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These are shots of the latest progress.







Note the date on the penny!!

Now we come to one of the more complicated parts of this project. All the other parts have been molded using block molds, or molds that are made as blocks of rubber encasing a master. The master is removed, which could involve cutting one or more slits into the side of the mold, and castings are then made by pouring material into the mold. The Command Sphere is too big to do that, economically.

To keep the amount of rubber needed to make a mold of this size to an affordable level, we have to make what is called a 'jacketed mold'. This is where a hard shell is used to hold a thin mold in its proper shape so accurate castings can be made. I missed getting photos of the first part of this process, but I will describe it as it is pretty simple.

The first thing to do is to mount the master solidly to a base. In the case of the Command Sphere, we need a bit of 'return' to the mold opening, so it is mounted first to a disc of plywood, to create a space under the part where we want rubber to partially cover the opening on the back.



Once this is done, the master is covered with a layer of plastic wrap (Glad Wrap, Saran Wrap, etc.). Next, an even layer of potter's clay (water clay) is built up around the master, completely covering it. The clay is cut with a special device resembling a big cheese slicer, so uniform layers of clay can be cut and applied. The clay is a substitute for the rubber, which will be poured last. Registration keys are also added in clay. Once this clay is in place, a dam is added for each place the mold jacket is to come apart. This dam is usually cut from cardboard or foamcore, and a layer of vaseline is used as a parting agent. Once the dam is in place, the jacket is built up over the clay and against the dams, in fiberglass. The same process can be done using plaster, but a plaster jacket is both heavy and breakable.

Each section of the jacket is built up this way. Once the first section has set up, the dam is removed and the mating surface is released (usually with PVA release) and the next section is built up the same way as the first. In the case of this mold, the jacket is in only two sections. I should also mention that the jacket is also layed up against the base the pattern is mounted to, creating a flange. Keep in mind that anything the fiberglass is touching has been released with either vaseline or PVA, so it won't stick. The only exception is the clay underneath, as it is still soft and nothing will stick to it!

Once all the fiberglass is cured, holes are drilled through the mating surfaces and the flange on the mounting board. These holes are for 1/4" or 3/8" bolts, which will be used for holding the jacket together later. Once this is done, the jacket is pryed open. The clay usually sticks to the jacket and the jacket will have to be washed thoroughly so no clay remains. Next, the pour hole is cut into one part of the jacket. We decided to position the hole over the window area on the master since this would provide more space for the rubber to flow into the mold. We also drill 1/16" holes, called vents, into both sides of the jacket. These are placed all over, in a sort of grid fashion. These allow air to escape as the rubber is poured and will weep rubber as the mold fills, telling us how full the jacket is getting.

At this point, the jacket is reassembled over the master. It is bolted together tightly, both to itself and to the mounting board.



The jacket is designed so that an even 3/8" of rubber surrounds the part, as this was the thickness of the clay.






Here you can see the bolts holding the jacket together and to the base.



As you can see in the photos, we originally planned on using a tall pouring pipe with a funnel at the top. This is done to provide a column of rubber above the mold, creating more pressure as the rubber flows into the cavity. We abandoned this at the last moment as we felt the pipe was not big enough in diameter to allow the rubber to flow quickly.

The sticks seen on the sides of the jacket are legs that will hold the mold upright when we are making the parts. Keep in mind that the mold is upside-down at this point!



We built a temporary 'bowl' of clay around the pour hole so the rubber would flow directly into the cavity. You can also see in this shot how we used screws to plug the vents once they started weeping.

One trick in making a mold of this type is to weigh a sample of the clay used to make the jacket and then weigh an equal volume sample of rubber. This way, the clay used in making the jacket can be weighed and the correct amount of rubber can be determined by establishing a ratio of the weights of the two substances. In the case of this mold, equal volumes of clay and rubber each weighed 170 and 100 grams respectively. This means the weight of the rubber was 58.8% of the weight of the clay. Once we weighed all the clay used to make the jacket, we just multiplied that weight by .588 to get the correct amount of rubber we should mix up to make the mold, since the mold and the clay take up the exact same volume. This number will change depending on the exact kind of clay used, but the process for figuring out the ratio of clay to rubber is the same. This is extremely valuable as it takes the guessing out of how much rubber to mix!

I'll have more later, when we open the mold and strip it off the master.

Here are the first photos of the model, completely finished, with the exception of a few small details.



This is the day after we stripped the mold. We poured some resin into the mold and slushed it around to coat the sides with an even layer. We did this 3 times, building up enough material so that the part would be strong enough to take out of the mold. Future parts will be epoxy and glass layups. (This is a bad photo, sorry...)





More later!

Kurt Zendler, one of the guys who did the patternwork on this model, is now in the process of painting it. Most of the work is in the Command Sphere, due to all the panels, but most of the ship is just a very light gray with some detail paint here and there. What you see here is just the first day's work. This should be finished by Wednesday and I'll have more shots then.





This particular model will be on display in San Jose, California, at the Tech Center Museum of Innovation (I hope I got that right...) starting May 8 or 9 and will be there until early September. This is part of an exhibit that opened in early February, but we sort of missed that date...

D'OH!!



This shot was done with the cheapo telephoto function of my digital camera. But its still pretty cool!

Scott
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