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Thread: Ok "metal masters" I got a difficult one here...

  1. #11
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    Is the air just applied to the end of the plank? That is not a very big area so how about some air nozzles in the sides further down the die? The air pressure would force the metal away from the plastic and maybe overcome the sticking, maybe.

  2. #12
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    If the plastic is being shot into a closed one-piece section of rectangular extrusion, with air pressure on the end, I'd call it a lost cause......

    Wanna talk about NO DRAFT?

    That would be kinda crazy... I can't imagine anyone who knows about molding really expecting that to work reliably in the long term..... Surely the guy cannot possibly be doing THAT.....

  3. #13
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    There was one of the Discovery "how it's made" type programs on a company making railway sleepers (ties) from recycled plastic, sounds like something similar.

    Nothing at all high tech, solid steel moulds, no draught or lubricant that I can remember. Once the plastic was in and had cooled to the point it was solid, they just used a hefty hydraulic ram with a flat plate on the end to force the sleeper out along its length.

    Seemed to work fine - they had a yard full of the things.

    Ian
    All of the gear, no idea...

  4. #14
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    Aluminum is not normally attacked by acids. You can leave aluminum in battery acid for days without visible corrosion. In order for the acid environment mentioned to be a problem there must be an electrolytic couple involved. If other metallic parts are in the bath such as a steel framework then the aluminum will act as one side of a battery with the acid bath as an electrolyte. Aluminum is more electronegative than steel and will corrode in favour of the steel. Using a pH adjuster will not cure the problem since ions will still be present to carry charge.

    Hard anodizing will work but that would be prohibitively expensive for such large pieces. The easy solution is to use sacrificial anodes which is standard practice for aluminum boats used in a marine salt water environment. Zinc is the best choice and is a cheap solution. Another possibility is to use active protection by reversing the polarity of the electrolytic couple. By connecting a current source to the aluminum framework and a lead plate in the tank stray electrolytic currents can be eliminated by driving the current flow from the lead plate to the aluminum. A microamp meter is used to adjust the amount of current until the aluminum is more positive than the lead plate. In a large apparatus such as this several lead plates will be required at intervals in the tank.

    Rain-X contains polydimethylsiloxane which is an industry standard mold release compound. Rain-X is already formulated with a carrier agent to provide wetting of the mold surface since it is intended for use on glass. It is also compatible with the plastics involved.
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  5. #15
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    1) Zinc. I was trying to remember how that worked. Its Zinc. That I can set up in no time flat.

    2) I am in with the local Stoner Brand Molding Chemical sales agent, I tried about a dozen formulas of release... none of them worked.

    3) Hydro ram... Might be an option, I'll have to look at the machine again.

    4) Splitting the profile... That might work. If nothing else it'll give him a way to split the dies open and possibly salvage part of the profile.


    Flowmolding is the bastard son of an injection mold machine and a profile extrusion line... all of the disadvantages of both, none of the advantages. It's like trying to mill a motorcycle helmet out of solid aluminum with a drill press.

    Seriously though... these sorts of "recycling" operations pop up all over the place they hire scum of the earth labor and go out of business as soon as the extrusion screw wears down from all the soup-can lids that get introduced into the material stream.

    It's a great idea, makes a great product but it requires considerable more investment than most of the operations around here are willing to make.
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  6. #16
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    Random thoughts on the Epic Bonding.

    I sort of have a vision of the process in my mind. My thoughts and possible solutions in no particular order are:

    Contamination. You are using waste plastic. Contamination from foreign substances or dissimilar plastics are changing the properties of you product subtly. Possible ways to combat are longer mixing or larger batches/mixed longer. Longer or bigger will dilute the impurities. Perhaps mixing at higher temperatures will make the mixture more viscous and aid mixing.

    Temperature too high. The mixture going into the mould is too hot and thus requires a longer time in the mould which make it stick. Perhaps precooling between the mixer and mould might help.

    Temperature differential in the cooler. The hot plastic comes into the mould and should immediately start cooling and thus shrinking. The coefficient of linear expansion for plastics are greater than aluminum. So in the theoretical world of coefficients the plastic parts should be rattling around within the mould after cooling. (I love theoretical worlds) Perhaps dropping the temperature of the cooling tanks 5 would provide for increased shrinkage of the plastic. Alternately cooling the input mixture 5 would accomplish the same thing. It's really the rate of heat transfer out of the plastic.

    Just some random thoughts

  7. #17
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    Rancherbill: I've gone up and down the process ladder with this one and they still stick. With my molding machine at home if I had this problem I'd shoot some release agent into the mold and adjust the mold surface temp, and if I still had a problem I'd adjust the injection pressure.

    Pressure on this machine is a function of the RPM of the machine. I've watched it for several hours now there are no jumps in RPM or head pressure, none that show up on the gauges anyway, and none that coincide with the sticking. My first thought when I heard his problem was "overpacking" and that I could process around it.

    I am on the way back from the fleet-supply center with a big bucket of Rain-X now I'm going to try that for him... after I get these zinc strips set up.
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  8. #18
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    So you've done all the process things.

    Now you are down to the material things. I would guess there can be variability on the recycled material.

    An idea might be a farmer chemical analysis of the sticking pieces.

    You could weigh some samples vs the norm. Maybe you could measure the density by submersing in a liquid and measuring the displacement. Burn some of the material in a controlled setting. The flame colour or temperature might be subtly different.

    I don't know anything about the feedstock but perhaps water or air bubbles are getting into, or not getting into the mixture because of feedstock variability.

    Just random first thoughts.

  9. #19
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    Maybe if you heat the plastic up enough to start getting small bubbles, when it cools it will shrink enough to create a space between the mold and the plastic.

  10. #20
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    Two other suggestions were to mix baking soda into the material and the acid-condition of the water will cause a foaming action hopefully preventing bonding. It's crazy... it is based on a sound "foaming" principle I have used in injection molding and it could lead to an explosion. Going to try this tomorrow.

    Second thing I'm going to do is go through his material supply (bales and bales of Blue Box material from the curbside program) and make sure he's running JUST HDPE and LDPE. No vinyl, no polypro, no styrene. After some reading these materials can cause issues in this sort of process. Certain reactions can take place in the presence of "other chemicals" such as soap and bug-killer and heat causing these materials to become adhesive.


    Lastly, I have my step-father and his Instrument And Control know-how coming out to look at the gauges. This is a simple machine... heat, RPM, and head pressure are the only gauges I have. Maybe I'm not picking up a process instability due to gauge error.

    I do NOT have an amp-meter... and that bugs me. In pure extrusion the amp meter and pressure meter are the two most important readouts they tell you how hard your screw is working... and that relates to your melt output... quality and stability of said melt.


    Did I mention I'm having fun doing this? I'm in my element here. Plastic is fantastic.
    This product has been determined by the state of California to cause permanent irreversible death. This statement may or may not be recognized as valid by all states.
    Heirs of an old war/that's what we've become Inheriting troubles I'm mentally numb
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