Abstract:

This video by Dan Gelbart details a novel high-resolution 3D printing method called "Cast to Center," developed by Rapidia, for producing metal and ceramic parts. The process utilizes inexpensive, high-resolution SLA resin printers to create scaled-up plastic masters, from which reusable silicone rubber molds are cast. A critical step involves filling these molds with a metal or ceramic powder slurry containing a specialized binder and freezing the entire assembly. This freezing creates a strong composite that allows complex geometries with undercuts to be demolded without damage. The frozen "green" part is then freeze-dried and sintered in a furnace (vacuum for metals, air for ceramics). This technique achieves micron-level resolution and excellent surface finish, surpassing typical direct metal 3D printing. While more manual and potentially prone to more distortion than Rapidia's standard printing process, it offers low equipment cost (particularly for ceramics, under $10k), suitability for small batches and exotic materials, and high throughput via multiple molds. Rapidia is making the process public to encourage sales of their sintering furnaces and binder/powder consumables.

Novel High-Resolution Metal/Ceramic 3D Printing via Casting

• 0:00 Introduction: Dan Gelbart presents a novel method for 3D printing metal and ceramic objects, focusing on achieving very high resolution (micron level) and excellent surface finish, superior to conventional methods.
• 1:13 Key Advantages: The process yields parts with an excellent surface finish that can be easily electropolished to a mirror finish, reproduces fine details and patterns, and handles complex features like thin walls, cross holes, and threads perfectly.
• 2:15 Process Overview ("Cast to Center"): Developed by Rapidia, this method converts high-resolution plastic SLA prints into metal or ceramic parts by casting a metal/ceramic slurry into silicone molds made from scaled-up SLA masters. It is enabled by the availability of low-cost (<$1000) high-resolution SLA resin printers.
• 3:30 Material Versatility: The same process can produce parts in various materials, including metals (demonstrated with 17-4 Stainless Steel) and ceramics (demonstrated with Alumina), and is compatible with any powder that can be sintered.
• 4:14 Step 1: Scaled SLA Master: The process begins with an SLA resin print of the desired part, scaled up to compensate for sintering shrinkage (e.g., 19% larger for 17-4 Stainless Steel).
• 5:20 Step 2: Silicone Mold from Painted Master: A silicone rubber mold is made from the SLA master. The SLA print must be painted or varnished beforehand, as photopolymers inhibit silicone curing. High-quality silicone like BJDino 30A is recommended for its strength and self-degassing properties.
• 8:38 Step 3: Prepare Slurry: A slurry is created by mixing metal or ceramic powder (10-25 micron recommended for high resolution) with a water-based binder (approximately 10% binder by powder weight) until a smooth, pourable consistency is achieved.
• 12:16 Crucial Freezing Step: The slurry is poured into the silicone mold and the entire assembly is frozen (typically at -40°C for at least 2 hours). This freezing step is critical because the resulting metal/water/binder composite is immensely strong, allowing complex parts with undercuts to be demolded while frozen without breaking, which would be impossible if demolded after drying.
• 15:03 Step 4: Degas and Pour: The prepared slurry is degassed (e.g., via vibration) and then poured into the assembled mold, often under vibration to ensure it fills all cavities and removes air bubbles.
• 18:01 Step 5: Demold Frozen Part: The strong, frozen part is carefully demolded from the flexible silicone mold.
• 19:03 Step 6: Freeze Dry: The wet, frozen "green" part is placed into a freeze dryer (a Harvest Right unit is shown) to sublimate the water content. This drying process takes approximately 20 hours.
• 19:32 Step 7: Sinter: The dry "green" part is placed into a sintering furnace. For metals, a vacuum furnace is required to achieve high density (~99%), taking about 12 hours. For ceramics, an air furnace can be used. A raft made of the same material, with alumina powder sprinkled as a separator, is used under heavy parts during sintering to prevent cracking due to shrinkage.
• 22:07 Cost Considerations: The total equipment cost for high-resolution ceramic 3D printing using this method (SLA printer, freeze dryer, air furnace) is under $10,000. Metal requires a significantly more expensive vacuum furnace (over $100,000), bringing the total system cost closer to $140,000.
• 22:56 Mold Variations: Simple part geometries (like nuts) can use one-piece, flip-out silicone molds, while complex geometries may require multi-part molds with injection ports and vents, filled under pressure.
• 24:52 Binder Details: The specialized water-based binder (available from Rapidia) is complex, fulfilling four roles: providing green strength, inhibiting ice crystal formation during freezing, reducing surface tension for good flow and bubble release (without weakening the frozen part), and enhancing shear thinning (reducing viscosity under shear, increasing when still).
• 28:13 Public Domain & Business Model: Rapidia developed this process and is making it publicly available (though they own the IP) to encourage sales of their furnaces and consumables (binder, powder).
• 33:06 Disadvantages: The process involves many manual steps and potential delays compared to more automated printing methods. There is potential for more distortion (hundreds of microns for complex parts) due to density gradients from particle settling before freezing. It is also less suitable for creating internal hollows or complex infill structures that cannot be easily cast.
• 36:19 Batch Production & Material Flexibility: A significant advantage is the ability to use multiple inexpensive, quickly reusable silicone molds for efficient batch production. The method works with any sinterable material and is particularly well-suited for small quantities or exotic/precious powders like platinum or gold, as it doesn't require filling large machine reservoirs. It also uses cheaper PM/MIM quality powder rather than expensive spherical free-flowing powder.
• 37:27 Low Part Cost: The cost per part is primarily the cost of the metal/ceramic powder and furnace utility, making individual parts very inexpensive, especially in batch runs.
• 38:28 Overall Summary: The method offers the highest resolution, best surface finish, and lowest cost per part compared to other techniques, provided the user can accommodate the manual steps and potential distortion.