Chapter 8 Summary of Manufacturing Processes MSEME 563

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Chapter 8 Summary of Manufacturing Processes MSE/ME 563 5 16 10 1

Chapter 8 Summary of Manufacturing Processes MSE/ME 563 5 16 10 1

8. 0 Manufacturing Processes for Thermoset and Thermoplastic Composites 8. 2 8. 3 8.

8. 0 Manufacturing Processes for Thermoset and Thermoplastic Composites 8. 2 8. 3 8. 4 8. 5 8. 6. 1 8. 6. 2 8. 6. 3 8. A 8. B 8. C Bag molding process Compression molding Pultrusion Filament winding Liquid composite molding processes (RTM, VARTM) Elastic reservoir molding Tube rolling Manufacturing processes for thermoplastic composites Manufacturing defects Tooling considerations 2

8. 2 Bag molding process • This is also known as the “Autoclave Molding

8. 2 Bag molding process • This is also known as the “Autoclave Molding Process”. • Objectives: Consolidate the laminate to specified degree (thickness) Fully cure the matrix material Ensure minimal porosity 3

8. 2 Bag molding process (contd. ) • Uses – Used predominantly in the

8. 2 Bag molding process (contd. ) • Uses – Used predominantly in the aerospace industry. – Used where a high production rate is not an important consideration. • Starting material • The starting material in this process is a prepreg containing fibers in a partially cured (B staged) epoxy resin. – Typically, prepreg contains about 42 wt. % resin. If prepreg is allowed to cure without resin loss, cured laminate would contain 50 vol. % fibers. – Since nearly 10 wt. % of resin flows out during molding process, actual fiber content in cured laminate is 60 vol. %, considered industry standard for aerospace applications. – Recent trend is to employ near net resin content, typically 34 wt. %, and allow only 1 to 2 wt. % resin loss during molding. 4

8. 2 Bag molding process (contd. ) • Procedure 1. Mold surface covered with

8. 2 Bag molding process (contd. ) • Procedure 1. Mold surface covered with nonstick Teflon coated glass fabric separator. 2. Prepreg plies laid up in desired fiber sequence and orientation. 3. Porous release cloth and a few layers of bleeder papers placed on top of prepreg stack. 4. Complete lay up covered with another sheet of Teflon coated glass fabric separator, caul plate, and thin, heat resistant vacuum bag. 5. Entire assembly placed inside autoclave where a combination of heat, external pressure, and vacuum is applied to consolidate and densify separate plies into a solid laminate. • Note: To prevent moisture pickup, prepreg roll on removal from cold storage should be warmed to room temperature before use. 5

3. 4 Bag molding process (contd. ) Schematic of bag molding process. P. K.

3. 4 Bag molding process (contd. ) Schematic of bag molding process. P. K. Mallick, “Fiber Reinforced Composites, ” Second Edition, Marcel Dekker, Inc. , N. Y. , pp. 374 (1993). 6

8. 2 Bag molding process (contd. ) • • • Bleeder: Absorbs excess resin.

8. 2 Bag molding process (contd. ) • • • Bleeder: Absorbs excess resin. Made from polyester mat or fiberglass coated with teflon (or mold release agent) or cotton. Barrier: Layer of material that limits the upward movement of resin and prevents resin from clogging the breather and vacuum lines (lets air to escape but does not allow passage of resin). Breather: Material acts as a distributor for air escaping volatiles and gases. Acts as a buffer between bag wrinkles and part surfaces. This is made from polyester felt, fiberglass, or cotton. Caul Plate: Made with metal or composite. Vacuum Bag: Made from nylon or co extruded nylon that has been heat stabilized. Peel Plies: Made from nylon, polyester or fiberglass fabrics. 7

8. 2 Bag molding process (contd. ) • Stacking concerns – Maintain proper fiber

8. 2 Bag molding process (contd. ) • Stacking concerns – Maintain proper fiber angle from ply to ply (use reference edge). – Eliminate trapped air between plies. – One chance only in lay up (“stuck” plies generally cannot be pulled apart without deforming prepreg). – Training is required to properly stack the plies together. – Only “matrix joints” allowed in plies. – One ply may require more than one piece of prepreg. – Matrix joint (matrix fills while curing). – Matrix joints do not cut across fibers. – Gap size should not exceed 0. 03 in. or 0. 76 mm. J. W. Mar and P. A. Lagace, “Advanced Composites, ” video course (reorder no. 676 2100), Manual, MIT Center for Advanced Study, Cambridge, MA, slide nos. 13 22 and 13 24 (1989). 8

8. 2 Bag molding process (contd. ) • During heating • As the prepreg

8. 2 Bag molding process (contd. ) • During heating • As the prepreg is heated, resin viscosity in the B staged prepreg plies: 1. Initially decreases, attaining minimum viscosity. 2. Increases rapidly (gels) as the curing (cross linking) reaction begins and proceeds toward completion. 9

8. 2 Bag molding process (contd. ) • In a two stage cure cycle

8. 2 Bag molding process (contd. ) • In a two stage cure cycle Two temperature ramps Two isothermal holds First ramp and hold allows resin to flow or bleed and volatiles to escape here viscosity initially drops and then dramatically increases Second ramp and hold is the polymerization portion of the cure cycle here viscosity initially drops slightly and then increases • Straight ramp-up cure cycle: This can be used with the net resin system. 10

8. 2 Bag molding process (contd. ) F. C. Campbell, “Manufacturing Processes for Advanced

8. 2 Bag molding process (contd. ) F. C. Campbell, “Manufacturing Processes for Advanced Composites, ” Elsevier Inc. , N. Y. , p 181 (2004). 11

8. 2 Bag molding process (contd. ) F. C. Campbell, “Manufacturing Processes for Advanced

8. 2 Bag molding process (contd. ) F. C. Campbell, “Manufacturing Processes for Advanced Composites, ” Elsevier Inc. , N. Y. , p 181 (2004). 12

8. 2 Bag molding process (contd. ) • High pressures (e. g. , 100

8. 2 Bag molding process (contd. ) • High pressures (e. g. , 100 psig) are commonly used during autoclave processing to provide ply compaction and suppress void formation. Autoclave gas pressure is transferred to the laminate due to the pressure difference between the autoclave environment and the vacuum bag interior. Translation of the autoclave pressure to the resin depends on several factors, including the fiber content, laminate configuration and the amount of bleeder material used. • The classical approach to applying autoclave pressure during the cure cycle is shown in the figure. • In this approach, during the ramp up to the first hold, only vacuum pressure is applied and maintained until the end of the first isothermal hold. At that point, autoclave pressure is applied, normally 80 100 psig for epoxies and the vacuum pressure is removed by venting to the atmosphere. • The rationale behind this approach is that vacuum will help to remove volatiles from the melting resin while application of the higher autoclave pressure would tend to trap them in the laminate. At the end of the first hold, full autoclave pressure is applied to insure that the laminate is well compacted before the resin viscosity rises to gel, otherwise the laminate will be poorly compacted and numerous voids and porosity. 13

8. 2 Bag molding process (contd. ) • • The approach to applying autoclave

8. 2 Bag molding process (contd. ) • • The approach to applying autoclave pressure, as shown in the previous slide, can cause problems in a production environment. If the autoclave contains a large number of parts with varying heat up rates, the actual point in time to vent the vacuum bag to atmosphere and apply autoclave pressure can be questionable. Because of different inertia (mass) of different parts, it will not be clear when hold period should start or when is the proper point to vent the vacuum bag and apply full autoclave pressure. Again, if the resin gels during this first isothermal hold with only vacuum pressure applied to the laminate, then the probability of gross porosity is very high. A second problem with applying only vacuum pressure during the initial portion of the cure cycle deals with hydrostatic resin pressure (HRP), as illustrated in the next slide. Even though a relatively high autoclave pressure (e. g. , 100 psig) may be used during the cure cycle, the actual pressure on the resin (HRP) may be significantly less. Because of the load carrying capability of the fiber bed in a composite lay up, the HRP is typically less than the applied autoclave pressure. With only vacuum pressure applied during the initial part of the cure cycle, the HRP on the resin can be extremely low, even negative. This is an ideal condition for void formation and growth if allowed to persist to high enough temperatures. The HRP is critical because it is this pressure that helps that keeps the volatiles dissolved in the solution. If the resin pressure drops below the volatile vapor pressure, then the volatiles will come out of the solution and form voids. 14

8. 2 Bag molding process (contd. ) 15

8. 2 Bag molding process (contd. ) 15

8. 2 Bag molding process (contd. ) • To circumvent both the problems in

8. 2 Bag molding process (contd. ) • To circumvent both the problems in a production environment as mentioned previously, a significant portion of the autoclave pressure can be applied immediately before initiating the heat up cycle. For standard epoxy systems, a full vacuum and 85 psig autoclave pressure can be applied through the first hold, and then the bag vented to atmosphere and 100 psig autoclave pressure applied before ramping up to the final cure temperature. • This approach, as shown in the figure here, applies full vacuum at the start of the cure cycle and also applies an autoclave pressure of 85 psig. The vacuum is again maintained until the end of the first isothermal hold and then vented to atmosphere while the autoclave pressure is increased to 100 psig. • This cycle was developed when a large number of parts on tools with widely varying heat up rates had to be loaded in an autoclave for a single cure. 16

8. 2 Bag molding process (contd. ) Autoclave Pressure 85 psig 100 psig 120

8. 2 Bag molding process (contd. ) Autoclave Pressure 85 psig 100 psig 120 350 Temp. (°F) 60 Temp. 240 RT Vacuum Pressure Time (Minutes) Cure Cycle With Pressure Applied From Start 17

8. 2 Bag molding process (contd. ) Rationale in the approach when pressure is

8. 2 Bag molding process (contd. ) Rationale in the approach when pressure is applied from start in the cure cycle (see previous figure): • • Vacuum removes volatiles from the flowing or melting resin. 85 psig autoclave pressure maintains a positive hydrostatic resin pressure (HRP) to keep deeper volatiles dissolved in the resin and initiating laminate compaction before gelation starts as seen by the rise in the viscosity. First ramp and hold also equilibrates temperature through the laminate thickness. At the end of the first hold, vacuum is released, the autoclave pressure is increased to 100 psig, and the temperature is ramped up to the final cure temperature to facilitate laminate compaction before the viscosity rises and full gelation takes place leading to completion of the cure of the part. 18

8. 2 Bag molding process (contd. ) • Typical two-stage cure cycle for a

8. 2 Bag molding process (contd. ) • Typical two-stage cure cycle for a carbon fiber-epoxy prepreg (figure, next slide): 1. First stage – Vacuum 25 in Hg, autoclave pressure 85 psig, temperature raised up to 130°C (266°F) @ 3 to 5 deg F per min. – Dwelling at this temperature for nearly 60 minutes until the minimum resin viscosity is reached (temp. , pressure and vacuum held constant). – During the temp. dwell, external pressure applied to prepreg stack causes excess resin to flow out into bleeders. 2. End of temperature dwell – Pressure increased to 100 psig, vacuum is released and temp. increased to actual curing temp. of resin (about 350 deg F) @ 3 to 5 deg F per min. – Cure temp. and pressure maintained for 2 hours or more, until predetermined level of cure has occurred. High pressures are used for ply compaction and suppress void formation. – Temp. slowly reduced @ 5 deg F per min to room temp. while laminate still under pressure at end of cycle. • Flow of excess resin from the prepreg is extremely important in reducing the void content in the cured laminate. 19

8. 2 Bag molding process (contd. ) Typical two-stage cure cycle for a carbon

8. 2 Bag molding process (contd. ) Typical two-stage cure cycle for a carbon fiber-epoxy prepreg. P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 375 (1993). 20

8. 2 Bag molding process (contd. ) • • Dwelling at temperatures lower than

8. 2 Bag molding process (contd. ) • • Dwelling at temperatures lower than cure temperature is important because (1) it allows lay up to achieve a uniform temperature throughout the thickness, and (2) it allows resin to reach a low viscosity. Void formation and growth in addition curing composites is primarily due to entrapped volatiles. Higher temperatures result in higher volatile pressures. Void growth will occur if volatile vapor pressure (void pressure) exceeds hydrostatic resin pressure (HRP), while the resin is liquid. The prevailing relationship for void formation, therefore, is Pvoid > Phydrostatic void formation and growth 21

8. 2 Bag molding process (contd. ) • Resin flow considerations • Resin flow

8. 2 Bag molding process (contd. ) • Resin flow considerations • Resin flow in lay ups depends on: • Lay up thickness • Heating rate • Pressure application rate • Cure pressure sufficient to squeeze out excess resin from 16 – 32 lay ups may be inadequate for 64 ply lay up. • If heating rate is too high, resin may start to gel before excess resin is squeezed out from each ply. • If the cure pressure is applied too early, excess resin loss would occur because of low viscosity in the pre gel period. If the cure pressure is applied after the gel time, the resin may not be able to flow in the bleeding cloth because of high viscosity. • Maximum cure pressure should be applied just before the resin viscosity in the top ply becomes sufficiently low for the resin flow to occur. 22

8. 2 Bag molding process (contd. ) • Common defects • Voids • Improper

8. 2 Bag molding process (contd. ) • Common defects • Voids • Improper cure • Defects related to ply lay up • Defects related to trimming operations • Residual curing stresses • Air, moisture, solvents absorbed/adsorbed during manufacturing • Foreign matter, debris, broken filaments • Filament crossovers 23

8. 3 Compression molding • Uses – Transforms sheet molding compounds (SMC) into finished

8. 3 Compression molding • Uses – Transforms sheet molding compounds (SMC) into finished products in matched molds. – High volume production of composite parts. • Advantages – Parts of complex geometry in short periods of time. – Can incorporate non uniform thickness, ribs, bosses, flanges, holes, and shoulders. – Can eliminate secondary finishing operations, such as drilling, forming, and welding. Sheet molding compounds (SMC): thin sheets of fibers, chopped or chopped and continuous precompounded with a thermoset resin. 24

8. 3 Compression molding (contd. ) • Procedure 1. 2. 3. 4. Placement of

8. 3 Compression molding (contd. ) • Procedure 1. 2. 3. 4. Placement of a precut and weighed amount of SMC (glass/polyester) onto the bottom half of a preheated mold cavity (figure, next slide). • Usually a stack of several rectangular plies called the charge. • The ply dimensions are selected to cover 60 – 70% of the mold surface area. Mold is closed quickly after the charge placement. • Top half of the mold is lowered at a constant rate until the pressure on the charge increases to a preset level. With increasing pressure, the SMC material in the mold starts to flow and fill the cavity. • Flow of material is required to expel air entrapped in the mold as well as in the charge. • The molding pressure may vary from 1. 4 to 34. 5 MPa (200 – 3000 psi). Mold temperature is usually in the range of 130 – 160°C (270 – 320°F). After a reasonable degree of cure is achieved under pressure, the mold is opened and the part is removed, often with the aid of ejector pins. 25

8. 3 Compression molding (contd. ) P. K. Mallick, “Fiber Reinforced Composites, " Second

8. 3 Compression molding (contd. ) P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 379 (1993). Schematic of the compression molding process. 26

8. 3 Compression molding (contd. ) • • Curing – Begins at the surface,

8. 3 Compression molding (contd. ) • • Curing – Begins at the surface, progressing inwards. – Occurs more rapidly at higher mold temperature (figure), however, peak exotherm temperature • May also increase (as in E glass laminates). • 200°C or higher may cause burning and chemical degradation in the resin. Avoid high molding temperatures with thick parts. P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 381 (1993). 27

8. 3 Compression molding (contd. ) Common surface defects in compression molded SMC Defect

8. 3 Compression molding (contd. ) Common surface defects in compression molded SMC Defect Possible contributing factors Pinhole Coarse particles, particle agglomeration Long range waviness or ripple Resin shrinkage, glass fiber distribution Craters Poor zinc stearate (used as a lubricant) dispersion Sink marks Resin shrinkage, fiber distribution, fiber length, fiber orientation Surface roughness Resin shrinkage, fiber bundle integrity, strand dimensions, fiber distribution Dark areas Styrene loss from the surface Pop up blisters in painted parts Subsurface voids due to trapped air and volatiles 28

8. 3 Compression molding (contd. ) • Examples of common defects a) Porosity b)

8. 3 Compression molding (contd. ) • Examples of common defects a) Porosity b) Blisters c) Fibers oriented parallel to edge (in SMC – R) d) Buckling (in XMC or SMC – CR, excessive resin flow in the transverse direction) e) Weld/knit lines f) Sink marks (in resin rich zone – CTE effect during cooling). Note the fiber rich zone in the rib base. P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 385 (1993). 29

8. 3 Compression molding (contd. ) • Advantages 1. Offers high volume production. 2.

8. 3 Compression molding (contd. ) • Advantages 1. Offers high volume production. 2. Offers production of low cost components. 3. Process offers high surface quality and good styling possibilities. • Limitations 1. Initial investment for equipment and mold is high. 2. Process not suitable for making small number of parts or for prototyping applications. 3. Molding of SMC provides non structural parts; but by utilizing ribs and stiffners, structural members can be produced. 30

8. 4 Pultrusion • • Uses Pultrusion is a continuous molding process for producing

8. 4 Pultrusion • • Uses Pultrusion is a continuous molding process for producing long, straight structural members of constant cross sectional area. • Pultruded products –Solid rods –Hollow tubes –Flat sheets –Various types of beams including angle channels, hat sections, and wide flanged beams. Recently pultruded processes have been developed for producing variable crosssections and curved members. 31

8. 4 Pultrusion (contd. ) • Major constituents – Longitudinally oriented continuous strand rovings

8. 4 Pultrusion (contd. ) • Major constituents – Longitudinally oriented continuous strand rovings (CSR). – Layers of mats or woven roving added at/near outer surface (figure), improving transverse strength. • Content – Total fiber content in pultruded parts can be as high as 70 wt. %. – Mats and woven rovings lowers longitudinal strength and modulus compared to 0° fiber strands. – Ratio of CSR and mats or woven rovings determines mechanical properties. P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 388 (1993). 32

8. 4 Pultrusion (contd. ) • Process 1. Polyester and vinyl ester are matrix

8. 4 Pultrusion (contd. ) • Process 1. Polyester and vinyl ester are matrix materials. 2. Epoxies have also been used, they require longer cure time and do not release readily from pultrusion die. 3. Application of thermoplastics (PEEK, Polysulfone) in pultrusion process are under development. P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 389 (1993 33

8. 4 Pultrusion (contd. ) (b) (a) (c) (e) (d) • (a) Resin bath

8. 4 Pultrusion (contd. ) (b) (a) (c) (e) (d) • (a) Resin bath • (c) Preformer • (e) Pull blocks –Resin –Distributes fiber bundles smoothly. –Pulls cured member. –Curing agent –Squeezes out excess resin. –Once, through the pull blocks, member is cooled in air or with water. –Colorant • (d) Preheated die –UV stabilizer –Final shaping and compaction occurs. –Fire retardant –Curing takes place. • (b) Thermoplastic surfacing veil –Added to improve surface smoothness. –Die length, die temp. , and pulling speed are controlled. UV protection: carbon black particles; Fire retardent: alumina trihydrate 34 P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 389 (1993).

8. 4 Pultrusion (contd. ) • Fiber wet out is the most important factor

8. 4 Pultrusion (contd. ) • Fiber wet out is the most important factor controlling mechanical performance of pultruded members. • Wet-out depends on – Initial resin viscosity. – Residence time in bath. – Mechanical action (looping of fibers) on fibers in bath. – Lateral pressure at resin squeeze out bushing. – Slower line speed and lower viscosity (favors resin penetration). – Higher line speed and higher viscosity (improves resin pickup amount owing to increased drag force). – Fiber and resin surface energies (determines amount of resin coating). 35

8. 4 Pultrusion (contd. ) • • Resin viscosity (400 – 5000 c. P)

8. 4 Pultrusion (contd. ) • • Resin viscosity (400 – 5000 c. P) – ≥ 5000 c. P results in poor fiber wet out, slower line speed and fiber breakage. – ≤ 200 c. P results in excessive resin drainage. Temperature – Figure shows temperature distribution along the length of the die. – Location of exothermic peak depends on the speed of pulling of the fiber resin system through the die. P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 393 (1993). Note: No external pressure is applied in pultrusion process. High internal pressure at the die entrance zone is due to the volumetric expansion of resin. As curing proceeds the pressure decreases. 36

8. 4 Pultrusion (contd. ) P. K. Mallick, “Fiber Reinforced Composites, ” Second Edition,

8. 4 Pultrusion (contd. ) P. K. Mallick, “Fiber Reinforced Composites, ” Second Edition, Marcel Dekker, Inc. , N. Y. , p. 392 (1993). • Viscosity change of a thermosetting resin in a pultrusion die. • At die entrance, viscosity first decreases and then increases at a short distance from the die entrance as resin cures. 37

8. 4 Pultrusion (contd. ) • • Defects found in pultruded products include: –

8. 4 Pultrusion (contd. ) • • Defects found in pultruded products include: – Fiber bunching – Fiber shifting – Folding of mats or woven rovings – Wrinkles These are related to pulling force applied to overcome: – Frictional forces of fibers against the die wall – Shear viscous force between thin resin layer and die wall – Drag resistance between fibers and back flowing resin at the die entrance. In addition to the above, defects include: – Interlaminar cracks – Extent of fiber/matrix wetting – Residual stresses 38

8. 4 Pultrusion (contd. ) F. C. Campbell, “Manufacturing Processes for Advanced Composites, ”

8. 4 Pultrusion (contd. ) F. C. Campbell, “Manufacturing Processes for Advanced Composites, ” Elsevier Inc. , N. Y. , p. 434 (2004). 39

8. 4 Pultrusion (contd. ) • Advantages 1. It is a continuous process and

8. 4 Pultrusion (contd. ) • Advantages 1. It is a continuous process and can be completely automated. Suitable for high volume composite parts. 2. Utilizes low cost fiber and resin systems. • Limitations 1. Suitable for parts that have constant cross sections along its length. Tapered and complex shapes cannot be produced. 2. High tolerance parts cannot be produced. 3. Thin wall parts cannot be produced. 40

8. 5 Filament Winding • • Description A band of continuous resin‑impregnated roving or

8. 5 Filament Winding • • Description A band of continuous resin‑impregnated roving or monofilaments is wrapped around a rotating mandrel and cured to produce axisymmetric hollow parts. • Uses Among the applications of filament winding are: –Automotive drive shafts –Helicopter blades –Oxygen tanks –Pipelines –Spherical pressure vessels –Conical rocket motor cases –Large underground gasoline storage tanks –Prepregs –XMCs 41

 • (b) 8. 5 Filament Winding (contd. ) • (c) • (d) •

• (b) 8. 5 Filament Winding (contd. ) • (c) • (d) • (a) • (e) • (f) (g) • Helical winding process a) Creels of fiber rovings. b) Fiber tension controlled by fiber guides or scissor bars. • (h & i) c) Rovings gathered into a band. d) Resin bath tank (resin, catalyst, pigments, and UV absorbers). e) Excess resin wiping device. f) Impregnated and wiped rovings gathered in a flat band. g) Carriage traverses back and forth parallel to mandrel. P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 395 (1993). h) Mandrel typical winding speed ranges from 90 – 110 linear m/min. i) After winding a number of layers to attain the desired thickness, the part is cured on the mandrel and the mandrel is then extracted from the cured part. 42

8. 5 Filament Winding (contd. ) • By adjusting the carriage feed and mandrel

8. 5 Filament Winding (contd. ) • By adjusting the carriage feed and mandrel rotational speed, any wind angle between 0° and 90° can be obtained. In polar winding, the carriage rotates about the longitudinal axis of a stationary (but indexable) mandrel. After each rotation of carriage, the mandrel is indexed to advance one fiber bandwidth. Polar winding pattern. P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 398 (1993). 43

8. 5 Filament Winding (contd. ) • Mandrel extraction – Collapsible mandrels (segmented or

8. 5 Filament Winding (contd. ) • Mandrel extraction – Collapsible mandrels (segmented or inflatable) are used for products in which the end enclosures are integrally wound, as in pressure vessels. – For low volume productions, soluble plasters, eutectic salts, or low melting alloys are used. 44

8. 5 Filament Winding (contd. ) Mechanical property variation in a filament-wound part as

8. 5 Filament Winding (contd. ) Mechanical property variation in a filament-wound part as a function of wind angle. P. K. Mallick, “Fiber Reinforced Composites, ” Second Edition, Marcel Dekker, Inc. , N. Y. , p. 397 (1993). 45

8. 5 Filament Winding (contd. ) • With conventional filament winding machines, the shapes

8. 5 Filament Winding (contd. ) • With conventional filament winding machines, the shapes that can be created are limited to surfaces of revolution, such as cylinders, cones, box beams, or spheroids (figure): – Cross sections of possible filament wound parts (a and b). – A cross section that cannot be filament wound (c). P. K. Mallick, “Fiber Reinforced Composites, ” Second Edition, Marcel Dekker, Inc. , N. Y. , p. 401 (1993). 46

8. 5 Filament Winding (contd. ) • Process parameters – Fiber tension: Maintains fiber

8. 5 Filament Winding (contd. ) • Process parameters – Fiber tension: Maintains fiber alignment and controls resin content. – Fiber wet out: Reduce voids. – Resin content: Good mechanical properties, weight and thickness control. • Material and process parameters to control fiber wet-out – Viscosity of catalyzed resin in resin bath: Determines temperature and cure advancement. – Number of strands in roving: Determines resin accessibility to each strand. – Fiber tension: Controls pressure on each layer or various layers. – Speed of winding and the length of the resin bath. 47

8. 5 Filament Winding (contd. ) • Common defects – Voids: May appear because

8. 5 Filament Winding (contd. ) • Common defects – Voids: May appear because of poor fiber wet‑out, the presence of air bubbles in the resin bath, an improper band width resulting in gapping' or overlapping, or excessive resin squeeze‑out from the interior layers caused by high winding tension. – Delaminations: In large filament wound parts, an excessive time lapse between two consecutive layers of windings can result in delaminations, especially if the resin has a limited pot life. – Wrinkles: Result from improper winding tension and misaligned rovings. • Note: Unstable fiber paths that cause fibers to slip on the mandrel may cause fibers to bunch, bridge, and improperly orient in the wound part. 48

8. 5 Filament Winding (contd. ) • Advantages 1. For certain applications such as

8. 5 Filament Winding (contd. ) • Advantages 1. For certain applications such as pressure vessels and fuel tanks, this is the only process that can be used to produce cost effective high performance parts. 2. Utilizes low cost raw materials and low cost tooling. 3. Process can be automated for the production of high volume parts. • Limitations 1. Limited to producing closed and convex structures. 2. Not all fiber angles are easily produced. Less than 15 degrees are not easily produced (geodesic path is preferred for fiber stability). 3. Maximum fiber volume fraction attainable is only 60%. 4. Difficult to obtain uniform fiber distribution and resin content throughout the thickness of the laminate. 49

8. 6. 1 Resin Transfer Molding (RTM) • RTM belongs to the general class

8. 6. 1 Resin Transfer Molding (RTM) • RTM belongs to the general class of Liquid Composite Molding Processes. • Advantages – Ability to encapsulate metal inserts, stiffeners, washers, etc. within a molded laminate. In liquid composite molding processes one does not use prepregs. – Can encapsulate a foam core between the top and bottom preforms of a hollow part, which adds stiffness to the structure and allows molding of complex three‑dimensional shapes in one piece. • Uses: The RTM process has been successfully used in molding such parts as: – Cabinet walls Water tanks – Chair or bench seats Bathtubs – Hoppers Boat hulls 50

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • • RTM (contd. )

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • • RTM (contd. ) Process 1. Place layers of mat, woven roving, or cloth in bottom half of mold. 2. Close mold and inject liquid resin (polyester or vinyl ester resins are commonly used for the RTM process) at pressures in the range of 69 – 690 k. Pa (10 – 100 psi). Resin spreads throughout the mold, displacing entrapped air and impregnating fibers. 3. Curing is performed at room or elevated temperatures. 4. Cured part is pulled from mold and trimmed. 5. For short injection time and good quality parts: low viscosity (100 – 300 c. P), high p, sufficient resin pot life, low volatile content, multiple injection ports and vents. 51

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) P.

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 405 (1993). 52

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) F.

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) F. C. Campbell, “Manufacturing Processes for Advanced Composites, ” Elsevier Inc. , N. Y. , p 333 (2004). 53

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) Edge

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) Edge Injection • Resin injected at one end of the part • Resin flows unidirectionally down the part length through the reninforcement • Air is vented at the opposite end Point Injection • Resin introduced through a port at the center • Resin flows radially into the reinforcement • Air is vented at the periphery of the part Peripheral Injection • Resin injected into a channel around the periphery of the part • Resin flow is radially inwards • Air is vented at the center of the part F. C. Campbell, “Manufacturing Processes for Advanced Composites, " Elsevier Inc. , N. Y. , p 334 (2004). 54

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • • RTM (contd. )

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • • RTM (contd. ) Earlier we saw that • For short injection time and good quality parts Low viscosity resin High p Multiple injection ports and vents Sufficient resin pot life Low volatile content 55

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) •

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) • Using preforms Instead of using flat reinforcing layers, such as a continuous strand mat, the starting material in an RTM process can be a preform that already has the shape of the desired product. • Advantages of preforms –Good moldability with complicated shapes (particularly with deep draws). –Elimination of the trimming operation, which is often the most labor‑intensive step in an RTM process. 56

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • • RTM (contd. )

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • • RTM (contd. ) Preforms Spray up on a pre shaped screen – 0. 5 to 3 in long fibers are mixed with resin and sprayed on to the screen. Continuous strand mat containing random fibers used on a pre shaped die. Woven fabrics stitched. Braided and textile weaving 57

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) F.

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) F. C. Campbell, “Manufacturing Processes for Advanced Composites, " Elsevier Inc. , N. Y. , p 307 (2004). Advanced textile Material Forms – Multiaxial Wrap Knit, Triaxial Braid, 3 -D Braid, and Knitted/Stitched. 58

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) F.

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM (contd. ) F. C. Campbell, “Manufacturing Processes for Advanced Composites, ” Elsevier Inc. , N. Y. , p 329 (2004). 59

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • Vacuum Assisted Resin Transfer

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • Vacuum Assisted Resin Transfer Molding (VARTM) F. C. Campbell, “Manufacturing Processes for Advanced Composites, ” Elsevier Inc. , N. Y. , p 349 (2004). 60

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • • RTM and VARTM

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • • RTM and VARTM Processes (contd. ) Advantages 1. Compared to compression molding and injection molding processes initial tooling cost is low. 2. Moldings can be made close to dimensional tolerances. 3. Can make complex parts at intermediate volume rates. 4. Can provide parts with good surface finish on both sides. 5. Higher fiber volume fraction, up to 65%, can be achieved. 6. Inserts can be easily incorporated into moldings. 7. Allows low volatile emission during processing. 8. Offers production of near net shape parts. 9. Can be automated. 61

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM and VARTM Processes

8. 6. 1 Resin Transfer Molding (RTM) (contd. ) • RTM and VARTM Processes (contd. ) • Limitations 1. Manufacture of complex parts requires a good amount of trial and error experimentation. 2. Tooling and equipment cost are higher than hand lay up and spray up processes. 3. The tooling design is complex. • Some issues in RTM process are Resin flow Curing Heat transfer in porous media 62

8. 6. 2 Elastic Reservoir Molding (ERM) • Process • Placed in a heated

8. 6. 2 Elastic Reservoir Molding (ERM) • Process • Placed in a heated mold, a sandwich of: – Liquid resin impregnated open celled foam. – Face layers of dry continuous strand mat, woven roving, or cloth. Sandwich foam: – Usually flexible polyurethane. – Acts as an elastic reservoir for the catalyzed liquid resin. Compression and curing: – The sandwich is pressed with 520 – 1030 k. Pa (75 – 150 psi). – Resin flows out vertically and wets the face layers. – Upon curing, a sandwich of low density core and fiber reinforced skins is formed. • • P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 409 (1993). 63

8. 6. 3 Tube Rolling • Process 1. Precut lengths of a prepreg are

8. 6. 3 Tube Rolling • Process 1. Precut lengths of a prepreg are rolled onto a removable mandrel. 2. The uncured tube is wrapped with a heat shrinkable film or sleeve. 3. Cured at elevated temperatures in an air circulating oven, as the outer wrap shrinks, entrapped air is squeezed out the ends. 4. After curing, the mandrel is removed and a hollow tube is formed. • Examples of products – Circular tubes – Space trusses – Bicycle frames 64

8. A Manufacturing processes for thermoplastic composites • The use of thermoplastic composites is

8. A Manufacturing processes for thermoplastic composites • The use of thermoplastic composites is becoming popular in aerospace and automotive industries because of their toughness, higher production rates, and minimal environmental concerns. In the commercial sector, the predominant thermoplastic manufacturing techniques include injection molding, compression molding and, to some degree, autoclave prepreg lay up process. However, most of the manufacturing processes for thermoset composites (e. g. , filament winding and pultrusion) are also used for thermoplastic composite parts. • Critical differences between thermosets and thermoplastics processing – Thermoplastic prepregs are not tacky (sticky). – Thermoplastic prepregs are not very flexible. – Processing temperatures and pressures of thermoplastics are much higher than thermosets. – Chemical reaction does not occur during processing of thermoplastics – they can be shaped and formed repeatedly by the application of heat and pressure. 65

8. A Manufacturing processes for thermoplastic composites (contd. ) • Thermoplastics can be processed

8. A Manufacturing processes for thermoplastic composites (contd. ) • Thermoplastics can be processed using metal working forming techniques: (a) Matched die forming (b) Hydroforming (c) Thermoforming P. K. Mallick, “Fiber Reinforced Composites, " Second Edition, Marcel Dekker, Inc. , N. Y. , p. 411 (1993). 66

8. A Manufacturing processes for thermoplastic composites (contd. ) • Thermoforming • This is

8. A Manufacturing processes for thermoplastic composites (contd. ) • Thermoforming • This is a common manufacturing technique forming unreinforced thermoplastic sheets into trays, cups, bathtubs, small boats, etc. When used for thermoplastic composites, various layers of lay up are consolidated into laminated sheet prior to thermoforming or lay up is pre heated to forming temperature, placed in the mold, and formed in the mold cavity by the application of vacuum, pressure or both. If consolidation takes place during thermoforming, the forming temperature is close to the melt temperature of the polymer. In the forming operation of thermoplastic composites, the sheet is both stretched and drawn into the final shape. However, since composites containing continuous fibers which are inextensible, it is not possible to stretch the individual plies in the fiber direction without breaking the fibers. Therefore, the composite lay up cannot be clamped and fixed at the edges. On the other hand, if the fibers are not in tension during forming, some of them may wrinkle. To overcome this problem, the lay up is placed between two thin, highly deformable diaphragms (such as superplastic aluminum alloys and polyimide films), which are clamped around their edges. As the forming pressure is applied, the deformation of diaphragms creates a biaxial tension in the lay up, which prevents the plues from wrinkling. • 67

8. B Typical Manufacturing Defects In composites, the range of flaws, which may be

8. B Typical Manufacturing Defects In composites, the range of flaws, which may be present and need to be detected is much larger and may include: –State of cure –Volume fraction –Orientation of fibers/lay up –Strength of bonded joints –De lamination –Fiber/matrix interface condition –Interlaminar cracks –Inclusions –Porosity/voids –Surface flaws (cracking, gel coat blisters, etc. ) –Fiber bunching –Improper cure of laminates –Fiber buckling –Knit or weld lines –Warpage –Fiber wrinkles –Residual curing stresses –Fibers oriented parallel to the edge in SMC R compression molded parts –Sink marks –Defects related to ply lay up and trimming operations 68

8. B Typical Manufacturing Defects (contd. ) • Spring - In In epoxies, volumetric

8. B Typical Manufacturing Defects (contd. ) • Spring - In In epoxies, volumetric shrinkage on the order of 1% to 6% occurs as they cure. Whereas the fiber reinforcement tends to limit this effect in the in‑plane direction, the through thickness shrinkage is largely unrestrained. In flat symmetric laminates, this has little effect, but in curved parts, it contributes to spring‑in effect. This can be illustrated by considering a laminate with a bend that is compressed through its thickness. If the bend angle is constrained during compression, the inside ply will be compressed while the outside ply is stretched. Spring‑in results when the imposed constraint is removed. It is therefore necessary to compensate for or accommodate spring‑in by adjusting the tool angle outwards by 1 to 5 , as shown in the figure in the next slide. The degree of compensation required is somewhat dependent on the actual laminate layup orientation and the laminate thickness. 69

8. B Typical Manufacturing Defects (contd. ) Spring‑In in a Composite Part F. C.

8. B Typical Manufacturing Defects (contd. ) Spring‑In in a Composite Part F. C. Campbell, “Manufacturing Processes for Advanced Composites, ” Elsevier Inc. , N. Y. , p. 213 (2004). 70

8. B Typical Manufacturing Defects (contd. ) Spring‑In Correction Factors F. C. Campbell, “Manufacturing

8. B Typical Manufacturing Defects (contd. ) Spring‑In Correction Factors F. C. Campbell, “Manufacturing Processes for Advanced Composites, ” Elsevier Inc. , N. Y. , p. 113 (2004). 71

8. C Tooling considerations • • What is the function of tooling in the

8. C Tooling considerations • • What is the function of tooling in the manufacture of PMC parts? Tooling provides a mechanism to give a part the desired shape at the end of the mold cycle. What single property should be considered as a critical factor in the choice of tool materials? The single property considered as a critical factor in the choice of tool materials is the match or differences in the coefficient of thermal expansion between the tool material and the composite part to be processed. What are three different types of materials used as autoclave tools? Three autoclave tool materials include metals, composites, and metal coated composites. What are the consequences of choosing improper tool materials? Differences in the coefficients of thermal expansion between the mold and the composite part can cause stresses in the part and in the extreme cases cracking of the part may occur. These stresses could also result in the dimensional changes or size of the part. Thermal correction for molds Engr. dimension x (CTEp – CTEt) x (Tgel – Troom) 72