Classification of solidification processes ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Principle of the process Design For Manufacturing (DFM) Casting Principle of the process Structure Process modeling Defects Design For Manufacturing (DFM) Process variation Module 4a

Principle of the Casting Process Module 4a

Casting – Principle and process Casting is a process in which the molten metal flows by gravity or other force into a mold and solidifies in the shape of the mold cavity. Physics: 1. Fluid flow and interaction with solid 2. Thermal dynamics Any material can be cast, as any material undergoes the liquid state and solid state. Module 4a

Basic casting steps (very simplified): Melt metal Pour it into mold Let it freeze Furnace Casting system ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Casting Principle of the process Structure Process modeling Defects and prevention Design and Design For Manufacturing (DFM) Process variation Module 4a

Casting system Module 4a

Casting System – (Generic) structure Basic components and their connections Make a cavity Module 4a

Principle of the process Design and Design For Manufacturing (DFM) Casting Principle of the process Structure Process modeling Defects Design and Design For Manufacturing (DFM) Process variation Module 4a

Process modeling Heat up metal to a certain temperature Pour molten metal into mold cavity Solidify molten metal Module 4a

Heat up metal Heat metal above the melting point Pour it into the mold cavity Heat energy required for heat for raising the temperature to the melting point, heat of fusion to convert the solid to the liquid, heat for raising the molten metal to the metal at the desired temperature ready to pour it into a cavity. Module 4a

Heat up metal 3.1 Heating Period Thermal Analysis (2) heat of fusion to convert the solid to the liquid 3.1 Heating Period Thermal Analysis (3) heat for raising the molten metal to the metal at the desired temperature ready to pour it into a cavity. heat for raising the temperature to the melting point Given T0, determine the total energy H, and then set up the furnace property Module 4a

Pour molten metal into mold cavity Flow rate at the base of a sprue (energy balance: Module 4a

Pour molten metal into mold cavity Mass balance Mass in = mass out Q=v1 A1 = V2 A2 v2>v1, so A2<A1, so the sprue must be designed as tapered Assumption: Fluid is incompressible. Module 4a

Pour molten metal into mold cavity Time to fully fill the model cavity Assumption: not consider the transient process of molten metal in the cavity. MFT is shorter than the actual time needed to fill up the mold cavity. Module 4a

Process modeling Heat up metal to a certain temperature Pour molten metal into mold cavity Solidify molten metal Module 4a

Solidify molten metal Chvorinov’s rule: the time needed for the molten metal to completely solidify in the mold cavity TST: total solidification time, min V: volume of the casting, in.3 (cm3) A: surface area of the casting, in.2 (cm2) N: exponent, 2 Cm: mold constant, min.in.2 (min/cm2) Module 4a

- thermal properties of the cast metal Cm is a function of - mold material - thermal properties of the cast metal - pouring temperature relative to the melting point of the metal. Determination of Cm can be done through experiment. The principle of such an experiment is to have a scenario that is the same as the casting and a known cavity geometry, to operate the process, and to record the Time TST. Module 4a

Implication of Chvorinov’s Rule: A casting with a higher volume-to-surface area ratio will cool and solidify more slowly than one with a lower ratio. Module 4a

Principle of the process Casting Principle of the process Structure Process modeling Defects Design and DFM Process variation Module 4a

Basic Problems in Casting Processes Defects Basic Problems in Casting Processes Incomplete filling of cavity Gaps in casting Incomplete filling is caused by too fast solidification and flow blockage. Solutions: multiple pouring cups, riser, etc. Gaps in casting is caused by a phenomenon called shrinkage. Solutions: riser, pattern allowance, etc. Module 4a

Shrinkage Liquid Contraction 0.5% Further Contraction Lack of molten metal Different metal thermal expansion coefficients Shrinkage handouts 3

Riser: conducive to overcoming defects 1 and 2 Casting Cup and Sprue to pour the molten metal Riser = Reservoir Module 4a

Summary of discussions so far Principle of casting process (casting is liquid to fill up cavity) Casting can be applied to any material Three processes: heat up, pour, solidify Defects (incomplete filling, gaps) Module 4a