When engineers and technicians pull their first sintered metal part from the crucible, the initial reaction is almost universal: “The part shrank by fifteen to twenty percent!” In traditional subtractive machining or plastic injection molding, shrinkage is treated as a dimensional flaw. In bound-metal additive manufacturing and powder metallurgy, shrinkage is the physical definition of success.

1. The Solid-State Sintering Mechanism: Why Shrinkage Happens

When Filamet™ is extruded on standard open-source FDM/FFF 3D printers, the as-printed green part consists of atomized metal particles packed tightly inside a polymer containment matrix. During debinding in a standard kiln, that polymer matrix burns away clean. As the temperature rises to the material-specific sintering plateau, solid-state atomic diffusion takes place.

Individual metallic particles bond at contact points, forming necks that broaden as atoms migrate across grain boundaries into interstitial voids. The microscopic pores collapse, and the part densifies from ~80% green density up to 98.5%+ theoretical density. That densification requires volumetric contraction.

2. Sintering Math: The Linear Oversize Factor Formula

To produce a sintered component that lands exactly on target CAD dimensions, CAD geometry must be scaled up prior to slicing. Do not make the common mistake of adding a flat percentage! You must always apply the linear Oversize Factor (OF) formula:

OF = 1 / (1 - S)

Where S is the linear shrinkage fraction:

• Pure Copper Filamet™: S ≈ 0.170 → OF = 1 / (1 – 0.170) = 1.205 (120.5% scale in slicer). Peak kiln soak: 1045°C (1913°F).
• Stainless Steel 316L: S ≈ 0.200 → OF = 1 / (1 – 0.200) = 1.250 (125.0% scale in slicer). Peak kiln soak: 1232°C (2250°F).
• Stainless Steel 17-4: S ≈ 0.200 → OF = 1 / (1 – 0.200) = 1.250 (125.0% scale in slicer). Peak kiln soak: 1260°C (2300°F).
• Bronze Filamet™: S ≈ 0.100 → OF = 1 / (1 – 0.100) = 1.111 (111.1% scale in slicer). Peak kiln soak: 885°C (1625°F).

Crucial Rule: Apply the Oversize Factor linearly to X, Y, and Z axes. Notice that total volume scales by the Oversize Factor cubed (OF³).

3. The In-Crucible Microclimate: Zero Hydrogen, Zero Shield Gas

All The Virtual Foundry sinterable metals achieve full densification in standard electric kilns using the patented in-crucible method. Parts are packed into refractory Al2O3 ballast sand within an alumina or ceramic crucible, capped with a top layer of TVF Sintering Carbon.

Under heat, the carbon getter reacts with residual oxygen to create a localized, non-explosive Carbon Monoxide reducing microclimate: 2C + O2 → 2CO. This eliminates the need for hydrogen retorts, forming gas lines, or million-dollar vacuum furnaces.

4. Radiation Safety Invariant

Notice: Rapid 3DShield™ filaments (Tungsten and Boron Carbide) are strictly non-sintered shielding assets. They are deployed directly off the print bed in their green state for immediate radiation attenuation. Thermal processing is prohibited.