Question

Draft angles, skrinkage allowance, section thickness, and sharpness of corners are amonh the issues that have to be considered when designing a part to be made by casting. Discuss each of these issues and its general significance in casting operations.

Answer 1

By draft is meant the taper provided by the pattern maker on all vertical surfaces of the pattern so that it can be removed from the sand without tearing away the sides of the sand mold and without excessive rapping by the molder.

Draft Angle is an important requirement in the die casting process. It is described as an angle or amount of slope that is incorporated into a wall of a die casting mold making the opening of the die cavity wider than its base. It is also known as the draft.

Die casting molds with surfaces perpendicular to the direction of ejection can damage the product and the mold. The metal formed will stick to the mold and an ejection by force and pressure causing an immeasurable damage. Additional cost will be incurred to produce another mold if it is reusable. Hence, a small angle must be applied on all surfaces.

During the cooling process, the castings will shrink towards the core at an acceptable rate. This is the reason why drafts are higher on the inner surfaces of the die. To avoid high pressure in castings, the inside corners must be protected with fillets.

SHRINKAGE ALLOWANCE:

All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage is of two types:

1. Liquid Shrinkage: it refers to the reduction in volume when the metal changes from liquid state to solid state at the solidus temperature. To account for this shrinkage; riser, which feed the liquid metal to the casting, are provided in the mold.

2. Solid Shrinkage: it refers to the reduction in volume caused when metal loses temperature in solid state. To account for this, shrinkage allowance is provided on the patterns.

The rate of contraction with temperature is dependent on the material. For example steel contracts to a higher degree compared to aluminum. To compensate the solid shrinkage, a shrink rule must be used in laying out the measurements for the pattern. A shrink rule for cast iron is 1/8 inch longer per foot than a standard rule. If a gear blank of 4 inch in diameter was planned to produce out of cast iron, the shrink rule in measuring it 4 inch would actually measure 4 -1/24 inch, thus compensating for the shrinkage.

WALL THICKNESS:

Generally, die castings consist of thin-wall structures that do not have any hard and fast rules for minimum and maximum wall thicknesses. It is important to design uniform walls throughout the part and where variations occur. This will ensure a smooth metal flow during filling and minimize distortion caused from cooling and shrinkage. A good mold filling will produce parts with excellent properties and few defects. The key is to design the casting so the entire mold fills before solidification begins. Failure to fill the whole mold first could lead to cold shuts (poor surface finish) in the casting. You can reduce the risk of cold shuts without any sharp or unnecessary corners, which impede the melts flow in the mold, by using radii.

Innovations in die casting technology make it possible to produce parts with minimum and maximum thicknesses that were unattainable a few years ago. Utilize this capability only when you find it necessary to enhance performance or to achieve economic benefits. Otherwise, stick with uniform wall thicknesses. You can make the metal flow better through the mold with thicker walls and ribs. When the main wall has protruding features, make sure they do not add significantly to the thickness of the wall. Excessive bulk can delay cooling.

SHARPNESS OF CORNERS:

Occasionally the die casting die may have sharply squared external corners at certain locations. At parting line locations and die block intersections, the designer must have this type of corner. On other corners, the design should incorporate radii to avoid premature die failure. It also reduces the chances of damaging the edge of the part during handling and assembly, and it enhances safety for the personnel who handle the material.

Solidification of molten metal always proceeds from the mold face, forming unbalanced crystal grains that penetrate into the mass at right angles to the plane of cooling surface. A simple section presents uniform cooling and greatest freedom from mechanical weakness. When two or more sections conjoin, mechanical weakness is induced at the junction and free cooling is interrupted, creating a “hot spot,” the most common defect in casting design.

In designing adjoining sections, replace sharp angles with radii and minimize heat and stress concentration