Back to part 6A
It’s good shop practice to gather all your masters for one or more jobs and to do your tool making at one or two sittings – you reap the benefit of not mixing more rubber than you need, and realize an economy of time as you dedicate yourself to specific tasks in mass. Here, several rubber tools have been poured together and are sitting under several shop incandescent lamps.
The heat from the lights accelerates the cure so I can get on with master extraction and lay-up of the second tool halves. I favor 3M masking tape to hold the liquid rubber as it cures. This tapes adhesive does not contain sulfur, one of the few things that will inhibit the cure of this type RTV rubber.
As soon as the rubber had cured hard – hard being a relative term here, the thing is, after all, ‘rubber’ – the entire affair was shoved into the refrigerator to cool the clay to the point where it would not adhere well to the rubber. Pulled from the fridge the rubber tool half was pulled carefully off the clay (in this case, but not always, the clay makes a clean break from the cured rubber, assuring an easy clean-up chore).
While still cold the masters were carefully pulled free of the clay and cleaned up with a lint free rag or paper towel. As seen here, some of the masters have already been transferred between clay backing and first tool half in preparation of making the second half of the disc tool. Note how the rubber has picked up, as positive dimples/tits the depressions (set into the clay with the shank end of a drill bit).
Preceding re-insertion of masters the entire flange face and cavities of the tool were liberally coated with silicon spray mold release. This to insure that the RTV silicon rubber of the second half disc tool would not adhere to the first half of the tool. Here I’m inserting the masters into the cavities of the first tool half. A 3M masking tape damn was then wrapped around the edge of the first tool half to contain the liquid rubber as the second tool half was poured and left to cure hard.
Note the dimples in the first half here – they create corresponding depressions in the second tool half. It’s this network of dimples/depressions that assures an interlocking network that assures exacting registration of the two tool halves when they are later clamped together for casting.
A picture is worth a thousand words, they say. Examination of the cast white metal ‘shot’ next to one half of the centrifugal disc tool used to form it. As the disc tool is set to spin on the spin-casting machine, molten metal is poured through the central sprue hole, where centrifugal force drives the metal instantly into the cavities where it freezes hard.
Removed from the machine, the tool is opened up and the parts – still attached to their runners and sprue – extracted. Note that even the complicated and thin of section chairs (the chair legs) shapes are easily captured using this type tool and casting medium. How long do you think those chair legs would last had they been rendered as resin? By the way, you can spin-cast resin, using the same tool. You simply have to keep the tool spinning till the resin hardens – tough on the machine!
Illustrating the differences in form and function of the disc type (above) and resin casting tools (below). The two resin casting tools are of the ‘open face’ type, consisting of only one tool element. 2
The wide openings represent the parting plane of the parts produced. Resin is poured in, a piece of wax paper laid over the flange, and a flat board placed atop to produce an even parting plane to the pieces made. Metal casting on the other hand requires a substantial ‘pressure head’, a force either achieved by a tall sprue column/pressure head (gravity pour) or centrifugal force.
OK, this second going over of tool making and casting is out of the way. Time to tell you a few things about what I expect you to do and NOT do with the knowledge I just handed you:
Don’t knock off model kits by taking their parts, using them as masters from which to form tools, and then casting duplicate kit parts of your own! At the very least, exercise the good judgment and courtesy not to steal from someone else. To take someone else’s work (it matters not if the item is or is not currently in production) and reproduce it without the originator’s permission is something I will not continence, no matter what excuse is offered by the offender!
I don’t teach you these techniques so you can reproduce kit parts without the originator’s permission! I teach you these things with the expectation that you will use these techniques to make duplicate parts of your own work. Your Own Work! Period.