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I've been doing some experiments recently, making tiny papier mache molds out of hot glue (the molds are for pieces of a tiny PM action figure that I wish to produce in quantities to use as a generic base figure, specifically separate legs, arms, head, and torso). While the results have thus far been promising (I've cast eight legs, six arms, four heads, and five torsos so far--I made the molds at different times which is why the number of limbs, etc. doesn't add up at the moment), I am having some difficulties with air bubbles in the molds. Any ideas how to get air bubbles out of hot glue? The stuff is so viscous, they can't excape very well on their own, nor does tapping the mold seem to facilitate their movement through the medium. I've tried varying where and how I pour the hot glue, when I make the mold, on the PM piece I'm covering, so that it slowly covers them, but I always get some air bubbles no matter what I try. Poking the bubbles with a needle doesn't help much, so, I've just been patching the molds, where necessary, which, while it works, is kind of annoying--I'd rather dispense with the bubbles altogether if possible.
I've been using a candle to melt the hot glue sticks--I know that hot glue becomes more fluid the hotter it gets, maybe that's the answer.
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That's a tough one! The glue cools faster than the bubbles can escape.
With a larger mold cavity, you might be able to fill the holes with more hot glue, but it would seem doubtful with molds so small.
Would you consider changing mold material? My first thought (and successive ones!) were to use polymer clay instead. If you aren't familiar with them, they are bought in small batches, easy to handle, stay soft until 'fired' in a regular oven or toaster oven. Once cured, they are almost indestructible, and they will take a very detailed impression. Mold release materials are cheap and simple: water or talcum powder or cornstarch (cooking type).
Generally speaking, there are firm polymer clays and soft ones. The softer ones are most suitable for molds due to their flexibility after hardening for easy removal of the casting. If flexibility isn't necessary, any available polymer clay will do.
One brand useful for molds is Super Elasticlay, mfg by Polyform (this company also produces Sculpey polymer clay and related products).
There are a ton of online tutorials.
Sue
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The reasons I like to use hot glue for molds are (1) that I can melt it down again and re-use it and (2) it has some flexibility/give to it which facilitates removing pieces from the molds more easily. I''ve tried making some molds out of various clays in the past without much success, but I attribute that more to my own shortcomings than the medium itself.
I've been using mineral oil as a mold release--it works pretty well, but everything tends to get pretty greasy.
Last edited by Patraw (2009-09-10 15:23:10)
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Okay, if you're determined.... ;-)
How do you melt your glue sticks, in the gun? You might try melting it in batch form. One thing that comes to mind is one of those small electric cup-warmers. If you're not familiar with them, they look like this: http://www.amazon.com/s/?ie=UTF8&ke … hsssyj02_e
In a thrift shop, I have bought them for $2. And I just happen to know how hot the contents gets from another experiment: 125F . (Hey, am I a fountain of useless information or what?)
I would avoid stirring or doing anything that would incorporate air bubbles, just let it sit and melt. ATTENDED! This isn't the time to go to another room and have a long conversation.
It seems you could pour the molten glue all over your model for a smooth mold lining, and then fill in with the glue gun where it wouldn't matter.
That's all I can think of.
Sue
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I cut glue sticks up into small chunks and then melt them inside a small metal candle tin, suspended over a candle flame. I don't stir the melted glue, or disturb it in any fashion, until it's poured. The air bubbles only occur during pouring. And, yes, I never leave a flame unattended; doing so indoors is foolhardy, especially with paper nearby.
Glue guns are too large/unwieldly for work this small. I fix any holes in the mold, that are a result of air bubbles, by spot "welding" tiny slivers of hot glue with the tip of a wood burner.
I've tried using wax for molds too, but those experiments didn't work particularly well either.
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There's a method that's used in silicone molding called "bombs away".
The idea is that you pour the silicone from high up, and as the liquid descends, it forms a thin stream that automatically eliminates the bubbles. It works a treat with silicone, but I haven't tried it with hot glue. The chances are that the glue will start to cool and solidify pretty quickly as it pours - unless you can get it REALLY hot, but then there are safety implications involved (how hot can you get the glue before it ignites?)
Like I said, I haven't tried this with hot glue, but it's worth thinking about. If you're used to using hot glue, you may want to experiment within the bounds of safety.
Most important, BE CAREFUL!
Not sure how much help this is, but I thought I'd throw it into the mix.
If you do try this, don't do it where there's any breeze, as this too will cool and solidify the stream of glue, and probably blow it off course.
Good luck
zyggy 
Last edited by zyggy (2009-09-11 09:34:59)
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I guess the stream dropped from a height would certainly provide you with . . a solid stream dropped from a height!
There used to be a molding agent (moulding UK) called Vinamold that melts under heat in a double boiler and takes forms well and is flexible. Alec Tiranti used to sell it. Maybe it has been superceded by other materials now but I'm sure there must be something suitable out there.
I'd avoid using mineral oil. It's not good stuff to get on your skin and, as you say, it is very greasy and could get absorbed into the paper. As Sue says, talc or similar may be a better option.
DavidO
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ziggy: Pouring it from a height sounds interesting, the only problem, as you've stated, is that it might cool down too much whilst airborne before it gets in the mold. I'll keep it in mind though, and give it a shot the next time I make a hot glue mold. That, and heating it up more, might do the trick.
dopapier: Why would mineral oil be bad for your skin (I imagine some people even use it as a moisturizer)? So far as I know, it's completely safe--you can even drink it to aid with constipation (or so the bottle claims). I had some concern that the oil might adversely affect the PM, but that didn't happen--the finished products aren't greasy/soft or anything. The oil that did get absorbed sizzles a bit when I go over the PM with a wood burner, which, I don't doubt probably adds to their flammability, but then, paper's highly flammable to begin with.
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Patraw, the mineral oil thing may be a translation problem. I faintly remember something about what the British call mineral oil being something like American motor oil, but I'm not sure.
DA-VID! What are you talking about?
Sue
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Well, I sculpted a male torso for a 2.5" scale figure last night and made another hot glue mold of it. I heated the hot glue up more, and also poured it into the mold from a greater height (which is no mean feat of accuracy, as the mold is around the size of a postage stamp), unfortunately, both tactics created more air bubbles, not less. I guess I'll have to stick to patching air bubbles holes.
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Pouring from a height will just allow it to cool faster, causing more problems.
Did you try heating it more and pouring as quickly as you could from the lowest possible height?
Sue
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As I said in my post, the 'bombs away' method works a treat for silicone rubber molds, where the liquid stays liquid for quite some time during and after pouring (almost 24 hours, actually, with the silicone I use!) and I suspected it would probably not work with hot glue, but hats off to you for trying!
Do you have to use hot glue? It does seem like you're creating big problems for yourself, especially as you're creating such small molds.
What about using latex rubber? Admittedly it does take longer to build up the mold in several layers, but you will end up with a bubble free mold (if done correctly) and one that's flexible.
I know you can't re-melt and reuse the latex, but it is fairly inexpensive - just don't get it on the carpet!
Good luck
zyggy
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Zyggy...
Aaaaah, now THAT makes more sense! Silicone rubber is different from silicone, and all you mentioned in your post was silicone.
Sue
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Oops, sorry Sue, I forgot that I was using shorthand when I made my original reference to silicone rubber molding.
I can see where the confusion would arise!
Zyggy
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CatPerson: Heating it up more resulting in more bubbles, because the hot glue started to boil. Pouring it at close to the mold as possible was what I was doing to begin with, but, like I said, the stuff is so viscous, it's very difficult for air to escape once it get trapped. The hot glue doesn't cool so fast that the 'bombs away' approach doesn't work, it too just resulted in more air bubbles, not less.
zyggy: I'm sure using another medium would probably work better for my mold making efforts, but I'm a stubborn (and cheap) person. 
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Here's a thought for you, Patraw - if your master models are heat resistant, can you lay your lumps of hot glue over the models, then direct your heat source (ie a hot air gun, or very powerful hair dryer) over the glue so that it melts directly onto your master?
Not having tried this, I haven't a clue if it will work, and I certainly wouldn't suggest it if your original is likely to suffer from it, but my thought is that directly applied heat would avoid having to pour and thus minimize the introduction of air bubbles.
Another variation of this would be to pre-melt the glue into a sheet (at whatever thickness would be appropriate), lay this over the original then hit this with the heat gun. Once that layer has set, you can thicken it up with further layers to your desired thickness.
Whaddaya think?
zyggy
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Zyggy's idea about form a sheet of glue and then using that for the first (smooth) layer might work. A piece of baking parchment might be a good surface on which to melt it into sheet form.
If your glue was boiling, it was too hot. I meant to melt it just a little warmer than the glue gun does. Maybe you could drop some fresh pieces into a small can (tin) and set the can in a some hot water in a pot. When the glue is fully liquified, pour it over the original model.
Sue
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Zyggy: Yup, I've already tried melting small pieces of hot glue directly onto the master sculpt with a wood burner. It works, but, I still get some air bubbles with that method too. The sheet of glue idea hasn't ever occurred to me though, I might give that a try, although I suspect the results would be more-or-less the same as melting smaller pieces directly onto the sculpt.
There's not much danger of the original getting damaged from the heat--I use a wood burner as one of my sculpting tools (which is why my unpainted PM stuff always looks charred). The only important thing is having oil or something on the sculpt so it doesn't stick to the melted hot glue, which will cause damage when removing the original from the new mold.
Catperson: Controlling the temperature of the hot glue is kind of a hit or miss thing, at least in my experience. You either get it too hot or not hot enough. I suppose I could stick a thermometer in it. Generally, the hotter the glue gets, the better the bond, although that isn't necessarily desirable for mold-making.
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In my former life one of the things I did was pot, polish and examine delicate samples under an optical microscope. Preparing these samples properly was very important. . Thin metal samples were placed at the bottom of a mold making machine, covered with finely ground acrylic plastic powder and heated as pressure was applied. This method worked well most of the time but occasionally a void (air pockets) would would appear next to the sample that needed to be filled to prevent distortion during the polishing process. The method used to fill these voids was a combination of vacuum and epoxy.
The void containing sample now potted in a 1" diameter and 1/2" thick acrylic disk with the metal sample exposed at its bottom surface was placed on a paper lip taped to a beaker containing two part epoxy. The lip was actually a large funnel shape angled toward the open beaker's rim made out of paper big enough to hold the offending sample.
This entire assembly (beaker with mixed 2 part epoxy and the sample balanced on the edge of the beaker's paper lip) was placed in a glass vacuum container and evacuated. The epoxy mix would foam as the air pressure was removed thus degassing the epoxy and pumping also removed the air in the offending voids of the sample.
The glass container was then tapped gently until the the sample slipped into the beaker and the epoxy. Less that a minute after that the pumping was stopped, air was allowed to rush back in which forced the now degassed epoxy covering the sample into the offending and evacuated voids. After removing the excess epoxy it was allowed to cure and polishing could be done safely with the voids nicely filled with epoxy.
I suspect this principal could be used to degas hot glue as well. A home or shop vacuum in combination with a wide mouth mason jar (used for canning) could be used. Modify the top (or make a new one from plywood or 1/4" thick acrylic sheet) to accept a garden hose and perhaps wires for a heater to keep the glue hot as it is pumped and degassed.
I have a home made adapter for my wet/dry shop vac that reduces the vac's hose diameter to a garden hose used to suck up water from a hard to get at shutoff valve in the ground. I suspect the small vacuum this would provide would be enough to remove the air bubbles.
In this particular application heat is your friend and boiling the glue (which is really degassing it) while under a vacuum may remove enough of the offending air to make a bubble free mold. Some experimentation with the variables of time, temperature and vacuum should provide good results.
Please feel free to ask questions if I have made any of the above unclear.
Bob C.
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Bob, thank you for the information!
"In this particular application heat is your friend and boiling the glue (which is really degassing it) while under a vacuum may remove enough of the offending air to make a bubble free mold."
I would NEVER have thought of that. As usual, I'm learning something new every day.
Sue
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Hi Sue,
Thanks for your kind words.
I'm a newbie to papier mache and performed a fair amount of on line research and study before I actually put paste to hands and paper. This site and you in particular have been very helpful in getting me started. Thank you and others for being here and willing to share your experience and knowledge so readily.
Bob C.
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I know some people that have rigged up some home-made vacuum casting apparati like that. Personally, it seems like a lot of trouble to go to rig something like that up, but I'm sure it would give good results. I'm looking for more of a quick and easy fix that doesn't involve creating/purchasing special equipment, but I thank you for the suggestion.
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Patraw, you might want to try some of the polymer clay that I suggested. The reusability of the hot glue might be overshadowing the frustration of getting poor results from it.
Now, I must admit that I am the queen of trying to use materials for other than their intended uses, but sometimes a material that is a little more expensive still works out better. Your time and frustration are worth something.
Maybe try it once and see how it goes?
Sue
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Catperson: I'll admit that clay probably would work better for mold creation--but, as I said, I'm stubborn (and cheap). Patching the air bubbles isn't a huge inconvenience (it generally only takes a few minutes, if that), it's just an annoyance. The molds give reasonably good casts--hot glue doesn't hold an extreme amount of detail, but they're serviceable.
These are pictures of the casts/molds I made for the recent Four Horsemen of The Apocalypse figures I added to my gallery:
Lately, I've also kind of been questioning if I'm really saving any time using molds. When one balances the time it takes to prep the mold, do the cast, and then wait for it to dry versus just sculpting it outright (which involves no drying time whatsoever for me due to the methodology I use), I can't help but feel I'm not really doing myself any favors in the long run. That said, sculpting multiples of the same thing is extremely tedious, which does make molding attractive.
Andrew11: I'm talking about the molds themselves here--I never get bubbles in my papier mache. Probably because I don't use any water--I use my white glue undiluted and only apply a thin layer to one side of my strips.
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I guess you might find that modelling without a mould works well. Out of a mould the air can circulate and give better drying. Recently I have become familiar with Nives' method and find that it not only gives amazingly smooth surfaces but also that fine modelling is possible. Some pieces I am working on are as small as those in your hand.
DavidO
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