Modifying molds to improve melt flow

[Ogajiga]

After 10 years of commercial pouring finally figured out that smoothing out a Do-it gate can greatly improve flow through it. Actually, opening my poured new No-Snag sinker mold was causing ugly gouges in the eye caused by the casting being forced off the pin at an angle due to mold sticking in both halves. So, alls I did was razor blade the rough spots out of the gate on ONE HALF OF THE MOLD and problem solved - the smooth gate half released the casting easily leaving it stuck in the rough half to be gently pliered out with no damage. But, the fringe benefit was that where formerly the melt was backing up at the gate requiring precision pour placement and drop volume to avoid gate freeze the melt was now flying through the gate with no back up even with only 1/2 the gate roughly smoothed. Been a lot of posts recently about tough pouring molds so this might be another solution possibility.

[reeves]

Hawn,

This is great. All the same things are always mentioned - mold hot enough, soot the cavities, cut a vent, heat the hooks and wireforms and oil the hinges. But this has never been mentioned that I can recall. Excellent discovery and hats off to you for sharing.

THANKS!!

[Ogajiga]

"Reeves", I also salute you for sharing hand casting tips even though you've graduated to spinning. Newer Do-its have way rougher cavity faces so every new mold I get needs to be resurfaced for more efficient castings release especially with cool expanding bismuth. Never messed with the gate because I figured that clean sprue break required a precision sharp edged transition where gate meets cavity. I think careful gate resurfacing might improve the flow and release properties of some other problem molds.

[sagacious]

Great tip, Hawnjigs. Years ago, I had a stubborn pistol-bullet mold that refused to pour well. I tried all the tricks, but it just wouldn't fill well. Not wanting to mess up the mold, I opened the gates by a couple hundredths of an inch. Presto! Problem solved! It made for faster pouring and better fill-out, and the lead would 'swirl-in' and fill the cavities very fast and very consistently. Now I don't even ask questions, I just recut the gates on any new molds, and go from there. Saves a lot of time and hassle in the long run.

I also recut the gates on all my Do-It molds. I measure the constriction at the gate, and re-drill (carefully!) with a bit 1/32" to 1/16" larger, just to true and smooth everything up. Be sure to clamp the mold shut while drilling. Then lightly 'clean-up' the gate constriction or funnel with a sharp 45* countersinking bit. Check for any tiny burrs, and remove gently with an exacto knife. The gates will be smooth and will pour faster. Applying a dry anti-friction surface treatment, such as moly disulfide, will further reduce friction and speed-up pouring. (ACTION MAGIC II® at Brownells Apply just part "B" to the gates and cavities with a cotton swab. A little goes a long way and lasts a long time.)

I do this with all my molds. Helps make the pours faster, more consistent, and trouble-free.

Good luck!

[Ogajiga]

"Sagacious", are you just drilling a larger hole down the middle of the Do-it gates and leaving the tapered ends intact? Can drilling the Do-it gates larger cause any pitting problems due to a widened sprue break?

[sagacious]

"Sagacious", are you just drilling a larger hole down the middle of the Do-it gates and leaving the tapered ends intact?

I always try to cover all the bases in my reply, but sometimes I miss a few. Let me back up and explain a little better:

Yes, the hole is enlarged slightly. The tapered end is intact, just smoothed slightly by the countersinking bit. The goal is to true-up and smooth-up the gate, not to remove a lot of material. Less is more here.

I enlarge the gate about 1/32" on a 1oz jig mold, and about 1/32" to 1/16" on a 3 or 4oz mold. For a smaller gate, I'd probably suggest a smaller increase in the gate aperture, such as the 1 or 2 hundredths increase I use for a small pistol bullet mold. You just want to true-it-up so that the gate is perfectly round and smooth. That has worked well for me, and just a small increase in the gate opening seems to make a disproportionate difference in pouring speed and smoothness.

I haven't tried-- and wouldn't recommend-- opening the gate on a mold that drops very small jigs. If your mold is cnc milled, I'd suggest against trying to "improve" it by opening the gate.

I'm not really sure if it's a simple increase in the aperture that makes the difference. I suspect it's also (mainly?) that the gate is made perfectly round, and now the tapered part of the gate is very smooth. The lead just swirls down into the mold very quickly, much more so than you'd expect from just an increase in diameter of a couple hundredths (1/32" is 3 hundredths). Precision pour placement became no longer required-- the lead swirls right in.

I think it has more to do with what you noticed: That a small burr or eccentricity can cause a flow disruption in a high-density fluid (molten metal) that's great enough to affect how the metal flows into, and fills, the mold. That's just my guess and working theory.......... where's Vodkaman when you need him lol?

Can drilling the Do-it gates larger cause any pitting problems due to a widened sprue break?

Good question. The sprue is only about 1/32" wider, which is (I'm guessing) perhaps within the tolerances you'd find in a production run. If a clean sprue break is critical, you may wish to test this technique with a old mold-- and your lead, pouring technique, and equipment-- before you commit more heavily.

These techniques have worked very well for me. If you're not careful, you could probably do more harm than good, so measure twice and drill once. Be careful not to drill into the cavity walls!! If you have a mold that's giving you pour hassles anyway, I'd measure the gates, get a drill bit 1/32" larger, a 45* countersinking bit in the right diameter, and give it a go.

Hope this helps, good luck!

[Ogajiga]

"Sagacious", I thought 1/32-1/16" seemed like a lot of metal to remove from gate walls until you reminded me you pour much bigger castings than my tiny head Do-it Pro RHs. The sum of my gate improvement experience right now is a few minutes razor blade shaving the gate walls of one half of one cavity in one mold, and I'm excited how such a simple effort can improve the pouring efficiency so much. I agree that slight aberrations on the gate surface can ruin pourability - many production Do-its I use have one cavity that never seems to fill out right despite appearing no different from others in the set. Never considered gate mods before and hopefully this will solve the problem.

Edit: leaving for a month tomorrow - catch ya all in July.

Edited June 3, 2008 by hawnjigs

[BLT]

Some of my molds call for a #1 or a 1/0 hook. I like to use the larger hooks and they have a hard time filling around the hook shank, and the barb does not come out right at times. would/could this help with that? The smaller hooks pour great. I have the same issue with 2 of my Hilts bullet head molds not making a good barb on them.

[sagacious]

...

I agree that slight aberrations on the gate surface can ruin pourability - many production Do-its I use have one cavity that never seems to fill out right despite appearing no different from others in the set. Never considered gate mods before and hopefully this will solve the problem.

...

Good luck. Hopefully there's a hardware store or machinist's supply in your area that carries wire-guage bits, or bits just a few thousandths over your gate diam. That way you can start small for your smaller molds.

[sagacious]

Some of my molds call for a #1 or a 1/0 hook. I like to use the larger hooks and they have a hard time filling around the hook shank, and the barb does not come out right at times. would/could this help with that? The smaller hooks pour great. I have the same issue with 2 of my Hilts bullet head molds not making a good barb on them.

I suspect your fill-out problem probably would not be solved with this technique. But I think there is an easy solution.

I also like to use larger hooks in some of my molds. Two problems present themselves immediately when using a larger hook.

First, the larger hook draws more heat from the lead as it fills the collar/barb area. It's important that the larger-sized hooks be warmed up slightly (not hot, but warm) before being placed in the mold. Seems like a small thing, but it can make a big difference.

And second, the smaller hook has a small gap (tolerance) between it and the hook channel. That gap allows air to vent from the mold. 1/1000" is equal to fifty-thousand air molecules stacked side-by-side, so a tiny gap can be critical, and vent a lot more air than you might have imagined. When the larger hook is placed in the mold, you lose that very tiny vent (gap) and the lead cannot flow fully into the collar/barb area before it starts to freeze-up. That prevents the barb from filling-out.

The 1st remedy is to (if your mold design allows it) drill out the hook channel with a wire-guage drill 5 thousandths larger than the diam of your larger hook. This needs to be done carefully, but is fairly simple. If done properly, it should not preclude useing the smaller hook. I have done this to a few of my molds, and it works fine.

The 2nd remedy is to scratch a vent line on the surface of the mold (not in the cavity) from the tip of the barb to the nearest edge of the mold. That slight groove will vent the trapped air and allow the barb to fill before the lead freezes.

One of these solutions should make your molds pour great with the larger hooks. Smoking the collar area makes a noticeable difference too. Hope this helps, good luck!

[LedHed]

HJ

Thnx for the tip - something else to look into with problematic pours...

BLT

If the hooks fit and the mold closes flush you should be getting good pours. You might try HJ's suggestion or heat up your hooks before you pour.

[BLT]

I'll try heating up the hooks first, thanks for the replies.