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Kolsterising meets tribopolymers

Kolsterising meets tribopolymers

Austenitic stainless steel shafts that have been surface hardened using the state-of-the-art kolsterising process and tribopolymer bearings make an ideal material combination that is unbeatable in the areas of wear values and corrosion resistance. In this article, Robert Dumayne of Igus explains why they work particularly well together in food industry packaging and filling machines.

Stainless steels are widely used in food and beverage manufacturing and processing industries. Most containers, pipework and food contact equipment in stainless steels is manufactured from either 304 or 316 type austenitic stainless steels – these steels are corrosion and oxidation resistant due to the presence of chromium that forms a self-healing protective film on the surface of the steel. 

However, while austenitic stainless steels meet the corrosion resistance required in most environments, their use is limited due to their low hardness, low wear resistance and risk of galling. Since these steels cannot be surface hardened by conventional heat treatment processes without reducing or losing the corrosion resistance, an alternative hardening process has been developed in order to extend their application range, called kolsterising.

The process of kolsterising is a low temperature surface carbon diffusion treatment that improves the material properties of austenitic stainless steel without affecting its corrosion resistance. The term ‘kolsterising’ can be traced back to a surface hardening method, based on the incorporation of carbon into steel, developed by the Dutch engineer Ben Kolster in the 1980s. The British company Bodycote now offers the process for industrial use under a registered trademark of the same name.

The benefits of kolsterisation are wide-ranging and include retention of corrosion resistance, improvement of wear resistance, including inside small cavities and bores, improvement of fatigue strength, prevention of galling, and retention of non-magnetic properties. Also, no chemical elements other than carbon are added during the process. As a result, kolsterising has become the accepted industrial surface hardening process for austenitic stainless steel in all applications and industries where components are subjected to wear and corrosion, including the chemicals industry, hydraulic systems, medical technology and the food and beverage industry, where today kolsterised components are manufactured on a large scale for use in the mechanical filling and packaging of a wide variety of products. 

In an industry reliant on quick, precise processing under hygienic and cost-effective conditions, the combination of kolsterised austenitic stainless steel and high-grade plastics is expected to become the standard. However, the technological innovation to be gained from the kolsterising process remains under-used in dynamic applications. Due to the diversity and variety of applications, finding the best individual solution and combination of kolsterised stainless steel and bearings is now a major challenge. This is where Igus tribopolymer bearings come into their own. 

For more than 30 years Igus has been researching and developing high-performance bearings made of tribopolymers under the brand name iglidur. The materials are based on Igus dry-tech bearing technology; in other words, the principle of incorporating dry lubrication. This is achieved by adding solid lubricant to plastic compounds, which reduces the coefficient of friction significantly. They provide the perfect polymer plain bearing for any shaft. 

Self-lubricating bearings made from iglidur exhibit the many characteristics that make kolsterised stainless steel ideal for the food packaging industry: low wear and abrasion properties, as well as resistance to corrosion, dust and debris, and chemicals. A number of iglidur materials are of food grade quality, which means they fulfill the American FDA standard and the EU regulation 10/2011 EC for repeated contact with food.

When operated with kolsterised shafts, the tribologically-optimised iglidur bearings ensure a reliable, maintenance-free dry run, which does not generate any friction on the bearing surface. This increases the service life of both components significantly, even allowing future forecasts increasing planning reliability for the user. In addition cleaning time and the associated costs are reduced.

Igus reached these conclusions through intensive functionality tests of kolsterised and non-kolsterised austenitic stainless steel shafts together with a range of iglidur bearings in its own test lab in Cologne. Specific test runs clearly demonstrated that the use of austenitic steel hardened using the kolsterising process, produces significantly lower wear to the bearing. The experiments included various materials that are commonly used in the packaging industry. The selected bearing materials are also used frequently and field-tested in other industries: iglidur X (the high-tech problem solver for high resistance to temperatures and chemicals); iglidur X6 (the endurance runner with an extremely long service life); and RN213 (a special iglidur mixture).

The test runs took place at a high temperature of 130°C, which is typical for many applications in the food and beverage industry. Resistance to high temperatures is a key feature of all iglidur materials. At constant load and surface speed parameters and after a travel distance/time of 36km or 100 hours, all three bearings in the test exhibited a lower wear rate when they were operated using kolsterised austenitic stainless steel shafts. The wear value, measured as a linear rate derived from the wear depth (µm) per complete rotational travel distance (km), dropped by at least 43% for all three bearings when compared with untreated steel shafts. The iglidur X6 yielded the best percentage value. Its wear rate dropped by approximately 52%. Kolsterising improved the wear behaviour of the bearings significantly. The conclusion of the tests is clear: kolsterised shafts and tribopolymer bearings make an ideal bearing combination offering reduced wear values and corrosion resistance.

The use of iglidur bearings together with kolsterised stainless steel is just one area of application for tribopolymers and this area has not yet been used to its full advantage in packaging or filling technology. The basic properties of corrosion and chemical resistance (including aggressive cleaning agents) perfectly meet the hygiene regulations of this industry. Material resistance guarantees interference-free operation without production shutdowns over the long term. Plastic components are also lighter than their metal counterparts. This lightweight design is ideal for use in high-speed applications, making the equipment more energy efficient. 

Igus offers individual tribopolymer components as well as complete systems. For those seeking design freedom, plastic bar stock with customised pre-forming can also be supplied. In the area of bearing technology, spherical bearings, drylin linear bearings, piston rings and xiros ball bearings are offered. Lightweight, easy-to-install e-chain systems can complement the system design for supplying energy and media. These systems can even be filled with space saving, oil-resistant chainflex cables or made available as pre-fabricated readychain units. 

The most recent example of the integration of various products made of tribologically optimised plastics into a complete system is the pikchain conveyor chain, a smart pick and place solution that cuts times by up to 30% and can be adapted easily to any application thanks to its modular design. 

Igus works closely with customers to realise the full potential of kolsterising for the food packaging industry. The specific requirements and problems from each application provide the basis for the development of technical and design solutions, as well as products that go beyond individual cases and are eventually incorporated into the Igus standard bearing product range.

Kolsterising improves the material properties of austenitic stainless steel and provides packaging machine manufacturers with a wide array of new design options. Machines and components become more corrosion-resistant and wear-resistant. When used in combination with iglidur bearings, these technical advantages turn into economic advantages with reduced maintenance times, cleaning times and downtimes, as well as a longer service life.

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