Geometry, Thickness and Structural Behaviour in 3D Resin Selection

Geometry and thickness: why the same material can feel rigid or flexible

• Any material may appear more or less rigid or flexible depending on thickness.
• Metal foils are flexible, but thick metal parts are rigid.
• Soft rubber films are flexible, but very thick parts are rigid.
• Materials typically appear flexible up to a certain thickness, and become progressively more rigid as thickness increases.
• Rigid materials in rod shapes can bend under pressure below a certain thickness.
• Flexible materials in rod shapes may not bend under pressure above a certain thickness.

Minimum feature size and the required rigidity

• Identify the smallest and thinnest part (the “minimum feature size”) of your 3D printable object.
• Identify the minimum required relative rigidity of that minimum feature size.
• Thicker feature sizes will appear more rigid due to their larger cross-sections.
• Avoid selecting excessively rigid materials when not required: excessive rigidity can reduce toughness and increase fragility (an “eggshell-like” behavior, as seen in glass and ceramics).
• Avoid selecting excessively flexible materials when not required: excessive flexibility can reduce toughness, flexural strength, and tear resistance (too-soft elastomers can become easy to break).
• For rigid materials, select the minimum rigidity that satisfies the smallest feature size, since thicker regions will naturally appear more rigid.
• For flexible materials, balance flexibility across the smallest and largest feature sizes to satisfy overall functional needs.

Practical thumb rules by decreasing Shore values (from D90 downwards)

• Above Shore D90: rigidity (Young’s modulus) increases, but excessive hardness can reduce flexural strength and impact resistance.
• Shore D70–D90: rigidity increases and mechanical properties are typically strong for many rigid materials.
• Shore D50–D80: in some systems, a softer resin (e.g., D60) can be more rigid and mechanically resistant than a harder resin (e.g., D80). This can occur with unique Engineering 3D resins containing resilient bio-based building blocks.
• From Shore D50 downwards: flexibility increases while mechanical strength typically decreases. For engineering, consider NextGen 3dresyns. For biocompatible flexible materials, consider Bioflex 3Dresyns and Bioflex 3Dresyns (MB range).
• Higher Shore A typically correlates with higher mechanical properties for many flexible and elastic materials. For maximum overall mechanical performance, choose the highest Shore A that still meets application needs.
• Shore A50 to A10: materials become very soft and typically elastic, often with reduced mechanical strength. For engineering, consider elastic NextGen 3Dresyns. For biocompatible elastic materials, consider Bioflex 3Dresyns and Bioelastic 3Dresyns.

Engineering functional materials: selection guidance

• Enginering 3Dresyns "like" are designed for engineering functional applications where very high durability and mechanical performance are required. These 3Dresyns contain bioplastics from renewable resources and are more environmentally friendly than petroleum-based plastics.
• These materials are supplied with access to multiple colors and functionalities and are available in different viscosity levels:
  • High viscosity versions can exhibit increased mechanical properties and are suited for printers with heating systems (printing above 25ºC).
  • Low viscosity versions can exhibit slightly lower (but still high) mechanical properties and are suited for printers without heating systems (printing below and above 25ºC).
  • Learn more: Benefits of printing with heated printers.

Other guidelines for selecting “green” biocompatible materials

Additional selection points for biocompatible materials:

• Monomer Based (MB) and Monomer Free (MF) 3D resins can exhibit similar strength for a given Shore value.
• Monomer Free (MF) versions do not contain monomers; consequently, the risk of causing skin irritation is minimized.