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 Post subject: Fiberglass bridging problem with a vacuum
PostPosted: Mon May 19, 2014 4:24 am 
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Hi guys,

I've got a problem with a step down from a top layer to the board core. Basically the top wood layer is routed at 45 degrees and this creates the changes in levels and a bridging problem for fiberglass.

The fiberglass does not conform to this shape well, creating an air pockets or lumps. Not everywhere tho. I cannot identify why is better at some places and more lumps at others.

I've tried very neat vacuum bagging, tensioned perfectly, and a folded vac bag to work out the lumps:

A) better results (maybe due to vine, duno:):

Image

B) not so good results, introduced wrinkles as well (all that on top):

Image

With B) the idea was to use move the vac film and to work out the lines where top layer bridges with the core. See how is folded where it should bridge.
Blue fabric is a perforated film.

Pls note, I've got very good results with sanding a top layer to like 30 degrees or less. But I trust there must be the way to vac it as is - on 45 degrees! Because some parts are puuuurfect. But not all.

Suggestions? How do you do it? Where I'm not doing it right? I'll post my idea as well.

Thx

D.


Last edited by downunder on Mon May 19, 2014 5:03 am, edited 1 time in total.

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 Post subject: Re: Fiberglass bridging problem with a vacuum
PostPosted: Mon May 19, 2014 4:37 am 
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Vacuum does not distribute perfectly, neither does the stress in the cloth, because of the outline and rocker curves, cloth properties, and other things.
You may do better rounding over the edges of the beveled parts.
But you can also use 3M No. 77 spray to adhere the cloth to the core before wetting out, whether you do open wet layup, wet layup plus vac, or infusion.
If you tack it down that way, it will stay down. A light dusting of this sticky stuff works wonders.
It's not cheating, promise.
Even pros do it.


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 Post subject: Re: Fiberglass bridging problem with a vacuum
PostPosted: Mon May 19, 2014 4:45 am 
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Tapering at 45 deg with a straight angle will leave a stress riser and potentiall point of failure. Taper your shape off so its not so pronounced then you will get a more evenly distributed flex pattern and stronger smoother to ride board.


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 Post subject: Re: Fiberglass bridging problem with a vacuum
PostPosted: Mon May 19, 2014 5:50 am 
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BWD wrote:
Vacuum does not distribute perfectly, neither does the stress in the cloth, because of the outline and rocker curves, cloth properties, and other things.
You may do better rounding over the edges of the beveled parts.
But you can also use 3M No. 77 spray to adhere the cloth to the core before wetting out, whether you do open wet layup, wet layup plus vac, or infusion.
If you tack it down that way, it will stay down. A light dusting of this sticky stuff works wonders.
It's not cheating, promise.
Even pros do it.


Ok, thx. Not sure when to do dusting and with what?

Rounding is fine, but...Pls read below

plummet wrote:
Tapering at 45 deg with a straight angle will leave a stress riser and potentiall point of failure. Taper your shape off so its not so pronounced then you will get a more evenly distributed flex pattern and stronger smoother to ride board.


Yes, you're both right.

However, some lumps are present on tips as well where the top is tapered to less than 1mm. Tiny air pockets tho.

Also, as said, some areas are perfect. Imagine ONE piece of wood. That is how it looks like, the FG conformed extremely well.

So I am after WHY? Why is it perfect on some spots and not on others?

What I think is due to the perforated vac film and FG tension.

Imagine this: a perforated film (PF or whatever you're using as a first glue contact), is pulled on one side and even if worked out with a roller or hand, it will be still pulled if there are any level changes.

That is based on <1mm level change as well. Bigger level change - bigger pull.

And we are after the compression. Not pulling or tension. The above fabric is relatively irrelevant. Use a hand towel paper, it will break when pulled, not creating any tension on a FG, or a perf. film.

My reasoning is this:

Image

Now the perf. plactic won't be pulled across the BOARD END and tensioned at the bottom creating an air pocket on the above (top air pocket).
It might be pressed with the above fabric instead.

Would that work?

I wonder how would one create this with a pneumatic press tho? Impossible task without the mold.


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 Post subject: Re: Fiberglass bridging problem with a vacuum
PostPosted: Mon May 19, 2014 6:53 am 
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This is $1000 brand new Nobile board with a >5mm level change and perfect finish.

The level change is 90 degrees:) How do they do this???? With a top sheet?! The question is: is this board actually glassed on top? Coz glass is fully saturated, not visible at all. Infusion? omg...

Attachment:
DSC_0115 (Small).JPG
DSC_0115 (Small).JPG [ 60.56 KIB | Viewed 378 times ]


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 Post subject: Re: Fiberglass bridging problem with a vacuum
PostPosted: Mon May 19, 2014 7:54 am 
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Where you have indicated the bridging in your diagram is where you should put a pleat in the vacuum bag. The bag will not stretch to go into recessed corners. You need the pleat to run along the bottom of the chamfer so that there is extra material available for the bag to be pushed into the recess.

If you are putting the vacuum bag directly onto the perforated film, the two will seal allowing no path for the air to pass through and create the full vacuum. You need to have a layer of material between the two to allow the air to have a pathway to escape.


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 Post subject: Re: Fiberglass bridging problem with a vacuum
PostPosted: Mon May 19, 2014 8:40 am 
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ronnie wrote:
Where you have indicated the bridging in your diagram is where you should put a pleat in the vacuum bag. The bag will not stretch to go into recessed corners. You need the pleat to run along the bottom of the chamfer so that there is extra material available for the bag to be pushed into the recess.


Thanks mate. Sure, the pleat was applied as per the above photo, under B). All around it.

That is exactly where I think the problem is. The vac bag can conform, the perforated film (PF) can't.

I can assure you, the A) results are much better than B). The setup is the same except lighter FG (which should conform even better). But it didn't. The lumps are longer and wider than with A).


ronnie wrote:
If you are putting the vacuum bag directly onto the perforated film, the two will seal allowing no path for the air to pass through and create the full vacuum. You need to have a layer of material between the two to allow the air to have a pathway to escape.
[/quote]

As seen, many layers: PF, peel ply, breather (or bleeder) are there.

Now the question for you guys: how much PF excess do you have all around?

Did anyone succeeded with good results? Good meaning no voids, lumps, air pockets.

Here, this is the void (5mm wood on top):

Attachment:
DSC05697 (Medium).JPG
DSC05697 (Medium).JPG [ 103.1 KIB | Viewed 335 times ]


This is easy to fix tho.

A simple resin infusion is a go. Drilling a tiny hole on top and bottom of the void and attaching a tiny vac plug indicated a pressure change, meaning the void is open all the way through. Which is good.

But this is a fix. I am after the cause.


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 Post subject: Re: Fiberglass bridging problem with a vacuum
PostPosted: Mon May 19, 2014 2:25 pm 
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B looks like maybe there was less resin in the part or you didn't get the vacuum pulled properly. One possibility might be that there was loose breather cloth under the vacuum port and it got sucked up and blocked the port?

When I said a pleat in the vacuum bag along the bottom of the chamfer, I mean a pleat exactly at the bottom of the chamfer and running exactly along the whole chamfer.

The perforated film can as you say bridge at the bottom of the chamfer. If it were me, I would cut the perforated film along the bottom edge of the chamfer and make a slight overlap of the PF along that edge.

Are you trimming the edge of the part before curing? I'm just wondering why you have a vertical edge in the laminate?


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 Post subject: Re: Fiberglass bridging problem with a vacuum
PostPosted: Mon May 19, 2014 3:06 pm 
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Yes
I think we are very near to identify the root of the cause. Cheers for that

Indeed, it might be the best to cut the perforated plastic.

It is also true that too tight vac bag might not conform to elevations, hence a pleat was tried on the crossing from a top layer to the core (say at the narrow line on the diagram).

So, not on the chamfer.
Thx for the tip. I will also try a proper breather coz this one was quite thin.

I leave some excess around the rails.


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 Post subject: Re: Fiberglass bridging problem with a vacuum
PostPosted: Mon May 19, 2014 4:03 pm 
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I suspect the fiberglass cloth is more the culprit here than the film or bag over it.
After all, FG is stiff yarns snaking around each other, and doesn't really like to bend in a tight radius at a sharp corner. And it is bending at a radius, unless all the fibers are broken. So some bridging, be it at millimeter scale or smaller, is going to occur.
The problem is twofold, first, bridging creates a stress concentration, second, it is visible.
If there is enough resin in the groove, the resin will form a fillet under the glass, solving both problems.
Whether you are wetting glass on a separate table or on the board, you can do a lot to avoid this
problem by loading the groove of the chamfered deck layer with resin before laying the cloth over it.
Vacuum will draw the excess resin through the PF or peelply and the bridging problem should "disappear."

The 3M 77 if you aren't familiar with it is a type of sprayed contact cement.
Spray the core with it (very lightly, sort of like spraypaint overspray) right before you lay the glass on. It will hold the FG down in contact with the core and help some against bridging.
The 77 is just another trick, mainly useful if you are laying dry cloth onto the core. It keeps the cloth aligned properly by tacking it down, and you can press the cloth into any grooves or chamfers and it will stick until the resin takes hold. It is not visible or harmful in the finished piece.
With 77 you may still have the tiny amount of bridging due to the stiffness of the glass, so you will have to make sure enough resin gets pushed under/through the cloth to fill the space.

If you just round over all grooves to a bit more radius, the glass may lay right against the core.

But looking back at your pictures, the most important factor may be you vac setup.
It looks like you are pulling air down the length of the board to a single vac fitting, with just a patch of breather under it to form an air passage. This can make the glass creep and undo all the above steps to avoid bridging. It is advantageous in normal vac bagging to make the air travel a short distance, and do so symmetrically to avoid disturbing the laminate. More breather and a central vac fitting should suffice, as in the picture, at left:
Attachment:
bridging.png
bridging.png [ 26.8 KIB | Viewed 266 times ]

At right is what I suspect happened to your board.
If you are aiming for a perfect-out-of-the-bag finish and want to keep the fitting off the board surface so it doesn't leave a mark, you can make vac channels down the sides of the board with rolled breather or spiral tubing instead and keep the vac fitting(s) off the board.
Your work in general looks very neat, you will be able to produce great looking boards when you get this dialed in.

Edit: final thought and sorry for such long answers, if the pics above of the board under vacuum (with wine glass) are not right after starting, but a while after the bag has been on, it may just be that you are a little lean on resin and need to use just a bit more to fill these little voids. I'd expect to see the resin spreading over the PF a bit more than it is in the pictures. Trust the vacuum to pull out the excess.


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