Excerpt from the January 2019 Additive Manufacturing article by Santosh Reddy Sama, Tony Badamo, Paul Lynch, and Guha Manogharan.
The market size of the metal casting industry was 20.23 billion USD in 2017 and is expected to grow annually at a rate of 8.87% to reach 39.94 billion USD by 2025. Engineered castings constitute about 90% of total manufactured goods and capital equipment. In United States, over 2000 metal casting facilities employ more than 200,000 people across the country. Despite having a casting market share of 80%, sand casting foundries across the globe suffer from long lead times, expensive tooling and limited flexibility. It is well known that the pattern making step in sand casting is the bottleneck and often, the most expensive component in low volume production runs. The need to remove the pattern from compacted sand mold to create casting cavity significantly restricts the geometries in traditional sand castings. The increased complexity of metal parts for industrial and mission critical applications demands new technologies. Recent advancements in 3D Sand-Printing (3DSP) which is a form of Additive Manufacturing (AM) bridges this technological gap through direct printing of sand molds in a layer-by-layer process.
Mold filling is a critical phenomenon that directly impacts the quality of castings. The filling process is typically comprised of free surface flow of the metal front inside the mold cavity. The exposure of liquid metals to air and moisture during free surface flows results in the oxidation of melt surface leading to the formation of surface oxide films. Folding of free dry oxide surfaces result in detrimental bifilms or double oxide films that act as cracks to initiate failures. The rate of bifilm formation increases with increase in turbulence as the oxide layers continuously stretch, rupture and regrow. Entrained defects viz. surface oxides, oxide inclusions, sand inclusions, blowholes, bubbles, bifilms among others are significantly detrimental to the mechanical properties of castings. Oxide films are found to adversely affect tensile strength, fatigue life, fracture strength, machinability and also act as initiation sites for shrinkage pore formation and hydrogen gas pore formation in castings. Therefore, it is of most importance to reduce defects due to entrainment since they cause 80% of the total effective problems in castings. Therefore, in order to make homogeneous castings with minimum scrap rate, it is critical to incorporate proper gating systems that facilitate effective control over mold filling by reducing velocity of liquid metal at ingate to less than 0.5 m/s, which will improve product quality and foundry productivity.
Since the sprue controls the fill rate of the casting, sprue is regarded as the "single most important part" and "most critical component" of the gating system. During the filling process, liquid fall will act as plunging jet if the sprue is not filled completely leading to gas entrainment. Therefore, the liquid metal is usually choked at the bottom of the sprue and the filling rate of the mold cavity is controlled by changing the cross sectional area at the sprue exit. However, Campbell notes that an ideal design should use the entire length of the sprue to control the flow rate. Campbell further mentions, "Although methoding engineers have been carrying out such calculations correctly for many years, somehow only the sprue exit has been considered to act as the choke." The authors in this research propose that if carefully designed, it is feasible to control the rate of the metal flow all through the length of the sprue via 3DSP. Therefore, in order to reduce casting defects, gating systems must be redesigned for 3DSP, which does not suffer from severe hard tooling requirements of conventional mold-making process.
In this research, numerical models for two complex sprue profiles are developed and their effects on casting performance are experimentally validated. Novel methods to fabricate complex conical-helix sprue channels (CHSC) via "hybrid molding" approaches are presented.
Section 2 presents a brief literature review of reported studies on optimizing gating systems.
Section 3 details the numerical models and optimization algorithm for the novel complex sprue concepts.
Section 4 details the design of experiments including steps to fabricate 3DSP molds and castings.
Section 5 presents both computational and experimental results and
Section 6 analyses the relevance of the results.
Finally, Section 7 summarizes this study along with brief discussion on future directions of this research.
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