In vacuum casting, Shore Hardness is the primary metric used to simulate production materials. We measure it on two scales: Shore A for soft, flexible materials (like rubber gaskets, ranging from 10A to 90A) and Shore D for rigid, structural plastics (like ABS housings, ranging from 40D to 90D). Choosing the wrong scale typically results in prototypes that are either too brittle for assembly or too soft to hold tolerance.
This guide translates technical resin numbers into tactile reality. You will learn to:
- Visualize the feel of specific casting resins before you order.
- Distinguish between the tricky “Overlap Zone” (Hard A vs. Soft D).
- Select simulants that mimic TPE, PP, ABS, and Polycarbonate.
- Adjust for geometry to ensure your wall thickness doesn’t alter perceived hardness.
Shore A vs. Shore D: What’s the Difference?
The Shore scale (Durometer) measures a material’s resistance to indentation. In the context of vacuum casting (urethane casting), we primarily use two distinct scales that occasionally overlap.
- Shore A Scale (The “Rubber” Scale): This measures softer, more flexible materials. If you need a part that can bend, stretch, or seal—like a keypad button, a tire tread, or a soft-touch grip—you are looking for a Shore A resin.
- Shore D Scale (The “Plastic” Scale): This measures harder, semi-rigid to rigid materials. If you need a structural component—like a handheld device casing, a bracket, or a snap-fit clip—you need a Shore D resin.
The Tactile Rule of Thumb:
If you can press your fingernail into it easily, it’s usually Shore A. If it feels like a hard plastic shell that resists your fingernail, it’s Shore D.
2026 Vacuum Casting Shore Hardness Chart
Use this chart to match a specific hardness value with a real-world object you are familiar with. This ensures you specify the correct polyurethane resin for your Yanmee prototype.
| Hardness Scale | Material Simulant | Real-World “Tactile” Comparison | Common Application |
| 30 A | Soft Silicone / Gel | Gel Insole or Rubber Band | Soft overmolds, damping pads |
| 40 A | Soft TPE | Pencil Eraser | Flexible gaskets, seals |
| 60 A | Standard Rubber | Car Tire Tread | Door stops, standard seals |
| 70 A | Automotive Rubber | Running Shoe Outsole | Industrial grommets, rollers |
| 90 A | Hard Rubber / PU | Shopping Cart Wheel / Shoe Heel | Bushings, heavy-duty wheels |
| 45 D | Soft PP (Polypropylene) | LDPE Tubing / Tupperware Lid | Hinges, flexible covers |
| 60 D | HDPE / PP | Inner Car Door Panel | Clips, containers, semi-rigid parts |
| 70 D | Nylon / ABS blend | Hard Hat / Cutting Board | Structural brackets, casings |
| 80 D | Standard ABS | Computer Mouse / LEGO Brick | Consumer electronics housings |
| 90 D | Polycarbonate (PC) | Bone / Plexiglass | Lenses, stiff structural frames |
The “Overlap Zone”: Hard Rubber vs. Soft Plastic
The most common confusion occurs between Shore 90A and Shore 45D. On paper, these two hardness levels are technically very close. However, they behave differently under stress.
- Shore 90A (Hard Rubber): Even though it is hard, it retains “elastic memory.” If you bend it, it wants to snap back to its original shape immediately. It feels dense but rubbery, like a hockey puck.
- Shore 45D (Soft Plastic): This feels more like a pliable plastic (like a milk jug). If you bend it past a certain point, it may yield or deform permanently rather than springing back instantly.
Recommendation: If the part needs to absorb impact and recover (like a bumper), choose 90A. If the part needs to hold a structural shape but allow some flex (like a cap), choose 45D.
Material Selection Guide: Matching Resins to Function
When requesting a quote, don’t just ask for “plastic.” Specify the target simulant to get the right physical properties.
1. Rubber-Like (Shore 30A – 90A)
These resins simulate TPE and TPU. They are excellent for elastomeric parts.
- Best for: Gaskets, buttons, overmolding, and wearable straps.
- Pro Tip: Low shore hardness parts (30A–50A) can be difficult to machine if post-processing is required. Vacuum casting is ideal here because the part comes out of the mold with the texture already applied.
2. Semi-Rigid / PP-Like (Shore 40D – 70D)
These resins simulate Polypropylene (PP) and High-Density Polyethylene (HDPE). They offer high impact resistance and flexibility without breaking.
- Best for: Living hinges, containers, and clips.
- Snap-Fits: This range is often the “sweet spot” for snap-fits because the material can flex to lock into place without cracking. If you are designing these features, check our guide on CNC machining for snap fit features to understand the geometry required to prevent stress failure.
3. Rigid / ABS & PC-Like (Shore 70D – 90D)
These are the most common resins used for functional prototyping. They mimic engineering thermoplastics like ABS, Nylon, and Polycarbonate.
- Best for: Electronic housings, automotive dashboards, and structural components.
- Clear Parts: If you need transparent parts (simulating PMMA or PC), we almost always use a rigid urethane in the 80D–90D range to ensure optical clarity and polishability.
How Wall Thickness Changes Perceived Hardness
A common mistake is selecting a hardness value based on a standard test block (usually 6mm thick) without considering the geometry of your actual part.
Perceived hardness is relative to thickness.
- A Shore 90D (very hard) resin will feel flimsy and flexible if the wall thickness is only 0.5mm.
- A Shore 60A (medium soft) resin will feel surprisingly rigid and blocky if the part is 50mm thick.
Design Advice: If you are designing a thin-walled part (under 1.5mm) and need it to feel rigid, you must select the highest Shore D available (85D+) or add structural ribs. Do not rely on the material hardness number alone.
Common Mistakes When Specifying Hardness
- Confusing Shore with Rockwell: Do not use metal hardness scales (Rockwell B/C) for plastics. They are not directly convertible. Stick to Shore A or D.
- Ignoring Heat Deflection: Two resins can both be Shore 80D, but one might melt at 60°C while the other withstands 100°C. Hardness does not equal heat resistance. Always specify if your part faces high temperatures.
- Post-Cure Drift: Urethane resins can harden slightly over time as they fully cure. A part measured immediately after demolding might read 78D but settle at 80D after 24 hours. At Yanmee, we account for this curing phase in our quality control.
Frequently Asked Questions (FAQ)
What is the difference between Shore A and Shore D?
Shore A measures flexible, rubber-like materials (ranging from soft gels to hard shoe heels). Shore D measures rigid, plastic-like materials (ranging from semi-flexible polypropylene to hard polycarbonate).
Is there a formula to convert Shore A to Shore D?
No, there is no direct linear conversion formula because the scales overlap in the middle. However, as a general rule, a Shore 95A is roughly equivalent to a Shore 45D, though their elastic properties differ.
Which shore hardness is best for clear parts?
For clear vacuum casting parts, we typically use resins in the Shore 80D to 90D range. Harder materials are easier to hand-polish to a high optical clarity compared to soft rubber-like materials.
What tolerance can Yanmee achieve on hardness?
We typically hold a tolerance of ±3 to ±5 Shore points. Factors like temperature, mixing ratio, and part thickness can cause slight variations, which is standard for the vacuum casting process.
Is Shore 80D the same as ABS?
Shore 80D is a hardness measurement, not a material. However, vacuum casting resins rated at Shore 80D are specifically formulated to simulate the feel and rigidity of ABS plastic.
Can I get a Shore A part that is also heat resistant?
Yes, specialized heat-resistant elastomers exist, but they are less common than rigid heat-resistant materials. You must explicitly state “High Temp” requirements in your RFQ, or standard resins will be used.
Conclusion
Selecting the right Shore hardness is the difference between a prototype that functions perfectly and one that fails in testing. Whether you need the soft grip of a 60A rubber or the structural rigidity of an 80D ABS simulant, the key is matching the resin to your functional requirements, not just a number on a spreadsheet.
Still unsure if you need 70A or 80A? Send us your STEP file. Our engineering team at Yanmee will review your geometry and application to recommend the perfect resin for your project.