How Cables Are Made - Step 6: Jacket Extrusion

When producing cables and wires, every little detail counts. In the sixth part of our series, you will learn how the jacket is manufactured and why it is important.

Sheath is applied with an extruder

The jacket consists of plastic that is liquified and applied with an extruder.

Multi-conductor cables - whether with or without braiding - require a jacket. This is what makes a cable complete, as it holds together its various components such as the conductors, braiding and filling materials. However, this is not the jacket’s one and only purpose. Jacketing also protects cables from mechanical stresses such as bending and torsion and makes them resistant to heat, cold, fire, chemicals, water or oil.

In order to fulfill the requirements of every application, the right jacketing material must be selected. The range of materials available is wide: in its production facilities, HELUKABEL works with more than 80 different plastics. The most common ones are:

  • Polyvinyl chloride (PVC): Often selected for its low cost and overall resistance to chemicals. However, its mechanical resilience is limited, and it is not always oil resistant.
  • Polyurethane (PUR): High-quality plastic resistant to abrasion, oil and chemicals, suitable for dynamic applications.
  • Thermoplastic elastomers (TPE): Resistant plastics suitable for high mechanical loads and for use in damp and wet rooms.

The wall thickness of the jacket can be between 0.2 and 5 millimeters, but most commonly, it is between 0.8 and 1.2 millimeters. HELUKABEL's thinnest jacketed cables have a diameter of just 2.4 millimeters, while the thickest ones reach a remarkable 78 millimeters in diameter.

Similarly to the conductor insulation process, the jacket is applied with an extruder. This consists of a metering unit for the plastic granulate, a screw that transports and heats the material, and a cylinder with a spray head that applies the viscous plastic uniformly to the passing cable. Afterwards, the cable runs through a water bath where the jacket cools down and hardens. The jacketed cable is tested with laser cameras and high-voltage testers and finally coiled before it reaches the final step in production: labeling.

Cable gets cooled down in a cooling trough
When the plastic has been applied, the cable runs through a water bath where the sheath cools down and hardens.

Ask the expert

Mr Sefuenc, what is the difference between the conductor insulation and the jacket?

It’s very simple: The conductor insulation has the task of galvanically separating the individual elements in multi-conductor cables. It is therefore applied to each conductor individually and influences the electrical properties of a cable such as the dielectric strength. The jacket, however, protects the inner parts of a cable from harmful environmental impacts as well as from mechanical stress. It has no electrical function. The manufacturing process, though, is the same: both involve a thin layer of plastic that is applied with an extruder.

What factors must be considered during the jacket extrusion process?

There are many parameters that have to be considered: for example, the diameter of the cable itself. The thicker the cable, the stronger the jacket must be to withstand all loads. Moreover, the wall thickness must be as uniform and concentric as possible to ensure harmonious functioning and machine processability of the cable, for example when stripping.

Portrait of Servet Sefünc
Servet Sefuenc has been Product Group Manager of Jacket Extrusion at the Windsbach plant since 2017. The certified industrial foreman (plastics/rubber) has been employed at HELUKABEL for more than 20 years.

How do cables actually get their individual color?

The color concentrate in the desired shade is added to the plastic granulate in the extruder's dosage unit. In the screw, under the impact of heat, a homogeneous and viscous mass is formed, which is similar to honey. This is applied using a spray head so that the cable comes out of the machine in the color ordered by the customer. Almost all imaginable RAL shades are possible. For some materials, special color batches have to be produced.