Issue 46

C. Bellini et alii, Frattura ed Integrità Strutturale, 46 (2018) 319-331; DOI: 10.3221/IGF-ESIS.46.29 323 or near the groove intersection. In the fourth and last machining phase, visible in Fig. 5d, a chamfer of 0.3 mm and 45° was milled on both side of each groove. The mould surface was treated before the stratification was carried out in order to make it unaffected by the dimensional variations that may occur during the cure process of prepreg. Once the correspondence between the CAD model and the produced mould was verified, a size was spread on the mould to fill the surface porosity, then a release agent was put to avoid part sticking at the end of the process; a layer of liquid wax was added, which was accurately laid down on the whole mould to make it still less porous. a) b) c) d) Figure 5 : Machining phases to produce the mould: a) cylindrical surface rough milling; b) cylindrical surface finishing; c) groove milling; d) chamfer milling . Design of tape stratification plan For the manufacturing of the lattice structure, to follow the grooves machined on the mould surface was necessary, stratifying the tape on the mould; for this operation it had to be taken into account the fact that the helical ribs had a radial thrust lower than the circumferential ones, therefore the stratification plan provided a first step in which all the helical ribs were stratified and then the circumferential ones; this sequence was executed for each one of the calculated layers of composite tape. The first step consisted in calculating the number of layers to be laid down to obtain the right structure dimensions, that were a rib thickness of 2 mm and a width of 5mm. Starting from the properties of the material used, the number of tape layers to be considered was calculated as described in the following. First of all, the weight per metre of the prepreg tape P C was determined from the fibre weight per metre P F and the resin weight content W R as: 1 F C R P P W   (1) The volume of a one millimetre of the rib V rib was calculated from rib width and thickness, indicated with w rib and t rib , respectively, according to the following relation: 1.04 rib rib rib V w t    (2) The weight of a one millimetre of the rib P rib was calculated multiplying V rib for the composite density ρ C by: rib rib C P V    (3)