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  • Injection Molding Nozzle flow

    I am new to the world of injection molding. I saw in a similar post that some of you have experience in this field.
    I have a Boy 15s that I retrofitted with a PLC. The PLC allowed me to add features such as having a motor spin to release threaded parts. I am using PVC. Between parts, I get about an inch of ooze. My first concern is the nozzle. The bore of the nozzle is large (0.109"). My sprue bushing starts at about 0.15". My machine shot size is 36 cm3. My part is about 14 cm3.
    Can I use a smaller nozzle bore? What would be the minimum bore?
    I fabricated the die with gearing to unscrew a pair of plastic parts
    My parts are perfect but I want to optimize my system.

    Having never seen this model of machine in operation, I only have you tube videos for dynamic reference.

    I used the original German/English processing schematic to make my steps. I may be doing something wrong. Here are my process steps. Please feel free to comment.

    1. Mold is open and an open switch is triggered.
    2. Motor spins and unscrews the part.
    3. Drop chute is now triggered by hand or by part dropping.
    4. Mold closes and close switch is triggered.
    5. High pressure clamp is activated.
    6. Nozzle is driven into mold and a switch is triggered.
    7. Injector cylinder injects about 40% of a shot into mold.
    7a. A switch is triggered to let PLC know that the Injector reached about 35%.
    8. Injector pressure is held for a few seconds to complete fill.
    9. Injector pressure is released.
    10. Auger spins and reloads to the point of setting a switch.
    11. High pressure clamp is released.
    12. Mold opens and nozzle retracts from sprue to return limit switch. (Nozzle retracts about an inch. Any more is wasted material. Any less will build up around the sprue bushing.)
    13. After open limit switch on mold and retract limit switch on nozzle carriage are set, ejector motor spins.
    Goto #3

    Should I add any more steps?
    I considered adding a dwell time after #10 to allow for the pressure to relax. I did not see one the original bilingual schematics.

    Since the parts are small, I plan to play with the order by injecting the resin and then using the high pressure clamp to "coin" the part.

    Since I am using PVC, I have to purge the system with PE after I stop for the day and spray all of the exposed parts with oil. It can take a couple of hours to get the machine flowing with PVC again. Therefore, I do not have time to try a new idea every day.

    Without adding a nozzle valve, I am considering machining a 1/16" (1.5mm) bore nozzle out of D2 steel. I could then possibly retract the nozzle only a few mm.
    Any thoughts?
    Thank you in advance.

  • #2
    Ah-nother vis-itor.... STAY awhile. STAY FOREVER.

    http://plastic-fantastic.weebly.com/

    This is what I do for a living, on paper anyway... Currently I'm between paying clients but --- lets see what we can do.

    Boy is a machine I know all too well. Can you post pictures, model, date of MFG, tonnage and shot size if you have them? Also details on your PLC. This will help me troubleshoot.

    First of all it should not take a couple of hours to get the machine flowing again. You may have a heater fault. Typically it takes an hour of heatup to get material moving again. Try this. When you finish your run, rotate the screw until no more material comes out, then "shoot it forward" to get the last bit out of the barrel. Trying to heat a barrel full of PE will give you fits.

    Next. You state you have what's called a "drooling" issue. If the machine oozes between shots we have a couple of remedy to try. We'll go down the troubleshooting list here.

    1) Do you dry your material at all in ovens or hoppers? If not, the material picks up atmospheric moisture, which boils off in the barrel.... creating steam pressure. This manifests as drooling. You can also have serious part stability issues too if you do not dry.

    2) Next step make sure you are running in the specified range of temperatures for your material. Having a properly functioning heater system is critical with this process. Make sure all your bands and all your controllers are working and your TCs are working.

    3) Close mold, inject sequence (boost-pack-hold-recharge-decompress). You should have a "suckback" or "screw retract" or "decompress" option somewhere. This is vital. After you pick up the next shot the barrel will be under pressure. Opening the mold and pulling the sprue will cause the material to drool if it is not decompressed. Check this setting. If your PLC doesn't have that function, add it. Event should happen after inject-forward, after screw-rotate-stop.

    4) I don't know where you live but there are non-mechanical drool-stop nozzles such as "Nylon Nozzles" and nozzles with reversed tapers internally. Nozzle-tips for a Boy are fairly inexpensive and can be bought from the major supply houses. Let me know your location and I'll get that information to you.

    Last bit of insight: Honestly, some material just drools no matter what we do. An inch is excessive though. That's a processing issue, and I'm fairly confident we can get it resolved.

    Don't hesitate to contact me at [email protected], as I said I'm between paying clients at the moment... this will give me something to work on until business picks up.

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