for the professional casters - WTF is wrong with this mold??

OdysseySlipways · Post by **OdysseySlipways** » Fri Aug 12, 2011 3:14 pm

I have never had such a probelm with a mold not working as I'm currently having with the Akira's saucer.

i did this mold the same as i would have with any other saucer, vented all the highest spots, inject the resin from down below, and out of about 12+ castings, not 1 has come out right.

i took some pictures yesterday starting with this one. it shows the mold (circled) in the tank and you can clearly see the reservoirs still have resin in them

http://i262.photobucket.com/albums/ii11 ... 93edit.jpg

(not the clearest picture) this one is with the mold taken apart but with the casting and vent stubs (circled in red) all still in place. i'm still showing there was enough resin in the stubs to flow back into the mold (if need be) but you can just make out the air bubbles (circled in black).

http://i262.photobucket.com/albums/ii11 ... 95edit.jpg

now mind you, the mold is in the tank when i fill it with resin so there's no chance of the mold getting slightly squeezed and sucking in air

this one shows the mold back in the tank and filled with resin, you can see the resin has overflowed the reservoirs with no problems

http://i262.photobucket.com/albums/ii11 ... ure496.jpg

now before i fille dit with resin, i enlarged the reservoirs a bit more and opened where it vents figuring if i needed more resin, it would have it, and the larger vent lines would allow any air to come out during injecting

now this morning i started on this tank in hopes i'd have a good casting....

here again is the mold in the tank, you can clearly see the serervois are all full except for the one, so i'm figuring good, this one should come out pretty good if not perfect.......

http://i262.photobucket.com/albums/ii11 ... ure497.jpg

just another view of the top of the mold with the rubber bands removed to give a clearer view of the reserviors

http://i262.photobucket.com/albums/ii11 ... ure498.jpg

felling pretty good about this casting until.........

http://i262.photobucket.com/albums/ii11 ... ure499.jpg

WTF????

i'm going to add 2 more reserviors and have them tie in where there air bubbles are.

but for the life of me, i don't understand why i am having this problem? I see no ari bubbles when i'd topping off the mold with resin (injecting it), once this mold is filled, i go and inject the resin for the 2 engineering hulls for this kit (and they both turn out just fine every time), then i go back to this one and give it another shot of resin (no air going into the the fill line, i make sure of that), the resin overflows from the top very freely, then the tank is sealed and pressurized (between 35 to 50 psi, so air pressure isn't the issue).

there is no undercuts or deep voids that could capture an air bubble durring filling. the mold doesn't leak out any resin, so that's not an issue

so, any ideas?

i can't even consider making a new mold until i know what's wrong with this one

Ziz · Post by **Ziz** » Fri Aug 12, 2011 4:53 pm

I wouldn't call myself a "professional caster" this early in the game, but from my own experiences so far, it looks like your problem may be that the vents you have aren't big enough to allow air bubbles to break through the resin to get through the vents. I've found that anything smaller than about 1/8" diameter isn't enough.

I've taken to casting my parts on T sprues whenever possible. Make a T shape out of .188 x .125 strip and anywhere from .040 to .080 strip depending on the thickness of your part. Find an edge to attach it to and then glue the whole assembly to a piece of sheet that fits the shape of your mold box bottom. Pour your rubber. When it's set, break down the mold box and split the mold down the sides from the ends of the T just enough to separate the mold enough to get the part out. Then use whatever system you like to hold the mold together for casting. As I build my mold boxes out of Lego blocks, I just build Lego brackets to hold the mold closed.

When I pour, I use a popsicle stick to pry the mold open near the lateral center of the T sprue until the mold is almost full. In a perfect pour, the mold squeezing back together is the right combination of pressure and resin overflow to fill the mold to the top. If not, just dribble a little more in the T sprue slot until the mold is full.

OdysseySlipways · Post by **OdysseySlipways** » Fri Aug 12, 2011 7:27 pm

ah, the squish mold method, i've heard of these mystical creatures, thought they were long extinct if nothing more than sometime told in old fairy tales...

since i started casting and learned of the injection method, i rarely (if ever) have bad castings (other than the first time casting a new part). and while using the injection method, most of my vents sometimes look like paper cuts because the pressure of the incoming resin pushed the air out of the mold cavities.

the vents on this particular mold are about 1mm wide (a bit wider than normal) and i could even fill the mold with resin if i were to pour it in through a vent hole if wanted to but i'm still having the issue of the big air bubbles.

i still don't have a clue where this trapped air is coming from or why i can't get rid of it

i'd try a stick, but i don't want the mold to misalign, but ti does give me an idea to try to cover 2 of the vents with an older mold to block them and then give it another shot to see if it'll force any trapped air out .....

(Lego's are GOLDEN)

DasPhule · Post by **DasPhule** » Fri Aug 12, 2011 9:50 pm

Looks to me like the mold is leaking.

Erin
<*>

kenlilly106 · Post by **kenlilly106** » Fri Aug 12, 2011 10:39 pm

It looks like to me that the mold is full of resin prior to it being placed under pressure, as the air bubbles collapse the resin volume decreases and slowly drops, there doesn't appear to be enough resin in the reservoirs to fill this space before the resin starts to set.

The reason I'm thinking its a volume reduction problem is that the air bubbles have a thin shell of resin like they were slush cast, that's leading me to think the void was completely full of resin before the level dropped.

As far as solutions, the only one I can think of this late at night is to increase the reservoir volume and height so the head pressure will keep the mold full when the bubbles get crushed.

Ken

OdysseySlipways · Post by **OdysseySlipways** » Fri Aug 12, 2011 11:41 pm

DasPhule, sorry, no leaks in the mold, was one of the first things i checked

kenlilly106, i thought about that to, so i cast the saucer by itself and it still came out the same way.

i have even been leaving the mold in the tank when i'm injecting the resin just to make sure i'm not making it gulp air in any way when i move it to the tank

the resin is still very fresh (?) when i put the lid on to pressurize it (i even have enough time to do more molds in that tank if i wanted to).

i did increase the reservoir just before this last casting, but it just doesn't seem to flow back in (i even increased the vent itself so it would do so freely).

I was thinking if i were to do another mold, it'd make it another inch taller to give the reservoir a bit more gravity.

it just kills me i had the same mold the last time i was making this kit and didn't have this problem (haven't seen the old mold, thinking it's been trashed for a while now or i would have looked at it)

kenlilly106 · Post by **kenlilly106** » Sat Aug 13, 2011 10:09 am

Is the mold in the pics the same design as your old mold that worked?

Have you changed resin suppliers, type, etc?

You mentioned casting the saucer by itself, from the pics it looks like that's all you're casting, is there more to the mold than shown?

Ken

modelnutz · Post by **modelnutz** » Sat Aug 13, 2011 10:18 am

If there's no mold release on the top surface of the mold ( or, if you can slice off a bit or rubber to make a clean fresh surface ) you can easly add to the top of the mold...Silicone sticks to it's self quite well. That way you can increase the area of the reservoiers.

Another trick is to fill the mold...put it under pressure for a few seconds...release the pressure, open the pot and check the level of resin...add more if you see the reservoirs are low then re-pressurize.

Have you tried rapping on the side of the mold as you're filling it ?
I've had molds that wouldn't fill completly unless they were agitated ( vibrated )...of course, they were more complex figure molds with a ton of small areas to fill ( ie..fingers,etc. )

OdysseySlipways · Post by **OdysseySlipways** » Sat Aug 13, 2011 12:38 pm

kenlilly106 - no it was the only one being done at that time (i just don't have room for all th emolds i currently use, so some stay in the tanks).

it's the same resin i've been using for a while now, Smooth Cast 321

and yes, this is the same way i had the mold the last time

modelnutz - adding rubber to the top of the current mold is an idea

never thought about the quick check after pressurising the tank, i'll try to keep that in mind for this afternoon's run

as to rapping on it, i'll try some Eminem (sp?), but i'll try that too. this morning after i filled it, i tool another mold, sat it on top of all the vent lines but 1 and gave it another shot just to see if any air might burp out, but got nothing, so i guess i have to wait until after 3 to find out (when i start my next run)

kenlilly106 · Post by **kenlilly106** » Sat Aug 13, 2011 4:57 pm

Ok, with the mold design and resin staying the same as before then what other variables or changes are there since last time?

Have you changed the way you mix the resin?

I still think that there are voids in the mold or air bubbles in the resin that are collapsing under pressure, causing the void space.

The only way I can think of trying to fix the problem is to increase the reservoir space on top so there will always be an excess of resin up there.

Ken

Joseph Osborn · Post by **Joseph Osborn** » Sat Aug 13, 2011 5:37 pm

I tend to agree with the theory of air in the resin causing more apparent volume than is really there, and then being pressurized with not enough of a reservoir to ensure the mold is filled. Is your batch of resin getting a little old? Does it foam up if you were to try to cure it without pressure?

modelnutz · Post by **modelnutz** » Sat Aug 13, 2011 6:02 pm

rapping ( no, not Rapping ) on the mold when filling should aleviate any air entrapment

Chris, one other note...when using rubber bands to hold your mold closed...be careful that you aren't over-tightening the mold...that can compress the rubber and pinch off air vents.

OdysseySlipways · Post by **OdysseySlipways** » Sat Aug 13, 2011 8:30 pm

kenlilly106 - as to any changes from the last time, i can't think of any. if i had this much trouble the last time, i'm sure i would have remembered this time around.

still mixing resin the same why i always have from the begining

as to air bubbles in the resin, if they are in there, they are small enough to escape my eye, and even if they where than small, the pressurizing of the tank would crush them

as to increasing the reservoir space, if the casting doesn't turn out when i go down there in just a bit, i'm going to add one more reservoir

Joseph Osborn - the resin is brand new, got it about a week ago (i've used resin that is up to 4+ months old where i have needed to filter it through cheese cloth to remove the crystalized crap and have it still work fine).

it doesn't foam up when curing without ressure, but it will have bubbles randomly through out it. I know resin doesn't like moisture and could foam a bit if water is mixed it. i haven't had an issue with either (not like i tried to find out) unless i'm too tired and forget to pressurize a tank.....

modelnutz - i know about over doing it with rubber bands, but have never managed to pinch off a vent yet

kenlilly106 · Post by **kenlilly106** » Sat Aug 13, 2011 8:57 pm

OdysseySlipways wrote: as to air bubbles in the resin, if they are in there, they are small enough to escape my eye, and even if they where than small, the pressurizing of the tank would crush them

I'm thinking of bubbles more along the lines of what you see in a meringue (for example), its technically a foam but you can't see the bubbles with the naked eye.

So while each bubble is small you get a few thousand (or hundreds of thousands) together and they start adding up to enough volume to make a void in the top of the mold when they all get crushed by the pressure in the casting pot.

Ken

Jeff Herne · Post by **Jeff Herne** » Tue Aug 16, 2011 9:59 am

In addition to what the others have said...

I have an auto buffer (random orbital type), clamped upside down in my vise on the bench.

After I pour, but before I pressurize, I sit the mold on the buffer and let it shake for 20-30 seconds.

Obviously, you need to hold the mold box otherwise you get the pulling-the-mixer-from-the-batter mess, but 95% of the time its enough to dislodge any trapped air.

J

EDIT*** After looking at your photos, I'd open up the sprue gates. What type of resin are you using? Smaller volumes of resin (like pour gates) will kick before the larger volumes do...maybe your channels are kicking before the reservoir has a chance to flow. Also...sometimes an escaping air bubble will block the channel, too, preventing the resin from flowing back in. And lastly, what pressure are you using?

OdysseySlipways · Post by **OdysseySlipways** » Tue Aug 16, 2011 10:49 am

(posted this over at Starship combat late last night)

and now as to the (current) culprit causing this problem........

first, i want to start back to yesterday morining when i went and cast these parts. with the mold in the tank, i filled it, filled the 2 other parts, then took another one of my smaller molds, covered 2 of the vent lines and filed the mold some more - to see if it would burp out any air bubbles ..... nothing.

so on the lid went and then pressurized.......

later i took out the mold and saw allthe reservoirs had dropped a little bit, and keeping my toes crossed (i needed my fingers to undo the mold), i opened the mold to find a perfect saucer!

seeing that it came out good, i tired it again the same way

later that night, i went and opened the mold again to find another bad casting

...... wtf???

i looked over the mold again seeing where air could be getting trapped even if unlikely and i ended up looking at the bottom of the saucer, near the rear.

if you look at the very last picture i posted in the beginning of this thread, you'll see the air bubble in the middle. now there is a vent in the middle, but it touches the rear of the ship, so i decided to nip it just a bit lower in case the problem is where that middle air bubble is.

so then i thought of what someone on another site said to try and just tap the mold on the side after i fill it to see if an air bubble becomes dislodged - so i did, and closed it up for the night.

the castings since then have come out just fine........... so far.

i'm thinking maybe my old mold may have stood slightly tilted to one side allowing the air to move up, out of the vents with no problems

8-16-11 - i just came up out of the basement and got nother, good, full casting!

Jeff Herne · Post by **Jeff Herne** » Tue Aug 16, 2011 10:56 am

Problem solved! Way to go...

I had a similar experience years ago with stacking molds into the pressure pot... But the way I stacked the molds caused a rubber band (I used the wide ones) to cover up a vent on the mold underneath. Air had no where to go and it caused 2-3 bad castings before I figured it out.

J