Mold help?

Discussion in 'Replica Props' started by Dainbramage546, Feb 29, 2012.

  1. Dainbramage546

    Dainbramage546 Well-Known Member RPF PREMIUM MEMBER

    Trophy Points:
    511
    I ran into a snag with my two part mold for my M8 avenger. The rubber coated the whole thing but we don't have enough to make the 1/2 around all sides (due to a slight miscalculation in the size of the box :facepalm)

    I need to get this done within the next couple days so I don't have the luxury (or the money) to buy more rubber or order anything online.

    Here in fargo we have most of the large hardware stores and hobby lobby and michaels and what not. I need something to make a shell over the rubber, what would everyone recommend? I'm thinking plaster but I'm not sure how well that would work with the rubber.

    [​IMG]

    Thoughts?
    Thanks!
     
  2. clonesix

    clonesix Sr Member

    Trophy Points:
    1,880
    Re: MOLD HELP???

    the issue would be the thin part of the rubber on the top of the pour. This is the outside face of the rifle, and most visible. Because the rubber is the thinnest there, it is most susceptable to distortion.

    plaster might work for you, depending on how careful you are with casting your piece.


    For future molds, I recommend making a matrix mold that will use less RTV.
     
  3. Finhead

    Finhead Sr Member

    Trophy Points:
    1,307
    Re: MOLD HELP???

    Depending on what kind of silicone that is I doubt that thin area on top is going to work for you, like clone said it's going to distort and or even tare at that point. Is it tin cure or plat?
     
  4. Dainbramage546

    Dainbramage546 Well-Known Member RPF PREMIUM MEMBER

    Trophy Points:
    511
    Re: MOLD HELP???

    In hindsight, I should have done a brush on mold. I'm thinking I should beef up the thin parts with some more mold rubber to be safe. Would that work better with plaster then? Also do I need to make registry keys on top of the rubber before putting on the plaster?

    Also, It's OOMOO 25
     
  5. Finhead

    Finhead Sr Member

    Trophy Points:
    1,307
    Re: MOLD HELP???

    Sorry to say but you wont get more than maybe 2 copies out of oomoo before that thin spots gives you real issues or splits, it's pretty fragile.
     
  6. Dainbramage546

    Dainbramage546 Well-Known Member RPF PREMIUM MEMBER

    Trophy Points:
    511
    Re: MOLD HELP???

    Would it survive if I built up dikes around the thin area and poured more over the top? Hobby lobby sells some silicon rubber, would be bad to buy some of that to apply over the top of the oomoo? Otherwise it looks like I'm going to be ordering more.
     
  7. Finhead

    Finhead Sr Member

    Trophy Points:
    1,307
    Re: MOLD HELP???

    The problem your going to have is the heat from the casting resin than anything. It will make the oomoo very brittle at thin areas after a few pours, just the way oomoo is. It really isn't a good silicone for doing large thick castings and not any cheaper than say MoldMax30 or 20. I do not think it will attach to itself once cured, I could be wrong on that though never tried.
     
  8. Dainbramage546

    Dainbramage546 Well-Known Member RPF PREMIUM MEMBER

    Trophy Points:
    511
    Re: MOLD HELP???

    Ah, I gotcha. Well, I guess at this point if I don't at least try to reinforce it this mold is screwed. I learned something. Too bad it was a very expensive lesson.

    Thanks for the input, I really appreciate it!
     
  9. T2SF

    T2SF Well-Known Member

    Trophy Points:
    856
    Re: MOLD HELP???

    I would put wood blocks in the corners to take up more volume!


    Also you can create a parting line in the middle of the gun after the silicone dries and then pour a 2nd half of the mold.
     
  10. Dainbramage546

    Dainbramage546 Well-Known Member RPF PREMIUM MEMBER

    Trophy Points:
    511
    Re: MOLD HELP???

    Yeah, that was my mistake. I definitely thought I had more rubber than I did.

    [​IMG]
    I decided to patch this side up and If I don't have enough rubber for the other side I'll order the pint kit of OOMOO to solidify it more. Hopefully I can get a couple casts out of this.

    For next time, what other kinds of rubber would people recommend? Because of the size and shape the M8 would it be better to do a brush on mold?
     
  11. Tarchinoko

    Tarchinoko Well-Known Member

    Trophy Points:
    566
    Re: MOLD HELP???

    seeing as you already have a print layer down id suggest just buying some cheap caulk silicone and applying that to thicken it up, oh and plaster works fine with silicone in my experience, might wanna try plaster strips. :)
     
  12. Beaker

    Beaker Active Member

    Trophy Points:
    342
    OOMOO is tough, as others mentioned, doesn't have the best tear strength... now oomoo does an OK job of sticking to itself depending on how long the first layer has been dry and what, if any contamination might be on the surface... I always try to have people replace OOMOO with Mold Star 15... same type of processing but light years better as far as tear strength and library life goes... (though DO NOT pout mold star on top of oomoo... huge sticky mess!)... I have seen people use the caulk method as described above though I have no data as to how well it works as I have never done it myself. Brush-on molds or Matrix molding definitely saves rubber as others mentioned...
    You can buy from:
    Reynolds Advanced Materials
    3920 Grape Street
    Denver, CO 80207
    (800) 603-3080
    (303) 399-0202
    FAX: (303) 388-1794
    Reynolds Advanced Materials | Everything for Mold Making & Casting

    Hope that helps
     
  13. Etewaf

    Etewaf Active Member

    Trophy Points:
    330
    If you go the route of starting over, it's not a complete loss. I've chopped up old/failed molds and used them to bulk up new molds that I've poured. As long as it's the same material, you should have no problems. I've never noticed oomoo having any trouble sticking to itself, even with molds that are over a year old. Just brush/pour on a thin detail layer, let it dry, and then dump in chunks (1/2 inch by 1/2 inch or so) of the old mold with the liquid. Good luck!
     
  14. Dainbramage546

    Dainbramage546 Well-Known Member RPF PREMIUM MEMBER

    Trophy Points:
    511
    The other layer of oomoo worked great, it didn't have any problems. Unfortunately the mold has a lot of soft spots on the inside where it wasn't mixed well enough. I think I'm going to just give in and buy some brush on rubber.

    Is Rebound 25 a solid choice?
     
  15. trooper

    trooper Sr Member RPF PREMIUM MEMBER

    Trophy Points:
    2,206
    i just got some for the first time, im not thrilled it, it's softer than i prefer, it's better than oomoo imo.
    i prefer polytek 71-20, it's spendy, but it's the best ive found, my molds come out perfect every time, you CAN brush on a thin layer and then pour a thick layer once the first one kicks off, if you dont have a vacuume chamber that is.
    plus the molds can last for years if you take care of them,ie using mold release, and depending on the amount of use.

    PLUS, you can recylce your old molds into your new ones, make a skin mold, then mix ground up old mold with new silicone and make a thiker layer :) saves big time on big molds. :lol
     
    Last edited: Feb 29, 2012
  16. Beaker

    Beaker Active Member

    Trophy Points:
    342
    How can a 25 durometer rubber be softer than a 20 durometer rubber :rolleyes
    Rebound also comes in a 40 durometer if you need...
     
  17. T2SF

    T2SF Well-Known Member

    Trophy Points:
    856
    Make a mini mock up. Buy the $20 kit. I was going to say something about the two colors in the first photo and that being a bad sign. But the 2nd pic shows a uniform color. It's harder than one thinks to mix Silicone. It's like mixing honey and molases. Both liquids stick to everything and is very different to manipulate than lower vicosity liquids. Practice on the small mock up! Figure out what you will stir with. With that much silicone you will need something really solid. I even found a wood dowel that worked really well but it would snap under more silicone.

    Again you could draw a parting line on your part and then aim to cut the silicone to that level and then pour your 2nd half.
     
  18. Dainbramage546

    Dainbramage546 Well-Known Member RPF PREMIUM MEMBER

    Trophy Points:
    511
    We used a container that was too small and couldn't stir it well without it flowing over the top, rookie mistake :facepalm I usually use the big paint stirring sticks that are usually free from lowes or menards, they work pretty well.
     
  19. Yodajammies

    Yodajammies Sr Member

    Trophy Points:
    1,046
    Try using a two-cup method when mixing things.

    Mix up the silicone in one cup, then when you're SURE it's completely mix, transfer to a second cup and mix it again. Transferring to a second cup is just about the only way to make sure the two parts are completely mixed.

    Better to spend an extra 5 minutes and 10 cents, then to eff up a mold because the silicone is not mixed properly. Ounce of prevention and whatnot.
     

Share This Page