PureFinish® is a post-processing technique. This technique allows stainless steel to be finished to a smoother surface with a low and reproducible SRI value below 0.010. In practice, this produces a surface with a very low degree of adhesion characteristics. The PureFinish® treatment has been extensively tested by TNO. TNO, a Dutch organisation is the leading institute for Applied Scientific Research. The SRI value (Soil Retention Index) was investigated because this parameter most accurately describes the relationship between surface structure, contamination and cleanliness.
Within the food and pharmaceutical industries hygiene is a top priority, not only for safety but also to prevent contamination and to ensure the quality of the product in terms of taste, aroma and appearance. The occurrence of contamination through bacteria, proteins or other elements must therefore be prevented. Process plants require a defined, homogeneous and reproducible surface for all parts that come in contact with food, beverage and pharmaceutical products.
Wet blasting and vibratory finishing
PureFinish® is an environmentally-friendly wet blasting process where media suspended in water is propelled onto the surface with velocity. Important parameters during the blasting process includes pressure, type of material, particle shape and size. Experienced engineers have also developed a process aimed at polishing smaller components using vibratory finishing techniques. During the vibratory process, grinding or polishing stones (chips) vibrate to produce a sliding, grinding movement which smooths the surface. This process ensures a high degree of homogeneity and reproducibility.
The Ra value measurement has been proven to be limited because it only provides the average value of the roughness, it says nothing about fluctuations of peaks and valleys within the surface. The formula for determining the SRI includes the Sa-value (roughness measurement of the surface in 3D), the frequency peak (Pc) and the corrugation (Mr2). The lower the SRI, the less contamination and the lower the sheer forces required to clean the surface. The latter is important to guarantee a proper “cleaning in process” procedure (CIP).
The above 3D topographic survey in thousands of millimetres is of a PureFinish® surface created by TNO. On such a small scale one sees a hilly landscape, which is created by this treatment. If we made a recording of a glass bead dry blasted surface, it would look like an Alpine region.
For further background information please click here for the optimisation of stainless steel surfaces
Post written by
Maikel van Orsouw
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