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Why Surface Finish Grades are a Key Part of Your Production Vessel

Topic: Equipment Design | Industry: General | Author: Timothy "TJ" Knob, Jr.
The surface finish grade is a crucial but perhaps overlooked component of your production vessel. The surface finish affects more than the look of your vessel; it’s also critically important to how your vessel accomplishes its intended task — or whether it’s able to at all.

A vessel’s surface finish will determine what products can be produced within that vessel. From pharmaceuticals to chemicals to food products, each industry has surface finish standards that must be met. And even beyond industry requirements, a vessel’s surface grade can have a big effect on how your product is produced.

How surface grades are measured 

At Lee Industries, we rely on a measurement called “roughness average” (or Ra) to determine the surface grades of our vessels. Equipped with a tiny needle, a tool called a profilometer drags across the microscopic peaks and valleys of the stainless steel used in the vessel, measuring the differences between the high and low points on the surface and generating an average measurement in micro inches. 

A smooth, shiny surface — a mirror finish — would produce an Ra measurement of around 6 to 12, while stainless-steel with a standard, brushed finish would likely produce a measurement of 20 to 30 Ra.

The key differences between surface grades

The most important distinction between surface grades has to do with sanitary versus unsanitary finishes.

A sanitary food grade finish typically falls in the 20 to 30 Ra range. Finishing a surface to this degree eliminates places for bacteria or other contaminants to accumulate. A rougher finish, perhaps one with exposed (unblended) welds or seams or an otherwise rough surface, could allow for food or pharmaceutical products to be contaminated during production.

A lower Ra finish can also lead to increased output from a given vessel. Even in applications that don’t require a sanitary or mirror finish, a higher-grade finish can make it easier to remove the product from the agitation surface, potentially saving a great deal of time during the cleaning process. This time savings can increase throughput and lead to higher overall yield. 

It’s not uncommon for a vessel to have multiple surface grade finishes throughout its construction. Typically, a vessel’s highest graded surface finish will be in areas of the vessel expected to come into direct contact with the product. In most cases, this means the agitator and interior surface of the vessel will receive the highest-grade finish. 

What to know about common surface finishes

The exact terminology may vary slightly depending on the manufacturer, but in general, most finish grades will fall into one of the following:

Finish Ra Measurement Application
Mirror Finish 6 to 12 Ra High-viscosity pharmaceutical and cosmetic products that may otherwise adhere to agitation or kettle interior surfaces
Sanitary Finish 20 to 30 Ra Interior and exterior surfaces in food and pharmaceutical applications
Mill Finish 100+ Ra Heavy industrial, non-sanitary products

It’s also important to be aware of how electropolishing can enhance a particular surface finish. This technique, which involves electrically charging the vessel can enhance a given finish’s non-stick capabilities, reduce friction on contact surfaces and improve long-term corrosion resistance. These high-level finishes are attractive for high-viscosity mixing applications, such as thicker pharmaceutical and cosmetic products.


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