Metal Casting Design: Principles and Considerations, Study notes of Mechanical Engineering

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EMG 5114 FOUNDRY

TECHNOLOGY

MAY 2025

CHAPTER 4: DESIGN FOR CASTING

Categories of design variables

There are three main categories of design variables in a metal casting process: (a) geometric features, tolerances, etc. that should be incorporated into the part, (b) mold features that are needed to produce the desired casting and (c) process parameter issues. ❖ Geometric considerations ❖ Shape, size ❖ Wall thickness ❖ Fillets ❖ Draft angles ❖ Mold features ❖ Material ❖ Parting line ❖ Feed system ❖ Process parameters ❖ Working temperatures ❖ Rates ❖ Delivery pressures

Why design considerations

1. Proper design ensures the final cast product meets required quality and performance standards, including mechanical properties and dimensional accuracy.
2. Adhering to design considerations reduces production costs by minimizing material wastage and optimizing resource use.
3. Good design practices ensure the casting process is feasible and efficient, avoiding common defects and issues.
4. Following design considerations mitigates risks, ensuring the safety and reliability of the casting process and the final product.

Part geometric considerations

1. Corners, angles, and section thickness. Sharp corners, angles, and fillets should be avoided as much as possible, because they act as stress raisers and may cause cracking and tearing of the metal (as well as of the dies) during solidification. Fillet radii should be selected to minimize stress concentrations and to ensure proper molten-metal flow during pouring. Fillet radii usually range from 3 to 25 mm, although smaller radii may be permissible in small castings and for specific applications. On the other hand, if the fillet radii are too large, the volume of the material in those regions also is large, and consequently the cooling rate is lower. Section changes in castings should be blended smoothly into each other. The location of the largest circle that can be inscribed in a particular region (Figs. 12.2a and b) is critical so far as shrinkage cavities are concerned. Because the cooling rate in regions with larger circles is lower, these regions are called hot spots, and can cause shrinkage cavities and porosity. Cavities at hot spots can be eliminated by using small cores, and although they produce cored holes in the casting, these holes do not affect strength significantly. It is also important to try to maintain uniform cross-sections and wall thicknesses throughout the casting, in order to avoid or minimize shrinkage cavities. Although they increase the production cost, metal paddings or chills in the mold can eliminate or minimize hot spots. 2. Flat areas. Large flat areas (plane surfaces) should be avoided, since (a) they may warp during cooling because of temperature gradients or (b) cause poor surface finish because of uneven flow of the metal during pouring. One of the common techniques for avoiding these problems is to break up flat surfaces with staggered ribs and serrations, as described below. 3. Ribs. One method of producing uniform thickness parts is to eliminate large, bulky volumes in the casting. However, this can result in a loss in stiffness and, especially with flat regions, can lead to warping. One solution to these problems is to use ribs or support structure on the casting. These are usually placed on the surface that is less visible. Ribs should, in general, have a thickness around 80% of the adjoining member thickness, and should be deeper than their strut thickness. It usually is beneficial to have the ribs solidify before the members they adjoin. Ribbing should not be used on both sides of a casting, and ribs should not meet at acute angles, because of complications to molding.

Parting plane location

• The parting line - a line or a plane separating the upper (cope) and lower (drag) halves of mold.
• In general, the parting line should be along a flat plane rather than be contoured.
• Effects of parting plane location ✓ number of cores ✓ use of effective gating ✓ weight of final casting ✓ method of supporting cores ✓ final dimensional accuracy ✓ ease of moulding

Parting plane location A part should be oriented in a mold so that the large portion of the casting is relatively low and the height of the casting is minimized. Part orientation also determines the distribution of porosity. For example, in casting aluminum, hydrogen is soluble in liquid metal but is not soluble as the aluminum solidifies. Thus, hydrogen bubbles can form during the casting of aluminum, which float upward due to buoyancy and cause a higher porosity in the top regions of castings; critical surfaces should be oriented so that they face downward. A properly oriented casting then can have the parting line specified; the parting line is the line or plane separating the upper (cope) and lower (drag) halves of molds. In general, the parting line should be along a flat plane rather than be contoured. Whenever possible, the parting line should be at the corners or edges of castings, rather than on flat surfaces in the middle of the casting, so that the flash at the parting line (material squeezing out between the two halves of the mold) will not be as visible. The location of the parting line is important because it influences mold design, ease of molding, number and shape of cores required, method of support, and the gating system. The parting line should be placed as low as possible (relative to the casting) for less dense metals (such as aluminum alloys) and located at around midheight for denser metals (such as steels). However, the molten metal should not be allowed to flow vertically, especially when unconstrained by a sprue. The placement of the parting line has a large effect on the remainder of the mold design; for example, in sand casting, it is typical that the runners, gates, and sprue well are all placed in the drag on the parting line. Also, the placement of the parting line and orientation of the part determine the number of cores needed, especially when it is preferable to avoid the use of cores, whenever practical.

Parting line and part

aesthetics

• Upon demolding, parting line forms as a contour around the part surface
• For parts requiring surface aesthetics considerations, the parting line should be located along the part edges and not on the visible surface

Feed system design

• Feed/gating system is the passage that guides the molten metal from the point of pouring into the cavity
• Comprises:
  • Pouring basin
  • Sprue
  • Runner
  • Gates
  • Risers
• Requirements needed in feed systems: ✓ Complete filling of mold in shortest time ✓ Metal should flow smoothly into the mould ✓ Metal entry into the cavity should be controlled ✓ Minimize temperature drop ✓ Maximize casting yield ✓ Aspiration of air eliminated

Gate and runner design

• Runner connects sprue to the gate
• Gates connect runner to the cavity
• Multiple runners and gates preferable for large parts – minimize temperature gradient
• Gates should be designed onto thick sections of the part
• Gates should not be attached to aesthetically sensitive part surfaces

Gate and runner design

• Gates are the connections between the runners and the part to be cast.
• Some considerations in designing gating systems are:
  1. Multiple gates often are preferable, and are necessary for large parts. Multiple gates have the benefits of allowing lower pouring temperature and reducing the temperature gradients in the casting.
  2. Gates should feed into thick sections of castings
  3. A fillet should be used where a gate meets a casting; this feature produces less turbulence than abrupt junctions.
  4. The gate closest to the sprue should be placed sufficiently away from the sprue, so that the gate can be easily removed. This distance may be as small as a few mm for small castings and up to 500 mm for large ones.
  5. The minimum gate length should be three to five times the gate diameter, depending on the metal being cast. The gate cross-section should be large enough to allow the filling of the mold cavity, and should be smaller than the runner cross-section.
  6. Curved gates should be avoided; when necessary, a straight section in the gate should be located immediately adjacent to the casting.
• The runner is a horizontal distribution channel that receives molten metal from the sprue and delivers it to the gates.
• Runners are used to trap dross (a mixture of oxide and metal that forms on the surface of metals) and keep it from entering the gates and mold cavity.
• Commonly, dross traps are placed at the ends of runners, and the runner projects above the gates to ensure that the metal in the gates is tapped from below the surface.
• A single runner is used for simple parts, but two-runner systems may be necessary for more complicated castings.

Gate design

• Gates are the connections between the runners and the part to be cast.
• Some considerations in designing gating systems are:

1. Multiple gates often are preferable, and are necessary for large parts. Multiple gates have the benefits

of allowing lower pouring temperature and reducing the temperature gradients in the casting.

2. Gates should feed into thick sections of castings

3. A fillet should be used where a gate meets a casting; this feature produces less turbulence than abrupt

junctions.

4. The gate closest to the sprue should be placed sufficiently away from the sprue, so that the gate can be

easily removed. This distance may be as small as a few mm for small castings and up to 500 mm for large

ones.

5. The minimum gate length should be three to five times the gate diameter, depending on the metal being

cast. The gate cross-section should be large enough to allow the filling of the mold cavity, and should be

smaller than the runner cross-section.

6. Curved gates should be avoided; when necessary, a straight section in the gate should be located

immediately adjacent to the casting.

Establishing good practice

• It has been widely observed that a given mold design can produce acceptable parts as well as defective ones, and rarely will produce only good or only defective castings. To check for defective ones, quality control procedures are necessary. Some common concerns are: - Starting with a high-quality molten metal is essential for producing superior castings. Pouring temperature, metal chemistry, gas entrainment, and handling procedures all can affect the quality of metal being poured into a mold. - The pouring of metal should not be interrupted, because it can lead to dross entrainment and turbulence. The meniscus of the molten metal in the mold cavity should experience a continuous, uninterrupted, and upward advance. - The different cooling rates within the body of a casting cause residual stresses; thus, stress relieving may be necessary to avoid distortions of castings in critical applications.

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