Help with new mould, venting?

[fred2892]

I am awaiting a group buy mould, but in the meantime I managed to borrow one that had been sent out early from the group buy.
Being new I cleaned and scrubbed with brake cleaner and heat cycled a couple of times in the oven.
I am having big problems trying to get fill out.
I started out with a 20:1 lead/tin mix at 720°. Mould was pre-heated to 350° on a hotplate. Pot is pid controlled.
Boolits dropped fine with no sticking or hang ups. The bases were all rounded with wrinkles on the boolit sides.
I went up to 745° with little improvement. Some bases started to look good but never all 4 cavities.
I played with the sprue plate tension, still got the rounded bases and wrinkles.
I gave up on this mix and went over to another pot with a lead/lino/tin mix which water drops at around 21 bhn.
Nothing changed, I went through the same routine of changing temperature and sprue plate tension but still the same rounded bases and wrinkles.
I gave up!!! During the post mortem I was measuring the few that did drop a decent base and noticed that any which had wrinkles up the side only had them on one half of the boolit from parting line to parting line.
I am a fairly experienced caster, own 60+ moulds from all the usual suspects and this is the first time I have been completely at a loss as to what to try next.
Only thing odd about this mould is the fairly shallow vent lines and smaller than normal sprue plate holes.
Any ideas??

NOE mould top, New mould bottom. Both are 30 cal. Notice difference in sprue plate holes, could this be the problem?

http://img.tapatalk.com/d/14/07/29/e2are2et.jpg

This is what I'm getting most of the time. When bases do fill out it is completely random, different cavites.

http://img.tapatalk.com/d/14/07/29/7u3aqu8y.jpg

[Cap'n Morgan]

Try adding a temporally shim under the sprue plate. This will immediately tell you if venting is the problem. If it doesn't improve the fill-out then your sprue holes are too small for the manner in which you cast (bottom-pour or laddle?) going from, say .12 to .17 will double the flow area. I always use quite large sprue holes, but then I only use two cavity molds - and a lot of people produce beautiful boolits from molds with rather small holes.

If venting is the problem a gunsmith checkering file can be used to cut venting lines across the top of the mold.

When I make blanks for my molds I run a fly cutter across the top .004 deep to ensure proper venting. In fact, I think the venting lines on the mold faces is pretty much superfluous unless the grease grooves are quite large and square (air traps when the mold are tilted during fill)

[fred2892]

I am bottom pouring. Tried both pressure and gravity casting, made no difference.
I'll try the shim idea tonight, thanks for the tip Cap'n.

[texassako]

Are you letting the lead swirl in and not go straight in the hole?

[fred2892]

Only when pressure casting with the sprue plate held tight up against the pot nozzle.

[Moonie]

Usually when I have this kind of issue I will break the edge between halves at the top using a stone. This normally provides enough venting to keep this from happening.

[fred2892]

Nailed it Cap'n, thank you. A brass shim under the sprue plate and this mould is transformed. Excellent fillout on bases and cavities. That's one trick I won't forget. Any reason why the temporary shim can't be permanent?

[Cap'n Morgan]

Good to hear it worked for you.

There's no obvious reason why the shim can't be permanent, except for the bases of the boolits becoming ever so slightly non-perpendicular. But a better solution would be grinding or milling a .002 - .004 step/recess across the middle of mold top, leaving the front and back end untouched for the sprue plate to rest on. Of course, the recess can just as well be cut in the sprue plate.

Since the base is indirectly filled (lead rising from below) the gap can be quite large before 'whiskers' become a problem.

[fred2892]

I cast a couple of hundred last night and only about half a dozen had rounded bases and all of those came from the cavity furthest from the shim. So the non perpendicular sprue plate is not too much of an issue. However, when my mould from this group buy finally turns up, I will certainly consider milling a step as you suggested. After all the game is all about consistency isn't it? And 4 cavities throwing slightly different bases isn't a good start in that consistancy ideal.

Thanks again,
Fred

[cainttype]

As mentioned earlier by Moonie, breaking the mating edge between the blocks under the sprue plate can often supply the additional venting you're after. It can be carefully done with a fine file, but I prefer to use a machinist block (any flat surface, a plate of glass is easy to aquire) with 200 grit sandpaper and carefully drag each block at about a 45 degree angle. It doesn't take much, is easily done, and can permanently cure the problem you're describing.

[fred2892]

As mentioned earlier by Moonie, breaking the mating edge between the blocks under the sprue plate can often supply the additional venting you're after. It can be carefully done with a fine file, but I prefer to use a machinist block (any flat surface, a plate of glass is easy to aquire) with 200 grit sandpaper and carefully drag each block at about a 45 degree angle. It doesn't take much, is easily done, and can permanently cure the problem you're describing.

Thanks, I will try that when I get my own mould, this one here is just on loan and I don't want to try anything irreversible on it.

[bobthenailer]

IMO i would stone the inside top edge of the mould blocks first ! as recomended by Moonie !

I personaly have done this on every one of my 25+ moulds from 2 to 8 cavity . also dont open up the sprue holes only as a last ditch effort as you will find the venting line will most likely fix your problem , also try pre heating your mould to 450* and cast as fast as you can with proper sprue solidfication from 5 to 10 seconds also speed of the alloy from the spout. let 500,000 boolets made talk from experince.

The venting process was covered many years ago in LBT's book , by Verl Smith , Im not sure which edtion though

[lavenatti]

I'm with Moonie too.

Just don't take too much off. I usually wait until after 5 or 6 casting sessions with the same problem before I try it. I have had the problem go away by itself after the mold was broken in.

[captaint]

A couple of weeks ago I was pouring up some boolits with a new NOE mold. Same problem. Rounded bases. Couldn't make them go away. Broke the top edges on the mold. No success. I was ladle pouring at the time. I usually bottom pour. Then I noticed, by accident, on one pour I kept the ladle off the sprue plate. Didn't let it touch. Boom, flat, square, filled out bases. First time I ever experienced that - but the ladle touching the sprue plate was the whole issue. Mike

[GP100man]

Slow the flow rate & let the alloy pour off the plate to warm it good.

Smallish plate holes when ya let it swirl will trap more air than trying for thru the hole.

These can be tried without any mods , but I`ll admit ,over 1/2 of mine have the mating edges broke to help venting.