Carbon Graphite Seal Quality Inspection Methods: 18 Tests from Raw Material to Finished Product

Quality is the lifeline of seal manufacturing enterprises. For carbon graphite seals — precision components used in critical equipment such as pumps, compressors, and vacuum equipment — any quality defect can lead to equipment downtime or even safety accidents. Huahao Sealing Co., Ltd. has established a full-process quality inspection system from raw material incoming to finished product shipment, covering 18 key inspection items. This article systematically presents these inspection methods, equipment, standards, and acceptance criteria, helping purchasers and engineers deeply understand the quality control essentials of carbon graphite seals.

1. Raw Material Inspection Items

1.1 Ash Content Testing

Ash content is a key indicator of carbon graphite raw material purity, reflecting the inorganic impurity content. Test method: place the sample in a muffle furnace, ignite at 850°C for 4 hours to constant weight; the residue is the ash. Our standards: general industrial grade ash <0.5%, high-purity grade <0.1%, ultra-high-purity grade <0.05%. Test equipment: Nabertherm L9 high-temperature muffle furnace from Germany, temperature control accuracy ±2°C.

1.2 Particle Size Distribution Testing

Raw material particle size distribution directly affects pressing density and sintering performance. We use a Malvern Mastersizer 3000 laser particle size analyzer with a measurement range of 0.01-3500 μm, capable of drawing complete particle size distribution curves. Standard requirements: D50 controlled at 20-50 μm, distribution width Span <1.5.

1.3 Volatile Matter Testing

Volatile matter content affects volume shrinkage during baking. Test method: heat at 900°C under nitrogen protection for 7 minutes, measuring mass loss. Standard requirement: volatile matter content <0.5%.

2. Physical Property Testing

2.1 Bulk Density Testing

Bulk density is the most fundamental physical property of carbon graphite materials. Test method uses geometric + weighing method: precisely measure sample dimensions then weigh to calculate density. Test equipment: Mettler Toledo ten-thousandth electronic balance XP204, resolution 0.1 mg. Our standards: carbon graphite density 1.70-1.85 g/cm³, metal-impregnated graphite density 2.0-2.5 g/cm³.

2.2 Shore Hardness Testing

Hardness reflects material resistance to indentation and wear. Using an HS-19GD Shore hardness tester, 5-10 measurement points are selected on the sample surface and averaged. Acceptance criteria: pure carbon graphite HS 40-60, resin-impregnated carbon graphite HS 55-75, metal-impregnated carbon graphite HS 70-90.

2.3 Compressive Strength Testing

Compressive strength is the foundation for bearing seal pressure. Using a universal material testing machine with loading rate 0.5 mm/min, record the maximum load at specimen failure. Standard requirements: pure carbon graphite ≥100 MPa, resin-impregnated carbon graphite ≥150 MPa, metal-impregnated carbon graphite ≥200 MPa.

2.4 Flexural Strength Testing

Flexural strength reflects material resistance to bending fracture. Using the three-point bending method with 30 mm span and 0.5 mm/min loading rate. Standard requirement: ≥40 MPa.

3. Microstructure Testing

3.1 Porosity Testing

Porosity directly affects material strength and permeability. Test methods: vacuum impregnation to determine open porosity, or Archimedes method for total porosity. Standard requirements: unimpregnated carbon graphite porosity 10-18%, impregnated porosity <2%.

3.2 Impregnation Rate Testing

Impregnation rate reflects the adequacy of the impregnation process. Test method: weigh before and after impregnation, calculate weight gain rate. Standard requirements: resin impregnation weight gain 3-8%, metal impregnation weight gain 15-30%. Our impregnation quality testing equipment is precise to 0.01 g.

3.3 Metallographic Structure Testing

Using a metallographic microscope to observe graphite microstructure, evaluating grain size, pore distribution, and impregnation phase distribution uniformity. Our Olympus GX53 metallographic microscope has maximum magnification of 1000×, equipped with image analysis software for automatic quantitative analysis.

4. Chemical Property Testing

4.1 Chemical Composition Analysis

Using X-ray fluorescence spectrometry (XRF) and inductively coupled plasma optical emission spectrometry (ICP-OES) to analyze material chemical composition, with focus on Fe, Si, Al, Ca, Na, and K element content. High-end applications also require GDMS glow discharge mass spectrometry analysis, with detection limits at ppb levels.

4.2 Corrosion Resistance Testing

Samples are immersed in specified media (such as 20% hydrochloric acid, 20% sodium hydroxide, acetone, etc.) for 168 hours, measuring mass change rate and strength change rate. Acceptance criteria: mass change rate <1%, strength retention >90%.

5. Dimensional and Geometric Tolerance Testing

5.1 Dimensional Accuracy Testing

Using coordinate measuring machines, micrometers, internal diameter micrometers, and other precision measuring tools to inspect key dimensions. Our Hexagon Global S coordinate measuring machine achieves measurement accuracy of 1.8 μm, meeting the highest precision requirements. Common inspection items include inner diameter, outer diameter, height, and thickness.

5.2 Flatness Testing

Seal face flatness directly affects sealing performance. Tested using laser plane interferometers or electronic levels. Standard requirements: general seal flatness <0.01 mm, high-vacuum seal flatness <0.003 mm.

5.3 Surface Roughness Testing

Using a Mitutoyo SJ-410 surface roughness tester to inspect seal face roughness. Standard requirements: sliding seal surface Ra 0.2-0.8 μm, static seal surface Ra 0.4-1.6 μm, particularly precise applications Ra<0.1 μm.

5.4 Concentricity and Perpendicularity Testing

Using coordinate measuring machines to inspect concentricity of inner and outer circles, perpendicularity of faces to axis, and other geometric tolerances. Standard requirements: concentricity <0.01 mm, perpendicularity <0.005 mm.

6. Functional Performance Testing

6.1 Gas Tightness Testing

Using helium mass spectrometer leak detectors to test seal leakage rates. General industrial seals require leak rates <1×10⁻³ Pa·m³/s, vacuum-grade seals <1×10⁻⁹ Pa·m³/s, ultra-high vacuum grade <1×10⁻¹¹ Pa·m³/s.

6.2 Friction and Wear Performance Testing

Using an MM-W1 vertical universal friction and wear testing machine to simulate actual operating conditions, testing friction coefficient and wear rate. Test conditions can be customized for load, speed, temperature, and medium.

6.3 Outgassing Rate Testing (Vacuum-Grade Products)

Using a quadrupole mass spectrometer to dynamically measure sample outgassing rate spectra, verifying whether vacuum-grade carbon graphite meets vacuum application requirements.

7. Final Pre-Shipment Inspection

7.1 Visual Inspection

100% visual inspection of surface quality, requiring no cracks, chipped corners, scratches, or inclusions. An industrial camera-assisted automatic inspection system provides inspection accuracy of 0.05 mm.

7.2 Identification and Packaging Inspection

Verify that product identification (grade, batch number, production date) is clear and accurate, and that packaging meets transportation requirements.

Conclusion

The 18 quality inspection items form Huahao Sealing Co., Ltd.'s rigorous quality assurance system. Every carbon graphite seal shipped undergoes strict inspection verification and is accompanied by a complete test report. We firmly believe that only through the ultimate pursuit of quality can we earn customers' long-term trust. For more detailed consultation about our quality inspection system, please contact our quality management team.