Base flashing with Mihec 4cav mold

[Echd]

A year or more ago I purchased a 359-125 from another member here. It wasn't in the best shape (Pins stuck ridiculously, good deal of dings, etc) but after some yelling and coaching it generally would cast well. The bullet design is absolutely top notch though and it shoots GREAT, so I am more than willing to put up with the mold's difficult temperament.

The past couple of attempts it has been casting poorly. The furthest cavity from the handles experiences massive base flashing- an amount that can't be just torn off or resized over. The first two cavities are always perfect, the third sees a small bit of flashing, but the fourth is unusable. There is also a visible join line on the bullet, as well.

The mold appears to close alright, but the far alignment pin appears to be problematic. However, I have bumped it back into what I believe is the proper position, but I continue to get terrible bases.

What else should I try to diagnose? I have heard people mention the washer under the sprue plate can be a problem, but I have cast a LOT with other mihecs and never had one wear out or anything like that.

[Beagle333]

Can you see light under the sprue plate? Is the top of the mold galled under the sprue plate around the screw? I have seen some molds that had some galling around the screw and if it was tightened too much, it would cause the far end to lift up off the blocks just a bit.

[Grump]

Depending on the fit of the sprue plate to the blocks, tightening the screw holding it on too much can result in the side away from the screw tilting up away from the blocks. Geometrically, that leads to exactly what you describe.

Yup, BTDT. More than once. Last time it was after trying to stone the sprue plate and the tops of the blocks to fit better--and I didn't notice that the corner of the sprue plate next to the screw hole was NOT parallel with the rest of the bottom of the plate. Polishing it made it worse, and going tight on the hinge screw for the sprue plate made it ever more worser.

One mould with a fairly thin sprue plate even became twisted up from the handle/hinge side of the blocks after a few slightly tilted whacks with the sprue "hammer", a 2-inch cylinder of heavy wood. After bending that corner back down and being more careful with my sprue cutting stroke, no problems with base fins again.

All of this MUST be examined in combination with the excellent advice above.

IF your blocks don't have a burr or lead flakes or similar holding them a bit open, then the most likely culprit for your parting line discrepancies is those alignment pins. Look closely at them--the one(s) nearest the handle hinge could either have a "ledge" or just be a hair too thick to go all the way into the alignment hole. Don't be afraid to tap those pins in or out until the mould blocks mate properly. The only light visible anywhere when they are closed should be through the vent hole--if any light goes through at all. It's sometimes quite hard to line up your eyeball perfectly enough to catch that tiny pinprick of light.

[44man]

Good answers all. I have learned that I can NOT make a sprue plate or top of mold flat by lapping or stoning. I make my own molds and plates, plates are thick stainless and any time I get a poor fit, the only fix is to mill the plate with an end mill.
The plate screw must be adjusted right or it will tip up at the other end. I don't like too loose or tight. It has to be JUST RIGHT. The Mark I eyeball to look under the plate works.
I never, ever touch mold tops with anything since I mill it flat before using the cherry, I would just ruin venting and flatness. Mill marks left are perfect vent lines.
Never polish a plate, the stroke is longer at the edges then in the center so you make a rocker. But edges must be rounded and polished. A burr at the hole takes a stroke or two.
If you use wood to cut the sprue, make little taps, never one whack. You should see little, tiny circular cuts on the lead.
The lessons I learned by making molds is so valuable it must be shared.
Our mold makers are so good I do not know why they don't add a list of things NOT to do. They start perfect and the owner is the cause of problems.

[silverjay]

Same mold same problem. I first tried to tighten the sprue plate and it got worse. Found I had some smearing at the first cavity that was pushing the plate up. Cleaned that up and they all drop good again.

[44man]

Same mold same problem. I first tried to tighten the sprue plate and it got worse. Found I had some smearing at the first cavity that was pushing the plate up. Cleaned that up and they all drop good again.
Common with glove cutting, let the sprue harden more. Speed casting will fail.

[Echd]

Disassembled and judiciously cleaned the mold. I did not notice any galling in any important locations. I reassembled it and left the sprue plate very loose- something which I never liked to do for whatever reason, but which has always worked just fine.

It appears the biggest problem was that one of the "female" alignment pin holders had been "backed up". I honestly never even knew those could move. I gave it a good thump with a punch and it was back in business- about 250 perfect bullets sitting downstairs.

Now just to wait for my new R8 (the tacticool performance center 627) to show up so I can slug it! This has been my go to bullet in the 9mm ever since i got it. Time to see how it acts in 38 and 357.

[44man]

Sounds good, glad it worked out.

[gtgeorge]

Glad to see you got it working for you as I think you got that mold from me? I run all my MP molds with loose sprue plates looking at the plate and backing off until it sits flat on the mold blocks. No daylight should be seen. I also found that particular mold can do what you described if filling the cavities too quickly. As for any loose pins (I remove them and clean them and use loctite). The loctite for retaining bearing races etc. May want to make sure the mold handles are closing the mold flush as well as the pins should not hit each other trying to close. But knowing you are not new to casting I am sure you know this anyhow :)

[Echd]

I think I might have! I have fed a lot of 9mms with that mold. I was pretty distressed when it started spitting out those bullets with the flashed bases.

Just as a matter of habit I always left my sprue plates loose enough to be turned by hand easily but not to where they flop about... even though I know the flopping hurts nothing, just every now and again I would do a pour with the sprue plate slightly out of alignment.

It appears to be working now. Honestly, I just had no idea the alignment pin holder (there is certainly a better name) could even move. That's the first time I have had to adjust one.