Improving the SubTech 1/60 ALBACORE phase-2 Kit, Part-4

A Report to the Cabal:

Completing work on the phase-3 tail-cone production tool took up most of my time the last couple of days. Within those hours I did sneak in some work on a special hybrid tool for my 1/96 SKIPJACK kit -- I'll also show off that to show the differences between that tool and the ALBACORE tail-cone tool. It should be most instructive to those of you who have a need to produce hybrid tools for your own projects.

OK, I understand that describing such a complicated and many stepped process as this is a bit tedious, even for you dedicated scratch-builders and I apologize to the majority of you out there who aren't. But, there are some guys who have been asking for the specifics of hybrid tool fabrication, so please be patient with me as I slog through these steps.

The prim objective on the ALBACORE job, initially, is the creation of phase-2
masters, tools, and production parts needed to fill an outstanding SubTech order,
so the short-term endgame here is to make phase-2 parts. So, I start with the

basic tail-cone production master (as needed for the phase-3 and on), make a production tool from that, then cast an epoxy cone in epoxy that becomes the phase-2 tail-cone after attaching two cast epoxy horizontal stabilizer masters, then used to produce the production phase-2 tail-cone master, and finally, from that tool (and copies of it) I'll produce production kit parts.

As we pick up this thread, I've already laid up the rubber over the tail-cone and SKIPJACK sail masters. Time to finish the first half of each tool, and to move on by completing the second half's of these tools.

Whew!

After building up the thickness of the rubber glove-mold with a second, thickened layer of rubber it was time to prepare the mold board for creation of the hard-shell, or mother-mold (some Brit's call it a 'case'). I laid down another rope of clay on the board to serve as a dam to contain the epoxy resin that would saturate fiberglass cloth, forming the GRP mother-mold. Keep in mind that this is the first-half of a two-piece hybrid tool, the same process will be employed (without need of the moldboard) to create the second half of the tool.

This is a hybrid tool for a SKIPJACK kit sail, I show it here to illustrate how the master looks when pulled away from one half of the hybrid tool.

The trick in tool fabrication is to minimize the size and amounts of bubbles entrapped within the rubber element. Entrapped air-bubbles, under pressure (or exposed to a reduction in pressure), cause the rubber element to deform, distorting the cast part it gives form to. When I mix the rubber with catalyst it's unavoidable to introduce air into the mix. However, to chase out the air-bubbles, once the rubber and catalyst are mixed together, the rubber is placed into a bell jar and the interior subjected to a hard vacuum. The high vacuum (must be over twenty-nine inches of Mercury) enlarges the bubbles to the point where they readily float to the surface, burst, and the liberated gas drawn out the vacuum pump.

If a rubber tool is to be, during the casting process, subjected to pressure, vacuum, or high heat (metal casting), then the rubber mix must be evacuated!

Creating the second half of the hybrid tool (I worked the SKIPJACK tool at the same time -- efficiency of multiple pieces undergoing the same work, can be your friend) went the same as building up the first half, but this time the work occurs atop the rubber and GRP flange of the first halves glove-mold and mother-mold.

Creating the second half of the two hybrid tools. With the 'brushable' rubber built up to an appropriate thickness, everything was coated with bowling alley wax, to provide a none-stick surface, and work got underway to build up the mother-mold GRP shells.

Before mixing up the epoxy I cut out strips and squares of seven-ounce fiberglass cloth. The best tool for this is a disc cutter -- an expensive pizza knife, if you will.

Constructing the mother-mold for the second tool half. The first layer to be built up against the rubber and flange of the first half is a thickened slush of catalyzed West System epoxy laminating resin cut with a significant amount of micro-balloon fillet material. I mixed in the balloons till I had a no-sag consistency to the resin. I then brushed this goo over the rubber and flange. It holds to vertical surfaces and forms a wide fillet between the right-angle edges. The objective with this first layer is to build up radius' and to make it easier for the glass cloth to conform without creating air-voids (bubbles) under the glass at sharp edges.

Notice that I've ground round indexing dimples into the first half's mother-mold flange. These will be captured in positive on the flange of the second half's mother-mold -- this to insure perfect registration whenever these two halves are assembled together as the tool is prepared to cast a model part.

A trick I learned from a model boat building symposium a few years back (the Hampton Roads Model Ship Society can be useful from time to time) is to use rice to determine the volume of rubber needed to fill a space. Just dump the rise in, then pour it out into the mixing container, mark the container at the height of the rice, get rid of the rice, then pour rubber and catalyst to the mark. Dirt simple and a neat way to economize on expensive mold making rubber.

I needed to mix up some silicon RTV mold making rubber to form the 'core' piece of the ALBACORE phase-3 tail-cone hybrid tool. I explained above how to use rice to make that determination. A very neat trick.

The appropriate amount of Polytek 71-20 ( http://www.bare-metal.com/polytek/ platsil.html ) mold making rubber, available from BareMetal foil ( http://www.bare metal.com/ ), is mixed up and poured within the interior of the phase-3 tail-cone masters interior. This rubber is a very thin, quick curing, 1:1 mixed, silicon formula

ideal for creating cores, such as the cone shaped rubber item needed to make the tail-cone tool able to produce thin-walled cast resin parts.

I used the same 'brushable' BJB rubber used on the hybrid tool to produce standard two-piece tools needed to make the required number of production masters of the rudders, stern planes, and horizontal stabilizers. Note that the masters were outfitted with the operating shafts as they were used to create the cavities within the tool.

The brass pin sticking out, perpendicular to the stern plane operating shaft, forms a cavity in the tool into which a mandrel is placed, preparatory to resin part casting, to form a bore that is later tapped to receive a 4-40 set screw in the production cast resin kit parts, the means by which the cast resin stern planes are secured to the operating shafts.

Before using the tools to create the production masters I cut in sprue and vent channels. The sprue channels on this set of intermediate tools are much oversized as compared with production tools designed to accept polyurethane casting resin. The epoxy I use to create master parts exhibits less shrinkage than standard production polyurethane casting resin, so I suffer less size distortion. However the epoxy resin (the same West System laminating resin I use for glass lay-up) is thicker, therefore harder to introduce into the closed tool, hence the need to cut in larger diameter sprue channels.

Note that one half of the tool has been outfitted with greased brass rods to form the bores in the cast epoxy masters that have been cast within this tool.

And the cast epoxy masters ready to be cleaned up and used to make the final phase-2 tail-cone production tools from which kit parts will be produced. A nice feature of the West System epoxy is its clarity, permitting you to see how the mandrels within the tool produce the bores in the parts that will later accept control surface operating shafts. Note the rather large sprue channels needed to introduce the thick epoxy laminating resin.

The resin takes about six hours to cure before the parts can be de-molded. Like normal resin casting, the tools are placed in pressure pots, pressurized to about two atmospheres, and left in that condition till the resin completes the majority of its state change (transitions from liquid to solid).

Now to assemble the cast epoxy resin tail-cone and two horizontal stabilizers into a production master.