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Injection Molding Press -- Speed and Pressures

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  • Injection Molding Press -- Speed and Pressures

    I am re-working a fairly simple ladder logic program for an injection molding machine and want to use stages. DL250
    I've got a decent handle on stages but have a few gaps. I'm sure I'll have many questions but first off: speeds and pressures.
    At each step of the molding process I will potentially be changing speeds and pressures. I'd like to have one sub-routine or stage for speeds and one for pressures. Both speeds an pressures are changed by combining 5-7 outputs (hydraulic valves).
    Some steps are sequential and some are parallel. For instance during the clamp closed step, the carriage forward, injection, screw rotate, suck back, carriage back, steps all can occur. Each of these steps could have independent speeds and pressures. The clamp closed and stuffer up/down steps are the only steps that can be combined in parallel with other steps.
    These steps can occur in automatic operation or in manual by operator using selector switches.
    Should I use stage or sub-routine for speeds and pressures?
    Would it be easier to set a bit stating what steps are active?

  • #2
    I would suggest not using subroutines. There are a lot of gotcha's with the way they operate. Stage would most likely be a better choice.
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    • #3
      A few questions come to mind on the Output end of things..

      What speed are we controlling.. ?

      How is the speed Called?.. is it 4-20 MA output to a Speed Valve?.. Or is it some sort of Multi Valved Affair.. with multiple outputs to get the speed you want.?

      Is there only ONE speed Valve set up.. or do the different devices have a speed control.. ?

      For smaller Ladder controls, I try to start out in Stage Programming.. but when things get complicated I revert back to Subroutines.. I can't help it.. That is my native language..



      • #4
        Speeds are controlled by six valves. You combine the valves to get the speed. Haven't actually figured out how the speeds work yet. Full hydraulic print is here (very large image)
        There are two pumps (I assume one is higher volume) that are split during the injection cycle. This may or may not change how the speeds work.

        Pressure is also controlled by six valves that are stepped at 2.2 bar increments. Pressures from 0-138.6 bars in 2.2 increments depending on the valve combinations. Pressure table

        At most every step on the molding process I need to change speed and pressure.


        • #5
          Originally posted by markbann View Post
          Haven't actually figured out how the speeds work yet.

          combine the valves using your pressure table... it's a binary chart, so you use OUTF over the output range

          Originally posted by markbann View Post
          At most every step on the molding process I need to change speed and pressure.

          have you looked into the EDRUM?

          On a much further note, have you looked into the Do-More CPU for this application? Just curious


          • #6
            If it were me, I would use a sequencer. I have made compression press control programs before using sequencers and it really makes things simple - this made it easier for the back shifts and so made for fewer midnight calls for me.

            Not sure why more programmers don't use sequencers.

            “Felix, qui potuit rerum cognoscere causas” - Happiness is understanding how things work.


            • #7
              All fairly straight foreward stuff..

              I see a 6 Bit word that is output from a Pressure curve table that you make.. to give you the pressure that you need at the particular point in the injection process.. and it is output from the table

              Next Question.. During the injection Cycle, is the pressure set by Tine from start, or do you have an Encoder hooked to the screw position, to shift pressure at different places in the Screw position


              • #8
                Haven't looked at either of those. Was hoping just to use what I have.


                • #9
                  Just using one pressure/speed during injection but if I need more I would do it based on a timer.


                  • #10
                    OK.. Lets go simple for now..

                    1) During Platten movement ( Before Clamp ) you have a certain Valve combo to give you relative movement during 'Auto' Operation.. It's unknown if you want that same speed during Manual Movement..
                    2) Before clamp you set another pressure to verify the mold is clear and ready for clamp..
                    3) After your 'Clamp Limit Switch' makes, you will hit a 'Boost pressure' to get the thing closed.. then turn off the Clamp Cylinder.

                    During Injection, you will ( Likely ) have an Initial injection Speed ( Pressure ) with a Holding Pressure after a certain time.. Again this will be done with Timers and likely limit switch conformation that the screw is where it's required..

                    Again the Screw Operations will be Speed/Pressure related.. back to a limit switch..

                    I'm seeing a truth table of Pressure Output, for different stages of machine operation.. with an E-Drum type of controller calling the different routines..

                    Can be done with Sub Routines that are like little Modules that only do certain things, then something calls them is a certain order..

                    I Like the sub routine type of programming.. it allows stand alone modules that can be proofed and debugged, then linked by a main program.. I'd be in the do more by now, but I'm to busy installing and programming to learn a new horse..



                    • #11
                      I use subroutines all the time but not in the conventional sense. They are called every scan - I use them mainly to separate large programs into sections on PLC's that do not support other structuring methods. I also never use stages either although some people like them.


                      • #12
                        What did this press originally have for controls? Do you have any documentation of the original sequence to go by?

                        “Felix, qui potuit rerum cognoscere causas” - Happiness is understanding how things work.


                        • #13
                          KB1GNI: Used an old Siemens PLC. It died. Don't much of the original documentation. Complete hydraulic, some mechanical but nothing on the original sequences.


                          • #14
                            What brand PLC? Some brands I've done (Cincinnati Milacron and NATCO) have really good sequence charts for the hydraulic valves. Others, not so much.

                            I never used stages because I didn't understand them at the time. Now that I can do both, I still wouldn't use them for this application. There are too many things that have to keep happening at the same time during the cycle. I used a series of holding circuits (rung 1 output holds in rung 2, rung 2 outputs hold in rung 3, etc). It's harder to initially debug this way, but a lot safer to man/mold/machine. There are just so many different scenarios between different molds, core sequences, ejector sequences, auto and semi-auto cycles, etc. I felt better programming this way so that if anything, anywhere went awry, I knew the machine would stop. With stages, you'd have to predict a lot of these circumstances and create logic to deal with them.

                            Also, I'd recommend writing the program to turn on bits for each of the machine functions (i.e. clamp close fast, clamp close slow, ejector retract, etc) then having a separate section of the program that determines which outputs are turned on during each function of the machine. It makes the program a lot simpler, and gives you a template that you can apply to other brands of machines with different hydraulic sequences.

                            Lastly, I'd highly recommend purchasing a copy of ANSI/SPI B151.1-2007 (Plastics Machinery -Horizontal Injection Molding Machines -Safety Requirements for Manufacture, Care, and Use). There are a lot of pretty specific safety standards in there that will help you do this properly. If you plan to add a robot interface, I'd also get a copy of ANSI/SPI B151.27-201X (Safety Requirements for the Integration of Robots with Injection Molding Machines). The standards are about $50 each at
                            Last edited by pressgrove; 12-13-2013, 04:05 PM.


                            • #15
                              Pressgrove: Press is Italian, by MIR. Has some fairly unique features. PLC is automatic direct.
                              Holding circuits vs stages:
                              Again, I know programing but have limited PLC experience. Turning stages off and on at least in theory looks to me like a simple way to do things. They put every action in discrete logical areas and I can turn on any combination. Maybe this is just my lack of experience with stages....

                              ANSI guides:
                              Good idea. I'm working from an already functioning (but limited) ladder program that has been working for a while and seems to adhere to standards. Some of the safety stuff is still original hard wired stuff and mechanical interlocks of course are in place. E-stop kills power and properly resets PLC. Logic for safety gates and doors works well also. As long as I can keep that logic I'm thinking I will be OK on safety.