Shore hardness selection and elastic part design of TPU flexible materials: application solutions from sealing rings to shock-absorbing pads

Introduction: Shore hardness system of flexible materials

TPU (thermoplastic polyurethane) is the most widely used flexible material in FDM 3D printing. Its core characteristics are adjustable hardness range and excellent elasticity. Shore Hardness is a standard for measuring the hardness of flexible materials and is divided into two scales: Shore A (for soft rubber) and Shore D (for hard rubber). The hardness range of TPU materials for FDM printing is usually 60A-95A (Shore A), covering a wide range from "soft rubber" to "hard sole". For every 5A increase in Shore hardness, the material stiffness increases by approximately 15-20%, and the elastic modulus increases by 20-30%. Choosing the appropriate hardness is the first step in designing elastic parts, which directly determines key properties such as sealing, cushioning, and wear resistance of the parts.

Performance comparison of TPU with different hardnesses

Shore 60A-70A (soft TPU): It feels soft, similar to an eraser or soft silicone. The elongation at break is 500-700%, the elastic modulus is 2-5 MPa, and the compression permanent deformation is large (20-30%). Suitable for manufacturing soft seals, shock-absorbing pads, soft hinges, and flexible protective sleeves. The disadvantages are poor wear resistance and low tear resistance. Shore 80A-85A (medium hardness TPU): Medium feel, similar to car tires or sports shoe soles. The elongation at break is 400-600%, the elastic modulus is 10-20 MPa, and the compression permanent deformation is 15-20%. This is the most widely used hardness range and is suitable for manufacturing sealing rings, shock absorbers, wear-resistant parts, and elastic buckles. Overall performance balance. Shore 90A-95A (semi-rigid TPU): It feels hard, similar to hard soles or engineering plastics. The elongation at break is 200-400%, the elastic modulus is 30-60 MPa, and the compression permanent deformation is 10-15%. Suitable for manufacturing rigid seals, engineering hinges, wear-resistant gears, and semi-rigid brackets. It is close to the performance of engineering plastics and still maintains elasticity. The hardness of TPU above Shore 95A is close to that of hard plastic, and its elasticity advantage is weakened. Rigid materials such as nylon or ABS are usually used instead.

Sealing ring design and hardness selection

Sealing rings are one of the most typical applications of TPU printing. Principle of hardness selection: Static seals (such as O-rings and gaskets) can choose a softer hardness (70A-80A), which can achieve sealing under a small amount of compression; dynamic seals (such as piston seals, rotary seals) need to choose a medium hardness (85A-90A) to balance sealing and wear resistance; high-pressure seals (such as hydraulic systems) need to choose a harder hardness (90A-95A) to avoid excessive deformation leading to seal failure. Design points: The cross-sectional shape of the sealing ring is recommended to be circular (O-shaped) or rectangular (flat gasket) to avoid stress concentration caused by complex shapes; the size of the sealing groove needs to be accurately calculated, and the compression of the sealing ring is usually 10-25%; the printing direction recommends that the sealing surface be perpendicular to the direction of the layer grain to avoid leakage caused by separation between layers; the sealing surface can be coated with silicone oil or grease to reduce friction and extend life. Case: A hydraulic cylinder company used 85A TPU to print piston seals to replace original nitrile rubber injection molded parts. The cost was reduced by 70% and the delivery cycle was shortened from 2 weeks to 1 day.

Shock-absorbing pad design and cushioning performance

The design of shock-absorbing pads needs to consider factors such as impact energy absorption, rebound speed, fatigue life, etc. Hardness selection principle: For high-frequency vibration environments (such as motor bases), choose a harder hardness (85A-90A) to provide sufficient rigid support; for low-frequency impact environments (such as buffer pads), choose a softer hardness (70A-80A) to provide better energy absorption; Heavy equipment (such as machine tool bases) requires a multi-layer design: 90A-95A TPU is used for the bottom layer to provide support, and 80A-85A is used for the middle layer TPU absorbs impact, and the surface layer uses 70A-75A TPU to protect the ground. Geometric design: Shock absorbing pads are usually designed in a cylindrical or prism shape, with a wall thickness of 8-15 mm. The recommended wall thickness is 8-15 mm. If it is too thin, the shock absorption effect will be poor, and if it is too thick, the stability will be insufficient. A honeycomb structure or a hollow structure can be designed internally to increase the amount of compression and reduce stiffness. The surface can be designed with anti-slip textures (such as diamond patterns and wavy patterns) to increase friction and prevent displacement. Performance test: It is recommended to test compression permanent deformation according to ISO 4652 standard (should be 100,000 cycles).

Key parameters for FDM printing of TPU

TPU printing is more difficult. The main challenge is that the softness of the material causes poor wire feeding and nozzle clogging. Equipment requirements: It is recommended to use a Bowden extruder or a direct drive extruder. Direct drive is more suitable for soft TPU (5000 hours. Case 3: A sports equipment company uses 90A TPU to print treadmill shock-absorbing pads to replace the original rubber pads. The shock-absorbing effect is increased by 15% and the weight is reduced by 30%. Case 4: A toy company uses 65A TPU to print soft doll joints to achieve integrated printing of movable hinges, reducing assembly time by 80%. Design suggestions: The wall thickness of TPU parts should be uniform to avoid stress concentration caused by sudden changes; the buckle design needs to consider the elastic recovery of the material, and the preload amount is recommended to be 15-25%; it is recommended to use an interference fit or adhesive (such as cyanoacrylate) to connect TPU to other materials, as the threaded connection effect is poor.