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An expanded and more complete version is available at: Pressboard vs Presspaper vs Transformerboard: Which One Does Your Project Actually Need?
Pressboard and transformerboard serve distinct functions inside power and distribution transformers, yet many sourcing teams treat them as interchangeable. Understanding the technical differences between these two cellulose-based insulation materials directly affects procurement cost, lead time, and long-term transformer reliability. This guide compares pressboard and transformerboard across manufacturing methods, IEC standards, thickness grades, and application suitability—so your engineering and sourcing teams can specify the right material from qualified suppliers.
What Are Pressboard and Transformerboard Made Of?
Pressboard is a cellulose-based electrical insulation material manufactured by dry-pressing high-purity kraft pulp into sheets ranging from 0.3 mm to 8.0 mm in thickness. The dry-pressing process yields moderate density and good oil absorption properties, making pressboard suitable for winding structures, layer insulation, and oil-duct formation in liquid-filled transformers. This material is standardized under IEC 60641, which defines three grades based on density and mechanical properties.
Transformerboard uses the same cellulose raw material but follows a wet-laid manufacturing process that produces substantially denser, mechanically stronger boards reaching thicknesses from 1.0 mm up to 100 mm and beyond. The wet-laid method—where cellulose fibers are deposited from a water suspension onto a forming cylinder, then pressed and dried—creates a more homogeneous fiber structure with higher tensile strength and dimensional stability. Transformerboard is governed by IEC 60763, which covers requirements for both pre-compressed and non-pre-compressed grades.
Both materials begin with the same raw material—cellulose—but the manufacturing path determines whether you get a cost-effective insulation sheet for general winding applications or a high-strength structural component for core clamping and support frames.
| Property | Pressboard | Transformerboard |
|---|---|---|
| Production Method | Dry-pressed kraft pulp | Wet-laid fiber deposition |
| Thickness Range | 0.3 – 8.0 mm | 1.0 – 100+ mm |
| Density | 0.9 – 1.2 g/cm³ | 1.0 – 1.3 g/cm³ |
| Governing Standard | IEC 60641 | IEC 60763 |
| Oil Absorption | High (suited for oil-immersed use) | Moderate |
| Typical Applications | Winding layers, cylinders, oil ducts | Clamping rings, support beams, end barriers |
| Mechanical Strength | Moderate | High (suitable for structural loads) |
| Cost Index | Lower | Higher (requires more processing) |
How Does the Manufacturing Process Impact Electrical Performance?
Dry-pressed pressboard achieves its final properties through mechanical compression of pre-dried pulp sheets between heated rollers—a process called calendering. This method results in a material with a natural oil absorption rate of 15-25% by weight when immersed in transformer oil, measured under IEC 60641-2 test conditions. Higher oil absorption benefits layer insulation because the oil-impregnated cellulose structure provides both dielectric strength and cooling channels within the winding stack.
Wet-laid transformerboard undergoes a fundamentally different formation. Cellulose fibers dispersed in water are deposited onto a rotating cylinder screen, forming multiple thin layers that are couched together to build the desired thickness. The wet sheet then enters a hydraulic press where pressures of 30-80 bar remove water and compress the fiber mat. For high-density grades such as POWERBOARD AK-type, the board is dried under pressure in multi-level heating presses, achieving a density of 1.2-1.3 g/cm³ and flexural strength exceeding 120 MPa.
This difference matters for procurement because pressboard insulation materials come in standard, pre-compressed, and formable grades—each suited to different voltage classes and mechanical requirements. Engineers specifying insulation for 400 kV power transformers typically select pre-compressed transformerboard for rigid components while using standard pressboard for winding insulation, balancing cost against performance across the bill of materials.
Which Insulation Board Fits Your Transformer Design?
Distribution transformers operating below 36 kV mainly use pressboard in thicknesses of 0.5-3.0 mm for coil-to-coil insulation, layer barriers, and oil duct formation. The material’s lower cost per kilogram and adequate dielectric strength (40-50 kV/mm in oil under IEC 60641-2) make it the economically rational choice for these applications. Engineers designing pad-mounted or pole-mounted units frequently specify 1.0 mm and 1.5 mm pressboard sheets for primary-to-secondary winding insulation.
Power transformers rated above 72.5 kV demand transformerboard for structural insulation components. Clamping rings must withstand continuous compressive forces of several tons while maintaining dimensional stability across temperature cycles from -25°C to 105°C. Only wet-laid transformerboard with thicknesses of 20-80 mm provides the compressive modulus (3,500-5,000 MPa) needed for these applications. End barriers in large power transformers—often 30-50 mm thick transformerboard—prevent partial discharge between phase windings and the grounded core structure.
For medium-voltage industrial transformers (36-72.5 kV), the optimal specification often blends both materials: pressboard for coil layer insulation and transformerboard for rigid spacers, lead supports, and core-to-coil barriers. This hybrid approach reduces material cost by 15-25% compared to all-transformerboard designs, based on bill-of-material analyses from transformer OEMs.
Manufacturers needing guidance on selection can refer to our detailed breakdown of how to choose the right pressboard insulation material, which covers density grades, surface finish requirements, and thickness tolerances for specific transformer types.
What Should Engineers Check When Sourcing Pressboard?
Five technical parameters determine whether pressboard meets your application requirements. First, verify density against your design specification—IEC 60641 Type B.3.1 requires 1.0-1.2 g/cm³ for general applications, while Type B.4.1 specifies >1.2 g/cm³ for high-mechanical-stress components. Second, confirm oil absorption rate: a value of 18-22% by weight (after 24-hour immersion at 105°C per IEC 60641-2) indicates adequate impregnation characteristics for oil-filled transformers.
Third, inspect thickness tolerance across the sheet. Reputable suppliers maintain ±5% variation across a 1,000 mm × 2,000 mm sheet, which prevents hot spots caused by uneven insulation in the winding stack. Fourth, request surface smoothness values: a roughness average (Ra) below 3.2 μm reduces partial discharge inception voltage in adjacent oil gaps. Fifth, demand batch traceability documentation—each pallet should carry a heat number linking back to raw pulp source, pressing parameters, and QC test results.
A certified supplier provides material certificates citing IEC 60641 compliance test data: tensile strength (≥80 MPa machine direction), elongation at break (≥3.5%), electric strength in oil (≥35 kV/mm for 1 mm thickness), and conductivity of aqueous extract (≤1.0 mS/m). Learn more about production quality standards in our article on how pressboard is manufactured, from raw pulp to finished transformer component.
SIDA supplies IEC 60641-compliant pressboard and transformerboard backed by the production capacity of Guangxin—our insulation systems specialist with 45,000 tons of annual pressboard output. Each shipment includes full material certification with traceability data. For a quote tailored to your thickness and grade requirements, reach our team at jessie.feng@sidanm.com or call +86-15958243831.
Why Do Leading OEMs Consolidate Pressboard Supply with One Partner?
Managing multiple insulation material suppliers creates hidden costs that procurement teams often overlook. Each additional supplier adds incoming inspection overhead, separate qualification testing, and logistical coordination. Consolidated supply—where one partner provides pressboard, transformerboard, transformer insulation papers, and related components—eliminates duplicate freight shipments and simplifies customs documentation for cross-border transformer projects.
Beyond cost reduction, a single-source partner reduces engineering risk. When the same supplier understands your pressboard requirements alongside your laminated wood and kraft paper specifications, they can identify material interactions before they become production problems. Cross-material expertise matters: a transformer using pressboard winding insulation and laminated densified wood clamping plates requires both materials to exhibit compatible oil absorption and thermal expansion characteristics. An integrated supplier validates this compatibility upstream.
SIDA operates under this consolidated-supply model through our four-shareholder structure. Guangxin produces insulating pressboard at scale, Fengbao manufactures complementary composite papers, Wanye handles precision processing, and Leadwin manages international logistics and standards compliance. This structure enables us to quote complete insulation packages—laminated pressboard for rigid components, PSP 3050 pressboard for winding structures, and precision-machined parts cut to your drawings—from a single purchase order with consolidated shipping.
To discuss your pressboard and transformerboard requirements, contact our export team via WhatsApp at +86-15958243831 or visit sidanm.com to browse our full insulation material catalog.
Frequently Asked Questions About Pressboard and Transformerboard
What is the difference between pressboard and presspaper?
Pressboard refers to insulation sheets with thickness greater than 0.3 mm, manufactured by dry-pressing or wet-forming cellulose pulp. Presspaper—also called press paper—describes sheets thinner than 0.3 mm, typically produced on cylinder paper machines and supercalendered for density and smoothness. Both are cellulose-based and governed by IEC 60641, but pressboard serves structural insulation functions while presspaper is used for inter-layer and conductor insulation in transformer windings.
Can pressboard be used in dry-type transformers?
Standard pressboard absorbs moisture from ambient air and loses dielectric strength when not impregnated with insulating oil. For dry-type transformers operating in air, manufacturers use pressboard that has been impregnated with epoxy resin or varnish to seal the cellulose fibers. This treatment blocks moisture absorption while maintaining the material’s mechanical properties. Alternatively, aramid-based materials such as Nomex replace cellulose pressboard entirely in high-temperature dry-type applications where thermal class exceeds 155°C.
How long does pressboard last in an operating transformer?
Cellulose pressboard in a properly maintained oil-filled transformer can remain functional for 30-40 years. The primary aging mechanism is thermal degradation of cellulose polymer chains, which accelerates exponentially above 95°C. The degree of polymerization (DP) of cellulose—initially 1,000-1,200—declines over time; when DP drops below 200, the material loses significant mechanical strength. Regular oil sampling and dissolved gas analysis provide early warning of pressboard degradation before mechanical failure occurs.
What thickness tolerances should I specify on a pressboard purchase order?
For pressboard under 3.0 mm thickness, specify a tolerance of ±5% as standard, with ±3% available from premium suppliers as a tighter option. For transformerboard above 3.0 mm, ±3% is achievable from qualified manufacturers. Include the measurement method in your specification—IEC 60641 references measurement at 23°C and 50% relative humidity after conditioning. Sheets measured in uncontrolled environments can show apparent thickness variations of up to 2% due to moisture content differences alone.
Does SIDA offer custom-cut pressboard components?
Yes. Through our Wanye division, SIDA provides CNC-machined pressboard and transformerboard components cut to customer drawings. We process rings, segments, support blocks, lead supports, and custom shapes from standard pressboard sheets as well as high-density transformerboard. Minimum order quantities start at 50 pieces for standard shapes and 10 pieces for complex machined geometries. Send your drawings to jessie.feng@sidanm.com for a quotation within 48 hours.
Summary: Choosing the Right Pressboard Supplier
Selecting between pressboard and transformerboard starts with your transformer’s voltage class and mechanical load requirements, not with a supplier catalog. Distribution transformers under 36 kV typically work well with IEC 60641 pressboard in the 0.5-3.0 mm range. Power transformers above 72.5 kV need transformerboard for structural parts and pressboard for winding insulation—a combination that rewards sourcing from an integrated partner who understands both materials.
SIDA brings four decades of collective insulation manufacturing experience to every order. Our Guangxin facility produces pressboard at 45,000-ton annual capacity with full IEC 60641 certification. When you source pressboard, transformerboard, and complementary insulation materials from one partner, you reduce incoming inspection overhead, eliminate multi-vendor logistics headaches, and gain a technical resource who understands how your materials interact inside the transformer.
Request a pressboard sample kit with material certification data by contacting our team at jessie.feng@sidanm.com or calling +86-15958243831. Visit sidanm.com to download product data sheets and review our complete insulation material portfolio.
