Patent Application
20140283987
The present invention relates in general to fillers for use with fasteners used on a structure's surface, and in particular the application of a filler used to fill the void over a countersunk fastener on the surface of an aircraft.
A typical manner of fastening one structural component to another is to employ fasteners, which are countersunk so that the head of the fastener is below the surface of the component being joined. In many aerospace applications, particularly airplane manufacture, whenever a rivet or other fastener is used on an exterior surface, the fastener is typically countersunk and a filler applied to fill the void over the countersunk fastener. The filler is used to fill the void and then any excess filler material removed so that the filler is flush with the surrounding aircraft outer mold line (OML).
The fastener fill material can be a polyester-based polyurethane thermoplastic (TPU) or other appropriate polymer fill material. The fastener filler material can be provided as a large sheet from which is punched a large number of hot melt filler “dots” having a size roughly equal to the size of the void and stamped from sheets of the filler material (for example, from 0.635 mm thick sheets of the polyester-based TPU material MERQUINSA PEARLCOAT® 125K A typical fastener fill installation process involves the installer applying a filler dot over the void above the countersunk fastener. Heat and pressure are then applied to the filler dot using a heated quarter-round platen, thereby causing the filler dot to melt and completely fill the void. Any excess filler material is then removed, usually with a skiving blade, so that the filled fastener void is flush with the OML of the aircraft.
Unfortunately, this process typically requires as much as two minutes per fastener. Because a large aircraft can have thousands or even tens of thousands of countersunk fasteners on the outside surface of the aircraft, this adds up to a considerable expenditure of time and effort. At full production rates, current fastener fill processes becomes a severe bottleneck in the aircraft manufacturing flow. To keep up with demand and avoid a bottleneck in the manufacturing process, fastener fill installation requires a fastener-to-fastener fill time of 30 seconds or less.
What is needed therefore is an improved process for applying filler to countersunk fasteners.
The present disclosure is generally is directed to applying a fill material to the void over a countersunk fastener so that the void is completely filled.
In one aspect, a method for rapidly adhering filled thermoplastic polyurethane (TPU) material over a metallic surface is provided, the method comprising placing a solid volume of a TPU over metallic surface; directing a laser toward said TPU; applying pressure on TPU; and irradiating said TPU material until the material melts and adheres to the metallic surface. In another aspect, the method comprises directing a laser toward said polymer material using a near-infrared radiation laser tool comprising a laser optics and fiber; a housing for holding said laser optics and fiber and maintaining a desired distance and orientation of the laser relative to the fastener to be filled; electronics for controlling said laser; a collimator; a beam expander; and a laser shield.
In another aspect, a laser apparatus for rapid fastener fill is provided, the apparatus comprising laser optics including a fiber-optical cable for producing a beam of radiation; a housing for holding said laser optics and fiber-optical cable; electronics for controlling said beam; a collimator; a beam expander; a laser shield; and a conformal dome or a flat pressure head for holding a solid portion of a filler material in place while the beam is directed at the filler material to melt the material and applying pressure to the melted filler material by pressing the filler material between the conformal dome or a flat pressure head and a substrate surface.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings.
The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing.
This disclosure, in general, relates to rapid fill system and in particular to an apparatus and method to rapidly install a polymer fill material over an installed primed fastener head so that the fill is flush with the surrounding aircraft outer mold line (OML).
For certain types of aircraft manufacture, it is desirable that each fastener head in the outer mold line be covered with a fastener fill material, such as a filler formed from a polymer material such as a polyester-based thermoplastic polyurethane (TPU), prior to application of outer mold line primer and final finish coatings. For some aircraft, this process must be applied to all fastener heads in the outer mold line. This is currently a time-consuming, labor-intensive operation that results in a high potential for variations in installation quality from fastener to fastener. Embodiments of the present invention provide an automated handheld tool and method of installation that allows fastener fill material to be applied more rapidly, preferably in less than 30 seconds per fastener, and more uniformly. As compared to an aircraft with approximately 40,000 outer mold line fasteners filled at current fastener fill installation times of approximately 2 minutes per fastener, embodiments of the present invention provide a potential savings of 1,000 hours per aircraft.
The current prior art process for filling these types of countersink voids suffers from a number of shortcomings. The process must be done almost entirely by hand and is thus expensive and time-consuming. Also, a thorough melt of the TPU material is critical to ensuring adhesion of the TPU material to the fastener head, however, the current process does not provide the installer with any indication of installation quality other than feel. Poor adhesion can result in fastener fill materials detaching due to thermal/pressure cycling. Incomplete fill can leave air pockets that can create pressure gradients at high altitude. Non-flush fastener-fill materials above the OML may affect aircraft aerodynamics.
Preferred embodiments of an improved method and apparatus for applying a filler to countersunk fasteners should thus be able to at least partially correct some of the deficiencies of the prior art. Preferred embodiments should be able to at minimum replace manual prior art installation described above with an automated or semi-automated process. Preferred embodiments should also be able to consistently apply filler with a fastener-to-fastener time of 30 seconds or less. Further, preferred embodiments should consistently and uniformly melt the TPU material to ensure adequate adhesion of the filler material without any air pockets within the fill. Further, preferred embodiments should allow any excess fill material to be quickly and uniformly removed to produce a smooth OML.
An installation tool according to embodiments of the invention should be able to handle a wide range of TPU dot types and size, in order to accommodate many different fastener and countersink sizes. TPU dots have various configurations depending on the specific needs. For example, as shown in
Some embodiments of an apparatus for applying the filler material should also be as short as possible, preferably less than 381 mm (15 inches) in length and should weigh less than 1134 gm (2.5 lb) for ease of use, because much of the installation process will take place in confined spaces or with an operator reaching overhead. Preferably the apparatus will also be easy to position over a TPU dot for installation. Embodiments that satisfy each of these general characteristics are described in detail below.
According to some embodiments of the present invention, use of an energy radiation source—such as a microwave energy source, infrared energy source, a near-infrared (NIR) energy source, an induction heater, or a laser—for uniform heating of the TPU filler material. The melted filler material is preferably forced into the countersink cavity by way of a conformal dome that applies pressure to the melted fill material uniformly, even on curved surfaces.
Peel tests were also conducted to check the bonding strength of the TPU dot material with carbon epoxy. The standard peel test method ASTM-D 3166 was followed. The test is used for fatigue properties of adhesives using plastic adherents. In this test, two plates of known thickness are used as adherents. Adhesive or bonding material is sandwiched between two plates. The thickness of the bonding material is maintained uniform. The test method covers the measurement of fatigue strength in shear by tension loading
The two plates were installed on Instron tensile test machine load cell via clamps. Instron strain rate was set low to provide relaxation to the bonding material. The force data was collected from the load cell into a computer and recorded real time.
As described above, a method of applying heat to the TPU material should consistently and uniformly melt the TPU material to ensure adequate adhesion of the filler material without any air pockets within the fill. Applicants have determined that a variety of methods could be used to accomplish this melting step, although several of these methods suffer from disadvantages that would make their use less than optimal under most circumstances.
In some embodiments, microwave-based melting can be used to melt the TPU material by microwave radiation coupled into the microwave cavity through a dielectric material would heat the underlying TPU material providing uniform melting. A preferred microwave system consists of a microwave cavity coupled to the aircraft substrate through a conductive boot. Preferably, a plunger residing inside the microwave chamber is used to press against the melted TPU dot to force TPU material into the fastener cavity crevices. It is preferable that the dielectric constants of the plunger material and the TPU material match as closely as possible to provide improved microwave energy transfer efficiency. For example, strontium titanate, with a dielectric constant of 255, could be used as a plunger material to match the dielectric constant of the TPU material.
Modeling performed by the Applicants determined that a microwave frequency of about 750 MHz would be optimal for melting the 0.64 mm thick TPU dots. In practice, however, it has been determined much higher power levels are required. Applicants theorized that the countersunk fastener itself will act as a heat sink, transferring microwave-induced heating away from the TPU material before it reaches melt/flow temperature. This heat sink effect can be overcome with greatly increased microwave power levels. Of course, equipment size and cost, as well as operator safety, becomes an issue in using a microwave system capable of producing sufficiently high power levels to overcome the heat sink effect and adequately melt the TPU material.
In some embodiments of the present invention, induction heating can be employed. Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to joule heating of the metal. The induction heater provides high frequency alternating current to an electromagnet. Magnetic materials improve the induction heat process due to hysteresis. Heat is generated by magnetic hysteresis losses in the material with high permeability, and as a result materials with high permeability are easier to heat via induction heating. Typically, induction heaters are used to heat bulk metallic materials such as iron rods, metallic bowls, etc., but can be used in embodiments of the present invention.
The Applicants have discovered that induction heating rapidly heats metallic objects within a wire wound core to extremely high temperatures by coupling a magnetic field to a metallic object and inducing eddy currents, which in turn causes resistive heating. Several metallic materials were heated with an induction heater to 149° C. (300° F., melting temperature of TPU material) to determine their respective heating rates. Carbon steel had the fastest heating rate, reaching 149° C. (300° F.) in 5 seconds. It was also found, however, that inductive heating is prone to result in much higher temperatures, which would damage the TPU material. Temperature profile of initial heating of carbon steel showed regulated temperature of 149±6° C. (300±11° F.). In this temperature profile, the carbon steel was heated rapidly at full power to 143° C. (289° F.). After crossing the 143° C. (289° F.) threshold, the duty cycle was lowered to 30 percent to reduce the heating rate. When the temperature of the carbon steel passed the 155° C. (311° F.) threshold, the induction heater was turned off to allow the carbon steel to cool. The temperature of the carbon steel was maintained at 149° C.±6° C. (300° F.±11° F.) using this variable duty cycle technique. Inductive heating also tends to generate a large amount of heat in order to melt the TPU and thus would require increased heat shielding or other forms of extensive heat removal to keep a hand-held tool cool to the touch and safe for the operator.
In some embodiments, the TPU filler material can be heated by way of infrared energy. For example, infrared energy could be supplied to the TPU material by using an incandescent IR system (for example SpotIR® 4150 of Research, Inc.). An infrared energy source would preferably be coupled with a plunger that is used to press against the melted TPU dot to force TPU material into the fastener cavity crevices. A preferred plunger material would transmit IR radiation to allow the underlying TPU material to fully absorb the energy, which would improve melting of the TPU material. Quartz is an example of a material that readily transmits IR radiation.
While IR radiation has been found to readily heat the TPU material to its melting point, Applicants determined a number of disadvantages of using an IR heating system with embodiments of the present invention, First, plungers formed of materials such as quartz, while adequately transmitting IR radiation, tend to undesirably stick to the melted TPU material. Further, Applicants have discovered that IR radiation tends to cause some undesirable charring when the TPU material is heated rapidly. This results from the IR heating mechanism, which heats the TPU dot surface via radiation followed by bulk heating of the remained of the TPU through conduction. When the IR energy is sufficient to rapidly heat the bulk of the TPU, the surface of the TPU dot is overheated and damaged. Applicants have discovered that for many polyester-based polyurethane thermoplastic materials this charring effect is especially problematic at energy wavelengths above about 2,000 nm. As a result, a notch filter is preferably used to restrict energy output to the near-infrared spectrum (800 nm to 2,000 nm).
Heating using energy in the near-infrared (NIR) range is more desirable because energy in this range actually passes through most non-metallic materials, including quartz and Teflon®, among others, because of low molecular absorptivity by the molecules of these material. However, NIR can effectively resonate metals and thus more effectively increase the temperature of such materials. The TPU dots will actually heat from the inside out, which causes the TPU material to melt without the surface charring caused by higher IR frequency radiation.
In a preferred embodiment of a near-infrared heating unit for use in a rapid fill system according to the present invention, near-infrared energy can be generated from a 300 W NIR emitter and focused through an optical notch filter into a light guide connector using a reflector. The notch filter will preferably pass only NIR wavelengths absorbed by the fastener fill material, converting unused energy to heat. This excess heat can be removed with a large heat sink and forced-air cooling. A liquid light guide (LLG) can be used to transfer NIR energy from the source emitter to the focusing optics of the rapid fill system. A preferred LLG uses liquid crystals that can transmit radiation at a wavelength of up to 2000 nm at 15 to 80 percent transmittance. One end of the LLG is preferably connected to the NIR source and the other to a compression head, described in greater detail below, which is used to maintain uniform pressure on the TPU fill material while transmitting the NIR energy to the TPU material without a major loss of NIR intensity.
One disadvantage of this type of NIR system, however, is that the higher wavelengths of NIR (1100 to 2000 nm) are detrimental to conventional liquid light guides. Unfortunately, those higher wavelengths are the wavelengths that most effectively heat the TPU fill material. As a result, such a NIR system requires a balance between heating times and damage to the LLG components.
In some preferred embodiments, the near-infrared energy can be supplied using an infrared laser source, rather that the incandescent NIR bulb of the previous embodiment. An advantage of using a laser as an NIR energy source is that the beam coherence allows it to be readily transmitted through a fiberglass light guide for great distances (more than 100 feet) without significant losses. Examples of a laser source suitable for use with embodiments of the present invention are the diode laser (DLR) and yttrium based laser (YLR) commercially available from IPG Photonics that can emit up to 100 W of collimated radiation at 975 nm and 1070 nm wavelength. A suitable laser system is preferably coupled to a fiber optics waveguide.
Fiber-optic material is a flexible, transparent fiber made of a fused silica to transport light at significantly farther distance. Fiber optic light guide made by fused silica is usually surrounded by a transparent cladding material and wrapped by a protective material outside. Cladding material is made of lower refractive index such that the light is kept within the core by internal reflection. The fiber-optic light guide connected to YLR and DLR laser varies from 50 μm to 200 μm in diameter. This light guide cable can extend more than 30 meters in length with very minimal loss within the travel path. One end of the light guide is connected to the laser source and the other end is usually connected to an optical train to expand the beam diameter to the required size. Optical train is comprised of a collimator and Galilei or Kepler type beam expander. The beam expander can vary beam diameter from 5 mm to 25 mm.
Applicants conducted an evaluation of the temperature profile of the TPU dot material for laser system. The laser system used for the evaluation was YLR, a yttrium based system, and DLR, a diode system. A narrow hole was drilled through the fastener and a thermocouple was installed at the backside of the fastener top surface. The thermocouple wire was connected to a microprocessor, which converted analog signal to digital. The microprocessor collected signal from a thermocouple every 0.1 second. The microprocessor was connected to a laptop computer via USB connection, which was programmed to start and stop data collection from a microprocessor and save data file into a computer. The data collection started just before the laser was triggered to provide energy onto the TPU dot material and stopped right after laser was turned off. The YLR type laser provided uncollimated light on to the TPU dot and DLR laser provided collimated light on the surface.
The YLR laser provided 1070 nm wavelength continuously at 100 W maximum capacity. The temperature results obtained by YLR laser system are shown in
The distance from the edge of the collimator to the TPU dot surface was set at 90 mm. The power settings of the laser were set to 40 W, 45 W and 50 W respectively for 12 mm head diameter fasteners. Temperature recording started just before the laser triggered on and stopped when the temperature of the TPU dot reached approximately 70° C. (158° F.). The target temperature was 170° C. (338° F.) with the high limit of 200° C. (392° F.). These are the temperatures when TPU dot material changes phase and starts charring beyond the limit. As seen from the chart, the temperature rose fairly quickly to 170° C. (338° F.) within 5 seconds at 50 W setting, within 6 seconds at 45 W setting, and within 8 seconds at 40 W setting. At the end of 15 seconds of heating, the temperature for 50 W was approximately 240° C. (464° F.), for 45 W it was approximately 220° C. (428° F.), and for 40 W it was approximately 200° C. (392° F.). The temperature drop was fairly slow compared to the rise. At the end of 60 s, 50 W power was at 60° C. (140° F.), 45 W power was at 50° C. (122° F.), and 40 W power was at 45° C. (113° F.). The dot surface profiles are shown on top of the curves in
The temperature results obtained by the DLR type laser system are shown in
With the YLR type laser, TPU dot material absorbed a higher level of energy from incident NIR light in comparison to DLR type laser. This is shown in the top images in
Charring in this case appears to be related to higher energy absorption. The TPU dot material is more transparent to the DLR laser light due to lower wavelength light, which does not burn the polymer matrix of the dot. A similar conclusion can be drawn at 45 W power level. The pictures of post-melt at 45 W power shows charring for YLR laser but at the same power setting, DLR laser shows no charring. DLR laser is thus more favorable in comparison to YLR laser (
Preferred embodiments of the present invention also make use of a compression head to center a TPU fastener fill dot under the compression head prior to fastener fill installation, and allow maximum transmission of NIR energy to quickly heat the underlying TPU fastener fill material and provide enough compression to completely flow the fastener fill material, filling the fastener cavity, during fastener fill installation.
It is desirable that the compression head use an optically transparent window that allows NIR radiation to pass with little to no energy loss. The typical wavelength range for NIR radiation used in embodiments of the present invention is from 400-2,000 nm. Suitable materials for use as a compression head over this wavelength range include NIR fused silica, sapphire, quartz, and UV fused silica among others. Quartz coated with a thin layer of Teflon (less than 0.05 mm) was used by Applicants for initial experimentation.
A Teflon coating can be used with any suitable material to prevent the TPU fastener fill material from sticking to the compression head window while installing the TPU fill material. Preferably, the rapid fill system comprises tooling developed around the window including brackets to hold an NIR temperature sensor to monitor the fastener fill temperature during melting and provide closed-loop feedback to the NIR source, along with a CCD camera to visually center the compression head over a fastener cavity and monitor the material as it is installed.
Suitable compression heads for use according to embodiments of the present invention can make use of a number of techniques to center the TPU dots under the compression head, including using multiple flat TEFLON-coated quartz disks to match varying TPU dot diameters, a single staggered TEFLON-coated quarts disk that could hold several TPU dot diameters using a single compression head; or a single free Teflon sheet formed into an inverted cone using vacuum to hold several TPU dot diameters, and applying compression using a quartz plate on the back-side of the TEFLON sheet.
Installation of a TPU dot on the countersink hole fills the gap effectively; however it leaves a non-flush surface with the aircraft skin. This extra TPU dot material needs to skive off to leave a flush surface. Prior art installation methods use a hot iron to skive the protruded TPU material. This hot iron is highly inconvenient and takes greater than 30 seconds to skive off the material.
In preferred embodiments of the present invention, thermoplastic skiving blades are used to remove excess fill material. Such blades can be formed, for example, from thermoplastics with glass transition temperatures that are above the melting point of the TPU fill material. For example, blades can be formed from PEI, an amber colored thermoplastic with glass transition temperature of 216° C. (421° F.) or from PEEK, an opaque material with glass transition temperature of 143° C. (289° F.). The retention of mechanical properties at high temperatures for PEEK and PEI make them ideal candidates for skiving. Both materials show good mechanical properties, high impact strength, and a high degree of chemical resistance. Both materials are widely used in medical, chemical, and aerospace industries.
These thermoplastic skiving blades are preferably used with some type of oscillating tool that vibrates the blade, allowing it to cut through the TPU material quickly and effectively. Several different blade designs were evaluated including double bevel and single bevel.
After skiving with different bevel types, it was found that the fastest, cleanest skive resulted from a single bevel design. The skiving process, including acetone wipe, was optimized to take 12 seconds. According to particular embodiments, combined with tool alignment and a dot melt recipe of 15 seconds or less, total install times including skiving are less than 30 seconds. For the smaller dot sizes, it was found that a narrow blade just larger than the dot size was ideal for optimal skive without OML surface primer damage. Larger dot sizes skived best with a larger blade that fully covered the dot material and was just greater than the dot diameter. Tailoring the size of the blade to each dot type minimized OML surface primer damage, and resulted in a flush dot surface with the primed panel.
In order to evaluate skiving blade material, vibration level, and angle of operation, Applicants performed a design of experiment (DOE—
Preferred embodiments of the present invention also make use of an alignment system, such as a sight glass and centering laser. Applicants have determined that even inexperienced users were able to show alignment times of less than 3.6 sec and accuracies of less than 0.7 mm when aligning a mock rapid fill system over installed fasteners in both upright and inverted orientations. An improved alignment system along with end-user training can be expected to further reduce alignment times and improve centering accuracies.
When the conformal dome is centered over the TPU dot, the tool is pushed forward to apply pressure on the TPU dot. Pushing the tool toward the surface brings flange 909 into contact with the rear surface of NIR shield 908. In some embodiments, there are one or more pins or electrical contacts on the flange such that the laser will not trigger unless the electrical contact is made. This ensures that the laser can only be triggered when the tool is pressed down onto a solid surface. In a particular embodiment, three contact sensor pins are mounted through the flange. The pins close an electrical connection when pressed against the NIR shield. The user must keep three pins fully depressed and also pull a trigger or activate a firing switch in order to keep laser on.
Once the laser is activated, the process should take anywhere from 4 to 7 seconds to completely melt the TPU dot inside the fastener cavity. In some embodiments, a user can then lift the tool away from the installed dot and push a second button by the handle to extend a skiving blade. A user can skive the extra material to flush the surface with OML.
In preferred embodiments, a computer program can be used to communicate with and control the laser unit for effective external control of the laser's power emission. The software is preferably able to communicate with the laser unit via simple code language allowing users to vary power instantaneously. This contributes to the ability to achieve optimal melt quality and strong adhesion to the fastener surface with high repeatability. In some embodiments, once properly positioned over a fastener, the tool can operate automatically to place a TPU dot over a sample, thoroughly melt the TPU dot, and apply sufficient pressure to completely fill the void over the countersunk fastener. In some embodiments, the tool can be placed over the correct location, such as a countersunk fastener, by hand. In other embodiments, the tool can be loaded into a robot arm which can automatically locate the next countersunk fastener to be filled and then properly position the two, before the rest of the process also proceeds automatically.
The particular heating profile, particularly the temperatures and power ramps, to be employed during automatic operation of the tool will depend, at least in part, upon the size of the TPU fastener being used. The following discussion describes a process that was employed to determine an optimum laser melt recipe for a particular TPU dot size; in this case the TPU dot was a specific for use with a 6.4 mm fastener head diameter. This process can be repeated for each size of TPU dot to be applied. Development of an optimized installation process for each TPU dot size/type involves several determinations, such as finding an upper and lower working range for the TPU dot (
The NIR laser set-up was automated through a computer program for recipe input. The laser collimator was attached to an 8× variable zoom beam expander, which was able to adjust the beam size from 5 mm (1×) to 40 mm (8×) in diameter. The beam size of this set-up is manually adjustable for various TPU dot sizes by turning a dial. For a specific size TPU dot optimization, the beam size was set to 2.5× (12.5 mm in diameter). The collimator and beam expander assembly were housed in a laser gun assembly. The laser gun assembly was mounted perpendicular to a breadboard on a test stand for ease of use. This allowed laser energy to directly target TPU dot in a vertical direction. The TPU dot was pressed against a 12 mm fastener under a 4 mm thick fluoropolymer. The fastener was drilled through from top to bottom for thermocouple insertion. The temperature data was collected via a thermocouple inserted towards the top surface of the drilled hole; the temperature of the bottom of the TPU dot was recorded in real time for each process run. The guide beam on the laser was used to align the TPU dot with the fastener head during tests.
Several TPU dots were exposed with a two-step laser power program for 12 seconds to determine upper and lower range of temperature to achieve optimal melt quality.
To minimize total process time of the recipe, it is desirable to reach the melting temperature quickly. Optimization of the initial power ramp rate was achieved by varying the initial power and observing corresponding TPU dot melt quality.
At 80 W/s ramp, 30 percent of the dot surface was charred around the center indicating very fast ramp rate. Some TPU material from the contact area formed a bond with the fluoropolymer and peeled off when lifted. At 60 W/s, TPU dot surface also charred around the center with approximately 15 percent of the TPU dot surface. At 50 W/s charring was very light and only accounted for less than 5 percent of the area in the center of the surface, indicating almost optimal ramp rate. At 35 W/s there was no charring observed. From this observation, it was determined that power ramp rate should be kept just less than 50 W/s for optimized dot quality.
In order to ensure a thorough dot melt with good adhesion, a pulsed power step was used in the recipe. This was achieved by inserting power on and off cycles for 0.5 seconds on, and 0.5 seconds off.
TPU dot melt temperature and initial power ramp are significant factors for the optimal process design. However, applied energy per unit volume of the recipe can also provide useful information. The relationship between TPU material quality (charring) and fastener surface adhesion are inversely related and an ideal process requires both of them to be of superior quality. A balance of these two factors can be achieved by observing energy per unit volume. It is anticipated that laser energy per unit volume of the TPU material would stay within a constant range across various dot sizes providing a benchmark for process optimization. During this time period, Applicants plotted a range of energy per unit volume for 12 mm diameter TPU material below which material could not melt properly and above which material overheated by showing surface degradation.
As shown in
After finding the upper and lower temperature, power ramp rate and energy per unit volume for 12 mm diameter TPU dot, an optimized process recipe was developed.
Using the energy per unit volume range established during the laser melt optimization task, laser melt recipes can be easily generated for new dot sizes. The process recipe can be developed by physically measuring each dot and calculating the optimal range of energy required for the laser melt recipe. As an example, the TPU dot with 9.75 mm diameter recipe calculation and optimization is demonstrated in
Using digital calipers, the dimensions of three dots were measured, and the total volume of the TPU material was calculated. Once the volume is known and using the energy range of 5800 to 6900 J/cm3, the lower and upper energy limit total for the laser melt process recipe can be calculated. The lower and upper limits of the calculated energy spectrum correspond with adhesion and melt characteristics. The lower energy limit may melt the dot with medium adhesion, and the upper energy limit may melt (or possibly char) the dot with a high level of adhesion.
From previous trials, it has been determined that an initial fast power ramp improves adhesion, and allows for short process recipe time. Therefore, Applicants developed process recipes with the highest power first, then decreasing in power for a few seconds, and finally holding the power level constant for the remainder of the recipe. The process recipe is developed by entering an initial power level and a total desired time frame. After adjusting these parameters and determining several recipes that fit the total energy process window, the recipes were then used to install the targeted dot with the installation tool to determine the best adhesion without charring in a short time frame (ideally less than 20 seconds).
Using this methodology, optimized laser melt recipes were determined for the 7.9 mm, 12 mm, 16.1 mm (torx) and 9.75 mm (torx) diameter TPU dots. A summary of the recipes is shown in
In addition to installation recipes, Applicants also developed removal recipes for the smaller dot sizes 7.89 mm diameter and 9.75 mm diameter. The dot removal recipe for each dot size is determined by setting the laser power level for a specified amount of time such that the upper limit of energy per unit volume (6900 J/cm3) is exceeded. In the case of 9.75 mm diameter dot, the removal recipe was 50 watts power for 12 seconds. The total energy of this recipe (600 J) exceeds the upper limit of energy (555 J).
It should be noted that beam size should also be taken into consideration for recipe development. If the beam area desired is greater than the dot surface area (by approximately more than 20 percent), energy loss outside of the dot area should be accounted for. In the case of 9.75 mm diameter dot, the beam diameter used was 11.7 mm, and the dot diameter was 9.75 mm. Applicants conducted a number of tests to measure the power output level of the laser at different beam diameter settings, and took these measurements into consideration when developing new recipes. Each system can perform differently based on specific system components such as the beam expander. The energy per unit volume requirement for each TPU dot melt recipe is used as a simple guide for recipe optimization. TPU dot melt trials are always required to optimize each recipe with an appropriate beam size, but following this straightforward method allows for ease of recipe optimization by a process engineer.
In step 79, the operator picks up skiving system. In step 80, the operator engages the skiving blade against the excess TPU dot material and removes excess prominent fastener fill material to flush with the aircraft OML. In step 81, the operator repeats this process at the next fastener site.
The invention has broad applicability and can provide many benefits as described and shown in the examples above. The embodiments will vary greatly depending upon the specific application, and not every embodiment will provide all of the benefits and meet all of the objectives that are achievable by the invention. Note that not all of the activities described above in the general description or the examples are required, that a portion of a specific activity may not be required, and that one or more further activities may be performed in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed.
In the foregoing specification, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of invention. After reading the specification, skilled artisans will appreciate that certain features are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, references to values stated in ranges include each and every value within that range.
As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of features is not necessarily limited only to those features but may include other features not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive- or and not to an exclusive- or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). Also, the use of “a” or “an” are employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.
Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments described herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
The present application claims priority from U.S. Provisional Patent Application No. 61/803,441 filed Mar. 19, 2014, entitled “RAPID INTELLIGENT FASTENER FILL SYSTEM,” naming Anjan CONTRACTOR et al., which application is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61803441 | Mar 2013 | US |
