Infill vs. Shell in 3D Printing: Understanding Their Roles and Differences

When designing for 3D printing, two terms appear repeatedly: infill and shell. These components are essential to every print, dictating strength, material usage, and build time. Below we explain what each term means, how they differ, and why they matter.

What Is Infill?

Infill is the material that fills the interior of a part. It is deposited layer by layer according to a pattern chosen by the slicer—commonly lines, grids, honeycomb, or gyroid. The user can specify an infill density, expressed as a percentage. A density of 0% produces a hollow part, while 100% creates a fully solid object. Typical industrial prints use 20–40% infill for a good balance between strength and material savings. According to research by Autodesk, a 20% infill in a tensile test can match the strength of a 100% infill for many filaments.

What Is Shell?

The shell, also known as the perimeter or outer layer, is the material that surrounds the part’s exterior. It forms the visible surface and the structural envelope. Shell thickness is usually expressed in the number of perimeters (e.g., 2 × 0.4 mm). A thicker shell improves dimensional accuracy and surface finish, while a thinner shell reduces print time and material usage. Like infill, shell settings are fully customizable in most slicers.

Why Infill and Shell Matter

• Strength & Stiffness – Shell provides the primary load‑bearing surface; infill contributes to overall rigidity.
• Material Economy – By reducing infill density, you can cut material cost while maintaining acceptable mechanical performance.
• Print Time – Fewer infill patterns and thinner shells translate directly into faster prints.
• Surface Quality – A well‑designed shell ensures a smooth finish and accurate dimensional tolerances.

Choosing the Right Settings

When planning a print, start with the shell: decide on the number of perimeters based on the part’s required surface finish and strength. Next, adjust the infill density to meet internal load demands without overspending material. Consider the infill pattern: honeycomb or gyroid often offer superior strength-to-weight ratios compared to simple lines.

Conclusion

Infill and shell are the two foundational layers of any 3D‑printed part. Infill fills the interior, while shell forms the outer shell. Mastering these settings allows designers to optimize for strength, cost, and print speed.

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