Customs: Resin leaking when casting under pressure

Discussion in 'Creative General Discussion' started by Venksta, Oct 12, 2010.

  1. Venksta

    Venksta Render Project Creations TFW2005 Supporter

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    I was wondering if any one could give me some pointers on an issue I'm having. I made some new molds yesterday, and did a test casting, under pressure. But when I went to demold them, I noticed the resin had leaked from the bottom of the molds. I made sure the two halves of the molds were tightly held together by pieces of foam board and rubber band. But I don't get how the resin could still leak through.

    I'm using Smooth Cast 326. It has about a 10 minute pot life, and 60 minutes before demolding. Anyways, I've had this issue before. So I was wondering if there is something I'm over looking? I can show pics of my molds if it helps. Any help is appreciated. :) 
     
  2. Insane Galvatron

    Insane Galvatron is not insane. Really!

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    Use Allumilite. Fully hardens in like 5 minutes, with only a 30 second working time. Plenty of time to pour it in, and it'll harden before it can leak out with that slow leak.
     
  3. Budokhan

    Budokhan Wheeljack's apprentice

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    Hey Man!
    Glad to be able to help ya on this one! Ive had a lot of experience with molding & casting, especially two-half molds! Throw up some pics & I'll see if I can help!
     
  4. convoymagnus

    convoymagnus Yeshua Is Mashiach.

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    Hey man, I am in if You need help too, vastly experienced as well, but as Budokhan said pics are needed. Now I looked that You use foam board, this could be a first point to correct, I have concluded that the best is to use to wood pieces (plywood or something not too thick) materials like styrene or thin stuff have the tendency to yield to the rubber band force and if the mold is not too thick it will get deformed and resin will leak.

    also try to put bands horizontally and vertically because if you do it only one way the mold will again deform and if it does not leak, at least will produce a lot of (thick) flashing in the parting line area.
     
  5. Bigbot3030

    Bigbot3030 Well-Known Member

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    Horizontal bands and foam core have worked great for me. I make the foam core more of a sturdy box so the walls use themsleves for support. Anouther thing I've done is I use to skimp on silicone around the parts, so it was easy to deform when band pressure was applied. Once I gave a little bit more meat around them, I had a whole lot less problems.

    one way i did some troubleshooting with some molds I made was make the box out of some sheets of acrylic, so I could see where the mold was slipping and allowing resin to pop out. that particular time is when i realized i didnt' have enough silicone around it, hard mold box be dmaned. :) 

    Randy!
     
  6. DanHibiki

    DanHibiki Dan Couga Mellencamp

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    Did you create your molds under pressure?
     
  7. convoymagnus

    convoymagnus Yeshua Is Mashiach.

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    This is another point to check...are Your molds thick enough?? when You try to save on silicone by making molds too shallow deformation and leaks happen. I have seen silicone makers that recommend at least 1/4 inch thickness from the master piece.
     
  8. Venksta

    Venksta Render Project Creations TFW2005 Supporter

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    Thanks for the input from everyone so far. I've attached photos of the current molds, and the foam core I'm using. Didn't put the rubber bands on, but I'm sure you guys have seen those before. :)  The mold layout is based on several designs I've seen by some Japanese model builders.


    Actually, like Allumilite, Smoothon has several types of resins. There are a couple of quick curing resins, that I've used from Smooth On. The 326, I'm using at the moment, is better suited for pressure casting, as it gives more time to allow for bubbles to be eliminated. This is especially great for translucent parts. I've tried the quicker resin for translucent parts, and it doesn't give enough time to the pressure to remove all the bubbles. Something I'm picky about.

    Hey dude, glad you can help me out. Photos attached. :D 

    Its kind of weird. With my first batch of Skyfall parts, I never had any leak issues. And I set my molds up the same way, using foam core as padding, and rubber bands. I actually used less rubber bands on that batch of castings, and didn't run into any problems.

    Thanks Randy. I always try to make my my molds thick. Mostly because I'm still inexperienced with mold making, so I'm not sure how thin is safe. So I've found it better to go thick, then thin. I usually give about an inch from the top of the master piece being used for the mold.

    Yup, I always make the molds under pressure. :) 

    Yeah, the current molds are about 1" thick, at the most, per piece.
     

    Attached Files:

  9. Bigbot3030

    Bigbot3030 Well-Known Member

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    Interesting. Did you have problems with both molds, or just the gun? I ask becasue while you did great work on indexing, I could see slippage and leak potential around the scope of the gun. There is nothing to lock in that side, and it's relatively thin between that and the edge.
    keep in mind the thinkness (i'm talking from the foam core to the split line of the mold) of each piece matters, as well as room around the parts. You've only got something like a 1/4" or less between that mold edge and your scope.
    the rest of it looks great. nice and clean, just the way it should be.

    Randy!

    Edit: what I said about around the scope applies to the "bottom" of the backpack mold as well. Close to the edge.
     
  10. convoymagnus

    convoymagnus Yeshua Is Mashiach.

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    this is true, try to get a little more thickness on those mentioned areas.

    Now one thing I see that might be causing the problem is the smoothness of the parting line, That mold is so perfect it seems it was machined. I never saw a mold like this(two part mold) in silicone rubber. I mean, silicone rubber molds depend on the uneven surface between halves to create a "lock" between them, yours are so smooth and flat just that pressure pushes resin in there specially if they are not pressed enough.

    You know, that perfection is good for steel diecast molds that are pressed by a machine to prevent leaks. but a silicone mold works different.
     
  11. Bigbot3030

    Bigbot3030 Well-Known Member

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    Yeah, the trick to that is making the surafce uneven enough to lock itself together, but not so may that you have to make sure every little nook and crany interlock perfectly together every time.

    I think you are on the right track. Wish I had that blaster now, his gun looks awesome.

    Randy!
     
  12. Venksta

    Venksta Render Project Creations TFW2005 Supporter

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    I had 4 sets of molds in total. The backpack was the least worse of them. It only leaked a little. But thanks for the advice, I'll look into fixing the mold now. :) 

    Yeah, the mold was made from a machined negative I created. Something I learned from another garage kit maker. I'll see if I can find a way to unperfect it. :) 


    Thanks again for the tips Randy. :) 
     
  13. thedreaded1

    thedreaded1 Well-Known Member

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    Can you tell me more about this machined negative?
     
  14. Venksta

    Venksta Render Project Creations TFW2005 Supporter

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    So I did some more test castings. One under pressure, and one with out. I used more rubber bands, and tied them vertically as well this time. Didn't get any leakage. :D  However, I'm still getting flash, as seen in the photos below. The parts with flash are done with pressure, and the ones that don't have it, were cast with out pressure. Between the two, the one under pressure filled out pretty good, and has great seem lines, but there are way to many bubbles. I'm hoping to figure out something so I can cast under pressure and not get the flash at all.


    Basically, I made a 3d model of the negative mold and got it printed. :) 
     

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  15. Fishdirt

    Fishdirt Tin Toy Transformer

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    I don't think there is a way around that.

    In injection molding we'd get too much flash if the material fed through too much or was contaminated. Maybe weigh out the amount of material you need and use that? Also maybe a quicker setting resin? Just guesses here.
     
  16. Bigbot3030

    Bigbot3030 Well-Known Member

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    your going to get flash becasue your surfaces between molds is so smooth, that and it's under pressurure. Like Fishdirt was saying, they get that with injection molds, and those are precision flat pieces of steel that have tons of force pushing them together.

    When you say bubbles, do you mean small bubbles in the casting, or the void at the base of the gun (top of the mold)?
    One thing about your set-up there, is the way you are doing it is better for casting without under pressure. You have a very large fill point with vents. But when bubbles work out of the resin and come out of the vent, there is no more material to be sucked back into the vent and take the place of the air. yes it should eventually come from your fill point and level out, but the shorter distance is the vent. What i do is I cut what we call a "suck-back trench" thats a little area above the vents for extra resin to gather in case this happens.
    I do it by simply cutting a "V" into the mold with it centered around the vent, and in this case i"d do it from vent to vent for the butt stock.

    But it sounds like you want perfect parts without flash, and so I'd recomend not doing vacuum pressure with these molds. You've got it to work, with more rubber bands, so roll with that this time.

    Randy!
     
  17. Venksta

    Venksta Render Project Creations TFW2005 Supporter

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    What I meant about the bubbles, was for the non-pressure casted parts. There are tons of small bubbles in the castings. Especially for the face casting. I had noticed many bubbles got trapped all around. I've never been able to do a bubble free, or close to bubble free casting with out pressure. Like the parts never turn out good enough, that I would release for sale.

    I also get the issue about the smooth surface creating the flash problem with pressure. I'll see if I can rough up the surface, so it mimics a clay bed better.
     
  18. Bigbot3030

    Bigbot3030 Well-Known Member

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    Ahhhh, I see. Yeah, its a trade off. I've just always done a clay bed and pressure casting, becasue while it takes more time, i've had a higher success rate. But i guess that my expereince with doing it any oher way is pretty limited.

    Randy!
     
  19. Venksta

    Venksta Render Project Creations TFW2005 Supporter

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    Decided to do some experiments, to see if I can get rid of, or minimize the flash, when cast under pressure. I doubled the foam core on each side of the molds, to two pieces per side now. It seemed to help. With the face parts, I get minor flash. For the gun, still getting the flash, but not as bad as before. Anyways, thanks for all the help Randy, as well as everyone else. :) 

    Now I just got get the red to match a bit better with the Blaster figure. :D 
     
  20. thedreaded1

    thedreaded1 Well-Known Member

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    I was wondering how you got your molds to look so pretty, lol.

    Have you ever thought of making your molds out of alumide? I haven't done much casting but would a "stronger" mold help reduce your bubbles? You might be able to add more pressure with C-clamps or something of that nature. It might also extend the life of the mold as well.
     

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