Bob Turner Lost Plastic Process

[bob turner]

Here are the modified trucks - I note that the piston rods can be adjusted to be horizontal, and that often it takes a photo to see the flaws. The larger PA attached is the sand cast CLW with Alclad II "stainless." The ATSF is, of course, nickel silver, but it corrodes rapidly (well, tarnishes is a better word). I shot the trucks with silver paint before re-installing, and am thinking of re-shooting the entire body silver to avoid ever having to attempt polishing it again. I will probably use Alclad II "chrome" on the sides, and silver paint on those parts that were painted on the real ones.

Anyway, here it is after four hours of fooling with a drill press and a hot soldering iron:



By the way, note that the trucks on the DRGW below are CLW trucks - arguably the best in O Scale. I cannot claim to be a great pattern maker, but I can say that I came close!

[De Bruin]

Looks great Bob, did you "pin" the cylinders on first or just tin up both the truck surface and mounting plate and soldier together? What kind of iron (torch?)

[bob turner]

Sort of half-pinned. I cut the mounting pads off the back of each cylinder, drilled and tapped 0-80, and used a stud for alignment. Half the cylinders needed new piston rods.

I think I posted most of this before - we did the trucks by first making plastic/wax masters for the frame, then casting that in bronze, then soldering the journals and brake beams on, then making a mold. I won't be doing it again, but if I ever did it would be with fake coil springs and brake cylinders on the master pattern.

But now we have 3 D printing - I am not at all sure how difficult the programming is, but once you get going you can fix mistakes more easily and cheaply. No more mold charge; you just print in wax and let the foundry pour.

[bob turner]

Oh - I mostly use an 80 Watt Weller. I have a 120 Watt iron, which I use mostly for running boards on locomotives (along with a "pencil tip" propane bottle) and a small Weller gun for wire hookup.

[E7]

But now we have 3 D printing - I am not at all sure how difficult the programming is, but once you get going you can fix mistakes more easily and cheaply. No more mold charge; you just print in wax and let the foundry pour.

I confess that I don't know much about 3D printing, but I've never heard of it using wax.

Another thought: Since shrinkage is only circa 1% with plastic burnouts, wouldn't it be more accurate using plastic?

[bob turner]

There is life here.
The shrinkage (4 1/2%) is due to the rubber mold. What pops out of that is encased in plaster. You lose a tiny percentage when the metal cools, and it really doesn't matter whether the part is plastic or wax.
Wax just takes less of a burnout = cheaper from an energy perspective.

[bob turner]

I always have trouble with nomenclature. Somebody correct me:

The original part is a "master." I think you can consider an original metal part to be a "master pattern."

A mold is made around that. Vulcanized rubber shrinks around 4 1/2%, but there are other compounds that reduce that shrinkage. As far as I know, the vulcanized mold lasts a lot longer - hot wax is shot in there.

Then you have the "investment" - normally plaster and silica - in which the wax (or plastic) parts are embedded.

Once the investment hardens, you bake the wax out. Apparently a microwave is good enough for wax, but if you are imbedding, say, an MTH PA, then you need a very high temperature oven and a "double burnout." Maybe 24 hours on your electric bill?

Finally you pour the metal. That requires special equipment - a centrifuge or a vacuum chamber.

Not an expert - never tried it. I am scared of stuff at 1200 degrees! I do weld, but that is more localized and contained.

[E7]

I would have bet you a beer that lost plastic was 1% shrinkage. I always thought the 3%-4%
was on the sand cast stuff.

You want to stay away from that molten metal for sure.

[bob turner]

We calculate 1/2% in the investment, for either wax or plastic.

If you think about it, the plaster really doesn't exert any force on the master, and once burned out the cavity in the investment is the same size, plastic or wax. Metal shrinks when it cools.

[E7]

I am scared of stuff at 1200 degrees!

Once you get above a certain number, I suspect it's all academic.

[E7]

Oh - I mostly use an 80 Watt Weller. I have a 120 Watt iron, which I use mostly for running boards on locomotives (along with a "pencil tip" propane bottle) and a small Weller gun for wire hookup.

As irons go, I like a 25 watt Weller, or my old American Beauty 50 watt with extended no tin tip from my old Telco tool kit.