Besides the degree of cleanliness – the removal of oil and grease, rust and mill scale, dust, and other contaminants – surface preparation specifications must also consider the surface profile and roughness relative to the coating to be applied. Rosler Metal Finishing builds shot blasting equipment to create the right surface profile on structural steel components as well as cleaning them in preparation for coating and painting.
This installment of our Structural Steel FAQ series will answer What is the optimum the surface profile for a structural steel component prior to painting and how is profile evaluated?
Optimizing for Painting
Shot blasting makes a surface rougher to increase the total contact area between paint and a work piece substrate and generally improves paint adhesion. A surface that is too smooth poses the risk of inadequate paint adhesion, while a surface that is too rough may not cover the peaks. The degree of surface profile required depends entirely on the coating to be applied.
Blade Technology: Straight vs. Curved Blades Explained
As an expert in the shot blasting industry,Rosler Metal Finishing knows about blade technology. All shot blasting machines require blades to propel media towards workpieces. While both straight and curved blades are used, each type offers advantages and disadvantages.
What’s the Difference?
Straight blades are, as the name suggests, blades that do not have curvature when viewed from the side and do not possess tangential curvature with respect to the turbine. Curved blades are blades that have some degree of curvature when viewed from the side.
As the newer design, curved blades are generally better than straight blades, but they also have some drawbacks related to longevity, maintenance, and cost of ownership.
As an expert in the surface finishing industry, Rosler Metal Finishing knows that all the expertise in the world won’t do any good if the surface of the work piece is not properly prepared. When it comes to structural steel, we receive many frequently asked questions about preparation. This installment of our Structural Steel FAQ series will answer How is the presence of dust on shot blasted structural steel components evaluated?
The Dangers of Dust
Blast-cleaned structural steel surfaces must be completely free of dust to ensure proper coating and painting.
Residual dust will reduce the adhesion of subsequently applied coatings and, by absorbing moisture, may promote the corrosion of the blast‐cleaned steel surfaces. The potential accumulation of dust is especially critical on horizontal surfaces, the interior of pipes, and in structural cavities.
Special inspections must be carried out to ensure that such areas are adequately cleaned and free from dust before painting.
Shot blast machines are often a considerable investment for companies. When these highly specialized and high investment pieces of equipment start to show signs of wear and underperformance, expert surface finishing companies such as Rosler Metal Finishing can help prolong the life and effectiveness of your investment by repairing and rebuildinga machine instead of replacing it.
Cost is often the biggest factor considered when rebuilding a shot blasting machine. Generally, rebuilds offer shorter turnaround times than buying a new machine. Rebuilds also come with the added benefit of not needing to integrate a new process since the process already includes a proven shot blasting process.
Levels of Rebuilds
The extensiveness of the rebuild process depends on your specific machine, its condition, and your expectations for longevity versus quick repair.
Different levels of rebuilds fall into three categories:
Surface preparation can account for up to 40 percent of structural steel painting and repainting jobs. As Rosler Metal Finishing’s Structural Steel FAQ series has already established, the life of anti‐corrosion coatings on a steel surface depends to a large extent
on how thoroughly this surface has been prepared for painting.
Properly evaluating the surface of structural steel surfaces for coating before and after shot blasting will help balance the cost of preparing, repairing, and monitoring structural steel throughout its impressive lifespan.
This installment of our Structural Steel FAQ series will answer How are rust and mill scale evaluated pre‐ and post‐blast?
Widely used standards were developed to visually assess the initial surface conditions and the quality of the required surface preparation relative to the initial steel surface conditions.
The dominant standards for evaluating rust and mill scale are ISO 8501‐1:2007 (based on the Swedish standard SIS 05 59 00), SSPC Vis 1‐89, and NACE. While different in some minor details, these standards are practically identical.
Shot blasting is a specialized surface finishing process where small metal (or mineral) pellets, called blast media, are thrown onto the surface of a work piece at incredibly high speeds, ranging from 200-800 feet per second. The impact on the work pieces from this process is what produces the desired surface finishing effect.
Shot blasting can help achieve surface cleaning, surface preparation, descaling, deburring, deflashing, and shot peening.
The process components of a shot blasting system include a shot blast machine, raw and finished work pieces, blast media, dust, and other byproducts.
Important factors that should be considered in the selection of any blast mediafor a particular application is the material and chemical composition, hardness, density, shape, screen size, and, last but not least, the hardness of the component to be blasted.
The term ‘media’ as used in mechanical surface finishing refers to the free flow abrasive or non-abrasive type of media which carries out the process required on the component.
So, what can be considered and used as a blast media?
Anything! That can be projected through a blasting system.
In addition to the pro and con evaluation of air blasting and turbine lasting found in our previous blog, these two methods can also be compared in terms of throwing velocity, applications, and industries.
In terms of throwing velocity. Media thrown by turbines immediately start losing speed the moment that the turbine blade releases it, producing higher intensity blast results closer to the turbine. Larger shot retains its speed better over a distance and is commonly used to maintain intensity while creating a larger blast pattern by positioning the turbine(s) farther away. In contrast, media thrown by air nozzles will continue to accelerate for the first 100-300 mm outside of the nozzle depending on blast pressure and media size and density until the compressed air fully dissipates to the ambient environment, meaning that your best blast results occur a distance away from the nozzle.
Blasting processes for surface finishing vary according to the size, quantity, composition, and desired finish of the work pieces in need of surface preparation. Air blasting and turbine blasting are two of the more common types of blasting. While there is some overlap between the two methods, each carries its own unique attributes and drawbacks.
When comparing these two blast methods, the number one thing to keep in mind is precision versus bulk. Air blasting provides precision surface preparation using a much smaller blast pattern compared to turbine blasting, which delivers large quantities of media over a wide blast pattern, thereby making it ideal for blasting large quantities of parts or larger individual parts.
“Finding a Better Way” isn’t just a tagline at Rosler Metal Finishing; it’s a mission. A recent preservation line developed by Rosler Germany is no exception.
The creation of a 740-foot-long (225 meters) shot blasting line with straightening equipment is the largest equipment project Rosler Germany has ever undertaken. Commissioned by Meyer Shipyard in Turku, Finland, the state-of-the-art equipment will be used to prepare and finish materials for building cruise ships.