Casting process

Casting process

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

Classification of solidification processes

Polymer modified cement

Module 4a 5

Casting Principle of the process

Configuration / Structure

Process modeling

Defects

Design For Manufacturing (DFM)

Process variation

Module 4a

Principle of the Casting Process

Module 4a 7

Casting – Principle of the 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.

Any material can be cast, as any material

undergoes the liquid state and solid state.

Physics: 1. Fluid flow and interaction with solid

2. Thermal dynamics

Basic casting steps (very simplified):

1. Melt metal

2. Pour it into mold

3. Let it freeze

Furnace

Casting system

Module 4a 9

Casting Principle of the process

Configuration / Structure

Process modeling

Defects

Design For Manufacturing (DFM)

Process variation

Module 4a

Casting system

Module 4a 11

Basic components and their connections

Make a cavity

Casting System – (Generic) structure

Module 4a

Animation

http://nptel.ac.in/courses/112107144/metalcasting/lecture5.htm#

Module 4a 13

Casting Principle of the process

Configuration / Structure

Process modeling

Defects

Design For Manufacturing (DFM)

Process variation

Module 4a

Process modeling

1. Heat up metal to a certain temperature

2. Pour molten metal into mold cavity

3. Solidify molten metal

Module 4a 15

-Heat metal above the melting point

-Pour it into the mold cavity

Heat energy required for

(1)heat for raising the temperature to the melting point,

(2)heat of fusion to convert the solid to the liquid,

(3)heat for raising the molten metal to the metal at the

desired temperature ready to pour it into a cavity.

Heat up metal

Module 4a 16

Heat up metal

3.1 Heating Period Thermal Analysis

)}()({ 0 mplfms TTCHTTCVH

(1) heat for raising the temperature to the melting point

(2) heat of fusion to convert the solid to the liquid

(3) heat for raising the molten metal to the metal at the desired temperature ready to pour it into a cavity.

Given T0, determine the total energy H, and then set up the furnace property

Module 4a

Pour molten metal into mold cavity

22

12

222

2

211

1 Fg

vPhF

g

vPh

Flow rate at the base of a sprue (energy balance:

ghv 2

Module 4a 18

Mass in = mass out

Q=v1 A1 = V2 A2

Assumption: Fluid is incompressible.

Pour molten metal into mold cavity

Mass balance

v2>v1, so A2<A1, so the sprue must be designed as tapered

Module 4a 19

Time to fully fill the model cavity

Q

VMFT

Assumption: not consider the transient process of molten metal in the cavity.

Pour molten metal into mold cavity

MFT is shorter than the actual time needed to fill up the mold cavity.

Module 4a

Process modeling

1. Heat up metal to a certain temperature

2. Pour molten metal into mold cavity

3. Solidify molten metal

Module 4a 21

Chvorinov’s rule: the time needed for the molten metal to completely solidify in the mold cavity

nm A

VCTST )(

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)

Solidify molten metal

Module 4a 22

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 23

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 24

Casting Principle of the process

Configuration / Structure

Process modeling

Defects/quality control

Design For Manufacturing (DFM)

Process variation

Module 4a25

Defect 1: incomplete filling of cavity

Defect 2: gaps in casting

Defects

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 26

Shrinkage:

The source of problems in any process involving the heat transfer

27

Liquid Contraction0.5%

Further Contraction

Lack of molten metal

Different metal thermal expansion coefficients

Shrinkage gaps and voids

Module 4a 28

Riser: overcoming the two defects

CastingRiser

Cup and Sprue to pour the molten metal

Riser = Reservoir

Module 4a 29

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, gap)