Bag Molding Process for Composites: A Detailed Overview

1. Chapter 8Summary of Manufacturing ProcessesMSE/ME 563 5-16-10

2. 8.0 Manufacturing Processes for Thermoset and Thermoplastic Composites 8.2 Bag molding process 8.3 Compression molding 8.4 Pultrusion 8.5 Filament winding 8.6.1 Liquid composite molding processes (RTM, VARTM) 8.6.2 Elastic reservoir molding 8.6.3 Tube rolling 8.A Manufacturing processes for thermoplastic composites 8.B Manufacturing defects 8.C Tooling considerations

3. 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

4. 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.

5. 8.2 Bag molding process (contd.) • Procedure • Mold surface covered with nonstick Teflon-coated glass fabric separator. • Prepreg plies laid up in desired fiber sequence and orientation. • Porous release cloth and a few layers of bleeder papers placed on top of prepreg stack. • Complete lay-up covered with another sheet of Teflon-coated glass fabric separator, caul plate, and thin, heat-resistant vacuum bag. • 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.

6. 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).

7. 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.

8. 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).

9. 8.2 Bag molding process (contd.) • During heating • As the prepreg is heated, resin viscosity in the B-staged prepreg plies: • Initially decreases, attaining minimum viscosity. • Increases rapidly (gels) as the curing (cross-linking) reaction begins and proceeds toward completion.

10. 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.

11. 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.) F.C. Campbell, “Manufacturing Processes for Advanced Composites,” Elsevier Inc., N.Y., p 181 (2004).

13. 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.

14. 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.

15. 8.2 Bag molding process (contd.)

16. 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.

17. Autoclave Pressure 100 psig 85 psig 120 350 60 Temp. (°F) Temp. 240 Vacuum Pressure RT Time (Minutes) 8.2 Bag molding process (contd.) Cure Cycle With Pressure Applied From Start

18. 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.

19. 8.2 Bag molding process (contd.) • Typical two-stage cure cycle for a carbon fiber-epoxy prepreg (figure, next slide): • First stage • Vacuum  25 in Hg, autoclave pressure 85 psig, temperature raised up to 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. • 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.

20. 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).

21. 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

22. 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.

23. 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

24. 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. 8.3 Compression molding Sheet molding compounds (SMC): thin sheets of fibers, chopped or chopped and continuous precompounded with a thermoset resin.

25. 8.3 Compression molding (contd.) • Procedure • 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 – 3000psi). 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.

26. 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.

27. 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).

28. 8.3 Compression molding (contd.)

29. 8.3 Compression molding (contd.) • Examples of common defects • Porosity • Blisters • Fibers oriented parallel to edge (in SMC – R) • Buckling (in XMC or SMC – CR, excessive resin flow in the transverse direction) • Weld/knit lines • 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).

30. 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.

31. 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 cross-sections and curved members.

32. 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).

33. 8.4 Pultrusion (contd.) • Process • Polyester and vinyl ester are matrix materials. • Epoxies have also been used, they require longer cure time and do not release readily from pultrusion die. • 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

34. 8.4 Pultrusion (contd.) (b) (a) (c) (e) (d) • (e) Pull blocks • Pulls cured member. • Once, through the pull blocks, member is cooled in air or with water. • (a) Resin bath • Resin • Curing agent • Colorant • UV stabilizer • Fire retardant • (b) Thermoplastic surfacing veil • Added to improve surface smoothness. • (c) Preformer • Distributes fiber bundles smoothly. • Squeezes out excess resin. • (d) Preheated die • Final shaping and compaction occurs. • Curing takes place. • Die length, die temp., and pulling speed are controlled. UV protection: carbon black particles; Fire retardent: alumina trihydrate P.K. Mallick, “Fiber Reinforced Composites," Second Edition, Marcel Dekker, Inc., N.Y., p. 389 (1993).

35. 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).

36. 8.4 Pultrusion (contd.) • Resin viscosity (400 – 5000 cP) • ≥ 5000 cP results in poor fiber wet-out, slower line speed and fiber breakage. • ≤ 200 cP 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.

37. 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.

38. 8.4 Pultrusion (contd.) • Defects • 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

39. 8.4 Pultrusion (contd.) F.C. Campbell, “Manufacturing Processes for Advanced Composites,” Elsevier Inc., N.Y., p. 434 (2004).

40. 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.

41. 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

42. 8.5 Filament Winding (contd.) • (b) • (c) • (e) • (f) • (d) • (a) (g) • Helical winding process • Creels of fiber rovings. • Fiber tension controlled by fiber guides or scissor bars. • Rovings gathered into a band. • Resin bath tank (resin, catalyst, pigments, and UV absorbers). • Excess resin wiping device. • Impregnated and wiped rovings gathered in a flat band. • Carriage traverses back and forth parallel to mandrel. • Mandrel typical winding speed ranges from 90 – 110 linear m/min. • 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. • (h & i) P.K. Mallick, “Fiber Reinforced Composites," Second Edition, Marcel Dekker, Inc., N.Y., p. 395 (1993).

43. 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).

44. 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.

45. 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).

46. 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).

47. 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.

48. 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.

49. 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.

50. 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