Composite Technology laboratory

1. Composite Technology laboratory Rene Herrmann 2011

2. Vacuum lamination setup and strategies • The products to be laminated have different shapes and curvatures. This results in unhomogenous fiber density along the laminated sheet. • Fiber direction must match as closed as possible the direction of the acting forces. • When placing fibers (textiles) on a curved surface two different things can happen, either the density of the fibers is decreasing due to spreading or the opposite.

3. Fiber placement • When placing dry fibers in a curved mold two things have to be considered. Vacuum bag has to be able to curve itself, meaning some folds are needed and the fibers have to be attached to the curved mold. • The folds in the curved mold have to be as big as hight variation requires in the mold, otherwise the vacuum bag will not align itself to the mold. The bag can even destroy itself. • The attachment of the fibers in a curved mold (or overhead) is done with a special glue called airtag which is to be disolved by the resin (once applied)

4. Texile overlap • When making large laminates texiles sheets are ultimately cut and when the next sheet starts an overlap is needed. This will however create a (unwanted) step. There are 2 options. • If the laminate is thick as compared to sheet thickness the overlap can be made zero, no step is formed and sheets in different layers lay in different positions. This requires thin sheets and the layup takes longer time as compared to thick sheets. • The other option is an overlap of the sheets, the length of the overlap has to be at least as long as the thickness of the sheet multiplied by the ratio of youngs modulus of fiber and resin. Ex:

5. Laminate setup • The straight panel requires no folds and the dry fibers rest on the release coated mold. • The curved mold requires folds for any high variation and special attention has to be paid to fiber alignement. When placing multiple layers of fibers a fold in the fibers of a low sheet can form, this has to be avoided by careful working. • The curvature of the mold may make the dry fibers to ’fall off’. An temporary attachment is needed, a glue that is later disolved by the resin, it is called airtack. • The mold can also be ’closed’ meaning the fibers go around and this creates a new problem (tubes).

6. Closed laminates setup • A laminate in which the fibers go all around the mold are used for manufacturing of tubes/pipes. • When these laminates are setup a proplem with fiber dislocations accurs. The dry uncompacted fibers are placed around using airtack and the worker skill to place the precut fibers. As later the vacuum is applied around the fibers are compacted much more and the fibers realign themselfs. • If the (tube) mold is male the fibers will ultimately undergo folding. • If the (tube) mold is female the fibers will undergo streching. • The realocation of the fibres in the mold must be controlled using a partial vacuum, otherwise surface quality of the product is less than optimal

7. Infusion strategies • Depending on size an complexity of the product different strategies can be applied. • The strategy used increases tool speed while insuring garantied wetting of all fibers without the risk of premature geling of the resin. • The speed of the resin flow is depended on viscocity and in turn on temperature. High temperature however results in earlier geling of the resin. • Strategies are either parallel or sequential. In parallel infusions multiple inlets are placed in sequential infusions also multiple inlets are placed bbut they are opened in sequence as opposed to simulatanously.

8. Parallel infusion strategy • The mold is analysed with a software for flow rates in the mold, taking into acount thickness (volume) variations and perhaps viscocity changes during the infusion. • The infusion is the fastest possible and therefore tool cycle time shortest, however the engineer is challenged by need to deterime the places of injection such that the piece is ’all wett’ at the same time, otherwise resin leaves the mold at some places to the resin trap unused while other places are noty yet wet.

9. Sequential infusion • The mold does not to be analysed, the engineer places more inlets than ever will be needed. • The infusion starts from a point where the laminate is either very thick or far away from the vacuum outlet. • As the infusion progresses a new inlet is selected plainly by the fact that the resin bucket first in use is now empty. A new resin mix is prepared and infusion continues on a suitable inlet closed to where the resin flow stopped. • The strategy is simple but slower, however the mixing of the resin (gel time) is not a problem because you mix a small amount that is sequentially injected.

10. Complex strategies • If the product is large and can not be studied a strategy is needed that works and is as fast as it can be made without the risk of premature geling. • These products use a sequential setup but start infusion simulataiously on may places. • In some cases (large thickness variation and complex shapes) multiple vacuum lines are applied and these lines are perhaps equipped with valves for sequential dragging.

11. See links • http://www.youtube.com/watch?v=Moh9WQ2ZnTo&feature=related • http://www.youtube.com/watch?v=fD8kmx96khY • http://www.youtube.com/watch?v=j7KNSMVSIX4 • http://www.youtube.com/profile?feature=iv&annotation_id=annotation_460156&src_vid=I5oJlc7c0tc&user=anthrakonimatagr#p/u/9/VXxUxTgcYLU