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Thread: Velocity-Pressure-Time Parameters

  1. #1
    Join Date
    Sep 2020
    Posts
    1

    Velocity-Pressure-Time Parameters

    ı use moldflow for analysis but there's some points that ı cant understand
    For fill analysis ı can use just one parameter(like ınjection time)
    However while ı am entering some values to ınjection machine, it wants velocity,pressure and ınjection time.
    If ı control fill analysis with ınjection time , appears some results in moldflow.
    Example; according to results that from moldflow, ıf ı keep ınjection time 3sn , ı need 30 mpa ınjection pressure and max velocity 107 cm3/s.
    should ı use this results while ı entering values to ınjection machine ı?
    If ı keep constant a parameter except ınjection time(example max pressure) and ı reduce or increase what would happen?(ı ask this because ı cant do that with moldflow. Software let just one parameter that ı can control fill stage.)

    sorry for my english
    thanks your answers

  2. #2
    Join Date
    Oct 2018
    Posts
    18

    Re: Velocity-Pressure-Time Parameters

    If you keep a constant parameter specifically Max Injection pressure, you risk the possibility of over packing the mold during the first cycles resulting in flashed parts, sticking or stuck parts inside the tooling and/or tooling damage. The mold flow software is telling you need the 30 mpa injection pressure and 107 cm/3s to fill the mold in 3.0 secs. These numbers seem high to me. Increasing the fill time in your mold flow program, you should see the injection pressure and velocity requirements go down. Without knowing the material, part information,tooling details and machine capabilities it is not possible to give a solid answer for a starting point. Other then using fill time to control the process is the least desirable, better to use shot position transfer or Peak Hydraulic pressure as a transfer point. If possible start with the shot size set less then a full shot and increase the amount over a few cycles till you have full parts. However under filled parts can stick in the molds as well depending on factors already listed. Its still better to start with lower pressures in most cases.

  3. #3
    Join Date
    Aug 2014
    Location
    Saint Paul, MN
    Posts
    166

    Re: Velocity-Pressure-Time Parameters

    I assume you are using two-stage molding (fill and pack/hold). You will typically control the fill stage by velocity. In this case, fill time and maximum injection pressure are machine outputs, not inputs. (This is why moldflow only lets you input one of the three parameters). What the machine is asking for is safety limits – you do not want the machine applying high pressure for a long time if a runner or a gate is blocked, and the screw cannot reach transfer position. So, if your maximum injection pressure really is 30 mpa, you can set the limit to 33 or 35 mpa and if your fill time really is 3 seconds, you can set the limit to 4 or 5 seconds.

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