Two part mold seams

Discussion in 'Materials' started by alien8, Jul 12, 2010.

  1. alien8
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    alien8 Junior Member

    Hi all , new member Brad here. I am not a boatbuilder but I been having a good time reading all the old threads and learning new stuff. I have been in the fibreglass game for 15 years (surfboard building).

    I have a small molding side project going on at the moment and need some advise. First off, I have never done any gelcoat tooling/molding before so its good to learn all about this side of the industry.

    I am making a two part female mold of a basic cylinder shape.Basically two U shapes joined together to form the cylinder and my question relates to the seams where the two halfs bolt together . I need to to achieve a seamless gelcoat look to the finished product. The cosmetic apearence of the gelcoat is of major importance. How do I achieve this? Could I use some kind of putty or silicon to fill up the small seam in the mold before I gelcoat/layup? If so should I apply the mold release over the putty or is it ok to gelcoat straight over it?

    Cheers, Brad.
     
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  2. waikikin
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    waikikin Senior Member

    Hi alien, use a bit of plasticine(your fglass supplier will have it) & wax over to smooth- & maybe a wipe with a pva release agent(use a sponge with some methelated spirit to apply) you still want the seam to be slightly "negative" to the mold surface- once demolded the then positive seam line on the product can be hit with fine wet sanded & easily polished. All the best with your endeavours from Jeff. PS: if you've been doing clear board work in the surf industy you'll adapt easily to the gelcoat world!
     
    Last edited: Jul 12, 2010
  3. tinhorn
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    tinhorn Senior Member

    I used to fill the parting lines on my multipart molds with gelcoat putty. I mixed Cabosil into the gelcoat until I had a stiff paste, then squeegeed the paste into the seams. I did this immediately before gelcoating so that my gelcoat was applied to uncured putty.
     
  4. alien8
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    alien8 Junior Member

    Thanks for the replys. I think the plasticine method sounds like the most suitable for my application if I am thinking this correctly. The molds need to be high turnaround.
    What is the benifit of using a gelcoat putty instead?
    I will be using alot of glitter/shimmer type gelcoats.Will the wet sanding of the seam cut into or affect the cosmetic appearence of the flake in the gelcoat?

    I have done tones of clear surfboard work in the past so I am pretty confident I can transfer the skill needed to get this new project right. I only thing I am a bit sketchy on is spraying the correct gelcoat thickness but there is heaps of info on here thats making it not so daunting. Thanks again guys!
    Brad.
     
  5. gonzo
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    gonzo Senior Member

    Wax and polishing should do the trick.
     
  6. patiras
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    patiras Junior Member

    If you are using glitter, pearl, flake or the suchlike, then you want to spray a clear gel first, and thick enough that when you clean off the flash line you don't go through the clear. If you expose the glitter flakes or pearl spheres then over time there is a good chance they will colour differently, and they can pick up dirt in a different way to a pearl that's completely encapsulated in gel. There's also different ways of spraying depending on the size of flake... If I understand correctly you are closing the mould, then spraying the gel; what's the diameter and length of the tube? If there's any doubt about getting a consistant cover, then think about backing up the flake with a solid colour.

    Al.

    al@trinitykayaks.com
     
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  7. tinhorn
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    tinhorn Senior Member

    You can sand it off, leaving a smooth surface. I've never used the plasticine technique--I think it might work good in combination with putty. I'm going to echo patiras's warning to make sure there is plenty of gelcoat to sand and polish at the seam.

    I've used gelcoat paste in two ways. On molds gelcoated and laid up separately, and then the parts joined while still in the molds, and also on molds which were bolted together and then gelcoated. I've never used it with clear gelcoat, though--some testing might be called for.
     
  8. alien8
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    alien8 Junior Member


    Yes Al, close the mold and spray the gelcoats/flake and lay up the cloth. Snap out the part and touch up the seam then polish the gelcoat. Sounds easy dosen't it:D
    The tube will be about 13'' diameter and that in length too and 1/8'' in thickness. Thanks for the tip about the backing colour, I did not even consider transparency of the layup, which would be an issue with how thin it is. Would using pigmented resin work instead of spraying a coloured gelcoat backing?
    All good, I learning lots of new stuff!

    Hey tinhorn, Yes I would say there will be a big pile of "prototypes" sitting in the junk corner before I pull somthing out of the mold thats passable. Like I said before the cosmetic appearence is of utmost importance with this project. The only thing I am worried about with a gelcoat putty is a discolouration between it and the sprayed clear gel. Sanding and polishing might take care of it but I wont know till I get going on the molds. I am pretty sure the part line on the mold should be know more than an 1/8'' x 1/8'' at the most so hopfully I can get away with as little sanding as possible.

    Cheers, Brad.
     
  9. alien8
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    alien8 Junior Member

    I have been rethinking the mold deign to try to eliminate one of the part lines. Instead of a two part mold I am wondering if a single mold with one parting line would work. A C shape instead of the two part U shape. Is this a crazy idea?
    I am thinking that the part line in the one part mold might just have enough relief to be able to release the postitive part out of the mold if the mold had a little flex in it. How flexable is tooling gelcoat, would it have a chance of cracking in this situation? obviously I dont plan to overflex the mold. This would cut down production time, which is an important consideration with this project. Thoughts?
     
  10. tinhorn
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    tinhorn Senior Member

    It undoubtedly would, but it's impossible to see air bubbles with pigmented resin. If your cosmetics are important, the last thing you want is a bubble beneath the gelcoat.

    Yup, a valid concern. I suggested it before I knew your gelcoat was clear. Forget the putty. I'd try the plasticine to reduce the size of the flash as much as possible. Brush gelcoat into the seam before you spray, simply to insure the seam has no air bubbles. Sand and polish the seam after demolding.

    I've never seen this done, but it's a clever idea. Make sure you use tooling resin in order to reduce resin shrink as much as possible. You don't want the mold pinching the part. You'll probably want the edges of the flanges as sharp as possible to reduce the amount of sanding on the part. Problem is, the sharper the flange, the easier it chips. You'll want to take extra care when you demold the part and slide it out of the mold.

    Me, I'd keep the mold light; veil, glass, CoreMat, glass. Then I'd build a couple extra molds and store them in a safe place along with the plug.

    Build lotsa molds and keep them circulating.

    I'll bet that you have a perfect part by the third pull.

    Edit:

    Having finished my beer, I've determined that gelcoating a one-piece mold would be a terrible job. You might save time overall by sanding and polishing a second seam per part. Hell, build both types and see which works better.
     
    Last edited: Jul 14, 2010
  11. alien8
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    alien8 Junior Member

    Hey tinhorn, some really good tips there. This is getting me closer to starting the molds. I have started the male plug so its good to be ahead of the game, so to speak.

    What do you mean by the mold pinching the part? Is that the mold shrinking on the plug?

    The idea with the one piece mold was to rebate and glue some thin sheet steel down the lenght of the plug to form the flange and putty the corners where it meets the plug to round it off a bit.

    In regards to gelcoating the one piece mold, would't it be the same as gelcoating the assembled two piece mold?
    Your right, I should build both.
    Cheers, Brad
     
  12. tinhorn
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    tinhorn Senior Member

    Yup, which could happen if you use polyester resin, or to a lesser degree, even vinyl ester. Tooling resin will give you minimal shrink. This might be a consideration for your parts, also, since resin shrinks but glass doesn't. (It's why we see the pattern of 24-ounce cloth in the sides of some boats.)

    Yes it would since you're gelcoating the assembled mold.
     
  13. alien8
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    alien8 Junior Member

    Mate, this has be a huge help. This is the kind of info that would have had me in tears if I learnt through trial and error. Many thanks!
     

  14. tinhorn
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    tinhorn Senior Member

    My pleasure. Best of luck to you.
     
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