Vent riser - Mold Modification

[midnight]

I was just reading a letter from Merrill Martin in the June 1990 issue of Precision Shooting magazine and I came across a mold modification I had never heard of before. He says foundry operators call it a "vent riser". To apply this to a bullet mold, he places the sprue plate, bottom side up, level in his milling vise and mills a half round groove from where the edge of the bullet base would be out to the edge of the sprue plate. The lead in the groove is sheared off when the sprue plate is opened and the bases are as flat and square as before the modification. He claims that no matter what pouring technique you use, the bullets come out "several" grains heavier than an unmodified mold. This is when casting a 400gr bullet for the 45-70. A heavier bullet can only mean a more dense bullet with fewer voids or other abnormalities. Has anyone here heard of this of tried it? It is simple to do. Maybe some of our moldmakers here could help us discuss this and see if it has any merit.

I just looked at the address label on the old Precision Shooting magazines and they were originally sent to Walt Berger of Berger Bullets & benchrest fame.




Bob

[bhn22]

I'm all ears, and I got my popcorn!
Attachment 97958

[Bullshop]

Its improved venting at the base. You can get the same effect by beveling the top seem between blocks. I have done it both ways to get better filled bases and both work.
I have also added vent lines to the block faces if there is a particular spot like one band that wont give good consistent fill. Vent that one band and the problem goes away.
There are only two reasons I can think or that cause poor fill, the mold not being hot enough and inadequate venting. Even with contaminants in the cavity you can still get a good fill if the venting is adequate to allow the gases from the super heated contaminant to freely escape.

[bhn22]

I think Harry Pope cut his molds with a similar vent, but in the top of the mold itself, from the pics I've seen

[bobthenailer]

I have stoned a vent line in the top inside halves of the mould blocks so there's a slight parting line going from the front to back for improved venting of the 25 or so moulds i have. i orgionaly got this idea from LBT's book 25 odd years ago.

[turbo1889]

I just loosen up the sprue plate and/or as Bullshop suggested just smooth off the top inside corners of the block faces to get good base venting.

[Cherokee]

I guess I haven't seen the need to do this for the base, I have encountered inadequate venting in the body of the mold and fixed that. I run with a lose sprue plate except for the Lee molds.

[Bullshop]

The single Pope mold I have seen had a fairly deep vent troth cut parallel to each outer edge of the blocks on top between the outer edge and the cavity. The normal depth vent lines going from the cavity to the vent troth.
I have a Victory mold done in the same fashion. Its more work to produce a mold with this type improved venting system but they work better.
I have not had any Mihec molds out lately but seem to remember his molds being vented similar to this as well.

[HeavyMetal]

I started Beveling the edge of the mold block, that surface area under the sprue plate in the early 80's most likely because I saw an interview with the owner of LBT.

It has been a very nice way to vent a mold block, never could figure out how to hold a sprue plate well enough to cut it in a mill and beveling the blocks has kept me from learning, LOL!

[Ben]

I'm with HeavyMetal on beveling the top edges of the blocks ! !

I use this technique on just about ever mold that I buy, new or used.

Makes all the difference in the world in well filled out bases.

Notice the metal removed at the tops of the blocks ( Just don't over do it, once metal is removed , it is impossible to put it back on the mold )

Trial and error casting will tell you when you've removed enough metal :

[btroj]

I've done the same thing Ben. I for a mould from HM moulds for my 9 mm that wouldn't give good fillout of the base until I beveled the blocks a bit. Took 3 tries to get it right went nice and slow. Made a frustrating mould enjoyable.

[mold maker]

There is a fine line (pun intended) between not enough, and way too much. It's much harder to determine the first, than the second.

[Ben]

Often times, it is amazing just HOW LITTLE metal needs to be removed to greatly improve venting. Don't get carried away with metal removal on the tops of the blocks.

[btroj]

Yep. Go slow. Take off a little then cast. If the mould wants more you will know. Like I said, I took 3 sessions of trial error to get it right.

You can take off more, you can't put it back!

[midnight]

It seems to me that milling the groove in the sprue plate would be less dangerous. Much easier to replace a screwed up sprue plate than screwed up mold blocks. I'm probably a little biased since I have a good mill, milling vise, parallels and some really tiny carbide end mills. I'll do it first on a Lyman mold since new sprue plates are readily available.

Bob

[StrawHat]

I am having trouble visualizing the sprue plate modification. If you can post a picture, before and after, it will help me.

Thanks.

I do break the top seam of the mold with a stone to help the venting. Just curious about this mod.

[Gtek]

I am in the block mod camp, too easy to run file or stone and break edge. I spend a lot of time centering and clean up work on plates to run ball mill across bottom. Gtek

[MtGun44]

Interesting info, I will have to try the bevel. What I have typically done is
to keep the sprue plate very loose, seems to vent fine that way.

Bill

[Ben]

Bill,

If you do have a mold that is recalcitrant when it comes to a good base fill out, breaking the top edges of the mold halves will change things.

Ben

[btroj]

I am having trouble visualizing the sprue plate modification. If you can post a picture, before and after, it will help me.

Thanks.

I do break the top seam of the mold with a stone to help the venting. Just curious about this mod.

Interesting info, I will have to try the bevel. What I have typically done is
to keep the sprue plate very loose, seems to vent fine that way.

Bill

I am amazed you haven't done this. Simple engineering......