Injection mold example

Discussion in 'Replica Props' started by TheSt.LouisKid, Mar 30, 2015.

  1. TheSt.LouisKid

    TheSt.LouisKid Sr Member

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    I forget what thread it was where someone asked to see this.

    I lot of people make a mold where you pour the resin in through the top and it fills up like a glass of water. I am sure everyone has noticed that bubbles can churn around in a glass of water as you are pouring. As we know bubbles are bad. :unsure

    This process reduces bubbles. With a syringe shoot the resin in where it flows through a runner that comes around under the part. Note how the gate is attached at the bottom. The arrows indicate where the resin flows. The mold fills and air makes it way out without churning. I have had many times where there are no bubbles at all depending on the shape of the part. The funnel shaped cut at the exit is a reservoir that will flow back if the resin has shrinkage. The finished part you see came from this mold. The part has a heat sink quality to the shape and the air made it's way out.

    So if you are feeling ambitious give this process a try.

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    robn1 and Sean like this.
  2. ultraman

    ultraman Sr Member RPF PREMIUM MEMBER

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    thanks for sharing!....educating people on different molding methods and prop builds is what we're all about!!!!
    the more you know the better off you are people!!!
    :cool
     
  3. robstyle

    robstyle Sr Member

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    should note, wear eye protection! Ive had to inject plastic into molds for super tiny detail castings and more than a few times has the material literally explode out of the mold when my thumb was a little to ambitious with the plunger.
     
  4. cavx

    cavx Master Member RPF PREMIUM MEMBER

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    How do you go when you reach the end of the plunger? I have used a syringe to inject resin into molds several times and with mixed results. Small parts seem fine. Larger parts, not so. The good parts is, liquids can't compress easy, so the air shoots out, but I found if I squeeze the plunger too much, it generates bubbles. There must be a small amount of air trapped in the syringe?

    Also how runny is your resin? The stuff I use is thick like honey. Think of how resin is just when it starts to kick, and that is what I have to work with from the get go. And so an injection system that forces the resin into the mold would be ideal. I kind of thought if it is possible to load tubes like those used for caulking and use a caulking gun to inject them.
    My train of thought is to load the tubes with mixed resin and degass the resin in the tube. The issue here is that you can't fill the tube, only quarter fill and have up to four ready to go to allow expansion during the degas.

    Has anyone done this?
     
  5. NormanF

    NormanF Master Member RPF PREMIUM MEMBER

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    I have only used that on two molds Neither one had the correct size opening for the syringe. If I was smart I would make a mold of the syringe tip so I could form a clay plug of the right size and use that while making the mold of the part. I guess I could just cut the tip off of one of them too.

    I did not fully read the previous post. I have degassed resin inside the syringe by putting my finger over the tip and then pulling back on the plunger. But I have only used it on small molds so I do not know about the caulk gun idea. I don't know where I would even get empty cartridges for that.
     
  6. paulp

    paulp Active Member

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    That's a nice idea. I've done injection before, but it's usually top down. HAven't tried from the bottom up. My to do list is getting longer.
     
  7. TheSt.LouisKid

    TheSt.LouisKid Sr Member

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    If it's getting thick already then maybe you are waiting too long to deliver it to the mold. Consider what you are doing and how you can speed it up. Also, I will at times put the resin in the refrigerator. When you use resin I am sure you all notice it gets hot as it hardens. If it is chilled then it can buy you some time. When drawing resin into the syringe the turn it over point up like a doctor and get the air out. When you shoot the resin nothing is forcing you to push the plunger until it stops so if you know there is air in the syringe then stop before it gets to the air.

    I have not done this. I think you are overthinking it. Your degassing in the tube is time when you should be putting resin in the mold.
     
  8. cavx

    cavx Master Member RPF PREMIUM MEMBER

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    The stuff I am using is a clear elastomer. It equal amounts by weight. Part A is thick like syrup. Part B is much thinner. It says to shake well before use and Part A, doesn't do anything when you shake it. The only way to ensure it is liquid is to turn the bottle upside down and then it takes close to 2 seconds to move the big air bubble. So I always pour this part first and tint it if required. Part B is fast. You can shake it up and pours like cordial.

    Once they are mixed, they are really thick. Think liquid soap. It pours but not well through holes. In fact it tends to catch the edge and build up creating an air lock.

    The stuff has a 15minute pot-life and why I can degas. Having mixed and poured parts without degassing this stuff is bad. No amount of tapping or vibrating the bench is shifting these bubbles. They have to be pulled out under vacuum.

    So idea is to mix in a tub and load the tubes, then degas. Now there is no air to make bubbles. If the tubes had a removable plug from the bottom (pulled at the last second when directly over the mold), it should start to flow. With the help of a plunger, it should force load bubble free.

    The other way I am thinking is to degas in the mold. Because I know how much product is needed, I could load 50% in the mold and the other 50% in cups to top up. It is messy this way as the vacuum expands the elastomer and sucks it out the holes in the top, but does give a good result. The sides of the mold would have to be built right up to catch the sucked out goo and let it run back in once the system is re-pressurized.

    The silicone used to make the molds was also degassed, so there are no bubbles trapped there either.
     
  9. TheSt.LouisKid

    TheSt.LouisKid Sr Member

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    Hmm. I dunno. Maybe research to see where you can get thinner resins in your area. It's good to try different stuff anyway.
     
  10. cavx

    cavx Master Member RPF PREMIUM MEMBER

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    There is one other supplier, from memory, his product was very similar.
     

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