Quality Control of Polymer-Based Rubber Components
Non-Contact Thickness Measurement on Polymer-Based Rubber Components Using Terahertz Technology
Polymer-based rubber materials are widely used in aerospace structures for vibration isolation, thermal insulation, electrical and environmental protection, sealing, and interface management. For these components, controlling thickness uniformity is important because local non-uniformity, air-gap-like defects, and interface discontinuities can affect fit, bonding, assembly repeatability, and service reliability. TeraNIM, Terahertz Time-Domain Spectroscopy (THz-TDS) used to perform non-contact inspection on low-loss polymer-based rubber components. .
As per our experience, below typical materials are widely used in aerospace systems. These materials thicknesses and material consistency directly influence quality, performance and reliability of aerospace assemblies. THz signals generate reflections from internal interfaces, enabling accurate non-contact thickness measurement, defect detection, and quality verification without damaging the component
Typical Materials:
- EPDM Rubber
- Silicone Rubber
- Nitrile Rubber (NBR)
- Fluoroelastomer (FKM)
- Neoprene
- Polyurethane Elastomers
- Rockasin
- Low-Density Ablative (LDA) Tiles
- Medium-Density Ablative (MDA) Tiles
In this application note, TeraNIM was used to perform non-contact THz thickness measurement on an EPDM rubber layer bonded inside a composite cylindrical aerospace assembly. The objective was to verify rubber layer thickness against the design requirement and support manufacturing quality control without cutting, deforming, or accessing both sides of the component.
Traditional Inspection & Challenges
Terahertz Inspection Workflow
- Defined Part type, material family, nominal thickness, tolerance, surface condition and acceptance criteria with below requirement.
- Thickness Mapping
- Multi-Layer Thickness
- Bonding Inspection
- Defect Imaging
- Check THz penetration, usable bandwidth, echo separation and signal-to-noise ratio for the selected rubber formulation.
- TeraNIM operates in reflection mode for single-sided inspection. A short THz pulse is directed onto the component. Reflections are generated at material interfaces where there is a dielectric contrast. The front wall and back-wall echo separation is used for thickness calculation.
- Thickness equation: d = (c × Δt) / (2n), where d is thickness, c is the speed of light, Δt is the time delay between front-wall and back-wall echoes, and n is the calibrated refractive index of the rubber material.
- Use a micrometer-verified reference coupon or destructive cross-section reference to calculate n for the selected material recipe
- Measured refractive index (n) and absorption across usable THz bandwidth (0.1–3.5 THz)
- Calibrate against known-thickness polymer-based rubber reference (2.230 mm, micrometer-verified)
- Identified front-wall and back-wall echoes; found the refractive index as (n = 1.66) for EPDM rubber
- Acquire front-wall and back-wall reflection positions per location
- Monitor timing drift and standoff/coupling stability
- Measured Thickness as 16.129 mm as show in the graph
- Generate report with settings, data, echo positions, thickness table and with remarks
Key Advantages & Practical Limitations
- Non-contact measurement reduces risk of deforming soft rubber components during inspection.
- Single-sided access is useful for bonded or installed parts where a micrometer cannot be used.
- The test result is traceable because waveform data, echo positions, settings and calculated thickness can be stored.
- The supplied case study showed clear echo separation with 1% agreement after calibration against a known-thickness reference.
- Thickness variation of approximately 0.25 mm was resolved across seven measured points on part
Conclusion
The conducted study on EDPM thickness measurement with TeraNIM THz-TDS system provides an accurate, reliable, and repeatable solution with non-contact thickness measurement. Calibration against a micrometer reference achieved with THz was validated successfully. The ability to perform single-sided, non-contact measurements makes THz-TDS particularly suitable for aerospace manufacturing, quality assurance, and in situ inspection of bonded or installed components. The results indicate significant potential for integrating THz inspection into routine production quality-control workflows.
Solution With
TeraNIM
Frequently Asked Questions
Can Terahertz NDT measure rubber thickness without contact?
Yes. For suitable low-loss polymer-based rubber materials, Terahertz Time-Domain Spectroscopy (THz-TDS) measures thickness by analyzing the time delay between front-wall and back-wall reflections after refractive index calibration.
What is the maximum rubber thickness that can be measured?
Polymer-based rubber components with thicknesses ranging from approximately 1 mm to 40 mm can be measured when the refractive index is around 1.5. Materials with a lower refractive index may allow greater penetration depth.
Can Terahertz inspection evaluate bonded or installed rubber components?
Yes. Reflection-mode THz inspection is a single-sided technique, making it suitable for bonded or installed rubber components where rear-side access is limited. Inspection capability depends on the material properties and component geometry.
Can Terahertz NDT replace all conventional rubber quality control methods?
No. Terahertz inspection is best used as a complementary quality control technique for non-contact thickness measurement, interface evaluation, and repeatable inspection. Conventional and destructive methods remain valuable for validation and specialized testing.
What factors affect Terahertz penetration in polymer-based rubbers?
THz penetration depends on material absorption, filler content, rubber formulation, moisture level, surface condition, component thickness, and the available Terahertz frequency bandwidth.