Henry W. Stoll

Tool Design and Construction for Sand Casting

Typically, the sand-casting tool design and fabrication process begins when the tool builder receives the part design from the client or design engineer. In general, the part design can be communicated in variety of ways such as by a physical part, a 2D engineering drawing, a 3D computer generated solid model, or other means that sufficiently conveys the design intent needed for tooling design and fabrication.

Rapid Tooling Processes

Sand casting is one of several near net shape manufacturing processes. In near net shape manufacturing, multiple copies of the product are produced by imprinting the shape of the tool or die on a suitable working material. Typically, the working material starts out as a liquid, powder, or pliable material that is eventually solidified or hardened after being formed by the tool. Net shape manufacturing processes include other casting processes such as die casting and investment casting, polymer molding processes such as injection molding, blow molding, and thermoforming, bulk deformation processes such as extrusion and forging, and sheet metal forming processes such as bending and deep drawing. Design and construction of tooling for near net shape manufacturing processes has traditionally been time consuming and costly. With the advent of modern computer-aided machining and fast freeform fabrication (FFF) technologies, many new and innovative approaches to making near net shape tooling have been developed. This has led to the rapidly expanding new field known as “rapid tooling.”

Sand Casting Processes

Sand casting is a mold based net shape manufacturing process in which metal parts are molded by pouring molten metal into a cavity. The mold cavity is created by withdrawing a pattern from sand that has been packed around it. Since the pattern imprint forms the cavity, the pattern creates the external shape of the cast part. If the part has undercuts or hollow internal regions, these can be formed by sand cores that are fabricated separately and then placed in the mold cavity. The cores are supported by core prints, and/or chaplets that allow the molten metal to flow between the core and the mold wall. In addition, cores may be necessary to produce a desired “zero” draft external surface, depending on the parting line selected. The parting line is formed by the interface between the cope (upper portion) and drag (lower portion) of the mold. The separate cope and drag are necessary to allow the pattern to be withdrawn from the sand and to allow the cores to be properly positioned within the mold.

Fast Freeform Fabrication Methods and Processes

Fast freeform fabrication, also called rapid prototyping, is the automatic manufacturing techniques using slicing and additive processes. The first techniques for fast freeform fabrication became available in the late 1980s and were used to produce models and prototype parts. With the fast freeform fabrication method, the machine reads in data from a CAD drawing, slicing the CAD model into a thin, virtual, horizontal cross-sections and lays down successive layers of liquid, powder, or sheet material, and in this way builds up the model from a series of cross sections. These layers, which correspond to the virtual cross section from the CAD model, are joined together or fused automatically to create the final shape. Figure 3.1 shows the principles of this slicing and additive manufacturing technology. The primary advantage to this slicing and additive fabrication is its ability to create almost any shape or geometric feature.

Evaluating Tooling Alternatives

Evaluation of tooling alternatives is the comprehensive mental exercise involved in selecting the most efficient way to produce a sand casting pattern. Selection of the proper tooling process can significantly effect the time, cost and accuracy of castings involved. Selection of an appropriate tooling alternative is typically performed by the tool builder. The process requires a sound understanding of the interactions between casting geometry and pattern design, required tooling properties, production volume, available shop capacity, available processes and process capability, and technical capabilities of the foundry involved.

Sand Casting Dimensional Control

Dimensional accuracy and variability are critical factors in the casting process that must be considered at each stage of the process. Dimensional accuracy is an indication of how close a casting dimension is to design intent (the actual target value). Dimensional accuracy is often referred as a system error. The main causes for poor dimensional accuracy in sand casting are pattern equipment errors, pattern wear, and casting contraction uncertainty. When properly understood and controlled, system error can often be corrected before production runs. Dimensional variability is the variation of individual casting dimensions about their mean. Dimensional variability is often referred to as random error. Many foundry process variables such as placement of cores and sand consistency contribute to dimensional variability.