With the propousal of making a urethane copy of the master, we built a "glove" type mold. This mold consist on applying several silicone coats along the master surfaces untill forming a overlayed sking of 7/8 mm thick. That silicone "glove" will be then wrapped on a divided in sections bondo mother. This kind of mold is the perfect mate for a rotocasting job.
First of all we placed the master in a stable and fixed position, using a long stick screwed inside the model. Then cleaned the whole master with the airgun with enough pressure to ensure a completely clean surface. Once clean, mixed the first silicone to apply. We waited around 30 minutes after mixing, this way the silicone wouldnt be completely liquid and would be easier to apply to vertical surfaces. The most critical layer is the fist one, because it will transfer the surface to the copy, so it needs extra attention to cover all the surface to replicate with a thinn and homogeneous silicone film.
Once this first coat gets sticky, its time to apply the next layer. And repeat this process until getting a "glove" thick enough to work with without breaking it.
In the last layer of silicone we added a previously casted silicone pieces as a reference for the position of the bondo mother about to be built around them. This bondo mother its divided in 5 sections to make it easier to disassembly the mold in each cast.
Once the mold is done, we can disassembly the mold for the first time, then remove the master from the inside and prepare everything for the casting to get the final piece.
We tried different urethane resins for the rotocasting process. The first try was with Rotocast resing of Smooth-cast. The copy was neat. Very few little spots to fill with bondo here and there, but not to much work needed. With the first cast practically finished and ready to paint, we noticed this resin has a very low melting temperature, what means that with 80ºC the helmet could suffer deformations. We werent sure about that "low melting point" so we changed the plan and tried another Smooth-Cast resin called Onyx Fast. Technically its not for rotocasting, but the viscosity and the fast cure time make possible to rotocast smooth enough thinn layers without any bubbles in the result.
This Onyx resin can be layered without laminating the result if applied when the previous coat is still sticky. The first cast of Onyx was as successful as the Rotocast one. The problem was the porous surface appearing all over the cast as a result of the sanding process. It is a little small holes finish all over the sanded surfaces. So, we decided to give it few coats of primer to solve the porous surfaces problem. It didn't worked, solved the little holes problem but the primer didnt stick well on the urethane plastic. We checked the Smooth-Cast data sheet of the resin and they say it is not a paintable material. Technically is only for black color casts.
At this point, we were stuck. We had to figure out how to make a rotocast copy with sandable surface and durability enough. After exhausting the possibilities, we reached to a possible solution. Applying the liquid primer directly inside the mold as a first rotocast coat, pouring out the excess before the primer cures. And then, when the primer is still sticky, continue with Onyx the desired coats overlayed on to the primer. The result: an allready primed cast ready to sand and paint with the same durability of a car.
Once we had an allready primed cast straight out of the mold, was time to start triming all the extra material. This includes: the front panel, the back panel and the inner extra material. The front panel trim results in a big hole in witch the formed visor will almost exactly fit. Removing the nape backpanel would give access to the inner controls and electronics and a entrance for the power ribbon coming from the gauntlet. The last trim is the inner extra, this will result on a homogeneus and smooth edge around the helmet.
What comes next is a total of 4-5 days of applying primer patches her and there and a scalated sanding from grain 400 to 1000 grain sanding paper: 400, 600, 800 and finally 1000 on all the helmet surfaces. If the geometry is continious enough, 1000 grain sanding paper finish should be enough for a guarantied mirror finish on the chrome.
With the cast trimmed out and sanded, we only had to drill the holes to attatch the ears, the control knob/leds and the cast would be ready for the paint job. A last intensive cleaning with filtered water, airgun and a silicone remover agent will get this unit ready for a high quality result on the paint job.