Three-dimensional (3-D) printing is defined by the American Society for Testing and Materials F42 committee as the fabrication of objects through the deposition of a material using a print head, nozzle or other printer technology. This article discusses a specific type of 3-D printing called binder jetting, which is the process of a liquid bonding agent deposited through a print head nozzle to join powder material. The material being bonded is sand, which can make a casting directly from 3-D printing.
How It Works
3-D printing works much like a 2-D printer in a home or office. However, instead of paper, this process uses paper- thin layers of a special foundry grade sand and a specifically engineered binder to create foundry mold components. The process has only a few steps that are repeated until the desired shape is produced. The sand is spread and the binder is selectively dispensed, guided by computer aided design (CAD) data.
The build platform is then lowered by a set increment and a new layer of sand is spread. The binder is then applied again to this new configuration using the CAD data. The process is repeated until the part is built to completion. At that time, the part can be extracted from the surrounding sand and used in foundry applications.
26-inch diameter 3-D printed sand core impeller with five veins, the drag view shown
Pump Component Molds
The sand, hardened with the specifically engineered bonding agent, can be used to make cope and drag mold halves and cores. These are the basics of foundry components. The printed sand can also be used as a representation of the part for form of mock ups or as an engineering model. It provides a tactile representation of the part that can be used for fit and even fixture setup in the machine shop. Painted, it looks like a sand casting and weighs about 0.06 pounds per cubic inch.
This sand can be used to make a core for any of the conventional molding techniques in a foundry and is safe for use with any alloy. This core could be complex and would not need any assembly because the 3-D printing process eliminates the need for conventional tooling, such as core boxes. One advantage of having no core assembly is that the internal cored passages of the casting are cleaner with no fins at the core assembly joints and no casting defects from poor core assembly or missed mudding at those joints.
This 3-D printing process results in less post foundry cleaning of the internal passages, which produces better internal pump passages. This internal core could also be made with no draft, which may provide better pump performance or efficiency.
26-inch diameter 3-D printed sand core impeller with five veins, the cope view shown
By producing a core that does not require a core box, the design and efficiency rule and are not constrained by manufacturability of the foundry tooling. This is not an all-or-nothing technology. It must be used with existing processes or as a standalone. Pump manufacturers and end users should incorporate it when and where it makes sense.
For cores that may be too delicate to ship, a solution is available. The core can be printed in a sealed box to guarantee intact delivery. Think of it as Matryoshkas, Russian nesting dolls. A 3-D printer can make all the dolls at once, at the same time and place. End users open the build box and remove the core. It is ready to use like any other core in a foundry application.
Another benefit of this printed core is the ability to change it whenever needed. No tooling charge is assessed to make a change. If the pump housing pattern is of sufficient design and all an end user needs is to upgrade the internal passages, then print them with no tooling and no fuss. This can be a great advantage for one-offs or samples.
28-inch diameter 3-D printed sand core impeller with six veins, the drag view shown
Many people like to try something before investing heavily in permanent tooling or to prove an engineering idea. The 3-D printing process allows for sampling without being penalized by the high cost of foundry tooling and the associated long-build cycle time for that tooling.
The printed sand could also be made into foundry molds. These molds can be printed to mimic any molding process so the foundry can gather valuable information as to mold fill, solidification and yield. This helps foundries refine, prove out and optimize the process before tooling is ordered, which eliminates tooling changes at the startup of production. For speed to market, 3-D printing can help when castings are required immediately and tooling is still weeks or months away from completion. One of the mantras of additive manufacturing is to make just what end users want when they want it.
28-inch diameter 3-D printed sand core impeller with six veins, the cope view shown
For legacy products, 3-D printing has great promise. Damaged foundry tools would not have to be replaced or repaired for those jobs that only run occasionally. Lost tooling would not need to be replaced. A company can print the solution and use whatever tooling is available.