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MME345 Foundry Engineering

Lecture 20

Casting Design Considerations 2Mould Features and Geometric Features Issues

MMEMMEmaterials & metallurgical engineering

Prof. A. K. M. Bazlur RashidDepartment of MME, BUET, Dhaka

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

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 20.1 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

Figure 20.2 (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

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Core Elimination

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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

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Figure 11-23 Using staggered ribs to prevent cracking during cooling.

Ribs

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Surface Integrity

Rules for changing section size

Rule A: Sharp re-entrant angles or small fillets are not recommended (Fig. 29A and B)

Rule B: A fair design results if both sections have a common centre line provided they are joined by a 15 deg taper or by a radius of 1 inch or more (Fig. 29 C and D)

Rule C: The best design is one in which the change in section takes place entirely on one side of the thinner section (Fig. 29E and F), and in which the junction is designed according to Fig. 30.

Rules for joining cylindrical sections of different diameter

Rule A: If d = 25 mm and D = 41 mm then join with a 15 deg taper

Rule B: If d = 25 mm and D = 51 mm, then join with a fillet having 12.5 to 28 mm radius

Rule C: If d = 13 mm and D = 41 mm, then do not join

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Figure 30 Typical guidelines for section change transitions in castings.

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Design for Functionality and Reduced Weight

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MME345 Foundry Engineering

Lecture 21

Casting Design Considerations 3Gating and Feeding Design Issues