Controlling exotherm in thick solid laminate with infusion

Discussion in 'Fiberglass and Composite Boat Building' started by Steve W, Jan 1, 2023.

  1. Steve W
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    Steve W Senior Member

    It's been a long time since i've been active on here but i have a situation that i have not encountered before so i thought i would seek out some thoughts. We have a 42ft swift trawler in the shop that ran up on a reef at speed. As a result we are going to be replacing a 17ft long section of the keel with a new part which we will layup in a splash mold, well, not quite. A simple splash mold was my suggestion but management wanted to digitize it and have a mold made using mdf which we will be assembling this week. I think its fraught with potential issues, but we'll see. The existing keel is solid glass with a thickness of 8mm on the sides. To achieve this i am planning to use 9 plies of 36oz quadraxial from Vectorply. I would like to infuse it but i am concerned about exotherm, particularly as it will be twice that thickness where it laps across the bottom. I have quite a bit of experience with infusion making parts up to about 12ft x 6ft but mostly cored parts so much lighter skins. I have not been in a shop that does large solid glass infusions so i don't know how they deal with exotherm. The distance from the keel C/L up to the cut line is 25" on each side so i have done a test infusion with ten plies 6" wide x 27" to see how it flowed and what thickness i would end up with and monitored the temperature with an Infared thermometer and was surprised to see that it didn't get as hot as i expected. It did go the whole distance with just a single feed line at the end. I do still plan on an intermediate feed line just to be safe. I am using VE infusion resin btw. If i were using a simple polyester/csm splash mold i would be able to blow fans over the vacuum bag and the outside of the mold to help cool it if need be but with this mdf thing it is quite thick and will act as an insulator and work against us. It also remains to be seen if we are even able to even make it vacuum tight, if not we will have to hand lay it, if we do that we can do it over a few days and exotherm would not be a problem.
    So, thoughts?
     
  2. wet feet
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    wet feet Senior Member

    I think you may face a more fundamental problem in that an MDF mould may not hold vacuum to the extent that you need.Have you tried applying a bag,or are you planning to envelope bag the piece?Obviously,it will be necessary to check the MDF for draft,unless it has a vertical split which would ease release but impede a strong vacuum.You recognise that this might be a challenge and I would draw your attention to the common use of MDF spoilboards on CNC routers that have vacuum tables as they allow enough airflow to permit the securing of the sheet being cut.It might just be less bother to laminate with a brush and roller as you will save the time taken to apply the vacuum bag as well as the cost of consumables.If you had enough overrun on the mould,it might be feasible to apply a couple of plies of CSM and use the overrun as a sealing surface for a bag.It might still be best not to apply the full laminate in one session for exothermic reasons.
     
  3. fallguy
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    fallguy Senior Member

    I think you have very valid concerns.

    Unfortunately, I don't know how to approach the subject. For me, I'd do it in three shots. And split the work to fit crew. So something like 3 days which would keep you in primary bonding windows..but this is not my area of expertise more than disliking 18 layers of heavy glass kicking and catchin fire.
     
  4. ondarvr
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    ondarvr Senior Member

    Use a CHP catalyst, or a blend of it. You'll be able to make a very thick laminate with a low exotherm.

    Unless you seal the mdf very well you won't be able to pull a vacuum.
     
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  5. Steve W
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    Steve W Senior Member

    Yes, this mold is a poor choice imho but it is what we have or will have. I can not realistically do an envelope bag as i would never be able to assemble it around the mold, which i expect to weigh around 1300lbs.The finished part should be under 400lbs as would a simple splash mold. As i said, i may end up hand laminating. Draft should not be a problem as the keel is not very deep before it starts to turn into the bottom but it is only about 2" wide. This is a perfect application for infusion with a splash mold as it would be made of just csm and polyester resin and be vacuum tight by its very nature.
     
  6. fallguy
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    fallguy Senior Member

    You can seal the mdf well enough. I made a sealed vac table with one inch mdf. I did 3-4 lifts of epoxy at 2 oz per yard. Since you need a vee; seal the mdf before building the vee as it'll hold more epoxy flat
     
    Last edited: Jan 1, 2023
  7. rxcomposite
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    rxcomposite Senior Member

    Are you using DMA DMA – Dimethylaniline | Composites Australia https://www.compositesaustralia.com.au/for-industry/health-and-safety/dma-dimethylaniline/#:~:text=The%20purpose%20of%20DMA%20is,and%20form%20a%20cured%20solid in your Vinyl ester mix? This can affect the curing rate/exotherm of your resin. We did a lot of mixture test with this because the process, tools, molds that was handed over to us was from Canada and it was cold over there. The rep that was sent to us was very concerned the laminate was not curing as expected.

    In our tests, low heat absorption/insulators will not absorb heat from the laminate and will let the mixture cure properly on its own. Mixture is important.

    Large surface areas tends to dissipate heat faster than the exothermic reaction of the resin. It is relative to the thickness of the laminate. Thus if you are making thick laminates, you must reduce cure times otherwise you risk getting a thermal runaway. We bump up the cobalt when the laminate is thin and reduce it when it is thick and we have to laminate in one go.

    Vacuum also plays a major part. Too high a vacuum sucks up the volatiles.
     
  8. wet feet
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    wet feet Senior Member

    I have a feeling that the OP may be proven to be right about a splash mould being a better idea.Although I would expect to have to make good the scraped areas on the actual boat before producing something suitable.It appears that he has to take a pile of MDF parts,assemble them accurately and apply a finish with maybe some corner fillets too.

    At the same time,somebody will have to get inside the boat and clear the area for the repair to take place.That,in itself,is a few days work as there will be an amount of hardware to remove,maybe an engine,some bulkheads to take the bottoms off and if he's really unlucky,there might be a holding tank that nobody had the foresight to pump out.In which case he ought to invite the management types to step aboard to help....

    Obviously,the new section will have to be bonded in place and this is where I wonder if the splash mould could have been the better option as it could have been chocked in place and laminated to the existing hull. Yes you would have to create some working space,but on the other hand,you would remove the finishing of the MDF and the alignment of the replacement section with the boat,prior to adding the bonding.I suppose it all comes down to the amount of access and what may be in the way.
     
  9. AndrewK
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    AndrewK Senior Member

    Steve, for a one off MDF mold I used to do either 2 coats of resin or a coat of resin and high build.

    I only ever did one VE infusion and that was a cored thin laminate sandwich so no help.
    But many years ago I did read an article perhaps Professional BoatBuilder where they were doing 1-2" solid glass VE resin infusion. If your resin supplier can not give you the detailed blend of catalyst required go directly to the manufacturer. Or a new supplier.

    Andrew
     
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  10. AndrewK
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    AndrewK Senior Member

    Do a search on Arkema BlocBuilder RC-50
     
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  11. Steve W
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    Steve W Senior Member

    Wet feet has it pretty much nailed. We have been off over xmas so will be starting on the mold assembly today. The boat has an extensive molded structural grid which precludes any repair taking place from inside the boat. We have established cut lines which are parallel to the C/L the length of the repair and fall outboard of the garboard radius where the hull is flat. If i had done a splash mold i would have scribed them into the hull so they would transfer to the mold and subsequently to the part. Once the damaged keel is cut out we will have pretty good access to the inside from the outside so we will be installing G10 bonding flanges to bond the new part to. We will then grind out the scarfs and tab from the outside. RX, i don't know the chemistry of the VE but i did have the supplier send me the data sheet so i will look further into it.

    Steve.
     
  12. KD8NPB
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    KD8NPB Senior Member

    Why are you worried?
     
  13. fallguy
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    fallguy Senior Member

    16mm of resin and glass would get smokin hot under many resin systems.. more so when the heat can't go anywhere and he doesn't want to burn the shop down
     
  14. KD8NPB
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    KD8NPB Senior Member

    With VIP, the resin content is low.

    Exotherm likely would not exceed 250F.

    I've done a similar laminate before. 3x E-3LTi 10800.

    85F ambient, 1.25% DDM9, infusing with Ashland AME1001.

    Exotherm was about 220 or so.
     

  15. Steve W
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    Steve W Senior Member

    What Fallguy said. In the bottom of the keel where the glass laps it will be 20 x 36 oz/yd2. If we were doing a hand layup we would be able to do it over a few days so excess exotherm would not be an issue. I have talked with my supplier and we have decided to use MCP catalyst which will help keep the temperature down. As i said, most of the infusions i have done have involved core with much thinner skins. I am still thinking of doing two separate infusions.
     
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