preventing voids in closed mold layup

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#1

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.

#2

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

#3

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.

#4

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.

#5

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.

#6

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.

#7

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 ?

#8

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.

#9

http://www.compositescentral.com/showthread.php?t=7007&highlight=wet+bladder

this thread is a good place to look for tips and good videos

#10

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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#11

Making a plug makes good sense, one of the complexities of this part is that both halves are layed up together, when cured the bladder must be removed, which would make removing a plug impossible. It has been fairly difficult to remove the bladder without harming it, I fused a “rip cord” to the nose of the bladder so it can be pulled from the inside out when the part is cured, this method and vacuum seems to be fairly consistent in getting the bladder out in one piece.

I greatly increased the clamping force and bolt locations, I was able to apply far more pressure on the bladder than ever before, and the parting line is mich more consistent and much thinner, all good. I also attempted to place the entire mold in a vacuum for 15 minutes after layup, while applying 30 psi to the bladder. I will post pics of the part once it is removed. I appreciate all the input!

#12

just as a note, stretchlon isn’t really self releasing which could explain some of your problems with removing your blader

#13

Have you tried applying vacuum to the bladder to help remove it? This way it will collapse, and should make it easier to remove

#14

Yes, that is currently how I remove the bladder, the cord also gives a way to help remove it by pulling it from the inside, which effectively flips the bag inside out when removed. I performed a few more runs to see where the issues are, here are some pics. There are a few problem areas that are consistently in the same location, indicating that the bladder is either not shaped correctly, or does not have enough pressure to push excess resin out of these areas. It is entirely possible that the resin is getting trapped in these areas and needs a way to evacuate. The red carbon stock was bagged for 10 minutes to evac air from the mold, then the bladder was inflated to 25 psi. The black carbon stock was placed under continuous vacuum and the bladder was allowed to inflate under vacuum, so approximately 14 psi from what I understand. Maybe a combination of vacuum for the entire cure cycle and applying additional pressure on the bladder will improve these areas?

#15

Have you considered making a silicone bladder? It seems that your current bladder (I believe its strechlon bag) is not inflating and applying pressure where you need it, the tight corners. Making a silicone one would have a near net shape of the ID and once pressure is increased should get all the tough corners that youre having issues with.

#16

Hojo,
Are you using silicone bladders or reusable bags? We are using silicone for reusable bags and they don’t seem to hold up as well as “promised”. I’ve been spending a ridiculous amount of money replacing bags on an almost daily basis. We are an industrial/production type operation and, unfortunately for me, my predecessor got us going down this path and I’m trying to find a way out while still maintaining volume.

I’d like to get back to LRTM but have some hurdles (product design changes, etc) to clear on that end. For now, I’m stuck with silicone so I’m looking for any suggestions that will help. Got any tips on prolonging the bag life? We’re using a platinum cure bag with a polyester resin.

I know this is a slight derail but it may help the OP if he goes silicone.

#17

I have considered it, and I think it is the next logical step. Since the issue is repeatable in the same locations, it is safe to say that the current bladder is not doing the job. The bladder isn’t 100% “anatomically correct”, and needs to be stretched to get into these tight problem areas, where a bladder that is formed or cast to the shape of the mold would work much better. Furthermore, there seems to be a build up of trapped resin in these areas, which compounds the problem, pressure and more direct contact of the bladder will hopefully solve it.

Any recommendations for materials for a silicone bladder? Should I make a cast of the inside of the shell, or do you need to account for the thickness of the material?

#18

There are some paintable latex rubbers that could potentially be built up in thickness. It would need some testing to see how much pressure they can withstand with certain thickness.

Also, since you wet lay the part there might well be these resin rich areas and if they stay like that with no escape path for the excess resin then the outcome likely won’t improve in these areas.

OOA prepregs might seem like a less headache prone way to go?

#19

sorry ,but i highly doubt a silicone bladder is going to solve the problem , you have a trapped zone. For one, you need to work the carbon more to get it into the corners better, that is quite a big void.

i would highly recommend trying an intensifier before you change bladder setups. use a solid intensifier just to try it out . make up a mix of cotton flox and micro balloons , and stick it in a small plastic bag. push this bag into the area that is causing the problems and allow to cure. use this intensifier to force the carbon into the corner , the bag will press against it and i can assure you will have a much better part

you havnt mentioned how you making your bladder ? do you apply wax to the bladder before use ? i find that not only does it allow for easier removal , but i also allows the bladder to slip into position.

#20

The intensifiers are a great idea. I made a few using body filler, just to try, I’ll bag them and wax the intensifiers and the bladder. I also put a few resin flow channels in these areas to help evac the excess resin and trapped air. The bladders seem to be coming out in one piece, using the vacuum and pull string method. Hopefully there won’t be any issues with the intensifiers getting stuck in the mold. Results will follow!