Applied processes and techniques in winding wire manufacturing

[12, UBA Germany, 2002] [54, BMLFUW Austria, 2003] [64, EWWG, 2004] [76, TWG, 2004]

[76, TWG, 2004]

This is a small industry and in Europe there are only about three producers of the machines used to make winding wires. The same technology will therefore be found in nearly all installations.

The manufacturing process for enamelled wires is generally linear: after the production of the bare copper wire by drawing or rolling, thermal cleaning or annealing of the bare wire is carried out, both of which are solvent-free processes. Enamel coating, drying and curing of the resin and then lubricant application are the following steps. Solvent use only occurs during enamel coating, and, depending on the specific process, lubricant application; drying and curing also gives rise to solvent emissions.

BVT-Merkblatt Oberflächenbehandlung mit Lösemitteln 105

3.2.1 Typical manufacturing process for enamelled wires A typical manufacturing process for enamelled wires is shown in Figure 3.1.

Wire rod (original dimension)

Rolling/drawing

Annealing

Coating

Enamelling oven

Lubricant application (only on round wire)

Repeated coating loop

Enamelled wire (final product)

Solvent

Thermal oxidiser

Lubricant Heat energy Enamel Heat energy

inert gas

Solvent

Solvents in use Bare wire in desired dimension

Annealed, clean wire

Figure 3.1: Typical manufacturing process for enamelled wires [64, EWWG, 2004]

Rolling and drawing

These are deformation processes. Rolling is a continuous or stepwise forming process under compressive conditions. The material can be rolled at ambient or elevated temperatures. Wires for enamelled and other insulated winding wires are preferably manufactured by a drawing process. The wire is drawn through dies which reduce its size and extend its length. Fine wires require several drawing stages to reach the desired dimension. The drawing process applies tractive and pressure stress to the drawn material.

Annealing

The wire drawing process is followed by annealing where the wire passes through heated tubes (containing an inert atmosphere to prevent oxidation) and leading to recrystallisation and softening of the wire.

106 BVT-Merkblatt Oberflächenbehandlung mit Lösemitteln Enamel coating

The next process step is enamel application followed by the drying and curing of the enamel film. The coating is normally applied to the moving wire by passing the wire continuously through a bath of enamel and metering the amount by an accurately sized die or by controlled application through felt pads. Each layer applied is typically 1 - 10 µm thick, depending on the wire diameter, to allow the solvents to evaporate as the wire moves through the enamelling oven.

Drying and curing

The coated wire then passes into the enamelling chamber, which consists of a heated chamber (horizontally or vertically arranged) where the solvent is evaporated before moving into a higher temperature zone (400 - 700 ºC) where the film is cured. The wire may then go back into the coating circuit for an additional layer of coating. In this continuous coating process, up to 30 applications of enamel may be applied until the desired layer thickness is obtained.

Recirculated airflow ovens are in use for contemporary wire coating processes. The direction of the recirculating airflow can be with or against the direction of the moving wire. Recirculating the airflow has the advantage that the vented volumes can be decreased. The solvent evaporation process leads to evaporation of the solvent and the air/solvent mix is usually treated in a catalytic oxidiser which ensures that residual solvent concentrations are below legal threshold limits (typically 20 – 30 mg organic C/Nm³). The process heat from the thermal oxidiser can be used in the drying process for the heat up of the circulating airflow (loop).

In gas heated machines, solvent-laden air is passed directly into a gas burner, where the design of the burner allows intimate mixing of the secondary air and oxidation occurs. To ensure that the solvent concentration does not rise beyond the lower explosive limit, it is important that excess air is supplied. Using electrical heating, the solvent-laden air is passed over heating elements to reach a temperature in excess of 500 ºC. This is generally sufficient to achieve complete oxidation. In all cases, electrically heated machines use precious metal catalysts. The efficiency of oxidation is sufficient to ensure that emission control limits for VOC emissions are always achieved. After thermal or catalytic oxidation of the solvents, the air stream is recirculated to provide complete or partial heating of the enamelling chamber. Exhaust gas velocities may be lower than other industries to ensure a maximum energy efficiency balance from the use of recirculated air. It is important that any oxidation takes place where the air stream is recirculated. Oxidation in the stack will produce only hotter exhaust air.

Contemporary ovens in use for very fine enamelled wires work with radiation or a combined radiation and circulating airflow.

The enamelling chamber is typically maintained under negative pressure to allow capture of any fugitive emissions from the enamel supply system and to contain any degradation products or products of combustion from entering the workplace air. An enclosure around the enamel applicator further reduces fugitive emissions from liquid enamel.

Lubricant application

Depending on the final product requirements, the enamelled wire may be lubricated before being wound onto a reel. Several lubricants contain different volatile organic solvents.

Typically paraffin is applied from an organic solvent with a solvent content from 98 to 99.9 %.

Lubricants, as concentrated emulsions, with a solvent content of between 50 – 95 %, water-based emulsions or even solvent-free hot melts are also used in this industry, though with limited success. With respect to the use of solvent-free alternatives, customer quality requirements with respect to thickness and evenness of the lubricant film have to be respected.

The lubricant is generally specified by customers. Generally, it can be stated that high quality requirements and thin wires are less appropriate for solvent-free lubricants. Wherever possible, customers should be encouraged to use products such as self lubricating enamels or solid wax.

BVT-Merkblatt Oberflächenbehandlung mit Lösemitteln 107 An alternative to solvent application may be the application of a molten lubricant to the wire surface by a lubricant wetted felt or by feeding the finished enamelled wire with a lubricant filament that melts due to the residual heat of the wire. The later technique applies the lubricant by looping the filament 2 – 3 times around the enamelled wire and moving it in the same direction as the wire, although at a lower velocity. Both techniques are not yet suitable for applying lubricants to fine wires with diameters of less than 0.1 mm.

3.3

Current consumption and emission levels in winding