Best way to mold this? Hopefully my last question for a bit!

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pandabarnes

Active Member
*** EDIT ***

This thread has moved on a little from the original post, as I decided based on the advice given, to start over!

thought I'd better update this post for people hitting the thread for the first time.

Thanks.


*** EDIT ***

OK, sorry to post with yet another mold related question but youtube has just confused me further, and the couple of books I have on moldmaking have also left me scratching my head.

I did post about this some time ago, but regarding a much smaller piece.

I'm making a love heart sweet, scaled up so it's about 9 inches diameter.

I'm working in polystyrene.

It's becoming annoying, everytime I sand back some filler, I dent another part, of ping off part of a letter.

Soooo... I thought, plaster would be more suitable to work with, and way better for the final piece.

But I'm not sure, can I just make a two part plaster mold of this? and if so, what release agent should I go for?

Silicone I assume will also work, but I'm looking for a cheaper way if possible, as the piece isn't finished yet, so really i just want this stage to change materials for me so it's easier to work.

Attached a photo of the piece, lots to do still. The back it the same as the front, but with no writing.

And yes, i know it's very cheesy! it's going to be a present for my GF if I ever finish it!

20150811_100922.jpg
 
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modelcitizen

Sr Member
questions.
is this the master or the mold?
are you doing the one piece or several copies?
related to the last question, is it even necessary to cast copies if it's just a one off?
i'm not much help but i can tell you if that is the master, you don't need a two part mold. one part will work.

on second thought, perhaps you should try to cast copies just so you discover how the process works practically.
 
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Mr Mold Maker

Sr Member
So what you're saying is you want to take the copy you have been tooling, and cast it in plaster to finish tooling it?
If that is the case, I'd buy some alginate and high quality medical plaster bandages and use it to make a waste mold. You mix up the alginate and put a nice layer on, and once it has gelled, add the plaster bandages. I usually do three ply for waste molds. Then flip it over and do the other side. The alginate won't stick to itself, but I'd recommend releasing the bandages.

Then just cast your stone and viola. Stone master.
 

Darktriumph

New Member
I am not sure exactly what you are going for but if it is the most accurate heart candy as you can get then there is an easier solution: here in the U.S. at least those little candies don't have recessed letters...they are printed on. So in theory for this one project you could just remove the letters and stencil them on after it is cast.

If it were me and I wanted the letters I would make the mold out of something more resilient than the foam...or sand more carefully.
 

pandabarnes

Active Member
Hey thanks for all the replies and advice.

The photo is the master, so I'm looking to cast that into plaster so it's easier to work, and plaster would be more suitable for the final piece.

Polystyrene was definitely a bad choice of material, in hindsight I'd go with something denser that wouldn't fill my flat with tiny white balls, and probably get everything smoothed out before adding the letters.

Sadly, the letters have to be there for it to be an accurate replica of the sweets we have here :(

I think I'm stuck with a two part mold as well because there's the heart shape relief on the rear too.

So what you're saying is you want to take the copy you have been tooling, and cast it in plaster to finish tooling it?
If that is the case, I'd buy some alginate and high quality medical plaster bandages and use it to make a waste mold. You mix up the alginate and put a nice layer on, and once it has gelled, add the plaster bandages. I usually do three ply for waste molds. Then flip it over and do the other side. The alginate won't stick to itself, but I'd recommend releasing the bandages.

Then just cast your stone and viola. Stone master.
That's exactly what I'm trying to do, alginate sounds like the way to go, and I think I have some laying around somewhere too so I'll give it a try.

Thanks!
 

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Plaster shrinks really bad. Try ultracal 30. But make sure your part has enough draft so that it will come out of the mold. And use a good release/ wax
 

clonesix

Sr Member
Hi Pandabarns

First question: No, you cannot make a plaster ( or any hard material) mold of that shape because of the vertical walls of the heart and letters.

The general rule in mold making: Hard part = Soft Tool; Soft part = Hard Tool Meaning: if you want to pull a ridged part out of a mold, the mold must be Rubber (soft)


Next on the list is that master: I don't know what type of putty you are using, but that looks like a sanding nightmare. Any time that the putty is MORE solid than the model, the model will sand prior to the putty. I believe that it would be far less work to start over with a new material, than to continue to try and smooth out that model. If you were to build a more solid model, you could eliminate the need for a mold altogether.
 

pandabarnes

Active Member
Thanks clonesix.

You know, I think you're right - I think I'm going to start over, much as it pains me, with a harder material, but I'm going to try and alginate cast this purely for the experience.

I think my big mistake (apart from using polystyrene) was that I attached the letters before I'd smoothed everything else out.

Any suggestions for a suitable material? I went with polystryene because it was cheap, and the sheets I had laying around were about the right thickness for the letters and heart shape.

I'm wondering if I should go MDF maybe for the main circle, then greyboard or styrene for the heart and letters. So I have a nice sold piece that I can sand back.

Thanks.
 

swgeek

Sr Member
I'd make the whole thing out of mdf. Like you said, I would leave everything separate until it's all smoothed out. Make sure you seal the mdf before you try to make it too smooth.
 

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jarvis

Sr Member
Does it really need additional cleanup? If you actually blew up a sweet heart it would look about as rough as this. I would just make a quick silicone mold and go right to plaster.
224445041_151b59b1ac_b.jpg
 

pandabarnes

Active Member
Thanks Jarvis - I was going for that bashed up look on some of the edges, but in the sweets I have, the space inside the heart shape is quite smooth.

The problem I've run into is by sticking the letters on too early, it's impossible for me to get that smoothness now - it's looking way too messy around the letters, I added some better pics to show what I mean.

Plus i keep knocking chunks out of it!

I also noticed I missed something when scaling up, although really it doesn't matter in this piece - the sweet is thicker inside the heart shapes than outside. Not sure how I missed it, but I'm going to try and address it in the remake.

I think I'm going for MDF until I'm happy, then cast in plaster or similar, and distress from there.

Not looking forward to cutting out the letters again! was hard enough in polystyrene!

20150818_112409.jpg20150818_112425.jpg20150818_112427.jpg
 

Exterminator

Active Member
What about carving hard foam? Easier than MDF, less messy than polystyrene. You should be able to coat it with fibreglass resin too.
 

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pandabarnes

Active Member
Sorry to keep coming back on this one - I've had a look around and I'm getting a bit confused about the high density foam - I keep finding stuff on rolls and that can't be right!

Is it the same stuff they use for insulation? I'm assuming it's going to be a lot more dense and a lot tougher than that.

I found this stuff, but it seems a little on the expensive side for 30cm x 30cm;

http://www.easycomposites.co.uk/products/pattern-making/high-density-polyurethane-foam-block.aspx

I also found these guys;

http://www.tridentfoams.co.uk/tricast.html

Just curious if I'm on the right track, at the moment I can't see how I'd get the correct thicknesses from something like this, whereas with MDF I can get thinner sizes, or layer up.

I'm keen to use the foam if that's the most suitable material, I'm just wary of spending on the wrong thing!

Thanks again.
 

Watson

Sr Member
RPF PREMIUM MEMBER
Why not have someone make a master for you from plastic or MDF that has been cut on a laser cutter. It would make your life so much easier when you go to make a mold
 

pandabarnes

Active Member
Thanks Watson,

Much as I love the laser cut and cnc stuff, I think this one needs to be hand made.

I should prob edit the original post as this thread has moved a bit sideways from the original post into the realms of building materials!

Thanks for replying tho.
 

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