Hi

I didn't know where to put this topic but finally went with the General Modeling section.
I want to make a mold and cast a building in dental plaster (maybe in resin, but it's pretty expensive, so I'm leaning more towards plaster). The problem is, that the piece I want to take the mold of is pretty big, actually it's a medieval tower that some of you might alredy know form the Scratchbuilt section. The tower is a two part piece:
- the lower part, dimensions: height: 15cm, width: 9cm, depth: 9cm (basicly it's a cuboid, that is empty inside, the walls are 2cm thick)
- the upper part, dimensions: height 4cm, width: 10cm, depth: 10cm (it's an upper platform)

I was thinking about making two molds, one for each part. I was thinking about something like this: The bigger part is placed in a container and then silicone rubber is poured inside the container. After the rubber sets the whole thing is taken out of the container and the original piece is removed from the mold. When casting with plaster, a bigger piece of styrene is placed in the middle of the mold so that less plaster is used when the cast is being made. Problems with this idea:
- a lot of rubber is needed
- the mold is pretty unstable on it's own, as I was planning on making the walls of the mold pretty thin (only 1cm) to use less rubber
- I have to use some "filling material" to use less plaster, as the mold is pretty much a big rubber container, with an open top, empty inside (textured from the inside)

The upper part mold would be done in a similar way, but it's smaller so it wouldn't be such a problem.

And here's a question to you all: have you ever made a mold based on a big piece?? If so what do you suggest?? I want to use a one part mold for the bigger part because I plan on selling that tower, and the less pieces you have to glue together the more happy the costumer is (plus making it a mutliple part model means everything should mathc perfectly and that another problem to take care of). If you have any ideas or suggestions, or maybe had a similar problem recently I would be very grateful if you could reply. Thanks on advance !!
If you need a photo of the tower, just drop me a PM or write something here and I will post a picture.

Cheers,
dsc.

If you are going to do it as a commercial thing...ie sell the remold as a kit, then bite the bullet and make yourself a decent mold that will stand up to repeated use. I realise this costs a lot more but in the long run it would pay off.

cheers
Cliff

Roo is right here. You can also get different grades of RTV. Some are very "stiff" which may be perfect for what you are doing as there really are no undercuts. As for using styrene in the plaster, thats strange, plaster is much cheaper than plastic anyday.

HTH

Gotta go with Roo on this as well. Is this going to be for a commercial application or just a one time deal? If it is a one time thing I have some input for you on ways to cheat. Standing by. "Q"

here is a product for you to check out....

its called por-a-mold by synair.. I got mine at Dick blicks.. i've been using the medium to make molds of small parts such as ammo boxs and radios for my parts bin from kit bits. and the molds are easy to make.. its a 1 to 1 ratio...

they also make a hard with a 2 gallon kit costing about $70.....

por-a-mold at dick blicks..

Howdy fellas,

I have a quick question...how expensive is "dental plaster" compared to resin??

Tread.

MrRoo: I'm not sure if I'll be able to sell anything, cause I don't know if anyone is even interested in buying such structures. I hope that someone will buy something from me and I'll be able to at least earn the money spent on the materials. The biggest problem is that the lower part is a bit too high and that is what causes the mold to bend.
You are right that I can use more silicone, which is a bit pricey but also should work better for multiple casts.

Sticky: I know that there are a lot of different kinds of RTV silicone, but I've already bought 1kg of a pretty decent silicone rubber produced here in Poland. I've worked with it before and it went pretty well, so I guess it should work also this time. If the mold is too bendy I can always attach it to a box of some sort and than fill it with plaster. When the plaster cures, the box is removed and the cast is taken out. I can use foamboard as the box material, which is pretty easy to work with and should be the right stuff to build the container from. I can use a two-side tape to attach the mold sides to the box and avoid all the bending.
As for the styrene idea, I know that plaster is cheap, but it would be pretty damn heavy when cast only in plaster. And also I would save some plaster for future casts which is cool too.

Matthew: It's not a one time deal, so I guess no cheating this time:) I'm still curious what you meant by that.

Delbert: thanks for the link, but unfortunatelly I live in Poland and I guess the shipping costs would be high enough to give me a heart attack. It looks like a nice material to work with, but the products available here aren't bad either. Dental plaster is cool when it comes to casting and the RTV silicone that I have is also good quality.
I will save the link though, might come in handy sometime.

Gordon: I buy my dental plaster for 5$ (5kg package, in my local dentist store). And a decent casting resin costs around 20$ for a 1.9lbs trial kit (smoot-on smooth cast 300 series trial kit). Plus I would have to pay around 18euros for shipping from Germany to Poland just to buy that trial kit, so you see that resin is a pretty expensive material.

Thanks for all the replies guys!! I wonder if anyone here has ever made a similar mold. Maybe we could exchange ideas and advice.

cheers,
dsc.