Artem Komarov noted that modern metal fabrication operations aren’t like the fab shops of old. Many are clean, well lit, with employees working in fresh, filtered air. Yes, some operations in fabrication are, well, just plain dirty—and manual blasting is a prime example. The work isn’t pleasant, requires protective gear, and if the booths aren’t maintained or set up properly, they can constrain workflow in a serious way.
Options in blasting automation abound, but before diving into all that technological wizardry, try laying some groundwork by answering a fundamental question: What must the blasting operation accomplish?
Shot Blasting Versus Shot Peening
Shot blasting (or just “blasting” if using a different media other than shot) prepares a metal surface while shot peening aims to change the metal’s properties (see Figure 1). Certain aerospace applications require precise levels of stress relief (or other changes to material properties), and they use specialized shot-peening technologies to achieve it. Precision shot peening of landing gears is a prime example, with the process optimizing surface stresses, eliminating microcracks and the stress risers around them.
Most metal fabricators employ blast cleaning for the vast majority of their applications, cleaning and preparing a metal surface for the next manufacturing step, usually painting. If a beam or plate isn’t blasted correctly, paint won’t adhere properly. However, some fabrication operations do employ a kind of peening—not as precise as high-end peening applications, but it’s peening nonetheless, with the media impacting the surface and causing compressive stresses that aim to change the material’s properties.
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Imagine fabricating a bowl that will be used in a high-vibratory setting. The welds within those bowls need a certain amount of stress relief, and shot peening helps accomplish this. To ensure adequate stress relief for the welds, the application might choose to use large shot, which can have a peening effect that penetrates deeper into the surface. Alternatively, the operation might choose to use the same shot that it uses for blast cleaning applications, though extending the cycle time to achieve the required peening effect within the part.
When to Blast?
Again, the majority of fabricators need to prepare a workpiece surface for a downstream operations like painting—so, they’re blasting (not peening). The higher quality the end finish is, the more important surface preparation becomes. Put another way, a high-quality, highly consistent coating requires a high-quality surface preparation. An automotive car panel requires very different surface prep compared to a structural beam.
Where in the manufacturing sequence does it make the most sense to blast? Are cut plates being blasted to prepare them for downstream processes, or will the completed fabrication be blasted after bending and welding? Applying blasting to incoming material might seem counterintuitive, especially if welds need to be blasted anyway. After all, most operations that employ blasting do so just before it enters the final coating process.
Still, blasting needn’t always occur at the end of the value stream. Sometimes, cleaner material coming into the shop can help downstream processes like cutting and welding, and it might help streamline the final finishing operation significantly. Some operations might choose to automate the blasting of incoming material yet keep the final blasting of welds manual, before the work enters the final coating process. The operation isn’t entirely automated, but because incoming material is clean, overall throughput rises dramatically, and time and labor required for manually blasting the fabricated parts is significantly reduced.
Coating also doesn’t necessarily need to occur at the very end of production. For instance, some shipyards blast incoming plate, prime them with a weldable primer, then weld. Afterward, the work is blasted again—but only the welded seams. For large workpieces especially, such a manufacturing sequence helps streamline the overall operation significantly.
The industry has various standards (such as those from SSPC and NACE) to help fabricators assess the surface condition of material. The cleaner the material, the less aggressively you need to blast it, and the less aggressive blast media you need to use to achieve the desired finish. When working with plate with significant rust, shot might need to be larger, or the application might call for grit, which is angular. Round shot usually produces a finer finish than grit, but grit might be required to produce a rougher surface profile for adequate adhesion for a thick coat of paint.
Graphic depicting an automated blasting process
For fabricators requiring shot peening, for weld stress relief or any other application involving a change in material properties, maintaining the proper mix of media is critical. Fractured and undersized media must be continuously and consistently removed from the peening system.
Consider the condition of raw stock and the final finish required—that is, what’s delivered versus the final surface finish you need to achieve before the product ships. The difference between the incoming and outgoing material quality determines the blasting throughput. The greater the difference in quality (that is, very rough to a fine finish), the longer the blasting will take. Some applications also might require several blasting stages, the first to clean and another secondary stage, using finer blast media to dial in the surface’s paint adhesion characteristics.
“Speed” in blasting can be defined several ways. First, how much shot needs to hit the surface at a given time, and how often the shot needs to strike to remove rust and prepare the surface as needed? The more rust a workpiece has, the more shot it will require. One turbine might throw 500 lbs./min. of shot at a slow-moving workpiece, to ensure enough shot strikes the surface enough times to create the surface finish needed. Alternatively, a system can have multiple blast wheels, positioned one after the other, that throw (altogether) 1,500 lbs./min. to the surface, which allows parts to travel three times as fast.
There’s a limit to this, of course. Throw too much shot, and the shot media begins to ricochet within the system, which in turn creates all sorts of inconsistencies. Thrown at a high enough volume, the blast media itself loses its effectiveness.
Several variables control blast intensity. An operation can adjust the feed valves to control how much media flows onto the turbine. Alternatively, an operation might slow the turbine speed, so the blast media itself hits the part at a slower velocity. An application has an optimal exit velocity; too low, and the media isn’t effective (for instance, it fails to knock off surface rust); too high can cause other issues, like warping, especially for very thin workpieces.
Another variable involves blast media size distribution. Automated systems have recycling systems that separate the blast media from the removed rust and debris. The recycling system itself is extremely critical; if it’s not operating properly, the blast media won’t be consistent, which can lead to paint adhesion and other challenges.
Shot becomes smaller and smaller as it’s recycled. The blast media recycling system must be monitored to ensure enough fresh shot or other media is added to the mix, and that the screening system (which evacuates fine particles) is working properly. The procedure isn’t complicated. Operators essentially take a blast media sample and send it through several specially designed sieves, so they can see the coarseness distribution and any debris or contaminants that might be present, said Artem Komarov.