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52.2 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.

52.2.1 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.

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-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

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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.

-air pockets

-solvents

-low molecular weight resin components

-etc.

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Vacuum line

•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.