Manufacturing Chapter 15

SQUEEZE CASTING AND SEMISOLID CASTING

- Advantages * High production * Thin-walled parts * Good surface finish * Dimensional precision * Good mechanical properties - Squeeze Casting * Large gate areas and slow metal velocities to avoid turbulence * Solidification occurs under high pressure * Intricate shapes with good mechanical properties * Reduced gas and shrinkage porosity

PERMANENT-MOLD CASTING(1)

- Also known as gravity die casting - Mold can be made from a variety of different materials * Gray cast iron, alloy cast iron, steel, bronze, or graphite - Most molds are made in segments with hinges to allow rapid and accurate closing * Molds are preheated to improve properties - Liquid metal flows through the mold cavity by gravity flow

VACUUM PERMANENT-MOLD CASTING

- Atmospheric pressure in the chamber forces the metal upward after the vacuum is drawn - Thin-walled castings can be made - Excellent surface quality - Cleaner metals than low pressure * Lower dissolved gas content - Better mechanical properties than low pressure casting

BASIC TYPES OF DIE-CASTING(2)

- Cold-chamber machines * Used for materials not suitable for hot chamber machines * Typical materials a.Aluminum, magnesium, copper, and high-aluminum zinc * Longer operating cycle than hot-chamber * High productivity

DIE MODIFICATIONS AND DIE LIFE(1)

- Die complexity can be improved through the use of * Water cooled passages * Retractable cores * Moving pins to eject castings - Die life * Limited by erosion and usage temperature * Surface cracking * Heat checking * Thermal fatigue

ADVANTAGES OF DIE CASTING

- High production rates - Good strength - Intricate shapes - Dimensional precision - Excellent surface qualities - Small-medium sized castings

BASIC TYPES OF DIE-CASTING(1)

- Hot chamber castings * Fast cycling times * No handling or transfer of molten metal * Used with zinc, tin, and lead-based alloys - Heated-manifold direct injection die casting * Molten zinc is forced though a heated manifold * Next through heated mini-nozzles directly into the die cavity * Eliminates the need for sprues, gates and runners

INTRODUCTION

- In expendable mold casting, a separate mold is produced for each casting * Low production rate for expendable mold casting - If multiple-use molds are used, productivity can increase - Most multiple-use molds are made from metal, so most molds are limited to low melting temperature metals and alloys

CENTRIFUGAL CASTING(1)

- Inertial forces due to spinning distribute the molten metal into the mold cavity - True centrifugal casting * Dry-sand, graphite or metal mold can be rotated horizontally or vertically * Exterior profile of final product is normally round a.Gun barrels, pipes, tubes * Interior of the casting is round or cylindrical * If the mold is rotated vertically, the inner surfaces will be parabolic

DISADVANTAGES OF PERMANENT MOLD CASTING

- Limited to lower melting temperature alloys - High mold costs * Mold life is strongly tied to cost * Mold life is dependent on the following a.Alloys being cast b.Mold material c.Pouring temperature d.Mold temperature e.Mold configuration * High production runs can validate high mold costs - Molds are not permeable - Limited mold complexity

DIE CASTING

- Molten metal is forced into the mold under high pressure - Held under high pressure during solidification - Castings can have fine sections and complex details - Long mold life - Typical metals cast * Zinc, copper, magnesium, aluminum, and their alloys

ADVANTAGES OF PERMANENT-MOLD CASTING

- Near- net shapes - Little finish machining - Reusable molds - Good surface finish - Consistent dimensions - Directional solidification

PERMANENT-MOLD CASTING(2)

- Process can be repeated immediately because the mold is still warm from the previous casting - Most frequently cast metals * Aluminum, magnesium, zinc, lead, copper, and their alloys * If steel or iron is to be used, a graphite mold must be used

CENTRIFUGAL CASTING(3)

- Semicentrifugal casting * Several molds may be stacked on top of one another * Share a common basin and sprue * Used for gear blanks, pulley sheaves, wheels, impellers, etc. - Centrifuging * Uses centrifugal acceleration to force metal into mold cavities that are offset from the axis of rotation

CENTRIFUGAL CASTING(2) PIC

- Specialized equipment - Expensive for large castings - Long service life - No sprues, gates, or risers

LOW PRESSURE PERMANENT-MOLD CASTING

- Tilt-pour permanent-mold casting * Mold is rotated to force flow into the cavity - Low pressure permanent-mold casting * Mold is upside down and connected to a crucible that contains the molten metal * Pressure difference induces upward flow * Metals are exceptionally clean because it is fed directly into the mold * Little or no turbulence during flow * Typical metals cast using low pressure process a.Aluminum, magnesium, and copper

DIE CASTING DIES

-Single-cavity die -Multiple-cavity die -Combination die -Unit die