ADM Abrasive Media Products
- EnviroStrip® Wheat Starch
- EnviroStrip XL-Corn Hybrid Polymer
- eStrip® GPX (starch-g-acrylic)
ADM abrasive media products are used for a variety of coating removal applications. The product line is designed to remove paint, primer, sealant and adhesive from aerospace substrates. Although somewhat similar to other abrasive blasting technologies, ADM abrasive media products meet stringent aerospace industrial blasting application requirements. This operational guideline is a basic overview of areas requiring attention when using ADM abrasive products for aerospace applications.
2.0 Process Equipment Considerations
The following describes the process equipment typically used for EnviroStrip and eStrip media. Each installation will include most of the equipment described herein, sized and specified to meet the application requirements.
Proper process equipment will accomplish the following:
- Recover and classify media to within the correct particle size range as determined by the application.
- Remove dust and paint particulate from recovered media mix.
- Clean and remove dense particle contaminants from recovered media (hangar applications).
- Use dry, oil-free, quality compressed air.
- Minimize process downtime for media recovery or pressure pot replenishment.
- Operate problem-free – FOD free
- Deliver consistent, repeatable process parameters to the nozzle (nozzle pressure, media flow).
2.1 Media Recovery
Recovery of media, whether from a blast room floor recovery system, hand cabinet or a closed-cycle unit, is performed pneumatically with a cyclone/blower combination. A centrifugal or positive-displacement blower provides negative pressure, conveying the media through a pneumatic recovery line.
As recovered media enters the cyclone separation stage, very fine particulate of media and paint dust are removed and sent to a dust collector. Airflow through the cyclone is usually regulated by mechanical adjustment, controlling the amount of media dust and paint removed by the cyclone. The remaining reusable media spirals down through the cyclone and is returned to the system for further processing.
On some systems, a rotary air lock or similar air lock device located at the bottom of the cyclone meters the media to a vibratory screen. The air lock allows the media to leave the cyclone without affecting the negative pressure in the recovery line or the cyclone itself. The air lock also regulates the flow of recovered media onto the vibratory separating device.
2.1.2 Vibratory Media Classification
Recovered media is typically fed to a multi-deck vibratory screen. Oversized media and foreign matter are removed with the upper screen. Fines (ie. typically less than 100 mesh U.S. Std. or 0.15 mm – depending upon the application) are removed through the lower screen. If dense particle separation is required, the usable media is split into two or three streams for subsequent dense particle removal. Smaller systems typically do not utilize a vibratory classification system, relying upon the cyclone separation system to remove spent media and paint dust.
2.1.3 Magnetic Particle Separation
Ferrous material can contaminate the media. This metal particulate can come from the parts being stripped or possibly from being tracked into the room under the operators’ shoes. Process equipment, when new, can also generate ferrous contaminants that end up in the recovered media. These ferrous contaminants have a much higher density than the blast media and should be removed.
A magnetic separator, also available in a self-cleaning version, is used to treat the media as it leaves the vibratory screens and enters the storage hopper. Media falls through a bank of magnets, which attract and retain the ferrous materials as the media passes through the device. It is an important housekeeping rule to check magnets for ferrous metals and clean units to maximize their efficiency.
2.1.4 Dense Particle Separation (DPS)
If required by specification, this final cleaning step removes remaining dense particulate matter from the recovered media. Most applications do not require this step as non-ferrous materials are not typically found in blast rooms or hand cabinet applications. When stripping large structures (aircraft) in hangars it is recommended DPS units be installed due to potential contamination from the concrete floor. Aircraft OEM and USAF specifications do allow an established amount of dense particulate to be in the media.
The DPS system takes each classified media stream and passes it over a fluidized bed which separates material more dense than the media. The media travels along an air fluidized deck, usually conveyed by a vibrating eccentric motion. The cleaned media streams are then transferred pneumatically to the storage hopper.
2.1.5 New Media Addition
An important parameter of media management is maintaining proper particle size distribution. While media fines are removed from the recycled media, new media should be added on a regular basis to maintain maximum efficiency. This can be done by adding media through the floor recovery system or into the hand cabinet work area. It is recommended that media be added often (“salted in”) in small amounts to insure a good mix.
A preferred method is the automatic (mechanical) addition of new media during processing of recovered media. The ideal addition point is after media classification and cleaning has been performed, during pneumatic transfer to the storage hopper. A small adjustable volumetric feeder, attached to a new media hopper, is calibrated to add new product to the recovered media in relationship to the amount of media being removed. The new media feeder interfaces with the on-off sequence of the blast nozzle.
2.2 Storage Hopper
After the classification and cleaning steps, the media is pneumatically conveyed to the storage hopper. Hoppers are typically inclined at 60° to 70° at the bottom to encourage good media flow from the storage vessel. Hoppers may also be epoxy lined. Flow enhancement devices located at the bottom of the hopper, such as air-fluidizing devices, can also be used to keep the media from “bridging” inside the hopper. Level sensors control the media processing system to prevent overfilling of the storage hopper. The media is transferred from the storage hopper to the pressure pot system on demand.
2.3 Pressure Pot/Media Delivery System
The pressure pot and media delivery system is the heart of the blast process. Properly designed equipment is crucial for achieving satisfactory process results. The components include a single or dual-stage pressure pot system, equipped with a volumetric starch media flow valve, transfer valves, depressurization or exhaust valves, and a compressed air feed to deliver the media under pressurized air.
The pressure pots specified must meet the standards in the country of use with a rated 8 bar (125 PSIG) maximum operating pressure. For example U.S. pressure vessels are constructed to standards set by the American Society of Manufacturing Engineers (ASME). The pressure pot system should be equipped with exhaust and transfer valves, which operate problem-free when using light abrasives. Condensation on exhaust valves within the pressure pot system must be avoided. This problem can occur during depressurizations if the exhaust system is not properly designed. The pressure pot must be designed to uniformly feed media into the media flow valve.
The media flow valve should be able to deliver a media flow-rate from 5 - 15 pounds per minute (2.3 to 6.8 kilograms per minute) depending upon equipment specifications and process application.
2.4 Compressed Air Supply
The compressed air needed to deliver the media under pressure must be dry, oil-free process air. The compressed air system should be properly installed to provide air at an operating pressure dew point not to exceed 40oF. It is recommended that the compressed air unit have an after-cooler to remove moisture prior to the pressure tank. A refrigerant or desiccant air dryer, installed on the compressed air line, should be located as close as possible to the blast equipment.
From the pressure pot outlet media is conveyed through blast hoses to the blasting nozzle. Blast hoses come in various lengths and ply. Typically thick ply hoses are used from the blast equipment to the blast room. A ‘whip hose’ (thin ply, flexible short hose connected to the nozzle) should be considered for the last 10+ feet to maximize the operators ability to carry and move the hose during blasting.
Blast hoses can develop wear spots at sharp curves or corners in the line. Due to pressure drop losses and hose wear, excessive coiling or curvatures in blast hose lines should be avoided if possible.
Several types of blast nozzles exist today. Conventional round blast nozzles, which have been in existence for decades, are used for manual stripping requirements. Round nozzles are characterized by the throat geometry and include straight-bore, single-venturi and double-venturi varieties. Single-venturi nozzles have convergent-divergent throats, which help to accelerate lightweight media particles to velocities sufficient to effect coating removal. Double-venturi nozzles have the same convergent-divergent throat, however air is entrained at the nozzle exit, which spreads out the media blast trace and reduces the unwanted hot-shot found with straight-bore or single venturi nozzles. Double Venturi nozzles are preferred for most blast applications.
The next generation of nozzles recently introduced are categorized as Flat Nozzles because of their linear blast trace. These nozzles can offer unparalleled productivity. Studies show that, in certain applications, the flat nozzles achieve a 100% increase in strip rates over conventional round nozzles. The Flat Nozzle uses similar media flow rates as round nozzles while typically removing more coating per minute (significantly reducing media consumption).
3.0 Operational Considerations
Media should not come into direct contact with water. Envirostrip XL-CHP and eStrip GPX are moisture resistant and can be dried and reused if they come into contact with water. Envirostrip Wheat Starch cannot be reused if directly exposed to water. It is a good housekeeping rule to try to keep rain and/or wash water out of the blast area. Compressed air, if properly designed with air dryers, should not create a problem.
If cleaning requirements mandate water to be used in the blast area, caution should be exercised. Media floor grates should be temporarily sealed to prevent water ingress into ducting. Water should not be used directly on the blast equipment. Floors should be sweep clean of all media and plastic sheeting should be used to prevent ingress of water into hoppers or blast equipment.
3.3 Waste Disposal
Waste material generated by the process is most often found to be non-toxic and can be disposed of as industrial waste. A “leachate” test is normally performed on dust samples to determine the level of heavy metals in the total mix. Operators using ADM’s abrasive products for several years have continuously tested material and, in most cases, have determined the metals (hexavalent chrome – typically found in aircraft primers), to be under the toxic level determined by U.S. State and Federal regulations.
4.0 Engineering Controls and Safety Procedures
Ignition properties of starch based media and its dust are less hazardous than Type V plastic media. Plastic media has been used throughout the world for nearly 20 years without incident. The safety precautions and engineering design required for starch based media compatible equipment is identical to plastic media. Consequently, at the very minimum, the same equipment precautions and equipment design should be employed for both starch media and plastic media stripping operations.
4.2 Explosibility Data
Several independent studies have been produced showing the ignition properties and explosibility data of starch based media. These studies show that ADM abrasive products, when purchased in available mesh sizes, are not explosible. These products cannot be ignited. It is only the very fine dust produced from the breakdown of these products after blasting that is ignitable (material at a size less than 120 mesh size US Std. >0.125 mm), and only when reaching a very dense concentration in the presence of a significant (high) energy source. Properly designed media/dust and electrical controls offered by equipment manufacturers manage dust concentrations and ignition sources to levels far below the upper limit requirements. Secondary safety systems such as blowout panels and ducting can also be utilized.
Several hazardous assessment reports are available from ADM and will be provided upon request.
4.3 Safety Controls
- All equipment exposed to the blasting media and process environment should be properly grounded. This includes the part being stripped, the operator, the nozzle and hose. Note that when stripping composites, electric (static – cold spark) discharge is more prevalent because the non-conducting composite parts can create more electrostatic build-up.
- Properly designed ventilation of the blast booth or enclosure, as well as good housekeeping rules in and around the blast facility, helps to ensure worker safety and high productivity.
- Equipment exposed to the blast process must meet a minimum electrical standard as provided for in the National Fire Protection (NFPA) Code and other applicable government regulations. This includes any lighting, switches, or other electrical equipment directly exposed to the process. Note that standard equipment design provides for the lighting to be usually mounted outside the blast enclosure and sealed off by a dust tight/dust proof enclosure.
- Dust collectors should follow all applicable NFPA and government regulations concerning dust handling and collection.
5.0 Start-Up Guidelines
5.1Cleaning Out and Preparation of New Blast Equipment (FOD Prevention)
- Run small amounts of media (100 Lbs +-) through system to remove metal shavings from equipment caused during manufacturing or installation of system.
- Do not overload system. Use only enough media to run through and clean equipment.
- Media should be recycled several times through system to insure all metal particles are removed. Add additional media and repeat process if metal particles are still present.
- Inspect media coming from the nozzle for metal particles – Recommend blasting into barrel or other collection drum for inspection of media. Use magnet to inspect for metal particles.
- Inspect the systems magnetic separator for particles and clean as required.
- Recommend removing all media used for this exercise prior to start-up. Place into trash barrel and discard due to potential metal particles in mix.
5.2 Calibrate Blast Pressure (Nozzle Pressure)
- The blast pressure displayed on the pressure regulator at the blast pot is not the actual pressure at the nozzle. There is a pressure differential (pressure drop) between the regulator pressure and the nozzle pressure.
- The PSI for the metal bond deflash process range from 30 PSI to 40 PSI. Nozzle pressures for other applications, including aircraft paint removal, range from 20 to 45 PSI.
- Utilize a needle gauge at the nozzle to calibrate correlation between regulator pressure and nozzle pressure. Recommend recording and displaying pressure numbers in the blast area. Recommend recording regulator pressure differential at various PSI in 5 PSI increments.. Normally there is a 5 PSI to 10 PSI pressure drop at the nozzle but this should be verified.
6.0 New Operator Process and Equipment Review
Topics to be reviewed with operators prior to using process
6.1 General Discussion
- Equipment Review
- Blast Hoses
- Media Feed Valve
- Blast Pot
- Pressure Vessel Fundamentals
- Inlet/Exhaust Valve Functions
- Cyclone Separator
- Metallic Separator
- Air compressor
- Dry Dryer
- Media Storage Hoppers
- Preventive Maintenance
- Required Service to Equipment
- Basics of Abrasive Blasting on Aerospace Substrates
- Fundamentals of coating removal
- Coatings and Paint
- Substrate Types
- Media Flow
- Air Pressure
- Impingement Angle
- Distance to Surface
- Operator Technique
- Operational and Safety Issues
- Operational Start-up and Shut-Down
- Blast Area Management
- Facility Requirements (EPA, OSHA, Local Regulations)
- Blasting Protocol
- Equipment Safety Check List
- Housekeeping Requirements
- Safety Considerations
- Media Storage
- Operator Protection
- NIOSH and OSHA Regulations
- Breathing Air Certification
- Protection Clothing
- Medical Considerations
- Emergency Situations
6.2 Process Familiarization and Training
- Operators should be shown each area and step of operation as reviewed in the General Discussion section. At minimum it should include:
- Blast Equipment
- Operator Safety Gear
- Operator Technique
- Media Management
- Operational Safety
- Housekeeping Requirements
- Each operator should display a full understanding of all pre-blast tasks by actually doing each required step of operation
- Each operator should be Breathing Air Certified prior to blasting
- Each operator should commence blasting on scrap/discarded parts and shall continue until operator displays a full understanding of the blasting task
- The operator should display ability to remove coating in accordance with specifications.
- Operator must display a full understanding of safety concerns and shall not take any action that may endanger other operators.
- A Management Plan covering all key areas impacting the blast equipment and process should be constructed to insure the continuance of a safe and productive environment.
† - EnviroStrip® and eStrip® are registered trademarks of ADM.
ADM provide no warranty nor indemnity to users and expressly exclude 1) all indemnities arising in law or otherwise in respect of the media supplied under a purchase agreement and, 2) any other warranty, express or implied including without limitation and, implied warranty of safety or fitness for a particular purpose arising in law of otherwise in respect of the media supplied to the purchaser. User recognizes and acknowledges responsibility for knowing, understanding, and satisfying all applicable safety regulations and environmental laws concerning dust contamination and exposure. User personnel and environment should be protected with proper engineering controls.
The information contained herein is correct to the best of our knowledge. The recommendations or suggestions contained in these guidelines are made without guarantee or representation as to results. Our responsibility for claims arising from breach of warranty, negligence, or otherwise is limited to the purchase price of the material. Freedom to use any patent owned by ADM or others is not to be inferred from any statement contained herein.