Hot melt Tape Solvent solution Fibers with clinging powders Commingled fibers Film stacking FIGURE 8.I Preliminary material combinations(PMCs). Compression molding of tape Compression molding of aligned fiber with clinging powders Compression molding of aligned commingled fibers Compression molding of stack of thermoplastic film and fabric Fiber placement(tape) FIGURE 8.2 Processing the simple form into the final part. 294
FIGURE 8.2 Processing the simple form into the final part. FIGURE 8.1 Preliminary material combinations (PMCs). 294
Preliminary Material Combinations(PMCs) 295 trix resin.Resin needs to exist in liquid form to wet the fibers.This can be done by heating and melting the resin and running the fibers through the bath of liquid resin.Hot melt processes are probably most common.In the hot melt process,the matrix is heated until melting.Its viscosity should become low enough such that flow to the surface of the fiber is possible and wetting can take place.Figure 8.3 shows a schematic of the fibers running through a melt of resin.This can also be done by dissolv- ing the matrix in a solvent and running the fibers through a bath of the so- lution.Solution processes are well established for thermosetting prepolymers.This process is used by Dupont to produce prepreg of Avimid K-III,a thermoplastic polyimide.The prepreg contains a sub- stantial amount of residual solvent and must be cured.Therefore the pro- duction of Avimid K-III composite structures must be conducted in a manner similar to thermosetting composites.The complication in this technique is that solvent needs to be subsequently evaporated,which may give rise to voids and residual solvents. Unidirectional tape is the most common form of thermoplastic ply.By convention,the tape is 0.127-0.152 mm (5-6 mils)and 7.62-30.48 cm (3-12 in)wide.To produce a 30.48 cm wide tape requires approximately 24 tows with 12,000 filaments each of 8 um diameter to the combining process.Conversely,wide tape can be filament wound from a single tow using a large diameter mandrel.This latter approach is convenient for ex- perimental ply production but may not be appropriate for low cost fabri- cation.Conversely,filament winding towpreg directly to produce a consolidated structure is potentially low in cost.This is due to process in- tegration.Fabric plies are difficult to produce from thermoplastic towpreg due to the stiffness of most towpregs.Consolidated towpreg, typically from slit tape,can be braided into two dimensional fabrics,but Fiber Pressure Hot melt of resin FIGURE 8.3 Impregnation of fibers by running fibers through a bath of melted resin
trix resin. Resin needs to exist in liquid form to wet the fibers. This can be done by heating and melting the resin and running the fibers through the bath of liquid resin. Hot melt processes are probably most common. In the hot melt process, the matrix is heated until melting. Its viscosity should become low enough such that flow to the surface of the fiber is possible and wetting can take place. Figure 8.3 shows a schematic of the fibers running through a melt of resin. This can also be done by dissolving the matrix in a solvent and running the fibers through a bath of the solution. Solution processes are well established for thermosetting prepolymers. This process is used by Dupont to produce prepreg of Avimid K-III, a thermoplastic polyimide. The prepreg contains a substantial amount of residual solvent and must be cured. Therefore the production of Avimid K-III composite structures must be conducted in a manner similar to thermosetting composites. The complication in this technique is that solvent needs to be subsequently evaporated, which may give rise to voids and residual solvents. Unidirectional tape is the most common form of thermoplastic ply. By convention, the tape is 0.127–0.152 mm (5–6 mils) and 7.62–30.48 cm (3–12 in) wide. To produce a 30.48 cm wide tape requires approximately 24 tows with 12,000 filaments each of 8 µm diameter to the combining process. Conversely, wide tape can be filament wound from a single tow using a large diameter mandrel. This latter approach is convenient for experimental ply production but may not be appropriate for low cost fabrication. Conversely, filament winding towpreg directly to produce a consolidated structure is potentially low in cost. This is due to process integration. Fabric plies are difficult to produce from thermoplastic towpreg due to the stiffness of most towpregs. Consolidated towpreg, typically from slit tape, can be braided into two dimensional fabrics, but Preliminary Material Combinations (PMCs) 295 FIGURE 8.3 Impregnation of fibers by running fibers through a bath of melted resin
296 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES FIGURE 8.4 Photo of a roll of unidirectional tape made of carbon/PEKK. three dimensional fabrics are difficult to produce.Figure 8.4 shows a photograph of a roll of tape. The speed of production of the hot melt process(meters per minute) depends on the viscosity of the melt,the thickness of the prepregs to be made and the applied pressure.Darcy's law can be used to estimate the rate of production as illustrated in the example below. Example 8.1 A hot melt process is used to produce prepregs for carbon/PEEK 0.2 mm thick.The temperature of the process is 380C giving rise to the viscosity of the resin of 1000 Pa(sec.A pressure of 1 MPa is applied to induce the flow across the thickness of the prepreg.Determine the maximum rate of production,if the length of the die is 50 cm and the permeability of the fiber preform is assumed to be 10-12 m2. Solution The fastest rate of production occurs when the resin has sufficient time to flow across the thickness of the layer.Using Darcy's law,one has: u=-K4 μAx where, u=the flow velocity across the thickness of the prepreg K=the permeability of the fiber preform Ap the pressure gradient across the thickness of the prepreg Ax=thickness of the laminate
three dimensional fabrics are difficult to produce. Figure 8.4 shows a photograph of a roll of tape. The speed of production of the hot melt process (meters per minute) depends on the viscosity of the melt, the thickness of the prepregs to be made and the applied pressure. Darcy’s law can be used to estimate the rate of production as illustrated in the example below. 296 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES FIGURE 8.4 Photo of a roll of unidirectional tape made of carbon/PEKK. Example 8.1 A hot melt process is used to produce prepregs for carbon/PEEK 0.2 mm thick. The temperature of the process is 380°C giving rise to the viscosity of the resin of 1000 Pa(sec. A pressure of 1 MPa is applied to induce the flow across the thickness of the prepreg. Determine the maximum rate of production, if the length of the die is 50 cm and the permeability of the fiber preform is assumed to be 10−12 m2. Solution The fastest rate of production occurs when the resin has sufficient time to flow across the thickness of the layer. Using Darcy’s law, one has: u K p x = − µ ∆ ∆ where, u = the flow velocity across the thickness of the prepreg K = the permeability of the fiber preform ∆p = the pressure gradient across the thickness of the prepreg ∆x = thickness of the laminate
Preliminary Material Combinations(PMCs) 297 Substituting in the values yields: 10-2m21MP 1000 Pa-sec 0.2 mm -=5x103 mm/sec Time required to traverse the thickness of the preform: h 0.2mm 1=-= -=40sec. u5×10-3mm/sec For the length of the die of 50 cm,the maximum rate of production R would be: R='=50cm or 1.25 cm/sec I 40 sec =1.25 cm sec 3.2.Fibers with Clinging Powders In the powder clinging process,the matrix powder is made to stick to the surface of the fibers.Figure 8.5 shows a schematic of the process. First,the dry fiber tow is fed from a creel to an air-conditioned spreader. The tow is spread to expose the fiber and grounded in order to pick up charge powder.By spreading a tow to expose virtually every fiber,it is easier to get the liquid resin to the surface of every fiber and it takes less pressure to force a polymer melt through a fiber bed.The fiber tow then enters into a heated chamber where matrix powder is electrified such that it carries an electrical charge,then it is fluidized.The powder is deposited on the band of fibers due to static electricity.At the next station of the Lean Phase 两州州 Air Dense Diffuser Plate Phase Electric Potential Let-off Air Comb Oven Take-up Winder Spreader Winder Air Tension Fluidized Pull Control Bed Rollers FIGURE 8.5 Process to get matrix powders to cling to fibers (reproduced from"The processing science of thermoplastic composites,"by J.D.Muzzy and J.S.Colton,in Ad- vanced Composites Manufacturing,T.G.Gutowski,ed.,with permission from Wiley Interscience)
3.2. Fibers with Clinging Powders In the powder clinging process, the matrix powder is made to stick to the surface of the fibers. Figure 8.5 shows a schematic of the process. First, the dry fiber tow is fed from a creel to an air-conditioned spreader. The tow is spread to expose the fiber and grounded in order to pick up charge powder. By spreading a tow to expose virtually every fiber, it is easier to get the liquid resin to the surface of every fiber and it takes less pressure to force a polymer melt through a fiber bed. The fiber tow then enters into a heated chamber where matrix powder is electrified such that it carries an electrical charge, then it is fluidized. The powder is deposited on the band of fibers due to static electricity. At the next station of the Preliminary Material Combinations (PMCs) 297 Substituting in the values yields: u = = × − 10 1 − 5 10 12 m 3 1000 Pa - sec MPa 0.2 mm mm / sec 2 Time required to traverse the thickness of the preform: t h u = = × = − 0 2 40 . mm 5 10 mm / sec sec. 3 For the length of the die of 50 cm, the maximum rate of production R would be: R L t == = 5 4 1 25 1 25 0 cm 0 sec . cm / sec or . cm / sec FIGURE 8.5 Process to get matrix powders to cling to fibers (reproduced from “The processing science of thermoplastic composites,” by J. D. Muzzy and J. S. Colton, in Advanced Composites Manufacturing, T. G. Gutowski, ed., with permission from Wiley Interscience)
298 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES reinforcement fiber clinging powder FIGURE 8.6 Fibers with clinging powders. process,the material is heated.The resin melts,flows and then wets the fibers.After cooling,the powder is fused into the fibers.After passing through a fluidized bed,the tows enter a tunnel oven to melt the polymer onto the fiber.After cooling,the towpreg is wound onto a take-up roll. There are advantages to the powder mixing process.By avoiding sol- vent or water in the combining operation,there is no need to remove volatiles.The extent of mixing between fiber and powder depends upon the extent to which the tow is spread.It is possible to spread the tow to ex- pose virtually every fiber,thereby achieving good mixing.Spreading the tow and not collapsing it when the polymer is molten leads to a flexible tow that can be braided or woven.The coated tow can be heated and cooled rapidly to minimize polymer degradation.The tow is not exposed to high stress,which minimizes fiber damage.Since powder coating can be accomplished quickly and continuously,dry powder combining pro- cess is potentially inexpensive. The production of towpregs using the electrostatic fluidized bed pro- cess has been demonstrated using numerous thermoplastics and thermosets as well as carbon,glass and aramid fibers.Good fiber wetting was obtained even when the particle size of the powder was substantially greater than the 8 um fiber diameter.In a commercial scale version of this process a line speed of 40 cm/s has been achieved while attaining 40%vol polymer content.Figure 8.6 shows a schematic of the fibers with cling- ing powders. 3.2.1.Slurry and Foam The above approaches appear to work particularly well for fine pow- ders below 25 um.Electrostatic cloud coating has worked successfully for powder well over 100 um.Since many polymers are difficult to grind, the ability to accommodate large particles is a definite benefit.Slurries and foams are being explored as alternative combining methods
process, the material is heated. The resin melts, flows and then wets the fibers. After cooling, the powder is fused into the fibers. After passing through a fluidized bed, the tows enter a tunnel oven to melt the polymer onto the fiber. After cooling, the towpreg is wound onto a take-up roll. There are advantages to the powder mixing process. By avoiding solvent or water in the combining operation, there is no need to remove volatiles. The extent of mixing between fiber and powder depends upon the extent to which the tow is spread. It is possible to spread the tow to expose virtually every fiber, thereby achieving good mixing. Spreading the tow and not collapsing it when the polymer is molten leads to a flexible tow that can be braided or woven. The coated tow can be heated and cooled rapidly to minimize polymer degradation. The tow is not exposed to high stress, which minimizes fiber damage. Since powder coating can be accomplished quickly and continuously, dry powder combining process is potentially inexpensive. The production of towpregs using the electrostatic fluidized bed process has been demonstrated using numerous thermoplastics and thermosets as well as carbon, glass and aramid fibers. Good fiber wetting was obtained even when the particle size of the powder was substantially greater than the 8 µm fiber diameter. In a commercial scale version of this process a line speed of 40 cm/s has been achieved while attaining 40%vol polymer content. Figure 8.6 shows a schematic of the fibers with clinging powders. 3.2.1. Slurry and Foam The above approaches appear to work particularly well for fine powders below 25 µm. Electrostatic cloud coating has worked successfully for powder well over 100 µm. Since many polymers are difficult to grind, the ability to accommodate large particles is a definite benefit. Slurries and foams are being explored as alternative combining methods. 298 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES FIGURE 8.6 Fibers with clinging powders