Surface roughness is an often-overlooked dimensional aspect of the manufacturing process. While more focus is generally given to the composition of a part and its strength, or to its measured dimensions and tolerances, a surface that is too rough can result in increased friction and premature failure of a part. Going beyond mechanical operation, high purity manufacturing requires smooth surfaces within the processing equipment to avoid contamination or build-up within it. Simply put, the smoother a surface is, the less likely it is that material will stick to it. And of course, the smoother the surface, the easier it is to clean.
How Smooth Is Smooth?
We interact with smooth surfaces throughout our everyday lives. Glass windows are smooth, skin is smooth, and the touchscreen on our mobile devices are smooth. Or are they? Anyone who paid attention in biology class knows that, when viewed up-close skin is not smooth. But touch screens? Those are manufactured with a controlled roughness because too high of a gloss makes the images hard to see. Additionally, the screen would be too slippery to accurately interact with the virtual keyboard. This roughness is measurable in microns (µ), which are .001mm, or micro-inches (µin), which are .0001”. But, while the roughness allows you to interact with the display, it’s also what allows you to leave smudges on the screen.
Different Ways of Measuring Roughness
Roughness itself is a series of microscopic “peaks and valleys” across a surface. This becomes clearer when viewed in cross-section. Surface roughness is calculated measuring the average of surface heights and depths across the surface. This measurement is most commonly shown as “Ra” for “Roughness Average” and that value is used to determine compliance of equipment with various industry standards. Formally, Ra is described in ASME B46.1 as “the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length.” Ra, Rq (RMS), Rv, Rp, Rz and some other parameters are two-dimensional in nature, being only concerned with ‘up and down’ measurements in a straight line. They do not take into consideration other components of the surface topography such as flaws, errors of form, or waviness (symbolized as Sa, Sq, Sz) that would be measured in a 3D evaluation. Two dimensional roughness measurements are usually taken across any grain that might be present.
For those who are fond of math, the formula to calculate the main height over the entire measured length or area is:
1/n * SUM(ABS[Zi-Zmean] from i = 1 to n
Previously, surface roughness was calculated by “Root Mean Square” or “RMS” which used the same measure of peaks and valleys but utilized a different formula. RMS is sensitive to larger peaks and valleys, where RA is not. RMS, or Rq, will mostly appear on older technical drawings as it has been phased out in favor of RA. Additionally, RMS was typically measured in inches while RA is usually measured in millimeters in most countries other than the USA as most industries now use metric. Many drawings in the USA will show metric measurements with English in parenthesis, i.e., “0.8 (32)”. Also worth noting, the measure of the absolute distance between the highest peak and the lowest valley is shown as Rz.
Ra is measured using a profilometer. This is an instrument with a stylus that travels across the surface and measures the difference in height between the peaks and valleys of the surface profile. ISO standards use the term CLA (Center Line Average), which is interpreted identically to Ra.
Why Does Ra Matter?
As mentioned earlier, a smooth surface makes it harder for the product within the system to stick to the sides of a vessel or piping. Similarly, should free iron or other unwanted material be introduced into the system, there is less likelihood that it will become embedded into the metal and become a source of contamination. With high-purity processes, any contamination can spoil an entire batch of products. While the cost clean and purge a system can quickly add up, there is also the cost of the lost production time to consider as well. Overall, the lower the Ra, the higher-purity production application of the vessel. Not only is it easier to clean, but a smoother finish means that it is easier to empty product. The savings in time between batches can increase the throughput of product, which benefits the bottom line.
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Surface Roughness vs Surface Finish
It should be noted that Surface Roughness differs from Surface Finish. The term “Finish” is used to describe the appearance of a stainless plate or sheet and can be highly subjective. Surface Roughness is objectively measured with calibrated equipment. Stainless steel with a 2B mill finish is a bright, relatively defect-free finish produced by a final cold roll pass using polished rollers. It does not show any grain and has been compared to a “cloudy mirror” in terms of appearance. Since thinner metal is passed through the rollers more times than thicker sheets, the thinner metal usually has a lower Ra and a more uniform finish. Any defects pressed into the surface will be revealed if the surface is electropolished.
Examples of Different Surface Finishes
Generally, the higher the level of purity demanded in the product, the finer the surface finish that will be required in the manufacturing equipment. As an example, 2B finish is used in baking equipment, food processing, tanks and vessels, pharmaceutical equipment and vacuum drum dryers. It is considered as smooth or smoother than a polished #4 finish and both are acceptable for meeting USDA standards. The Ra for a 2B finish is typically 0.3 (12) to 1µ (40) depending on the gauge of the metal.
Other finishes and their roughness averages for comparison:
- A #1 finish, sometimes called Hot Rolled, Annealed, and Pickled (HRAP), is plate material as it emerges from the mill. It is very rough, in the neighborhood of 3.2 (125) to 12.5 (500) Ra and has not had any mechanical finishing work performed such as the application of abrasives.
- A #4 finish is a straight-grained finish commonly thought of as ‘brushed’ finish, as are #3 and #6 finishes. A standard #4 finish could be about 0.8 (32) Ra, while a #4 Dairy or Sanitary finish have an roughness average between 0.3 (12) Ra and 0.4 (16) Ra.
- Two other finishes; #7 and #8 are buffed. The surface of #8 is very nearly flawless. The Ra on a #8 finish would be 0.025 (1) Ra.
There are many other finishes available of course, but for bio-pharmaceutical use (injectables, otic solutions) 0.38 (15) Ra and electropolished is usually specified and is codified under BPE SF-4. Powder and tablet manufacturers may be able to use a slightly rougher surface of around 0.5 (20) Ra under the BPE SF-2 standards as it is not electropolished.
The surface finish of a vessel, as well as its Ra, determine what product can be produced within it, and as stated above, increasing levels of purity require increasingly finer surface finishes with lower Ra numbers. Each industry has specific finish standards that must be met. Sanitary food grade finishes generally fall in the 0.5 (20) to 0.7 (27) range. This range eliminates places where bacteria or other contaminates can gain a foothold. The Ra can be lowered by employing a combination of chemicals and electricity to carefully dissolve the surface of the steel. This process is known as electropolishing. Only between 5 to 10µ of material is actually removed, and that is primarily the high peaks of the surface. The results makes the valleys much shallower by comparison. The surface roughness can be reduced by up to 50%.
Electropolishing is not the correct solution for heavily damaged surfaces such as caused by physical impact, welds or chloride micropitting. In those cases, mechanical polishing such as sanding or grinding may need to be employed to reduce the Ra to near the desired range. Once that is accomplished, then electropolishing is performed. While electropolishing delivers an overall smoother surface, it also removes any embedded debris such as abrasive dust or metal fines that may have been burnished into the surface.
Ultimately, the thickness of the stainless steel also plays a factor in both Ra and electropolishing as the thicker metal is capable of withstanding more processing to achieve better smoothness.