JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010 - 55

Their favourite system is based on a modified hydraulic or mechanical forklift with a series of 6-metre aluminium platforms articulated between the lifts. It is important to weigh down the lifts or anchor them, especially when working at heights of 2 metres or more. Model and tooling construction The production of master models follows the same trend in production boat building in that 3D milling is taking over manual model construction. Tooling requirements in one-off advanced marine composites are similar to what is used in production boat building with only a few differences. Such differences come from considerations of cost, timing and precision. it to float without hard anchoring spots to avoid mould skin distortion through differential heat expansion and the risk of marking the component. Specialised grades of tooling carbon prepreg specifically designed for this purpose and curing at no more than 60°C are used with further post-cure at higher temperatures to achieve the necessary properties. Although other methods exist such as wet-preg with high-temperature tooling resins or even infusion, tooling prepregs present clear advantages in tool construction due to the easier handling of the materials and absence of drawbacks associated with wet resins or the risk of porosity when using infusion. Curing oven requirements Oven control systems should give the operator a complete view of the process limits in real time with an automatic facility to control the heating curve based on a selection of those values. This means that enough thermocouples have to be positioned at the most critical points and spread out over and under the component surface in a pattern that allows the operator to verify that the airflow is correct and sufficient to effect the temperature distribution required by the curing process. To guarantee uniformity of results over large surfaces, it is necessary to direct the hot air to produce a turbulent flow that eliminates hot and cold spot formation. Because every component is of a different shape and mass, it is important to check every time that there are no areas of uneven distribution. One of the issues that influence temperature distribution in a curing process is a large mould mass. Because the mould absorbs heat just like the curing prepreg, it can adversely affect the curing curve, delaying the temperature climb in certain areas and hence distorting the ramp rate and overall curing curve. In extreme cases, this can result in poor laminate quality if the resin flow does not occur when and as required by the process. A very high airflow is a guaranty of success. Given the characteristic curve of low-temperature prepreg cure, where the requirement is normally for a ramp rate of 0.5 to 1°C per minute up to the dwell temperature and then the same to the cure temperature, the oven power requirements are very much dictated by the combined mass of the component and mould and the process temperature rate of climb. Typically, for a 25-30 metre oven with an internal volume of 1300 m3, the power requirement can be estimated at 450-500 kW. Provided there is sufficient air flow, this power will only be required during the initial temperature climb. To maintain a temperature of 85-90°C in such circumstances, a power rating of 190-250 kW is sufficient. Poor airflow slows the heat transfer to the component, which can be further delayed by a large mould mass. In such cases, it is difficult, if not impossible, to achieve even a temperature climb rate of 0.5°C per minute. By the time certain parts of the component arrive at the dwell temperature (say 70°C) where the Oven control system screen shot showing the relevant parameters The number of units to be built in production invariably justifies the use of sturdy female composite tooling built to last many demoulding cycles, whereas in one-off construction the rule is to use light wooden male plugs over which the components are constructed. There is a scenario where the use of female moulds is becoming more common in one-off construction, i.e. when the structures use carbon prepregs with elevated temperature cure coupled with the need for absolute shape precision and a lightweight surface finish. It is common to opt for female moulds in the construction of ACC or VOR70 hulls, especially if two identical hulls are ordered. Carbon tooling materials are normally used for their physical behaviour similarities to structural prepregs, having very close coefficients of thermal expansion. Three critical considerations should prevail in tooling design for this application: - The tool skin should be totally non porous to avoid vacuum leakage. - The tool should be light to reduce its thermal mass; usually a skin thickness of no more than 6 to 10 mm is adequate. - The tool should be supported by a frame structure that allows No55 February - March 2010 / jec composites magazine

JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010