Sand casting, also known as sand molded casting, is a metal casting method whereby sand is used as the mold material. Sand is a cheap material and in combination with clay (as the bonding agent) is it often used for mold making for metal casting. The proces exists in traditional manufacturing for a long time, but the making of the mold can now be done using 3D printing machines, which has economic and design benefits.
Using sand as a mold material for metal casting is a very old process. The process goes back all the way to ancient China. In the movie Andrei Rublev by Russian filmmaker Andrei Tarkovsy, the process of casting church bells during the Middle Ages is depicted brilliantly in the last sequence of the film.
Difference between mold, cast and core
A mold is the impression of an object in a material (in the case of sandcasting the material is sand), from this mold the cast is made, by hardening the mold to withstand heat and presssure.
The core is used in casting and moulding processes to produce internal cavities.
Benefits of 3D printing mold for casting
An important advantage in using 3D printing machines for making sand molds is that one can create complex geometries not possible with conventional casting.
Also there are economic benefits. Additive Manufacturing shortens the production process for molds and cores since only a CAD file is needed to start production. They can be built in days, instead of weeks. Also it is easier to make changes to the design.
In the research paper “Economies of Complexity of 3D Printed Sand Molds for Casting” it is described when it makes sense to use additive manufacturing methods over traditional casting or when to use a hybrid method: a combination of conventional and additive manufacturing processes
Why make molds for metal casting when you can 3D print metal directly?
In the foundry industry the making of the molds take up the highest part in the total cost structure. So if with additive manufacturing one can print models direct in metal without making molds, one can ask: why would you make molds any longer?
The answer is that using molds allows for large series production, which in many cases is still cheaper and faster using traditional methods, even though AM as a whole is moving into the direction of large series production.
The combination of making molds with 3D printers and then using those molds to do traditional metal casting can be called a hybrid method. There is nothing wrong with that: additive manufacturing does not have to replace all methods of production. Sometimes one method is more efficient then another, sometimes a combination makes the most sense, we could call this hybrid manufacturing.
Sand casting: 3D Printing molds from sand
The most common materials used for making sand molds are Silica (SiO2) sand, a sand found on the beach and also the most commonly used sand, and Zircon sand.
Additive machine manufacturer Voxeljet offers 3D printing systems for creating sand molds and cores for metal casting using binder jetting, whereby silica sand is built up layer by layer until the desired part has been constructed.
The layers of sand are bonded using a binder for strength.
Voxeljet uses proprietary Furan Direct Binding (FDB) silica sand in their process.
With the VX4000 printer large formats are possible: molds of up to 4 metres long and 2 metres wide and one metre high can be constructed. Or alternatively small series of molds in one print session.
Steps of the casting process
1. 3D Printing molds and cores in a layering process.
2. Blacking: the printed core is coated with a black wash in the foundry so that it can withstand the high heat.
3. Mounting the core: The 3D-printed sand core is subsequently inserted into a conventionally produced mold.
4. Casting: All common alloys can be cast with the 3D-printed molds and/or cores. Different sand granulations can be used to influence the surface quality.
5. Removing the core: Like with conventional production methods, 3D printing of sand molds and cores is a lost-mold casting method.
6. Post-processing: The component is post-processed after it is removed from the mold.
Sand casting is an example how a very old process is adopted and innovated by additive manufacturing.