in Vol. 16 - January Issue - Year 2015
Airblast Rooms and Evolution Towards Automation
Full floor media reclaim
Manual "Sweep-in" of blast media
Basic airblast room with media reclaim system
Airblast room with robot for nozzle manipulation
Recovery system with vibratory classifier for media size classification
Blast cleaning as a process started with the operator manipulating a blast nozzle in the open and cleaning his target with sand as the abrasive. The concept of an enclosure and recyclable abrasive followed, gradually progressing to recycling automation such as full-floor, partial floor etc. The industry and its users have travelled a long way since then in terms of equipment, diversity of applications, and repeatability of the process. CNC shot peening equipment in aerospace, shot peening of medical implants, and introduction of robotics in blast machines are some of the steps in this evolution. The future certainly is filled with promise. Repeatability, accuracy, consistency of finish and speed are all being addressed by equipment manufacturers. Innovation is now seen on a global platform with quality equipment being manufactured in more than just select parts of the world.
Through these developments, Empire has invested engineering time into the highly versatile yet fairly rudimentary Airblast Room. Such design improvements have made these rooms easily affordable to the smallest of blast businesses with features lending themselves to a DIY (Do-It-Yourself) concept, while others continue to extend the room¡¦s reach to blend in automation to manipulate the blast nozzle and parts being processed.
Some of the features in an airblast room offer the scope for automation. However, it has to be seen whether doing so is practical for use in Shot Peening and specialized cleaning applications.
Main elements of airblast rooms
The most basic of airblast rooms is equipped with a chute in one corner into where the operator directs the blast media upon completion of the blast cycle (or when running out of abrasive, whichever happens first!). Different stages of reclaim automation exist, from the simple chute to a full-floor recovery system where media automatically drains into the lower reclaim system components. If your expectation is total operator uninvolvement from handling abrasive, the latter is the only choice. The two common styles of reclaim system are ¡V vacuum, involving a cyclone reclaimer and exhaust fan to move the abrasive, and the second being a mechanical style with a bucket elevator and airwash separator. The former is generally used to convey non-ferrous and small-sized ferrous abrasive. Mechanical style reclaim systems are commonly used to transfer ferrous abrasives.
In addition to the reclaim system, airblast rooms are designed with means to handle the part being processed, on a work car, monorail, or in some cases using a forklift truck. If you are planning for your airblast room to assume a different role than manual blasting, start with the basic configuration and build up on it. Let us discuss some possible modifications.
Blast room for shot peening applications
Shot peening is a process that has to be monitored and measured very carefully. A slight change in process parameters could cause major fluctuations in peening results, leading to incorrect residual compressive stress being imparted on to the component and possible premature component failure, sometimes even fatal. The main process parameters to be controlled include media velocity, media shape and size, and media flow rate.
Media velocity is controlled through air pressure (direct proportionality) at the blast tank / pressure vessel with a closed feedback loop such that the tank pressure is compared with the pre-set value and the system shuts down if the variation is not within the allowable deadband. All airblast rooms are supplied with a pressure vessel. Not all airblast rooms are fitted with a PLC for controls. This retrofit to the blast tank is relatively simple, but the PLC required to enforce the feedback loop is a critical part of this retrofit, requiring control system updates.
Media size is monitored using a vibratory classifier located downstream to the cyclone reclaimer or airwash separator in case of a mechanical reclaim system. Inclusion of a vibratory classifier will ensure that consistent size of abrasive (glass bead, ceramic, steel shot or conditioned cut wire) is always available for every consecutive peening cycle. Adding a classifier is not a very difficult proposition, though minor modifications will be required to the reclaim duct or bucket elevator to house the additional height.
Media flow rate is monitored using flow control valves located at the tank outlet, one for each blast nozzle. Such flow control valves are also supplied with controllers that provide a closed loop for media flow through each outlet / nozzle.
Sophisticated peening applications with airblast rooms
Modification of an airblast room for shot peening or specialized cleaning (grit blasting, etching etc.) almost always involves automating the nozzle movement. This is accomplished through a (i) roof-mounted nozzle carriage retrofit, or (ii) robot mounted inside or outside the airblast room. Both styles of nozzle handling arrangements ensure accurate and repeatable positioning of the blast nozzle with respect to the component being peened or cleaned. The importance of the optimum blast angle and consistency of standoff distance cannot be stressed enough. Automated handling systems ensure that these variables are maintained within the required tolerances.
Nozzle carriages are preferred when the part geometry is not very complicated, and when the part style can be expected to be the same over large volume runs. With the development of blast cleaning and shot peening equipment, machine users are starting to appreciate the flexibility of industrial robots over nozzle carriages. This is also true for job shops and manufacturing facilities that experience a wide variation in part styles (and sometimes sizes). An industrial robot, whether mounted inside or outside the airblast room, provides the flexibility to follow the contour of a part with relatively easy 'teach' techniques. Therefore, inclusion of a robot in an airblast room is a very effective means to convert your room to a highly flexible, automated machine. Such rooms will have to be modified to allow robot arm entry (in case of an external mount robot), flexible seals and proper protective cover for the robot to minimize damage from dust and abrasive.
The simple airblast room that has served your purpose for decades can be given a new life when converted to peen or clean in auto mode with these retrofits. That said, it is also not reasonable to expect such retrofits to be always feasible. Current rate of utilization and condition of the airblast room may not make it viable to carry out any modification.
After any such retrofit, the room can continue to be used for manual operations as well. However, change in media size / type on a regular basis to accommodate this flexibility is not advisable. Media contamination can wreak havoc on your peening results and saturation curves. Therefore, you will need to consider (a) the financial feasibility of adding a classifier, flow control valves and air pressure monitoring arrangements, and (b) utilization factor in auto mode, before embarking on this challenging modification.