Vacuum Infusion - Decks

[FLexpat]

I decided to use vacuum infusion to make replacement decks for my 23 Sceptre but I couldn’t find a whole lot of useful info how to do it so I figured I’d start a separate thread on vacuum infusion. I will probably be going back and editing my posts to make them more understandable and fill in missed details. Assuming this is successful, this thread will only get into how I’m teaching myself, what I learn, and how I use vac infusion to make the decks (or what abject failure looks like). Either way, I’ve already learned a lot. Hopefully I get decks done and back to my rebuild thread soon.

Vacuum infusion (VI) seems to go by a lot of different names like Vacuum Assisted Resin Transfer (VART), Vacuum Infusion Process (VIP), Vacuum Injection Molding, etc. – seems like a bunch of folks are going acronym happy about it, but it all seems generally the same to me. Lay your glass and core out on an impermeable surface, seal the other side (flexible plastic sheet or mating mold surface), pull a hard vacuum on it with a vacuum pump, hook up a resin supply, let resin fill the glass/core up, and let it cure. It’s a bit beyond vacuum bagging where you lay everything up with resin by hand, seal it up and pull a vacuum to have atmospheric pressure compress it all with an air-free cure. Vacuum Infusion uses atmospheric pressure to ‘inject’ the resin, keep bubbles out, compress everything and make a good layup. Like everything else there are some details that can turn into nightmares and wreck the project and/or tools. One of the best threads I’ve read is at boatdesign.net; https://www.boatdesign.net/threads/infusion-plan.43380/.

My 23 currently has the decks cut out and is partially covered on a trailer in Park City, UT – that is kinda important since the atmospheric pressure at ~6800’ is less than at sea level; 11.5 psia instead of 14.7 psia so it limits what my vacuum pumps can do and limits how much force atmospheric pressure can apply on my layups. And I’m trying to get stuff done before it gets too cold and the snow starts.

I am using vinyl ester instead of epoxy since I want to be able to gelcoat it without worrying about bonding to epoxy. Since this is infusion, I am using ultra-low viscosity infusion resin instead of laminating resin.
The basic plan for the decks is (top to bottom):
7725 2x2 twill (to minimize print-through)
1808 (0/90, +csm)
1400 (+/-45, no mat)
1400
1808
¾” double cut Corecell (A-500) – Wanted M-80 but I got a bunch of A-500 really cheap on ebay
1808
1400
1808

I am doing the layup upside down so that I get a smooth surface on top (hopefully). I am beveling the edges of the Corecell at 45 deg so that the bottom and top layers of glass tie together for >2” all the way around the core. There is a ~1.5” lip all the way around the cockpit for the new decks to rest on. I left lips around the fishbox and fuel tank hatches too. Details on that go into the rebuild thread when I get a chance. The original deck had a 3/8” core and this one has ¾” so that affects the stringer clearance – more on that later.

There is a bunch of contradicting info out there on resin uptake for glass and not much info on resin uptake for the Corecell. I ended up making a stupidly complex excel spreadsheet to estimate high and low for uptake, then started experimenting.

I am running 2 vac pumps in series behind a resin trap. I can get to ~22” Hg, ambient here is ~23.2” so that works out to something like 30mm Hg which is ok since the styrene in the resin doesn’t boil til 5mm. For reference, ambient at SL is 29.9”.

Trial 1; 12x12 layup of all glass, no core
Goal – figure out better resin uptake numbers and get a quick peek at what I’m planning wrong – unfortunately I only took 1 pic of this test

First I cut all the glass and weighed it with a digital food scale – it was 6.4% more than the spec weights.
I taped a sheet of polyethylene sheet plastic to a table top since I don’t have a good layup table set up yet. (Don’t do this if you want a good part – the poly wrinkles and seems to be affected by the resin)
I put a layer of peel ply down, the stack of glass, then another layer of peelply over the top of everything.
I just used a piece of slit tube (instead of spiral wrap) for the resin inlet – won’t do that again. The inlet was full length on one side of the glass stack.
I used MTI (gas permeable) tubing for the vac line – along the opposite end of the glass.
I covered it all with Strechlon 200 plastic and stuck that to the board with tacky tape.
I pulled a vacuum on it and used an ultrasonic leak detector (ebay) to find leaks and patch them with tacky tape. Did a leakdown test for ~15 min and it held vac.
It was late afternoon, ~77F in garage and I decided to run the test… spec gel time for 1.2% MEKP at 77 is 35 min.
My calcs for resin included the tubing volume and some residual but the total range was a high of 30oz and a low of 17oz; the wide range was due to the crazy estimates of uptake ranging from a ‘general’ 1.5 times glass weight to 0.45 times glass weight (infusion specific). I mixed up 20 oz and rolled the dice.

Results and lessons:
My glass weighs 6.4% more than spec.
Polyethylene sheeting is good for some things, but not this.
Remember to have a clamp or clothespin handy to hold the infusion tube in the resin tub.
Resin took too long to get through/out of the lousy inlet tube – when it was starting to gel about 4” into the panel I had to use a piece of pipe to roll it into the glass towards the MTI tube – that almost worked but I should have used ½” OD spiral wrap as the inlet in the first place. Also start the infusion when it is cooler and let air temp increase.
The strechlon sealed the top half of the MTI vac tube – I love the MTI, but it needs breather on top of it.
Short version on resin uptake – RESIN UPTAKE FOR INFUSION IS WAY LESS THAN A HAND LAYUP/CURE. I will now use about 0.5-0.6 oz resin/oz glass (VAC INFUSION ONLY!), peel ply takes about 0.25 oz/ft2.
The glass stack was thinner than I expected (better numbers in Trial 2) and amazingly strong – even with a lousy infusion.

Next up – Trial 2; 12x35 glass with 9x30 core (it worked much better and I took more pics).