Process temperature and pressure for forming
Composite helmets are made from strong fabric impregnated with phenolic resin and cut into hexagon/octagon shapes. Straight radial cuts are also made such that hexagon/octagon shapes called pinwheel comprises of crown from which radiates a plurality of straight edge petals. Numerous pinwheels are superposed by placing the crowns on top of one another such that the petals are in staggered relationship. The result is place in a hated mold which comprises match steel dies in compression press and then it is subjected to heat and pressure to form helmet.
The blanks are arranged in stack with their centers aligned, the subsequent blanks are rotated by a predetermined angle with respect to the previous blank. The angle should not exceed 90° or 180° mostly the blanks are rotated at 55° with respect to the previous blank. To make a spherical helmet the blank is placed into a concave cavity with part spherical surface with little pushing force applied. An additional layer is provided at the crown portion of the preform which is preheated and taken to the mold comprising convex lower molding tool and concave upper molding tool, then it is pressed to the desired helmet shape by pressing the tools together and heating at around 140-160°C for about 15 minutes. The curved cuts allow the resin to flow horizontally all over the helmet hence even distribution of the resin. Resistance to the impact of helmets can be predicted more easily than the prior art because the amount of adjacent lobes overlap is predictable.
In thermolplastic prepreg stacks the processing temperature is approximately 30°C than matrix melting temperature, thus showing that hot diaphragm forming is one-step process for solidifying. In modern days hot diaphragm is prevailing in helmet structure manufacturing using thermosetting prepreg. During hot diaphragm forming process tool deformation is attained by heat and vacuum pressure, and in plane deformation behavior between adjacent plies and compaction behavior through thickness direction occur to form curved structure.