Vacuum infusion vs hand lay up problem

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#1

Hi to all,

Two days before i infuse a 10m hull. It’s my first time which use infusion for such big scale project.

I haven’t any problems during the infusion but today when i put out the vacuum bag i noticed that the thickness of the product is about 5mm. In other way when use hand lay up was about 10mm.

The question is : the mechanical properties of the hull will be better with infusion or not ? Some people say i’ll not have tensil because of the thickness

I have use about 5500 fiberglass (matt,triaxial) units total and also vacuum resin.

I’ll apriciate if you send me your opinions.

Regards

George

#2

It’s been recommended to me that when infusing I should add an additional layer of material due to the additional compaction of the laminate as compared to vacuum bagging. True the laminate isn’t as strong because of the infused product being thinner. But by adding the additional layer your putting in material that actually has more strength in comparison to more resin in vacuum bagging. I was told this and this has also been my experience. I hope I helped.

#3

This is a problem often seen in laminate design. Many confuse strength and stiffness. Most boats are built with more than enough strength but they sometimes suffer from lack of stiffness. Stiffness will be drastically reduced if your laminate thickness is cut in half. Your 10mm panel is about 8 times stiffer than your 5mm panel.

When biax glass first started being used as a replacement for chop/woven roving laminates many people fell into the trap of looking at the weight of the material and figuring it was a one for one swap (After all, biax is stronger, right?). The problem was that the finished thickness was much thinner with the biax and panel stiffness suffered as a result. Flexible panels lead to fatigue failure. Lots of people were caught with their pants down due to this. The same issues arise when swapping middling fiber content, hand laminated composite for high fiber content, infused laminates.

You can solve this at least two ways. 1. a lot more stiffeners (stringers, bulkheads, etc) or, 2. add laminate to rebuild the thickness.

#4

Thanks for your replies,

Some other told me that with vacuum no air bubbles exist between fiberglass and the strenght is better than lay up.

Your thoughts ?

Regards

#5

The fiberglass of the hull are :

  1. Gel coat
  2. Csm 30units (x1)
  3. Csm 225units (x1)
  4. Csm 450units (x1)
  5. Triaxial 900units (x1)
  6. Csm 450units (x4)
  7. Triaxial 900units (x1)
  8. Csm 450units (x2)

Total : 5200units

Of course i put four stringers and 5 bulkheads but i don’t know if it is ok for this laminate thickness.

Thanks

#6

Infusion will almost certainly ensure that there are little to no air bubbles or voids in the completed part if done correctly. A properly hand laid part can be nearly free from voids and bubbles as well if care is taken during layup and properly rolled out. One major benefit of infusion is the amount of prep time you have before the resin is brought into play, you can lay up all your laminate and bag and perform dry runs and check for potential issues. When laying up by hand, you are working against the clock, and this often requires more hands on deck (pun intended) than with infusion. The downside to infusion is cure times, flow media, resin flow channel placement, and more can be very critical, if there is an issue you may not be able to resolve it and it will result in a failed attempt.

Infusion is not voodoo, but it is not a process that should be over simplified, it takes some skill and knowledge.

#7

I have done a lot of conversions of hand laminates to infusion.

You are right that thickness can be decreased by 50% due to the pressure of the vacuum bag. This leads to overly flexible panels, which can fail. (as mentioned before)

Lets see what advantages and comparisons can be made:

with infusion:
-resin uptake dramatically decreases.
-void content drops from approx 3% (hand layup) to less than 0,5%
-much less resin rich areas, due to better compaction, leading to a better ILSS.
-ability to use much heavier fabrics, saving on prep time.
-less thickness, due to compaction of the laminate. Resilient materials such as CSM, CFM show a much larger reduction in thickness than less resilient materials, such as wovens, NCF and core materials.

Let’s address the thickness issue:
-decreased thickness leads to more flexibility, even with the same amount of glass.
-tensile strength remains roughly the same.
-in single skin laminates tensile is hardly the limiting factor, where stiffness is.

So the key is to regain stiffness back. As stiffness is dominantly a result of thickness (much more than materials used) it is easiest to make the part thicker. This can be done in 3 ways:

-use more glass. In you laminate scheme I clearly see a system of using some finer layers to maintain surface quality, and a sort of sandwich of Non crimp fabric (NCF) sandwiching a couple of layers of matt. You could opt for more layers of CSM, perhaps some more NCF which will also add to the stiffness, strength and depending on fiber orientation on torsional stiffness.

Another option is to add a core material. A specialised infusion core is Lantor Soric. This core is very easy to use and give good results. Use it in the middle of the stack.

Other options are hard cores, such as PVC foam.

Every option should be calculated in terms of cost, labour and performance.

No time to elaborate now…

#8

Thanks herman for the details, it’s very usefull.

I’m thinking to add a 3mm lantor soric xf to add thickness in the hull. Before and after this i put triaxial 900 to close the core.

Also i’m thinking the deck construction. If i put the same fiberglass es as the hull will be very lengthy.

Your thoughts ?

Regards

#9

Not this simple- depends how the part is loaded. If all you are doing is loading the laminate directly in bending then you can see a little bump in stiffness from a contact layup due to the increased thickness.

But if the laminate is acting as the outer layer of a sandwich or part of a monocoque structure then the denser, stronger, laminate is significantly stronger since not only does the laminate have higher material allowables but it’s further from the neutral axis so the stiffness goes up.

For most structures the laminate will fail in compression or inter-laminar shear, consequently the better quality (higher strength and stiffness) laminate wins out over the slightly thicker laminate.

And this doesn’t address fatigue. The higher quality vacuum bagged or autoclaved laminate certainly wins out here.

So in general, parts cured under pressure are superior for load bearing applications.