Hot-Air Winding of Bobbinless Coils Using Self-Bonding Wire｜Uniform Bonding with Double Heaters and Eliminating the Annealing Step

PRONEC CO., LTD. provides contract manufacturing of bobbinless coils using self-bonding wire, produced with a double-heater winding method at our factory in Thailand.

With hot-air winding, self-bonding wire is bonded uniformly during the winding process itself, eliminating the need for a subsequent annealing step. This keeps coil quality consistent while improving production efficiency.

This article explains the hot-air winding technology used for bobbinless coils made with self-bonding wire, how double heaters achieve uniform bonding, the benefits of eliminating annealing, and our quality control system.

Table of Contents

• Uniform Bonding with Double Heaters and Improved Reliability of Bobbinless Coils
• Winding Without a Subsequent Annealing Step
• Range of Bobbinless Coils We Support with Self-Bonding Wire
• In-House Winding Machines and Dimensional Control from the Design Stage
• Automated Testing for Resistance, Withstanding Voltage, and Impulse
• Prototype Consultation and Transition to Mass Production
• Frequently Asked Questions

Uniform Bonding with Double Heaters and Improved Reliability of Bobbinless Coils

When manufacturing bobbinless coils using hot-air winding, insufficient bonding during winding can cause the coil to swell or deform during reliability testing under real-world conditions, or under high-temperature, high-pressure conditions.

To address this, PRONEC has equipped its winding machines with a dedicated double-heater system to improve bonding efficiency during hot-air winding.

The double heater delivers heat evenly and consistently to the bonding layer of the self-bonding wire, reducing delamination between layers and localized areas of incomplete bonding. This allows us to produce bobbinless coils with excellent shape stability and structural strength, even without a bobbin.

Winding Without a Subsequent Annealing Step

If bonding during winding is insufficient, an additional annealing step using an oven or similar equipment is needed afterward. However, adding an annealing step extends production lead time and increases manufacturing cost.

With PRONEC's double-heater winding method, self-bonding wire receives sufficient heat at the same time as it is wound, keeping the bonding condition stable. Because the bonding process is completed during winding itself, the subsequent annealing step can be eliminated entirely.

This reduces the number of process steps, shortens production lead time, and lowers costs in mass production.

Range of Bobbinless Coils We Support with Self-Bonding Wire

As demand grows for space-saving designs and higher electrical performance, bobbinless coils need to support a wide range of wire types — including microfine wire, Litz wire, and flat wire.

PRONEC supports bobbinless coil production using microfine wire with a conductor diameter of φ0.02 mm or larger, Tetron-covered Litz wire, and flat wire, among others.

When larger-diameter Litz wire is used, the bonding layer can stick to the winding jig during winding, causing problems removing the workpiece or even halting the production line. To solve this, PRONEC has developed and adopted a specially coated, highly durable winding jig in-house.

Even with specialty wire materials or particularly demanding specifications, we can maintain stable continuous production by designing jigs suited to each product's shape and production conditions.

In-House Winding Machines and Dimensional Control from the Design Stage

For coils with unusual shapes or strict tolerance requirements, standard off-the-shelf winding machines alone are sometimes unable to reproduce the optimal production conditions.

PRONEC modifies general-purpose winding machines in-house, and also designs and builds dedicated cutting equipment and automatic coil press machines ourselves. We fine-tune equipment conditions to match each product's specifications in order to maintain consistent winding quality.

Because Thailand's production environment is easily affected by outdoor temperature, controlling temperature within the production area is also important. Our 24-hour production lines use a central chiller air-conditioning system during the day, and package air conditioners in combination at night depending on which lines are running, to minimize temperature fluctuation.

In coil production, variation between wire material lots, along with stretching and shrinkage caused by tension during winding, can cause a difference between the dimensions on the drawing and the actual finished dimensions.

For this reason, PRONEC considers wire material properties and winding conditions from the design stage, and proposes approaches that account for the dimensional changes likely to occur in actual production. Reviewing tolerances and drawing dimensions in advance like this reduces issues at the prototyping stage and supports a smooth transition to mass production.

Automated Testing for Resistance, Withstanding Voltage, and Impulse

Inspecting coils before shipment requires testing multiple characteristics, including resistance, withstanding voltage, insulation resistance, and impulse. Performing these tests separately by hand can increase takt time and carries a risk of judgment errors.

PRONEC has built an inspection system that uses a shared measurement probe across these inspection and testing steps, covering everything from automatic switching between test equipment to automatic removal of the finished product.

Products whose measurements meet the criteria are automatically removed from the line, while products that fail are sorted into a dedicated reject box. We also use the "QC Pro" automated measurement software to record, calculate, and analyze measurement data automatically.

This reduces human error and variation in judgment, while building a quality control system that makes inspection data easy to search and trace.

Prototype Consultation and Transition to Mass Production

When considering a new development project, or converting an existing coil to a bobbinless design, confirming that the design is sound with a prototype before moving into mass production is an essential step.

PRONEC supports prototype production of bobbinless coils to your specified requirements. In some cases, if a similar existing sample is already available, we may also be able to provide it for evaluation purposes.

Checking bonding condition from double-heater winding and dimensional accuracy at the prototype stage allows us to identify potential issues before the transition to mass production. PRONEC supports the entire process, from prototyping through to building mass-production lines in Thailand, to minimize development risk and support the launch of new products.

※For more details, please download the document or get in touch through the form below.

Frequently Asked Questions

Q1: Can we consult about dimensional variation during hot-air winding as early as the drawing stage?
A1: Yes. Before prototyping, we assess the dimensional changes that may result from wire specifications, number of winding turns, and winding conditions, and propose winding parameters such as tension and traverse pitch for your review.

Q2: If we use the double-heater method, is there an additional cost for annealing?
A2: No. Because double-heater winding completes the bonding process during winding itself, a subsequent annealing step isn't needed — eliminating the additional cost and working hours that annealing would otherwise require.

Q3: Can the transition from prototyping to mass production be handled in Thailand?
A3: Yes. We can handle everything in Thailand, from modifying winding machines and producing dedicated jigs through to the various tests required, based on the conditions confirmed during the prototype stage, to support a smooth transition to mass production.

※For more details, please download the document or get in touch through the form below.