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Kang

How metal minis are made

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In lost wax casting the original (we could call this a green) is the wax piece, usually built from something called victory wax. This is not the same wax you use for making candles. In the case of the lost wax process the original is completely destroyed, hence lost wax. That's why its important that the casting be as near perfect as possible.

 

In the case of the process used by Reaper and other miniature companies you are close to being correct. The green is cast in a single mold, this becomes the master mold. From this enough castings are made to fill a production mold. These are master castings (and occasionally these are what are given to the artists to paint, I have one from Crocodile Games). These master castings are cleaned up of imperfections, like mold lines, and these are then used to create the production mold or molds. The production molds just start out as two hard rubber discs and heat and pressure are used to press these together around the castings to make the molds. Once that is complete then the channels and gates are cut into the mold so that the liquid metal can fill it. The same metal used to create the miniatures you buy is the same metal used to cast the masters.

 

Thank you Heisler for the Model Railroad info as well as the above quoted post.

 

In the Jewelry industry, I know the wax is put into a dental plaster "flask" (metal cup).  The plaster is then heated to near the molten temperature of the metal being cast (to give a better cast and prevent backsplash), and then the jewelry is spin cast in a centrifuge in a metal "flask," or it can be pressure cast (I've worked personally with two different spin-casting centrifuges but no pressure cast ones).  The dental plaster completely disintigrates in water at that temperature, leaving just a tiny bit imbedded in the metal, which is sanded and polished off of the jewelry. 

 

From here, many jewelers can and do make silicone molds of the pieces, injecting liquid wax into the mold to be then placed into the flask that fits their centrifuge.  They will make "trees" of the molds and then re-cast trees in much larger flasks.  The excess metal from the trees, much like how miniatures are cast, is then put into the next casting if it's clean (or it's cleaned up and then used).

Edited by Darkmeer
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Yeah, burning out an actual original that's sculpted directly in wax without taking a mold off it first is a pretty ballsy move I must say!  But apparently that was the original method used in ancient times...  A lot of sculpture these days is obviously done in other mediums, and a mold is made for casting reproducible waxes that are used (and "lost") to make the mold for metal casting.  Sometimes some of the final details of a sculpture are carved on the wax and not the original, but that is not always the case either.  Now we have modern materials available like RTV silicone for making molds, so more trial and error is possible, and lost wax castings can be produced in numbers greater than one much more easily (but still very cumbersome compared to spin-casting into rubber molds as is possible with metals that don't get hot enough to destroy the rubber, ie. the stuff used for making miniatures).  Test pieces can be cast to ensure the gating and venting is all set up to enable a successful casting, so a mistake doesn't mean starting from scratch and having to recreate an entire original sculpture  Because of this, lost wax is now used in industry to make huge numbers of complex engine parts, etc., as well, though sand casting is also used a lot for that sort of thing.  But a wax casting will often require less machining after being cast, and can produce castings with undercuts etc. more easily.  Often several test pieces are poured in case they get the gating and venting wrong and the casting doesn't come out right.  

 

I've read a lot about investment casting and watched hours of video, but I have so far only tried lost foam casting (and a couple of anthills, and a couple of Prince August molds that are my only non-aluminum castings so far), which sounds a lot more similar to lost wax casting than it actually is.  I'm eager to try some lost wax casting eventually in my backyard foundry, but I have to build a bunch of equipment first (like a kiln for burning out the wax and preheating the molds, flasks to contain the investment mold, etc.) and figure out where I can get the wax and the investment mixture at a reasonable cost, or else make my own.  I'd also like to build a new furnace that could survive the melting of bronze and other metals that need more heat than aluminum - right now my furnace is lined with a simple mixture of sand and fireclay with just enough water added so I could ram it into place in a ring-shaped form inside a round steel shell (basically a big metal bucket with fire resistant walls a couple inches thick inside it).  I use charcoal for fuel and an old hair dryer attached to a metal pipe leading into the furnace as a blower to get it burning hot enough to reach pouring temperature.  I could get it hot enough to melt bronze I don't doubt, but my furnace wouldn't last too long if I started doing that very often... 

 

I know some hobbyists and backyard dabblers on budgets like mine have experimented with different homebrewed mixtures of beeswax and pine resins and various other waxes, and 3D printed PLA plastic parts replacing the wax patterns used for investment casting are also gaining popularity among hobbyists and backyard foundry guys now that 3D printing is becoming closer to being widely affordable in the home setting.  But well-tested waxes and investment powders rated for different metals and temperatures are certainly available commercially for those who can afford them or are not so much interested in experimenting as in getting good results on the first try.

 

Thanks for the tip about dental plaster!  I have a bunch at home from casting dungeon bits in HirstArts molds, but somehow I never thought of that.  Makes sense though...  Pretty sure that if/when it's used in investment casting it is probably mixed with other ingredients (particularly if a vacuum assist is used in filling the mold rather than a centrifuge effect, as the mold must be porous enough for the vacuum to do its work), but that is a starting place for me if I ever decide to try mixing my own homebrew recipe!

Edited by Kang
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re: safety equipment:

Our casters are required to wear safety goggles or approved safety glasses, gloves, and full length pants with closed toed shoes.  Aprons are provided but not necessary - I can speak from experience that spatter to the torso is just as harmlessly deflected by a shirt as it is by the apron. Remember that we are discussing very small quantities - typically 6oz or less at low temperatures.  The amount of exposed skin, other than the face, is only a small patch right around the elbow.  We have never had a damaging injury - in part because tin has surface properties like water - it beads up and rolls like water would. So when it hits your skin or clothing, it beads up and rolls off. It's really amazing, actually.

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The video explains how a mold for casting one copy of the green is made, but he doesn't go into detail about how the other (ie.) 11 copies of each piece appear in a 12 piece mold.  ReaperBryan said above that they don't use lost wax, so I'm assuming that they pour 12 single copies of it using that first mold the same way that is shown in the video (possibly using a different low temperature alloy, though perhaps not) and use those to make the mold with the multiple copies.  Or else they make 12 wedge shaped rubber molds of the green and glue them together into a wheel shape somehow.  Or something.

We cast about 15 copies from the master mold, and then rigorously clean those masters to remove the artifacts from the mold making process - mold lines, tool marks, etc.  These perfect 1st Generation Copies are inspected by our Masters Dept. Head and approved one by one - individual masters are judged on their own merit, and if one is not good, we will melt it and make another try.  Once approved they become our Masters, and al future molds of the same part are made from this set of Masters.  For further insurance, after the mold is made, 30% of the Masters are taken off site and stored elsewhere, at a secret facility, in case of emergency, crisis, or disaster.

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That is good to hear, especially about the goggles and the no sandals rule (though videos of 3rd world foundries ALWAYS show a guy pouring metal in bare feet, I guess the thinking is that bulky PPE keeps them from leaping to safety fast enough or something.  I'll keep my boots when I pour though, thank you very much!).  I had a feeling you were underdressed during that tour compared to Reaper's typical everyday casting wardrobe.  I can understand that, you might make the tourists a little nervous if you put on an aluminized space suit to do your demo and they didn't get to wear one too!  :) 

 

I have only worked with hotter stuff, that is pretty cool what you're saying about splashes just bouncing off harmlessly!  Now I'm imagining "water" fights in the casting department!  OK maybe not...

 

edit - just saw your follow up about the masters, Fascinting stuff!  Thanks again,

Edited by Kang

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Meanwhile, I was making beer on Saturday and gave myself a first degree burn with a jet of hot (~212F) water. I'm probably not the right person for the Reaper production line. :)

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Thank you Heisler, for saving me from having to ask a stupid question!  I was wondering about that.

 

One thing that does have me going 'hmmm', though...is there a keying process or somesuch to getting the two halves of the mold to match, and how do you get 2 halves without losing material thickness-wise?

 

Pardon my cluelessness...the only casting I've done is sand casting, and the last time I did that was...cripes, 1995?  I was in 7th grade...(and no, they didn't let us handle the metal...just make the molds)

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The video explains how a mold for casting one copy of the green is made, but he doesn't go into detail about how the other (ie.) 11 copies of each piece appear in a 12 piece mold.  ReaperBryan said above that they don't use lost wax, so I'm assuming that they pour 12 single copies of it using that first mold the same way that is shown in the video (possibly using a different low temperature alloy, though perhaps not) and use those to make the mold with the multiple copies.  Or else they make 12 wedge shaped rubber molds of the green and glue them together into a wheel shape somehow.  Or something.

 

We cast about 15 copies from the master mold, and then rigorously clean those masters to remove the artifacts from the mold making process - mold lines, tool marks, etc.  These perfect 1st Generation Copies are inspected by our Masters Dept. Head and approved one by one - individual masters are judged on their own merit, and if one is not good, we will melt it and make another try.  Once approved they become our Masters, and al future molds of the same part are made from this set of Masters.  For further insurance, after the mold is made, 30% of the Masters are taken off site and stored elsewhere, at a secret facility, in case of emergency, crisis, or disaster.
You know, just to be extra, extra safe, you need to store the other 70% of the masters at a separate, even more secret location. I just happen to know of the perfect place...
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Thank you Heisler, for saving me from having to ask a stupid question!  I was wondering about that.

 

One thing that does have me going 'hmmm', though...is there a keying process or somesuch to getting the two halves of the mold to match, and how do you get 2 halves without losing material thickness-wise?

 

Pardon my cluelessness...the only casting I've done is sand casting, and the last time I did that was...cripes, 1995?  I was in 7th grade...(and no, they didn't let us handle the metal...just make the molds)

The two mold halves are keyed together when they are made, by introducing a series of lock nuts that hold fast to one side and make an indent on the other.  

 

The mold is made under pressure, in the vulcanizing process, and throughout the process the rubber goes first to a liquid state then to a solid state. In that liquid state, there is no agitation or movement, so there's no movement within the mold box, but the high pressures force the gooey rubber into the cracks, also coincidentally forcing the two halves as close as physically possible with no gapping or overlap.  A simple layer of powdered Talc (not "Talcum Powder" - talcum powder has fragrances and other additives, this is the purer version of the finely ground mineral talc) between the two layers prevents integration while molten. When the pressures kick off the vulcanization reaction, and the rubber goes from gooey to hard, the halves will be able to be peeled apart because this layer of talc prevented them from becoming a unified solid.

 

So, now, one might say that on a very small scale, there would be a "powered fine talc layer"-width "gap" between the two halves once reassembled. And I suppose that might be true, but it is not sufficient for metal to "escape" through.  Also, unless a mold is being made form a 4th or higher generation spin*, the "compression" caused by removing that layer when the final pour occurs is so small as to be invisible.

 

*If we made masters and then had a sculptor do a conversion directly to the master instead of the green (which converting the original green is the normal process) and then converted the new master again, this might happen. Which is why we don't do that.  We do occasionally remaster a part. Remastering usually occurs when either the green was damaged (either due to errors in mold making or rough handling in transit) OR when a joint does not fit or was deformed under pressure. The messed up part might be resculpted on the master instead of on the green, and we might make a second go. I am aware of figures where a 3rd remaster has occurred, but I'm fairly sure that we'd just go back to the green if we ever needed a fourth remaster or higher.

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Man 4 years makes a lot of difference! I was so much younger!

 

Is it the four years or the three Kickstarters (and the stresses and headaches that come with them)? 

 

Thomas the Death Engine.

 

I'm still mad that we've had him for over  year and never bothered to add Angry Face to the front.

 

Reapercon contest? Design the best angry face for Thomas and Izzy will paint it on?  :devil:

 

 

Izzy will already design a much better angry face than I ever would. I say cut out the middle man.

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That's an informative read...thanks for filling in the gaps!

 

That plus the videos paint a clear picture; I was picturing trying to take the newly-cast mold through a thin saw or wire to split it back apart, which didn't compute.  I've also got massive respect for the casting department at this point...I doubt I could keep up with 1500 spins a day (give or take for breaks), even with practice. 

 

Didn't expect Thomas to be that small, either.

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That's an informative read...thanks for filling in the gaps!

 

That plus the videos paint a clear picture; I was picturing trying to take the newly-cast mold through a thin saw or wire to split it back apart, which didn't compute.  I've also got massive respect for the casting department at this point...I doubt I could keep up with 1500 spins a day (give or take for breaks), even with practice. 

 

Didn't expect Thomas to be that small, either.

It takes some time to work up to that.  Your first 30 days, by the end of the 30 day period you need to be at ~900 spins a day, then ~1200 a day by the end of your second 30 days, and after 90 days, be and stay at ~1500.

 

And trust me, week one being at ~900 is darn impossible.  It was hard dork getting up to speed, and I know that while I was above minimum by the end of my 90, I missed 900/day at the 30 day mark, and almost missed 1200 by the 60 day mark.  I didn't start hitting target until around day 50.

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I imagine it's one of those things where there's a lot of muscle memory and learned instinct involved...that a lot of the speed relates to how much you can do without having to think about it.  Whether the talc layer is good, fitting the halves together without having to look closely, knowing instinctively when it's seated in the machine correctly and how much metal to dip, etc.

 

That's the way my cabinet factory job was...except the machine I was on required a 2-man team [one feeding, one checking for flaws], and the experienced guys liked to play the trial by fire game.  Whenever they were asked to work with a new guy, they cranked up the belt and went as fast as they could, whether you could keep up didn't matter...and some of these guys could feed 30 boards a minute.  If you had to stop and think 'is that the wood grain, or a feather [toothpick-or-thinner strip of a separate board]', the feeder would turn up the belt speed and launch the next one into your gut.  If you had to question if a knot was too big [once you were there a while, you learned that if it was bigger than the tip of a wood crayon, it was out...tap and go] there were 3 boards on the floor, and bending over to pick them up meant getting brained by a 4th and yelled at by management for not being able to keep up, no matter whether it was your 2nd day or your 2nd year.

 

Keeping up was all about being able to run on instinct.  You didn't have time to think, it had to be all subconscious...I imagine doing castings is similar.  Granted, I'm sure morale was better; I doubt Reaper runs on a "you are expendable and should be thanking us for the opportunity to be horribly maimed by our intentional OSHA violations" mentality.

Edited by Foxden Racing
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