JEC COMPOSITES MAGAZINE - Issue #52 - October 2009 - 63

contamination and blocking is virtually eliminated. However, as previously stated, the density of the reel (or spool) is of paramount importance to optimise winding (see Figure 5). Tension requirements To achieve optimum density, it is necessary to ensure that the prepreg is transported through the slitting machine at a constant tension, especially through the slitting zone. Then it should be rewound under the required tension profile to achieve the necessary density of the finished package, whether it is a reel, a pancake or a spool. The rewind tension can vary from a constant tension profile to a constant torque profile (100% taper). When rewinding reels or pancakes, it is important to use a duplex winding system incorporating differential winding to allow individual tension control of each rewound package during the rewinding process. When spooling, here again, it is of paramount importance that each spool has its own tension control as close as possible to the rewinding position. In each case, there is a benefit in taper tension winding to reduce the compressive forces as the package diameter is built up. Taper tensions of 1% or 2% are totally inappropriate. You would need to be winding with a taper in the region of 20% to 60%. However, it should be remembered that the two limits of such winding packages are between constant tension and constant torque. Therefore, 100% taper is a constant torque profile, not a reduction from start tension to zero tension at the final diameter. To optimise the wound package, it is important to reduce any unsupported web as the material reaches the rewinding position, while ensuring that the correct web tension has been vised. For any wound package, it should be ensured that the density of the reel (or spool) is such that it can be transported to the next process without telescoping or distortion, and to ensure that the finished product can be unwound or de-spooled without causing any failures during the next process. To achieve this, winding tension is particularly important for the final packaging of the product. Slitting the material is critical, and it is important that the minimum amount of distortion and damage occurs to the material during the slitting process. It is advisable to use a shear-cutting or burst cutting principle when processing prepreg materials and the blades will need to be cleaned manually on a regular basis. However, if tangential slitting is used, as opposed to bottom-knife wrap shear slitting, it will significantly reduce any potential contamination of the female bottom knife. If suitable cleaning liquids can be found to physically clean the male and female knives, these could be used to continually wipe the blades clean during the slitting process. However, silicone sprays and liquids must never be used for this purpose, as they would contaminate the prepreg itself. Once the material has been slit, each of the slit widths should be treated separately as soon as possible from a tension point of view, and the minimum possible bend radius imparted to the product itself. Therefore, largediameter rollers are essential, and the diameter depends on the thicknesses of prepreg material being processed. A compact design of the web transport system is important. The number of rollers that the open face of the prepreg material has to make contact with should be minimised and a secondary liner should be inserted as soon as possible. Contact: neal.r@drc.co.uk More information: www.drc.co.uk No53 November - December 2009 / jec composites magazine

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JEC COMPOSITES MAGAZINE - Issue #52 - October 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #52 - October 2009