seam line on cast slug

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airgunfans

Member
Jun 8, 2018
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Macau
Being a total newbie to slug casting, I have started trying it with a 8-cavity , .22 cal mold from MP Mold. As expected there is a seam line on all slugs but I wonder how big the step along the seam line is considered acceptable. I have tried to measure it by clamping one half of the mold on a machining table, put the other half on the top and move it back and forth with my hand and measure the amount of play with an indicator. The reading is 0.05 ~ 0.06 mm. This is measured when the mold is cold. I am not sure whether it will be larger or smaller when the mold is heated up.

May I know if such amount of play is normal ? 

It seems that it is possible to reduce the amount of play by pressing the alignment pins towards the seats with a vise but I am not sure if it is a good idea.

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you know it has been a long time since I cast bullets and a 8 cavity mold is fairly ambitious but good for you, 2's and 4's were used for H&G 68's .45 acp and 9MM for me

now your talking .002- on the mold and that ain't much

so back in the old days anything we cast we sized and of course they got lubed at the same time

aren't the air gun slugs being sized or are the raw castings be used

I any case that is a good looking mold
 
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I wouldn't try to press that alignment pin in, it's probably OK. Even with the mold closed if you see a very small hair line space at the bottom of the cavity (that would be the nose of the pellet/bullet) that is OK and actually helpful to allow the bullet to fill the mold completely. There are many reasons to have a seam line. Alloy/lead temp, debris inside the mold halves, not lubing the alignment pins, handle hinge and connecting points to the mold too tight. The mold halves should be a little loose on the handles, so they can find and move into position on the alignment pins.

Believe it or not, how you pour the lead into the mold is a big factor. I've always used Lee bottom pour furnaces. If I let the lead stream enter the mold through the center of the sprue hole I often get a seam. There's just a little too much pressure. I offset the pour (bounce it off the sprue hole actually) the seam goes away and the bullets are perfect. 

Leave a little metal on top of the sprue. Lead will shrink slightly as it cools. You can actually see it draw down that puddle a little as it cools. It only takes 2 seconds for it to cool. It's very noticeable with bigger bullets 38's, 44's 45's etc. but still a factor with smaller .22 bullets/pellets.

If you use a dipper you will have to develop your own method. Experiment and see what works. 
 
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I use the same mold and get great results. I did have one casting session where I couldn't close the gap between the plates tight enough. I ended up with some slugs too big to use, I then noticed there was something smushed onto the plate not letting them come together tight.

Been all good since and pumps out beautiful slugs fast, I use a ladle and works fine.

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Thanks for all the replies. Indeed it's a learning process. After posting I tried casting with the mold again and this time I did not soot the cavity as suggested by the maker of the mold. Immediately, I ran into mold sticking problem and I had to smack the handle VERY HARD to get the cast separate from the mold.

Then disaster happened. Because of the violent smacking actions, the aligment pin moved outward and made it impossible to close the mold completely ! I thought that's the end of it. The mold was permanently damaged after less than 30 pours and producing just a handful of usable slugs. 

As there was nothing more to be lost, I heated up the mold , put it into my table vise and pressed the two halves together. Much to my pleasant surprise, the play previously seen is now gone ! The fitting between the mold halves is now perfect ! I went on to cast some slugs and this time I sooted the cavities to ensure good release. DA DA !!! . No more seam lines ! All the slugs look just perfect ! I am so pleased with the quality of the mold that I ordered another one of slightly smaller head diameter ( 5.52 mm ). 

Yes , I am using a Lee bottom pour furnace which is another big headache. The temperature control is just hopeless. With the temperature control knob set to about 5.5 ( slightly above middle ), the lead will solidify completely after a while before the thermostate kicks the heater on and melts the lead again. I am planning to fix it by installing a PID controller and that will be another learning process .....
 
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