Rigid graphite cylinder insulations are shape-retentive insulating materials made of graphite fibers and carbon binder. Based on high purity graphite fibers which have been milled, carbonized, mixed with proprietary resins, and formed into different sized cylinders according to the requirements of the customer.
The material then go through a process polymerization and drying. They are then carbonized followed by a final heat treatment to between 1500 ºC and 2800 ºC depending on the process application.
Its properties are governed by the precursor, the type and quantity of the binder, its compression ratio and thermal treatment. The material is used in the form of self-supporting cylinders for high temperature applications in tubular furnaces.
In case of special applications, where process gas infiltration of the insulation heat pack is problematic, the using of sandwich structure is recommended. Sandwich structured cylinders are made from alternating layers of rigidized felts and graphite foil.
Strict process control ensures a homogeneous material with excellent thermal properties resulting in improved hot zone temperature uniformity. Thermal properties, physical resistance and dimensional stability may be further enhanced by the application of graphite foil or CFC coating on one or both surfaces.
Rigid graphite cylinder insulations have excellent machining properties. It can be easily machined by such conventional methods as sawing, cutting, drilling or milling. The using of diamond tools is recommended, the edges have to be sharp at all time.
Available Types, Sizes and Main Properties
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Cylinder Type
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Wall Thickness
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Diameter
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Height
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Pure surfaces (no foil or CFC)
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30-120mm
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200-1500mm
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300-2000mm
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Graphite foil on one side
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Graphite foil on both sides
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CFC on one side
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CFC on both sides
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Graphite foil on one side, CFC on the other
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Properties
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Value
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Bulk density (g/cm³)
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≥0.18
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Flexural strength (MPa)
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1.5 - 3.2
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Thermal conductivity (W/mK)
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0.15 - 0.30
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Tensile strength (MPa)
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2.8 - 6.8
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Ash content (%)
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≤0.05
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Carbon content (%)
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≥99
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Processing temperature (°C)
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1500 - 2800
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Thermal expansion coefficient (10-6/K)
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0.8 - 6.9
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Characteristics and Benefits
- Low thermal conductivity allows high temperature furnaces to be designed and built with thin layers of insulating material. Moreover, the graphite foil-faced cylinders ensure heat reflection into the furnace interior, and the foil acts as a convection barrier
- Low heat capacity. The material has low density so the small mass of the heat insulating layer allows rapid heating and cooling of the furnace.
- High thermal stability. Rigid graphite cylinders are stable up to 2800 °C in vacuum and under inert atmospheres
- Shape-retention. The insulating cylinders do not compress or shrink under normal operating conditions. The bulk density thus remains unchanged throughout the entire insulating layer. The insulating properties stay constant, and no voids, channels or hot spots occur, especially in case of sandwich style
- Low adsorption of gases and vapours. Due to their small specific surface area, rigid graphite insulations adsorb only insignificant amounts of moisture. This allows short evacuation times in vacuum furnaces
- Erosion resistance. The carbon binder secures the felt fibers, thus reducing the amount of particulates entering the furnace atmosphere as a result of high gas velocities. Felts protected by CFC or graphite foil coated surfaces perform even better
- Excellent machining properties. Rigid graphite cylinders could be machined by using conventional methods such as cutting, drilling, sawing and milling. The clamping pressure should be distributed over large areas to reduce the risk of creating indentations
Applications
- Vacuum heat treatment
- Vacuum brazing
- Hard metal sintering
- Non-oxide ceramic production, e.g. aluminium-nitride, silicon-carbide, boron-nitride, etc.
- Precision casting units
- Carbon fiber production
