When studying materials for 3D printing, especially in the Resin 3D Printing group—whether for models, engineering, prototypes, or functional parts—we often see hardness values indicated on the bottle or in the datasheet, such as Shore A 50, Shore A 80, Shore D 75, or Shore D 85.
Many people might mistakenly believe that "the higher the number, the better or more durable the material." In reality, Shore Hardness is simply a measurement of "the surface hardness of a material in resisting indentation," not an index for the overall strength of the workpiece. Choosing the correct Shore value is the key to selecting the right resin for your needs, reducing the cost of trial and error, minimizing breakage, and maximizing performance.
What is Shore Hardness?
Shore Hardness is an international standard used to measure a material's ability to resist indentation or surface deformation using a device called a Durometer. The working principle of a Durometer is as follows:
- Press an indenter into the material's surface with a specified force.
- Measure the depth of the indentation.
- Convert the result into a value ranging from 0–100.
Basic reading rule: Lower number = Softer material / Higher number = Harder material
• Shore A 20: Very soft, similar to gel or soft rubber.
• Shore A 70: Moderately flexible, similar to a plastic eraser.
• Shore D 80: Hard and rigid, similar to engineering plastic.
Internationally recognized Shore Hardness testing standards
Hardness testing is commonly referenced against international standards such as ASTM D2240, ISO 868, and ISO 48-4. The most important thing when checking resin specifications is to ensure the scale is always indicated alongside the number.
• ❌ Spec written as just Shore 80 (unclear and cannot be used for reference)
• Shore A 80 or Shore D 80 (accurate and immediately understandable)
Deep Dive into Shore A: Scale for Flexible & Elastic Materials
: Shore A is designed to measure soft, flexible materials or elastomers. Parts printed with resin in this scale excel in flexibility, shape recovery, shock absorption, and compression resistance.
• Materials commonly using the Shore A scale: Flexible Resin, Elastic Resin, TPU, Silicone, Rubber
Comparison table for Shore A sensitivity levels
What types of applications are suitable for Shore A resin?
• Shore A 20–40 (focus on softness and shock absorption): Suitable for seals, gaskets, and shock-absorbing parts like soft cushions.
• Shore A 40–70 (focus on balance): Suitable for wearables that come in contact with the skin, flexible prototypes, and protective padding.
• Shore A 70–95 (focus on durability and shape retention): Suitable for shoe soles, grips, and external protective parts that require high durability without breaking.
Deep Dive into Shore D: Scale for Rigid & Hard Materials
: Shore D is used to measure hard, rigid materials capable of bearing high loads. It is commonly used for materials requiring structural stability, resistance to torque and shear, and is suitable for engineering applications.
• Materials commonly using the Shore D scale: Standard Resin, ABS-like Resin, Tough Resin, Engineering Resin, Nylon, Polycarbonate
Comparison table for Shore D sensitivity levels
What types of applications are suitable for Shore D resin?
• Shore D 50–70 (focus on geometric dimensions and aesthetics): Suitable for figurines, architecture, and display parts that require sharp detail and ease of printing.
• Shore D 70–85 (focus on toughness and load-bearing): Suitable for functional prototypes, electronic housings, and assembly parts.
• Shore D 85 and above (focus on extreme strength): Suitable for jigs, fixtures, prototype molds, and industrial parts that must withstand continuous pressure.
Summary Comparison: Shore A vs Shore D
⚠️ Deep Reality: Is a higher Shore value always better? The short answer is "not necessarily" because Shore values only measure surface hardness and do not indicate the material's toughness or brittleness. Look at this example:
Consider this example:
• Shore D 88 Resin: Very hard surface, excellent shape retention, but could be "brittle and shatter" if impacted from a height.
• Shore A 65 Resin: Soft surface, flexible, and won't shatter when dropped because it "absorbs shock well," but may not be able to support heavy structural loads.
Other Technical Properties to consider alongside Shore values:
If you want to choose professional-grade resin, it is recommended to check the datasheet for these values:
• Tensile Strength: Ability to withstand pulling forces before the part breaks.
• Elongation at Break: Maximum percentage of elongation before the material tears (very important for Shore A group).
• Impact Strength: Ability to withstand sudden impact.
• Flexural Strength: Ability to withstand bending forces.
• Heat Deflection Temperature (HDT): The temperature at which the material begins to deform under heat.
3D Printer Resin Selection Guide by Application Category
To simplify purchasing resin for 3D printers, we can categorize resins by their primary functional properties:
1. Flexible Resin (Focus on flexibility)
• Applications: Rubber seals, shock-absorbing pads, wearable cases, or prosthetic models.
• Point to note: Focus on the Shore A value (recommended range 30A – 80A) alongside high Elongation at Break. The higher the percentage, the further the material can stretch without tearing.
2. Standard / Modeling Resin (Focus on models and display)
• Applications: Figurines, Art Toys, architectural models, shape prototypes.
• Point to note: Focus on the Shore D value (recommended range 75D – 85D) for ease of support removal, good surface finishing, sharp details, and printability.
3. Tough / ABS-like Resin (Focus on assembly and mechanical prototypes)
• Applications: Light box cases, drone parts, clips, snap-fit assemblies.
• Point to note: Focus on the Shore D value (recommended range 70D – 85D) but must consider Impact Strength as well to ensure that the parts won't turn brittle or break during assembly or screw fastening.
4. Engineering Resin (Focus on engineering and industry)
• Applications: Jigs & Fixtures, production line parts, equipment requiring heat or chemical resistance.
• Point to note: Choose resin with Shore D 80 and above, with high Tensile Strength and HDT (heat resistance) to maintain shape under real-world operating conditions.
5. Casting Resin (Resin for casting – Exception)
• Applications: Jewelry molds, dental work (Dental Wax).
• Recommendation: This group "should not use Shore values as a primary decision criterion." Focus instead on Burnout Performance, Ash Content (must be as low as possible), and Surface Finish to ensure smooth, flawless metal castings.
💬 Frequently Asked Questions (FAQ)
1. What Shore value is suitable for each type of 3D printing application?
: There is no single best Shore value for all tasks because each resin type is designed for different purposes. Always select based on the nature of the work:
- Flexible Resin: Recommended range Shore A 30–80 for applications requiring flexibility.
- Standard / Modeling Resin: Recommended range Shore D 75–85 for models and display pieces.
- Tough / ABS-like Resin: Recommended range Shore D 70–85 for assembly and functional prototypes.
Engineering Resin: Usually Shore D 80+ for engineering and industrial use. Always consider other mechanical properties alongside hardness.
2. How does Shore Hardness differ from Tensile Strength?
: Although both are mechanical properties, they indicate different things:
- Shore Hardness: Measures surface hardness when resisting indentation.
- Tensile Strength: Measures the ability to withstand pulling forces before breaking.
For example, some resins may have very high Shore D values but poor tensile or impact resistance, making them easier to break than expected.
3. Why does high Shore D resin still break easily?
: Because Shore values do not measure "toughness" or "impact resistance," only surface hardness. If a resin has low Impact Strength or low Elongation at Break, the part may be hard but brittle, causing it to shatter under impact.
4. Does Shore Hardness affect 3D printing quality?
: Indirectly. Resin hardness affects material behavior during and after printing, such as support removal, part extraction, shrinkage, surface finishing, and post-curing. Choosing the appropriate Shore value helps achieve better print quality.
5. Can Shore A and Shore D values be directly compared?
: No, because they are different measurement standards using different indenters and test ranges. For example, Shore A 90 does not mean it is harder than Shore D 70.
6. Does the Shore value change after Post Curing?
: Yes, it can change, especially in Resin 3D Printing. After post-curing, the polymer structure undergoes further crosslinking, making the material generally harder compared to its pre-cure state. Always check the datasheet to see if values are pre- or post-cure.
7. Do Flexible Resin and TPU use the same Shore scale?
: Generally, they both use Shore A because they are flexible materials. However, the same Shore value does not mean they have the same feel or properties, as they depend on the material composition, internal structure, and other mechanical properties.
8. Is choosing resin based on Shore value alone enough?
: No. If selecting professional-grade resin, review the Technical Datasheet (TDS) for these properties:
- Tensile Strength
- Elongation at Break
- Impact Strength
- Flexural Strength
- Heat Deflection Temperature (HDT)
Reading the datasheet alongside Shore values helps minimize errors in material selection.
9. What is the lowest Shore A and highest Shore D?
: If focusing on detail, surface smoothness, and ease of sanding, a resin in the Shore D 75–85 range is generally recommended, as it offers a balance between hardness, detail retention, and printability.
10. What Shore D should I choose for models?
: There is no single "better" value; it depends on the usage:
- If you want softness, flexibility, and shock absorption → Choose low–mid Shore A.
- If you want hardness, rigidity, strength, and precision → Choose higher Shore D.
Therefore, "choosing for the specific task" is always more important than picking the highest number.
Conclusion
Selecting the right resin for a 3D printer should not just be about "which resin is harder." Start by correctly understanding Shore Hardness, as it measures "surface resistance to indentation," not total material strength. The basic principles are:
- Shore A → Used for soft, flexible, and resilient materials like Flexible Resin, Elastic Resin, and rubber-like materials.
- Shore D → Used for hard, rigid, and high-strength materials like Standard Resin, Tough Resin, and Engineering Resin.
Crucially, Shore A and Shore D cannot be directly compared due to different standards. Choose resin based on the application rather than just the number. For professionals or those wishing to reduce trial-and-error costs, check the Technical Datasheet (TDS) for other mechanical properties such as:
- Tensile Strength
- Elongation at Break
- Impact Strength
- Flexural Strength
- Heat Deflection Temperature (HDT)
Ultimately, a good resin selection is not about finding the hardest material, but finding the right balance of hardness, toughness, flexibility, and real-world utility to ensure durable and high-performing prints.
References
- ASTM International. ASTM D2240 – Standard Test Method for Rubber Property—Durometer Hardness International standard for Shore hardness testing of rubber, plastics, and elastomers.
- International Organization for Standardization (ISO). ISO 868 – Plastics — Determination of Indentation Hardness by Means of a Durometer (Shore Hardness) Standard test for plastic hardness via the Shore method.
- International Organization for Standardization (ISO). ISO 48-4 – Rubber, Vulcanized or Thermoplastic — Determination of Hardness Standard for measuring hardness in rubber and elastomers.
- ZwickRoell. Shore Hardness Testing (ASTM D2240 / ISO 868 / ISO 48-4) Guide explaining the working principle of Durometers and selecting Shore A vs. Shore D scales.
- Intertek. Shore Hardness ASTM D2240 Test Guide Covers testing procedures, reading values, and precautions for interpreting Shore data.
- UL Prospector. ISO 868 – Shore Hardness Overview Material database and hardness interpretation for plastics and engineering materials.
