Mold Cavity Problem

[Lonestar22]

I cast last evening using the RCBS .32 cal 77gr RN mold I bought from a member of the Cast Bullet Forum. The mold is an older model, as it came in the white cardboard box RCBS used many years ago. I cleaned the mold thoroughly before use. The mold is in excellent condition.
After pre-heating the mold for about 45 minutes, I began casting. I'm using Lyman #2 alloy, and the alloy temperature was 650*F when I began. Since I cast using one mold at a time, I wait 15 seconds after filling the mold before removing the sprue, and emptying the mold.
The front cavity (the one furthest from the handles), cast a perfect CB, with clear sharp edges and a perfect base. The back cavity is where the problem is. The body of the bullet is filled out, but the base looks like alloy poured before properly heated. The base area is not filled out at all.
I checked the vent lines on both mold blocks and they are cut true across the block faces. I did notice the vent lines were cut very close together in the bullet base area, compared to the rest of the vent lines. I think this may be due to the relatively small size of the CB. I checked the bottom of the sprue plate with a steel ruler, holding the ruler perpendicular to the sprue plate base at eye level, moving the ruler across the plate looking for irregularities, but found none. The mold halfs close tight without any voids, and the alignment pins line up perfectly.
The pour holes and counter sinks in the sprue plate are smaller than say a 9MM sprue plate. But again, I think this is due to the small CB.
Not being able to find any physical issues, I switched to the casting process. I increased alloy temperature from 650*F in 50*F increments up to my maximum of 800*F, and the problem with the rear cavity did not go away. I tried casting at a faster rate, and at a slower rate, neither had any effect on the rear cavity.
Finally since I want to try the CB's in my Colt Pocket Pistol, I turned this 2-cavity mold into a single, and cast about 100 CB's using the front cavity only. I've been casting 28 years, and this is the first time I have experienced this problem.
Your thoughts and comments please.
Tim

[wiljen]

My first thought would be trade the sprue plate for one with large holes and see if that helps. I'd hate to ruin the original plate, but would bet someone like Red River Rick (Kal.castpics.net) could make a new plate with a larger diameter hole that might enhance fillout.

[WILCO]

1) Make sure the mold is thoroughly cleaned.
2) Pour a heavier sprue.
3) Loosen the plate hinge screw to allow for more escape of air under the cutoff plate.

[Lonestar22]

Thanks for the ideas guys.
Wiljen
I thought about replacing the sprue plate with one having larger pour holes, but the time was late, so I'll have to try your recommendation during my next casting session. I did find the sprue plate is quite a bit thinner than the current RCBS plates. The old plate thickness is .127" vs. .180" thickness of the newer sprue plate. Not sure if thickness has anything to do with my problem.
Wilco
I had tried all three of your suggestions last night, but unfortunately none had any positive effect on the problem.
Tim

[Echo]

My WAG would be as Wilco #2 indicated - pour a heavier sprue. Is the sprue dimpling in as the boolits cool?

[GLL]

If venting is not the issue try a spouted ladle like the RCBS model and pressure pour that back cavity !

Jerry

[Lonestar22]

My WAG would be as Wilco #2 indicated - pour a heavier sprue. Is the sprue dimpling in as the boolits cool?

I pour a generous puddle of sprue that covers both cavities, as is my normal casting practice.
I have tried the heavier pour of sprue, without success.
Tim

[Lonestar22]

If venting is not the issue try a spouted ladle like the RCBS model and pressure pour that back cavity !

Jerry

Boy, I don't like the thought of going back to ladle filling a mold, but, if push comes to shove, that's what I'll do. However it will be my last ditch fix. BTW- can you pressure pour using a bottom pour pot?
Tim

[deltaenterprizes]

Is your lead stream in the center of the hole in the sprue plate? If it is not try doing it and if it is. let it hit the edge to make the lead swirl as it is filling the cavity.
I was casting 25ACP bullets and the only way I got complete base fill out was to use pure linotype.
Lyman #2 should have plenty enough tin.

[Lonestar22]

Lonestar 22,

I belive that GLL is on the right track.

All my moulds are RCBS, with the exception of one lone Lee .30 caliber. I normally cast the cavity farthest away from me first and then fill the cavity closest. Don't know why, habit I guess. However, I have a couple of moulds that will not completely fill out the the base of the nearest cavity using this procedure. I found that I can get complete fill out if I cast the nearest cavity first and then the cast the farther cavity.

I haven't found any explanation as to why this is so, but it works. Just the nature of the beast I suppose.

PB

I too have molds that act up and require changing of the pour sequence. I tried the switch on my 'problem' mold, and it did not fix the problem.
Tim

[Lonestar22]

Is your lead stream in the center of the hole in the sprue plate? If it is not try doing it and if it is. let it hit the edge to make the lead swirl as it is filling the cavity.
I was casting 25ACP bullets and the only way I got complete base fill out was to use pure linotype.
Lyman #2 should have plenty enough tin.

I tried a straight pour into the center of the hole, a 'bank shot' off the edge, making the stream swirl in. I tried tilting the mold in different directions, and none of the above fixed the problem.
Tim

[462]

Lonestar22,
Yes, you can pressure pour using a bottom-pour pot. Put the pot's spout into the sprue plate's hole and lift the handle. Or, vary the distance between the two.

[mooman76]

I have found that when you have one cavity that consistantly pours good and the other bad(base) especially the one closest the handle that loosening the sprue plate seems to help. Sometimes to the point where you think it would be too loose. Something to do with venting I think.

[shaggist]

Could the handles be siphoning heat away from the rear cavity, cooling it too much and too quickly?

[Le Loup Solitaire]

A problem with smaller caliber bullets is that they do not have enough mass to give off enough heat to the blocks. It is a major problem with even smaller calibers especially 22's. But if one cavity is casting perfect bullets then the other should do so. Try all of the following; Raise the heat above 800--the worst that can happen is that A) it might work, B) you will get frosted bullets, #2 pour the difficult cavity first and see what happens--also alternate which cavity you pour first. #3 loosen the sprue plate so that it swings free of its own weight, when the mold is tilted....but not up and down sloppy loose. #4 try different size lead streams starting from minimum dribble first and then increasing it a little at a time. Also pour alloy all over the sprue plate. #5 when the sprue puddles are liquid, let the mold drop gently so that it hits the top of an ingot with a slight jolt...this trick has benn used by some folks to improve fillout. A thicker sprue plate might improve performance as it holds more heat. Give RCBS a call or e-mail them and explain the problem; they may send you a thicker plate with larger sprue holes to work with. Their customer service is good. Somewhere between all of the above is the answer to your problem. It is a matter of trying combinations of all of the possibilities; it will take time and patience if you have one or both. There will be a fair amount of cussing untill you hit the right combo. The alloy/melt is either running into trapped air or cooling/setting up before it has a chance to flow completely and fill out the problem cavity. The mold is good, but you have to outsmart it. Easier said than done I know, but there is an answer. LLS

[Lonestar22]

Thanks to everyone who offered solutions to my mold cavity problem. I have my work cut out for me, and will report back with what worked to fix the problem.
Thanks again,
Tim

[Edubya]

Lonestar, You've got a few more years' experience than I have and I know that you said that you've cleaned the mould thoroughly, I have but one more suggestion: use a #2 pencil to follow the vent lines. I don't know exactly what this performs, other than a minimum of abrasion and graphite application, but it worked for my one problem mould.
Good Luck,
EW

[Lonestar22]

Lonestar, You've got a few more years' experience than I have and I know that you said that you've cleaned the mould thoroughly, I have but one more suggestion: use a #2 pencil to follow the vent lines. I don't know exactly what this performs, other than a minimum of abrasion and graphite application, but it worked for my one problem mould.
Good Luck,
EW

EW,
While I do have a number of years of experience, I am always open to new ideas and help when I have a problem. That said I will also include your suggestion on my list of things to do as I work towards a solution to my problem.
Tim