Casting – Additive Manufacturing Style

2020-09-23

Lee-Bath Nelson  

detail of sand mold for casting 3d printed on exone

About a year ago, Aya Bentur gave an interesting overview of sand casting and the use of Additive Manufacturing (AM) to manufacture sand molds. In the world of AM a year is a long time – time to get updated on the latest. Certainly using additive manufacturing for sand molds has advanced – one foundry told the National Association of Manufacturers that in 6 years it’s been using 3D printers the percentage of their sales that involve 3D printing went from 2% to 40-50%. In addition, new ways to help casting with AM have come to the fore. Here is our update.

A to Sand to Zinc

As in any additive manufacturing technology, sand casting has advanced in the past year. Beyond new printer announcements, sand 3D printer OEMs have turned their focus to the business side as well. To make sure 3D printed sand mold are available to all manufacturers, ExOne has announced a North American network of sand 3D printers that can be used to order a sand mold close to you (ExOne printed sand molds with different binders below and mold detail above). The network includes over 40 sand 3D printers and supports castings of multiple metal materials from Aluminum to Zinc. In their press release, ExOne note that creating an on demand network (together with a digital inventory of mold designs) can help overcome supply chain and other failures that have been plaguing the industry during the COVID-19 pandemic. This is important for the foundry business but also for their customers – the manufacturers. The network helps foundries and manufacturers find a suitable sand 3D printer in your area so that even if you don’t own a sand 3D printer (as Ford does, for example) you can enjoy its benefits.

exone 3d printed sand Materials Binders

In addition, like many 3D printer OEMs, ExOne is highlighting software capabilities. In this case, it is simulation capability that can show how a mold would behave during the pour without 3D printing it.  Simulation is important to ascertain good casting and also a steady not too rapid pour speed. The pour speed is especially important for a high quality cast part. If the molten material enters the mold too quickly or unevenly it creates metal turbulence which in turn can cause mold erosion, essentially changing the molded part – clearly a bad thing.

Insert Core

Cores and inserts are used in casting to create a cavity (e.g., the internal part of a funnel) and they too can be 3D printed. Since these internal formations may be very complex, sometimes they are the only parts that are 3D printed. Ceramics 3D printing is often used for the cores and inserts, more on this below. Designers and engineers often go to great lengths to avoid needing a core as it is a complication: it requires another part, costs rise, and the operator must place the core correctly for a successful cast – room for human error. Some foundries have developed their own proprietary processes to reduce human error and improve the resulting part quality. In fact, some – like Kimura Foundry in Japan  – have developed proprietary sand which they believe improves part quality and simplifies the casting process (cranes cast with such molds by Kimura in pic below).

kimura cranes cast from 3d printed sand molds

Invest and Cast

Investment casting is another way to cast metal parts. In this method a meltable mock up of the desired part or item is created, it is buried in sand (or other powders) and then as molten metal is poured the item melts away and is replaced by the metal. Investment casting has been popular for ages and is widely used in jewelry making, for example, but also for industrial parts in various metals such as aluminum. In a recent “Cool Parts Show” (a show I think is indeed cool!) Stephanie Hendrixson and Peter Zelinski show parts of an FUV (Fun Utility Vehicle) by Arcimoto that were investment cast. The part is topologically optimized and 3D printed in plastic and then investment cast. The disposable parts (called lost parts) have traditionally been made in wax (because of its low melting point) but lost parts 3D printed in PLA work as well. (Below is a topologically optimized Arcimoto part.)

topologically optimized arcimoto part

Ceramics or Sand?

More recently 3D printed molds from other materials have started to compete with sand molds for casting. Specifically, ceramics molds made of calcium sulfate. In a recent paper, a group from the University of Leon compared such molds’ performance with traditional (not 3D printed) sand molds when it comes to casting aluminum alloys. The researchers focused on the main issues in casting: reducing porosity, improving the microstructure, controlling the dimensional quality and limiting defects. Many of these issues can be addressed with advanced design of the mold – something 3D printing can enable in general (ceramic or sand). The process the researchers chose is illustrated below – it is important to note the simulation step which enabled optimizing the rate of pouring, as much as the technique allowed. Also note that the pattern used to create the traditional sand mold was a 3D printed item (in FDM on an Ultimaker, but smoothed chemically with dichloromethane vapor after printing to remove the visible layers).

comparing traditional sand casting and 3d printed ceramics mold

Because additive manufacturing gives more design freedom, the speed at the gate (through which the molten metal is poured into the mold part) was better optimized in the ceramics mold. This one difference is the root of most of the improvements that the researchers saw in the ceramics mold when compared with the sand one. 3D printed ceramic molds were equal or better than traditional sand molds in all metrics checked except porosity. The reason for the increased porosity is the binder in the mold. Although the mold went through a 250 degree heating to get rid of most of the binder, some remained and caused surface porosity in the cast part. In addition to all these physical considerations, there is a time element as well: the AM method took 67% less time than the traditional sand casting method!

I’m Melting…

Finally, as someone who loves loves loves lava, I couldn’t end without a video of molten metal – it is the closest thing to lava that is man made. Below is a video of molten metal being poured into a sand mold – enjoy!

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