Scaling Up with naphtha

[only87]

This seems to be ghosting around in the forum and pops up here and then so here's a thread for it.

If you soak a silicone mold in a naphtha bath, the mold will soak itself with the fluid and increase in size / scale to about 130% of its original size. The final results may vary with different materials / silicone.

I wouldn't really advise anybody to this trick because it isn't as easy as it sounds and even if you're doing everything right it's only a 1 to 5 shot, me thinks. This method is seriously for the patient minds. Also your model may turn out too big and you're in for shrinking run after all.

The main problem is that after the mold increased it is soaked full with the oily fluid which perspires from the mold when using it for casting then. This results in a rather raw surface of your final casting, detail will eventually get lost, the casting may very well be totally unusable.
Haven't found a real good solution here yet.

However I found this method useful for some projects of mine. For me, it is a good way to get the basic shape of a model in solid cast already with a little detail, so I can work on it fiurter, especially if there is no other model around. The Furutas come to mind here, some are too small, some adjustments to the proportions in need and the detail is good for a final touch, but the prototype is very exotic and no better or bigger model around.

The whole effort is very time-taking and a hit and miss game so I haven't used it for a long time.

Pictures to follow.

[skunk]

only87 said:

Håkan, the soaking method is the least predictable of all "size adjusting methods" I know of. I would only use that one if there is no other easier shrinking method and that means no good model of it to shrink. Of all those 1/100 Kyoshos there are some bigger scale models that are at least equally good in terms of quality.
I'll open a thread on the topic in the workbench section.

Bah, humbug. Maybe I will have to get some Hydrospan.

Thanks for the info though, and as you say, it might be good for those models that only exist in smaller scales. I am thinking of the tiny tiny Land Cruiser 60 series in particular.

[only87]

Nah, I don't want to discurage. The method is just not miraculos, that's what I wanted to say. Hydrospan might be better, haven't tried it.
Anyhow, here's some models I effectively "blew up" with this method.

First one is Kyosho, one of their 1/100 models, a Nissan 240Z:




The casting didn't turn out too bad, even most of the detail is there, but I'd still say shrinking a 1/64 or 1/72 model would have been easier and been more accurately if the base model was right. Model has already been sanded and putty used on on some places, one door sill turned out too long and had to be reworked.

Here's one I had no real other choice, a Mitsubishi Fuso cab, some Korean (? Kingstar is the brand) diecast in 1/100. There are some models in 1/50 but they're a lot more expensive and offer lesser or just plain wrong detail, so 1/100 was the way to go. It's also not that easy to get some good measurements for these kind of model (Japanese trucks of the 70's...), if you're buying old sales catalogues for hard cash you normally don't know what you'll get if you find any, so scratchbuilding wasn't the better or easier option. If you got a scaled model you only need one measurement and can verify the scale.
Here everything went quite nicely, the surface turned out a bit rough but all the detail more or less was carried over, the grille and the back of the cab transferred nearly excellent which was my main concern. That and that the basic shape and scale was right. Be aware that the pics show the already reworked cab, apart from other works the surface has already been cleaned from the meanest craters which were an effect of the naphtha getting in contact with the resin.





And now the last one, a Nissan Silvia from Furuta. I liked what the method did to the casting here. The thick bumpers turned out very fragile as did the mirrors, so the casting turned out even better than the original model. The solid block has already been reworked, the shape of the side windows corrected and a little this and that. Just haven't come around to give the last finishing touches to it and hollowing it out.



These turned out good and are now well on their way but as I said there was some trial and error before. I learned from that which way I had to put the mold in the naphtha bath so it soaked in consistently, I knew if I had to bend the mold this or the other milimeter and also the final resin casting which I normally do bend again. A bit of putty and sanding went into them and I even pulled out the saw for the Fuso twice. Just with naphtha and hexing things would look a bit different.

[skunk]

Oohhhh... nice - I love construction shots. And all models that I am interested in, which makes things that much nicer.

Is this how all the craters occurred in a certain mazda coupe I know of? For if that's the case, I am definitely going to give Hydrospan a shot first.

[hofan]

Hello:
Your models are very nice. I do not recall if there is a lotus esprit in 1/72 scale to shrink it? have you worked on one before or the model does not interest you at all?

[only87]

hofan said:

Hello:
Your models are very nice. I do not recall if there is a lotus esprit in 1/72 scale to shrink it? have you worked on one before or the model does not interest you at all?

I haven't done anything about the Lotus yet, except from buying one in 1/72. The maker is Kyosho (like the 1/100s) and the model looks really good. I won't touch that one until I got some of my other projects done, I do not possibly have another square inch on my workbench for another project. So chances are high that someone makes that model before me.

[hofan]

Hello:
Glad to know that it is in your plans as I am not sure that someone else will do it. Is the one you will do the V8 or the S1?
Regards

[only87]

That would be the S1. If I remenber correctly the V8 was considered by another private small-series maker, but I don't know if that is still the case.

[Christian]

I have been thinking about this. While the samples you show look great, it really seems to be a hit and miss method.

How about finding a material that extends during the setting? That way normal silicone could be used. I am not thinking about Hydrospan, more along the lines of plaster, which I know extends to a (small) degree when it hardens. I am not a chemist though ...

[Lee]

You want a small expansion......... put water in the mold and freeze it. Ice expands too. ;D

[Christian]

But can you make a mold of the ice model before it melts? ;D

[Lee]

christian said:

But can you make a mold of the ice model before it melts? ;D

Sure, as long as you keep it in the freezer. But there is nothing stopping the ice from expanding into the open end of the mold..........

[only87]

Boys, how about barm then?

I was thinking about using my method the other way around one day. Using the silicone as casting material and blowing the actual casting up. Don't know if that would make any difference though.

[skunk]

That sounds interesting, except then you'd have to cast silicone on silicone twice, which isn't a lot of fun in my experience.

[only87]

Unless you make the mold out of ice water or barm, you're right. So be it. I think I'll find a way somehow. I wouldn't totally dismiss the ice idea totally, I kinda like it.