Start with the introduction
The following is a quick photo presentation of how spin-casting tools were worked up and used to produce white metal parts. Specifically, making the tools and model parts that fit within the Observation Compartment (OC) – the figures and furnishings housed within the bow of the DeBoer fifty-seven inch long SEAVIEW model kit. -
I’ve covered the process and tools associated with the task of casting resin and metal (and metal spin-casting in particular) in other articles I’ve authored for CultTVman (part-2 of this article and the DOVE epic, to be specific). For the in-depth word and picture description of the process I invite you to look there
The second step to producing cast metal parts is the tool (mold). The tool imparts the shape/form of the desired model part. Specifically, molten metal is introduced into the tool where it undergoes a state change from liquid (molten) to solid (frozen). And that’s the emphasis here: the creation, from high temperature tolerant silicon RTV rubber of spin casting tools from which are cast very intricate, thin walled section, delicate model parts used to furnish and outfit the interior of the SEAVIEW Observation Compartment (OC).
The figures (the creation of four 1/96 Officer figure masters was covered in part-5), two types of chairs, computer reel-to-reel machines, drafting machine (for the Navigator’s dead-reckoning plot), FS-1 hangar access hatch and hand wheel, three types of ash-trays (hey, it was the evil 60′s), a drafting triangle (atop the dead-reckoning trace chart), two ‘manuals’, two clipboards, and two lengths of hydraulic/electrical conduit (seen over the computer and fire-control station bulkheads).
Why fabricate the SEAVIEW model parts from metal and not resin?
Resin cast items become a very difficult medium to handle, file and sandpaper as the part size diminishes – if too small, the parts can be damaged simply through the extraction process from the tool that gives them form. A more robust substrate is required to produce, in mass, model parts of small size and/or narrow cross section. The low temperature melting alloys are ideal for such model part production.
But, one does not need to embrace an entirely new technology, equipment, and tools to make low melt metal parts. In fact, the same type mold material and techniques can be used to produce the masters and tools from which either resin or metal parts are rendered.
The BJB TC-5050 RTV silicon mold making rubber I use is suitable for white metal casting as well as resin and GRP forming.
Previously I described how I fabricated the ten ‘girder’ masters that make up that complicated truss work seen under the window frames of the OC. Those masters were mounted onto a disc shaped moldboard. Backing clay was pressed on the underside of each master to prevent the liquid mold rubber from getting to the back-side of each, permanently encapsulating the master in the solidified rubber – bad! Here I’m pouring catalyzed rubber over the exposed face of the masters to form the first half of the two-piece spin casting tool.
After the first half of the RTV silicon rubber tool cured hard, the masking tape damn was unwrapped, the rubber pulled clear of the moldboard, the masters removed from the tool half, any remaining clay scrapped out, the flange face and cavities of the tool half given a heavy coating of silicon spray ‘mold release’, and the masters re-inserted in preparation for pouring of the second half of the disc shaped metal casting tool. To insure an exact registration between the two halves of the disc tool an array of circular depressions were cut into the flange face of this tool half. These depressions would form a system of indexing ‘pins’ to the flange face of the second tool half.
The ‘end game’. In foreground is the resin ‘foundation’ piece that holds the six key girder pieces (the ones that butt up tight against the inside frames of the four windows). To the left is a freshly cast white metal shot of girder pieces still attached to their sprue/channel network. Another girder piece/sprue shot still within one half of the tool used to create these intricate, yet tough model parts. The girder pieces, when assembled later, were held together with thick formula CA adhesive.
Assembly of the ten girder pieces together to form the truss array became a relatively simple task when the cast resin foundation piece is used, acting as a supporting and alignment jig. I found that nearly the entire interlocking array could be dry-fit together without glue. It was like sticking Lincoln Logs together (man… did I just date myself!)… OK, Lego’s, for you youngsters out there. You get the idea.
Here is perhaps the best shot of the completed chair, computer reel-to-reel machines, hatch, ashtrays, drafting machine, clipboards, drafting triangle, conduits and figure masters. Before committing a spin casting tool master layout to clay I first lay everything out on a piece of paper that has been marked off with concentric ink circles. The outer circle defines the outside edge of the disc (maximum size my spinning machine – a modified blood separation centrifuge – can handle).
The inner circle denotes the point where centrifugal force is so weak as to not produce the required force to assure an adequate fill of a part cavity. Once happy with an even layout of masters within the two concentric circles, I then transferred the masters to a circular disc covered in clay, and prepared it for creation of the first RTV silicon rubber tool half.
The masters have been set, approximately halfway, into the masking clay that covers the moldboard. The clay prevents the applied liquid rubber (which will form the first half of the tool) from getting severely under each item, encapsulating it, making separation of master from tool half all but impossible. The end game is to produce two disc tool halves, each possessing a cavity representing about one-half of a masters volume.
Though RTV silicon rubber is very flexible and will tolerate undercuts, a good tool designer will endeavor to eliminate, or to at least limit, both undercut and draft by careful tool design. The ability to work the clay to mask effectively is an acquired skill. How would you have engineered a two-piece tool to capture a four-legged chair equipped with an open seat back? I did – eight of them. In one tool!
Pouring catalyzed BJB TC-5050 silicon high temperature RTV rubber onto the face of the clay masked masters, producing the first half of the spin-casting tool.How much undercut it too much? Make a few tools and find out. Don’t have the experience yet? Take the time you currently waste in the chat rooms and boards and direct that energy to the practical exercises of model building! There are no worthwhile schools out there teaching how to design and build successful resin, GRP, or metal casting tools – it’s an acquired skill, learned ‘the hard way’. You want easy – stick to the keyboard!
On to part 6B