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There are four basic methods for laminating composite fabrics.
This paper will discuss, in simple terms, these techniques, giving an overview of each method. These methods include overlay, vacuum bagging, pressure molds, and a few hybrid methods.
Choice of fabrics
Choosing a fabric type is mostly dependant on two factors – weave type and thickness. Determining the weave type is based on your aesthetic and conformability requirements. The most common fabric chosen for aesthetic applications is typically a 3K 2×2 twill for carbon fabric applications. This fabric provides the most dramatic look of all weave types. For a more sophisticated look, a harness satin H8 or possibly harness satin H4 is sometimes used. For maximum conformability, a harness satin fabric is the most flexible of all the fabric weaves. Which harness satin (ex. 4 vs. 8) conforms the best to complex curves is dependent upon the specific fabric. The next most flexible fabric is generally a twill weave (note that a 4×4 twill will be more flexible than a 2×2). The least conforming fabric is a plain weave. It is no accident that the harness satin is the loosest of all weaves, while the plain weave is the tightest of all weaves. Therefore you can generally conclude that the looser the fabric weave, the more conforming it will be too complex curves and the more easily it frays (at the edges), while the tighter the fabric weave, the less conforming to complex curves, but it is likely to fray the least amount of all the weaves at the edges.
What thickness you need in a particular fabric is dependent on your application. For cosmetic purposes using carbon fiber fabric, a 3K carbon is often an ideal choice. For structural applications, the most cost-effective solution is to use the thickest possible fabric. Thicker fabrics are cheaper per pound than multiple layers of thinner fabrics, although thinner fabrics will generally conform better to complex curves than thicker fabrics. For more information regarding choosing the correct fabric, please see our paper “Weave Definitions” at: www.sollrcomposites.com/fabricchoice1.html
The Overlay Method
The overlay method is the simplest of all the laminating methods. Generally, it involves finding an existing piece and sanding it lightly, then the composite fabric is laid over the top of this existing piece, and resin is applied. Finishing such a piece using the overlay method generally involves one of two techniques. The first is sanding and/or buffing the finished overlay composite piece to a shine. The second option is to sand the piece smooth, then apply a final coat of resin or add a clear coat, typically of urethane for epoxy, or a polyester clear coat for a polyester-based resin. Sanding into carbon fiber in order to smooth the surface is commonly not recommended since it can destroy the cosmetic appearance. In order to provide a smooth outer layer without cutting into the carbon fiber outer layer, some add a final layer of fiberglass since all fiberglass turns clear when the resin is added to it, and one can sand into the fiberglass to make a smooth surface without touching the underlying carbon material. The main advantage of using the overlay technique is its simplicity.
The overlay method is commonly used when one custom piece needs to be made, or a small number of custom pieces need to be made. The main disadvantage of using the overlay technique is that results can be inconsistent and one often needs to be at least somewhat “crafty” in order to be able to create professional-looking pieces.
Vacuum bagging is by far the most complex and expensive of all the methods but usually results in the best final product (after a well-designed mold and your processes are well established). The first step in vacuum bagging is to create a perfectly designed reverse mold of the final piece which you intend to make. This mold can be made out of virtually any material, anything from silicon rubber molds to composite or even water-soluble materials. The second step involves laying your composite fabric(s) into your newly created mold, then applying either a release fabric for fairly flat products or a peel-ply for complex and curvy applications. A release fabric is typically a plain weave nylon-treated fabric that allows the resin to pass through it, but the release fabric itself will not stick to the composite product. A peel-ply is a stretchable rubber-like membrane with small holes space throughout the membrane, allowing the resin to be sucked through those holes. Behind the peel-ply or release fabric, you place a breather fabric. It often resembles a baby’s blanket and may even be called that. The purpose of the breather fabric is to absorb the excess epoxy being pulled through the release fabric or peel-ply. Behind the breather, the fabric is the vacuum bag itself. This acts as a permanent barrier and helps create an airtight chamber so that the resin can be sucked away from the product. Sealing the bag to the mold requires special sticky tape. This tape provides an airtight seal between the mold and the vacuum bag itself. This tape is commonly referred to as sealant tape.
If producing large numbers of identical units, such as if you intend to go into production making one specific piece or product, vacuum bagging is an ideal method. The disadvantages of using a vacuum bagging method are that it often requires a great deal of effort to create a perfect mold; it also often requires adjusting the vacuum bag line(s) and possibly adjusting the individual suction of each line. Because of this, it is common to go through at least three to five pieces until you perfect your product and are ready to go into production. Therefore this method is generally not recommended if your intention is to create only a few specific pieces.
Pressure molds provide the advantages of vacuum bagging without necessarily the expense. Pressure molds are ideal for flat applications. The technique involves creating a male and female mold and sandwiching your composite material between them, thereby pressing the two molds together to create your final product. For flat applications, one can use materials for molds that are as simple as two pieces of flat lexicon (plexiglass) by treating each side of the plexiglass with a release agent prior to inserting the composite fabric (with resin). One can therefore quickly and easily create perfect and flat composite pieces. Complex products can also be made by creating both male and female molds that perfectly fit inside each other. The main disadvantage of pressure molds for complex pieces is the complexity of making the molds themselves. The main advantage is the elimination of all the vacuum bagging supplies to create your product.
The Hybrid Methods
This is a cross between the overlay method and vacuum bagging. This involves creating a table with holes in it, similar to a peg board, laying your overlay piece over the top of this table, applying your composite fabric, and laying a stretchable plastic over the top of the composite. Finally one either uses vacuum suction, to suck the plastic down onto the piece or simply stretches the plastic over the top of the piece. In this method, one also treats the plastic with a release agent prior to use. The main advantage of this method is its simplicity. This technique works quite well for products that are fairly flat, and/or a convex shape. For products that have complex curves or holes in their center, you will find that the plastic will not conform well to the complex shapes, therefore this method has limited use but is ideal for some applications. Note the use of treated stretchable plastic sheeting used in this method can also be used in the overlay method mentioned above with the potential for exceptional results.
Another option for the hybrid method is to use a tubular vac bag. This eliminates
the need for the table mentioned above. The ends of the tubular vacuum bag can then be sealed either with vacuum bagging sealant tape or by heat sealing the two halves together similar to the way common vacuum food storage bags are sealed.
Choosing the right technique depends on the number of identical products you intend to make, how crafty you are, how much time you intend to invest (upfront), and how sensitive you may be to the cost of producing your pieces. For a small number of products, the overlay or hybrid methods are typically desired. For larger numbers of products, typically the hybrid or vacuum bagging is the most effective.