Why woukd a cast bullet grow in dia??

[Dom]

I have a new Alum. 2 cavity HP bullet mold listed at .434 dia, & 300+ grs. . The first 5 or 6 bullets out of the warm mold drop right at .434. As the mold heats up bullets start dropping at.436, then .438, finally stabilizing between .439 & .440 . After this, they produce beautiful .440 bullets from then on. Way too big to size down thru my .330 sizer. I'm using my stash of decades old scrap , clip on WW's. No surprises here. I also have other 44 cal molds from this same mold maker that work perfect. I'm not posting the name of the mold make because they are in the process of making & shipping me a new mold. Could this be a problem with the aluminum alloy in the blocks. Any thoughts.

[JonB_in_Glencoe]

new Alum. 2 cavity HP bullet mold
It's not closing correctly...and whatever issue is there, it's worse when it's hot.

Check alignment pins. Check HP hardware, depending on design there is different things to look for, but a problem like you describe, the problem should be obvious. Also, Check mold face for alloy splatters.

[Dom]

All the items you mentioned have been looked at. NO splatters. Blocks appear to close tight , with no light coming thru.I have many aluminum molds & have never had a problem like this before. I can set the mold aside , let it cool down, & the next 5 or 6 bullets come out right at .434. Then as the mold heats up, the bullets grow in dia.

[charlie b]

Since it is being returned will be interested if they say anything about it.

If I let my cadence vary I will get slightly different weights/diameters, but, not the extent you mentioned. Maybe .001" or 1gn.