Using new and innovative technology can be risky, but it can often be rewarding. European electric-motor manufacturers Bosch, Siemens and Volkswagen realized this in 2011 after discovering hot crimping.

For decades, these companies used machines or chemicals to strip the enamel coating off the ends of magnet wires before crimping them to copper ring terminals or sleeves. Hot crimping, in contrast, vaporizes the insulation during crimping of the wire and terminal or sleeve.

By eliminating the stripping step, hot crimping enables manufacturers to increase productivity. Equally important, hot crimping improves weld strength by minimizing stress on the copper in the wire and terminal.

These two benefits have convinced Bosch, Siemens and Volkswagen to increase their use of hot crimping in 2012. Equally impressed with hot crimping is an American company in the motor industry that will begin using the technology this summer, and several U.S. automakers, which are testing it.

A Type of Resistance Welding

In 2010, Strunk Connect Automated Solutions GmbH & CoKG introduced hot crimping after years of improvement. The technology is based on the company’s resistance welding process, which has been used by manufacturers in the automotive, medical and solar industries since 1984.

Similar to resistance welding, hot crimping relies on heat generated by the electrical resistance of the material being welded and the force used to hold the materials together during welding. Unlike resistance welding, hot crimping uses specially designed welding heads that join the magnet wire to the terminal with nearly zero contact resistance.

“This produces very high conductivity without any voltage drop,” says Heinz Bockard, managing director of Strunk Connect Automated Solutions. “The hot-crimped joint has very high tensile strength and will not weaken over time due to vibration or temperature. It is one solid piece. If you try to pull it apart, you will break the wires but not the weld.”

Hot crimping can join standard magnet wire, high-frequency magnet wire (for current above 10 kilohertz) and uninsulated copper wires. Hot crimping can weld individual magnet wires as thin as 30 AWG or a wire bundle as thick as 180 square millimeters (stranded or braided).

Magnet wire is an insulated copper or aluminum electrical conductor used in electromagnetic equipment like motors and transformers. The wire is wound in coils to generate an electromagnetic field. A copper terminal, which can be coated with tin, nickel or silver, is attached to the free end of the wire so it can be connected to a source of electrical power.

How it Works

Hot crimping can be done using semiautomatic benchtop machines or fully automated systems, which only require the operator to input three application-specific welding parameters (current, pressure and time) before activation. Cost can range from $65,000 to $500,000.

When using a benchtop welder, the worker places the coil wound with magnet wire into the machine, inserts open wire ends in the terminal and activates the weld cycle. Within milliseconds, the welding heads release enough current to produce a temperature of about 700 C at the terminal. This temperature is hot enough to vaporize the insulation and weld the wire ends to each other and the terminal. A fume extractor exhausts the vapors.

Once welding is complete, a two-stage cool down of the terminal begins. Within a few seconds the terminal cools down to about 100 C. After a few more seconds, it cools to 45 C and can be manually removed from the machine.

Semiautomated systems have a small pallet that moves the component into and out of the hot-crimping machine. The worker inserts the wire ends into the terminal, activates the machine and removes the component from the pallet after it cools. Fully automated systems also automate the insertion of wire ends into the terminal prior to welding.

All welding data is stored in the machine and can be easily accessed. Manufacturers can use this data to quickly verify or change welding parameters, or for product traceability.

Initially used on transformers for hybrid vehicles, hot crimping now welds wires used in a wide range of motors, stators, generators and chargers. One manufacturer hot crimps 20 or more 16 to 20 AWG wires at one time. Another manufacturer hot crimps wire for electric motors ranging from 1 to 350 hp. A third manufacturer crimps wire for a generator that is 3 feet in diameter.

Manufacturers interested in hot crimping can receive a welding sample by sending Strunk the terminal and connecting wire or wires. Strunk will send back a sample weld within two weeks.

For more information, call 860-227-0683 or visit Strunk also will be displaying the hot crimping process at the 2012 Electrical Wire Processing Technology Expo in Milwaukee on May 9-10.         

A Primer on Magnet Wire

Magnet wire is used in a variety of electrical equipment, including motors, transformers, inductors and speakers. Magnet wire is made of copper or aluminum and coated with a very thin layer of insulation. The insulation can be enamel or a polymer film (polyurethane, polyamide, polyester or polyimide).

Most magnet wire is made of fully annealed, electrolytically refined copper. Aluminum magnet wire is less common. This is because it has a lower electrical conductivity, and its cross-sectional area must be 1.6 times wider than copper wire to achieve comparable DC resistance.

Smaller-diameter magnet wire (20 to 36 AWG) tends to have a round cross-section. Thicker wire is often rectangular (with rounded corners) or square to more efficiently use available winding space.

To ensure the best connection between the wire and terminal, the insulation must be removed. Strunk Connect Automated Solutions GmbH & CoKG makes hot crimping machines that vaporize the insulation during crimping without damaging the copper.

However, hot crimping is not the only way to remove insulation from magnet wire. For example, Spectrum Technologies has designed a laser-based system to completely and cleanly remove enamel insulation. Designed for high-volume applications, the modular system can be used as a standalone process or integrated into existing production lines.

Alternatively, the Eraser Co. Inc. offers a variety of mechanical and chemical methods for stripping magnet wire insulation.

Available in benchtop or portable models, wheel-based machines gently abrade insulation from the wire. Some units use fiberglass wheels to strip the wire, while others feature abrasive wire-brush wheels.

“Fiberglass wheels rotate at high speed and generate frictional heat that softens the insulation,” says John Smith, lab technician for Eraser. “Then the ‘nap’ of the wheel wipes it away, producing a clean polished surface with no risk of nicking the copper.”

Wire-brush wheels are more abrasive than fiberglass wheels and remove insulation by the cutting action of their many bristles. These wheels are recommended for use on thicker insulations and larger-diameter wires where a roughened surface is more acceptable.

Machines with stripping blades also come in benchtop or portable units. These machines can perform window and close-up stripping of magnet wire.

Window stripping, which removes insulation and exposes the bare wire at a location other than the ends, can be accomplished by removing the machine’s strip length stop. Close-up stripping is possible because the machine’s blade tips are located directly behind the see-through safety guard. Blade depth, pressure and strip length are adjustable on all units.

Eraser also offers eight chemical-stripping pots, which hold caustic chemicals heated to 70 to 800 F. A worker dips the wire ends in the pot for a specified time before removing the wires and rinsing them in water.

Smith says Eraser’s machines are used in many industries, including automotive, military, medical and speaker manufacturing. Eraser’s customers include automakers General Motors and Ferrari, as well as speaker manufacturer Bose.

“Stripping many magnet wires at once remains a big challenge for manufacturers,” says Smith. “Some of our customers process litz wire, which can consist of hundreds of fine strands of insulated magnet wire. Our machines can strip hundreds of wires at one time—wires as small as 30 AWG.”