Casting a Tunnel Portal

Discussion in 'Scratchin' & Bashin'' started by George D, Jan 17, 2004.

  1. George D

    George D Member

    I thought someone might be interested in the process I used to make the mold that was used to cast this tunnel portal. The portal casting is HO. The casting needs a little more sanding then painting, and it will be ready to go.
  2. George D

    George D Member

    For materials, I used some scrap wood, 0.030 styrene sheets and the glues in the picture. I use non flammable contact cement when I glue styrene to another material. I’ve heard that contact cements like Walthers Goo will attack plastic. I can’t confirm it, but why take the chance? The two on the right are for styrene. The Model Master liquid cement has more viscosity and dries slower than the Pro Weld and is good for fastening the layers of styrene together.
  3. George D

    George D Member

    It all started when I needed a single track tunnel portal for a layout that I’m helping to build. A while back, I built a mold to cast some double track portals for our club layout, so I figured why not change the dimensions and make a mold for a single track portal. Building this was pretty straight forward. In fact, I put this mold together in an hour or so. My previous experience did save me time in designing the thing.

    Here’s a picture of the base of the mold with the major dimensions added. It’s designed so the portal casting will be taller than it has to be. You can always saw a little off the bottom with a hacksaw to make it fit. You may be wondering about the yellow styrene. The short story is that it supposedly came from an office where it was used as drawer dividers. It’s .030 thick. Evergreen styrene can be used too.

    The picture doesn’t show the perimeter walls for the mold because they are removable. This feature makes removing the casting from the mold easier.
  4. George D

    George D Member

    I started by cutting out the tunnel opening from a piece of styrene. I use the scribe and snap method of cutting styrene. Maybe a quick explanation of this should be covered. To cut styrene, scribe a line where you want the cut and snap it apart. I use an Xacto knife with a #11 blade. Scribe the line, along a steel ruler, using the back of the knife point - not the sharp side. Don’t press hard, just make several easy passes. Then bend the plastic back away from the scribed surface. Maybe a tutorial on working with styrene would be useful – anyone?

    To scribe the upper half of the circle, I used a set of dividers. Then, the vertical lines were scribed on a tangent to the circle. Snap it out carefully and clean up the edge with sand paper. One caution, make sure the dimensions of the bottom of the tunnel opening are equal to or maybe 1/32” larger than the circle diameter at the top. The only way to remove the casting from the mold is to slide it up along the base and a narrow bottom won’t clear the wider top.

    The piece that’s been cut out is used for the mold, not the left over piece in this picture.
  5. George D

    George D Member

    This picture shows that the mold is made in layers. It’s four layers high. The top (fourth) layer is the tunnel opening. It’s hidden inside the white styrene wall. The piece for the opening is glued on the vertical centerline of the third layer, and it’s flush with the bottom.

    The tunnel “walls” are formed by the white piece of styrene. I used 3/8” wide, 0.30” styrene for this. The dimensions aren't critical but it has to be wide enough to handle the thickness of the casting. I started with one end flush with the bottom of the third layer and glue it around the outside edge of the opening piece. This step goes a little slow. You just have to work it carefully, making sure it stays tight against the opening piece. I use heavy toolmaker’s angle plates from Micro-Mark,, to help keep the wall piece vertical while the cement dries. The bracing is made from scraps. They reinforce the wall when the plaster is poured. I don’t know if they’re necessary, but it only takes a few minutes to add them. Trim off the other end of the wall piece once the cement has set and everything is tight.

    The third layer is the portal face and the second layer is for the top trim piece of the portal. This may be clearer when you look at the picture of the finished casting. The first layer is the base - the sides for the mold sit on it. Don’t pay any attention to the line in the left of the picture, it was drawn when I was laying things out, but wasn’t used.
  6. George D

    George D Member

    As I said earlier, the walls of the mold are removable. They are made from scraps of wood I ran through the table saw and they turned out to be 3/4”X7/8”. Like the portal wall piece, they need to be large enough to handle the thickness of the casting. I glued ½” wide strips of styrene to the wood with contact cement. This is so the plaster won’t stick to it. The styrene is glued flush along one side of the wood. Later you’ll see that the flush side goes down. The picture shows two of the four walls.
  7. George D

    George D Member

    Here’s the mold with the four walls clamped in place.
  8. George D

    George D Member

    To provide clearance for the clamps under the mold, some scrap wood was glued to the bottom of the mold. I used contact cement for this.
  9. George D

    George D Member

    Here’s a closer shot of the mold with the walls clamped on.
  10. George D

    George D Member

    Here’s a shot of the mold filled with freshly poured Hydrocal.
  11. George D

    George D Member

    Let the Hydrocal sit over night. That way you know it’s strong enough to take the stress of removing it from the mold. Take the four walls off and push up on the two bottom edges of the portal. The casting should come loose and slide out.

    Using this technique, it’s simple to make other things such as “concrete” retaining walls. If you need a “cement” tunnel portal give this a try.

  12. Arlaghan

    Arlaghan Member

    Great How-To, George!

    I've done this trick before with plaster of paris and cardstock. The plaster soaks the cardstock so it's a one shot deal. Your mold is certainly much nicer! :D
  13. Ralph

    Ralph's for fun!

    Great info and nicely presented George! An excellent "how to".
  14. George D

    George D Member

    Thanks guys. I left out an important step after the Hydrocal is poured. This applies to pouring plaster in any mold. Tap the bottom of the mold with the handle of a screwdriver for a minute or so. This eliminates bubbles in the casting. In fact, you can see the bubbles come to the surface as you do it.

  15. Matthyro

    Matthyro Will always be re-membered

    An excellent tutorial George. Thanks for posting it.
  16. Fred_M

    Fred_M Guest

    Cover the cardstock with clear packing tape. Will not soak up water and the plaster will not stick to it. DASH

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