Casting voids (zinc)

[Evil Dog]

Trying to cast my 2 1/4" cannon ball with zinc and am running into a casting void problem that so far I haven't been able to overcome.  Perhaps one of you might have a good suggestion or two.

The mold itself is machined steel,  just under 3 1/2" diameter and just under 4" long.  The ball cavity is 2 1/4" with a 1/4" long 1/2" diameter sprue and a 1/2" deep 1" diameter reservoir after the sprue hole.  The mold halves are held together with a vise grip "c-clamp" plier.

I am using virgin 97% zinc alloy melted in a cast iron pot 4 1/4" wide by 3 1/2" deep (inside measurements) placed on a large cast iron propane burner.  I flux the zinc with beeswax the same as I would do if it was lead or wheelweight.  I can get 5 pours out of a full pot.

The first pour is made into a cold mold.  Pick up the pot of molten zinc with a large pair of channel lock pliers and pour directly into the mold trying to pour very slowly.  The pot is returned to the propane burner and I wait about a minute before opening the mold.

The same procedure is followed for the second and subsequent pours.... by the third pour the mold is hot enough that I will smoke a pair of very heavy leather gloves when I touch the mold.  When I notice the reservoir above the sprue start to shrink I will add a bit more zinc as necessary.

After the balls are cold, the sprues are removed with a hacksaw.  Just as often as not I will see a void just below the sprue.  Weighing the balls on an electric scale, they have been ranging 1 pound 6.6 ounces to to 1 pound 6.8 ounces.

Cut one open that did not show an opening where the sprue was cut off and found what I consider to be a rather sizeable casting void.



The void is approximately 1/2" wide by 7/8" deep.

I haven't had a chance to try it yet, but GGaskill suggested playing a propane torch over the sprue reservior to try keeping it melted until the ball solidifies more.  The idea being that the sprue has been hardening before the rest of the ball.  Sounds logical to me and will be giving that a try.

Any of you that cast your own zinc balls have any other ideas?  I sure would appreciate it.

[Cat Whisperer]

A couple of observations and thoughts, no solutions.

Preheating the mould may be a good idea.  Cooling slowly will allow metal to flow in while the outside cools.

I've read that often to get good castings of large spheres in zinc and iron that it works better with a core.

Ggaskill has mentioned about using a large feeder to allow metal to flow into the cavity as it cools.

Really good precision zinc castings are PRESSURE FED.  Perhaps a longer sprue (and a riser) to add some pressure.

[guardsgunner]

I do not have the answer either.
We ran into the same problem casting projectiles. Void in the nose.
With and without cores. Ours were about 1oz to 1.5 oz. difference and we have shot them anyway. We have not cast since.
My thought to improve was to vent the mold better be adding small grooves across the inside of the mold to allow air to escape.. Like those on a lyman bullet mold. (perhaps by using a cuter. drill as a mill cutter.)
If that doesn't work, I'm going to try adding an air vent which travels on an angle from near where the sprue comes in. Hopefully this will allow the air to escape faster and not get trapped in the material.

[jeeper1]

I am no expert by any means, I only use lead for casting and then only for bullets, but it looks like the sprue area is cooling too fast. I am wondering if increasing the diameter will let the zinc stay molten long enough to eliminate the void.

[Will Bison]

Hey, I recognize that cannon ball, I cast a whole bunch just like that. If you can get that hollow cavity to come out a little better you have the ‘makins for Shrapnel Shell.

I have three observations: First, I think your temperature is a bit low. Zinc has a low thermal inertia meaning it transfers heat quickly. Lead does the same thing which is good in one respect in that it solidifies in the mould in a reasonable length of time. This allows for rapid casting. The down side is it gives up heat to the mould and starts to solidify in the sprue hole. We want the mould to get up to temp also so that means everything needs to be a little hotter.

Second, make a pouring instrument that works just like a Lyman lead dipper. It should hold at least double the amount of metal required for one ball. This type dipper introduces the metal from the bottom. An open top dipper pours from the surface of the molten metal and all the crud on the surface goes right in the mould. A closed bottom dipper does two things; it keeps the crud out and provides some head pressure on the molten metal. The head pressure is very important, the higher the column of molten metal is in the dipper, the more pressure is applied. My dippers are made from black iron gas pipe. I use a nipple of about ball diameter and screw a reducer on one end. Screw a smaller nipple of about sprue hole diameter in the other end of the nipple. The smaller nipple is cut as short as practical and the end ground to fit the sprue hole. Its gonna leak a bit just like a small lead dipper. On the top end of the large nipple, screw on a pipe cap and then cut out a hole near the top of the large nipple so the metal can run in. I weld a long steel handle on the side with a ring on the end for gripping. These are two handed dippers. Set the dipper on the pot when melting the zinc so that the dipper comes up to temp with the metal.

Third, get a bigger pot to melt the metal in. You will need enough surplus metal in the pot so you can fully submerge the dipper. The larger pot also helps to keep a more stable temperature as you donÂ’t have to keep adding metal every few balls.

IÂ’ve also learned that two people are better than one when casting any large size balls. One person tends the dipper and pot while the other tends the mould. Getting a good rhythm going really helps produce good balls. I also find that itÂ’s better to cast a bunch of balls in one session as opposed to casting a few at a time over several sessions. An extra set of hands really helps.

There is a learning curve involved so practice and experimentation are required. I can tell from the photo that you almost have the knack.

Bill

[Powder keg]

So let me see if I understand what your saying. You build a funnel out of black pipe that fits the sprue hole? Does the metal freeze in the funnel neck? If it does I guess it would be no problem to just stich it in the pot and remelt it. Does the dipper have some sort of valve, or do you pore into the dipper? Thanks for your discriptions.Later,

[jeeper1]

After reading Will Bison's remarks I went back and looked at the whole picture instead of focusing on just the void. It looks like the exterior (what I can see of it) does have wrinkles from a cold pour. both the mold and zinc does look to have been too cold. What Will said is right.

[Evil Dog]

Just finished pouring 3 balls.... the first poured into a cold mold, the second and 3rd just as soon as the mold was put back together after gently removing the previously cast ball (gently is with a 1 1/2 pound brass hammer applied to the end of the mold).  On all 3 pours I had one of those "Quick Fire" propane torches played right over the sprue and reservoir.  The first pour solidified fairly quickly even with the torch on it... cold mold.  The second took probably 20 or 25 seconds to solidify under the torch... mold real hot to the touch through heavy leather gloves.  The third pour took well over a minute to solidify under the torch... heavy leather gloves immediately smoke when the mold is touched.  Asbestos foundry gloves are a lot more comfortable at this point when holding the mold to whack with the brass hammer.

After the balls were cool the sprue was removed with a hacksaw and the balls weighed on one of those electric postal scales.  First ball = 1 lb 6.7 oz, undoubtedly a casting void in that one.  Second ball = 1 lb 6.8 oz, very suspect for having a casting void.  Third ball = 1 lb 7.3 oz.... a full half ounce heavier than the second ball !!!!!

As Kelly Bundy used to say on "Married With Children"...... "Urethra... I have found it !!!"

Will be interesting to see how the weights turn out on a 5 ball run.

I sure do appreciate all of the ideas you guys have been coming up with.

[GGaskill]

Here is a picture of the Lyman casting dipper (in the foreground) and some fragments of other stuff.

[Powder keg]

Thanks for the picture G. Ithink I have it now. Good Idea!!

[Will Bison]

I've found that it takes about six balls cast in sucsession before they start to come out nice. Lead balls are about the same, after about five or six the mould heats up and they pour well.

To answer Powder Kegs question, the dipper is kept in the melt pot except to pour. That eliminates freezing in the neck.

As molten metal and mould come up to temp, the balls will come out of the mould easier.

Bill

[HotGuns]

Just a thought that I haven't seen mentioned...

As one that casts lots of bullets for various guns, you might notice that all of the mold manufactures have their molds vented.

I dont know if it matters for cannon ball making since Im fairly new to the sport, but I do know that the vents allow gas to escape from the mold so that you dent have voids or cavitys in the bullet.

Back in the good ole day, the molds weren't  made with the precision that they are nowadays. The process of manufacture and the looser tolerances of the day were sufficient to let gas escape.

The vent lines are only a couple thousanths deep. Not enough for metal to escape, but definaltey enough for gas to get out.

Ive made a few molds and the way I vented them was to take a height gage with a carbide scriber and scribe them. I usually use 1/8 increments scribing across the face of each mold...it seems to work well. The first mold I ever made I didn't have any vent lines and it wouldn't cast bullets worth a dang. After venting the mold, it made all the difference in the world and my voids went away.

Anyhow...its something you may want to look at...

[Cat Whisperer]

Just a thought that I haven't seen mentioned... you might notice that all of the mold manufactures have their molds vented....

And the alternative use of both a sprue and riser - to provide both the reserve of metal to flow back into the cooling alloy in the cavity and to povide a large escape path (the riser) for air being displaced (and expanded) by metal coming in the sprue.

[HotGuns]

Ahhh...I see.

So if I decide to make a mold for my Coehorn mortar I wont need to vent it ? You are saying that the sprue and riser serve to vent the mold right ?

[Cat Whisperer]

Ahhh...I see.

So if I decide to make a mold for my Coehorn mortar I wont need to vent it ? You are saying that the sprue and riser serve to vent the mold right ?

Sprues and risers are generally used in sand casting.  If they're placed right, the air flows out and the metal flows in (not interfering with each other) and both supply metal to fill the mould as the metal cools and shrinks.

Metal moulds, as for casting bullets, most often (but not always) are split and usually filled from one end or the other.  The air must have a place to escape.  If the halves don't fit exactly there is often enough space for it to slide out between the mould halves; but modern moulds ensure that by having very small grooves to assist.

Some metal moulds (often with zinc) are pressure fed and the air is under much pressure and will escape through the cracks between the mould halves.  There is often a bit of flash (thin wafers of zinc) also.  Ask anyone who's worked in a zinc casting plant about the sharpness of the zinc flash and how it goes right through gloves.

Zinc shrinks a bunch.  Allowance must be made in mould/alloy temperature for the metal from the sprue and riser (or use of pressure casting) to keep from having voids in the finished casting.

[GGaskill]

I'm not sure you want to vent a mould used for zinc.  You will have a problem with the liquid metal overflowing through the riser unless the mould is fairly thick on the top.  
 
I think Evil Dog's problem is related to the sprue solidifying before the rest of the metal in the mould has solidified, which gives the cavity.  The mould would have to be very smoothly finished at the edges to be trapping air.  My opinion is that you need to keep the sprue liquid until the whole casting in the mould has solidified, which can be done by externally heating the sprue or using a closed pour dipper like a Lyman which can apply its heat to the sprue.

[Evil Dog]

I think that the problem is pretty much solved now.  Cast a run of 5 today and 4 of them are keepers.  All 5 would have been keepers if I had pre-heated the mold a bit longer.

The key is definitely keeping the sprue and reservoir melted while the rest of the ball solidifies.  While picking up a lug wrench at the local Auto Zone auto parts store today I found a torch head that is just the ticket.  The Magtorch MT 245 C Heavy-Duty Pencil Flame Torch Tip.  Ran $20.00 but the burner head is close to 3/4" diameter and puts out one serious flame.  It will use either propane or MAPP gas (propane was plenty hot enough) and to top it all off, it is USA made !!!!

Of the 5 balls I cast tonite, they weighed 1lb 6.9oz, 1lb 7.4oz, 1lb 7.5oz, 1lb 7.5oz and 1lb 7.5oz in the order they were cast.  I figure the first one just heated up the mold to operating temp and that monster torch kept the sprue and reservior well melted on the next 4 castings.

Casting 4 or 5 a day will soon have a goodly supply of fodder for my half scale Napoleon......... and soon to be 1861 Seige Mortar...... but that's another story.

[Cat Whisperer]

 ....  Casting 4 or 5 a day will soon have a goodly supply of fodder for my half scale Napoleon..........

I hope you talking about shooting them on the range and not urban redevelopment!

Good technique on heating the sprue resevour - one commonly used style of large-flame tourch is called a rose-bud.

[Will Bison]

Hey, Hey. Sounds like yer gettin the hang of it E D. That's great.

[Double D]

E.D. tell me about you casting equipment.  What are you using for a pot and burner?  Can post a picture of your equipment.

For the group, about these zinc fumes.   If I go casting zinc in my back yard in urban Northern Virginia in the middle of a housing project, am I going to poison all my neighbors and be on the 11 o'clock news?

[GGaskill]

I seriously doubt it.  But you may have to deal with some NIMBYs.  Get some leftover Col. Sanders and put it on the side of the burner to camouflage the smell (you shouldn't get any significant smell from the zinc anyway unless it gets way too hot.)  If you get complaints, tell them you forgot the fire for a few minutes and burned the food.

[Cat Whisperer]

We cast zinc inside the plant with only a minimum of ventillation (and you don't even notice it walking a few feet away!).  It does take some of course, but outside you have little to worry about.

I burn brush every year in my back yard and watch the wind very carefully.  With about 100' from the house to the alley, I've found that if there is a little wind I need to get about 50' from the house and can avoid smoking the neighbors.

Smelting lead, however, takes only a little breeze to dissapate.  I do it in the drive, which is 20' from my house and 30' from the neighbors.  No problems - it dissapates quickly.

The biggest hazzard will be if you are smelting lots of used zinc castings and are burning off lots of oil and crud - it will generate a bit of smoke and smell.  

But for casting, with no more than an iron pot and a propane burner with a light breeze you should have no problems.

[Evil Dog]

Well Double D.... as you can see, my setup is certainly nothing fancy.



A single cast iron propane burner with a cast iron melting pot.  Holds enough zinc to cast 5 balls... probably between 8 and 9 pounds.  The channel lock pliers are used to pick up the pot and hold it while pouring into the mold.  A machined steel mold with the halves clamped into a vise-grip "c-clamp" pliers.  A handheld propane torch to pre-heat the mold and keep the sprue from setting up too fast.  The brass hammer is used to seperate the mold halves and knock the ball out of the mold.  There is a piece of what looks like concrete wall board covered with a piece of sheet metal to keep from burning the work bench.  In front of the burner you can clearly see where the mold was once set while still quite hot.



I use 2 different sets of gloves.  The pair on the right is a very heavy set of welding gloves.... use them while pouring.  The pair on the left are very heat resistant foundry gloves... uses them whenever I will actually have to touch the hot mold.

I usually cast 5 balls in a row, the first one normally being culled.

[Double D]

Thanks E.D.   What size balls are you casting and how much zinc does you pot hold?  

I am thinking of doing 6 PDR balls that just happento also weigh 6 lbs.

[Evil Dog]

I'm casting a 2.26" ball nominally weighing 1 lb 7.5 oz.  I can get 5 balls from the pot with some zinc still left so I would guess the capacity of the pot to be 8 to 8.5 pounds.  I'm sure there is some formula for figuring out the weight of any given zinc sphere, but I have absolutely no idea what it is.

One limiting factor that you might run into is the heating capacity of your burner.  I tried using a 5 quart cast iron pot.... held plenty of zinc but my burner just didn't put out enough heat to keep it melted.  A larger pot should be easier to keep at a constant temperature though.

[GGaskill]

Formula for the volume of a sphere is 4/3 * pi * Radius ** 3, which is read "four thirds pi R cubed."  Density of ZA-3 is 6.7 gm/cubic cm or 3.873 oz per cubic inch.  So a 2.25" solid shot of ZA-3 would be 4/3 times pi times 1.125 cubed cubic inches, which works out to be 1.33333 times 3.141592 times 1.423828 which equals 5.96412 cubic inches.  Multiply that by 3.873 oz per cubic inch and we have 23.099 oz or 1.4437 lbs or 1 lb 7.1 oz.  A 3.5" spherical zinc shot would weigh 5.434 lbs by calculation.

[John N]

GGaskill,

Thanks for the formula. It's neat to see the math match the results Evil Dog has achieved. If fact, if you recalculate for Evil Dog's 2.26" solid shot, the volume equals 6.04399 cubic inches which weighs 23.408 ounces. That's 1 lb 7.41 oz which is almost exactly the weight of the balls Evil Dog is producing.

[Evil Dog]

Thank you George.  Once again I am totally in awe.  Knowing what the actual weight should be really does help.

[Double D]

Looks like my spring scale isn't to far off, thanks for the arithmetic George.  


So what do you fellow think I should look for in the way of pot size, burner and ladle for  6lbs  shot?

[cannonmn]

I had a similar problem with large voids in 8-pounder balls, about 4" diameter.  I asked a very experienced projectile caster who told me he was pretty sure I was pouring the zinc too hot.  We were pouring it at about 850 deg. F.  He advises the voids can come from the high shrinkage of zinc, if it is too hot it shirnks so much it pulls voids inside the castings no matter what else you try to do to avoid it.  He casts zinc so it is barely beyond the slush stage.  He does not cast it unless there's a ring of unmelted zinc around the edge of the pot, showing him that's the lowest temperature he can cast zinc at.  He also uses an acetelene torch to keep the zinc at the top of the mold hot enough so it won't freeze before it should.  He keeps the mold at a temperature that gently boils water it is put into, but not hot enough to form a steam jacket around the submerged mold.  So he has to cool the mold every few pours to keep the temperature down.