【3D Printing Knowledge】The Eight Steps of Metal Casting: Into the Foundry!
The 3D printing process for investment casting is increasingly becoming the choice of artisans, engineers, and industrial manufacturers. Using a 3D printer and PolyCast, users can test designs, reduce expensive manufacturing costs, and shorten lead times. The more you understand the relevant 3D printing processes, the better you can avoid early design errors and obtain more perfect finished products under the same conditions.
Below is a technical analysis of the process from 3D printing to metal parts, accompanied by video explanations:
Part 1: 3D Printing with PolyCast
PolyCast is a filament specifically designed for investment casting, which almost completely burns out within the mold cavity, leaving less than 0.003% ash. (For LulzBot 3D printers, you can also use Cura LulzBot Edition to select automatic printing parameters for PolyCast.)
．Determine the printing orientation based on the prototype requirements; fewer support structures result in a more aesthetically pleasing surface and shorter printing time.
．Use minimal infill (i.e., 10%) and wall thickness (2-3) for 3D printing to promote complete burnout of the prototype within the mold cavity.
．Add shrinkage compensation to the STL file to reduce dimensional errors between molten and solid metal. This can be done by modifying the model dimensions with a compensation factor related to the metal/alloy, usually between 1.007-1.030. (For example, for steel, the compensation factor is 1.025-1.030; if the metal part is 1 meter, the printed pattern size should be 1.025-1.030 meters.)
．A layer height of 0.1-0.2 mm is ideal, contributing to print resolution and post-processing.
． PolyCast easily absorbs moisture, so it's recommended to store the filament in a dry environment (relative humidity not exceeding 20%), such as a desiccant box.
Part 2: Polishing and Sanding
The key to investment casting is a smooth object surface. Coating and spray polishing are two recommended methods. It is advisable to first use 800-grit sandpaper to remove initial surface marks or seams, and then clean any dust from the object.
1) Dip Polishing:
．Hang the object with a thin wire.
．Dip the printed object in isopropyl alcohol for 5-10 seconds. Increase the number of dips as needed for more polishing effect.
．Allow the object to dry for 20-30 minutes, then determine if the above steps need to be repeated.
2) Spray Polishing:
．Use Polymaker's Polysher alcohol polishing machine.
．The machine automatically performs alcohol spray polishing for 20-40 minutes while maintaining dimensional accuracy.
After completing the above steps, place the object in a 40℃ dryer for about 1 hour to ensure complete solvent evaporation and surface hardening, or leave it to air dry overnight.
Part 3: Wax Tree Assembly
The wax tree structure holds the castings in place and ensures that metal flows smoothly and evenly into the parts. Wax rods or sprues form channels for metal flow.
．Minimize bends in the sprue to reduce the impact of decelerated metal flow on the part.
．Use a small torch on the final part to eliminate gate defects or holes from the wax tree.
Part 4: Ceramic Dipping
After the 3D printed prototype evaporates during burnout, the ceramic shell becomes the mold for casting metal.
．The mold is made by repeatedly dipping the wax tree into a ceramic slurry and silica, ensuring even coating and minimizing gaps in both steps.
．At least 5 layers are needed; for complex objects, 7-9 layers are recommended to prevent damage to the mold shell during casting.
．It is crucial to allow each layer to dry completely before repeating this step. Once the shell reaches the desired thickness (average about 9.525 mm), it can be burned out.
Part 5: Complete Burnout
Sinter or harden the ceramic shell and burn out the internal 3D printed material to form the mold cavity.
．Place the object into a kiln, opening downwards, and heat to approximately 1100-1200℃ for about 40-60 minutes.
(Note: Exact time and temperature depend on the type of kiln used and the metal part being produced.)
．After completely burning out the 3D printed material, allow the mold cavity to cool completely.
Part 6: Casting
The mold cavity is preheated, ready for molten metal to be poured in.
．If there is any residual ash or debris from burnout in the mold cavity, it must be cleaned.
．Place the mold cavity in a kiln to preheat for pouring metal; temperatures generally range from 550-1100°C.
．Molten metal flows through the sprue and into the mold cavity, allowing the mold and casting to cool and shrink at the same rate, forming a dimensionally accurate part.
．Allow the metal to cool and fully solidify. Different curing times will be needed depending on the material and thickness of the object.
Part 7: Cleanup
Remove the metal part from the ceramic shell and perform precision machining.
．Once the cast object has completely cooled, remove the mold from the metal part, or use other tools such as pneumatic jacks or high-pressure water jets.
．Remove the sprues by sawing, cutting, grinding, or using a plasma cutter.
Part 8: Post-processing of Metal Parts
The final appearance depends on the chosen material and the intended use of the part. For some materials, a patinated effect may be applied or retained, while some metal parts only require a layer of anti-corrosion protective coating.
．For bronze parts, a layer of potassium sulfide is applied; after baking in another oven, a layer of ferric chloride is added, and finally, a protective agent is applied to maintain the surface luster.
．For steel products, especially functional parts, only a layer of protective agent is needed.
The cost of 3D printers is more economical compared to machines costing millions, making them applicable to companies of all sizes to reduce costs and accelerate innovation. 3DMart will continue to share all the latest news and technical tutorials related to 3D printing. Follow our fan page to get first-hand updates and free events. If you have any needs, we also provide printing services and industrial-grade 3D printers.
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More metal casting finished products: See metal parts here! A collection of 3D printed castings!
Related information: https://www.lulzbot.com/learn/tutorials/3d-print-patterns-investment-casting?pk_campaign=newsletter_feb19&pk_medium=email&pk_source=phplist&pk_content=polycast