Wet blasting offers a highly efficient and cost-effective means of surface preparation and finishing for the aerospace industry, which is highly regulated due to the extreme need for safety. Paul Rawlinson, Aerospace Specialist at Vapormatt, looks at innovations in wet blasting and reveals how wet blasting machines out in the field that may be perhaps 40 years old can be upgraded to incorporate some of the new technology that enhances controllability and, crucially, traceability.

Wet blasting, similarly to dry blasting, is the use of an abrasive media and compressed air to achieve a desired finish on a chosen surface. However, unlike dry blasting, wet blasting uses a third element - liquid - to enhance the levels of surface finish that can be achieved. Developed in the early fifties by the Ashworth family, the introduction of liquid allowed component manufacturers to achieve much more consistent and uniform finishes with the water offering a cushioning effect. Wet blasting offers great versatility as users can control various flows and pressures to make the processing of several substrates and components of various complexities much easier.

By altering the parameters to offer a processing environment that is either very aggressive or delicate, the blast slurry can flow over components to achieve a range of finishes. Typically, aerospace users will benefit from a controlled peening effect on turbine and fan blades to achieve a gentle and uniform abrasion on composite frames and blades in preparation for bonding or painting.

At the core of all wet blasting systems is the slurry. It is the "life blood" of the process, with the pumping system its "heart". However, failure to control the concentration of this slurry can impinge on consistency if not done correctly.

Depending on the application, users will typically look to hold a concentration of 15-20% abrasive media; however, this will be continually fluctuating as media breaks down, leaving smaller particles in the process along with potential contaminants from the component being processed. To control this, aerospace manufacturers are opting to install elutriation towers into their wet blasting systems. As a patented innovation, these elutriation towers can calibrate and condition the media and slurry consistency to ensure very tight abrasive tolerances by removing particles that are not of the right size from the machine.

Based on Stokes' law, elutriation towers can be set at 1 mesh size lower than the rating in the Federation of the European Producers of Abrasives (FEPA) specification. Controlling the consistency of slurry is a particularly important element when wet peening turbine or fan blades, as a change in concentration will affect the intensity levels and potential integrity of the components.

Constantly measuring concentration is another important control element. Advanced wet blasting machines come with accurate flow meters, used to detect the slightest drop in concentration, and can automatically dose to ensure concentration stays the same. These advanced meters will continually be monitoring slurry concentration, whereas some machines may only do this every few hours. During this time, the concentration can drop significantly, causing great differences in the overall finish of the component.

However, monitoring is not simply enough. With continual information being fed back to inbuilt HMI’s pertaining to the slurry concentrations, water pH, air flows to individual nozzles, and wear on abrasive hoses and soap concentrations, advanced wet blasting systems are able to run for extended periods of time with minimal user interference. Innovations include automatic abrasive and soap dosing for controlling concentrations, automated adjustment of air and slurry pressures to ensure each nozzle is of the same process intensity, and wear tests to help plan services and preventative maintenance.

By upgrading or specifying a wet blasting machine to include self-monitoring, systems will alert manufacturers to even the slightest amount of wear and be able to advise when parts should be replaced. With a range of add-ons and optional features available, old and existing wet blasting machines can be retrofitted to bring greater process control and, ultimately, improved component quality to the aerospace industry, where meeting specifications is a key factor for success.

In the aerospace industry, wet blasting suppliers have to be able to offer highly controlled systems that can reliably and repeatedly provide the standard of finish that is demanded by OEM's and MRO companies.

The wet blasting process is established but, as with most blasting processes, often regarded as crude. As production methods and technologies have improved, wet blasting manufacturers have had to develop new and innovative ways to offer this control. A key differentiator in the provision of these developments is not only how well they work, but also how easily they can be incorporated into existing, perhaps outdated, wet blasting machines.

For Information:
Vapormatt
Monarch Centre, Venture Way
Priorswood Industrial Est., Taunton
Somerset, TA2 8DE, England
Tel. +44.1823.257976
E-mail: sales@vapormatt.com
www.vapormatt.com