How to Choose Diamond Dotted Paper for Transformer Windings

By Chengrui Insulation Material Co.,Ltd
May 08 2025

Diamond dotted paper is one of the most widely used insulation materials in oil-immersed transformer windings.

But in practice, many buyers and even some new engineers use the term too broadly. They know diamond dotted paper (DDP) is used inside transformers, but they are not always sure where it should be used, what problem it solves, or how to choose the right type for a specific winding structure.

This article is not a general product introduction.

It is a practical guide to help transformer manufacturers, repair companies, and insulation buyers understand how to choose diamond dotted paper more accurately for real transformer applications.

What Diamond Dotted Paper Actually Does in a Transformer

Diamond dotted paper is an electrical insulation paper coated with heat-curable resin dots arranged in a regular diamond pattern.

During the winding drying and heating process, the resin softens and cures, creating bonding points between adjacent layers. This helps the winding become more mechanically stable after processing.

In simple terms, diamond dotted paper provides two functions at the same time:

electrical insulation between layers
bonding support for winding stability

That is the main reason it is widely used in oil-immersed transformer coils.

It does not simply separate layers like ordinary paper. It also helps hold the winding structure together after curing.

Where Diamond Dotted Paper Is Commonly Used

Diamond dotted paper is mainly used in transformer windings where both insulation and bonding are required.

Typical applications include:

interlayer insulation between winding layers
layer insulation in oil-immersed transformer coils
insulation positions where mechanical stability is important after drying
winding structures that need better resistance to movement during operation or short-circuit stress

For many transformer designs, DDP is chosen when the manufacturer wants the winding layers to stay more firmly bonded after thermal processing.

When Diamond Dotted Paper Is the Right Choice

Diamond dotted paper is usually a good choice when the application requires:

stable interlayer insulation
bonding after heating or drying
improved winding rigidity
better resistance to layer displacement
compatibility with oil-immersed transformer production processes

If the insulation position is flat, layered, and part of the coil build, DDP is often more suitable than flexible wrapping materials.

When Diamond Dotted Paper Is Not the Best Choice

DDP is not the right material for every insulation position inside a transformer.

It is usually not the best option when the job requires:

flexible wrapping around irregular shapes
tight bending around corners or leads
insulation for highly curved surfaces
rigid structural support like barriers, blocks, cylinders, or angle rings

For those cases, manufacturers often choose other transformer insulation materials such as crepe paper, kraft paper, pressboard, or fabricated insulation components depending on the specific function.

This is an important point:

DDP is a winding insulation and bonding material, not a universal replacement for all transformer papers or boards.

Five Key Factors When Choosing Diamond Dotted Paper

1. Transformer Type

Start with the transformer itself.

Is it a distribution transformer, a power transformer, or another oil-immersed design?

Different transformer types can have different winding sizes, thermal conditions, and mechanical requirements.

The more demanding the winding structure is, the more important correct DDP selection becomes.

2. Application Position in the Winding

Do not ask for diamond dotted paper based on name only.

You should first identify exactly where it will be used:

between which layers
on which winding
in what thickness range
under what process conditions

The same material may perform differently depending on the position in the winding system.

3. Required Thickness and Width

Thickness and width affect both processing and final insulation structure.

A thinner grade may be suitable for compact winding designs.

A thicker grade may be selected where stronger insulation spacing is needed.

Width should match the actual coil design and slitting requirement.

For bulk production, stable tolerance is often just as important as nominal thickness.

4. Heating and Drying Process

Diamond dotted paper works through a curing process.

That means your drying temperature profile matters.

If the heating process is not matched properly with the resin system, bonding performance may not be ideal.

When discussing with a supplier, it is helpful to provide your usual coil drying or baking conditions.

5. Mechanical Stability Requirement

Not every transformer winding needs the same bonding strength.

If your design faces higher short-circuit stress, stronger mechanical support in the winding may be more important.

In that case, DDP selection should not be based on price alone.

It should be based on the final winding stability you need after curing.

Common Mistakes When Buying Diamond Dotted Paper

Mistake 1: Treating All DDP as the Same

Some buyers think diamond dotted paper is a simple commodity product.

In reality, the base paper, coating system, curing behavior, thickness tolerance, and process consistency can all affect performance.

Mistake 2: Choosing Only by Thickness

Thickness matters, but it is not the only factor.

The real application position, winding structure, and curing process also matter.

Mistake 3: Using DDP Where Flexible Wrapping Is Needed

DDP is mainly designed for layered winding insulation with bonding.

For curved, uneven, or lead-wrapping positions, a more flexible insulation material is usually the better choice.

How to Judge Whether a DDP Supplier Is Reliable

For transformer manufacturers, a good DDP supplier should offer more than just a quotation.

You should also look at:

consistency of base paper quality
coating uniformity
stable thickness tolerance
experience in transformer insulation materials
ability to support bulk orders
sample support for testing
clear technical communication

A reliable supplier understands not only the material itself, but also how it is used inside transformer windings.

Final Thoughts

Diamond dotted paper is not just another insulation paper.

Its value comes from combining insulation and bonding in one material, which makes it especially useful in transformer winding systems that require mechanical stability after drying.

The best way to choose diamond dotted paper is to start from the actual winding application, not just the product name.

When the transformer type, insulation position, thickness, width, and process conditions are clear, it becomes much easier to select the right grade and avoid unnecessary trial and error.

If you are sourcing diamond dotted paper for transformer production, prepare the application details first.

A technically clear inquiry will always lead to a better recommendation than asking for price alone.

FAQ

Q1. What is diamond dotted paper used for in transformers?

A: Diamond dotted paper is mainly used for interlayer and layer insulation in transformer windings where bonding is also required after heating or drying.

Q2. Is diamond dotted paper used in oil-immersed transformers?

A: Yes. It is widely used in oil-immersed transformer winding systems, especially where the coil structure needs insulation plus bonding stability.

Q3. Is diamond dotted paper suitable for lead wrapping?

A: Usually no. For flexible wrapping on leads, corners, or irregular parts, more flexible materials are often preferred.