Building Masters For A 1/48 SEAWOLF Pump-Jet, Part-2

It came time to take the three blade masters and make rubber tools from which I could cast the required number of white metal pieces. Those metal blades would then be assembled, with the aid of specialized assembly jigs, around Renshape 40 cone sections, forming the two stator masters and the single rotor master.

It goes without saying that you have to exercise some care when working with molten metal: don't drink it, sling it at friends, and don't pour it on the cat. If you're ever going to wear eye-protection in the shop, this is the time! A heavy work apron is a must too. And an old Molder's trick: wrap masking tape around the cuff of your pants to pull them tight around the top of your shoes, guess why!

And whatever you do, don't get any water or other liquid on the molten metal or it will explode in a shower of terribly hot droplets of very angry metal -- very nasty!

Let me reiterate the endgame here, as what I'm presenting may be a bit confusing to some of you: I need to produce cast resin pieces that will assemble into a practical 1/48 scale SEAWOLF class submarine model pump-jet (PJ). As I want to make multiple units I had to come up with a repeatable process, in this case a set of rubber tools from which to produce the production parts. But, first, and what you're looking at here, is creation of the many subassembly masters needed to produce the four principle working masters (the forward hull taper with attached pre-swirl stator blades, the rotor hub with its attached blades, the dunce-cap with its attached post-swirl stator blades, and a two-piece shroud). The rubber tools here are not production tools, they are master subassembly tools -- their job will be done once I have the required number of stator and rotor blades.

Later installments will detail the creation and use of the production masters and production tools.

There are three multiple subassembly items to the PJ masters I'm building here: the rotor blades, the forward pre-swirl stators and the post-swirl stators. So, I built a master of each type blade -- a twisted piece of annealed brass sheet built up Evercoat filler and worked with file and sandpaper to achieve the correct foil sections, and Renshape 40 for the two stator blade masters. This meant making a rubber tool from each master -- using a rubber that can tolerate the heat involved as I cast the required number of pieces from white metal. Here you see the three masters, each mounted on its own moldboard and surrounded by a cylindrical containment -- the containment, or dams, needed to hold the freshly poured rubber while it changed state from liquid to rubbery solid, a process that takes about twelve hours.

This is how I secure each blade to the moldboard. What you don't see here is a short length of 1/16" brass wire fit into the base of the blade and into a hole atop the moldboard. The pin holds the blade master some standoff distance off the moldboard. The clay, built up between the moldboard and base of the blade master will give form to the eventual tool sprue hole through which the molten metal will be introduced. Care was taken to use a non-sulfur bearing clay. Sulfur in contact with most RTV rubber systems will retard the cure of the rubber, resulting in a God-awful mess!

I use BJB Enterprises, Inc. TC-5050 blue RTV silicon rubber for my tools that will be used to cast low temperature alloys, such as white metal (Tin/Antimoney) and Lead. This mold rubber, once cured hard, has a working temperature of about six-hundred degrees! The TC-5050 rubber and catalyst is mixed to a ration of ten-toone. Note that I use a postal scale to measure out the rubber and catalyst. It's vital that metal tooling rubber, once mixed with cattiest, be subjected to a hard vacuum to chace out all entrapped air bubbles, a process called de-airing. This requires subjecting the mixed rubber to a hard vacuum until most of the entrapped bubbles have been evacuated -- a minimum vacuum of twenty-nine inches of Mercury is required. All tools that will be later used for either metal or pressure resin casting must be de-aired.

White metal is typically a ninety-five percent Tin to five percent Antimony alloy and melts at around five-hundred degrees. The melting can be done in one of the wife's saucepans in the kitchen (make sure she's out shopping when you do this!) or you can do what I do and employ a gun bullet melting pot like the one seen in background. The Alloy I use is simply 'leadless solder,' available at any hardware store. Leadless solder is white metal, boys and girls! The three rubber tools in the foreground have been brushed inside with Talc. Talc is hygroscopic and pulls moisture out of the tool cavities -- water, even the small amount in the atmosphere that condenses onto the cavity surfaces of the tool will react with molten metal, imparting pits on the surface of a cast part. Always sprinkle on, brush around, and bang out the talc prior to a metal pour.