Excerpt from the December 2018 Engineering Product Design article.
Green sand casting is the most versatile among the manufacturing methods that enables manufacturing of very detailed and complex parts from various metals and their alloys.
A casting design would be useless unless the design can be made in the foundry, satisfying all the functional requirements within budget. The cost effectiveness of the process depends highly on the design as this reflects the final cast. The decisions made during the design changes would make a big impact on the cost and some of the disadvantages of sand casting can be avoided by designing the part to suit casting. However, there are key sand casting design guidelines to keep costs down. Engineering product designers should understand the reasons why some of these techniques are used and consider those during the design stages of the cast.
Key design elements of sand casting:
- Draft angle
- Parting line
- Undercuts and cores
- Cross sections
- Wall thickness
- Corners and angles
- Junction design
- Casting allowances
Draft angle is the angle applied or allowed on all vertical faces of a pattern to aid easy removal from the sand mold without damaging its walls. Angle required depends on the molding process, cast design and pattern depth inside the mold.
Design engineers often overlook this aspect of sand cast design even though it is critical for a successful cast. Allowing suitable draft angle and utilizing the tapered surfaces in the design will increase castability and reduce tooling cost due to increased metal flow and ease of tooling.
If it does not critically affect the functionality of the design, a draft angle as per ISO standards will help produce cheaper and consistent sand molds. Generally, foundries as a rule of thumb use 1o to 1.5o of draft angle under normal conditions.
A parting line in sand casting is the borderline in which draft angles change direction. Although the foundry will have the knowledge and experience of placing the parting line, engineering product designers should be aware of parting line placement as it dictates the quality and the cost of the cast. Parting line should be wide, short, horizontally flat and placed as low as possible. Change in parting line placement will affect core usage, gating placement, weight of the cast and dimensional accuracy.
Undercuts & cores
Undercuts in sand casts are features that prevent and stop the pattern from being removed during the mold making stage. Usage of core sand loose pieces increase production time and cost. Parts should be designed in such a way that they reduce or eliminate core usage. Early parting line definition helps to understand the features to avoid undercuts.
Uniform cross sections, also referred to as uniform wall thickness is generally preferred but they are unfeasible in many engineering product designs. The principle requirement is to avoid leaving thicker sections of the casting isolated when cooling. The thicker section takes longer to cool while all the metal around it had already solidified. As the thicker section continues to solidify it cannot “feed” from the sections around it leading to defects such as porosity or tearing. It is worth discussing with the foundry about thickness limitation of your material before deciding.
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