What causes finning on the seams?

[cabezaverde]

I have a couple of molds that give me finning on the seams. What is the major cause of this? The faces seem clean, and the molds seem to close correctly.

[MtGun44]

Typically not closing fully, which is often due to a dab of lead adhering to the joining face
or need for lube on the alignment features of Lee molds. For lube, I strongly recommend
Bull Plate Lube from Bull Shop, link at bottom of page.

Look closely at parting faces and alignment features, maybe with magnification to see what
may be holding the blocks open a bit.

Bill

[excess650]

This usually happens when the blocks aren't completely closed, alloy is really hot, or too much head pressure when pouring.

Lyman molds can develop raised spots around the alignment pin holes from repeated use and no lube. Stone the face to remove the burrs. If there is lead splatter between the blocks, it can do the same, regardless of brand. Check the edges of the faces for nicks, etc.

If you have a really hot alloy that flows really well, it can seep more easily than a cooler alloy with more antimony content. If you're pressure casting, quite often you'll see finning or whiskers if the vents are too large for the amount of head pressure, alloy temp, etc.

[cabezaverde]

Molds are Lymans. I had not thought about pressure - I am a dedicated bottom pour guy.