Haven't learned a thing so far!

[Elric]

Had to toss in a flash-bang...

Suffering from lack of alcohol (think of Bender from Futurama)...

Is a mould's casting performance a interplay between amount of alloy, temperature of blocks, and thermal mass of blocks?

If so, then how do we crack the code?

Cadence depends on the time needed for the sprue to cool enough. Folks alter that with fans, casting with two or more molds, or a moistened towel...

Has anyone with a PID looked into the optimal temps for bullet drop? This may turn out to be the upper limit of mold temp where you can reasonably expect to whack the sprue plate and cleanly cut the sprue. Any lower temp starts cutting into the speed of casting, because the mould will drop below te temp needed for good fill.

Let the sado-mathification begin!

 

[fiver]

@ 375 for steel and about 425 for aluminum.
I don't got a PID thingy but that's bout where I run most of mine.

the aluminum ones vary some depending on the grade of aluminum used
[I can run some of them just past the shiny stage but nowhere near hot]
the better ones are more like in the 375 area the steel [mehanite] molds operate in.

 

[Elric]

The specific heat will vary due to the thermal mass of the blocks. Steel will retain more heat per unit of weight than aluminum. Different grades of aluminum, different retention.

Each pour of a specific weight of alloy will transfer some of the melt's thermal energy into the blocks, which are radiating heat to the air, based on block surface (mostly, you could also figure sprue plate and transfer to the mold "jaws"). Each pour will add a reasonably consistent amount of thermal energy, directly related to the alloy, melt temperature, number and volume of cavities poured.

So... for each block size and composition, let's say an RCBS double cavity mould, the radiative and conductive heat losses will be (relatively) the same, for like moulds.

 

[Rick]

The specific heat will vary due to the thermal mass of the blocks. Steel will retain more heat per unit of weight than aluminum. Different grades of aluminum, different retention.

Each pour of a specific weight of alloy will transfer some of the melt's thermal energy into the blocks, which are radiating heat to the air, based on block surface (mostly, you could also figure sprue plate and transfer to the mold "jaws"). Each pour will add a reasonably consistent amount of thermal energy, directly related to the alloy, melt temperature, number and volume of cavities poured.

So... for each block size and composition, let's say an RCBS double cavity mould, the radiative and conductive heat losses will be (relatively) the same, for like moulds.

And . . . Ambient air temperature. The temp of the room your casting in will have a huge effect on heat loss from the blocks.
.

 

[Brad]

Drafts make a difference too, think of the fan.

I don't have a mould temp probe so I don't know what specific mould temp is "best" for each mould.

I do use a PID for my pot and have found I get bullets just as good at 700° as I did at 725°. What I don't get at 700° is the extra heat gong to the mould. I can cast at a faster pace and keep a similar good mould temp.

I think that bullet size, think mass, makes a bigger difference than block size. No way does my 120 gr 30 cal 6 cav Lee cast the same as my 420 gr 460 mould. One I can go fast as I can and never have problems, the other will over heat in a heartbeat. All a matter of the heat poured into the mould.

I spend less time fretting over all this and more time paying attention to the time it takes for the bullets to harden. I cut the sprue as soon as it hardens but sometimes the bullet is still shiny and not fully hardened. I watch for that and take it as a signal to slow down a bit.

 

[fiver]

I try for a consistent pace and adjust the heat additions temperature.
that way I can maintain a comfortable pace and consistent count, I may adjust my sprue cut count slightly.
but I also mostly cue off the sprue flash over and feeling to the sprue cut.

 

[yodogsandman]

What about the sprues? Adding them right back to the pot or not to maintain temperatures. I add very little back, while I cast, when the temp is below 20*F.

 

[Kevin Stenberg]

I use both a PID on the pot, and a NOE temp. monitor. I have found that each mold prefers different temperatures. I use the mold temp. monitor only as a guide for what temperature the mold drops the most consistent bullets at.
The only molds I don't use the monitors on are my brass Mihec molds. I just can't get myself to drill into them for the sensor.

 

[Elric]

Drafts make a difference too, think of the fan.
I think that bullet size, think mass, makes a bigger difference than block size. No way does my 120 gr 30 cal 6 cav Lee cast the same as my 420 gr 460 mould. One I can go fast as I can and never have problems, the other will over heat in a heartbeat. All a matter of the heat poured into the mould.

Well, the amount of heat emitted by the blocks is fairly constant (radiated, convected, or conducted) so the addition of heat via melt will be the significant variable. No effort to account for the temp reduction of the melt from adding sprue, at least I don't return the sprue to the pot until I am down to about 1/3d full, and then I take a break after adding a few ingots and sprue. Let it come back up to temp, flux, then return to casting.

For myself, I run two fairly comparible moulds (try to cast similar amounts of melt), they are placed on a wooden table, and any air flow depends on the weather.

If one sets up a cadence, the amount of heat lost will generally be the same. Just for snorts 'n grins, the radiant heat will be IR, the convection will be affected by the air temp and velocity, and convection will depend on the surface the blocks rest on during the cadence. No takers for mathification?

 

[fiver]

I could do it but I can't quite reach my pencil and sharpener from here.

I do return my sprues back to the pot, but I doubt a 250-f half oz sprue is gonna affect 40 lbs of alloy.

I did a write up a while back on the LEE 20 lb pot and how long x amount of alloy dropped the temp of the pot by 20-f or more.
an ingot of just 4 oz's at room temp will drop it's temp by 20* for about a full minute easily, especially if it's between heat cycles.
I pretty much relegated my 20 lb pot to the smaller diameter boolits and found a 1/2" thick steel top for it to increase it's mass and give me a place to pre-heat ingots after that little experiment.
I prefer to keep my pots topped off as I go for several reasons.

 

[9.3X62AL]

"Sado-mathification".......I like that. I am among the least scientifically-minded members here, I'm pretty sure. Bullet casting to me is much more art form than a mathematical equation. I don't use a thermometer of any kind at all, and let each mould's preferences dictate how I proceed. I do use the sprue puddle flash-over to help time the sprue cut more properly. My mentor that got me headed correctly when I started this madness c. 1981 told me that you can only get just so much from a book or an instructor--he said something to the effect that experience with the metal and the tooling was the best teacher of all, and that the moulds themselves held or shared the best info obtainable if you let them do the talking during the casting process.

 

[Brad]

Al, that is my feeling exactly. All the math in the world can tell us what needs to be done but it can't do it for us. This is where the art of casting comes in.

 

[Ian]

Set pot temp to 100°F higher than liquidus temp of alloy.

Pour lead. Let solidify. Cut sprue and observe base, dump bullet and observe fillout/sheen, close mould, fill, repeat.

The remainder of the "math" has to do with how you time each of the operations, how you control the alloy stream going into the mould, and how large a sprue puddle is needed. I calculate the next pour timing based on the results of the last, and during the fill and wait operation that's mainly what I'm thinking about. Once everything is right, I maintain it exactly. That's all there is to it.

 

[Rick]

Well, the amount of heat emitted by the blocks is fairly constant (radiated, convected, or conducted) so the addition of heat via melt will be the significant variable.

That would only be true if casting in a climate controlled environment. Heat in winter, AC in summer. Some people cast out doors or in a 15 degree garage in winter and 95 degrees in summer. That will have a huge effect on how fast a mold looses heat. If casting outdoors any breeze blowing will also increase heat loss, winter or summer. Add to that most non PID controlled pots will have at least a 25 degree plus & minus temp swing will add to variations in mold temp.
.

 

[Ian]

+1 Rick. Drafts, ambient temperature, and also RELATIVE HUMIDITY have a HUGE effect on mould temperature. Felix Robbins used to comment how casting in a "condensing atmosphere", i.e. while it was raining, was "pure magic". I've found that to be true as well because of how humid air absorbs heat vs. dry air.

 

[fiver]

I think it is just before it rains that's best.
in our dry climate here I keep a big pot of water on the stove in the shop in the winter and have one of those water filled ac type units on in the summer.
it stays about 65-f most of the time in there which is super nice but getting the humidity up before casting time really does help thing go along smoothly.
it doesn't help me keep comfortable in any shape or fashion as I end up taking a shower right after casting but it sure is noticeable if I don't run the water works in there.

 

[KHornet]

Really agree with Brad on this one! Not sure that casting is an art form, but if
it isn't, it probably comes close when one is casting bullets from 22 to and thru
58 cal. A person with only one or two molds, can probably master them. But
when you have quantities of molds, when casting all of them may be in the
area of an art form. Math was never my subject.

Paul

 

[RBHarter]

Armed with the above its a wonder that I ever get a good bullet . I'm going to have to learn it all over in just over a yr and a half .
I'm at 4000 ft 17-21% humidity a miserable wet day is above 35% . When it does rain I've seen the temperature drop 30* in 20 minutes .

The shop fan does make nicer sprue cuts .

 

[quicksylver]

Here's some data to work with..
I have about a dozen Al. molds..
My alloy is 4-4-92
I run a probe on my molds and a thermo in the pot.
I maintain my pot at between 675 and 700
I have marked all my mold boxes with the temps the mold likes both alloy and mold temp
The molds run anywhere from 338 to 402
Some actually have two "sweet" spots.
The probe has really made a difference for me...