Molding a bondoed object???

[laxman09]

Hey guys, i'm going a bit off topic with what i am molding, but i've been on this site and trust the advice from here over that of people from the other forums! I'm making a custom motorcycle tail for my cb900f out of a polyurethane foam base (used foam-it 10) and then proceeded to bondo over it to remove the pores and get a nice finished looking surface. it'll be primered before i mold it. Only problem i'm having right now is trying to figure out how i'm going to mold the thing!

Basically, i only need one pull from said mold, and will be casting in either fiberglass or onyx fast. With the experience y'all have in your molding pasts, what would you suggest i mold the bondoed object in? i was thinking about doing a plaster of paris mold because i've got a bunch of this stuff i need to get rid of (and its easy and cheap) but how would that work out in terms of molding and casting? alternatively i can make a mold out of silicone and i know that would work, but i have just a small supply of it and i dont need to make replicas of the tail (dont want to waste the material on a one-time production). so either situation seems to have its ups and downs. any better options are very, very welcome

i tried looking online but i must be terrible at it, i couldnt find any super relevant information that helped me out!

mold masters, I NEED YOUR HELP!


p.s. if this thread is in the wrong place then i apologize, wasnt sure what category this might fit under.



tl;dr

need to mold an object made from bondo, trying to cast in onyx fast or fiberglass. how can i mold this sucker??






will add pics if people demand to see pretty pictures instead of words

 

[clonesix]

Bondo is not the issue so much as shape. When you say "tail", are we talking fender, or something else? Picture always appreciated. If we are talking about the rear portion, behing the seat, it is a 3-sided box with maybe some sharp angles. Sorry, not sure what this part looks like.

Are there any areas that would lock a part into it? Gypsum is not a bad material for making a mold, but Plaster of Paris is about the worst choice possible. Better to choose something tougher, like hydrocal. If I were doing a one-shot mold in plaster, my only concern would be to let it dry completely to remove all msture, and seal it several coats of lacquer, and release it with several coats of PVA prior to laminated any fiberglass into it.

I have done this with foam models, even with undercuts, by digging the foam out of the mold, and breaking the mold after the lamination. It is TRUELY a One-Shot mold.

If you are worried that you may need a second part, or that you will ruin the foam model, then do the mold in two pieces to release well.

Once again, Bondo shouldn't be a problem, but be sure to wax and release the model well.

 

[hydin]

There's always the option of a silicone sleeve style mold.

Either way, yea plaster of paris kinda blows for mold making.

Chris

 

[laxman09]

Bondo is not the issue so much as shape. When you say "tail", are we talking fender, or something else? Picture always appreciated. If we are talking about the rear portion, behing the seat, it is a 3-sided box with maybe some sharp angles. Sorry, not sure what this part looks like.

Are there any areas that would lock a part into it? Gypsum is not a bad material for making a mold, but Plaster of Paris is about the worst choice possible. Better to choose something tougher, like hydrocal. If I were doing a one-shot mold in plaster, my only concern would be to let it dry completely to remove all msture, and seal it several coats of lacquer, and release it with several coats of PVA prior to laminated any fiberglass into it.

I have done this with foam models, even with undercuts, by digging the foam out of the mold, and breaking the mold after the lamination. It is TRUELY a One-Shot mold.

If you are worried that you may need a second part, or that you will ruin the foam model, then do the mold in two pieces to release well.

Once again, Bondo shouldn't be a problem, but be sure to wax and release the model well.

pics for you sir, i knew i shouldve included detaiils about the form, shame on me! you can see here, several undercuts, main one being the rear light housing, i still need to do some foam work on it before i bondo that part of it, but it will end up going inward with a slight bevel on the sides. have some undercuts on the bottom of the sides too as shown in pics. so ultimately i'm looking at two lockable points, i think i may have to make it a 3 part mold... whats hydrocal and where can i find it??

There's always the option of a silicone sleeve style mold.

Either way, yea plaster of paris kinda blows for mold making.

Chris

damn.. here i was thinking i could actually use it for something! haha

for the sleeve mold, how thin do you think i could get away with on the silicone? maybe 1/8" with some upside down trapezoidal shapes to lock into the support shell?



thanks for the insight and advice guys, i really appreciate the help with this!

 

[Duncanator]

Darn, after seeing the photos I can't recommend my first thought which was to vacuform your pattern, and then do your casting or fibreglassing into the vacuform shell. So much for that...

You could do a sleeve mold. That wouldn't use very much silicone. I typically use square blocks rather than trapezoids, but to each his own. The mother mold (support shell) grips the silicone blocks just fine. You could use your plaster for the mother mold, but I'd recommend re-enforcing it with shredded hemp to make it stronger. The plaster shell would likely be faster to make than a fiberglass shell, which is what I usually use.

…and 1/8 inch thickness for your silicone should be adequate.

 

[laxman09]

Darn, after seeing the photos I can't recommend my first thought which was to vacuform your pattern, and then do your casting or fibreglassing into the vacuform shell. So much for that...

You could do a sleeve mold. That wouldn't use very much silicone. I typically use square blocks rather than trapezoids, but to each his own. The mother mold (support shell) grips the silicone blocks just fine. You could use your plaster for the mother mold, but I'd recommend re-enforcing it with shredded hemp to make it stronger. The plaster shell would likely be faster to make than a fiberglass shell, which is what I usually use.

…and 1/8 inch thickness for your silicone should be adequate.

I was hoping so bad to be able to vac form, but the ones i have access too are too small!

so it seems like a sleeve mold might be my best bet, at least this way i'll know for sure im getting the surface quality im looking for! do you mean that you usually use plaster for your mother mold, or fiberglass? i'm able to do either, i guess the answer of which to use would probably be "how long do you want the mold to last" haha

 

[Duncanator]

Ha Ha! Yup, the plaster won't last as long as the fiberglass, but it should be more than adequate for getting a few castings out of your mold. Plus you said that you've already got some lying around, so why not?

I usually use fiberglass for my mother molds, but I've also used plaster, hydrocal and even urethane resin with a TON of filler in it to make it a paste.
I always use shredded hemp in the plaster and hydrocal molds the make them stronger. Even if they crack, the hemp strands can hold them together.

 

[robn1]

I'd say a sleeve mold also. For the sleeve mold, I'd brush on a thin detail coat first. It'll be runny to start with, just keep brushing it back up until it's cured enough to stay put, about 20 minutes or so. When that first layer is pretty firm follow up with a heavier layer thickened with Cabosil or a thixotropic additive. 1/8 inch will do fine.

Plaster is fine for a mother mold, I've used kid's craft plaster from Walmart and it was fine. If the shape doesn't allow for a clean release you can make the plaster mother mold in sections. Easiest way to mix and apply is by hand, keep a large bucket of water handy to clean you hands in. Apply the first layer at least 1/2 inch thick and let it dry. Add another layer reinforced with hemp or burlap, let it dry then follow with one more layer to seal it in.

 

[clonesix]

View attachment 374925View attachment 374926View attachment 374927

pics for you sir, i knew i shouldve included detaiils about the form, shame on me! you can see here, several undercuts, main one being the rear light housing, i still need to do some foam work on it before i bondo that part of it, but it will end up going inward with a slight bevel on the sides. have some undercuts on the bottom of the sides too as shown in pics. so ultimately i'm looking at two lockable points, i think i may have to make it a 3 part mold... whats hydrocal and where can i find it??

Now that I can see what we are trying to mold, I can say that it is too complex for a plaster mold. I can also say that you have a LOT of block sanding to do, so we have plenty of time to plan the mold.

For future projects, Bondo is a terrible sculpting medium! It is an enormous amount of effort to get things straight, smooth, and symetrical. If you do any future fenders, just go with a Chavant medium hard, and a warming oven. You will be much happier with the results.

Back to our regularly scheduled program, already in progress: I suggest spending the money on some silicone RTV, and doing a brush-up mold, with minimum of 6 mm, 10 mm better. If money is an issue, then you may wish to give the thinned silicone caulking method a try. I have never done it, but I hear it works for cheap molds, and you only need one pull out of this.

Once a good layer of rubber is brushed onto the form, and ALL the undercut ares are filled, then you can make a two-piece, burlap-backed, plaster shell.

Hydrocal is much tougher stuff that plaster of paris. google gypsum suppliers in your area. If there is nothing else, look for a good tile supplier, and they will carry a better gypsum than PoP.

Good luck

 

[laxman09]

Ha Ha! Yup, the plaster won't last as long as the fiberglass, but it should be more than adequate for getting a few castings out of your mold. Plus you said that you've already got some lying around, so why not?I usually use fiberglass for my mother molds, but I've also used plaster, hydrocal and even urethane resin with a TON of filler in it to make it a paste.I always use shredded hemp in the plaster and hydrocal molds the make them stronger. Even if they crack, the hemp strands can hold them together.

true, i could probably just make a containing box and suspend the mold in that box, pour strategically, end up with a glove mold and a 3 part mother. sounds like a good plan to me, easy!

I'd say a sleeve mold also. For the sleeve mold, I'd brush on a thin detail coat first. It'll be runny to start with, just keep brushing it back up until it's cured enough to stay put, about 20 minutes or so. When that first layer is pretty firm follow up with a heavier layer thickened with Cabosil or a thixotropic additive. 1/8 inch will do fine.Plaster is fine for a mother mold, I've used kid's craft plaster from Walmart and it was fine. If the shape doesn't allow for a clean release you can make the plaster mother mold in sections. Easiest way to mix and apply is by hand, keep a large bucket of water handy to clean you hands in. Apply the first layer at least 1/2 inch thick and let it dry. Add another layer reinforced with hemp or burlap, let it dry then follow with one more layer to seal it in.

good to know that 1/8th will work, now to calculate and see if i have approximately enough to do it!

Now that I can see what we are trying to mold, I can say that it is too complex for a plaster mold. I can also say that you have a LOT of block sanding to do, so we have plenty of time to plan the mold.For future projects, Bondo is a terrible sculpting medium! It is an enormous amount of effort to get things straight, smooth, and symetrical. If you do any future fenders, just go with a Chavant medium hard, and a warming oven. You will be much happier with the results.Back to our regularly scheduled program, already in progress: I suggest spending the money on some silicone RTV, and doing a brush-up mold, with minimum of 6 mm, 10 mm better. If money is an issue, then you may wish to give the thinned silicone caulking method a try. I have never done it, but I hear it works for cheap molds, and you only need one pull out of this.Once a good layer of rubber is brushed onto the form, and ALL the undercut ares are filled, then you can make a two-piece, burlap-backed, plaster shell.Hydrocal is much tougher stuff that plaster of paris. google gypsum suppliers in your area. If there is nothing else, look for a good tile supplier, and they will carry a better gypsum than PoP.Good luck

well actually i put a layer on top of the foam that was already sculpted to the shape i wanted, then disc sanded to a certain point, then knocked out the disc sander marks that come with disk sanding real quick with 80 grit paper. just need another layer in places to fill in spots that didnt get filled good enough!but i agree, i hate bondo work for things like this, unfortunately i dont have access to an oven or good clay yet, school doesnt start our auto projects till november so i cant nab any free clay right now seems like silicone mold is the way to go, fortunately the sample packs from smooth on are only $30, that plus the stuff i have right now might just be enough to git 'er done. might as well do it the right way the first time ay!updates might come as soon as this weekend to maybe next weekend, gotta work on this inbetween classes!

 

[robn1]

...good to know that 1/8th will work...

A half inch or so is better. But for a smooth part with no surface details, and if you only need one or two pulls, 1/8 will do.