Trying to cast a model car shell. Confused.

[TrueDave]

I am having a logic block on this.
You know what a model car shell looks like right? Its a convertable . I can understand casting the top . Also to get the wrap around details I think a mother mold with a thin silcone skin would be good.

BUT. I just cant figure out what I should do with the underside. It cant be a solid positive. Too much resin, heavy and no room for legs of the Megos.

I've been looking around the Car maker sites but I'm lost.

I reallly suspect it isnt this hard.
Yes it will be a "kit" I'll share as with other Forum members if they are interested.

I just want to make a car like you see in WalMart at summer time. You know. A lot like the old stuff? A piece for teh inside seats a windshield and some pegs for teh wheels.
Also I dont want to cast the wheels ar sterring wheel . I need to fins a hobby supplier that does that.

Thanks !

[TrueDave]

sealed the "cockpit" duct taped the wheel wells . Filled it with cement.
Wife told me to make the "bar of soap" i was complaining about and carve away from the positive.
Thanks !

[ctc]

>I just cant figure out what I should do with the underside.

Do it like a model kit: essentially flat bottom that attaches to long pins from the top. (Or you could make the pins/columns separate parts that fit in.)

Don C.

[TrueDave]

Casting it that thin is what scares me Don. I have molded and done the FIRST casting , I can get some tools and teh Resin is soft enough I can carve it out like Ceramic Class. But Then I worry about casting a thin shell.
Like I posted. I filled the shell with cement. blocked off the wheel wells.
BELIEVE me :-) Ill post again ( whining) once the casting and sanding are done.

[AAAAA]

when doing resin thin casting you need a Medium in the resin for strenght and rigidness,
something like carbon fiber, fiberglass or Nylon
the best way to do something like your doing is VAC-U-FORM

[CrimsonGhost]

Pictures would help in understanding exactly what your original looks like, but I think you should do a rotocast. You pour in your resin and roll your mold around so the resin get into every corner and keep rolling it until the resin cures. I would recommend doing 3 or so thin layers as opposed to 1 thick one. It might take a couple of tries to get what you want, but I've had good results doing it by hand.

Anyway, I think that will get you where you want to go.

[pmwasson]

Instead of casting, can you using vacuum forming to make the car shell?

[ctc]

Hmmmm....

Fiberglass would work, but it's tough to work with. EXTREMELY hard, and can react with other plastics. (My very first customs were fiberglass. Took HOURS to hollow out a torso....)

>Instead of casting, can you using vacuum forming to make the car shell?

That'd be your best bet; if you can set up the vaccum-former. I've never gotten it to work, but back in the day a friend of mine used to vaccum-form large vehicles for our Warhammer 40K games. He'd.... er.... appropriate the ads from the city bus to get material. You might want to reinforce 'em with a wood frame; maybe making a wood chassis with wheels, seats, etc and designing a snap or screw on vacform cover.

Don C.

[Tothiro]

If the mold with casting material inside is too heavy to hand turn for a slush casting (roto) You might also try a two part push mold. Basically this is a stationary mold, set up much like the one piece I think you're planning already, except it has a second component that drops down inside and squeezes the resin up, around and out of the mold. Imagine it like the mold is two dixie cups, and your resin is water in the first cup, with the second being pushed down on top...
The set-up just requires a little forethought, because the top half must be rigidly mothermolded, and static (it can't sag in the center or the top and bottom half will touch). Now, you mentioned this was a convertible, so that makes things a lot easier on you (at least for the top of the car) because you can build the mold so that the top mold half is actually touching and supported by the bottom in the middle - since you're casting an elaborate donut, there's no car there.
With the base half of the car (undercarriage, chairs, etc) you really only need the top to have detail, depending on how you plan to fix the axle points for the wheels. I mean traditionally, the metal rods would pass through and sandwich between a housing engineered by the top shell and base fitting together, right? So the bottom of the car is usually pretty vanilla. For this part you could more easily do a traditional slush mold, where you're just rocking the mold left, right, and around to make sure everything is evenly covered. The trouble spot would be the depth of the seats because they'll pool so much resin, but that's why you hand rock it.
Otherwise, you could mold the base in a two part mold, the same as the top, but you might just consider two circular cut outs in the base (under hood and trunk) for the mold halves to touch. Does that make sense?

Like I said with this style you have to make the mold a little differently (especially because of the size) because the top half has to be so rigid. I'd suggest some type of keyhole in the silicone for the top, so for instance when you've poured it, drop down and suspend some things in it which will get stuck in - like the heads of carriage bolts, or something with a big flange, mushroom shape. If you hang them with string or wire, you can remove that once it's cured and then pour a thin mothermold out of actual resin before you do any demolding... slushing the thing as best you can to keep the mothermold thin and cut down weight. This way the mothermold will have the bolts locked into it, and they will in turn secure the silicone to keep it from drooping. There are other solutions to that, and I'm sure somebody might have a more elegant one...

Vacuform is good, and will yield the exterior you're looking for, but it is A) tricky to set up and operate, and B) won't have any interior details, like mounting pegs, attachment points, etc. You'd have to resin/epoxy hand-build all of the internal structure. It sounds to me like you want to do the sculpt work once, and then just pull a shiny plastic piece from the mold that you can trim, screw together and play with, am I right? Stickers not withstanding...

There was a material called Aquafirm or something that they sold a decade plus ago... I don't know if it's still around. Basically it was a fiberglass-like concept but poured like water. You could slush it solo, or reinforce it with strips of cloth, more like plaster work really than fiberglass. I should research that and see if it's still sold.

Let us know how things go with this, because eventually I'll have to tackle it large scale too for my Landspeeder.

ADDED:
Another light mold rigidity idea might be to take thin metal plates with large and numerous cutouts in it (thinking some rigid waffle cut material or the like) and suspend that into the poured silicone... if it's well encapsulated and the cut outs are thick enough that the silicone doesn't tear out, you should be good.
-OR-
you might consider a silicone paste for the top mold half instead of a pourable - something that will set up with less material could give you a concave mothermold space, and here you could use the same wingnut/carriage bolt head/ whatever keyhole idea.

[TrueDave]

Hmmmm....

Fiberglass would work, but it's tough to work with. EXTREMELY hard, and can react with other plastics. (My very first customs were fiberglass. Took HOURS to hollow out a torso....)

>Instead of casting, can you using vacuum forming to make the car shell

I vacuformed a Laser rifle back in school. Then I was a member on the Creepy Crawlers forum for a while. Did a bunch of Vacuforming then. I have the plans for my own machine in a Stage Prop MoldMaker book.
I dont think It's suitable for this.

CTC I dont know if youve done this or not but I casted some Mugato feet and kept the holes intact by inserting metal rods through them when i made the mold. I havent resculpted a Torso yet but I think the same thimg would work. that way you spend less on materials and less post casting work(drilling) .

I am SORRY We don't know how to post pictures yet.

[TrueDave]

TOTHIRO:
WOW!!!!!!
I have read your post. I had my wife read it. And I'm rereading it for the fourth or fifth time. Let me get it all in my head before I comment.

If I did a Slush with the "Bar of Soap" I could make it hollow and dremel off the underside after the casting was pulled from the mold.
But I like the Two Part sluch Mold idea.
I was playing with a Turkey Juice needle. ( not a baster smaller) All this time I thought injection molding was not something I had the facilities for. Now I realize its just FORCING material in with air vents in the mold ( I suppose)
The turkey Sringe Im going to try for getting Hydrospan in TINY molds.

I need to reread these posts again. Thank You so much. If it all works I really hope people like my custom. !!!!!!!
We Will learn how to post pictures!!!!!