Step 80 : Hull molds.
Time to complete : 30 hours.
Total time to date : 684 hours.
Time to finally put to use that hull plug that has been sitting around so long by making the hull molds from it. The technique is the exact same as when the molds were made for the tower, only of course on a larger (and therefor more difficult to handle) scale. To recap:
1) 1/2 of the plug is to be exposed for the making of the rubber mold. I chose port and starboard halves, but I see no reason why an upper and lower configuration couldn't be used. This is done with the use of a parting board. A hole is cut in the board the shape of the plug, with the plug positioned inside it, exposing one half of it. The remaining gaps are filled in with Clay to make a seal. For this hull plug, I built a box to contain those foam packing peanuts that I put inside. This foam supported the plug from underneath. You do not want the parting board to be supporting the plug, it acts as a shield only. Before putting the plug in the parting board, I used 1/4" masking tape down the exact centre, all the way around the plug. This tape is going to leave an indentation in the mold, which will be cast onto the part, looking like a line. This line will be a reference to where the part needs to be sanded down to. Without this reference line, getting the part sanded down to the proper size after removing from the mold would be a HUGE hassle, if not almost nearly impossible to do. This will become evident when I show the casted hull parts and the removal of the excess material.
2) With the plug now in position, time to get out the gooey stuff. I used Smooth Sil 920 from Smooth-On as my initial RTV glove. I just brushed it on with a paintbrush after mixing it 1:1. I did not use any release agent of any kind. This glove is going to capture all the fine details of the plug. Let cure overnight.
3) With the initial RTV glove cured, I applied two coats of Rebound 25 from Smooth-On. This stuff is mixed and applied the same way. It acts as a rubber reinforcement to the somewhat delicate initial glove, giving the whole rubber mold some thickness and strength. This stuff is orange coloured. Each coat is left to cure over night. Rebound 25 WILL stick to the Smooth Sil 920, forming one unified piece of rubber.
4) The rubber mold is now basically done. I trim the excess away around the plug leaving a nice clean edge. Now a hard back is needed to provide the rigid support for the rubber mold. Without the hardback, all you have is a floppy rubber "thing" that is useless, if you can imagine a person without a skeleton......
......anyway..... Now I apply two layers of polyester fiberglass resin / fiberglass cloth, letting each layer cure, which takes about 4-5 hours. When the whole thing cures, it's hard and is perfectly conformed to the rubber mold, providing what is essentially an exo-skeleton for the rubber mold. I prefer the fiberglass hardback over other materials such as plaster due to it's light weight. With the sizes I'm dealing with on this 1:25 scale U-boat, I have to consider such things. One of those molds complete with the hardback and stand weigh about 25lbs. I could probably add a zero to that if it was a plaster hardback.
5) Before removing this whole thing from the plug, I will put a base on it, making sure it's level. I use Gorilla Glue for this and let it set overnight.
Those are the steps to make a mold for 1/2 the plug. Now comes the difficult part, extracting the plug from this stuff you've piled on top of it. Granted, the Smooth Sil 920 does not stick to the plug, but there is a bit of a vacuum created that keeps everything stuck down. It's a matter of fiddling and prying, and pulling to get them separated. The tough part is not to do any damage to your plug or the hardback in the process. It can be done, I did it twice, once for each side. To make the other half, repeat the process by inverting your parting board, and positioning the other side of your plug. I used a particle board parting board, which was crap. It broke when I was removing my plug so I had to fix it too. If I was doing it again, I would use 5/8" plywood, maybe even 3/4". I cheaped out on the particle board and it cost me time and effort.
This pic shows the parting box, parting board (which is basically the lid for the box), and all the layers of rubber as well as fiberglass hardback complete with stands applied. Under all that stuff is the plug. This is now ready for removal.
This pic shows the two port and starboard molds. The upper one has cross supports already in place. These supports ensure the correct shape of the hardback, given that the hardback does have a little bit of flex in it. I thought about simply making the hardback thick enough that it wouldn't flex, but I decided that this would not only make it much more difficult to remove from the plug/rubber but as well be considerably more costly / time consuming to make. I concluded that I would probably add the supports ANYWAY to a thicker hardback to make sure things were bang on. I figure that even a thick hardback would flex a little. Making and installing the braces was not difficult, and took about an hour and once in place, the hardback is very rigid, as in....it does not move at all. The same supports will be put on the bottom one. The bottom mold has the rubber pulled away from the hardback to show how flexible that rubber mold actually is as well as exposing the hardback underneath and how it conforms. The rubber mold has captured all the details of the plug very nicely.
These molds are ready to start producing some hull castings. Ooooo Goody
More to come.
"I don't have anything else planned for this afternoon." - Lt. Col J.O.E. Vandeleur
A Bridge Too Far (1977)