- ▸Baked carbon graphite rings have 10%-20% porosity; the impregnation target is open porosity below 2%, meeting the density requirements referenced in GB/T 566
- ▸The vacuum stage typically requires -0.095 MPa or better held for 30-60 minutes to evacuate air from the interconnected pores
- ▸Impregnation pressure is commonly 0.6-1.0 MPa held for 1-4 hours; penetration depth grows with holding time
- ▸Resin viscosity controls penetration: 30-80 mPa·s is typical; above 150 mPa·s thick-wall rings cannot be fully penetrated
- ▸Curing heat-up rate should stay at 5-10℃/h; heating too fast vaporizes volatiles and re-creates porosity
- ▸Two to three impregnation-curing cycles are typical, each adding about 3%-8% weight; a stable weight gain means full penetration
A carbon graphite ring is first "baked" and then "soaked". During baking the binder carbonizes and leaves a network of interconnected capillary pores — typically 10%-20% porosity. Until those pores are filled, media will wick straight through the ring wall, and the ring cannot seal. Impregnation fills the pores with liquid resin or metal; vacuum-pressure processing is the most reliable route today.
I. Why Vacuum Comes First
At atmospheric pressure, resin only penetrates 1-2 mm by capillary action, because trapped air pushes back against the liquid.
The vacuum removes that air and opens the channels. Standard practice:
1.Preheat the rings to 60-80℃ and load the autoclave — warmer resin flows better
2.Pull vacuum below -0.095 MPa (roughly 5 kPa absolute) and hold 30-60 minutes
3.Insufficient vacuum is the most common failure cause — residual air is driven deep into the pores during the pressure stage and blocks them
Holding time scales with wall thickness: 30 minutes is enough for walls under 20 mm; heavy-wall or large-section rings need 60 minutes or more.
II. Pressure Stage: Pressure and Holding Time
Once vacuum is established, resin is drawn into the vessel, then pressure is applied to push resin from the OD surface deep into the pore network.
- Common pressure is 0.6-1.0 MPa; high-density parts may use 1.2-1.5 MPa
- Hold time is 1-4 hours depending on wall thickness and resin viscosity
- More pressure is not always better: thin rings can distort or micro-crack under excessive load
Penetration follows capillary-flow physics, and holding time contributes more directly than pressure. Two hours of hold versus thirty minutes can mean several millimeters more penetration. Weight gain is the practical indicator: the first cycle typically adds 3%-8%; a second-cycle gain below 1% signals full penetration.
III. Curing: The Underrated Step
Curing is not just "drying". Resin polycondenses and releases small molecules; if the temperature ramps too fast, those volatiles vaporize inside the pores and create bubbles, destroying part of the impregnation.
1.Heat up at 5-10℃/h with staged soaks
2.Final cure temperature depends on the resin system; phenolics typically cure at 150-180℃
3.Leave machining allowance after curing — the resin-rich surface layer is removed in precision grinding
IV. Resin Viscosity vs. Porosity
Viscosity is the physical bottleneck. Impregnation resin is usually controlled at 30-80 mPa·s; above 150 mPa·s, even correct pressure cannot penetrate walls thicker than 20 mm.
The impregnate-cure-reimpregnate cycle typically runs 2-3 times. The final target: open porosity below 2% with no leakage in hydrostatic or air testing. Well-impregnated epoxy or phenolic graphite can rise from 150-200 MPa compressive strength as a blank to the 250-300 MPa class, with improved chemical resistance as well.
V. Quality Control Points
- Sample each batch for porosity (water absorption or mercury intrusion)
- Record weight gain per ring; under-weight rings are re-impregnated
- Re-measure dimensions after curing — resin expansion can shift size by 0.05-0.1 mm
Huahao Sealing completes impregnation, curing, lapping and inspection in-house at its Lu'an, Anhui plant, and ships custom impregnated graphite rings with per-piece porosity and pressure-class verification. Vacuum level, pressure, hold time and curing curve are interlocked; if any one of them slips, the final porosity figure cannot be recovered downstream.