Repair sag of papermodel

Discussion in 'How Do I...' started by johanvanacker, May 20, 2019.

  1. johanvanacker

    johanvanacker Member

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    Step 2: insert foam in the model.

    Holes in the feet of the dino for the tube that will deliver the foam from the can.
    I used low expansion PU foam for this (1-component).

    IMG_1173.jpg IMG_1174.jpg IMG_1175.jpg

    Because the model is already completed and closed, it is impossible to evaluate the amount of foam that is inserted in the model, so I did a few test burst outside the model to check on the quantity per burst and the amount of expansion after each burst.

    Finally decided to start the process and not much happened at first...
    then foam came out at a few spots indicating that I might had underestimated the amount of foam already in the model.

    IMG_1176.jpg IMG_1178.jpg

    I waited some time and more foam came out from the feet. The foam from hole at the mouth had stopped. Apparently it was already hard.
    After some more time, foam started exiting from a small hole at the back leg. It acted as some sort of overpressure valve where the excess foam could escape.

    IMG_1179.jpg

    Waited a few hours and then the foam seemed to stop exiting from the hole at the legs.
    Overall the model kept it's shape well and I was relatively pleased with the result.
    The small hole at the legs was going to be easy to repair.

    Nothing prepared me for what I woke up to the next morning ...

    It seemded that the foam was far from being completely hardened the previous evening and had continued to expand during the night.
    The result is shown on the pictures below. ruptures on the belly between the legs and also on the back of the dino.
    The PVA glue treatment probably prevented the model from completely bursting open.
    The model looks a bit more swollen (like the T-rex from Toy Story), but it is not noticeable if you hadn't seen the before version.

    IMG_1181.jpg IMG_1182.jpg

    Still thinking of how I am going to fix this ...

    On the positive side, the whole model is now very hard and completely filled with foam. You could use it as a baseball bat ...
    Papa Mashy and lyter1958 like this.
  2. zathros

    zathros -----SENIOR---- Administrator Moderator

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    Skin it with the same some reptile skin!! :)
  3. Revell-Fan

    Revell-Fan Co-Administrator Administrator Moderator

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    You may say you gave the Rex too good a meal... I'm sorry for this to happen but Zathros is correct. Print some sections on regular paper, cut them to shape and glue them to the surface to cover up the fissures. You learn from this. I would never have thought that the foam woud expand further overnight.
  4. johanvanacker

    johanvanacker Member

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    The technical sheet of the foam specified that the curing time is +/- 45 min. So after more than 2 hours, I thought it had finished reacting. The small hole at the belly probably got blocked due to hardened foam and the rest of the expanding foam could not escape anymore and probably caused the expansion of the complete belly of the dino.
    Anyway, good idea to cover the rips with some thin paper from the original file. I still have it in my collection, so as soon as I find the time ...
    Revell-Fan likes this.
  5. zathros

    zathros -----SENIOR---- Administrator Moderator

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    I would have only filled the legs. That would have given any expansion more than enough room. Sorry I didn't post that, but I assumed yoou were only going to fill the legs. I would really wet the patch pieces so they can follow the contours, and print using thin paper, which sill shape easier. It's really not so bad though, to be honest. :)
  6. Papa Mashy

    Papa Mashy New Member

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    Thank for sharing the foam experience. I look forward to seeing how you remedy the issue.
    That is the key in most things.
    When I was doing my joinery apprenticeship, one of the old boys said to me " Anybody can make a mistake, but it takes a craftsman to put it right".
    thumbsup
  7. daishi

    daishi Member

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    Yeah I could have been a bit more explicit too, sorry. :(
    the foam expanding overnight is new to me too, so at least now we all know to look out for it ;)
  8. johanvanacker

    johanvanacker Member

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    No need to apologize, I knew that was the risk. Hope you all can learn from my mistake.

    @zathros: First plan was to only fill the legs, but somewhere between planning and execution, the plan changed... after all, I had a full can of foam :)
    Even low-expansion foam still expands a lot...
    I still believe that strengthening the model with PVA glue saved it from completely bursting open. Now the rips occurred only at the seams between paper pieces.
    In hindsight, a better plan would have been to first fill one leg, wait a day and then fill the other leg. But that would have cost me another can of foam since the foam hardens once you start using the can.

    @Papa Mashy: A teacher one said something similar: "A good carpenter knows how to hide his mistakes"

    It's a bit busy for me now, but I will definitely post pictures of the end result.
  9. Gandolf50

    Gandolf50 Researcher of obscure between war vehicles... Moderator

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    Actually, if you take the nozzle off right after use and clean it with lacquer thinner or nail polish remover cover the spout on the can with some plastic wrap and rubber band, you can get away with saving the rest of the can for later on. Not sure how MANY days but best to use up within a day or two.
    zathros likes this.
  10. zathros

    zathros -----SENIOR---- Administrator Moderator

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    Hey, you learned a good lesson, so did the rest of us.. I clean those cans and at the most, I get maybe three days out of them, but then they solidify, and become useless. I believe this is done on purpose, and maybe the twp art foams mixed for aircraft use would be better, though far too expensive. I made the rudder for a Zenith Zodiac with a friend using this stuff, and e filled the aluminum rubber, finished with blind pop rivets, making an incredibly strong rudder. Maybe the thicker Gap Filling Crazy Glue, with an accelerant, applied when the leg is in the proper position would work better, then an armature built. Either way these models could use some kind of inside structure and the designers never supply that. Popsicle sticks work really well.

    I fixed the air pump pulley on my M.G. Midget 30 years ago using Gap Filling Crazy glue and the accelerant, to make it harden instantly, I built the surface up. It has never come apart. The porous nature of the cast part probably allowed for an incredible mechanical connection. I used the air pump to filled the rubber raft I used to go boating with, I had removed all other tubes, and it made a great volume of air pump. I'd have a line of people asking me to inflate their various fill up tubes and such. Lesson learned, and the next time, just doing the legs, with a couple of Popsicle sticks in there, so as to give you something to glue the rest of an armature too, will give you the results you wish. You may be able to go in there with a sharp and dig out enough material to close those gaps. Just a though. :)