04-13-2009, 09:45 AM
Josh, I made my moulds from .060" sheet styrene - look in the Yellow Pages for a plastics supplier, as it's much cheaper buying it in 4'x8' sheets than as small sheets from the hobby shop. For casting, I used a product from Canada Gypsum called Durabond. U.S. Gypsum has the same stuff, under the same name, too. The name is followed by a number, which represents the setting time, in minutes. I used Durabond 90, the same stuff that I used for my landforms and rivers on the layout. It sets in approximately 90 minutes, at which time you can remove the mould. When fully cured, it is extremely hard and durable. Various setting times are available, as quick as 20 minutes, and it can be had, I think, in 2 and 5 lb. boxes, or in 33 lb. bags - the bigger the quantity, the lower the cost per pound. It will set no matter to what consistency it is mixed, although very thin mixtures don't seem to be quite as strong. When fully hardened (it depends on the thickness), it is difficult to file or sand, although, when first removed from the mould it can be carved or scribed to represent cut stone. It takes water-based stains reasonably well, although it's best to use a fairly wide brush to apply it, in order to avoid blotchiness - use a thin mixture and make several applications, as required.
I used vegetable oil, applied with a paper towel, as a mould release, although my moulds were very simple shapes - it didn't seem to affect the plaster's ability to take a stain.
To make the bridge spans modular may be as simple as casting a "key" (a simple protrusion) extending across one end, with a corresponding keyway (depression) in the opposite end. Since some modellers may wish to shorten their spans (a mitre saw - hand-powered
or hacksaw should work) I'd leave placement of the piers up to the individual. You could build the abutments with a "seat" to accept the end of the spans, though, which would both support the bridge and prevent it from moving sideways.
To join the spans to the piers and abutments, white glue or yellow carpenters' glue should work well - I used yellow glue to glue the footings to both the Durabond scenery and the plywood riverbeds.
Wayne
I used vegetable oil, applied with a paper towel, as a mould release, although my moulds were very simple shapes - it didn't seem to affect the plaster's ability to take a stain.
To make the bridge spans modular may be as simple as casting a "key" (a simple protrusion) extending across one end, with a corresponding keyway (depression) in the opposite end. Since some modellers may wish to shorten their spans (a mitre saw - hand-powered
or hacksaw should work) I'd leave placement of the piers up to the individual. You could build the abutments with a "seat" to accept the end of the spans, though, which would both support the bridge and prevent it from moving sideways.To join the spans to the piers and abutments, white glue or yellow carpenters' glue should work well - I used yellow glue to glue the footings to both the Durabond scenery and the plywood riverbeds.
Wayne