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52.2 COMPOSITE MANUFACTURING
52.2.1 Manual Layup
Commonly used for polyester and fiberglass
- 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.
- 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.
- expensive equipment is not required, but a vacuum pump is often use for epoxies, and some polyesters.
- condensation type cross linking (of the polymer matrix) cannot be used because pressure would be required to remove entrapped condensate.
- 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.
- 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.
- 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.
Disadvantages of Prepreg layup,
During layup the fibre orientations are often arranged at multiple angles.
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,
52.2.2 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.
52.2.3 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.
52.2.4 Vacuum Bags
Application of a vacuum to the resin helps eliminate residual materials/gas trapped in the uncured resin.
2. Remove prepreg materials from the freezer. Allow the materials to warm to room temperature to reduce condensation - this would contaminate the materials.
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.
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.
52.2.5 Autoclaves
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.
52.2.6 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.
52.2.7 Pultrusion
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,
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")
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
- fibre and resin might accumulate at the die opening, leading to increased friction causing jamming, and breakage.
52.2.8 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.
52.2.9 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,
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)
52.2.10 REFERENCES
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.
52.2.11 PRACTICE PROBLEMS
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,
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.
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?
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