thick card will absorb the water and turn to mush. depending on the mixture you've made, it might last long enough to do the job or it might fail before the concrete is set. i'd bet the latter.

i have used chipboard molds before and while they didn't turn to mush, they did stick to the concrete. My advice: line the inside of your formwork with plastic if you want it to look clean.

Chhers for the quick response guys. Would card lined with acetate/acrylic be fine then?

should work. at least thats what my studio prof. told me

you can use vaseline to release the form.... i used formica and lined my molds/forms when i did a few counter tops..... depending how complicated it is, use acrylic........... can also wrap he inside with aluminum foil.....or syran wrap

b

hmmmmmmmmm...concrete countertops.

i used gator board and it worked well. i slightly lubricated it with vegetable oil. don't go too heavy with vegetable oil because it can stain.

try acrylic/plex; the surface will come out the smoothest and cleanest...not to mention you can laser cut the pieces easily.
it also makes a really strong mold that wont buckle easily.

also, instead of concrete....try rockite. it looks much nicer b/c its finer, unless your model is huge/you don't have enough money.

^^^^second the plexi idea, I''ve never used chipboard but I suppose it could work depending on the amount of concrete you are using. Remember to spray the mold with something like PAM, or vaseline before you pour the concrete.

To follow up on justavisual's suggestion: do a test pour with acrylic first, as the concrete surface you get may end up being more glass-like than you want - it almost doesn't look like concrete any more when it's cast on plexi. You could use the laser cutter (or an exacto) to score the surface of the acrylic to make surface texture, I suppose.

Rockite with acrylic/plex is the way to go!!

After quite a few failed attempts, I most successfully (and surprisingly) made the mold out of foam core (spraying it with PAM before pouring). The foam core is "slick" and non-porous and so it peels right off and because it's "foam" core, you can kind of smush it(technical term) to get it out of crevices. I didn't reinforce the concrete with anything as it was never more than about 1/2"-1" thick and it has held up- my mom still has it hanging on the wall in her house- it was too heavy to hang on the fridge ;). By the way, I used quick-crete. You have to work really fast and make sure you really tap out the bubbles, but it dries in just a couple minutes.

2 tiles- poured into mold- quickcrete


good luck...

faom core will work..use a spray on vegitable oil. You need to reinforce it, though

use acrylic or some type of polystyrene (petg works well)

you will get very very smooth surfaces and crisp edges if done right

no oils needed

watch the mix on the concrete..... just get portland cement and some play sand or white silicone sand....... figure out a decent mix.... the regular concrete has tooo many rocks and shyt

b

yeah, stay away from concrete mix!

i would use pourstone in school. nice consistency and grey color.

oh, and experiment with different liners, etc.

depending on the look you want, different form liners will give you different results. try acrylic, mylar, plastic grocery bags, maybe some strips of basswood (for that board formed look), etc.

I normally use hydrocal instead of concrete, and I would recommend it if it suits your purpose. it cures faster and is more dense.

for a mold, I use foam insulation boards. why? the blue boards are:

(1) dense enough to give the finished product a smooth surface (but watch out for the printed side of the board - it can transfer to hydrocal)
(2) rigid enough to cut with a bandsaw but soft enough to cut with a knife
(3) easy to assemble in the final mold (using hot glue)
(4) easy to test as a mold (just fill with water to make sure you are "air tight"

now, I've never coated the insides with anything, and *most* of the mold comes off pretty easy. You can find yourself digging it out for a while, if you have some, shall we say, less-than-exposed voids in the final form you want.

I would think the concrete might work with the blue insulation board also. just make sure you shake out the air bubbles (or, if what you are doing is large enough, get a vibrator).

I used gatorboard and wax paper. I only made 2 concrete models though.

why concrete - try using plaster of paris about 1/2 the weight and dries fairly quickly

About 10 years ago, my thesis studio did a site model out of concrete. It was about 3 feet x 3 feet x 5 feet tall (one continuous pour). We basically did this over Christmas break. The model was much, much more complex than I am writing below, but it is almost impossible to explain fully in writing, so I will only explain the top portion of the model. I'll see if I can post some images later.

1. Made a plexiglass site model (positive). Each plexiglass building was built so that it could be pulled out of the wax negative form independently (and carefully).
2. Made a form with the plexiglass positive and poured a was model (negative).
3. Make a plexiglass box with the wax negative at the bottom and pour concrete (without aggregate, mixed with plaster).
4. Dig out the wax to reveal the concrete model.

"2. Made a form with the plexiglass positive and poured a was model (negative)."

Sorry, make that a "wax model (negative)."

you can try and use chip board and put clear boxing tape on both the outside and inside . it last long enough to pour around 5-10lbs of (quikrete-quickset fine sand mix) the dry time is about 24 hours to pull off the chip board and tape but it comes out pretty clean. 

this is the most cost effective way to go as i tried silicone molds, resins, acrylic, rockite, plaster, bass wood.

quickrete quickest fine sand mix - 12$

chip board(48" x 36") - 4$

tape - 4$

pigment color change - 5$

laser cut - depends what you need 

so your test model comes out to be no more than 25$ :) there are some proffesor who demand alot and dont care about cost so i tried my  

what would people recommend using for small scale models? I'm making a 1:100 model of a fairly small building, not much longer than 500mm. Is it worth using concrete or is there a better alternative?