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Guest Post: Two Laminate Samples, One “Core” Lesson

Our friend Chris Rogers from ExploreComposites.com joins us today with this piece. Make sure to check-out more of Chris’ posts and great information at ExploreComposites.com .

Part of the idea with the Laminate Samples is to show how materials
and processes work – but there’s another part: showing how to mess stuff
up! I have been building in (mostly intentionally but sometimes not)
mistakes and demonstrations of common problems that are easy to have.
From vacuum bags that are too small, race-tracked infusions and pinholey
surfaces, there are plenty of mistakes to make even on a flat panel.
Most are pretty obvious once you see them so I’m making sure to show as
many as I can without making the samples themselves totally useless.

The Core Concept – Bleed Holes!

This post is about two samples: #18 and #24. They share the same
core (1/8″ / 3mm Divinycell H80) and both use 200g / 6oz skins laminated
with epoxy and vacuum bagged. One is e-glass so you can see through
it. The other is spread-tow carbon, and you can’t see through it but
you can see the big mistake!

The main idea here is that when you’re vacuum bagging foam (or balsa)
cored parts, you need a way for the air and excess resin on the mold
side of the panel (the first skin) to get out. This is why core needs
to have either holes or slices with a scrim. Both ways allow air and
resin to escape. In #18, I drilled a bunch of holes on 50mm / 2″
centers to let that happen and it came out great. On #24 I didn’t – and
the bottom skin was full of too much resin, trapped air, nasty surface
defects and at least one fairly large non-bond / void area. Ouch! And I
had to go and do it with that pretty expensive spread tow… but it makes
for a better lesson that way.

Look at that mess – the excess resin and trapped air forces too much
thickness in the bottom skin. When I tap this area it sounds hollow
like there’s a huge air bubble between the carbon and the foam – because
there is!

The Videos

The good one…

…and the bad one:

Pre-drilled Holes

Most foam core manufacturers sell foam with pre-drilled holes. This
is great because it saves a ton of time – drilling thousands of little
holes is no fun. Usually the holes are the right size for infusion,
where the holes allow resin to get to the bottom skin. They’re also
fine for bagged wet layup, where they let air and excess resin out. For
pre-preg, you may be able to get away with smaller holes.

Hole Spacing and Layout

The ideal size is about 1mm / 0.04″ for bagged wet layup. If you
have good control of your resin wet-out ratio this will be fine. If you
just pile the resin on there like I did in #24 – you’ll need bigger (or
more) holes. Thicker core or core bedded down with thicker filled
resin will need larger holes too. The weight penalty for larger holes
decreases if you are using a core bedding mix that has a low density
filler.

Hole spacing depends on the core thickness, bedding material and how
much you expect to bleed off the bottom skin. I’d suggest keeping it
closer then 100mm / 4″ so that there is never more than 50mm-ish of
distance for air to travel. For filled core bonding resin, go closer.
Really, unless you have tested it – 50mm / 2″ spacing is ideal. If you
are building big stuff you should totally test it!

I also suggest sticking to a regular pattern. It is appealing to
just blast them all random and assume that averages will work out in
your favor. The big problem is making sure you don’t leave “islands.”
Draw a grid and be systematic. If you have many sheets to do, stacking
them and using a pattern and a long bit can speed the work.

Vacuum Level

And about vacuum level and pressure – you can bleed off too much
resin if you really hammer on the vacuum. For stuff like this, 15inHg /
500mbar or so is fine. I have seen people use much less for bagging
wet laminates and things come out fine! Check out what happens when you
really crank up the vacuum in
Laminate Sample #5: Vacuum Bagged Wet-layup Carbon/E-Glass with Foam Core.
That carbon really shows how resin starved the e-glass is. Its easy to
think things are great with carbon but fiberglass telly you what you’re
really getting in there!

Note: If you are thermoforming your core, you should get
un-perforated core and then drill the holes after you form and slump the
core. They close up when you bend the panels and redrilling thousands
of holes is bad news.

I hope that helps explain the importance of perforating core with bagged wet layup. Nothing like seeing it on video!

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