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Old 10-10-2015, 09:39 AM   #1
Compositeman

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Default preventing voids in closed mold layup

I have been working on some various mold designs, and trying different methods of layup using a closed mold with a bladder. One issue that I frequently run into is voids in the mold, where the material (fiberglass or carbon) doesn't make contact with the mold surface leaving a void and a part that is unusable, or one that needs a great deal of finish work to be salvaged. I am currently laying up the part wet, both sides have material pre-cut, one side flush with the mold edges and the other overlapping, the bladder is then inserted in one side and the mold is bolted closed and the bladder inflated. The bladder is ran to around 15 psi, material is SL200 (stretchelon) and I can't seem to get the SL200 to go much higher pressure than that before it fails. I have tried SL800, but it doesn't seem to be flexible enough to get into the recesses of the mold interior. I am considering attempting to vacuum bag the mold to pull the air out of the mold with the bladder inflated to see if this can be fixed, there is about a mil gap between the mold faces for air and resin to evacuate.

Do I need a better bladder system where I can increase the pressure? Would bagging the mold and allowing vacuum to inflate the bladder (vs pressure) be best? I attempted a carbon layup without any tint or without pre gellcoating the mold to see where the issue lies, the part is painted so if carbon is used, it doesn't need to be cosmetically perfect right out of the mold. Below is a view of the molds and the part, you can see where the excess resin was trapped and where voids between the mold and material is.


Last edited by Compositeman; 10-10-2015 at 09:47 AM.
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Old 10-10-2015, 11:23 AM   #2
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how are your molds bolted together ? are they backfilled ? is there any flex whatsoever when the mold is inflated ?
i have found that if there is even the smallest section where you dont have overlap of the fabric, the bladder will pop there , you must ensure the bladder sits entirely on the fabric and doesnt touch the mold anywhere along the parting line

remember F=PxA , so there is ALOT of force even only at 1 bar ... vacuuming the entire setup will do the same as pumping to 30 psi , your bladder will fail (if not supported properly)

if you have area's that are problematic, like where the tight corners which fill with resin , the you need to either use an intensifier , or just use a bit of black sploog in that area before you layup , going with higher pressure wont push that out ... when its white it attracts your attention , when its black you dont notice it

i pump my moulds to 45 psi , and yes it does make a nicer part, but it doesnt eliminate problem areas
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Old 10-10-2015, 01:45 PM   #3
sammymatik

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If your'e only putting in 15 psi, then why not just use vacuum instead? With 29" you'd get 14.5 psi, which is nearly the same. And you might not have so many issues since you can bag around the out side of the mold which would apply equal pressure and clamp it shut.

I think that's what you were planning to do according to your post?

When we do bladder molds, we use pretty thick aluminum molds and then pump to 150psi... parts come out beautiful class A with no post processing needed.

Last edited by sammymatik; 10-10-2015 at 01:46 PM. Reason: added a sentence
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Old 10-10-2015, 07:15 PM   #4
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Quote:
Originally Posted by Blackrat View Post
how are your molds bolted together ? are they backfilled ? is there any flex whatsoever when the mold is inflated ?
I have large T nuts epoxied into one side of the mold, through bolts are spaced every 8 inches. The back of the mold is reinforced with 18lb foam, so there is a little flex if the pressure is too high. I also intend on installing additional T nuts spaced closer together for more clamping force.

Last edited by Compositeman; 10-10-2015 at 07:54 PM.
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Old 10-10-2015, 07:21 PM   #5
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Quote:
Originally Posted by sammymatik View Post
If your'e only putting in 15 psi, then why not just use vacuum instead? With 29" you'd get 14.5 psi, which is nearly the same. And you might not have so many issues since you can bag around the out side of the mold which would apply equal pressure and clamp it shut.

I think that's what you were planning to do according to your post?

When we do bladder molds, we use pretty thick aluminum molds and then pump to 150psi... parts come out beautiful class A with no post processing needed.
I was planning on allowing the bladder opening extend outside of the bag and not inflate as you mentioned, let vacuum do the job. There are two issues with production molds out of aluminum at this point; (1) cost is too high, at a later date it will be something I will try (2) the mold has a fairly aggressive texture in a few areas, I have been told it cannot be machined in, but could be penned or punched to transpose onto the part. I suppose we could use a CNC aluminum mold with soft inserts where the texture needs to be? I have kicked around the idea of pouring molds out of a epoxy corian mix, which would be a lot more rigid than the glass molds but still allow the texture to be cast in.

Last edited by Compositeman; 10-10-2015 at 07:57 PM.
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Old 10-11-2015, 10:32 AM   #6
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I could see a texture giving problems, like knurling that some rifles have on handles. I guess that sort of thing could be machined, but would probably be extremely expensive. I like your idea of the insert. Might take a little tinkering around with but sounds like it'd be a good solution. Then you could 3d print the insert and then use the printed piece or cast it in silicone.

And yes, the alulminum molds are expensive but I guess you've costed them out at whatever volume you expect to make. It's a bit of a trade off on price but you get great durability, long life, and excellent finished parts. I"m in the same boat though, I"d only have an aluminum mold made if I was absolutely to the final production run.

Well it's easy enough to just vacuum bag a part and try it. I think the results would be similar. Maybe the stretchalon would work better in getting into every crevice with vacuum as well? Or you can switch to a non stretch bag as long as you have enough excess bag to get into the nooks and crannies.

Sounds like you have some good ideas about how to solve the problem.
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Old 10-11-2015, 11:34 AM   #7
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there is no ways a bolt every 8 inches is gonna keep a glass mold together , you can add two more bolts between those

the mold is opening and thats why your bladder is popping

is the butt of the stock cut open so you can get inside ?
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Old 10-11-2015, 02:12 PM   #8
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Quote:
Originally Posted by Blackrat View Post
there is no ways a bolt every 8 inches is gonna keep a glass mold together , you can add two more bolts between those

the mold is opening and thats why your bladder is popping

is the butt of the stock cut open so you can get inside ?
That makes sense. When I close the mold and bolt it down with the current amount of bolt locations, there is a mil or less of gap, but when the part comes out there are areas where the flashing is significantly thicker indicating the mold is flexing. The mold is open in the back, I use a urethane plug in the rear to seal the bladder, or the bladder can extend out and be sealed to be exposed outside of the vacuum bag.
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Old 10-13-2015, 04:11 AM   #9
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http://www.compositescentral.com/sho...ht=wet+bladder

this thread is a good place to look for tips and good videos
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Old 10-13-2015, 12:19 PM   #10
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Default preventing voids in closed mold layup

Use a foam core shaped to intensify pressure in your trouble areas an simplify the shape the bladder has to conform to. One method is to pour a urethane foam cast inside your mold once cured cut the needed area from the cast give it a quick sand to prep the surface for bonding lay up your part insert foam components and bladder close mold inflate bladder. Now your bladder has to conform to a more generic shape and you should be able to exert more force on your part and intensify the pressure on the laminate in your troubled areas.


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