Engineering
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Magnet Assisted Composite Manufacturing: A Flexible New Technique for Achieving High Consolidation Pressure in Vacuum Bag/Lay-Up Processes
Chapters
Summary May 17th, 2018
A new technique for applying consolidation pressure on the vacuum bag lay-up to fabricate composite laminates is described. The goal of this protocol is to develop a simple, cost-effective technique to improve the quality of laminates fabricated by the wet lay-up vacuum bag method.
Transcript
The overall goal of this procedure is to use a magnet assisted composite manufacturing or MACM technique to significantly improve the quality of wet lay-up vacuum bag composite laminates by applying a high consolidation pressure during laminate fabrication. The main advantage of this technique is that it is a low-cost, simple way to manufacture composite laminates and it can be used to fabricate large, geometrically complex parts with relative ease. This method is broadly applicable not only in wet-layer vacuum bag processes but also in other composite manufacturing techniques such as out-of-autoclave prepreg curing and vacuum assisted resin transfer molding.
Layer preparation and magnet arrangement and placement are difficult to learn without visual demonstration. We believe this presentation will help other researchers implement MACM to make high-performance composite laminates. To begin the procedure, carefully place 25 one-inch by one-inch by 1/2-inch N52 Neodymium-Iron-Boron permanent magnets in a five by five square configuration in alternating polarity on a 12-inch by 6-inch-by 3/16-inch magnetic steel plate.
Use a rotary fabric cutter to cut six plies of eight-inch by six-inch of plain weave glass fiber, then stir together 40 grams of epoxy resin and 10.96 grams of resin hardener at 350 rpm until fully mixed. Degas the resin for 15 minutes to remove trapped air introduced during the mixing process. Then, place a 0.3-millimeter thick eight-inch by six-inch aluminum caul plate pre-coated with PTFE release agent in the center of a 10 1/2-inch by 8 1/2-inch perforated release film.
Tape the edges of the caul plate to the release film with half-inch wide polyester tape. Next, fix an adhesive-backed flexible silicone heat sheet to one side of a 1/4-inch thick 400-series tool steel plate. Cover a 17-inch by 11-inch area on the other side of the steel plate with 0.003-inch thick non-porous PTFE-coated fiberglass release film.
Outline the area with 1/2-inch wide double-sided sealant tape designed for vacuum bagging. Apply just enough of the degassed resin on a six-inch by eight-inch area to wet out a single ply of the plain weave glass fiber. Place a single ply of the fabric on the resin and use a roller to press and squeeze out excess resin.
Pour additional resin on the fabric and use a squeegee to spread the resin over the fabric until the fiber bed is fully saturated. Repeat this process for the remaining five plies of glass fiber, using approximately the same amount of resin for each ply. Then, place the caul plate on a fiber preform.
Tape down the edges of the attached release film with polyester tape. Place two layers of 16-inch by 10-inch breather cloth on top of the release film. Place the bottom piece of a two-piece aluminum twist-lock vacuum valve on the breather cloth ensuring that the valve is at least six inches or 15.3 centimeters from the saturated preform to avoid contact with excess resin.
Remove the sealant tape backing. Place vacuum bagging film over the assembly and press the edges firmly against the sealant tape. Then, connect the top piece of the vacuum valve to a vacuum pump equipped with a pressure regulator.
Cut a small slit in the vacuum bag film over the bottom piece of the valve. Insert the top piece of the valve into the bottom piece, and gently twist the top piece to lock it in place without wrinkling the vacuum bag. Start the vacuum pump, wait for the pressure to stabilize at 93 kilo Pascals, and check the system for leaks.
Clamp all four sides of the bottom plate to a support base to immobilize the assembly. Allow the laminate to cure under constant vacuum for 45 minutes at room temperature. Then, carefully place the array of permanent magnets on the vacuum bag, taking care to ensure that the magnets are properly aligned with the fiber stack.
Ramp the temperature of the bottom plate to 60 degrees Celsius at five degrees Celsius per minute. Cure the laminate for eight hours at that temperature. When curing is finished, release the vacuum, remove the vacuum bag, and demold the composite laminate.
To begin the measurement procedure, secure a steel bottom plate to the movable crossbar of a mechanical testing instrument equipped with a 1, 000-pound force load cell and a linear variable differential transformer. Attach a steel top plate to the load cell. Ensure that the bottom steel plate is at least 25 millimeters from the top plate.
Then, carefully place one Neodymium-Iron-Boron permanent magnet on the bottom plate. Begin moving the bottom plate upward at one to two millimeters per minute. Record the generated magnetic force and the displacement over time.
Continue monitoring the magnetic compaction force measurements until the magnet almost contacts the top plate. Then, stop the test and calculate the magnetic compaction pressure versus gap. To begin the density determination, cut three specimens from a laminate sample according to ASTM specifications.
Place each specimen in a beaker containing 300 milliliters of an aqueous transparent heavy liquid. Place the composite specimen in the beaker and ensure that the specimen floats on the liquid. Add three milliliters of distilled water to the liquid and stir the solution at 1, 000-rpm for five minutes.
Continue adding water in this way until the composite specimen is suspended in the mixture, indicating that the density of the solution is equal to the density of the specimen. Use a specific gravity cup to determine the density of the solution. To begin the resin burn-off procedure, place each specimen in a separate porcelain crucible.
Measure and record the weights of the specimens as well as the crucibles, then place the crucibles in a furnace. Heat the furnace to 1, 112 degrees Fahrenheit or 600 degrees Celsius at about 10 degrees Celsius per minute and allow the resin to burn off for four hours. Afterwards, turn off the furnace and carefully open the furnace door.
Allow the furnace to cool to room temperature. Then, remove the cooled crucibles from the furnace and weigh the recovered glass fibers to calculate the weight fractions of the fiber and resin. The measured magnetic pressure generated by a Neodymium-Iron-Boron magnet as a function of the gap between plates agreed well with the supplier-provided data sheet, confirming that increased magnetic pressure is applied to the laminate as the thickness of the lay-up decreases during the curing process.
The moderate decreases in lay-up thickness of the two random mat laminates during consolidation resulted in notable increases in magnetic pressure. The plain weave laminate lay-up thickness decreased only slightly, resulting in a small increase in magnetic pressure during consolidation. Applying magnetic compaction pressure in the range of 0.2 to 0.4 mega Pascals significantly reduced the average laminate thickness for both plain weave and random mat glass fiber laminates.
This was reflected in a notable reduction in resin-rich areas and in void volumes. This change was observed across multiple resin systems. The increased fiber volume fraction and the decreased void volume fraction of the laminates fabricated with MACM resulted in significant improvements in the flexural strength and flexural modulus relative to the conventionally produced laminates.
After watching this video, you should have a good understanding of how to use permanent magnets to apply a high consolidation pressure during cure of a laminate in the wet lay-up vacuum bag process. The high consolidation pressure considerably enhanced the properties of these composite laminates. We are very excited about the use of magnetic pressure in manufacturing structural, high-performance composite laminates.
We believe that this method can be adopted in other composite manufacturing processes. Don't forget to be extra careful during arrangement and placement of permanent magnets as they generate a very high pressure. Precautions such as wearing impact-resistant gloves should always be taken when performing this procedure.
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