Make Metal Parts or Scale Production with Casting
3D printing is optimal for objects where customization is required, but multiple copies of the same design are often better produced using a conventional casting technology. In this tutorial, you’ll learn about three of the main casting processes which will both expand your material capabilities to a variety of metals and plastics, and allow you to create large quantities of the same object.
3D printing is associated with low initial investment, high cost per part, and long production times, though this dynamic is changing with new releases like the Formlabs Fuse 1 SLS printer which rapidly produces consumer grade parts at moderate costs. For applications where you’re creating multiple copies of the same part, casting processes can prove both faster and more economical.
Urethane and Silicone Casting
Urethane and Silicone casting are optimal for cases where you need to manufacture medium volumes of the same design or when your application requires material properties that aren’t accessible via 3D printing. These processes generally involve mixing two components in a specific ratio and then pouring them into a mold to harden. A number of different products exist on the market, and there are a few things you should keep in mind while deciding which is best for your application.
Urethanes and Silicones are two chemically different materials that can be used in similar processes. Silicones often exist in two classes, tin cure and platinum cure. Tin cure silicones tend to degrade more quickly but are often associated with lower costs, whereas platinum cure silicones cost more but degrade less.
You might run across the acronym RTV which stands for ‘Room Temperature Vulcanizing’, and this means that the material doesn’t need to be heated to cure. This is important to keep in mind as many Silicones and Urethanes do require heating to properly cure.
Choose Your Materials
Start off by deciding which material properties your final design requires. Urethanes and Silicones both come in a variety of mechanical properties from rigid to elastomeric. Once you’ve decided on which is best for your project, you’ll want to choose a compatible mold material.
A general rule of thumb is to use elastomeric mold materials when casting rigid objects and rigid mold materials when casting elastomeric or flexible objects. You’ll also want to pay close attention to whether your material is platinum cure or tin cure as they’re incompatible, and both of your materials must be one or the other.
Producing the Mold and Casting
Mold design is a complex discipline that often requires dedicated engineers to execute correctly. Stay tuned for upcoming tutorials on designing your own simple molds. Make sure to include drain holes in your design so that when you pour your casting material, air has the ability to flow out allowing the Silicone to fill the cavity.
When designing your mold, visualize how the casting compound will flow, and avoid cavities that won’t fill or features that will inhibit you from pulling apart your mold.
Silicones and Urethanes come in a variety of different mechanical properties and also vary in terms of curing requirements. Pay attention to the ‘pot life’ of your material which refers to the workable time or how long you have until it begins to thicken and becomes challenging to work with. Note, pot life is not the same as overall drying time, and most materials require longer than their pot life to fully cure.
Also pay attention to the curing temperature of your materials. RTV silicones cure at room temperature, but many materials require higher temperatures to catalyze the curing reaction.
Directly 3D printing metal parts requires expensive machinery and dedicated operators, but low-cost solutions exist for those looking to experiment with metal part production. Pewter is a metal alloy predominantly comprised of tin that has a low melting point compared to other metals. With new material releases like High Temperature Resin from Formlabs, pewter can be poured directly into 3D printed molds.
Two different techniques can be used for designing molds to be used with pewter, shell molds and pull-apart molds. In a pull-apart mold setup, the two halves of the mold are compressed and then pewter is poured. Once the pewter cools and hardens, the two halves of the mold are separated to release the finished object. This technique allows for re-use of the mold, but is restricted to simple geometry that’s devoid of drafts and undercuts.
Shell molds offset material from your design such that the hollow cavity on the inside matches the shape of your original model. Pewter is poured into the mold and once hardened, the mold is broken away from the pewter. These are sometimes called sacrificial molds as they only survive a single use. Shell molds tend to be the best option for any geometry that’s too complex for a pull-apart mold.
Both techniques yield a solid pewter object at a quality that closely resembles that of the original printed object. For a more in-depth tutorial on the process of designing your own molds, check out Formlabs’ original tutorial on pewter casting.
Investment casting is a process by which a master object is surrounded in a ceramic like material called investment, and then burned out leaving a hollow cavity behind. Investment casting is the technique of choice for jewelers and dentists looking to create high resolution objects in a variety of precious metals.
The process starts by printing your object in a wax-like material. SLA printers are best for resolving the high resolutions typically required for jewelry and dentistry, and Formlabs Castable Resin is well-optimized for this process. Once the object is printed, a wax sprue is attached that will provide a path for molten metal to flow later in the process.
The printed object is surrounded in investment via one of two process. For most small objects, the object is placed in a metal flask and then the flask is filled with investment. For larger objects, a shell casting process is used, whereas the object is coated in a thin shell of investment. Once the investment has cured it’s put into a oven so that the printed object can fully burn out.
Castable Resins are designed to vaporize instead of burning as to minimize the residue left behind in the investment. When done correctly, the final result is the investment with a hollow cavity left behind where the printed object once was.
After the printed object has been fully burned out, the mold is ready for casting. Because the materials used in investments have very high temperature resistance, a wide variety of metals can be used including copper, silver, and gold. The metal solidifies quickly and after it’s fully cooled, the investment is broken away revealing the final object. From there, a variety of polishing and finishing techniques are used to prepare the object for customers.
In an investment casting process, it’s important to pay close attention to the type of investment you’re using. You’ll want to make sure that the investment is compatible with both the 3D printing material and can withstand the molten material you’re looking to pour.
These three techniques provide methods for manufacturing copies of the same part at high volumes, and expand your material options to a variety of silicones, urethanes, and metals. Casting is a cost-effective solution for producing high part volumes, but 3D printing is continuing to catch up to the speed of casting processes with new autonomous printing setups like the Form Cell.