METAL COATED RAPID PROTOTYPE MATERIALS FOR SPACEFLIGHT APPLICATIONS

The use of Rapid Prototype (RP) processes and materials for qualified spaceflight components will present NASA with a solution to many of its unique hardware development challenges. Previously published literature has documented the advantages of RP technology, such as decreased model cost, improved schedule and design flexibility.

The technology includes a wide variety of base materials that can be chosen to best suit the application. Metallic base materials can reduce manufacturing costs for some complex parts, but are too heavy to be practical for spaceflight related structures. Polymer bases offer relatively low weight and cost, but have not been extensively used for manned spaceflight hardware because of their inability to meet NASA's stringent flammability, electrical bonding, and environmental requirements.

In order to use these products for space flight applications, specific strategies and methods needed to be developed to address these concerns. This article documents the results of a program developed by a space experiment project that, in conjunction with the NASA Glenn Research Center's Safety and Mission Assurance Directorate, has created and implemented a process control system that includes the metal plating of many of those components and certifies them for use in flight experiment hardware.

BACKGROUND

The motivation to produce space-qualified RP components came from the development of an experiment to characterize smoke created in microgravity. The project, the Smoke Aerosol Measurement Experiment (SAME), requires smooth, aerodynamic flow ducts in order to minimize the particulate losses in the system. The overall volume of the experiment hardware is limited by the internal size of its carrier, the Microgravity Science Glovebox (MSG), which also limits mass. The experiment team chose to mitigate these challenges by making parts from Stereo Lithography (SLA) material, which is an industry term for a family of RP liquid, UV cure epoxies formed into parts with heat from an infrared laser. SLA materials offer relatively higher temperature resistance, smooth non-porous surfaces, and the ability to clear unfused materials easily from internal volumes. This material can also be polished during the post processing stage to create a transparent surface that allows the inspection of inaccessible internal features, as shown in the photograph above. SAME includes both plated and unplated parts, dependant on the usage and location.

SAME RP smoke sample manifold with 1/16-in. internal passages.

Once this material was identified as the most appropriate, the SAME engineering team determined to discover a process to coat it with a flame retardant material, preferably metal, in order to meet NASA's safety requirements for flammability. Relying upon the accepted precedent of using 0.003-in. thick aluminum tape to cover flammable components for spaceflight items, the team used this dimension as the plating thickness goal.

Nickel was chosen as the exterior surface due to its hardness (making it more resistant to damage) and its inherent electrical bonding superiority.

Many methods were utilized in an attempt to plate the RP parts. Direct vapor deposition onto the parts was tried, but by itself the process could not provide a coating that was thick enough to protect the part. Furthermore, if used as a basis to provide a conductive surface for subsequent other plating, it did not adhere well enough to the polymer base to provide effective coverage.

In another approach, conductive paint was applied prior to nickel plating. The team found that a uniform paint thickness was difficult to achieve, and that non-uniformity resulted in incomplete coverage and tolerance issues. The surface bonding of the latex base of the paint to the surface of the SLA part was also an issue.

Eventually, the SAME team located a vendor that provided a solution. By a clever application of techniques learned from years of experience in the making of plated custom manufactured polymer parts, the plating company developed a multi-stage process to provide a finished nickel coating at a final thickness of 0.004 in. +0.003 in./- 0.001 in. Furthermore, because of the company's experience in the plating of irregularly shaped objects, they had the skills and technology to plate the complex geometries required by SAME.

To plate these RP components the temperatures and currents used in the plating process are carefully controlled to stay below the softening point of the SLA material. The thickness of the plating is controlled in part by the use of specially designed electrodes, shielding and current thieves that combine into a single design to provide a uniform field around the part. The process contains the following steps:

1. Cleaning: the parts are cleaned with acetone, if required.
2. Etching: the parts are then sand blasted with alumina, and cleaned with alcohol.
3. Metalization: a Room Temperature Electroless Nickel (RTEN) process is used to apply a conductive coat directly to the RP substrate.
4. Racking: this is the point where the current producing system is designed, based on the contours of the part. Considerable care needs to be taken in this step to insure that a uniform plating thickness will be achieved.
5. Electroplating with a 'high throw' copper bath.
6. Low temperature Nickel plating.

DEVELOPMENTAL TESTING

In order to determine if the process and parts would be able to be certified for flight, a series of tests was outlined through discussions with the ZIN Technologies engineering team and NASA GRC Safety and Mission Assurance Division. Two distinct types of samples were manufactured. The first was a sample coupon of the type commonly used for Upward Flammability Testing, which is a flat 2.5 in. x 12 in. x (desired thickness) 'plate' that can be hung above an ignition source. The second, used primarily for impact and long term creep testing, was a 2-in. diameter x 6-in. x 0.125-in. wall tube section. Several samples of each type were plated for use in testing.

Nickel coated RP shield.

To verify the durability and effectiveness of the plating, a plan was developed to test properties of the manufactured part.

These tests were as follows:

• Surface adhesion tests: a tape pull test based on ASTM B 571-97'03 Test 11 was performed on the various sample parts. This test involves adhering a 1-in. x 1.89-in. square of tape to the test article and quickly pulling it off at a 90° angle followed by a visual inspection for flaking.
• Impact resistance testing: A 1 lb. hammer attached to a rotating shaft is allowed to fall over a 1ft distance and impact the plated part, which is then inspected for damage.
• Bend testing: A sample coupon was deflected 2 in. downward over a 4-in. rod. In some instances, the sample did break but the plating did not significantly delaminate.
• Heat quench testing: this is a test plan borrowed from the metal plating industry. A sample is baked to a set temperature and then quenched in cooler water. The resulting temperature shock and contraction tests the adhesion of the plating.
• Thermal cycling: eight cycles were performed on the test articles from -1.1°C – 54.5°C (32.7°F – 88.3°F) with a one hour dwell time at each extreme.
• Self Extinguishment testing: The plating was intentionally removed from one end of a test coupon and it was then subjected to the same ignition source and configuration as the Upward Flame Propagation Test (UFPT) to determine if it would ignite and continue burning. It did not.

A view of SAME showing several Ni-clad RP parts, including EMI shields.

Samples were also manufactured for formal UFPT at the White Sands Test Facility. Sample coupons 0.25 in. and 0.125 in. thick with plating thicknesses of 0.003 in. and 0.004 in. were sent for testing to NASSTD- 6001 Test 7 at 10 psia and 30% O2, which was determined to be the worst case environment aboard the ISS. All passed and an 'A' rating for flammability was granted for parts manufactured in that manner.

FLIGHT CERTIFICATION PROCESSING

Based on the results from the developmental testing and the experience gained from the manufacture of previous parts, a verification plan and process was developed for the qualification of the flight parts. It involved the following steps:

• Process control. All processes were documented within the project's ability to control and define each one. A specification document was created by the plating organization to describe their process and for use as a reference within the project's CM system. Process plans were created to provide traceability for the base material for the RP process. Cure times and cleaning were controlled. The system that was implemented provides traceability for all flight parts to the process or materials source.
• Dimensional checking. Parts were checked after the initial fabrication but before being sent out for plating, with the dimensions recorded on a hard copy print. For close tolerance fits, some parts were lightly machined at this point. After the plating process, the part was re-measured and the plating thickness was checked, verified and recorded.
• Plating adhesion durability. All flight parts are tested for surface adhesion using the tape pull test described previously.
• Sacrificial parts. Sacrificial (back up) parts are provided of each type in the event that destructive testing is required to verify the thickness of the plating at a more detailed level.

All parts are thoroughly inspected before assembly into the experiment components. All non-plated parts have limited exposure to the atmosphere and ignition sources, and are heat sinked to metal.

The assembled system was rigorously tested in the lab, and followed a standard process for the verification of flight parts, including vibration testing to protoflight levels (in stowed condition), thermal cycling, EMI, Acoustic, Sharp Edge Inspection and Offgas Testing (NAS-STD-6001 Test 1).

ADDITIONAL TESTING PERFORMED

During the course of the project development, several other tests were performed on the RP material. There were:

• Workmanship vibration testing. Since SAME uses Ni-clad RP parts for enclosures, the project elected to perform workmanship vibration testing on a mass model of the largest and heaviest of those enclosures, utilizing an unplated RP part. This was done not only to test the material in a 'worst case' condition, but also to have the ability to visually detect any cracks or separations that occurred during vibe.
• Threaded insert pullout testing. Samples were manufactured and destructively tested to determine the actual pullout strength of threaded inserts installed in the RP materials. The findings were that the results had good matching with the theoretical calculation that utilized the published strength of the material.
• Tensile testing. Since the UV cure time utilized for these parts (30 min.) was at variance with the cure time listed in the strength tables for this material (90 min.), a tensile test was performed on samples made expressly for that purpose. It was found that the reduced cure time had no negative influence on the ultimate strength of the material, and experience showed that the shorter cure time parts had slightly greater toughness.

BENEFITS FROM THE USE OF RP MATERIALS

By developing and following this customized qualification process, the SAME engineering team has been able to gain many of the benefits provided by RP technology. Designs were created based on the geometry required for the part purpose, unconstrained by the limitations of tooling and manufacturing costs. This allowed designs with many complex curves and internal geometries that would not be possible if those parts were manufactured by any other means.

Analysis of the models used for manufacturing, versus the weights of the actual Ni-plated RP parts shows that the weight savings, on average as compared to aluminum are approximately 25%. This can vary, however, and is highly dependant on the actual plating thickness applied to the part. Parts that had geometries that made it more difficult to apply a uniform field were generally heavier in order to assure that there was a minimum plate thickness of 0.003 in.

Manufacturing costs are roughly estimated to be only about 15% (or less) of those anticipated for such custom, low quantity parts made from conventional materials. There are cost savings and some efficiencies of scale that can be gained from the manufacturing of multiples of identical parts since there are set-up costs associated with both the manufacturing and plating processes, but they are not as great as the ones achieved from mass production techniques. Furthermore, since the manufacturing is performed using the actual design model, paper drawings are only required for the documentation of the plating thickness and inspection. Even the most complex parts can be formed and plated in less than three weeks, including shipping. Typically, all data transfer, estimating and ordering are performed electronically, over the web.

TYPICAL USES FOR RP PARTS

The SAME project utilized primarily plated and in some instances, unplated RP parts in every situation that they determined it to be practical to do so. Most of the equipment enclosure bodies are made from Niclad RP. Typically, enclosures of this type would be hogged out from large blocks of aluminum, or made from sheet metal and machined afterwards. Rapid prototyping allowed each enclosure to be custom sized to match the needs of the specific device(s) and have individual features (such as cooling fan holes and mounts) manufactured as a part of the original build.

As referenced previously, Ni-clad RP materials were used for ducting for the smoke created by the experiment. This was especially important for the custom designed elbows placed at the inlet and outlet to the smoke storage chamber. They must not only cause a change in the direction of the flow without disturbing its laminar flow streams, but the Inlet Elbow needed to incorporate a flat lens in the outside of the bend to direct light upstream to the sample. This part, and the curves that it required, could not have been machined.

SAME also took advantage of these materials to create EMI shields. It was realized after preliminary EMI evaluation was performed that certain components of the equipment needed shields that contained relatively complex geometries. Ni-clad RP materials offered an excellent, low cost and low weight solution to this problem.

The primary use of the unplated parts in SAME was for manifolds. SAME contains two basic small internal diameter manifolds in a device called the Thermal Precipitator. Typically, manifolds are made by drilling blocks of metal, then plugging the access holes with a threaded plug.

With SLA materials, the internal passages could be configured to fit the need, without extra external perforations. This proved to be an extremely cost effective solution to this requirement.

Other possible uses not explored by the project include complex spacers to support components and/or structures inside of larger assemblies and as replacements for cast or welded parts that do not have extreme temperature or pressure requirements.

FOLLOW-ON RESEARCH

SAME has done some research and developed a certification process for Ni-clad RP materials only insofar as it pertained to and was justified for its particular mission. While the project team feels that certain useful precedents have been determined, there is a large body of research that could be performed to better understand the material, its other potential uses and its limitations.

More testing could be performed to better determine all of the physical properties of this composite material. Research could include the creation of analytical models to test and verify the correlation between them and the actual parts created. Some of this testing could be performed at more extreme temperatures to determine the usable temperature range.

More UFPT testing could be performed to determine the actual minimum thickness of metal that is required to protect a part from ignition. Since the plating is a major component of the overall part weight, this could be an important consideration.

Some concerns have been raised regarding the long term creep that the RP material has, especially at elevated temperatures. SAME has done some testing in this regard, but has made no attempt to explore the outer limits of temperature and compression caused by packing.

CONCLUSION

The SAME team, which includes the Glenn Research Center Quality Division, has developed a program that it has used to verify the expected performance of parts made from RP materials with a nickel plated surface within context of the development of a space experiment.

It has shown that, even including the expense of developing the techniques and methods for the manufacturing and the verification program, it has been a cost effective solution to the hardware requirements for SAME, and has had a significant positive impact to the overall program.

Although there are many aspects regarding these materials that could benefit from further research, the responsible organizations feel that the larger aerospace community could benefit from a wider application of this technique for manufacturing and hope to encourage that endeavor.