Why Transformer Manufacturers Use Diamond Dotted Paper

Transformer manufacturers use diamond dotted paper because ordinary insulation paper can separate winding layers, but it cannot bond them together.

Diamond dotted paper does both.

It provides electrical insulation between transformer winding layers and, after curing, the epoxy resin dots bond the winding structure to improve coil stability.

This is the main reason diamond dotted paper is widely used in oil-immersed transformers.

For standard transformer windings, it helps solve a practical problem: keeping the winding layers insulated, stable and well bonded during long-term operation.

What Is Diamond Dotted Paper?

Diamond dotted paper, also called DDP insulation paper, is an electrical insulation paper coated with B-stage epoxy resin in a diamond pattern.

The base paper is usually:

electrical insulation kraft paper

cable paper

transformer insulation paper

The resin is coated in a regular diamond pattern rather than as a full-surface coating.

This design is important.

The resin dots provide bonding strength after curing, while the open spaces between the dots allow transformer oil to penetrate the insulation structure.

That is why diamond dotted paper is suitable for oil-immersed transformer insulation systems.

How Diamond Dotted Paper Works

Before curing, the epoxy resin dots remain dry and stable on the paper surface.

This makes the material easy to handle during transformer winding.

During the transformer drying and hot pressing process, the B-stage epoxy resin softens, flows slightly and cures.

After curing, the resin bonds the winding layers together.

This helps improve:

• winding layer bonding

• coil mechanical stability

• winding structure strength

• short-circuit resistance

• insulation reliability

• long-term transformer performance

The diamond pattern is not only a surface design. It is a functional structure for bonding and oil penetration.

Where Diamond Dotted Paper Is Used

Diamond dotted paper is mainly used in oil-immersed transformer windings.

Typical applications include:

winding interlayer insulation

conductor insulation

rectangular copper conductor wrapping

rectangular aluminum conductor wrapping

coil bonding

winding structure reinforcement

oil-paper insulation systems

For distribution transformers, DDP insulation paper is often used because it provides a good balance of insulation performance, bonding strength and cost.

For transformers requiring better thermal aging resistance, thermally upgraded diamond dotted paper may be selected instead.

Why Not Use Ordinary Kraft Paper?

Ordinary kraft paper is useful in many transformer insulation positions.

It provides electrical insulation and good oil compatibility.

But it does not provide bonding.

Diamond dotted paper is selected when the winding design requires both insulation and bonding.

The key difference is simple:

Kraft paper insulates. Diamond dotted paper insulates and bonds.

Why the Resin Pattern Matters

Some buyers think the diamond pattern is only for appearance.

It is not.

If the paper were fully coated with resin, oil penetration could be reduced.

If there were no resin, the paper would not bond the winding layers.

The diamond pattern provides a balance between the two requirements.

It creates resin bonding points while keeping open areas for transformer oil.

This is why diamond dotted paper is especially suitable for oil-immersed transformer winding insulation.

Common Specifications of Diamond Dotted Paper

Diamond dotted paper is usually supplied in rolls, slit rolls or custom widths.

Common reference specifications include:

Item: Diamond Dotted Paper

Base Paper: Electrical kraft paper / cable paper

Resin Type: B-stage epoxy resin

Resin Pattern: Diamond-shaped dots

Coating Type: Single-side or double-side coating

Common Thickness:  0.08mm, 0.13mm, 0.15mm, 0.17mm, 0.20mm, 0.25mm, etc

Custom Width: Available

Typical Thermal Class: Class A, around 105°C

Application: Oil-immersed transformer winding insulation

Single-Side and Double-Side Diamond Dotted Paper

Diamond dotted paper can be supplied with single-side or double-side resin coating.

Single-Side Diamond Dotted Paper: Single-side DDP has epoxy resin dots on one side. It is used when bonding is needed on one contact surface.

Double-Side Diamond Dotted Paper: Double-side DDP has epoxy resin dots on both sides. It is used when bonding is required on both sides of the insulation layer.

Before ordering, buyers should confirm whether their transformer winding design requires single-side or double-side coating.

This is a small detail, but it can directly affect bonding performance.

Typical Curing Conditions

Diamond dotted paper uses B-stage epoxy resin.

The resin needs suitable temperature and pressure during transformer drying and hot pressing.

Typical curing reference conditions may include:

These are reference values only.

Actual curing conditions should follow the supplier’s technical datasheet and the transformer manufacturer’s production process.

Correct curing is important because insufficient curing may reduce bonding strength.

Diamond Dotted Paper vs Thermally Upgraded Diamond Dotted Paper

Ordinary diamond dotted paper is commonly used in standard oil-immersed transformers.

Thermally upgraded diamond dotted paper is used when better thermal aging resistance is required.

The difference is not the diamond resin pattern.

The real difference is the base paper.

If the transformer requires longer insulation life or higher thermal aging resistance, thermally upgraded DDP may be a better option.

FAQ

Q1: Why do transformer manufacturers use diamond dotted paper?

A: They use it because it provides both electrical insulation and bonding between winding layers after curing.

Q2: What is the purpose of the diamond pattern?

A: The diamond pattern provides bonding points while keeping open areas for transformer oil penetration.

Q3: What is the difference between DDP and thermally upgraded DDP?

A: The main difference is the base paper. Thermally upgraded DDP uses thermally upgraded insulation paper and provides better thermal aging resistance.

Q4: Can diamond dotted paper be customized?

A: Yes. Diamond dotted paper can be supplied in rolls, slit rolls, custom widths, single-side coating and double-side coating.

Conclusion

Diamond dotted paper is used in transformers because it solves a problem ordinary insulation paper cannot solve.

It provides electrical insulation and, after curing, bonds the winding layers together.

This helps improve winding stability, coil strength and long-term insulation reliability in oil-immersed transformers.

For standard transformer applications, ordinary diamond dotted paper is a practical and cost-effective choice.

For higher-load or longer-life transformer designs, thermally upgraded diamond dotted paper may be more suitable.

Xuchang Chengrui Insulation Material Co.,Ltd. supplies diamond dotted paper for oil-immersed transformer applications.