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Casting advice
Old 09 Mar 2009, 18:47   #1 (permalink)
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Default Casting advice

Split from an older discussion. - Khanaris

let me say that I currently work in the mass production of cobalt parts, and have worked in injection molding and plastic molding. some of what I say my not help or may need to be scaled down to individual production.

more will be added to this post as time permits and questions rise, when completed I will post "completed" at the top of this post

I would say that if you are going to be casting a piece, and you only want one, single piece, use plaster or something super cheap rather than bother investing in more expensive materials for producing multiple pieces. a good mold done correctly will yield 30 pieces or more per build

Pewter casting:
The material pewter is a very soft metal made of copper lead and tin, predominantly tin. This combination makes it melt at very low temperatures, (pure tin will melt as low as 60 F) making it easy to work with, and inexpensive to make or obtain. since pewter is one of the easiest metals to work with, you can melt pewter on your kitchen stove, but I would recommend that you keep your metalworking and food preparation areas separate (for those of you who are married and intend to stay that way don't steal equipment and utensils from the kitchen). Work in a well-ventilated area, and don't forget your safety goggles. To melt pewter, put it into a small, handled container, I would suggest that you buy or make this container for heating, and make sure that has a pour spout on one end for easy pouring. The larger the flat surface of your container, the more metal you can melt (so if you only want one small model at a time get a tiny pan). Again due to low melt point, a steel or cast iron pan will easily work for this application, and a steel rod will work for stirring (a stirring rod is critical, so don't try melting without one) when melting and casting only work at a work bench, old table, or specially designed melting stand, liquids can and do splatter and spill! For pouring use a shallow baking tray filled with sand underneath your mold, or better build a 3" deep box to set your molds on.

the two methods that I would recommend for you are to use are sand casting and rubber mold casting (lost wax will work if you want to add a bunch of extra steps, because you will need to make a wax model of your pewter model in order to make another pewter one)

of these two sand casting will work the fastest (least amount of time for production) while rubber casting will probably be less expensive.

for sand casting you will need to either find a way to bury your mold exactly halfway in two different sides, or find a way to separate your molds at exactly their middle.

when melting, you will want to be warmer than the absolute minimum ie. if you are told to get the pewter to a certain tempurature in order to melt it, heat to 15ish degrees (F) warmer (you can go even hotter if you want, but you shouldn't need to). If you are using a rubber mold (which will melt at a very low temperature too) put it on a hot plate and warm it as much as you can (I think rubber liquefies and burns at 100F, but I am not sure so try and find out how hot you can go on a practice piece of rubber). on all other molds you will want to heat the molds to be hotter than the temperature of the metal because the mold will cool as soon as you take it away from its heat source, and because it has a larger surface area it will cool really fast (it takes 2 minutes for a 45lb mold to lose 1500F of heat) if you are doing this by yourself and you take a long time to pour you will want to have your mold really hot. The vast majority of casting defects (90%) happen at the pouring stage, so try to eliminate the possibility of them happening anywhere else before that point. In industrial casting a mold should be cast in less than a minute (up to several hundred pounds) so you have to be quick. get several friends to help you cast, one to grab the mold, one to pour and one to time you, from the time that you remove the mold from its direct heat source to the time you finish pouring, you should have taken less than a minute.

crucible with pour spout: this is the key to the operation, and can range in size from a 2oz container (single model), to a small saucepan (small application) to a much larger one such as (I use) 40lb (volume of metal) ceramic crucible to as large as a several ton crucible (that require robots to lift). can be made of steel, or ceramic, though for pewter casting steel will work (most casting applications require ceramic for cleanliness purposes)
stirring rod: in order to keep your metal in a constant liquid state you will need to keep it stirred once melted! , do not bother to start stirring until it is rather liquid, and only to keep a skin from forming until you pour your metal, and to remove large unmelted chunks.

here are some other resources that may help out but I am not certifiably recommending that they are entirely accurate

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Old 14 Mar 2009, 03:57   #2 (permalink)
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Default Casting advice

Heat would be the biggest factor simply turning the pewter to liquid is not hot enough, another couple 100 degrees would be desired, i have cast brass and aluminum before and i assure you, it's not about pouring as soon as you can.
I used to leave the blast furnace on all day the metal would be molten and red this was the only way to ensure consistant casting. For single models cast in metal i would recommend plaster of paris as it is capable of standing upto molten brass so i think pewter will be fine, for easy plaster of paris casting there is an extra step or two.
Lost Wax casting as hardcore described.
#1: Cast your model in parafin wax, this is easily heated and if you're getting clean casts out of resin there should be no problem with wax (you can even make a few wax casts at this time really)
#2: mix up a good batch of plaster of paris and submerge the wax models into the plaster of paris, leaving an input and output channel for the metal (straws will work fine if the model doesn't have anything to this extent) vibrate the molds/tap them. to get out any air bubbles
#3: let the plaster of paris dry for around 2 weeks at ambient temp and humidity you can speed this up by leaving it in the oven at very low for 2 days or so, or even placing it infront of a vent to speed things up(if you want to get really technical you can weight the mold each day and note when it stops losing moisture)
#4: heat the mold until the wax is completely liquid and pour this wax out.
#5: Heat the pewter until it is liquid then wait 5-10 more minutes, and then pour the mold it
#6: let cool down for a bit and then just break the mold it's only Plaster.
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