Step 19: Chapter 3- Helmet Molding 3: Jacket Molding the Front
Now that you've finished the silicone mold of the front, you'll need to make what is called a jacket mold (or mother mold). The jacket mold provides rigid support for the flexible silicone mold inside. This will prevent the mold from flopping around during the casting process (which creates warped or broken models). The size and weight of the finished mold calls for a light weight solution that is still strong and rigid, so we're going to use fiberglass.
First some safety things to get out of the way....READ THIS.
Before we get started you should know that the fiberglass resin is highly toxic and should only be used in well ventilated areas. The fiberglass mat cloth is also unsafe to handle with unprotected skin. I use disposable vinyl gloves. It is quite literally fibrous glass and can irritate the skin and damage the eyes and the lungs. Make sure you use a respirator and not a dust mask. A simple dust mask is not adequate. You do not want fiberglass in your lungs, in your eyes, on your skin, or in your hair. I like to wear a ball cap and hoodie to keep it out of my hair and use goggles instead of safety glasses to keep it out of my eyes. I've had flakes of fiber float over the edge of my glasses while cutting cured fiberglass with a Dremel. Rinsing glass particles out of your eyes is NOT fun.
Also, do not try using fiberglass resin in the cold, since low temperatures will impede curing.
As a rule, I do not ever work with fiberglass in temperatures below 60 degrees. Anymore.
The nozzles shapes on the front of the helmet do not allow for a single part jacket mold. The underlying silicone mold is flexible enough to be pulled around. The cured fiberglass is not. The fiberglass jacket would be mechanically locked around the nozzles and be unable to be pulled away from the back. What we will need to do instead is build the front jacket mold in two pieces that screw together. To do this, you'll need to create a parting wall of clay down the center of the face (as pictured) with key shapes that allow the two shells to interlock.
Begin cutting your fiberglass mat into a pile of 4x4 inch squares. Cut enough squares to cover your mold and allow for about an inch of overlap on each side. Once you have finished, you will need to mix the fiberglass resin that you will use to laminate the fiberglass mat. Fiberglass resin must be mixed with a catalyst (or hardener) before it will cure. You can make a mixture that is anywhere from 4:1 (four parts resin, one part catalyst) or 10:1. The working time for a mixture of resin is different from the cure time and is somewhat short. It will begin to congeal into an unusable goo after about 5 minutes so you will need to work quickly. It will not completely cure however for another few hours. You can extend the working time of your resin by using less catalyst and by using a wide and shallow container. Containers that are tall and narrow will cause the resin to catalyze faster because it has less surface area to vent the heat of the reaction. A 10:1 mixture will take a long time to cure and will be sticky for quite awhile. This can be good if this is your first time using fiberglass or are working on a large area.
Place the fiberglass cloth and use a chip brush* to soak the cloth with resin. You will notice the fiberglass mat go from opaque to clear as the resin catalyzes the cloth. Overlap and laminate your pieces of fiber cloth and continue working in your resin until you've covered the entire work space. Work the cloth and resin snugly around the silicone key shapes that you imbedded in your rubber mold. This will allow the silicone to register with the fiberglass jacket. Allow this to cure for 2-4 hours. Fiberglass will cure faster when using more catalyst (and in warmer temperatures). The result should be a hard, glassy shell. Remove the clay wall and use a Dremel and cutting wheel to cut away the rough, fibrous edges. Use some sand paper to blunt the edge so you do not cut yourself while handling the finished shell (cured fiberglass is very sharp).
Apply a coat of parting wax to the part of the fiberglass shell that will interlock with the surface of the other half of the finished shell. Allow it to dry for 5 minutes and then brush on (or spray) a coat of the PVA solution. You'll need to apply these both to insure that the fiberglass shells do not cure together. You only want the shells to interlock- not cure together. Think of the parting wax and PVA as the mold release spray that you used earlier.
Once this is done, follow the same steps on the other side. Apply your glass cloth and brush in the resin until the other half of the front is covered and resin has cured into a completely rigid shell. Take special care again to laminate the fiberglass around the silicone key shapes. Using the edge of other half of the shell as a guide, cut away the fibrous edges and sand it down. Once you have finished, drill several holes along the seams of the interlocking shells, and thread the holes with screws and wing nuts. These will hold your shells together while you are casting.
* Do not spend too much money on chip brushes since they will be ruined by the resin after it cures.