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Next ClassMME 345, Lecture 45

Casting Design Considerations

Successful casting practice requires proper control of a large number of variables

characteristics of the metals (or alloys) casts

method of casting

mould/die materials

mould/die design, and

various process parameters

Flow of the molten metal in the mould cavities, the gating systems, the rate of

cooling, and the gases evolved would influence the quality of a casting

All casting operations share the characteristics of phase change and

thermal shrinkage during the casting cycle

But each process will have its own design considerations

Introduction

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The general design considerations in casting include:

1. Design the part so that the shape is cast easily.

2. Select a casting process and material suitable for the part, size, mechanical properties, etc.

3. Locate the parting line of the mould in the part.

4. Locate and design the gates to allow uniform feeding of the mould cavity with molten metal.

5. Select appropriate feeder geometry for the system.

6. Locate mould features, such as sprue and feeders, as appropriate.

7. Make sure proper controls and good practices are in place.

Design Considerations in Casting

Two types of design issues in casting:

1. Geometric features and tolerances incorporated into the part

2. Mould features that are needed to produce the desired casting

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Design Issue: Mould Features

1. Parting line

2. Directional solidification

3. Pattern withdrawal (Draft/taper allowances)

4. Dimensional tolerance (machining, shrinkage allowances)

5. Surface finish

6. Core design (core elimination)

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Figure 46.1 Redesign of a casting by making the parting line

straight to avoid defects.

A part should be oriented in a mould

so that the large portion of the

casting is relatively low and the

height of the casting is minimized.

In general, the parting line should be

along a flat plane rather than be

contoured.

The parting line should be placed as

low as possible relative to the casting

for less dense metal (such as

aluminum alloys) and located at

around mid-height for denser metals

(such as steels).

Parting Line

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Figure 46.2 (Top left) Design where the location of the parting plane is specified by the draft.

(Top right) Part with draft unspecified. (Bottom) Various options to produce the top-right part,

including a no-draft design.

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Internal Soundness – Directional Solidification

Figure 46.3 Formation of shrinkage cavities

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Figure 46.4 Schematic examples of design sections showing

correct arrangements to improve casting soundness

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Figure 46.5 Necessity for adequate draft

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Figure 46.6 Cross section of pressure vessel design

showing added padding to feed an insulated heavy section

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Figure 46.7 Examples of padding for enhancement of feeding characteristics

in roll and gear wheel castings

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Core Elimination

Figure 46.8 Eliminating core

Figure 46.9 Simplification of a base plate design to eliminate a core

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Figure 46.10 Design illustrating reduction in number of cores

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Design Issue: Geometric Features

1. Jointed sections (eliminating hot spots)

2. Surface integrity

3. Design for functionality and reduced weight

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Joined Sections

Figure 46.11 Basic design rules for X, Y and T-junctions15/24

Figure 46.12 Basic design rules for T-junctions

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Figure 46.13 Y- and T-junction arrangements

Figure 46.14 Cored opening to improve X-junction

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Figure 46.15 Grid design with and without X-junction

Bad design

Good design

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Figure 46.16 Using staggered ribs to prevent cracking during cooling.

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Ribs

Figure 46.17 Rib to increase rigidity and decrease weight.

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Figure 46.19 Changing section thickness

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Figure 46.20 Typical guidelines for section change transitions in castings.

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End of Syllabus

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