Introduction

When entering the extruder’s crosshead, conductor cores must have a certain specified mechanical elongation and temperature. They also must be clean and dry. Therefore, heat treatment plays an important part in the quality of the final product. Maschinenfabrik Niehoff, one of the world’s leading manufacturers of machinery for the wire and cable industry, has developed a continuous resistance annealer specifically designed for copper strands.

By using this annealer, it is possible to anneal strands and to feed an extrusion line with strands having a constant temperature. The new annealer type RT 202.2 can be synchronised with all existing extrusion lines (Figure 1).

The challenge

During continuous annealing, the wires of a strand are normalised. After that, they are quenched in a liquid coolant. The risk that residual moisture remains inside the strand is high. During the subsequent extrusion process, the moisture vaporises and forms bubbles, which negatively affect the extrusion quality. Therefore, it is critical that the stranded conductor leaves the annealer in an absolutely dry condition. Moreover, the strand must have the temperature required for the extrusion process.

The solution

Niehoff’s specifically developed continuous annealer for copper strands type RT 202.2 is divided in a main annealing zone and a pre-heating zone (see Figures 1 and 2). When passing through the main annealing zone, the strand is annealed and then cooled by a coolant liquid. When passing the pre-heating zone, the strand is again heated electrically to a temperature of more than 100°C. The residual moisture inside the strand evaporates, yielding a practically dry conductor.

Figure 1: The RT 202.2 type resistance annealer for copper strands

As temperature can be still too high for the following extrusion process, the strand is cooled by using compressed air wipes until reaching a specified final temperature. In the cooling device the length of strand and therefore the run-through time of the conductor but also the cooling ventilator performance can be adjusted. Therefore, the cooling effect can be precisely controlled. A non-contact temperature measurement device located at the annealer outlet and a closed loop control system maintain the required outlet temperature. A digital display indicates the temperature of the strand before entering the extruder’s crosshead.

A variety of applications

The resistance annealer RT 202.2 is ideally suited for compacted strands which need to be annealed before extrusion. It can be used also as a pre-heating unit for strands when disconnecting the main annealing zone.

In this case, a closed-loop control system combined with the temperature-measuring head automatically regulates the outlet temperature. The annealer is driven by an AC motor with integrated dancer. Therefore, the annealing speed can be synchrionised with any extrusion line. All operating and maintenance elements are easily accessible.

Figure 2: Annealing section of the RT 202.2 resistance annealer

Technical data of the RT 202.2

The resistance annealer RT 202.2 is designed for a strand cross section of 0.22 to 2.00mm2.
The maximum speed depends on the strand cross section or the maximum strand pay-off speed.

At a cross section of e.g. 0.75mm2 the maximum speed is 1,200m/min, at a cross section of 2.00mm2 the speed can be up to 360m/min.

Sophisticated annealing technology

For decades, Niehoff is playing a key role in the development and introduction of drawing and annealing systems for wires made of copper and other non-ferrous metals. In practice, the heat treatment of copper wires by the continuous resistance method consists of pre-heating, main annealing - in which re-crystallisation of the drawn wire takes place - and in some cases post-heating. The annealing sections are broken down accordingly into three zones.

When the wire leaves the main annealing zone it is cooled with a cooling emulsion, which is then blown off by means of air nozzles. To ensure that no residual moisture remains on its surface, the wire is heated once again in the third zone (post-heating). All the moisture is thus vapo-rised, and the wire reaches the take-up station in completely dry condition. In the case of wires with a diameter of more than 0.25mm, the residual heat in the wire is sufficient to vaporise the moisture. These wires can thus be reliably annealed in just two zones, i.e. without additional input of energy.

It is a special feature of the RM series of multi-wire annealers from Niehoff that they can be automatically switched to 2-zone or 3-zone annealing mode in order not to waste any energy.

Annealing systems for different wire materials

Niehoff’s experience accumulated and the findings made during the industrial use of copper wire annealing provided an important basis for adapting these technologies so that they could also be used to anneal aluminium, aluminium alloy and copper alloy wires. The R 501 inline continuous resistance annealer for drawn copper wire can work at a maximum speed of 40m/s and has proven particularly successful. Based on the R 501, special versions for aluminium, aluminium alloys and copper alloys are available.

As these materials have different properties, the annealer design must be adapted to the respective material. Aluminium and aluminium alloys have a tendency to oxidise very quickly at high-temperatures in the presence of air and moisture. The oxide layer thus formed not only makes the wire useless - it also damages the contact pulleys in the continuous annealer. Therefore it is required that the annealing is realiably performed in a protective atmosphere. Annealing in a protective atmosphere is also necessary for copper alloys.

At the same time, it must not be forgotten that numerous alloys such as brass and various grades of bronze display poorer electrical and thermal conductivity that pure copper. Consequently, the wires need to be more intensively heated in order for the heat treatment to be successful. On the other hand, the heating has to take place at high speed because of the relatively short time spent by the wire on its way through the annealer, which in turn calls for a higher electric voltage. Furthermore, an annealer based on such a design needs a special cooling zone for fast cooling of the wires back to starting temperature.

Annealing of aluminium and aluminium alloy wires

Some aluminium wire producers are using single-wire and dual-wire drawing lines for aluminium rod, which consist of an MSM 85 rod break down drawing machine and an RA 501 continuous annealer (see Figure 3). This annealer is based on the above mentioned 2-zone annealing principle. An encapsulated protective atmosphere section through to the end of the annealing operation prevents all possible oxidation of the wire surface.

Figure 3: RA 501 continuous annealer for single-wire and dual-wire aluminium rod wire drawing lines

Each contact pulley is driven by its own AC-motor, thus resulting in a wire surface of outstanding quality and in a long life time of the contact bands. Automatic cleaning of the contact bands and an oscillating movement of the wire on the contact pulleys also help to prolong the life time. The machine is designed for maximum speed of 40m/s. The 8-wire drawing line for aluminium wires type MMH 104 can be combined with an RMA 201 continuous resistance annealer (see Figure 4).

Figure 4: 8-wire drawing line for aluminium wires with an RMA 201 continuous resistance annealer

This annealer, specifically designed for aluminium and aluminium alloy round wires, works by the two-zone annealing principle, which promotes the economical and ecological consumption of annealing current. The annealing sections are kept short, and the entire wire section up to the end of the annealing operation is fully sealed off from the outside in order to avoid that the wire surface becomes oxidised. Nitrogen is used as the protective atmosphere.

Each contact pulley is driven by its own AC-motor for optimum adjustment of the wire speed to the properties of the various wire alloys. An automatic contact pulley cleaning system and further special details in the wire guidance help the contact pulleys to achieve a long life time. The RMA 201 annealer accepts wire with a diameter from 0.15 to 0.60mm. The wire speed depends on the material and the diameter of the wire. The machine is designed for a maximum speed of 31.5m/s.

About the company

Niehoff develops and builds machinery and lines for drawing, annealing, tin-plating, bunching, spooling and braiding of non-ferrous wires, plus machines for the stranding and spooling of insulated conductors. The machines and lines are equally good at meeting the needs of ecology as well as economy. Beside all the equipment required for the production of wire and cables - except extruders - Niehoff and its partners can offer reliable consulting and comprehensive support. Services range from basic design and planning to the handing over of complete turnkey cable factories.

Maschinenfabrik Niehoff GmbH & Co. KG with headquarters in Schwabach near Nuremberg, Germany, is certified as complying with the quality standard DIN ISO 9001 and the environmental protection standard DIN ISO 14001. It has around 550 employees and is represented by subsidiaries and service companies on all the major markets.