How Prepreg DMD Improves Insulation and Bonding in Dry-Type Transformer

In dry-type transformer production, low-voltage coils are often made with copper foil or aluminum foil. These foil windings need insulation materials that can do more than simply separate conductive layers.

The insulation layer must stay flat during winding, bond well after curing, and help the coil keep a stable structure during service.

This is where Prepreg DMD is used.

Prepreg DMD is an F Class flexible composite insulation material made from DMD base material impregnated with modified epoxy resin. It is pre-cured into a B-stage semi-cured condition. Before final curing, it remains flexible enough for winding and forming. After heating and pressure, the resin cures further and bonds the insulation structure together.

For dry-type transformer manufacturers, the main value of Prepreg DMD is clear:

It provides electrical insulation and heat-activated bonding in one material.

Why Is Prepreg DMD Used in Dry-Type Transformers?

Prepreg DMD is mainly used in dry-type transformer foil windings because it can provide:

• F Class 155℃ insulation performance;

• dielectric strength for foil layer insulation;

• bonding between copper or aluminum foil layers after curing;

• improved coil compactness;

• good flexibility before final curing;

• low shrinkage during curing;

• stable processing for winding and forming.

Ordinary DMD can provide flexible insulation, but it does not provide the same bonding function. Prepreg DMD is selected when the transformer coil needs both insulation and bonding after heat treatment.

What Is Prepreg DMD?

Prepreg DMD is made by impregnating DMD base material with heat-resistant modified epoxy resin.

The DMD base material has a three-layer structure:

Polyester non-woven fabric + Polyester film + Polyester non-woven fabric

After impregnation, the epoxy resin is pre-cured into a B-stage condition. This means the resin is not fully cured yet.

At this stage, the material can still be cut, wrapped, formed and used in winding production. During the customer's heating and pressure process, the resin cures into the final C-stage condition and forms bonding strength.

This structure makes Prepreg DMD different from standard flexible insulation paper.

It is not only a separator between conductive layers. It also becomes part of the bonded coil structure after curing.

Main Technical Features

Where Prepreg DMD Is Used

1. Dry-Type Transformer Foil Winding Insulation

This is one of the most important applications of Prepreg DMD.

In dry-type transformer low-voltage coils, copper foil or aluminum foil is commonly used as the conductor. Prepreg DMD can be placed between foil layers as interlayer insulation.

During winding, the material needs to stay flexible and flat. During heating and pressure, the epoxy resin cures and bonds the foil winding structure.

The material helps improve coil integrity and reduces the risk of loose insulation layers after curing.

For this application, buyers should confirm foil material, coil size, insulation thickness, winding process and curing condition before ordering.

2. Bonding Insulation Between Copper or Aluminum Foil Layers

In foil winding structures, bonding performance is important.

If the insulation material only separates the foil layers but does not bond properly, the coil may not have enough structural integrity after curing. This can affect handling, assembly and long-term stability.

Prepreg DMD can bond with copper foil, aluminum foil or adjacent insulation layers under suitable heat and pressure.

Its modified epoxy resin cures without releasing small-molecule volatile by-products, and the shrinkage is low. This helps keep the insulation layer stable during the curing process.

3. Motor Slot, Turn and Phase Insulation

Prepreg DMD can also be used in B to F Class motor insulation systems.

In motor production, insulation materials may need to be inserted into slots, shaped around coils or used between phases. The material must be flexible enough for processing and stable enough after forming.

4. Electrical Equipment Insulation Structures

Besides transformers and motors, Prepreg DMD can be used in electrical equipment that requires both insulation and bonding performance.

5. Transformer Barrier Cylinders and Formed Insulation Parts

Prepreg DMD can also be used in some transformer insulation structures such as barrier cylinders or formed insulation parts.

Before curing, the material can be shaped or wrapped. After curing, the epoxy resin helps hold the formed structure.

Prepreg DMD vs DMD Paper

Many buyers ask whether Prepreg DMD is the same as standard DMD.

The answer is no.

They may share a similar DMD base structure, but their functions are different.

Storage and Handling Suggestions

Prepreg DMD contains B-stage epoxy resin, so storage condition is more important than for ordinary DMD.

Recommended storage and handling points include:

store in a clean and dry environment;

avoid high temperature;

avoid direct sunlight;

keep away from moisture;

keep the roll sealed before use;

avoid heavy pressure on rolls;

use within the recommended storage period;

test bonding performance if stored too long.

The typical storage life is about 6 months at room temperature, but actual shelf life should follow the supplier’s datasheet and storage condition.

When to Choose Prepreg DMD

Prepreg DMD is suitable when the application requires:

• F Class 155℃ insulation;

• flexible material before final curing;

• bonding after heating and pressure;

• dry-type transformer foil winding insulation;

• copper foil or aluminum foil interlayer insulation;

• improved coil compactness;

• low shrinkage during curing;

• good dielectric performance;

• formed insulation structures.

It is especially suitable for dry-type transformer coils where the insulation material must bond with foil layers after curing.

FAQ

Q1: What is Prepreg DMD used for?

A: It is mainly used for dry-type transformer foil winding insulation, copper or aluminum foil interlayer insulation, motor slot insulation, turn insulation, phase insulation and bonding insulation structures.

Q2:What is the difference between Prepreg DMD and ordinary DMD?

A: Ordinary DMD mainly provides flexible electrical insulation. Prepreg DMD contains B-stage epoxy resin, so it provides both insulation and heat-activated bonding.

Q3: Can Prepreg DMD bond copper foil and aluminum foil?

A: Yes. Under suitable heating and pressure, the epoxy resin cures and can bond copper foil, aluminum foil or adjacent insulation layers.

Conclusion

Prepreg DMD is not just another type of DMD insulation paper.

Its key value is the combination of F Class electrical insulation and heat-activated epoxy bonding. Before curing, it remains flexible for cutting, winding and forming. After heating and pressure, it bonds the insulation structure and helps improve coil stability.

This makes Prepreg DMD especially useful for dry-type transformer foil winding insulation, copper or aluminum foil interlayer insulation and electrical insulation structures requiring bonding after curing.

For buyers, the most important selection points are application position, thickness, width, resin condition, curing process, bonding requirement and storage condition.

Chengrui supplies Prepreg DMD, ordinary DMD, NMN, NHN, Diamond Dotted DMD and other flexible insulation materials for transformer, motor and electrical equipment manufacturers worldwide.