Front cavity on Lyman 4 cav not filling

[happy7]

I got a new 429421 mold in a four cavity and was able to cast good bullets in short order, except the first cavity. The mold would not fill, like what happens when you cut the flow too soon and leave a too small sprue. However, I was leaving big sprues and still it would not fill. The rest of the bullets was perfect, just the base of the bullet was consistently rounded. The other three were always fine. I tried loosening the sprue plate but did not help. Finally I started filling back to front (using RCBS promelt). That helped and was getting mostly well filled out bullets. Anyone know what could be causing this?

[Onlymenotu]

*umm* my guess and it's just a guess,,,, is venting issues......... filling it last it has no where to vent to...... filling it 1st...... vents to the other cavities...... again just my guess,,,,,,, i'm sure other with more experiance will chime in soon

[grumpy one]

It sounds as if the mould is not hot enough around that end cavity. It might be a venting issue, but the fact that is is an end cavity is suspicious. I suggest you look closely at the venting but don't make any design modifications - just ensure the existing vent lines are clear. Then run the mould hotter.

[Shuz]

What is your alloy temp?

[John Boy]

It sounds as if the mould is not hot enough around that end cavity
Grumpy's on the hunt! Best way for these multi cavity molds is to pre heat them on a hot plate. This way, the mold is fully heated

[Jon K]

All have good points, I have had same with new Lyman multi cavity molds, clean again w/brake arts cleaner- cavity & vent lines, pre-heat, and you may have to run the alloy temp high until the mold warms up. Often times the first session will do as you described, then next session works like a charm- just needed break in session.

Jon

[happy7]

These ideas make sense to me. Especially the vent issue. My lyman thermometer says temp is around 720. If I run it hotter its gonna frost pretty bad I think. I'm gonna clean it good and inspect the vents. It will probably be a week before I have time to try this mold again.

[klw]

I got a new 429421 mold in a four cavity and was able to cast good bullets in short order, except the first cavity. The mold would not fill, like what happens when you cut the flow too soon and leave a too small sprue. However, I was leaving big sprues and still it would not fill. The rest of the bullets was perfect, just the base of the bullet was consistently rounded. The other three were always fine. I tried loosening the sprue plate but did not help. Finally I started filling back to front (using RCBS promelt). That helped and was getting mostly well filled out bullets. Anyone know what could be causing this?

Years ago I bought two H&G four cavity moulds with each cavity cut for a slightly different bullet. In both cases the bullets cast from the front cavity had a noticeably lower visual acceptance rate than bullets cast from the other three. Never figured out why. Bullets were different enought to easily tell apart but not all that different in shape or weight. I've long thought that this is just a characteristic of four cavity moulds.

I asked Wayne Gibbs about this. But he had made very very few moulds of this type so he wasn't sure what was going on.