Vacuum Infusion - Divinycell/Glass Bonding

I'm a builder in Florida and we're working on a new center console line. I've got an experienced crew that is has a lot of experience in vacuum infusion. The problem is that I don't. They're insisting that the Divinycell coring we're using on the hull sides does not require any type of mat or csm between it and the 1708 glass below. I'm finding conflicting information from vendors, other builders and online. Has anyone had any experience with this?

I believe if I was handlaying this we'd have an issue between #3 and #4 below but it appears as if the skrim and tack glue could provide enough bond between that point with vacuum infusion?

Layup stack is:

1. Gelcoat
2. Skin coat chop
3. 1-layer 1708
4. layer Divinycell
5. 1-layer 1708


Any input would be greatly appreciated!

 

There doesn't have to be any cohesion problem between layers 3/4 or 4/5. It would be a different matter to ask if the 1708 fabric is much stronger than necessary, ie a lesser layer might be enough, perhaps saving you weight and labor.
Normally, less resistant layers are placed adjacent to the core than those that are far from the core. But it is not because of problems between the layers and the core.

 

1708 already has 3/4 CSM on one side, stitched to the 17oz biax. Your stack isn't 5 layers it's 7:
1. Gelcoat.
2. CSM, whatever weight you decide to use as a skin coat.
3. 17oz. biax
4. 3/4oz. CSM
5. Foam
6. 3/4 CSM
7. 17oz. biax

You could of course decide to use the CSM side of the 1708 against the gelcoat and skip the extra skin coat. Do a test with your chosen resin, see how well it bonds the biax to the foam.

 

I have a suspicion that if the cured skin coat were to be left out,the result might show some print through of the biax.If the cosmetic aspect isn't important you might save a bit of time that way but it definitely would be best to do a test piece to find out.

 

Bond will be fine between core and 1708, tack glue does not help here it can only make things worse if too much used.
I suspect you will have unacceptable print through if only relying on the CSM side of 1708, best to do a test sample.
Also if test sample confirms that skin coat is required you may want to use CFM continuous filament mat rather than CSM as it aids infusion.

 

Gelcoat and Spray chopped strand mat is manually applied and partly cured. It is not part of the infusion process.

 

So, the only thing I am curious about is the use of the 1708. I'm just a wet bag builder, but assuming the csm is out to the gc means the biax is closer than further to neutral axis, and 8 oz of csm is a bit!

You don't say the direction of the inner layer, but the 8 oz or so of mat inside seems a bit silly, based on my experience, others ought to remark or the person who specified the laminate ought to..

Maybe it is the right formula. RX will know for sure.

 

If I were to engineer this, I would put a light CSM in between foam core. This is to transition the shear stress curve from the high modulus skin to the low mod core. This is consistent with engineering practice of putting a relief/fillet on the junction on web and the flange. Most Class rule would also require a light CSM (<300 gm/m2) in between core and skin.

As to debate whether this is superflous or not, Ondarvr claims they have done some test and it increases the adhesion between core and skin. I tend to agree with him. The short coarse fiber in the CSM acts as a bridge by embedding itself in the coarse core surface and relatively smooth fabric skin. This can be quantified by a drum roll test or as simple as gripping the skin with a plier and try to separate the skin from the core. Catbuilder did some test and claims it matters not as difference is quite small.

Problem arises when doing it with infusion. CSM is porous and resin flow starts to race whenever there is a highly permeable layer and not so on the next layer. I can get by with a veil cloth. It reduces the thickness and restricts the flow.

AndrewK is an experienced infusionist and is a "no brainer" to him as there are methods by which this can be mitigated. There is a lot of other parameters in the process that he can adjust.

 

With hand lamination the CSM comes into play with polyester or VE, not so much with epoxy.

With infusion it's not as much of an issue, the bond tends to be good.

In hand lamation it's difficult to consolidate the fabric and get uniform contact with the core, so you end up with gaps that either fill with resin or nothing. Core bond type products are made to help with this problem. If you do have a resin rich layer at the interface of the fabric and core, which is normal with hand lamination, it fractures and fails rather easily. A layer of CSM will conform to all of the irregularities in the fabric layers and core. And it also puts glass fibers into an area that would just be resin without it.

With infusion the core is forced down into the laminate so there normally isn't a resin rich layer to fail.

With either method the actual bond strength of the resin is the same, it's just that one method leaves a resin rich layer that fails and the other doesn't.

With epoxy the resin is strong enough to not fail in resin rich areas.

 

Agree with what RX is saying, with Ondarvr when infusing this statement needs more consideration and discussion.

"With infusion the core is forced down into the laminate so there normally isn't a resin rich layer to fail.
With either method the actual bond strength of the resin is the same, it's just that one method leaves a resin rich layer that fails and the other doesn't.
With epoxy the resin is strong enough to not fail in resin rich areas".

Infusing at full vacuum can lead to not enough resin at the interface reducing the sheer strength even with epoxy resins, this is my observation by pulling apart my off cuts.

Infusing the scrim backed block core FBG is using will improve the sheer strength because the slits will be fully filled and bridge the two laminates on either side of the core, also all of the open cells on the core surface get filled with infusion.
That is the positive of the process, reduced layer of resin at the core interface is a negative.
By sing 1708 the incorporated layer of CSM is sufficient to overcome the very thin amount of resin at the core interface.
My preferred way to overcome this is once the infusion is almost complete is to reduce the vacuum to 85% before the resin inlets are closed so that slightly higher resin ratio is achieved.

I just realized that FBG is infusing the hulls not a console so my answer stands if the resin is VE. But if poly then do some testing as the incorporated CSM in 1708 may not be enough.

Andrew

 

My comments were on the general principles of the bond. The exact combination of resin, gel time, core, vacuum, part size, skill and shape will influence the actual outcome of all aspects of the part.

A resin starved laminate will have more issues than just a poorly bonded core, although that bond may be the first failure point.

 

As for surface profile. For infused cosmetic part parts this always comes up, and the solution sort of backtracks on the advantages of infusion.

Less resin and more fibers increases print, so does moving the fibers closer to the mold surface. So special resins, barrier coats, skin coats, and certain fabrics are introduced to help limit the print.

These all add steps, cost and weight, which is commonly what infusion is intended to reduce.

At the start of the conversation customers typically say they want the best surface possible. But when they're shown how to get a mirror like finish, they don't like the cost and/or complexity of achieving it. What frequently happens is they choose a "good enough" surface profile that takes less than 40% of the effort and cost of the mirror like finish.

 

To make sure there is no confusion, what I am saying is for high quality infused laminates not something that is resin starved.

The resins that we typically use VE and epoxy to build boats with lack strength and elongation at break for the fiber to resin ratios achieved with full vacuum. My assertion.
And I am not saying that these resins are not fit for purpose either.

My advice is:-
1. to infuse at full vacuum and than reduce it at the end and accept a slight increase in the resin ratio.
2. OR do as RX said add a light slightly higher resin ratio CSM/CFM/veil at the core interface and maintain full vacuum.
3. OR use a high spec stronger and tougher resin and full vacuum.

 

Maybe it goes without saying, but for #3, above, I would include using bleeder release as a requirement~as you know, I am referring to my experience wet bagging, so if infusion is different, correct me as needed...I don't even know if media variation affects this, etc. All I know is I pulled too much resin into breather at full vac. I can also delete the comment, if you like.

 

Hi fallguy, perforated release film is used only if over the top infusion media is used. When using the grooved infusion core then it is not required but good idea to place under the resin feed lines.
Release film for infusion is a bit different to the one used for wet bagging it has a lot more closely spaced holes. You can use this film for wet bagging but not the other way around.