sandblasting internal cores.

   • Surface Cleaning:

     • Shot blasting (steel shots).

     • Sand blasting (sand particles).

     • Tumbling (rotary barrels).

   • Gate/Riser Removal:

     • Cutting (bandsaws, torches).

     • Grinding.

   • Finishing Operations:

     • Machining (for dimensional accuracy).

     • Heat treatment (for improved properties).

     • Painting/coating (for corrosion resistance).

2. Inspection and Testing:

   • Visual inspection (surface defects).

   • Dimensional checking.

   • Non-destructive testing:

     • X-ray radiography.

     • Ultrasonic testing.

     • Dye penetrant.

     • Magnetic particle.

   • Destructive testing (sample castings).

3. Applications by Process:

   • Sand Casting:

     • Engine blocks, cylinder heads.

     • Machine tool bases.

     • Large valves, pumps.

   • Die Casting:

     • Automotive parts (transmission cases).

     • Appliance components.

     • Power tool housings.

     • Consumer electronics.

   • Investment Casting:

     • Turbine blades (aerospace).

     • Surgical instruments.

     • Jewelry, dental appliances.

     • Firearms components.

   • Centrifugal Casting:

     • Pipes, tubes.

     • Bearing bushes.

     • Cylinder liners.

4. Advantages of Casting Processes:

   • Complex shapes possible (internal cavities).

   • Very large parts can be made.

   • Suitable for brittle materials.

   • Economic for mass production.

   • Wide range of metals and alloys.

5. Common Defects and Prevention:

   • Porosity: Proper degassing, directional solidification.

   • Shrinkage: Adequate risers, proper gating.

   • Cold shuts: Proper pouring temperature, adequate fluidity.

   • Misruns: Sufficient metal, proper mold design.

   • Sand inclusions: Proper sand properties, mold coating.