As someone who’s been deeply involved in the field of TPE (Thermoplastic Elastomer) overmolding onto nylon for years, I know firsthand how tricky hardness adjustment can be. You want a softer touch, but you worry about wear resistance. You aim for high rigidity, but then face issues like delamination or cracking. After countless production runs and iterations, I’ve developed a comprehensive, systematic approach to help engineers fine-tune the hardness of their formulations effectively—balancing tactile comfort, durability, and production efficiency.
This guide is broken down into seven key parts:
Hardness measurement standards
Factors that influence hardness
Polymer system selection
Use of fillers and plasticizers
Process parameter optimization
Formulation workflow
Real-world case studies and an FAQ section for common challenges
Tables are included throughout for clarity, and important points are highlighted in bold. External data links are added for further validation.
1. Overview of Hardness Standards and Testing Methods
Before diving into formulations, you must first understand what hardness range you’re aiming for, and under which standard.
| Standard | Shore A | Shore D | Applicable Norms |
|---|---|---|---|
| Target Materials | Soft elastomers, silicone | Hard plastics, overmolded TPE | ISO 868, ASTM D2240 (source) |
| Scale Range | 0–100 A | 0–100 D | – |
| Typical Use Case | 20–80 A | 20–60 D | – |
| Test Conditions | 23°C, 50% RH | 23°C, 50% RH | ISO 7619-1 (source) |
- Shore A: Used for softer, more flexible materials like grips, buttons, and seals.
- Shore D: Used for rigid applications like structural overmolding or hard panels.
- ⚠️ Tip: Always condition samples in a standard environment (23°C, 50% RH) for 24 hours before testing to avoid measurement errors of ±3A or ±3D.
2. Key Factors That Influence Hardness
Hardness in TPE materials results from a combination of formulation and processing, not a single variable. Here are five major contributors:
Ratio of hard to soft segments in the polymer
Use of plasticizers and flow modifiers
Content and type of inorganic/organic fillers
Degree of crosslinking or dynamic vulcanization
Processing conditions (temperature, shear, cooling rate)
| Factor | Increasing This Factor | Effect on Hardness | Notes on Trade-offs |
|---|---|---|---|
| Hard segment content | Higher | Hardness ↑ | Toughness ↓, impact resistance ↓ |
| Plasticizer content | Higher | Hardness ↓ | Heat resistance ↓, migration risk ↑ |
| Inorganic filler | Higher | Hardness ↑, wear ↑ | Flowability ↓, processing viscosity ↑ |
| Crosslinking (TPV) | Deeper cure | Hardness ↑, heat ↑ | Cycle time ↑, throughput ↓ |
| Shear rate | Stronger shearing | Slight hardness ↑ | Potential foaming or surface defects |
3. Selecting Polymer Systems and Balancing Hardness
A. Blends & Copolymers
SEBS/PP blends: By adjusting the ratio of SEBS (soft phase) to PP (hard phase), you can tune hardness between 30–70 Shore A.
TPU/Nylon overmolding: TPU ranges from 70–95 Shore A; but overmolding to nylon also depends on melt compatibility and flow.
B. Hard/Soft Segment Ratio
Example: For a grip requiring 60–65 Shore A, use 40% SEBS + 60% PP. If you need to lower it to 45–50 Shore A, adjust to 50% SEBS + 50% PP.
Hard phase (PP or PA): Increases rigidity but reduces flow, so higher temperatures may be needed.
Soft phase (SEBS, TPU): Reduces hardness but may compromise thermal stability.
4. Plasticizers and Fillers: How to Use Them Correctly
A. Plasticizers
Common Types: Phthalates (DOP, DINP), polyethers (PEG series)
Dosage: Limit to ≤15 phr; every additional 5 phr typically reduces hardness by 2–4 Shore A
Note: In high-temp or long-term applications, plasticizer migration is a concern—polyester-based or high molecular weight plasticizers are better.
B. Fillers
Calcium carbonate / Talc: Cost-effective hardness boosters, with minimal impact on flow. Add at 10–30 phr for a 1–3 A increase per 10 phr.
Silica (white carbon black): Increases hardness and strength but raises melt viscosity. Recommended range: 5–15 phr.
Glass fiber: Strong rigidity booster, significantly increases hardness, but reduces elasticity.
5. Process Parameters and Their Impact on Hardness
| Parameter | Adjustment Direction | Effect on Hardness | Recommended Practice |
|---|---|---|---|
| Extrusion temp. | Increase | Hardness ↓ (softening) | Keep mid-zone < 230°C |
| Screw speed | Increase | Slight hardness ↑ | Optimal: 30–60 rpm |
| Back pressure | Increase | Hardness ↑ (mixing) | Keep between 5–12 bar |
| Cooling rate | Faster cooling | Hardness ↑ | Water temp not below 15°C |
| Hold pressure | Longer time | Hardness ↑ (densification) | Add 1–2 sec for molded parts |
🔍 Best Practice: Adjust one variable at a time, and observe Shore A/D shifts of 0.5–1 unit to determine trends.
6. Workflow for Developing a TPE Overmolding Formula
Define the target hardness range
Choose a base polymer blend based on your goal (soft feel or rigid support)
Adjust plasticizers and fillers, record every formula iteration
Fine-tune process parameters on lab-scale extruder or molding machine
Scale-up to small batch (5–10 kg), check hardness consistency
Document the SOP, including allowable hardness range (±2 Shore)
7. Real-World Case Study
Scenario: A consumer electronics brand needs a TPE grip overmolded onto a PA6 shell with a final hardness of 50–55 Shore A.
Formula Development:
Initial: 45% PP + 55% SEBS → 58 Shore A
Add 10 phr PEG → drops to 52 Shore A
Add 10 phr calcium carbonate → slightly increases to 54 Shore A
Lower extruder mid-zone temp to 190°C; increase back pressure to 10 bar → final hardness stabilized at 53 ± 1 Shore A
Result:
Hardness: 53 ± 1 Shore A
Yield rate: improved from 90% to 98%
Client feedback: Comfortable grip, wear-resistant, 15% higher assembly efficiency
8. FAQ – Common Questions About TPE Hardness Adjustment
Q1. Why does the same formula show different hardness in different batches?
It’s often due to raw material inconsistencies or temperature variations during processing. Always revalidate with a small test batch.
Q2. How do plasticizer and filler interact in affecting hardness?
Use this rule of thumb:
Δ Hardness ≈ –(Plasticizer phr ÷ 5) × 3 A + (Filler phr ÷ 10) × 2 A
Q3. Does hold pressure during molding affect final hardness?
Yes, longer hold time allows tighter molecular packing—raising hardness by 0.5–1 Shore A.
Q4. What’s the most stable way to control hardness long-term?
Use TPV systems (dynamically vulcanized blends)—though costlier, they offer long-term stability and reduced migration.
Q5. Any regulatory concerns with plasticizers?
Yes—phthalates are banned in the EU. Use non-phthalate or bio-based plasticizers to meet global standards.
Each product and application is different, and there’s no one-size-fits-all recipe for hardness control. Only by combining thoughtful formulation, controlled processing, and careful validation can you truly dial in the perfect feel and performance. I hope this breakdown gives you the confidence to tackle your TPE-Nylon overmolding challenges head-on.
If you’ve encountered specific issues or want to share your findings, feel free to drop them in the comments. Always happy to exchange ideas with fellow professionals.