Effective tailoring of raw materials yields optimum strength

Over its lifetime, a retractable cooker control knob gets pushed in and pulled out thousands of times. This puts enormous stress on the mechanism responsible for this movement. In addition to the excellent slip properties needed for easy operation, these knobs must exhibit high mechanical strength and high torsional strength – the basic prerequisites for a long service life.

Hurst + Schröder GmbH are one of the world's leading manufacturers of illuminated and non-illuminated control knobs for cookers and ovens, washers and dryers, commercial cooking appliances, heating units, switch units and small appliances. Scores of renowned manufacturers of household appliances, including such names as BSH Bosch and Siemens Home Appliances, appreciate the impeccable workmanship and long service life of these products made in Werdohl, located in Germany's North Rhine-Westphalia.

On top of the strict quality assurance procedures already in place at Hurst + Schröder, BSH have developed their own method for testing the custom-designed components used in their appliances. In this method, designed to simulate the high stresses to which the components are subjected, the extended knob mechanism is clamped at the back end and exposed to elevated temperatures. A torque defined by BSH is then applied to the knob.

The mechanism must withstand this stress. When made of non‑reinforced POM, the component failed the test.

So Hurst + Schröder sought the help of Hamburg-based plastics distributor K.D. Feddersen to find a way to optimise the material. Given the material requirements, the distributor recommended a Hostaform® product with a glass-bead filling, based on an easy-flowing POM copolymer made by Ticona.

“In addition to higher mechanical and torsional strength, the primary criteria that led us to choose a glass bead-reinforced POM were the continued favourable slip characteristics (lack of slip-stick effect) compared to PBT and the extremely low warpage,” explains Stefan Oelze, application engineer at K.D. Feddersen.

“After we switched to the new material, the BSH IC-5 series cooker knobs passed the strength test with flying colours and are now being mass-produced with the glass bead-filled Hostaform®,” proclaims Willi Schwarz, design engineer at Hurst + Schröder.

Cooker knob mechanism used in the IC-5 series from BSH Bosch and Siemens Home Appliances, made by Hurst + Schröder GmbH out of glass bead-filled Hostaform®