TL;DR:
- Transformer pressboard is high-density, pure-cellulose insulation board used for main insulation and mechanical support in oil-immersed transformers.
- IEC 60641-2 defines the test methods; grades such as B.2.1, B.3.1, and B.4.1 set density and application, while pre-compressed board improves dimensional stability.
- Thickness from 1.0 to 6.0 mm maps to different jobs — winding spacers, end pressure plates, and oil-duct structures.
- The standard sheet is 1160×2200 mm; confirm density, thickness tolerance (±5%), moisture content, CNC machining, and Incoterms before you order.
Transformer pressboard (transformerboard) is a high-density, pure-cellulose insulation board that provides the main electrical insulation and mechanical support inside oil-immersed power and distribution transformers. Buyers rarely fail on what it is — they fail on which grade and thickness to specify. Choosing the wrong density or tolerancing a sheet too loosely leads to dimensionally unstable barriers, blocked oil ducts, and costly rework. This article explores the IEC 60641-2 pressboard grades, how to choose the right thickness from 1.0 to 6.0 mm, and a practical sourcing checklist, so you can write a specification your supplier can quote accurately.
What Is Transformer Pressboard (Transformerboard)?
Definition and Material Composition
Transformer pressboard is made by pressing layers of unbleached sulphate (kraft) cellulose pulp into a dense, rigid board, then drying and calendering it to a controlled thickness. Unlike paper, which is thin and flexible, pressboard is thick — typically 1.0 to 8.0 mm in single sheets — and is used where both dielectric strength and mechanical load-bearing are required. In an oil-immersed transformer, pressboard forms winding cylinders, angle rings, end collars, and insulating barriers that hold the winding in place while separating high- and low-voltage circuits. SIDA supplies transformer pressboard (PSP-3050) and G4 high-density transformerboard for these structural applications.
How Pressboard Differs from Insulation Paper
Pressboard and insulation paper come from the same cellulose family but serve different roles. Insulation paper is thin — 0.05 to 0.5 mm — and used for conductor wrapping and layer insulation. Pressboard is thicker and denser, so it provides the structural strength that paper cannot. A typical pressboard has a density of 1.0 to 1.3 g/cm³, giving it roughly ten times the mechanical stiffness of ordinary paper at comparable area. That is why it is specified for load-bearing spacers and barriers rather than flexible wrap. In short: paper insulates conductors, while pressboard both insulates and supports the winding assembly.
IEC 60641-2 Pressboard Grades Explained
What B.2.1, B.3.1, and B.4.1 Mean
The IEC 60641 series governs pressboard and presspaper for electrical use: Part 2 specifies the test methods, and Part 3 defines requirements for individual material types. Grades such as B.2.1, B.3.1, and B.4.1 are pressboard types distinguished mainly by density and processing. B.2.1 is a medium-density board for general barriers and spacers. B.3.1 is a higher-density, calendered board for structural components that must hold shape under clamping. B.4.1 is a high-density, heat-stabilized board specified where dimensional stability matters most. Higher density generally means greater strength and better oil-impregnated dielectric performance — at a higher cost.
“3.1A Grade” and Pre-compressed Pressboard
You will often see “3.1A” or “3.1B” on inquiry drawings rather than a plain “3.1”. These suffixes are supplier-specific variants of the B.3.1 grade, usually indicating the moisture state or a tighter density band — so a buyer should always ask the supplier to state the exact density range and tolerance behind the suffix. Closely related is pre-compressed pressboard, which is compressed under controlled pressure and heat so it stays dimensionally stable in service. Pre-compressed board typically holds thickness relaxation to about 1% under clamping load, versus 3–5% for standard board — a decisive advantage for oil-duct spacing.
Key Parameters: Density, Tolerance, and Moisture
Three parameters decide whether a grade works in your design. Density, measured in g/cm³, sets mechanical strength and dielectric performance. Thickness tolerance is normally ±5% for board above 1.5 mm and ±7.5% for thinner sheet; premium suppliers can hold ±3% for board over 3.0 mm, which matters when you machine tight-fitting components. Moisture content is the third check — as-delivered board may contain 5–9% water, but for transformer use it must be dried below 1% before oil impregnation. Confirm all three on the supplier’s pressboard certificate of analysis.
How to Choose Thickness: 1.0 / 1.5 / 3.0 / 6.0 mm
A Real Inquiry Example: the Aljizzi Specification
A recent buyer inquiry — from Aljizzi — is a useful worked example. The request combined multiple thicknesses in one order: 1.0, 1.5, 3.0, and 6.0 mm, all in the standard 1160×2200 mm sheet size. Mixed-thickness orders are common because a single transformer uses thin board for winding spacers and thick board for end plates and structural barriers. When you bundle thicknesses this way, specify each thickness as a separate line with its own tolerance and quantity, so the supplier can price the order correctly instead of quoting a single blanket grade.
Matching Thickness to Application
Thickness follows function. Thin 1.0 mm board is used for winding spacers, cooling-duct separators, and small barriers where flexibility and low mass matter. The 1.5 mm sheet is a workhorse for general barriers and machined components. Around 3.0 mm you move into structural territory — end collars, angle rings, and components that must resist clamping pressure. At 6.0 mm and above, pressboard serves as end pressure plates and heavy support members. Pair the right board with matching spacers and angle-ring strips and crepe paper tubes to build a complete oil-duct system.
Standard Sheet Size 1160×2200 mm and Custom Ranges
The 1160×2200 mm sheet is the de facto standard pressboard format, sized to feed CNC routers and press tooling efficiently. Single sheets typically range from about 0.5 mm to 8.0 mm thick, while flat boards and laminated transformerboard run much thicker — up to 50 mm and beyond. If your drawing calls for a non-standard sheet size or a thicker laminated build-up, ask the supplier up front; most factories, including SIDA, will cut or laminate to your dimensions with an adjusted lead time rather than forcing you onto a stock size.
Pressboard vs Transformerboard: Naming and Supplier Differences
Pressboard, Transformerboard, and Presspan
The three terms describe essentially the same material and are often used interchangeably, which causes quoting errors. “Pressboard” is the IEC term. “Transformerboard” is the trade name popularized by Weidmann for its high-density pressboard, and is now used generically. “Presspan” is an older term for thinner, calendered pressboard and presspaper. When you request a quote, use one term consistently and confirm the grade and density, rather than assuming the supplier reads “transformerboard” the same way you do. For a fuller breakdown, see pressboard vs transformerboard.
Chinese vs European Suppliers: Pucaro and Weidmann
European suppliers such as Weidmann and Pucaro set the benchmark for premium transformerboard, with tightly controlled density and documented IEC test data — at a price to match. Chinese manufacturers now supply the same IEC 60641-compliant grades at a more competitive cost, which is why so much volume has shifted to Asia. The practical difference is in specification discipline, not material quality: when buying from a Chinese supplier, insist on a clear IEC grade reference, a stated density range, and a mill test certificate rather than a bare “transformerboard” description. This closes the gap that the premium brands historically protected.
Sourcing Checklist: What to Confirm Before You Order
Technical Specifications to Confirm
Before issuing a PO, lock down five technical points. First, the IEC grade — for example B.3.1 — and its exact density range. Second, thickness tolerance: ±5% is standard above 1.5 mm. Third, surface finish, especially if the board will be oil-impregnated or glued. Fourth, whether you need CNC machining to drawing, which turns a board order into a component order and changes pricing. Fifth, the test report: confirm whether the supplier certifies density, moisture, tensile strength, and dielectric strength per IEC 60641-2. SIDA provides CNC-machined laminated wood components and pressboard parts to drawing.
Commercial Terms: MOQ, Lead Time, and Incoterms
On the commercial side, confirm minimum order quantity, lead time, and delivery terms before you commit. Stock pressboard ships quickly, but custom thicknesses or CNC-machined parts add lead time. Standard boards are usually quoted EXW or FOB China, while many buyers of finished components prefer CFR or DDP to fix a landed cost. Clarifying Incoterms up front avoids surprises at the port — a CFR quote does not include destination duties, whereas DDP does. Lock these terms alongside the technical spec so the quote you approve is the price you actually pay.
Frequently Asked Questions
What is the minimum order quantity for pressboard?
MOQ depends on thickness and whether you need stock sheet or CNC-machined parts. Standard pressboard sheet is usually flexible — often a few hundred kilograms or a mixed pallet — while custom-machined components may carry a higher MOQ because of tooling and setup. The best approach is to send your full drawing and quantity with your inquiry; a supplier like SIDA will quote the lowest workable MOQ for your specific part rather than a generic number.
How long is delivery for transformer pressboard?
Stock pressboard in standard thicknesses and the 1160×2200 mm format can often ship within days. Custom thicknesses, pre-compressed board, or CNC-machined components typically add one to four weeks depending on drawing complexity and order volume. Air freight and express options shorten the door-to-door time for urgent orders. Ask for a production-and-shipping timeline at the RFQ stage so lead time becomes part of your sourcing decision rather than a surprise.
Can you CNC-machine pressboard to my drawing?
Yes. Pressboard and laminated wood are machined into spacers, angle rings, end collars, and other components on CNC routers and lathes. Send a DXF or PDF drawing with dimensions and tolerances, and the supplier will quote both the material and the machining. Machining is where a one-stop supplier saves time, because the material and the finished component are quality-controlled under the same roof rather than handed between two vendors.
What does the test report include?
A proper mill test certificate for pressboard should report density, thickness, moisture content, tensile strength, and dielectric strength, tested according to IEC 60641-2 methods. For pre-compressed board, dimensional stability — thickness relaxation — should also be listed. Before accepting material, match the certificate values against the grade you specified, particularly density and moisture, because those two numbers drive how the board performs after oil impregnation in your transformer.
Summary: Specifying Pressboard That Performs
Transformer pressboard is a specification-driven material: the grade, density, thickness, and tolerance you write down determine how the board behaves in service. Start from the IEC 60641-2 test methods and Part 3 grades — B.2.1 for general use, B.3.1 for structural components, and B.4.1 where dimensional stability is critical. Match thickness to the job, from 1.0 mm spacers to 6.0 mm end plates, and confirm the standard 1160×2200 mm sheet or request a custom cut. Finally, lock the technical spec and the commercial terms — MOQ, lead time, and Incoterms — in the same quotation.
For buyers sourcing IEC 60641-compliant pressboard and CNC-machined transformer components, SIDA offers one-stop supply from raw material to machined part. Contact SIDA to discuss your pressboard requirement:
- 📞 +86-15958243831
- 📧 jessie.feng@sidanm.com
- 💬 WhatsApp: Chat on WhatsApp
- 🌐 sidanm.com
References
- International Electrotechnical Commission. IEC 60641-2: Pressboard and presspaper for electrical purposes — Part 2: Methods of tests. IEC webstore.
- International Electrotechnical Commission. IEC 60641-3: Pressboard and presspaper for electrical purposes — Specifications for individual materials. IEC webstore.
- IEEE Xplore. A study of industrial manufacturing and testing procedures for press board transformer insulation. https://ieeexplore.ieee.org/document/7853291
- Weidmann. Transformerboard — Technical literature. transformerboard.pdf
- ACC Insulations. Pre-Compressed Pressboard for Transformer Insulation. https://www.accinsulations.com/pre-compressed-boards
- SIDA. Pre-Compressed Pressboard: Technical Guide & Buyer Checklist. sidanm.com
