Pressboard vs Presspaper vs Transformerboard: Which One Does Your Project Actually Need?

Ordering the wrong cellulose insulation product costs transformer manufacturers in two ways—paying for transformerboard where standard presspaper would suffice, or specifying pressboard that fails mechanically in a structural application designed for high-density transformerboard. These three materials share the same raw material and the same IEC 60641 standard, but their manufacturing processes create fundamentally different products suited to different roles inside the transformer. This comparison gives procurement teams and design engineers a decision framework based on thickness, mechanical load, voltage class, and cost—so your purchase order matches what your transformer actually requires.

What Is the Physical Difference Between Presspaper, Pressboard, and Transformerboard?

Presspaper is the thinnest of the three, manufactured on cylinder paper machines where cellulose fibers are deposited from a water suspension onto rotating screen cylinders. Multiple layers are couched together while wet, then dried and compressed through supercalenders—heated rollers that densify the paper to 0.9-1.2 g/cm³ while smoothing its surface. Presspaper thickness ranges from 0.05 mm to 0.5 mm, and its thin, flexible nature makes it suitable for conductor wrapping, inter-layer insulation, and applications where tight bending around small-radius conductors is required.

Pressboard starts with the same kraft pulp but undergoes dry-pressing rather than wet-laid formation. Pre-dried pulp sheets are stacked and compressed between heated platens under pressures of 20-50 bar, producing sheets from 0.3 mm to 8.0 mm thick. The dry-pressing process creates a material with moderate density (0.9-1.2 g/cm³) and naturally higher oil absorption than transformerboard, which benefits oil-immersed transformer applications where the paper must saturate with insulating oil to achieve full dielectric strength. Pressboard serves as layer barriers, winding cylinders, and oil duct spacers in transformers up to 245 kV.

Transformerboard is the heavy-duty option. Cellulose fibers are formed through a wet-laid process where the fiber suspension deposits onto a rotating cylinder, building up layer by layer until the desired thickness is reached—sometimes exceeding 100 mm. The wet sheet enters a hydraulic press at 30-80 bar, which removes water and compresses the fiber mat to densities of 1.0-1.3 g/cm³. High-density grades undergo additional pressing while drying in multi-level heated presses, achieving flexural strength above 120 MPa. This combination of thickness and density makes transformerboard the material for structural insulation components that must carry mechanical loads—clamping rings, support beams, end barriers, and core-to-coil insulation in power transformers rated above 72.5 kV. Our detailed comparison of pressboard versus transformerboard differences covers the manufacturing distinctions and their impact on procurement decisions.

Which Material Fits Your Transformer’s Voltage Class and Mechanical Requirements?

Distribution transformers operating below 36 kV use presspaper for conductor insulation at thicknesses of 0.05-0.13 mm and pressboard for layer barriers at 0.5-2.0 mm. The mechanical loads in these smaller transformers—winding compressive forces typically below 5 MPa—fall well within pressboard’s mechanical capability. Specifying transformerboard for a 500 kVA distribution unit adds unnecessary material cost without a corresponding improvement in transformer performance. Presspaper wraps around rectangular and round conductors with bend radii as tight as 1.5 times the conductor thickness, a forming capability that thicker pressboard cannot match.

Medium-voltage power transformers (36-72.5 kV) introduce the first applications where transformerboard becomes economically justified. Winding clamping rings must withstand continuous compressive forces of 5-15 tons during operation and substantially higher forces during short-circuit events. Pressboard at 4-8 mm thickness handles the moderate clamping forces in this voltage class, while transformerboard end barriers at 10-20 mm thickness prevent partial discharge between phase windings. The hybrid specification—pressboard for layer insulation and oil ducts, transformerboard for structural rings and end barriers—optimizes material cost by applying the more expensive material only where the mechanical loads demand it.

Power transformers above 72.5 kV make transformerboard the primary structural insulation material. A 400 kV transformer’s end barriers—typically 30-50 mm thick transformerboard panels—must maintain dimensional stability across temperature cycles from -25°C ambient to 105°C top oil temperature without warping or delamination. Only wet-laid transformerboard with controlled moisture content below 4% at the time of assembly provides this thermal-mechanical stability. Types of insulation pressboard come in standard, pre-compressed, and formable grades—pre-compressed transformerboard specifically addresses the high-voltage segment where partial discharge at the oil-cellulose interface becomes the limiting design factor.

What Does Each Material Cost and Where Can Procurement Save Money?

Presspaper commands the lowest per-kilogram price of the three materials at approximately $4-8/kg depending on thickness and quantity. A typical distribution transformer uses 5-10 kg of presspaper per unit for conductor wrapping and inter-layer insulation. Pressboard costs 20-60% more than presspaper of equivalent thickness because the dry-pressing process consumes more energy and yields lower throughput than the continuous paper machines that produce presspaper. Transformerboard at thicknesses above 10 mm costs 60-150% more than pressboard because the wet-laid formation and hydraulic pressing cycles limit production speed and the thicker boards require longer drying times that tie up expensive multi-level heating presses for hours per batch.

Procurement savings come from two strategies. First, avoid over-specifying. Many transformer designs call for transformerboard where pressboard meets the mechanical and dielectric requirements—particularly in the 36-72.5 kV range where the margin between pressboard capability and transformerboard necessity is narrow. A design review that replaces 20 mm transformerboard clamping rings with 12 mm pressboard in a 50 MVA transformer saves $300-500 in material cost per unit without compromising electrical performance or mechanical safety factors. Second, consolidate purchases across material types with a single supplier. Custom electrical insulation pressboard cut to your drawing dimensions from an integrated supplier eliminates the scrap losses and labor costs of in-house cutting while consolidating presspaper, pressboard, and transformerboard onto one purchase order. SIDA provides all three materials with batch-level certification—contact jessie.feng@sidanm.com or call +86-15958243831 for a consolidated quotation covering your complete insulation material requirements.

How Do Quality Requirements Differ Across the Three Materials?

Presspaper quality focuses on surface smoothness and thickness uniformity. The supercalendering process that gives presspaper its smooth surface also determines how consistently the paper feeds through automated taping heads—a roughness average (Ra) below 2.0 μm prevents tape-head jamming and ensures uniform tension during conductor wrapping at speeds up to 60 meters per minute. Thickness tolerance of ±5% across the roll width prevents the cumulative build-up errors that create uneven insulation walls in multi-layer windings.

Pressboard quality verification centers on density consistency and oil absorption rate. How pressboard is manufactured determines its density profile—sheets from the center of a dry-pressing stack typically achieve higher density than edge sheets because the pressure distribution in the press is not perfectly uniform. A qualified pressboard supplier measures density at five points across each sheet and rejects panels where the coefficient of variation exceeds 3%. Oil absorption measured by weight gain after 24-hour immersion in transformer oil at 105°C per IEC 60641-2 should reach 15-25%—values below this range indicate insufficient porosity for adequate oil impregnation in the finished transformer.

Transformerboard demands the most rigorous quality control because its structural function means that a defect in the board translates directly to a mechanical failure risk in the transformer. Each panel undergoes ultrasonic inspection for internal delamination—air gaps or resin-starved regions within the board thickness that are invisible on the surface. The compressive modulus measured perpendicular to the board surface must exceed 3,000 MPa at the rated clamping pressure to prevent creep relaxation over the transformer’s 30-40 year service life. Batch traceability extends from the finished board back to the raw pulp shipment, pressing parameters, and drying cycle data. For projects requiring materials for pressboard oil duct spacers and structural components, request the full manufacturing record with each shipment to satisfy your quality audit requirements.

Frequently Asked Questions About Presspaper, Pressboard, and Transformerboard

Can presspaper be used where the design originally specified pressboard?

Only if the application involves pure dielectric separation with zero mechanical load. Presspaper at 0.25 mm thickness provides equivalent oil-impregnated dielectric strength to pressboard of the same thickness—both achieve 40-50 kV/mm under IEC 60641-2 test conditions. However, presspaper cannot substitute for pressboard in any application where the insulation material carries compressive, tensile, or bending loads, because presspaper lacks the structural stiffness and creep resistance that pressboard’s dry-pressing process imparts. Winding cylinders, layer barriers thicker than 0.5 mm, and any component that must maintain its shape under its own weight require pressboard or transformerboard.

What is the shelf life of pressboard versus transformerboard?

Both materials have indefinite shelf life when stored in sealed, moisture-barrier packaging at 20±5°C and below 50% relative humidity. Once the packaging is opened, pressboard absorbs moisture from ambient air at approximately 1-2% per 24 hours at 60% relative humidity, while high-density transformerboard absorbs moisture more slowly—approximately 0.5-1% per 24 hours—because its denser structure limits water vapor diffusion. Opened material intended for oil-filled transformers should be re-dried at 105°C for 24 hours before winding if it has been exposed to ambient air for more than 72 hours, regardless of which of the three materials is being used.

Does the IEC 60641 standard cover all three materials?

IEC 60641 covers presspaper and pressboard. IEC 60763 separately covers transformerboard because the wet-laid manufacturing process and resulting material properties differ sufficiently to warrant a distinct standard. When specifying materials on a purchase order, referencing the correct IEC standard ensures that the supplier applies the appropriate test methods and acceptance criteria—IEC 60641 for presspaper and pressboard up to 8 mm, IEC 60763 for transformerboard from 1 mm upward. Material that meets IEC 60641 requirements does not automatically satisfy IEC 60763 criteria, and purchase orders that reference the wrong standard may result in receiving material tested to criteria that do not adequately assess the material’s fitness for its intended structural application.

How do I specify the right surface finish for each material type?

Presspaper typically specifies a supercalendered finish with Bendtsen roughness below 150 ml/min, providing the smooth surface required for automated taping and consistent oil impregnation. Pressboard can be specified with either a smooth press-plate finish (roughness below 3.2 μm Ra) for general applications or a matt finish (roughness 6.3-12.5 μm Ra) when the application requires enhanced oil penetration or adhesive bonding to adjacent surfaces. Transformerboard surface finish depends on the pressing plates used during manufacture—mirror-finish plates produce a glossy surface suitable for oil-immersed applications, while sandblasted plates create a textured surface that improves epoxy adhesion in dry-type applications where the board may be bonded into a composite structure.

Summary: Making the Right Material Choice for Your Next Transformer Order

Presspaper, pressboard, and transformerboard solve different problems inside the transformer. Presspaper wraps conductors and separates winding layers in every transformer from 50 kVA pole-mounted units to 500 MVA generator step-up units. Pressboard builds the insulation structure—cylinders, barriers, and oil ducts—in distribution and medium-power transformers where the mechanical loads remain moderate. Transformerboard carries the structural loads in power transformers where clamping rings, end barriers, and support frames must withstand tons of continuous compressive force over decades of thermal cycling. The procurement decision comes down to mechanical load, not dielectric performance—all three materials insulate adequately when properly oil-impregnated. The money saved by specifying pressboard where transformerboard is unnecessary, or presspaper where pressboard is overkill, drops directly to the bottom line.

SIDA supplies all three materials from integrated production lines with 45,000-ton annual capacity in pressboard for transformer manufacturing. Our technical team reviews your transformer design to identify specification opportunities—where pressboard replaces transformerboard, where standard grades replace premium grades, where consolidated ordering across material types captures volume discounts. Request a material consultation and consolidated price quotation from jessie.feng@sidanm.com or call +86-15958243831. Download product datasheets covering our complete presspaper, pressboard, and transformerboard portfolio at sidanm.com.