What is Elboard?

Elboard is a solid electrical-quality fibrous laminate material made of cellulose pulp.

The manufacturing technology has similarities with papermaking but also with processes for making plastic laminates or fibreboard.

Figeholm Elboard is made of pure cellulose fibres only. There is no filler material or added bonding agents. The mechanical strength comes from the fibres themselves and from the adhesion between them.

Elboard is marketed either as flat sheets, as semi-finished rods or laminated blocks, or as finished, shaped components for insulation systems.

Raw Material

The best combination of electrical and mechanical strength is obtained with unbleached sulphate-process conifer-tree pulp from sub arctic forest.

Figeholm takes its material from a few selected mills, known for stable quality and very low electrolytic impurity content.

Manufacturing process – different grades

The pulp is dispersed in clean water and milled in ”raffineurs” to suitable fibre length composition. Forming of the material is similar to papermaking: A thin web of fibres is taken up from the thin suspension in an outlet box on to the wire mesh of a forming roll. From the roll the web is transferred to a loop of felt as carrier.

Single-sheet material, is made in a discontinuous process. The wet webs from the forming rolls are wound up in many turns on a large collecting cylinder. When enough thickness of material has been built up, a knife blade cuts up the wet, felt-like deposit and lets it fall out on a horizontal table as a large sheet. The building of next sheet then immediately begins, without stopping the rotation. The length of the sheet is equal to the circumference of the collecting cylinder.

The forming process results in a material with all fibres oriented in the plane. Within the web layers the orientation along and across the direction of movement (machine direction, MD – cross machine direction CMD) is kept as equal as practically possible (minimum anisotropy).

The raw sheets leaving the machine hold about 70% water. They are taken further in different ways to form different grades of material:

Low-density board – the raw sheet is dried in a tunnel oven and forms a soft and flexible material, easy to wrap and strap but with moderate tensile strength and little compression stiffness.

Pre-compressed board – drying is performed under compression at an elevated temperature in a hydraulic stack press with heated plates between the sheets. The plates are covered with a wire mesh to let the water escape, and this leaves a characteristic cloth mark in the surfaces of the board. Pre-compressed board is a material with maximum density, tensile strength, compression stiffness, and flexural strength.

Low-density sheet

This grade of material is mainly used by transformer manufacturers for making mechanically shaped parts or for mouldings.

Strips of thin-gauge material may be shaped between rollers to form small straight L-profile edge coverings or round angle rings.

Pre-compressed, high-density board and laminated material

This material is developed for applications where maximum stiffness and tensile or flexural strength is desirable. It is the right material e.g. for spacers in windings, subjected to compression by electromagnetic forces from through-fault overcurrent, and for barrier cylinders in large transformers.

Moulded L-profile bars of pre-compressed board are used as structural members for the supporting of winding connections (“cleats and leads”).

The material is also well suited for all types of former cylinders for windings.

Laminated blocks

(thicker than 8 mm) are glued together from several sheets of board. They are used for winding press plates and for parts of cleats and leads structures.

Pre-compressed board and laminated blocks replace natural wood and plywood where dimensional stability, strength against compression, and superior electrical insulation properties are wanted. The material is fully impregnable with insulation oil and is free from knots or crevisses which could give rise to partial discharge inception in the major insulation structure of the transformer.

Machining of parts from pre-compressed laminate gives excellent results, but only when sharp tools and correct cutting speeds are used. Final dryout of block laminate parts in large transformers requires particular considerations because of the anisotropic diffusion properties.