The role of wet blasting in the post-processing of AM parts. Colin Spellacy, the Head of Sales at Rosler in the UK describes the processes which are taking place in the traditional manufacturing paradigm.
Additive manufacturing continues to disrupt the traditional manufacturing paradigm, and every day secures a more robust foothold as a production technology. The reason for this is driven by advances in build processes that promote increases in speed, accuracy, and repeatability of production, and therefore increased yield ratios.
As is now well-known and accepted, AM is important as it facilitates the creation of geometrically complex parts and components. AM reduces waste and allows for the creation of lighter structures with an obvious importance in the automotive, aerospace, and medical sectors. AM also allows for the simple production of replacement parts impossible or uneconomical to replace using traditional processes, meaning that machines can be repaired not replaced. All such advantages along with AM’s ability to democratise and localise manufacturing with all that implies in terms of shortened and domestic supply chains means that as a technology, its future is assured.
AM and post-processing
One area where significant issues reside in AM parts is in the surface finish as they come out of the build chamber or off the build plate. Whether plastic or metal, AM-produced parts require primary post-processing processes to remove powder or physical supports. But even then, AM parts are characterised by a relatively poor look and feel, layer steps often being obvious, and surface roughness often being significantly high, which can affect aesthetics as well as functional performance. This means that in most instances, they require secondary post-processing to enhance surface form and finish.
With a significant part of the cost of a finished end-use AM part being the cost of post-processing, the fight is on to develop efficient, repeatable, and automated AM post-processing techniques. A number of mass finishing technologies are already used such as vibratory finishing, tumble finishing, shot blasting, and shot peening and chemical smoothing technologies all maintain a foothold.
Wet blasting is a clean, reliable, repeatable, and accurate process that creates parts with a consistently superior surface finish than alternative processes, and is ideal for parts made using direct metal laser sintering (DMLS) and selective laser sintering (SLS). Wet blasting is when water and abrasives media work together, and is particularly well suited for the finishing of delicate, precision-produced parts. The process is characterised as being dust free, can use very fine abrasives, and uses very low volumes of abrasives due to the protective water layer. It also produces what is perhaps the most important AM post-processed part characteristic, a superior surface finish.
Wet blasting typically sees a mix of between 10-40% abrasive media to water. The slurry is pumped to the blast gun and then accelerated to a high velocity using compressed air through a blast gun nozzle that is directed at the part. The blast media impact on the workpiece creates the desired effect, be that cleaning, surface smoothing, coating preparation, cosmetic surface texturing, or peening.
Advantages of wet blasting
Wet blasting has benefits for the characteristics of the workpiece itself, but also for the processing environment. The over-riding advantage is its gentleness, but also the process produces no dust, which prevents electro-static issues and therefore removes the need to consider regulations concerning explosive atmospheres. There is also no media impingement using wet blasting, an ever-present problem with dry blasting where media particles can embed in the workpiece surface compromising surface integrity.
Dry blasting generates significant heat during post-processing which when combined with the high impact energy of the media on the workpiece can lead to the warping and bending of delicate parts. Wet blasting by contrast is a cooler gentler process that immediately washes away anything that is adhering to the work piece.
In terms of surface finish, wet blasting promotes smoother finishes while being more precise, consistent, and repeatable. Finishes are fine and uniform, with low surface roughness of 100µm Ra being easily attainable.
When looked at through the prism of exacting tolerance attainment, superior surface finish, applicability to delicate and geometrically complex parts, and repeatability, wet blasting is a compelling technology for the post-processing of AM parts and components while remaining adaptable and cost-effective.