A 1/48 SEAWOLF Pump-Jet, Part-9

A Report to the Cabal:

It's been quite a trip. First the design work, the masters, and the tools. And now, finally, production of the actual working parts, cast from polyurethane resin. The job is almost done.

Let's take a look at this, the second-from-last operation needed to create a

practical 1/48 SEAWOLF class submarine pump-jet: All cast resin pieces required to make up one 1/48 SEAWOLF class pump-jet, behind them the rubber tools used to create them. Two of the tools required only basic pressurization to insure bubble free parts -- the duct tools (one for the forward half of duct, the other for the after half of duct).

The two stator blade equipped items and the rotor had to be formed using the spin-casting technique -- a centrifuge sets the disc shaped tool in rotary motion, centrifugal force driving resin into the thin stator and rotor blade cavities of the tools. The lightweight displaced air-bubbles travel to the center of rotation, where they exit through the sprue hole as make-up resin is introduced (this requiring starting and stopping the centrifuge a few cycles, but with practice, it can be done before the resin hardens).

The rotor tool is a simple two-part type. Take note of all the indexing tits and pits that insure very tight registration of the two tool halves when assembled.

Here I'm showing off the central brass hub foundation piece. It will be encapsulated permanently within the resin rotor hub -- its tough metal presenting a low wear matting surface to the propeller shaft. The brass hub foundation also provides an ideal medium from which to drill and tape a transverse hole, needed to accept the propeller shaft-to-rotor securing set-screw. The hub foundation piece slides over the bore mandrel, the mandrel removed prior to extraction of the solid rotor casting.

Once assembled, with mandrel and hub foundation inside, the same PVC pipe containment ring that was used during the tool making operation, is used to hold the hull halves together. It's important, before the tools is created, to index the inside of the ring with some sort of mark (I scribe a line) so that later assembly of the tools to its ring is the same - PVC pipe is extruded and is notorious for being ovoid, not round, in section!

Just prior to setting the spin-casting tool on the rotary bed of the centrifuge. Note that the bed has a big piece of #100 sandpaper glued to its surface -- this non-slip surface insures that the tool will not slide off-center as I bring up the rpm's. You goof up a spin-up setup and you eat rubber!

Atop the tool, to the left of the projecting bore mandrel, can be seen the sprue hole through which the mixed, still liquid, resin will be introduced. As this hole is so close to the center of rotation there was no need to provide a closure plug -- this tool can be brought up to speed after filling with no escape of resin out the sprue.

The two other tools that required spinning had closure plugs owing to their sprue holes being located well outboard of the center of rotation - with those tools the procedure was to fill the tool with resin as much as possible, spin the tool, stop it, quickly add make-up resin (the initial spin forced bubbles from the extremities of the cavity towards the center of the cavity, under the sprue), and set the tool back spinning and to keep it spinning till the resin change state from liquid to solid.

De-molding the cast resin rotor part was identical to the initial extraction of the rotor master. Note some flash (resin overflow into the tightly spaced flange areas of the upper and lower tool halves) clinging between blades on the cast resin rotor. In the bore of the rotor you can just make out the encapsulated brass insert.

Work left to do is to remove the flash with knife, files, and sanding tools. Then a thorough scrubbing with steel wool saturated in fresh lacquer thinner to remove all mold release silicon oil. Then, an initial priming, followed by filler, putty, sanding, and more priming. All that work repeated until an unblemished part is achieved.