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Thread: Help: Acrylic Black Specks and Pitting

  1. #1
    Join Date
    Dec 2016
    Posts
    121

    Help: Acrylic Black Specks and Pitting

    Hello everyone, and Help!

    We have a couple of jobs that run in different machines, that run same material. These parts are opaque and called 'Windows". I am not able to get rid of black specks and pitting (small imperfection, kind of looks like surface struck by a needle) We scrap 20 to 25% of product for mostly these 2 issues. The black specks and pitting are not in the same spot every time.

    Picture of the smaller part attached, (did not try to get picture of defects, as they are small and likely too small to capture well in picture.) Other part is similar, just larger, and has similar defects.)

    - Pictured job runs in a 40 ton, and shot size is at 100% of screw capacity (not ideal, I know), Other job in a 88 ton and at 30% shot capacity.

    - edge gated,
    - Hot Sprue/Inlet
    - Material, Acrylic (Acrylite H15, compounded dark grey) dried at 180F for 4 plus hours, and I believe that material is properly dried.

    I have tried many variations of process parameters, so here are the most typical settings....

    Barrel Temps 470 front - 460 Rear, 2 zones dme at 410F
    H20 180
    Back Pressure, 25%, (sorry, working on getting an exact number for this)
    Fill Time - 1.5 seconds.
    Overall Cycle time 35 seconds.

    Running Virgin, have filtered the pellets for fines coming in with raw material. Have pulled and cleaned screws too! Nothing has worked.

    Thx!, Joel
    Attached Thumbnails Attached Thumbnails window1.jpg   window2.jpg  

  2. #2
    Join Date
    Oct 2018
    Posts
    21

    Re: Help: Acrylic Black Specks and Pitting

    Have you looked at the defect under magnification to see if it's a void or something foreign?
    Any chance dust could be getting into the tool while it's open and molding around it
    What about ejector pin flash, that's dislodging, staying in tool and molding around it

  3. #3
    Join Date
    Aug 2019
    Posts
    26

    Re: Help: Acrylic Black Specks and Pitting

    Was the dryer and hoses cleaned properly from the last job? I know you said you cleaned the screw and barrel, but how about the feed throat from the hopper into the press? Usually it's a square with circular opening. Under and around that circular opening there's creases. If you rub your finger inside of there, there's usually always some build up that will randomly get pulled into the new material.

    Fairly difficult to see the defects - but why the 50 degree jump from rear to front zones? Does it freeze off if lowered at the tip / front?

  4. #4
    Join Date
    Dec 2016
    Posts
    121

    Re: Help: Acrylic Black Specks and Pitting

    Hi,

    Yes, Dryer recleaned, hoses recleaned, loader removed and put in hopper via clean bucket, with no difference. Can't see any ej pin flash, dme lowered for stringing, but no difference seen in parts otherwise.

    Thx!
    Joel

  5. #5
    Join Date
    Jun 2014
    Location
    EU
    Posts
    90

    Re: Help: Acrylic Black Specks and Pitting

    Hi,

    two things.

    1) it looks like wrong mixing - the screw is not able to mix the colour into the natural homogenously. Than when injected the non disperged colour makes the dark marking.
    2) to much shear during screwing put inside the tesion which is than relaxed by micro orientation or micro cracking - changing the colour (optical properties) for light coming thru.
    2a) this can appear during cooling time
    2b) after some time of cooling at area temperature

    s:
    1) check the mixing, the full screw capacity isnīt good so try to proove that on the larger machine.
    try to setup the rounds / backpreassure to have the colour-homogenous film when purging out.


    2) a) if its there after ejection - try to prolong the coling time to 1 minute or more (this depends on the wall thickness).
    b) if the part is ok after ejection and the issue appears later - put the parts into the oven tempered about 210°F for 5 mintes, than check

    opt. additional share can be there applied also by wrong hot runners chanels, sharp edges 90° curves.

    Acrylates are very sensitive to any share.

    J.

  6. #6
    Join Date
    Dec 2016
    Posts
    121

    Re: Help: Acrylic Black Specks and Pitting

    OK J

    The material is pre-colored grey, thx for the feedback!

    Joel

  7. #7
    Join Date
    Aug 2011
    Location
    Upstate of South Carolina
    Posts
    652

    Re: Help: Acrylic Black Specks and Pitting

    Has this ever ran good? The pitting sounds like gels or unmelted pellets. The nozzle tips looks incorrect for the sprue too. I would use the Eliminator from Sam Kern or properly sized full internal nozzle tip AND nozzle body! What is the extruded pressure when running? It should be around 800-1200psi or so. If higher then resin is being over heated. Is this resin UV or FR? That will cause it to be VERY shear sensitive and degrade. I think the water temp could be hotter. Are the parts hazy when ejected? That’s typical for this resin because the part isn’t fully setup but you can drop them into clean plastic totes with clean water.
    Rick.

  8. #8
    Join Date
    Dec 2016
    Posts
    121

    Re: Help: Acrylic Black Specks and Pitting

    Thanks Rick, ... will investigate your ideas. Joel

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