Weight variation due to led pot temp variations?

[Krag 1901]

In and effort to get better accuracy I started weighing my bullets and grouping them by weight in Lots. I realize that different alloys will cast different weight but I've been using Linotype and getting different weights in the same casting session of over several grains with nominally 155-200 gr molds. I kind of expected my Lino to cast lighter weight than nominal, but I get significant variation as much as 5-10+ grains with a "200" mold. ( don't have my notes handy)

Realizing that the pot temp cycles and pot levels and adding lead changes temp, How much does it effect bullet weight?

PS I use a Lee 10 pound pot and have no PID controller.

[Mitch]

by the same casting session do you mean the alloy is all the same?As in from a lage batch of ingots.If so i do not think that the temp would make 10 plus grains of variation.i would be loking at the bullets alot closer.Things like the diameter and the fill out.>the mold not closeing tight every time or bad fill out will cause the weight variation.

[whisler]

Also not casting a large enough sprue can cause void in the bullet base which obviously affects weight.

[ryanmattes]

Just a thought experiment, to help you visualize:

As a liquid gets hotter, it expands. So you pour an amount of very hot liquid lead into the mold to fill it, where it cools and contracts slightly. Meanwhile, a relatively cooler pour of lead will not be expanded as much, so more (mass) will fit, and it will contract less when cooling.

Your mold will also expand or contract with temperature, so mold temp variations can also affect both size and weight.

Based on temperature fluctuations, I may get +/- 1.5 grains on some molds, although the ones that heavy or light are rare. I'll usually sort them by weight, and keep anything withing +/- 0.5gr of the most common weight. So with my 125gr mold for 9mm, I may get 90%-95% of my bullets at 127.2+/- 0.5gr. So I call 'em 127gr, toss the handful outside that range into my lead scrap bucket, and call it a day.

5-10 grains is quite a swing. I don't know what would cause that, but it's most likely a combination of things.

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[Tazza]

Just a thought experiment, to help you visualize:

As a liquid gets hotter, it expands. So you pour an amount of very hot liquid lead into the mold to fill it, where it cools and contracts slightly. Meanwhile, a relatively cooler pour of lead will not be expanded as much, so more (mass) will fit, and it will contract less when cooling.

Your mold will also expand or contract with temperature, so mold temp variations can also affect both size and weight.

Based on temperature fluctuations, I may get +/- 1.5 grains on some molds, although the ones that heavy or light are rare. I'll usually sort them by weight, and keep anything withing +/- 0.5gr of the most common weight. So with my 125gr mold for 9mm, I may get 90%-95% of my bullets at 127.2+/- 0.5gr. So I call 'em 127gr, toss the handful outside that range into my lead scrap bucket, and call it a day.

5-10 grains is quite a swing. I don't know what would cause that, but it's most likely a combination of things.

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That is a very interesting idea. I generally cast at as low of a temperature as i can to make it so i don't need to slow down as much as obviously, the lead needs to cool enough to cut the sprue and drop projectiles when solid and not liquid. Using hotter metal, may contract a little more when cooled to release from the mould better, in theory at least, unless the mould shrinks at the same rate.

The main cause of weight differences i get i believe are from lead build up behind the sprue plate, this makes the base of the projectile just a little longer, enough for a s light variation.

[Krag 1901]

When I cast boolets I generally use two or three molds and Pour one, set it aside to cool and pour another then set it aside and cut the sprue from the first, dump the boolets, and repour, set it down and then cut the sprue from the second, dump the boolets and repour. My molds are in good shape and I generally let the sprue solidify before I set the mold aside, cycling thru them this way I keep the molds hot and once in a while have to retighen the sprue plate but not often. My fill out is good, except when the molds are dead cold, then I have to recast the first couple of pours generally.

My Lee 309-200-R, for instance, can pour a boolet from 187 to 199, gn some times lighter, with no visual defects? Sometimes I get the odd flier, that goes 4-6" out of my generally 2-3 inch groups and I attribute some of these to boolet defects not dumb nut shooting.

I just wondered how much difference 10-40 degrees(?) would make?

[charlie b]

I suspect you are letting the mold cool too much. I use one mold at a time for that reason. Easier for me to get a good rhythm. I have also found that different molds like different timing. It is a little slower but I find I reject fewer bullets.

FWIW, my Lee 180gn mold would throw bullets in a 2.0gn spread. The NOE 210gn mold is a little better than that. I could probably do better if I were more careful with the pour.

PS preheat your molds on a hotplate. Makes 'start up' so much easier. Experiment with the temp setting until your mold is at or slightly above casting temp.

[Conditor22]

I believe a hot mold will expand a little also so with the lead expanding and shrinking and the mold expanding then with any variation in the alloy temperature you could get a swing in weights.

Are all the boolit bases flat with sharp edges? (small sprues, cool sprue plate, slow casting can cause this)
IF the bases have a divot in the middle (cut the sprue too soon) or a lump in the middle (cut the sprue too late or sprue plate is too lose) the weight will vary

how many cavities do the molds have and what make and model are the molds? I've had molds where the cavities didn't match, some Lyman 4 cavity molds were over 7 grains difference between the 4 cavities in the same pour.

Precision casters use a temperature-controlled casting pot, only cast 1/2 a pot before refilling and letting the pot return to temp, don't return sprues to the pot while casting, use a hot-plate to get the mold to temperature and maintain the mold temperature, and have a temperature probe in their molds to assure consistent mold temperature.

[Krag 1901]

I believe a hot mold will expand a little also so with the lead expanding and shrinking and the mold expanding then with any variation in the alloy temperature you could get a swing in weights.

Are all the boolit bases flat with sharp edges? (small sprues, cool sprue plate, slow casting can cause this)
IF the bases have a divot in the middle (cut the sprue too soon) or a lump in the middle (cut the sprue too late or sprue plate is too lose) the weight will vary

how many cavities do the molds have and what make and model are the molds? I've had molds where the cavities didn't match, some Lyman 4 cavity molds were over 7 grains difference between the 4 cavities in the same pour.

Precision casters use a temperature-controlled casting pot, only cast 1/2 a pot before refilling and letting the pot return to temp, don't return sprues to the pot while casting, use a hot-plate to get the mold to temperature and maintain the mold temperature, and have a temperature probe in their molds to assure consistent mold temperature.

I use Lee two cavity molds. Aluminum so they heat and cool quickly, how quickly ??? I'll try using one mold next time I cast and see if that helps. I let the pot warm up after loading new ingots or sprues so don't thank that is the problem. Unless it is?

[Conditor22]

your mold (s) will cool down when you do this and you have to start the whole warm-up the mold - get into a rhythm process again.

Hotplates are cheap and a great benefit to casting. You usually can get good boolits on the first or second cast when preheated on the hotplate. (I set my hotplate to 400°)

discard the first few casts each session giving yourself time to get into a rhythm and get the heat of your mold stabilized.

[Krag 1901]

My timing in mold alternation is about 60 seconds from one mold to the next. I move pretty fast. I get good fill out and cut sprues, very few divots on the bases. Most of my "wait time" is letting the sprue set up.

I've got some IR thermometers and I'll check to see how well they work at casting temps and check the molds and pot temps to see how they measure.

[243winxb]

Use just 1 mold & mark the cavity for ID. Or drop bullets on 2 seperate piles. 1 pile for each mold.

No 2 cavities are the same.