1.2 COMPOSITE MANUFACTURING
• The basic process involves,
1. creation of a mold/form
2. Preimpregnation of the fibres (or the later addition of resin)
3. Applying fibres to the mold or form
4. Applying resin if not a prepreg
5. curing of composite in oven (possibly an autoclave)
6. finishing to remove excess, etc.
1.2.1 Manual Layup
• Commonly used for polyester and fiberglass
• Wet Layup
- the dry fabric, or mat is laid in the mold. Resin is then poured on and then rolled or squeegeed evenly over the surface, with attention to removal or air pockets. This is done in layers until the part is done. Fabric can be prewet before laying to allow better fibre/matrix ration control. A parting agent, such as silicone is applied to the mold to allow easy removal or the finished part. Vacuum bags can be used to: remove trapped air/voids trapped in the matrix that weakens the composite; pull the fabric to the mold; compress the composite layers.
- molds are often made from wood, plater, plastic, composites
- the surface of the part that touches the mold will be the good surface (take a very good opposite of the mold). The back surface will be rough.
- Curing is often done at room temperature, but hot air blowers and infrared lamps can accelerate the process.
• Advantages of wet layup
- tooling can be made of any material that can withstand a small pressure.
- tooling can be easily changed.
- expensive equipment is not required, but a vacuum pump is often use for epoxies, and some polyesters.
- curing ovens are not required.
- highly skilled workers are not required.
• Disadvantages of wet layup,
- condensation type cross linking (of the polymer matrix) cannot be used because pressure would be required to remove entrapped condensate.
- the techniques lead to a great deal of variation between each part.
- resin content tends to be high because pressure compaction is not used.
- voids are common in the matrix.
- the strength of the materials tends to be poorer compared to other composite methods. This is in part because the fabrics have a tight weave and are hard to impregnate with resin.
- resin might run when on non-horizontal surfaces, causing pooling of resign. In these cases higher viscosity resins are often used.
- there is more shrinkage in volumes with higher resin contents.
- only one finished surface is possible.
• Prepreg layup
- the fibres are purchased with resin already mixed. They commonly come in various widths (3 to 72 inches) and have a leathery feel. They are slightly tacky so that they will stick when formed. (The resins can be thermoplastic or thermoset). After layup the part is vacuum bagged and oven cured. The prepreg materials degrade over time, and should be kept in cool environments.
• Advantages of prepreg layup,
- because the resins are mixed by the manufacturer, the ratio of components is more closely controlled. The manufacturer also ensures better distribution of the resin in the cloth. The manufacturer also performs most of the operations normally hazardous to health.
- Automated machines can also be used to overcome efficiency problems
- typically this method gives better physical properties than the wet layup method
• Disadvantages of Prepreg layup,
- vacuum bagging is required to properly consolidate layers, and remove voids.
- expensive curing ovens must typically be used.
- the vacuum bagging procedure leaves room for more scrapped parts.
- it can be difficult to bag complex parts.
• During layup the fibre orientations are often arranged at multiple angles.
e.g. 90, 45, -45, -45, 90, 0 degrees
• Typical fibre content in the matrix is 60%
• Typical desired maximum of air/voids in the matrix is 0.5%. There is about a 7% loss of strength for every 1% of voids, up to 4%.
• Disadvantages of manual layup methods,
- these methods tend to be slow compared to automated methods
- surface finishes are not the best possible
- long cure cycles are required
1.2.2 Automated Tape Lamination
• Basically does layup with automated machine.
• An overhead gantry moves a tape application head across the mold, and up inclined faces to apply a prepreg tape, 3” width is typical. Cutting and trimming is done automatically.
• NC programs direct the tape layup, often in geodesic paths.
• This methods saves time, increases part consistency and precision, but requires programming and is unable to handle some complicated parts.
1.2.3 Cutting of Composites
• Cuts can be made with common utility knives, carbide disc cutters (pizza cutters), etc.
• Multiple sheets can be cut at the same time, reducing cost and increasing consistency.
• more advanced cutters use ultrasonics, water jets (care is required not to wet the materials), die cutting, laser cutting, etc.
1.2.4 Vacuum Bags
• Application of a vacuum to the resin helps eliminate residual materials/gas trapped in the uncured resin.
- air pockets
- low molecular weight resin components
• Basic steps are,
1. Coat the mold with a mold release agent. This allows the part to easily separate later.
2. Remove prepreg materials from the freezer. Allow the materials to warm to room temperature to reduce condensation - this would contaminate the materials.
3. Build up the layers of the part. Inserts, ribs, etc. may be inserted at this stage.
4. Put a layer of release film on the part. This allows resin to flow out under vacuum, and leaves a good surface for subsequent composite layers to bond to.
5. Add the bleeder layer. This layer will soak up excess resin. It is typically a mat of cotton, polyester felt, or fiberglass (with teflon coat), etc.
6. (Optional) Add a layer of barrier to prevent resin movement to the vacuum valve, but allow air movement. A resin trap should be used in the vacuum system if this step is omitted.
7. (Optional) Add a layer of breather material. This will act as a buffer between the wrinkles in the bag, and the part surface. It also allows better distribution of the vacuum.
8. Apply a sealant around the edges of the part. This can be a tape.
9. Insert thermocouples and any other monitoring devices into the assembly, and ensure that they will not allow air leaks at the sealant. These will be used to monitor cure rates, and control oven temperatures.
10. Put the vacuum bag over the part, and seal at the edges. A typical material is nylon. The vacuum is then applied, and possibly a curing oven is used to accelerate curing.
• Basically an oven that also uses pressure.
• The part is placed in the pressure vessel, and heated, pressure is applied simultaneously. Vacuum bagging can be used to increase the heating effects.
• The heat accelerates the curing of the thermosets, or melting of the thermoplastic resins.
• The pressure helps bond layers, and remove more voids in the matrix.
• Inert gases are often injected to prevent fires.
• Although autoclaves are expensive, they produce better parts, and can process many parts at the same time.
1.2.6 Filament Winding
• Basic (Typical) Process - A tape of resin impregnated fibres is wrapped over a rotating mandrel to form a part. These windings can be helical or hooped. This continues until the part is thick enough. There are also processes that use dry fibres with resin application later, or prepregs are used.
• Parts vary in size from 1” to 20’
• mixtures of hoop/helical layers, and layers of different materials allow higher strengths in various direction, and resistance to impact damages.
• geodesic paths are commonly preferred with this approach.
• winding speeds are typically 100 m/min.
• typical winding tensions are about 0.1 to 0.5 kg.
• to remove the mandrel, the ends of the parts are cut off when appropriate, or a collapsible mandrel is used when the parts must remain intact. (one way to do this is with low melt temperature alloys).
• entire parts on mandrels can be cured in autoclaves when desired. A rotating mandrel will help reduce the resin flow effects caused by gravity.
• inflatable mandrels can also be used to produce pretensioned parts that are designed for high pressure applications, or parts that need a liner, and they can be easily removed.
• this method is well suited to round parts, or parts undergoing high hoop stresses.
- can handle a wide variety of part sizes
- parts can be made with strength in several different directions
- high percentage of material usage
- forming after winding will allow non-cylindrical shapes to be made
- flexible mandrels can be left in as tank liners
- reinforcement panels, and fittings can be inserted during winding
- parts with high pressure ratings can be made
- viscosity and pot life of resin must be carefully chosen
- NC programming can be difficult
- Some shapes can’t be made with filament winding
- Factors such as filament tension must be controlled
• Basic principle - fibers are brought together over rollers, dipped in resin and drawn through a heated die. A continuous cross section composite part emerges on the other side.
• Some points of interest include,
• Hollow parts can be made using a mandrel that extends out the exit side of the die.
• Variable cross section parts are possible using dies with sliding parts.
• Two main types of dies are used, fixed and floating
- Fixed dies can generate large forces to wet fibre
- Floating dies require an external power source to create the hydraulic forces in the resin.
• Multiple dies are used when curing is to be done by the heated dies.
• Up to 95% utilization of materials (75% for layup).
• Most fibres are suitable for this process
• Resins must be fast curing because of process speeds.
• Rollers are used to ensure proper resin impregnation of the fibre
• Resins can also be introduced in the die if perforated metal surfaces are used. Prepreg parts are also used.
• Material forms can also be used at the inlet to the die when materials such as mats, weaves, or stitched material is used.
• For curing, tunnel ovens can be used. After the part is formed and gelled in the die, it emerges, enters a tunnel oven where curing is completed.
• Another method is the process runs intermittently with sections emerging from the die, and the pull is stopped, split dies are brought up to the sections to cure it, they then retract, and the pull continues. (Typical lengths for curing are 6” to 24”)
• Typical parameters for,
- speeds are 0.6 to 1 m/min
- thickness are 1 to 76 mm
- diameters are 25mm to 5m
• double clamps, or belts/chains can be used to pull the part through. The best designs allow for continuous operation for production.
• diamond or carbide saws are used to cut sections of the final part. The saw is designed to track the part as it moves.
• these parts have good axial properties
- good material usage compared to layup
- high throughput
- higher resin contents are possible
- part cross section should be uniform
- fibre and resin might accumulate at the die opening, leading to increased friction causing jamming, and breakage.
- when excess resin is used, part strength will decrease
- void can result if the die does not conform well to the fibres being pulled
- quick curing systems decrease strength
1.2.8 Resin-Transfer Molding (RTM)
• Basic principle - A mold is filled with fibre, it is closed and resin is injected. The mold is often in vacuum before injection. The pressure of injection wets the fibres.
• This process was used to make car body panels.
• The fibre in the mold can be any that holds its shape during the injection. Layers are often stitched, and bonded.
• Inserts/ribs/etc can easily be put into the mold before it is closed.
• most resins can be used, but low viscosities are useful.
- Very large and complex shapes can be made efficiently and inexpensively
- production times are very short compared to layup
- low clamping pressures
- better surface definition than layup
- inserts and special reinforcements are easily added
- operators may be unskilled
- A large number of mold materials may be used
- part consistency is good
- worker exposure to toxic chemicals is reduced
- Mold design is complex
- material properties are good, but not optimal
- resin to fibre ratio is hard to control, and will vary in areas such as corners
- reinforcement may move during injection, causing problems
1.2.9 GENERAL INFORMATION
• Resin curing is best done through slow heating, rapid heating will reduce final strength of the part.
• The composite sheets may be strong, but in thin layers they are less capable of resisting bending moments. To overcome this a honeycomb core can be used inside to increase bending resistance. Typical core materials are,
- PVC foams
- aluminum honeycombs
- paper honeycombs
• Joining of composites may be done using adhesives,
• There are a wide variety of techniques for joining composites, beyond those shown here. Most attempt to maximize contact areas by using tongues, oblique planes, etc.
• Composites may also be joined with mechanical fasteners, (NOTE: use drilled holes, instead of trying to warp fiber about hole - this leads to resin rich areas)
Mallick, P. R., Fiber-Reinforced Composites; materials, manufacturing and design, Mercel-Dekker Inc., New York, 1988.
Mallick, P. K., and Newman, S., Composite Materials Technology, Hanser Publishers, New York, 1990.
Schwartz, M. M., Fabrication of Composite Materials, American Society for Metals, Metals Park, Ohio, 1985.
Strong, A. B., Fundamentals of Composite Manufacturing, Society of Manufacturing Engineers, Dearborn Michigan, 1989.
1.2.11 PRACTICE PROBLEMS
a) List at least 5 advantages of composite materials.
b) List at least 3 disadvantages of composite materials.
2. For thermoset polymers, what effects does cross-linking generally have on the material properties?
3. Which type of glass is good for applications that require,
a) Low cost?
b) Operate at high temperatures?
c) Are resistant to corrosion?
a) List 6 different forms (other than single filaments) that composite fibres may be purchased in.
b) What form of composite fibres are best used for pultrusion?
5. If you were making boat hulls with pre-preg composite fibre and large moulds, what steps would be followed?
6. Indicate if the following parts are best made with pultrusion/filament winding/resin transfer moulding.
rocket engine tanks
car body panels
a mast for a sail boat
7. A composite section has a honeycomb core 1” thick and can withstand a maximum bending moment of 10KN. How much thicker/thinner would the honeycomb have to be to withstand 1KN?
8. TRUE / FALSE - Multi-directional fibres can be used with stereolithography to increase part strength.
9. What are the major factors that weaken composites? Explain the effect of each.
10. Describe the difference between alloys and composites.
11. Describe the properties of the matrix and fiber materials, and then describe why their combinations is so desirable.
12. What properties does a honeycomb core contribute to a composite part?
13. List 10 products that you have purchased or used that are made of composite materials.
14. What are the advantages and disadvantages of composite materials. What design considerations can be used to overcome the disadvantages?
15. A composite has more than one type of fiber. Why would this be desirable?
16. A part is made of a composite material that is 40% fibers (by area) with a Young’s modulus of 300 GPa, and a matrix of 60 Gpa. The UTS of the fibers is 2000 GPa and 100 MPa for the matrix. If the total cross sectional area of the part is 2cm by 0.2cm, what is the effective stiffness and failure load?
17. Calculate the percentage increase in strength of nylon when e-glass fibers are added.
18. List 5 parts that benefit from the anisotropic properties of composites. Explain why.
19. Corrugated cardboard and composite honeycomb have similar construction. What are the similarities and differences in behavior?
20. List 8 different types of composite manufacturing processes and give an example of a part they are well suited to.
21. Composite materials typically cost more than metals. why are they preferred?
22. List 10 factors determine the strength of a composite materials and parts?