Casting Design Rules c203r

CASTING DESIGN RULES

Do we need stringent tolerances? • Requirements for close tolerances significantly affect the cost and delivery time of castings. • Close tolerances could result in the need for special casting processes. • Tolerance must always be greater on dimensions across the parting line …... In case of sand mould, cope and drag parting surfaces will not close together exactly and precisely at all points from mould to mould.

• In castings, heat transfer and solidification of the casting are two closely related areas • Look for those areas in the mould where heat transfer is delayed due to restricted surface area. • These are hot spots in the mould, can cause a delay in the solidification of the casting in those areas and thus be a source of casting defects. • The hot spots may produce defects visible on the casting or can set up internal stresses that could later cause the casting to fail.

Consult a foundryman or patternmaker 1. 2. 3. 4. 5. 6.

General design Type of pattern needed Metal shrinkage How the casting is to be moulded Location of parting line Design of a gating system to ensure proper filling of the mould cavity

7. Placement of risers to provide molten metal as the casting solidifies 8. Number of castings to be produced in each mould 9. Feasibility of making on a moulding machine 10. Accessibility of parts for cleaning 11. Location of machined surfaces 12. Dimensional tolerances which can be held

Rule 1: Avoid sharp angles and abrupt section changes: • Solidification of metal always proceeds from the mould face • crystal grains penetrate into the liquid mass at right angles to the plane of cooling surface

A simple section presents uniform cooling and a casting free from mechanical weakness.

• When two or more sections con, free cooling is interrupted at the junction, creating a “Hot spot”.

Metal structure is also influenced by the solidification range of the alloy poured.

Rounded corners avoid local structural weakness.

• Avoid abrupt section changes; eliminate sharp corners at ading sections.

• The difference in the relative thickness of ading sections should not exceed 2:1. • Where a greater difference is unavoidable, consider design with detachable parts. For example, the Guideways of machine tool beds can be bolted, etc. • When a change of thickness is less than 2:1, it may take the form of a fillet • where the difference is greater, the form recommended is that of a wedge, with a taper not exceeding 1:4.

Replace sharp angles and corners with radii.

Local structural weakness

Local shrink weakness

Too large a fillet not desirable

Replacing all sharp angles with radii, minimizes heat and stress concentration.

Avoid acute angles in deg ading sections

Avoid core design which does not present a cooling surface.

Rule 2: Fillet all sharp angles Fillets have three functional purposes: • To reduce stress concentration in the casting in service. • To eliminate cracks, tears and draws at reentry angles. • To make corners more mouldable and to eliminate hot-spots.

Too large fillet radius, exceeding section thickness, causes “Hot Spot” at junctions and tends to cause structural weakness.

Other examples where fillets can help solve casting problems are those of V and Y sections.

Rule 3: Bring the minimum number of sections together • A well designed casting brings the minimum number of sections together avoiding acute angles. • Sections are no thicker than necessary to achieve the desired strength • Sections are evenly proportioned to avoid local slow cooling.

Adding too large fillets aggravates defect

Avoid concentration of metal by staggering cross- ribs

Cored hole helps speed up solidification where a number of sections con

Circular web with ading sections is preferred

Rule 4: Design all sections as uniform in thickness as possible. • Failing this, all heavy sections should be accessible for feeding by riser.

Deg with uniform section thickness, saves weight, material, machining costs and results in a stronger casting.

Hydraulic coupling

Correct Design

Rule 5:Avoiding shrinkage cavities

Use metal padding/chills to increase the cooling rate in thick regions and develop steeper temperature gradient in a casting to avoid shrinkage cavities.

Rule 6: Dimensions of Inner walls should be correctly proportioned • Inner sections of castings, due to complex cores, cool much slower than outer sections and cause variations in strength properties. Rule: • Reduce inner sections to 9/10th of the thickness of the outer wall. • Avoid rapid section changes and sharp angles. • Wherever complex cores must be used, design for uniformity of section to avoid local heavy masses of metal.

When the diameter of the core is less than the section thickness of the metal surrounding it, the core becomes overheated slowing down the solidification rate and can cause “Hot Spot”.

Rule 7: Do not use bosses, lugs and pads unless absolutely necessary

• Bosses, pads and lugs increase metal thickness creating hot spots. • When used, they should be blended by tapering or flattening the fillets.

When several bosses or pads are on one surface, they should be ed if possible, to facilitate machining.

• This aids in removing possible hot spots and also allows for moving hole locations.

Rule 8: Design ribs and brackets for maximum effectiveness • 1. 2.

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Ribs have two basic functions: Improve stiffness, and Reduce weight Ribs and brackets when incorrectly used can create moulding problems, shrink defects and localized hot spots which can create localized structural weakness. Ribs in compression, in general, offer a greater factor of safety than ribs in tension

• Thickness of ribs should approximate 0.8 of the casting thickness. • Avoid complex ribbing when possible to simplify moulding procedure and assure more uniform solidification conditions

• Avoid concentration of heat by providing cored openings in webs and ribs.

• Use oval shaped cored holes with the longest dimension in the direction of the stresses.

When deg gears, flywheels and spoked wheels: • Use odd number of curved spokes.

Curved spokes dissipate additional stress.

• Provide for all cross sections to cool as evenly as possible by avoiding excessive sectional variation. • Blend sections of varying sizes carefully. • A balance between the section size of the rim, spokes, and hub must be attempted in order to minimize stress and avoid chances of cracking.