Resin Shell Molding
Resin shell molding is a precision sand casting method that produces castings with excellent surface smoothness and higher precision compared to traditional sand molding. These precision qualities are applicable to a wide range of alloy metals, offering greater design flexibility than die casting and lower costs than investment casting.
Overview of Resin Shell Molding
The key feature of resin shell molding is the use of fine-grained, high-purity sand to provide smooth surfaces and dimensional accuracy for cores and similar castings. Traditional sand molds cannot use such fine sand as it significantly reduces mold permeability, leading to short-run castings or castings with gas defects due to delayed air and mold gas escape. However, the significant feature of resin shell molding is that , a shell with a thickness of only about 10 millimeters (0.4 inches). The ability to produce such thin external shell molds has been a revolutionary development in metal casting. The concurrent development of plastics, such as thermosetting resins (e.g., phenolic resins), provided the foundation for shell formation. In shell molding, fine sand is coated with a thermosetting resin that provides the relatively high strength required for the production of thin-section or shell molds.
To produce precision castings, the mold should accurately replicate the pattern details and size requirements, which are achieved through shell formation. This is because the resin bonding is formed when the mold contacts the heated pattern. Additionally, the mold separates from the pattern without the need for mold expansion as in wet sand molding. These features also apply to the production of cores. By using zircon sand instead of silica sand, casting precision can be further enhanced. This occurs because the expansion of zircon sand due to the heat generated by casting metal is lower and more predictable than that of silica sand.
Resin Shell Molding Materials
Shell molding is used for both ferrous and non-ferrous metals. The most commonly used materials include cast iron, carbon steel, alloy steel, stainless steel, aluminum alloys, copper alloys, and more. Typical part sizes produced using this process are medium to small, requiring high precision, such as gearbox housings, cylinder heads, connecting rods, and lever arms.
Resin Shell Molding Process
First, metal molds are prepared based on the shape and size of the part. These molds have the opposite shape of the final part and can accommodate the expanding material.
Shell Mold Preparation
A special silicate or siloxane-type resin is applied to the surface of the metal mold to form the shell mold. This resin is a high-temperature, melt-resistant material capable of withstanding the high temperatures and pressures during metal melting.
Shell Mold Baking
The prepared shell mold is baked to ensure it fully cures and to remove any remaining moisture. The baking temperature and time may vary depending on the resin material used.
The de-waxed shell mold is placed in casting equipment and heated again to ensure the shell mold reaches the appropriate temperature range. Then, molten metal is poured into the shell mold, filling the entire cavity. After the metal cools, the desired parts are formed.
Shell Mold Breakage
After the metal has cooled and solidified, the shell mold becomes fragile. Special mechanical equipment or vibrating devices are used to break the shell mold to separate the cast metal components, which need further cleaning and processing. This may involve removing residual shell mold material, grinding or trimming surfaces, performing heat treatment, or other post-processing operations to achieve the final part shape and performance.