Are cables with PUR sheathing always better than cables with PVC? And is stainless steel always preferable to die-cast zinc or plastic for connector housings? Such black-and-white thinking does not work for cables and connectors either. There are no simple rules when it comes to selecting the right materials for connection solutions.

These examples show just how big the differences are, even in the basic requirements: The food and beverage industry requires materials on which bacteria cannot take hold and which do not soften when exposed to hot steam and aggressive cleaning agents. In train carriages, on the other hand, pipes have to comply with strict fire protection standards. This naturally has consequences for the choice of materials - including plugs, cable bushings, seals and other accessories.

More than just insulation

The agony of choice begins with the insulator: not only does it have to provide electrical insulation, it also determines the quality of transmission in data transmission cables such as Ethernet. The dielectric constant of the plastic should be as low as possible, which applies to polyethylene and polypropylene. If flame-retardant insulation material is required, halogens are added to the plastic. However, halogens are prohibited where people are present - in buses, for example - because they form toxic fumes in the event of a fire, which combine with the fire department's extinguishing water to form corrosive vapors.

Manufacturers such as Lapp therefore use HFFR (Halogen Free Flame Retardant) plastics, which are non-toxic. However, they require a filling level of up to 60 %, which in turn can impair the mechanical properties of the plastic. So-called synergists are therefore a new trend: combinations of two substances that together provide better flame retardancy than either of the two starting materials alone.

Coat for protection

In the weathering test, the resistance to ageing is tested in sunlight (UV) and under the influence of ozone and climate.

© Lapp

The sheath has no electrical function, but rather protects the interior from environmental influences such as abrasion, chemicals, cleaning agents, UV light or temperature. Depending on the application, cables must be immune to lubricating oil, grease, cleaning agents or other aggressive media. PUR is considered the robust 'workhorse' of sheathing materials. However, it is difficult to process - both in the manufacture of the cable and during assembly, as the jacket is difficult to cut. One compromise that combines the high resistance of PUR with the simple processing of PVC is the 'Ölflex 408P' and 'Ölflex 409P' cable types from Lapp, for example, which have a PUR outer sheath and a gusset-filling functional layer made of PVC.

Resistance to biological influences such as microbes or fungi is important for the food industry. Sheath materials made of special TPE are ideal here; with the 'Robust' cables from Lapp, for example, microbes hardly find a foothold and the cables are easy to clean. The smooth surface of this special thermoplastic elastomer is achieved thanks to a special mixture of additives that fill microscopic gaps in the material and which remain bound in the plastic matrix even when cleaned vigorously with a steam jet.

Stainless steel - good but expensive

There is no way around stainless steel for connectors, at least when connector housings or cable/hose bushings need to withstand chemicals or cleaning agents. It does not rust and there is no coating that could flake off. However, standard V2A stainless steel is not chemically resistant enough when the food industry uses hypochlorous acid for cleaning. For such cases, V4A is a more resistant alloy that is extremely hard and can withstand knocks or cleaning with hard brushes.

The resistance of plastics to oils, cleaning agents and chemicals is tested at room temperature or elevated temperatures.

© Lapp

But stainless steel, and V4A in particular, is difficult to machine. With an untreated surface, V4A is rougher and the friction is higher, so it is not possible to turn a screw into a thread as it would get stuck. This is why Lapp subjects V4A stainless steel products to a special surface treatment that reduces friction. Stainless steel is not an option for rectangular connectors, as the metal is difficult to machine due to its hardness. It would therefore have to be milled from a solid block, which would be too expensive. For this reason, Lapp has taken a different approach: The housing of the 'Epic Ultra' rectangular connector is made of nickel-plated die-cast zinc. This material is corrosion-resistant, which is important in salt spray on oil drilling platforms or in the food industry, for example.

Plastic housings, some of which are also resistant to acids and alkalis, are an alternative. However, cost savings are not necessarily to be expected - plastic enclosures can also be expensive because special, complex materials are sometimes required. In applications where electromagnetic compatibility is important, plastic is unsuitable anyway.

Author:
Bernd Müller is a freelance journalist from Bonn.