What am I doing wrong here?

[garehan]

Guys, I have been fiddling around with composites for a while and I keep running into the same problems, despite changes in technique.

Let me explain what I'm doing right now. I'm laying 3 layers 6oz/yd^2 twill weave carbon fiber into a silicone mold that's about 6"x20" and 1/4" deep. It is a female mold. There is about a 1/8" radius at the edges of this "box." I trim the carbon fiber so that it is just the right size to fit into the box, the fabric is less than 1/8" larger than what the finished part will be. I am not using a gel coat, as I've found that when gel coat comes into contact with the silicone mold, it will never dry, even when removed from the mold. After I lay a layer of the carbon fiber, I wet it out with epoxy and a foam brush. Once all three layers are applied and a generous final layer of epoxy is added. Then I apply an over-sized piece of perforated peel-ply, followed by an over-sized piece of felt. I place the mold, carbon, etc into a stretchlon vacuum bag. I then run my vacuum hose into the bag and bridge the gap between the hose and the bleeder felt with a piece of resin infusion matrix. Finally, I seal the bag and pull approximately 12psi of vacuum (or 25 inHG as per the gauge I've placed on the pump I built). The epoxy I am using is listed on fiberglast's site as "20 minute cure time," but I suspect that that is based on autoclave curing, because I notice about a 2hr cure time (and yes, I'm sure my mixture ratio is correct). I wait about 24 hours and remove the part from the bag.

Now the problem...

At the edges of my mold, where I mentioned the 1/8" radius, I often get voids where the carbon fiber has not laid flat against the mold. I got rid of most of this problem by cutting the fabric to nearly its finished size, before laying up the part, but I till see the voids in several places, just a bit smaller. I also occasionally get voids on the flat surface of the mold. I have tried using excess resin, I have tried massaging the wet layup thoroughly before vacuum bagging, I have tried massaging the layup while under vacuum. None of these methods have gotten rid of the voids. Below are some pictures of the voids on my most recent piece.

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http://i45.photobucket.com/albums/f59/thegarehanman/100_0779.jpg

http://i45.photobucket.com/albums/f59/thegarehanman/100_0780.jpg

http://i45.photobucket.com/albums/f59/thegarehanman/100_0781.jpg

http://i45.photobucket.com/albums/f59/thegarehanman/100_0783.jpg

Does anyone have any ideas about what I could do to avoid these flaws? I tried resin infusion, but that got rid of the big voids at the cost of many many small ones throughout the part. I was thinking perhaps a wet layup followed by perforated peel-ply and resin infusion matrix(instead of bleeder felt) might help get rid of the air bubbles, but that's just a hunch.

I'd love to hear the pro's opinions.


thanks for your help,
russ

 

[PanelGuy]

Depending on what you have tried...

My guess would be that since carbon is difficult to wet out it is happening over time. As well, depending on how mach resin is in your breather at the end, you may be drawing too much out of the system and in to your breather/bleeder.

First I would questions the need for the bag. Depending on the desired resin content...it may not be needed. You may be able to get by with using"just the right amount" or a little more of the epoxy and make sure you use it all.

Regardless...I would wet out the Carbon on a sheet of cardboard...brush some epoxy on the sheet lay all three ply side by side, spread more on each, work it in, fingers are some times better than brushes...Consider wet out in a tape laying process and that the epoxy is driven in to the plies with rollers. If you let things sit, flip and rub it helps let things settle in. The desireable quality of the stiffness of the fiber makes it tough to get things in the spaces between fibers. You may also want to try reducing the viscosity of your resin to get it in the lay up better.

Now lay each layer in and rub or roller well between each to ensure good compactin and air release. This may do it, pretty common in open molding.

If you want to go down the bagging route there are many things to consider in cluding a lighter breather/bleeder or more ports. Personally I have had more success in a project like this with a nylon peel ply and a reuseable breather. I make these by putting the thick bat beather in a bag with perforated film on one side and unperforated on the other. you really want the breather to allow the vacuum to evenly distribute in the bag, not soak up adhesive. This way you use far less adhesive.

For the size of you lay up just peel ply may be sufficient. The other VERY useful thing, if you enjot the very unique cosmetic quality, is the use peel ply with bubble wrap. If you go bubbles down you can remove a ton of resin, bubbles up maintains your resin content. Either way the pattern is cool.

The last item is concerns the seal. Assuming your bag is sealed and intact, is your mold? The third picture almost looks like things were fine and a micro pore in your tool opened and allow a bit of air in. that is also typical sometimes of small leaks on edges, it does not take much. You could seal a bag on and submerge the tool in water and see if you draw in? You definitely should be able to seal the tool and draw a full vacuum on a breather from end to end and have all gauging read the same. My first thought would be to cut some RTV with about 30% toluene to make a thin "laquer" and put a coat or two on the outside of the tool. It does get little bubbles but with the mold and two coats the likelihood of the connecting is small. A leaks in the mold would show easy if you found your defects are all in the same spot.

Sorry, ran longer than I meant. Let us know if you get success.

 

[garehan]

Thanks for the reply.

I can't let the lay up cure in open air, as without the vacuum, it will not completely conform to the mold. I haven't tried it, but have noticed that very thing happening during the lay up process.

As for holes in the tool, that is not possible. I'm using a two sides bag, that is not attached to the tool. Instead it completely surrounds the tool. Air is definitely getting into my vacuum system, but in very very small quantities. I have about a 4 gallon reservoir on my pump, which kicks on when the vacuum drops about 5 inHG. The system will only kick on every hour and a half or so.

Just for arguments sake, I will check my parts to see if the problematic locations are consistent. I think they will be somewhat consistent on the edges, simply because the tighter corners tend to give me the biggest problems as far as conforming the material to the mold.

Thanks for you help. I'll get back to you once I've tried again.

peace,
russ

 

[PanelGuy]

It may be beneficial to try just running a band of breather around the perimeter to get pressure in the corners better. the part is small enough that you may not need it in the middle of the part, just use a sgueegee on the bag to work out bubbles, kind of like resin transfer, only "transfer" the epoxy more directly.

Soetimes a light brushing of the epoxy into the mold first can get the surface wet-out.

6 oz carbon is always a pain to get to wet out. Lots of what i have done is cost both sides and work it in either by hand, with a roller or squeege and then place in the mold. It does yield a less porous part. The is important if you do not bag it, you will be resin rich but have a more cosmetic part. As well perhaps less viscosity OR a wetting agent/silane in your a side may help a little. Check with your fabric guy to see if the epoxy is compatible?

You may also wish to review what adhesive you are using as well as environment. If you hagh a high dewpoint situation you may get moisture in things which cause stickness and off gassing.

 

[garehan]

I've been brushing epoxy into the mold before laying these parts (laid 3 now). The issue with that is that the epoxy does not adhere to the silicone mold at all, and actually puddles up, rather than spreading out. I really like using the silicone though, as demolding the part is ridiculously easy. I hope I don't have to give up the silicone. Although at the same time, I wonder if it's the silicone that's off gassing. I'm lead to believe it's not, or if it is, it's minimal, because the voids at the sharp corners are loads more common for me than that void I showed in the middle of the layup.

I'm wondering, what if I were to vacuum bag to only bag the very first layer, let that cure, then lay and bag the subsequent layers all together. This would leave the first layer to be more easily manipulated by the force of the vacuum bag.

Thoughts?

peace,
russ

 

[Compositepro]

Your photos show air bubbles in pools of resin on the part's tool surface. You are adding excess resin and not removing it suficiently with your bleeding process.

You must understand that a vacuum bag is not a vacuum chamber. There is no change in the pressure on your part between being bagged and not bagged. The bag only changes the way that forces are applied to certain areas. for example if you vacuum-bag an inflated birthday balloon it has no effect on the balloon.

Presaturate your fabrics on a sheet of plastic film. Grooved rollers often work better than bristle brushes and I don't think foam brushes would work well at all. The purpose of these tools is mainly to push fibers down into the resin and to the tool surface. To do this the resin must be able to flow up into the grooves or between bristles. Then you have to be able to lift the tool without sucking the ply up with it. After preliminary saturation you can place a second film on top and roll the air out. This will let you see areas that don't have enough resin and you can get the resin content fairly uniform.

Saturation of fabrics takes time and that depends on resin viscosity. Capillary action is the main mechanism for resin getting into fiber bundles. If you saturate between clear film you will see bubbles form under the film. This is due to resin wicking into the tows and displacing air to the weave opennings in the fabric.

If you are only applying 20 in.Hg vacuum you cannot create voids by boiling the volatiles in your resin. You must have enough bleeder to soak up all the excess resin without becoming saturated at any point.

Composite lay-up requires a lot of skill that takes a long time to acquire. You would probably be able to figure things out faster if you were working with fiberglass on clear glass tooling. You will be able to see how bubbles move.

Remember that pressure is force per unit area and that pressure is always the same on both sides of a vacuum-bag film. It is atmospheric pressure on the outside. On the inside it is compaction pressure on your breather fibers plus the air pressure between the fiber (your vacuum is merely lower pressure air). The pressure on your impregnated laminate consists of two components. It is the sum of the compaction presures on your fibers and the hydrostatic resin pressure. These add up to atmospheric pressure.

Without the vacumm bag you cannot compact fibers because the air would simply flow between them. Resin and air must be able to flow toward the vacuum source for fibers to compact.

You are making a very thin laminate. This calls for a closer perforation spacing than thicker laminates.

 

[garehan]

Thanks composite pro. From what you're saying, it seems maybe I should go the route I was thinking of going earlier and hook up my resin trap between the pump and bag, and use infusion matrix rather than bleeder felt and porous Teflon rather than perforated peel-ply, so that the resin gets removed from the bag and doesn't block the flow of air from any parts of the bag. I'll also wet out the material before laying it in the mold, like you and panel guy have recommended.

Also, do you think think increasing the cure time would help? I figure with the infusion matrix and porous Teflon, the added time would be added insurance that any air in the laminate would be removed from the pressure.

thanks again,
russ

 

[PanelGuy]

When the material hits its gel time you are done removing air....cure time comes after. As far as changing the cure time it really is dependent on your adhesive system. If ankything you may be better served with a littel lower viscosity or higher energy (added suracing agent) to saturate the weave.

Like pro said, rollers or squeegee to saturate, you need to work the resin in. I prefer fingers, but I am pretty old school on lay-ups like this, you can actually feel the difference because when the fiber bundles are saturated the resin is lubricating and things feel softer rather than the stiffness you normally get with carbon.

 

[berkshire]

To add to what Panel Guy and Composite pro have said, make sure your first layer is completely wetted out before adding the next layer, keep the resin down to the minimum needed to wet the fabric. Too much resin encourages the fabric to float on a sea of resin, which if it contains a bubble, makes it harder for your bagging system to remove since that much more material has to flow through your bleeder which can only hold so much.
Also when starting the vacuum on your bag, make sure there is enough slack in the the bag material to avoid bridging the corners, this is another cause of voids.
B.E.

 

[garehan]

Ok, sounds good. I did notice the "bridging of the corners" that you mentioned in my first bag and have avoided it since.

 

[stimpee]

I am going to go along with the bridging comment, but not limit it to bridging of the bag.

I have seen many many parts come out with voids in inside corners due to bridging of the PEEL PLY on the part as it compacts into the mold under vacuum. The vacuum bag will typically stretch a bit, but the peel ply will not. This presumes that your plies are well seated into the corners/radii however.

I would recommend that you cut your peel ply into several pieces. In this case, I would cut a piece that fits into the main surface of the part, and then cut 4 pieces for the 4 sides. Let the side pieces overlap well onto the main surface section.

Then, when the vacuum bag pulls down, the peel ply will slide against itself allowing the bag to stretch into or fill into the corners.

Steve

 

[kognition]

It kind of looks like your fabric is not coming into contact with the mold in the corners. You def need to press those corners in before infusion. Squeegee the excess resin out of the corners, because it will puddle up in 90 degree corners if you use stretchlon. I also think that the three layers are creating resistance in the corners. You can also create a male
pattern to press into the female portion of ther mold to help get the fabric into the tight radius corners. Have you considered using one layer of twill and 7781 laminating glass for the backing? You may have to press a male blank into the mold while under vacuum until cure just to keep the pooling from happening.

 

[cpinz]

You could also use some filler along the edge, mixing resin and microfibres/microballons. In this way you obtain a tough material and avoid to accumulate resin in excess in areas where cracks will form surely.
It will help also during the vacuum bagging since it will be a base where the bag will adhere and contour the mold.

Cpinz

 

[sailorguy]

You are leaving too much resin in the mold and you aren't holding the fabric against the mold, especially in the corners. You need to treat this as a hand lay up in which, HOLY SUCKING PLASTIC, BATMAN! you have a vacuum bag to hold those fibers against the mold after you push them against the mold. Obviously the vacuum bag can't reliable press the fibers in place by itself. You have to do that. In other words, the vacuum bag can't make up for poor hand layup techniques.

Are there voids in your mold? Was the silicone thoroughly degassed before it was cured? If not, you may be sucking air out of voids in the mold.

Don't wet out in the mold. Wet out on cardboard, formica, polyethylene, steel or something else that will allow you to wet out your material, then lift it off your substrate and leave excess resin behind. If you lift off the material and it's dry underneath, your material isn't wet. (You may need to add a veil layer to control the thickness of the resin at the finished surface. Otherwise you will probably get a lot of pinholes.)

Forget paintbrushes. They are expensive and ineffective. I wouldn't use a roller either. Use a squeegee. You will get better, more even distribution of the resin and better wet out.

Wet out one layer at a time, then stack the pieces in the mold separately, or at least fold back the edges all around and lay them in the corners separately. Layers of fabric won't slide against each other very well even wetted out. Since they won't slide, they act as a group and are much less flexible than the pieces are separately. You might add a bit of excess resin in the corners as you lay the last layers of fabric in the corners of the mold (see below).

Try laying the fabric in on the bias if you can tolerate that. The fiber will be bending around a radius which is slightly larger.

You need to find or make a tool to press the fiber into the corners. A fender washer set in a groove cut lengthwise in a dowel, and spinning on an axle (e.g., bolt) will allow you to push the fabric into the corner. It probably won't stay, but you can do it again after you put your peel-ply and absorbent fabric in.

For your peel-ply, use a nylon fabric that is porous over the entire surface.

Use strips of peel-ply and absorber at the corners to allow the peel-ply and absorber to slide and adjust themselves if they want to. This will reduce the chance of bridging, which is one fundamental problem.

The other solution to the bridging is this: When your bag is loaded, use your fender washer tool to press the bag, absorber, peel-ply, and fabric into the corners of the mold. Pull your vacuum down slowly and keep working the tool around the edges of the mold. Don't press hard. It shouldn't take much to get bag and fabrics against the mold. You are trying to guide them, not nail them down.

If you have added excess resin at the edges AND you are working your fabrics and vacuum bag into the corner of the mold, you should see the excess resin show up in the absorber. Hence the slight excess resin mentioned above. The bag, peel ply, and absorber need to be tight and slightly lumpy right into the corners (my absorber is slightly lumpy when sucked down). If it isn't, release the vacuum and once again roll the bag and fabrics down with your tool while you pull down the vacuum.