Venting Mold

  • Last Post 01 September 2017
John Alexander posted this 28 August 2017

I have a single cavity mold that casts great bullets easily with the exception of the base.  To get complete fill out of the base I have to cast very hot and leave a huge puddle over the sprue hole which uses up three times the lead required for the bullet.

I wouldn't mind a little rounding of the base which I don't think affects accuracy of a gas check bullet but only one side of the base is rounded so the base looks a bit oblong.  I don't know if this affects accuracy either but it seems likely.

I think better venting of the top of the mold will cure this but haven't gotten around to trying yet -- but will soon

I would appreciate any advice from shooters who have cured a similar problem.


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rhbrink posted this 28 August 2017

File a very small 45* angle on the top of the mold where the two halves come together can also make the sprue hole bigger.

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onondaga posted this 29 August 2017


If you have to pour puddles big to get the mold to fill out, venting will cool the pour faster in the mold and make fill-out worse, yes, worse, and will not help fill-out. You may luck into something that helps you cure your thermodynamic problem, but a change in your casting method is the cure, not cutting vents in mold blocks. A good caster can cast any bullet mold with NO vents.

Big sprue puddles aren't a problem for me so your method is different, I just put them aside and use them in the next melt. That is not a sacrifice to me to get good bullets and most of my molds need that.

I haven't posted on swirl casting in a while but that is the most successful way to cure the zone cooling you have. You describe zone cooling on one side. Swirl casting swirls the metal rapidly in when you pour and counteracts cool zones of the mold during casting. Your turbulence pattern when you pour is the cause of your cold zone. Swirl casting also evenly warms the mold well when you do it and adds to the cure.

To swirl cast use no longer than a 1/4 to 1/2" stream from the pot nozzle or ladle, Tilt the mold 5 Degrees and flow your pour off center by half of the sprue gate hole diameter and into the high side. It is good to partially hit the high side of the funnel shaped gate hole Do not gurgle the metal into the hole, pour accurately off center into the high side.This immediately flows the metal into the mold swirling fast. The swirling corrects your other pour method that has caused zone cooling. Despite any inconvenience you have with big puddles, they serve a very important purpose. The puddle is a reservoir to flow in as cooling shrinks metal in the mold as it cools and the reservoir feeds to compensate. So the biggest puddle you can balance is a GOOD THING for good casting. The Lyman pressure casting method does a similar service by keeping the flow connected as the metal cools in the mold. But you can't swirl cast when pressure casting because of the tilt and off center pour needed to initiate swirl.

This is nothing new in casting and treatises were written in Hieroglyphics on swirl casting and casting reservoirs thousands of years ago by the Egyptian Pharaohs metal workers who were executed for miscasts when they failed with Pharaohs gold. I taught this stuff to Dental students and technical students as it also relates to metal flow in centrifugal casting in the Dental Lab.

Mold makers base the diameter of their sprue gate holes to make them 30-50% larger than the molten metal flow in. If your gate hole is smaller than that relation to your flow diameter, fix it. Air has to be able to get out as you pour metal in. Don't rely on vent holes to do that, pour better.



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OU812 posted this 29 August 2017


Adjust the sprue plate for a looser swing open fit.

My bases start to get rounded when pot level drops and flow rate gets lower. Flow rate may be too low. Adjust flow rate for more flow. Too much is not good either. Spout hole may need to be cleaned.

Some molds can be difficult (especially using bottom pour pots). When all else fails try using a ladle.

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Starmetal posted this 31 August 2017


For the problem you described the number one thing I've found, and as mentioned, is to just "break" that top mold edge corner. I use a fine flat stone.  This has worked everytime for me when I had that problem. 

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onondaga posted this 31 August 2017

Cutting the mold edge to create a vent may help but does not address the pouring method error that causes cold shorts and zone cooling. Learn the swirl casting method I described and you cure the problem for all your molds, not just the one you needlessly mutilate by cutting.

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TRKakaCatWhisperer posted this 31 August 2017

Hmmmm.  Bottom pour or ladle? 

I use bottom pour.

I watch several things when switching to a different mould, as all affect how it fills out.

At some time in the near future I'll take notes on what I've done with each mould.

First and easiest to adjust is the distance from the spout to the sprue plate.  This can vary from contact and holding to 1/4" 1/2" 3/4" below.

Temperature of the alloy.  A bit slow to adjust, obviously.  Might could be different for 1:20 vs WW + tin.

Adjusting the flow rate valve - more open for bigger moulds.

Then the size of the puddle on top of the sprue plate.

The time that is spent in the mould cooling.  Sometimes when I'm impatient I'll use a muffin fan.

At some point I'll add the thermocouple to the mould.  I want to gain consistency in weights.

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onondaga posted this 01 September 2017

 TRKakaCatWhisperer, You said,

"Temperature of the alloy.  A bit slow to adjust, obviously.  Might could be different for 1:20 vs WW + tin."

Try casting at the ideal pot temp for your alloy or any bullet alloy. You can determine that precisely if you have the gear needed:

Turn your fresh potful of cold alloy to 750-800 F. and and put a thermometer in. when the alloy is fully fluid and cleared by fluxing, unplug the power and watch. Note the temperature when any solidification starts on the surface like a crystalline pattern or wrinkling. The ideal pot temperature is 100 degrees F. hotter than the temp you recorded at the first sign of solidification.

That temp is ideal for a 1-2 cavity molds dropping 3-4 times a minute. 4-6 cavity molds will need about 30 degrees hotter. Incorrect cadence is the cause of this method failing.



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TRKakaCatWhisperer posted this 01 September 2017

good summary.   I gauge cadence by the quality of the shearing of the sprue.  Too early and it smears (and the bullet is not yet hardened appropriately), too late and it takes a mallet.  Just right and it's easy and the sprue sheared surface is clean and smooth.  That likely figures out to fit your cycles per minute.


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onondaga posted this 01 September 2017


Yes, the appearance of crystallization on the cooling surface of a sprue puddle is very visual and with a conscientious observation a caster can set the time to open the sprue cutter cleanly by using a gloved hand. That is ideal, but I keep a mallet handy to be sensible.

An important step passed over by lesser skilled casters is the mount of time between opening the cutter to the opening of the mold handles to drop bullets. Molds that hold more heat need to allow the metal more time within the mold to solidify. Some of my six cavity molds need up to 10 seconds after cutting before I open the handles to drop bullets or the bullets will deform from slumping before solidification. Yes, you do have to fit these steps into your casting cadence of 3-4 drops per minute when your pot temp is idealized 100 degrees F. higher than first solidification of the alloy in your pot.


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