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Thread: Back Pressure

  1. #1
    Join Date
    Sep 2020
    Posts
    28

    Back Pressure

    When we should use back pressure?
    How does it affect the product?

  2. #2
    Join Date
    Aug 2011
    Posts
    616

    Re: Back Pressure

    You should always add Back Pressure. There is a way to set back pressure but for now since you are a beginner, set it to about 7000 Plastic Psi or about 700 hydraulic psi.
    If you do not add back pressure you can have splay on the parts. You can also have shot to shot inconsistency.
    Suhas

  3. #3

    Re: Back Pressure

    Depending on what molding machine your working with you will likely be limited to approximate 300 psi maximum (hydraulic pressure), 150 to 200 psi of back pressure is typically a good range for most materials. Back pressure is intended to provide some resistance against the screw during shot size recovery to improve the melt and shot size consistency as Suhas stated. Excessively high back pressures will introduce greater wear on the screw tip and check ring.

  4. #4
    Join Date
    Dec 2012
    Location
    Serbia
    Posts
    98

    Re: Back Pressure

    Usually there is recommended values for back preassure in material datasheet. Is most of the time OK value.

  5. #5
    Join Date
    Jun 2021
    Posts
    3

    Re: Back Pressure

    As the above have stated back pressure should always be used. When using a natural material and a master batch/pigment back pressure will help these mix into a uniform colour.

  6. #6
    Join Date
    Aug 2018
    Posts
    1

    Re: Back Pressure

    I make closures, high cavitation molds (32, 64, 72, 96, cavities), hot runner systems. A lot of the processors don't use back pressure because it results in gate strings that are difficult to remedy. How can I fix this to show the others how it could be a benefit.

  7. #7
    Join Date
    Sep 2018
    Location
    Portland, OR
    Posts
    44

    Re: Back Pressure

    What benefit are you hoping to get by changing the process?

  8. #8
    Join Date
    Aug 2011
    Location
    Upstate of South Carolina
    Posts
    685

    Re: Back Pressure

    Do you have a shut off nozzle? Without it, and without the proper software in some machines, you cannot continue to recover the screw while the mold opens and the parts are ejected; for the absolute fastest cycle times. You leave the nozzle open, while you decompress the screw, shut the nozzle, and you can recover the screw while the mold opens, ejects parts, and even closes back.
    To do this properly, you need more than one motor/pump on a hydraulic press (one runs clamp and the other the injection unit) or on a hybrid you need electric screw drive!
    Back pressure is a MUST for a stable process AND consistent dimensions!
    Rick

  9. #9
    Join Date
    Aug 2011
    Location
    Upstate of South Carolina
    Posts
    685

    Re: Back Pressure

    Do you have a shut off nozzle? Without it, and without the proper software in some machines, you cannot continue to recover the screw while the mold opens and the parts are ejected; for the absolute fastest cycle times. You leave the nozzle open, while you decompress the screw, shut the nozzle, and you can recover the screw while the mold opens, ejects parts, and even closes back.
    To do this properly, you need more than one motor/pump on a hydraulic press (one runs clamp and the other the injection unit) or on a hybrid you need electric screw drive!
    Back pressure is a MUST for a stable process AND consistent dimensions!
    Rick

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