This video by Dan Gelbart details a novel, high-resolution 3D printing method for metals and ceramics called "Cast to Center," developed by Rapidia. The technique utilizes inexpensive, high-resolution SLA resin printers to create oversized plastic master models. These masters are then used to cast silicone rubber molds. A unique aspect involves filling the molds with a water-based metal or ceramic powder slurry containing a specialized binder, freezing the slurry within the mold at -40°C, and then demolding the resulting strong, frozen composite part. This frozen part is subsequently freeze-dried and finally sintered in a furnace (vacuum for metals, air for ceramics). This method offers superior resolution (micron level) and surface finish compared to conventional direct metal 3D printing, can work with any sinterable powder, and allows for complex geometries including undercuts due to the strength of the frozen material. While the process involves multiple manual steps and can exhibit more distortion than direct printing methods, it provides a low-cost entry point for high-resolution ceramic additive manufacturing (under $10k total) and is well-suited for small quantities, exotic materials, or high-throughput batch production using multiple molds. Rapidia has made the process publicly available to drive sales of their furnaces and consumables.
Novel High Resolution 3D Printing Method for Metals and Ceramics
0:00 Introduction: Dan Gelbart introduces a novel 3D printing process for metals and ceramics offering very high resolution (micron level) and excellent surface finish, superior to other known methods.
0:54 Key Advantage - Surface Finish: The method produces parts with an excellent, shiny, and smooth surface finish, allowing for easy electropolishing to achieve a mirror finish, particularly useful for medical parts or injection molding dies.
1:54 High Resolution Details: The process accurately reproduces fine details, resolution test patterns, thin walls with high aspect ratios, and complex features like cross-holes and cross-threads.
2:15 Process Origin: This method, called "Cast to Center," was developed by Rapidia and relies heavily on the availability of inexpensive (<$1,000), high-resolution SLA resin 3D printers.
3:24 Material Conversion: The process effectively converts plastic SLA prints into metal or ceramic parts using the same core method. It works for metals (e.g., 17-4 Stainless Steel) and ceramics (e.g., Alumina, Zirconia).
4:14 Step 1: SLA Model Creation: Start with an SLA model scaled up by the required shrinkage factor (e.g., 19% larger for 17-4 Stainless Steel) to account for material shrinkage during sintering.
5:26 Step 2: Mold Preparation: The SLA model must be sprayed with paint or varnish to prevent inhibition of the silicone rubber curing process.
6:04 Step 3: Silicone Mold Casting: Create a silicone rubber mold from the painted SLA model. BJDino's 30A blue silicone is recommended for its self-degassing property, strength, and tear resistance.
8:02 Step 4: Demolding the Master: Cut the silicone mold to remove the rigid SLA master part. The soft silicone maintains its shape accurately.
8:44 Step 5: Prepare Metal/Ceramic Slurry: Mix metal or ceramic powder (10-25 micron recommended for high resolution) with a water-based binder (approx. 10% by powder weight) until it has a smooth, pourable consistency like honey.
12:16 The Crucial Freezing Step: The key innovation is freezing the slurry in the mold. The frozen mixture acts as a strong composite material (metal/ceramic particles reinforced by ice), allowing complex parts with undercuts to be demolded without breaking before drying.
13:38 The Binder's Functions: The binder provides green strength (after drying), inhibits ice crystal formation (which would degrade frozen strength), reduces surface tension (for bubble release), and enhances shear thinning (low viscosity when vibrating/pouring, high viscosity when still to prevent settling).
15:03 Step 6: Degas and Pour Slurry: The slurry is degassed (typically by vibration) and poured into the reassembled silicone mold, often under vibration to ensure complete filling and remove air bubbles. A raft can also be cast for sintering support.
16:43 Step 7: Freeze: Place the filled mold into a freeze dryer (e.g., Harvest Right) pre-cooled to at least -40°C for a minimum of 2 hours until completely frozen.
18:01 Step 8: Demold Frozen Part: Carefully remove the frozen part from the flexible silicone mold. The strength of the frozen composite prevents damage to delicate features.
19:03 Step 9: Freeze Drying: Place the wet, frozen part back into the freeze dryer and run the drying cycle (sublimation of ice), which takes about 20 hours.
19:32 Step 10: Sintering: Place the dry "green" part (on a raft with alumina powder separator) into a sintering furnace. Metals require an expensive vacuum furnace ($140k total system cost). Ceramics can be sintered in a much cheaper air furnace ($4k furnace, <$10k total system cost for ceramics). The sintering cycle takes several hours (e.g., 12 hours).
22:56 Mold Variations: Simple geometries can use one-piece, flip-out molds that don't require cutting. More complex parts may need multi-part molds with injection ports and vents, filled under pressure using a syringe.
28:13 IP and Business Model: Rapidia owns the intellectual property for the process but has released it into the public domain to encourage sales of their vacuum sintering furnaces and consumables (binder, powder), which they will make available.
33:06 Disadvantages: The process involves many manual steps; there is more potential for distortion compared to direct printing methods (due to slight settling before freezing); it is less suitable for parts requiring complex internal hollows or infills that cannot be cast.
36:19 Advantages Revisited: The process is good for batch production (many inexpensive, reusable molds can be filled simultaneously); it works for any material that can be sintered; it is suitable for small quantities and exotic/precious materials where filling a printer reservoir is impractical; material (powder) cost can be lower than bound filament or slurry.