MP mold question

[Lefty SRH]

I have 3 MP molds made by MiHa, love all 3 and they make GREAT boolits! I have noticed something odd about the latest one. Its a 4 cavity 330gr solid Ruger ONLY (normally 297gr HP, 454-297). The back cavity (the one closest to the handle pivot bolt) won't fill out reliably, I mean bad! Not the case where 2% TIN would fix, I mean the base is very rounded.
What do I need to fix this small problem? I seems to happen when my 20lb pot is less than half full.
My alloy is straight COWW, running about 680 degrees, running a pretty fast flow, NOT pressure casting, etc. I'm leaving plenty of sprue puddle too. Anything else?
My other 2 MP molds run all 4 cavities just fine.

[BoolitBill]

You might try to use a ladle to cast that mold with. I always get better results with larger boolits (Mihecs 350 gr. 45/70 boolit as an example) using a ladle.

[Lefty SRH]

Why would the back cavity be more difficult than the other 3? I've tried pouring it first, second, third, doesn't seem to matter.

[Carolina Cast Bullets]

First thing that comes to my mind is possible warped sprue plate.

Close the mold, close the sprue plate and look to see if any light shows between the plate and mold top. If you see light or uneven light, sprue plate is warped.

simple replacement

Jerry
Carolina Cast Bullets

[mpbarry1]

I actually noticed that with my HG 68 mold. If i get a rounded base, it is in the number 4 hole. My theory is that there is less brass to hold heat back there. I can usually fix the problem by making sure that i pour a large sprue every time over hole number 4. Good luck.

[white eagle]

you mold aint hot enough
I have the same problem till my mold gets up to temp
try it

[Lefty SRH]

tried more heat and it frosted the other 3 cavities pretty bad

[Lefty SRH]

I'm considering taking the sprue plate to work and opening the countersunk hole a little bit. It doesn't do it when the pot is more than half full. I'm thinkin it needs more pressure when its poured into that cavity.

[rintinglen]

You are very likely right on target. As the lead level diminishes in the pot, the head pressure of the pour decreases, making rounded bases more likely. I have a 4 cavity LBT that does the same thing with the number 2 cavity. Once the pot level falls an inch or two, I start getting bad boolits.
I follow Bruce B's tip of prewarming lead ingots on top of the pot to allow me to keep the lead level in the pot up while casting to limit the problem. Pouring a larger sprue might also help.

[longbow]

I think you should check sprue plate tension. If the sprue plate tension is too high it may be preventing air from escaping under the sprue plate as the cavity fills. The fact that is happens as your pot level drops may be a hint.

I usually set my sprue plates so they will almost swing open under their own weight and get good results that way.

Alternately if it is a venting issue, using a jeweler's file or any very fine small file you can put a very slight bevel on both top edges of the mould where it meets the sprue plate. I am talking just remove the sharp edge bevel ~ not a big flat. This provides a small vent right under the sprue plate. I have had moulds where this has helped.

Also, as BoolitBill says, try casting with a ladle. I find I generally get better fill out using a ladle.

I would try loosening the sprue plate a little first to see how that goes. If that doesn't work then try ladle pouring. If that solves it then venting, fill rate or both are likely the culprits. These are both easy to try and no mould alterations required.

If that doesn't do it I would bevel the top edges of the blocks ~ very small and very carefully.

Longbow

[GLL]

I have the very same mold.
Experiment with that added 2% tin and use a bored out RCBS ladle with a rapid cast rate !
That is what I use all of the time for all of the brass MP 4-cavity molds that produce large bullets . Great fill-out and clean drops!

Jerry

[Iron Mike Golf]

Lefty,

I had that on my 432-256 solid (the #503 clone). I also broke the top edge of the parting line. Now that's part of my new mold prep for MP molds. A couple of strokes on a finediamond stone. I just drag the block perpendicular to the top parting line.

Before I started the venting work, I'd pressure cast that cavity and after a couple of casts like that, it seemed to cast good bases for a session. I also now run my sprue hold-down screw looser. Pretty much just loose enough for the plate to free swing.

I'm not sure if the sprue hold down adjustm ent, the venting, or both fixed it for me.

[seagiant]

Hi,
I had that same problem and it was because the sprue plate was not completly flat! Might be something to check or just lap it for a secure mind. There is no reason you should have to use a ladle to get it to work!

[Springfield]

I always loosen the sprue plate first, then open the sprue plate hole, then last I will lightly file the edges of the mould. The first 2 are fixable/not permanent, the last is not.

[Cadillo]

I cast with a ladle and until I got my first Mihec mould, most of mine were Lymans. Until I tried casting with my Mihecs, I had always filled the cavities front to back or coming toward the handles. I always had trouble with that rear cavity on the Mihecs until I reversed my fill order and went from the rear near the handles and then forward. I really don't know why this works, but suspect it has something to do with heat retention. This little change moved me from frustration to happy success with my Mihec moulds.

Good Luck!

[Lefty SRH]

AS said before its the cavity closest to the handles and/or closest to the sprue plate pivot bolt. I'm not 100% sure as to what edges you are recommneding to file. I have noticed that there is NO light what so ever under the sprue plate and this problem cavity. There's a smidge of light under the other/forward 3 cavities.
Is the group suggesting that air is getting trapped between the lead and the sprue when the cavity is filled and NOT venting out?

[captaint]

As others have said - my step one would be to loosen the sprue plate some. Next, I would break that top edge where the mold halves meet. Doesn't take much. enjoy Mike

[white eagle]

tried more heat and it frosted the other 3 cavities pretty bad

I was referring to the actual mold temp
it may also be a technique issue as well
mine are ladle cast and after I get the mold to temp get 4 good boolits \
every time

[dragonrider]

Mold temp is more important than alloy temp. IMO your mold to a bit cold. Use a (I can't believe I am going to say this) HOT PLATE........if needed. Next get a file, a very smooth, fine cutting file and champfer the parting line edges across the cavities. Don't need much .010" at most but try for .005" Heat and venting will take care of your problem. And yes there is a such a thing as too much heat.

[geargnasher]

My MP H&G #68 copy did the same thing, it's sprue plate tension causing the cavity closest to the sprue plate pivot to not fill the base. I also noticed that the sprue plate was lifting off the blocks by about .030 at the far end! Boolits from the farthest cavity weighed almost six grains more. There was no lead under the plate, but it had galled slightly under the screw from being too tight despite using plenty of sprue plate lube on the pivot.

Ultimately the problem was the sprue plate pivot bolt was too tight, which I think it sort of has to be since there's no bolt head on the end opposite the pivot to tuck the sprue plate underneath, so I took it apart, dressed the galled surfaces, tweaked the sprue plate flat again, broke the edge of the blocks slightly, ran the sprue plate with just barely any tension on the wave washer, and proceeded to burn the daylights out of the back of my left hand with the wildly-swining sprue plate several times (I'm a slow learner). I ended up drilling a hole in the handle tang and installing a long roll pin for a stop so the sprue plate wouldn't swing open so far.

I may yet pull the roll pin out of the RH block and tap the hole for a bolt, then drill and tap for a setscrew for the bolt, because gravity is the only thing keeping the sprue plate flat on the blocks.

Gear