Sand casting is an old method of fabrication that has been updated through the use of 3D printing for making patterns. In traditional sand casting, metal components are produced by pouring molten metal into molds, where it solidifies. The mold is created by compacting sand around a pattern placed in a mold box. After the compaction, the pattern is withdrawn, leaving a cavity in the shape of the pattern, into which the molten metal is poured. Patterns are typically made from wood because it is easily obtained and machined. It also has high compressive strength along its grains that makes it well-suited for compaction molding with little or no failure during the operation.
However, there are several drawbacks to wood pattern making. The wood has to undergo a good deal of machining and tooling before it can be removed from the mold after compaction. In countries like Nigeria, CNC machines aren’t often available, so the machining and tooling process can take weeks or even months. Even though 3D printing and casting has been underway for a number of years; using FDM for casting is relatively new and we don’t often get findings and learnings from the shop floor.
In a paper entitled “Fused Deposition Modeling Printed Patterns for Sand Casting in a Nigerian Foundry: A Review,” a group of researchers discusses using FDM 3D printing for making casting patterns.
“The FDM technique is gradually being adopted for pattern making in sand casting,” the researchers state. “It is believed that printing of patterns with FDM can reduce lead-time and poor dimensional accuracy that are associated with traditional pattern making.”
3D printing has its own drawbacks, including poor surface finish, different compressive strengths of the PLA pattern at different values of process parameters, delamination of the PLA pattern while in storage and high surface friction between the sand and the pattern wall, leading to mold damage on removal of the pattern from the mold.
“This requires further investigation and solution to encourage the adoption of FDM on a large scale for sand casting,” the researchers add.
In the paper, the researchers investigate the problem of surface friction on the pattern walls.
“The sand cohesion is especially around the grooves, corners and fillets,” they point out. “These defects in the mould cavity as a result of the withdrawal of stuck PLA-pattern often lead to casting failure thereby causing a repetition of the whole process.”
One solution is to optimize layer height, which reduces the grooves or serrations on the PLA pattern walls. Those grooves and serrations cannot be completely eliminated through the 3D printing process, however, so the researchers suggest post-processing to smooth the walls and make cleaner patterns. Further research is suggested as well in order to determine the optimum infill for a 3D printed pattern, as low infill saves time and material but may not be strong enough to withstand compaction pressure.
3D printing is not recommended for all types of pattern making, but for complex patterns that have cavities or fillets, FDM 3D printing can save both time and money. The time- and cost-effectiveness also depends on whether support structures are involved, and how much post-processing is required.
In short, both traditional and FDM pattern-making have their benefits and drawbacks, but the researchers suggest that using 3D printing to replace wood pattern making can be viable in some circumstances. It’s easy to point to 3D printing as better than traditional manufacturing, but that’s not always the case – only in certain situations. A combination of the two technologies may end up being the most effective for sand casting in the future.
Authors of the paper include P.I. Anakhu, C.A. Bolu, A.A. Abioye, and J. Azeta.
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