Vol. 3, No.3 Fall, 1998

A QUARTERLY NEWSLETTER FOR THE AIRCRAFT PAINT STRIPPING INDUSTRY

Increasing Aircraft Dry Stripping Efficiency Through Automation Presented by John Oestreich, CAE Electronics Ltd., at the 1995 DOD/ Industry Advanced Coatings Removal Conference. Written by Pierre Dicaire and John Oestreich. Reprinted with permission from Proceedings of the 1995 DOD/Industry Advanced Coatings Removal Conference.

ABSTRACT Recent development efforts for Starch Media Dry Stripping

(SMDS) confirm that significant gains in production efficiency are achievable through automation. The primary benefits of automa­tion and its impact on stripping operations are discussed in quanti­tative terms. The paper demonstrates that the automation of the SMDS process provides major reductions in stripping cycle times at competitive operating costs. Most importantly, this approach avoids the need to dedicate a hanger to dry stripping of transport aircraft.

INTRODUCTION Until 1994, development programs and industrial use of

Starch Media Dry Stripping (SMDS) processes were carried out manually. This paper will examine the major efficiency improve­ments possible through the automated application of SMDS. Future industrial benefits from the Closed-Cycle (automated) application of SMDS on large aircraft will also be reviewed. Finally, the economics of automated SMDS will be presented and compared to chemical paint stripping.

BACKGROUND Since the commercial introduction of EnvirostripTM Starch

Media in 1990, developments have led to its efficient, problem-free use on aircraft components and airframes. Dedicated industrial users such as Beech Aircraft and Northrop Grumman have helped to advance the SMDS process. The creation of the Envirostrip Test Center in Montreal has provided a test site for further process validation. In late 1994, CAE Electronics Ltd. of Montreal initiated the development of automated systems for the application of the SMDS process. CAE, a world leader in commercial flight simulators and control systems, will market these systems and the Envirostrip Starch Media worldwide.

In 1994, Boeing approved the use of SMDS on thin metallic skins for an unlimited number of strip cycles'. Boeing's evaluation showed no degradation in metal fatigue-life with repeated applica­tion of the SMDS process. Since then, Boeing has published its multiple-cycle approval for SMDS on composite structures2

• For the first time, a single paint stripping process can be repeatedly and safely applied on the most sensitive Boeing aircraft structures. Similar approvals are expected from AIRBUS and other major OEM's later this year.

Until recently, the development and use of the Starch Media Dry Stripping process had been carried out in a manual mode. Labeled «operator tolerant" because of its low potential for damag­ing metallic structures, SMDS has not required tight control of application parameters. However, recent development work with SMDS in the automated mode has shown productivity gains which translate into shorter cycle times and major savings in labor, media and waste disposal costs.

During the last five years, companies in North America and Europe have built and demonstrated full-scale development proto­types of mobile, closed-cycle, automated dry stripping systems for large aircraft. Each new prototype has shown improvements in effi­ciency and capability over its predecessor. The feasibility of apply­ing SMDS in a closed-cycle automated mode of application has been demonstrated throughout these programs. The strengths and weaknesses of different design options are now better understood and will benefit the development of the next generation of systems.

EFFICIENCY ADVANCEMENTS FOR SMDS Three recent developments have improved the productivity

ofSMDS: 1) Media particle size control

Tests performed by ADM/Ogilvie demonstrated that proper media size management can lead to increases in paint stripping efficienci. Paint removal rates simifar to those achievable with plastic media blasting (PMB) are now possible with Starch Media. Regular replenishment of working media and proper

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particle size classification ensure optimum paint removal efficiency. This productivity is achieved without any detrimental effect on clad surface roughness or residual stress measurements.

2) Media flow rate regulation Early process development efforts clearly demonstrated the benefits of regulating media flow rates for certain applications. A stable, non-surging media flow is essential for the safe use of SMDS on sensitive composite materials. In the automated mode, the tight control required for parameters such as media size, nozzle pressure, nozzle distance/angle, and end-effector traverse­speed must be matched by similar control of media flow. Properly-engineered media flow regulating devices have recently been developed through the efforts of several dry stripping equipment manufacturers.

3) Efficient nozzles Until 1994, round venturi-type nozzles were used for aerospace dry stripping applications. These nozzles were developed in the early 1950's for rugged sand-blasting applications at operating pressures of 90 psi and higher. When used with lighter abrasive media at lower pressures, however, these nozzles produce a "hot spot". This drawback limits the productivity of dry-stripping processes and leads to inefficient use of media and compressed air. This has been overcome through re-designed nozzles which disperse the flow of media particles more uniformly across the spray pattern.

Advanced Computational Fluid Dynamics (CFD) software facilitates the design of innovative nozzles with complex geometries. The most promising prototypes converge the media particles into a flat, uniform, spray pattern three inches in width.

The nozzles developed by CAE using CFD software have been designed primarily for automated SMDS application. They must travel over the surface at a precisely controlled travel speed while maintaining nozzle orientation, angle, and distance. Unfortunately these nozzles are currently not practical for use in a manual mode because an operator cannot adequately control their orientation and travel speed. Operator fatigue and uncomfortable working positions amplify this problem.

Table 1 shows that a 2: 1 increase in strip rate is achieved with automated use of flat nozzles versus hand-held round venturi nozzles. For example, when stripping a USAF MIL-spec paint system (MIL-C-83286, MIL-P-23377), paint removal rates double with the newly designed flat nozzles when compared to conventional round nozzles. Strip rates on the Boeing paint scheme (BMS 10-79, Type 2, BMS 10-60 type 2) can be increased to 3.0 fe/min with automated flat nozzles.

AUTOMATION ELIMINATES ERGONOMIC INEFFICIENCY When a hand-held process is used to strip large aircraft,

typical production data show that no more than 60% of optimum strip rates (shown in Table 1) is obtained. The operators' stripping efficiency during actual paint stripping is measured by taking the total depainted surface area divided by the total number of operat­ing nozzle hours. This reduction in performance can be attributed to operator fatigue and poor ergonomics of working around large aircraft. Automated systems on the other hand achieve maximum strip rates by eliminating ergonomic factors.

The production efficiency of manual processes must also take into account total labor-hours associated with the dry strip­ping task. Activities such as recovery of media from the hanger floor, rotation of operators to reduce fatigue, housekeeping, and equipment supervision add considerable labor-hours to manual stripping operations. The overall production efficiency in terms of strip rate must then be measured as the depainted surface area divided by the total labor-hours involved with the dry stripping task. Alternatively, one can look at the total time a worker is operat­ing a nozzle, which we can call the "on-stream" ratio. This on­stream time is invariably less than 50% of the total labor -hours used for dry stripping.

With automated systems, on-stream ratios in excess of 80% can be achieved depending on the number of systems employed. Automation improves nozzle efficiency, eliminates ergonomic fac­tors, and provides higher on-stream ratios, delivering a potential sixfold efficiency gain over conventional hand-held round nozzles. This efficiency improvement directly impacts media consumption/ cost and air compressor utilization.

As a result of these product~vity advantages, the automated SMDS process will be among the lowest cost aircraft paint-stripping processes available for the foreseeable future.

Table 1. Comparative stripping rates (complete paint finish removal)

Paint System Round DV Nozzle Manual (fe/min)

Boeing*

USAF MIL-Spec**

* BMS 10-79, Type 2, BMS 10-60 type 2 ** USAF Mil-C-83286,Mil-P-23377

30 psi

0.9

0.6

40 psi

1.2

1.0

Aircraft Paint Stripping News a Fall, 1998

Flat Nozzle Automated (fe/min)

40 psi

3.0

2.0

BENEFITS OF CLOSED-CYCLE/DUST-FREE SYSTEMS A contained process using closed-cycle, dust-free applicators

offers several advantages. Large hangers do not need to be dedicated to the dry stripping operation, allowing other maintenance tasks, including painting, to be performed in the same facility. These mobile systems can also be used in different locations, performing partial to complete stripping of airframes without totally disrupting other maintenance activities.

An automated closed-cycle process contains the paint and media dust within an end-effector enclosure, preventing emissions to the work environment. This method of applying the SMDS process avoids facility modifications and investment in large venti­lation systems needed for an open-blast dry-stripping facility. It should be noted that meeting anticipated EPA particulate emission standards would require High-Efficiency Particulate Air (HEPA) dust filters on ventilation systems. Reductions in facility housekeep­ing, facility downtime, and main~enance costs for process equip­ment are added benefits.

The closed cycle feature facilitates control of several process parameters. A compliant closed-cycle end-effector must travel in constant contact with the aircraft surface to maintain the seal required for a dust-free operation (see Figures 1 & 2). The blast nozzles are rigidly attached to the compliant blast/recovery enclo­sure. Nozzle orientation, angle, and distance to the surface are there­fore maintained at optimum values at all times. With such tight control over process parameters, the maximum possible paint removal rates are obtainable.

Figure 1. CAE Automated Closed-Cycle Dry Stripping End-Effector

Media management, important for efficient use of Starch Media, is simplified when SMDS is applied in a closed-cycle mode. Media contamination risks are reduced since the media is not exposed to external contaminants. Approximately 3,000 Ibs of media are used at any given time with each mobile automated sys­tem versus the more than 100,000 Ibs used for large multiple nozzle open-blast systems.

When dry stripping in an open-blast environment, masking the aircraft to prevent media ingress typically requires as many labor-hours as the paint removal operation itself. Masking require­ments are significantly reduced with closed-cycle SMDS. For example, when only the fuselage of commercial aircraft are being stripped, wings and engines will not have to be masked.

The total time to strip an aircraft will be compressed due to the fact that closed-cycle dry stripping can proceed concurrently with masking and de-masking operations. Using several automated systems simultaneously will allow operators to further reduce total strip times.

ECONOMICS OF AUTOMATED CLOSED-CYCLE SMDS The economics of any paint -stripping technology should

address the operating costs, crew size, cycle times, and investment pay-back.

The operating costs of automated SMDS for stripping a C-135/KC135 size aircraft are presented in Table 2. The cost per square foot of $3.33 USF compares very favorably to methylene chloride and nonHAP chemical stripping processes.

Aircraft Paint Stripping News II Fall, 1998

Table 2. Operating Costs for Automated Closed-Cycle SMDS (in US Funds)

Aircraft: C-135/KC - 135 (10,000 fe)

Item Quantity Unit Cost Cost Starch Media 7,5001bs $ 1.40/1b $ 10,500 Sanding Material $ 2,000 Masking Material $ 3,000 Waste Disposal (incl. paint) 7,8001bs $ 0.50/1b $ 3,900* Direct Labor Masking 120 labor hours $ 30/hour $ 3,600 Stripping (Automated) 96 labor hours $ 30/hour $2,880 Hand Sanding 111 labor hours $30/hour $3,330 Demasking 24 labor hours $ 30/hour $ 720 Supervision 48 labor hours $ SO/hour $ 2,400 Power 10,000 kwh $ O.lO/kwh $1,000

Total Cost (per A/C) $ 33,330 Total Cost $/fe) $3.33

Assumptions: • 90% of the aircraft stripped via automated, closed cycle SMDS • 10% of the aircraft stripped via hand-sanding, or closed-cycle manual SMDS • Strip-rates: Automated SMDS = 2.0 ft2/min/nozzle • Hand-sanding = 0.25 ft2/min (dustless)

* Note: Several users of Starch Media are fInding that the dry waste from their SMDS operations can be classifIed as non-toxic according to environmental regulations; they can therefore dispose of this material as general industrial waste with a net cost to strip a C-135/KC-135 of $2.95 US/ft2.

CREW SIZE For a C-135/KC-135 aircraft, application of closed-cycle

automated SMDS and manual sanding activities can proceed simul­taneously with masking and de-masking. As shown in Table 3, a projected crew of nine (9) workers per shift will be capable of completely masking, stripping, and demasking a C-135/ KC-135 aircraft in seven (7) shifts using one automated system.

STRIP CYCLE TIME Several production scenarios for C-135/KC-135 aircraft are

presented in Table 4. The automated SMDS operation, applicable to 90% of the aircraft surface, dictates the duration of the aircraft stripping operation. All other activities (Le. preparation, masking, demasking and hand-sanding) can be conducted concurrently with automated stripping.

This analysis shows that a stripping operation based on one automated system can complete a C-135/KC135 aircraft within 56 hours, or less than three days. By comparison, methylene chloride­based chemical stripping requires 4 to 5 days to strip the same air­craft4

•

An additional reduction of 1 to 2 days in the critical path of the aircraft maintenance cycle is possible when the automated SMDS operation is overlapped with other maintenance tasks, or when several automated stripping units are used simultaneously.

PAY-BACK Direct operating costs for methylene chloride/phenol

chemical stripping5 can range from $3.00/ft2 to $8.00/ft2, while non­HAP chemical stripping, if effective, can range from $5.00/ft2 to $1O.00/ft2. As presented in Table 2, direct operating costs for Automated Closed-Cycle SMDS are expected to be in the range of $2.95 US/ft2 to $3.33 US/fe as shown in the example with a C-135/ KC-135 aircraft.

Automated, closed-cycle SMDS is very competitive with chemical paint stripping alternatives. Payback periods relative to currently used processes will depend on: l. The number of automated systems used (purchased or leased) 2. The savings with automated SMDS relative to current stripping

costs 3. The number and size of aircraft stripped per year 4. The value of reduced strip-times/downtime for an operator

Aircraft Paint Stripping News II Fall, 1998

Table 3. Crew size Activity Phase of Strip Operation

Crew Size workers/shift

Preparation and Masking ...... ................. ........ .. .4 Stripping Automated SMDS ............. ..... ......................... ... 2 Manual Sanding .............. ...... ... .................... ... .. 2 Direct Supervision ..................... ..... ........... ....... 1

Total Crew ... .. ... .......................... ...... .................. 9

Table 4. Automated Closed-Cycle SMDS cycle times

C-135/KC-135 (10,000 fe)

No. Automated Systems 2 3

Calculated strip-cycle (hours) 48 24 16

Repositioning "penalty" (hours) 8 4 2

''Adjusted'' strip-cycle (hours) 56 28 18

No. (8-hour) shifts 7 3.5 2.25

No. (3-shift) days 2.25 1.2 0.75

Figure 2. CAE Automated Closed-Cycle Dry Stripping System

CONCLUSION The Starch Media Dry Stripping process has the broadest

technical acceptability of all aircraft paint stripping processes. It offers unsurpassed, long-term compliance with foreseeable environ­mental and worker health and safety (OSHA) regulations. Recent technical advances for the efficient use of SMDS combined with the availability of mobile, closed-cycle, automated application systems now offer the best overall economics for stripping large aircraft.

REFERENCES 1. Boeing Commercial Aircraft Group, Boeing Process Specification

BAC-5725, "Stripping Organic Materials" (All models, All ' Airplanes), Seattle, WA, Sept., 1993.

2. Boeing Commercial Aircraft Group, Boeing Composite Stripping Specification - Wheat Starch D656993, Seattle, WA, March, 1994.

3. Oestreich, J., An Investigation of Envirostrip Starch Media Coating Removal from 2024-T3 Aluminum Alloys, 1994 DoD Advanced Coatings Removal Conference, New Orleans, LA, May, 1994.

4. See, D.W. et al., "Large Aircraft Robotic Paint Stripping (LARPS) System", 1992 DoD Advanced Coatings Removal Conference, Orlando, FL, May, 1992.

5. Wasson, E., "Dry Stripping the C-5 and the B-52 in the World's Largest Dry Stripping Installation", 1993 DoD Advanced Coatings Removal Conference, Phoenix,AZ, May, 1993.

Aircraft Paint Stripping News II Fall, 1998