Surface Preparation - It's a blast!01 October 2006

So what exactly is industry's hang-up with surface preparation? Unfortunately, it's that the process doesn't add any value to the finished component. However, ignoring this important operation is likely to be a very costly decision, if it jeopardises the optimum adhesion conditions required for subsequent coating operations.

The primary concern with surface preparation is the presence of surface contaminates, which come in many forms. Generally speaking, they fall into three categories: heat scale, surface discolouration and oxides; solvent soluble contaminants, such as cutting oils and silicone oils; and water-soluble contaminants, such as chlorides and sulphates from rain water (if stored outside) and from general handling (sweat).

Historically, blasting is the most immediately identifiable method of preparing surfaces before applying coatings, such as paint or lacquer. The three most popular blasting machine variants in use today are: dry blast machines, which use a centrifugal wheel (turbine) to propel the media; air blasting, which deploys compressed air; and wet blasting equipment that combines water and air (or abrasive media) to blast, clean and degrease simultaneously. Depending on the type of finish required, and the volume of work being processed, the choice of equipment is vital.

"The growing range of blast equipment now available means plant engineers can choose the machine most suited to their application," explains Mike Speak, UK general manager at Wheelabrator Group, a global designer and provider of surface preparation equipment.

"The latest generation of wheel blast machines can handle far higher volumes than previous technology," he adds, "whereas development of low-cost, entry-level machines continues to widen the choice available."

Present-day equipment offers meticulous control, particularly air blast machines, to allow the precision finishing and repeatability required for further processing. Consistency is a vital requirement in surface preparation and only component tests will identify the optimum method.

Ahead of the field
If all this sounds off-putting, it begs the question: why has blasting retained its appeal? Are there any alternatives? Well, there are, but none that surpasses blasting for speed. Blasting is also less labour intensive and therefore more cost effective than manual cleaning with wire brushes, needle guns or abrasive wheels.

Among the other advantages of blasting are:
- Thoroughness - blasting reaches the entire component surface, even corrosion or contaminants 'hiding' in surface depressions
- The impartation of a key or anchor pattern to the cleaned surface
- It is far more environmentally friendly than chemical cleaning methods.

Today, blasting is far from the aggressive, uncontrollable process only fit for removing rust and coatings from steelwork it was considered to be only a few years ago.

According to Airblast, a UK leader in the surface finishing equipment industry, advances in machine design, the introduction of new types of media and improvements in process control mean that blasting can prepare many material types and produce special surface textures. The company says that the process is now suitable for applications once unimaginable and is used in industries as diverse as electronics, medical device manufacturing, aerospace, defence, motorsport, jewellery and architecture.

Automation has also played a major role in the development of new machines. Input and output conveyors, integral loaders, and pick and place robots reduce the pressure on workers tasked with filling machines, lowering exposure to potential strains and sprains. Overhead transport systems, a popular choice for large components and fabrications, can route, position and load/unload parts or assemblies automatically. Also, most blasting equipment can now be readily integrated with other processes, such as painting and drying, to create a more streamlined operation.

"Robotics are used increasingly by plant engineers to load input conveyors with parts ready for blasting or to transport components via buffer areas before using them to load components directly into machines," says Wheelabrator's Speak. "In addition, robotics also allow more specialised or highly focused blasting to be performed on specific areas of components."

The challenge for blast-cleaning equipment manufacturers is to offer new functionality and performance that is beyond the reach of the competition. At Wheelabrator, there has been much recent focus on minimising the effects of wear, a natural occurrence with wheel blast machines.

Along with constructing critical areas from high wear-resistant materials, such as chrome and tungsten carbide, its machines can be fitted with automatic wear monitors on the deflector and vibration monitors on the wheels. The captured data is then processed by a PLC.

The efficiency of this shot-blast equipment can be optimised by performing a preliminary blast pattern analysis and simulation. This software-based process allows different intensities to be considered and, if required, to change the wheel or nozzle positions to maximise performance. It also allows the way workpieces are suspended or moved to be adjusted.

This virtual blasting process defines parameters specific to each workpiece that are then trialled in practical blasting tests. These tests must produce perfect surface preparation results and cause as little deformation to the parts as possible.

New controllable wheels give engineers the ability to modify the operation of these machines to adapt to different components, particularly intricate ones, as one setting may no longer be suitable for all. The wheels have a two-stage abrasive acceleration feature that, combined with a regular blast pattern, makes them highly efficient.

One of the latest major developments at Airblast is the introduction of stainless steel grit blast media. When preparing non-ferrous components or assemblies for painting, contamination problems meant that, until recently, the only blast media available was aluminium oxide grit. Aluminium oxide has one inherent problem: it is extremely friable, breaking down on impact to create large quantities of dust that requires extracting so not to reduce visibility and increase wear on blast nozzles, guns, hoses and couplings.

The new Hichro stainless steel grit from Airblast contains 2% carbon and 30% chrome, maintains 8 on the Mohs scale of hardness and has a bulk density of 4,750kg/m3. It is available in a range of sizes and unlike aluminium oxide, it relies on inertia to prepare surfaces, not sharp cutting edges. It is extremely durable, has a low breakdown rate and generates very little dust.

According to Airblast, the chief benefits to the customer include reduced usage: extended use by key customers has proven that, on average, for every 33 tons of aluminium oxide used, only 1 ton of stainless steel is required. Results also show that a 50% reduction in cleaning time is achieved, leading to significant savings in energy and compressed air consumption.

Major savings
Airblast offers two examples of the savings on offer. One customer, a supplier of tanks for the storage and transportation of food grade liquids and chemicals, has a blasting facility that is used to produce a profile of just 0.045mm on new and refurbished tanks prior to re-coating (profile is the distance between the highest and lowest points - peak and trough - on the surface of the metal measured after blasting).

Having switched from aluminium oxide to Hichro stainless steel grit 12 months ago, the new company is reporting reductions of 60% in blast media costs, 70% in blast nozzle wear, 95% in waste disposal costs, and a vastly reduced maintenance programme.

A second new user of Hichro, a European repairer of shipping containers, previously used 95kg of aluminium oxide to prepare each container. This has been reduced to 2.88kg of Hichro. The company also reports that blast hoses, filter cartridges and other consumables usage has been reduced by half. The company has estimated a saving of €47,720 in year one and projected savings of €152,720 each year thereafter.

With savings such as these on offer through new blasting innovations, discerning plant engineers would be wise to think carefully about allocating future capital spend to 'value-added' processes only.

High-pressure wet blasting
Blasting is playing an increasingly important role in the aerospace sector. One application involves highly-used frames, cases, bearing housings and turbine blades that are thermally coated with a layer of special plasma coating after shot peening to improve resistance against erosion, and frictional wear and tear. As part of regular inspection and maintenance, this coating must be renewed regularly, requiring the removal of the old coating and re-preparation of the exposed surface.

For this specific application, Lufthansa Technik in Hamburg, Germany, ordered a Rösler high-pressure wet blast system,with integrated process water treatment. Not only is wet blasting environmentally friendly, but it also produces much faster results than the conventional chemical coating removal method deployed previously.

At Lufthansa, a crane loads the components to be blasted on to a turntable in the blasting chamber and a pneumatically sealed door closes automatically prior to the start of processing. The environmentally-friendly water treatment system uses proven technology from vibratory finishing that has been modified to suit high-pressure, wet-blasting applications.

The 4,000bar needed to remove the plasma coating and re-prepare the surface is generated by means of a high-performance pump and is sprayed on the component by a high-speed rotary nozzle, manipulated by a six-axis robot, at a speed of 3,000rpm. The effect achieved guarantees a quick, precise and, at the same time, gentle coating removal, without causing deformation of the component surface. Depending on the type and thickness of the coating, removal speeds of up to 75mm/sec are achieved.

SOE

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