Additive manufacturing of sand cores and molds has attracted the attention of many foundries in recent years due to the process’ unique ability to form core shapes that cannot be manufactured using conventional coremaking techniques. It does this by a process called “binder jetting”: a reactive resin, most commonly, a furfuryl alcohol-based (FA) binder formulated for the application is deposited on a substrate. Typically, the substrate is a silica sand that has been pretreated with an acid catalyst, but it can be a number of other aggregates used in metalcasting, such as zircon and synthetic ceramics.
The FA-based binder is similar to the more commonly known furan no-bake binder that is cured using a sulfonic acid, usually toluene sulfonic acid in water. Once the binder contacts the catalyst it cures in much the same way it will do in a no-bake foundry application.
First, a layer of the pre-treated substrate is spread evenly across a fixed space commonly called a “build box.”. Build box sizes can range from 300x200x150 mm all the way to 4000x2000x1000 mm. The thickness of the pre-treated substrate varies, but usually it is about 0.3 mm.
Next, a print head using technology similar to common ink jet printers, “jets” the reactive binder onto the sand. But, note that it is the manner in which the binder is deposited on the layer of sand that makes the process unique. The actual part geometry is created by the print head jetting the binder only onto the layer of sand in the required shape. This process is repeated until the final 3D part geometry is created.
Advantages of this process are well-known: The part complexity of a casting can be much greater than in typical sand casting because the need for draft and parting lines is greatly reduced; more complex shapes can be created; multiple cores can be combined into one; and multiple different core geometries can be combined into the build-box volume. These are just a few of the advantages.
There are some disadvantages, too: the build box for binder-jetting sand cores and molds is comprised of both bonded and un-bonded substrate. Once the printing process has been completed, removing the core shapes from the build space can be time-consuming and detailed. The cured shapes need to be handled carefully and un-bonded sand carefully brushed or vacuumed away from the part.
While the core shapes and geometries are cured, some of the reactive furan binder can migrate away from the desired geometry and also cure, creating a “sticky” sand that can be more difficult to remove from the part, and certainly more difficult to remove than fully uncured sand.
The furan process described above is the most commonly used binder/catalyst used in the U.S. Compared to other processes known in Europe, such as Direct Croning, and direct silicate, binder-jetted furan is cured when the printing process has been completed. While strength will increase over time, the shapes can be handled. The Direct Croning process requires developing a usable “green” or uncured strength sufficient to allow part extraction, followed by post-curing to reach usable strengths for casting.
Several independent foundry binder suppliers have developed specially formulated furan binders to work in the binder-jetting process. Quite different physical and chemical properties are required to work successfully in the jetting process. Characteristics such as dynamic viscosity and surface tension are critical characteristics that must be carefully controlled.
These commercially available binders and catalysts are at the point of equivalency with those supplied by OEMs. However, care must be given to OEM warranty requirements if and when substituting any consumable used for 3D printing.
The economic viability of 3D sand printing depends in part on productivity, mostly being driven by the printing speed of the printer itself. But, being able to quickly extract cured cores and molds has an obvious effect on emptying a build box and beginning the next print.
Continuing development efforts are focusing on reducing resin migration while improving part clarity and speeding part extraction. HA International is committed to extensive research into new materials for the binder jetting process.
Doug Trinowski is the Director of Special Projects R&D for HA International. Contact him at [email protected], or visit www.ha-international.com