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Sidewall of my Sn3 pilot model.

Sidewall of my S-scale pilot model

! On3 MOLDS AND CASTING !



    The Denver & Rio Grande - DURANGO and GUNNISON Roundhouse!


! LET'S MAKE SOME MOLDS !

    With the patterns complete we now move on to making the rubber molds. I'm using Dow Corning HS II RTV base with the Clay catalyst, which is a very strong and flexable silastic moldmaking rubber. Since I have already composed an article on mold making I will direct you there:     Crow's How 2 - Mold Making

Durango rounhouse construction.

The new rubber is very flexable.

    Here's a look at some of the new molds I've made. That green sort of growns on you. After 20 years some of my original 3110 RTV molds are finally breaking down. Subjecting them to an alchohol wash (de-bubbling agent) didn't help. I'll be more careful with the new ones. The HS II requires vacumm de-airing in the process. 3110 does not. The HS II is very stong and flexable.

Durango rounhouse construction.

When complete the patterns are sealed with Scalecoat Sanding Sealer.

Durango rounhouse construction.

The patterns do not have to be glued down to the base but there's a risk of them shifting out of place and/or breaking.

Durango rounhouse construction.

Here's a series of parts that will all be on the same mold. I've glued them to the plywood base.

Durango rounhouse construction.

Be sure to fill any openings underneath the patterns with putty, wax or other.

Durango rounhouse construction.

Petrolum jelly is used as a releasing agent on the patterns and boards.

Durango rounhouse construction.

Simple dams are built with stripwood. I'm leaving a 1/2" wide boarder around each piece and about 3/16" of thickness.

    I'm using Dow Corning's HS II silicone rubber for these molds. It's softer and more flexable than the 3110 I've normally used. The HS II requires more careful measurement and vacuum de-airing which the 3110 does not (though its recommended). We'll see how it goes. Of course there are a number of choices and brands. This is just what I use for Hydrocal casting.

Durango rounhouse construction.

HS II RTV rubber base and Clay Catalyst.

Durango rounhouse construction.

One scoup of rubber base is about 100 grams.

Durango rounhouse construction.

The rubber and catalyst is carefully weighed out for the 10:1 ratio.

Durango rounhouse construction.

Once mixed the HS II rubber is vacuum de-aired to eliminate bubbles.

Durango rounhouse construction.

The rubber is poured into the mold box.

Durango rounhouse construction.

Just a little more.

Durango rounhouse construction.

The mold box is vigorously tapped to help any trapped air bubbles rise to the surface.

Durango rounhouse construction.

In order to get a perfectly smooth back on the new mold a sheet of styrene is placed over the rubber and then a flat board followed with some weight. I'm using a drill press vise for this.

Durango rounhouse construction.

In the morning a stack of molds for the Durango rounhouse are ready to have the patterns removed.

Durango rounhouse construction.

Here's that sheet of styrene being removed.

Durango rounhouse construction.

We have a nice flat back.

Durango rounhouse construction.

The excess rubber is trimmed off.

Durango rounhouse construction.

The new mold is gently pulled away from the pattern.

Durango rounhouse construction.

Being careful not to damage the pattern.

Durango rounhouse construction.

Almost have it.

Durango rounhouse construction.

Now we can reproduce those parts.

Durango rounhouse construction.

On some larger pieces I do not glue them to the board. Some rubber may migrate underneath the pattern. Before removing the pattern from the new mold the excess can be simply removed with your thumbnail or a razor.

Durango rounhouse construction.

Here are the inspection pit side walls.

NOW WE CAN DO SOME CASTING

    So now we can start castings. I'm always excited about new molds. I've been working on these patterns for months now so finally we can start pouring and make dozens of them if we want.

Durango rounhouse construction.

Durango rounhouse. Mold sink. Don't do this in the kitchen sink. Not if you want your wife to ever talk to you again. I've plumbed in an old sink and have a trap underneath it to catch all the plaster chunks and yuk.

Durango rounhouse construction.

Here's the trap. About once a year it needs to be cleaned out. One of those dirtiest jobs you see on TV.

Durango rounhouse construction.

Close-up of a four pocket mold. These are for Gunnison's front portal.

Durango rounhouse construction.

I've just poured the Hydrocal in these molds and tapped them vigorously to help remove any trapped air bubbles. In a moment they will begin to harden.

Durango rounhouse construction.

Before the Hydrocal gets too hard the backs are scraped smooth. It must be done gently on the HS II as it is much softer than the 3110 rubber I'm used to.

Durango rounhouse construction.

Now we let them set up.

Durango rounhouse construction.

In 15-20 minutes the castings are hard enough to remove from the molds.

Durango rounhouse construction.

The castings are gently removed from the molds.

Durango rounhouse construction.

The molds are rinsed off and the process can start one again.

Durango rounhouse construction.

All that HO scale stuff looks so small! Where the heck am I going to stack the new castings? Somethings got to give.

Durango rounhouse construction.

The casting sink corner.

NARROW GAUGE RESEARCH CENTER

Durango rounhouse construction.

Here's the research library for the Durango roundhouse.

Durango rounhouse construction.

Some sketch notes.

Durango rounhouse construction.

The laser computer desk.

Durango rounhouse construction.

We're using Corel Draw to run the laser. Looks like the roof panels for Gunnison'e extended stalls are on the screen.

Durango rounhouse construction.

Durango rounhouse.

Durango rounhouse construction.

Durango rounhouse.

Durango rounhouse construction.

Durango rounhouse.

Link to next section:     Constructing the Base    

Link back to:     Durango Roundhouse Page    

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  C. C. CROW     P. O. BOX 1427      MUKILTEO, WA   98275   USA