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Thread: Hot Runner-Nozzle Heater

  1. #1
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    Sep 2015
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    Hot Runner-Nozzle Heater

    Dear Everyone,

    Good day!

    First I would like to thank everyone especially this site to where I learned the way of scientific molding.
    Now, for my concern, we have molds in our customer that reportedly experiencing frequent busted nozzle. At least two to three times of having this problem for a month of production.
    When we asked them, they mentioned that it is due to type of heater used that is prone to problem. The existing nozzle heater is an INCOE cartridge heater. Now we are planning to reply the whole runner system to make the nozzle heater with cable heater (tubular type). The modification is costly and we need justification that the problem is really because of the type of heater.
    Honestly, I am not familiar with the hot runner system cause for in seven years in the industry, I only handled cold-runner type.

    Your insight on this concern is highly appreciated.

    Thank you very much every one!

  2. #2
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    563

    Re: Hot Runner-Nozzle Heater

    Hello Sammy,
    Thanks for signing up here. In regards to your question, when you say nozzle, are you saying the one on the machine? Please give us a clearer pic and we can give you some suggestions. Yes, it is expensive. I have a feeling that you may just be Pressure limited.
    Let us know.
    Suhas

  3. #3
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    Re: Hot Runner-Nozzle Heater

    Suhas,

    The nozzle I am referring is for the hot runner nozzle.
    Does processing under pressure limited situation weighing

  4. #4
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    Re: Hot Runner-Nozzle Heater

    If the process is using all the pressure the machine has then you are exerting all that pressure in the hot runner nozzle trying to fill the plastic in the last point to fill. Example, if you have a 20 inch long part with one gate on one end, by the time the plastic travels 20 inches not only has the plastic cooled down but because of fountain flow the flow channels is much smaller. Therefore you need a lot of pressure to fill the last say 1 inch. But now the plastic is still molten in the hot runner and so now you are putting all the machine pressure into the hot runner system and can damage it.
    In my opinion, it is not the heater, it is the mold design, placement of gates and of course could be part design.
    Here is what you do quick: Is the required injection pressure = the max machine pressure?? If yes, that is the problem.
    Regards,
    Suhas

  5. #5
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    Re: Hot Runner-Nozzle Heater

    Hi,
    Suhas has right, we need usually more information. what type of materiál are you processing? what temperatures are set/get from the thermocouples for heaters (sometimes happend when you use wrong type of thermocouple you get different REAL temperatures even that required are shown at display). Also what is busted? freezed?

    J.

  6. #6
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    Re: Hot Runner-Nozzle Heater

    Are the failures in the same location? If so that may be a combination of excessive injection pressure and the duration of time the manifold sees that pressure, poor mold or hot runner design, manifold supports. machining limitations, poor fit or expansion consideration of the hot runner when up to temperature. Are you using the correct temperature range in the manifold and barrel and not trying to push cold resin too far into the mold?
    More info would help with a much more exact root cause.
    Rick.

  7. #7
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    Sep 2015
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    Re: Hot Runner-Nozzle Heater

    Dear Rick and Suhas,

    The material is PP and the melt temp is set from 210 to 220 deg. C while the hot runner heater is set from Zone 1 to 7 with 366, 365, 351, 280, 327, 351 and 370 deg. C respectively.
    The design of heater system was made by INCOE, so we have no further details of its effectiveness except in some of our molds, with INCOE hot runner also but we do not know if the design is similar, we barely experience the same problem.
    For the max injection pressure, the max setting is set to 210 bar and actual is 160 bar so I think the process is not pressure limited.
    The failure usually occured at zone 3, 5 and 6.
    I just termed it 'busted' or broken (not working). I do not know if I used the term correctly for this problem.

    In addition, I attached the picture of the drops layout.

    The you so much guys.

    Click image for larger version. 

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ID:	106Click image for larger version. 

Name:	7 Drops HR.jpg 
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  8. #8
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    Re: Hot Runner-Nozzle Heater

    So what you're saying is that you have to replace the heaters because they've stopped heating? Were those temps in C? If so perhaps you should increase the barrel heats so you run the mold heats lower. Also do you use heat transfer paste on the heaters you replace? Not anti-seize as it will short heaters out when it gets hot due to the conductivity of the metal flakes. Lastly what are your water temp settings? Does the mold have any internal water leaks that could be grounding the heaters out? I've seen some molds leak only when under tonnage but be perfect when checked in the shop. Also be careful of the wire routing when replacing heaters and tape them up with high temp fabric tape made for the job.
    Rick.

  9. #9
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    Sep 2015
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    Re: Hot Runner-Nozzle Heater

    Dear Rick,

    Yes. That is actually what is happening. We are also looking for the possibility of suggesting to change the setting to increase the barrel temp instead of the runner heaters.
    Yes. The temperature is in Celsius. The setting of coolant for the core and cavity is set at around 25~28 deg.C (chilled water). As for the heat transfer paste, I am not sure if they are using such but I am glad you mentioned it. I will take a sneak peak on their PM process to see if they do.The last time I checked the condition, we have seen no leaking water in anywhere that may close to the heater but we are looking for the possibility of moist (from air) that may develop as heat and very cold medium interacts. Do you think this is reasonable to consider?

    Thank you very much!

    Sammy

  10. #10
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    Re: Hot Runner-Nozzle Heater

    First rise temperatures on barrell to 235-240 °C.
    Second the temperatures on HR are too high. Set them +5-10°C higher than barell temperatures.

    What is the closing/opening mechanism on HR nozzles? are they self closing, pneumatic or hydraulic controled?

  11. #11
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    Re: Hot Runner-Nozzle Heater

    Sammy the extreme high heat is shortening the life of the heaters but there is another method that really reduces heater life span, not using soft start when the mold heats are turned on!! Check and most likely you'll find out the cold start or evaporation mode is disabled to start the mold sooner. When cold, some moisture will be in a heater and if the amount of current is too high, that moisture when released can short the heater internally. I'm sure the thermal shock hurts the heater elements as well; room temp one moment and red hot the next. Also I think that the mold might suffer from setting it in the press without pre-heating the hot runner.
    The reason I asked about the water is there may be too much surface contact between the cold section of the mold and the hot runner. That's another reason why the heats are so high as the manifold is losing so much heat to the mold steel. I worked on an early manifold system that used the gas inside to transfer the heat around the entire system. We had an issue with too much heat transfer so we made insulating crush rings out of alternating pieces of brass and stainless steel. No more issues. Last possibility: when pulling the mold for change over, depending on mold design and water fittings it could be easy to not drain and blow out the water lines, disconnect tyem while under pressure and the cold water spray, run, drip, or soak onto the hot runner.
    Rick.

  12. #12
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    Sep 2015
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    Re: Hot Runner-Nozzle Heater

    Rick,

    That is quite an explanation but for that, I really got the point.
    Tomorrow I will visit the plant for they reported another similar problem. I will apply all the information I have learned and see what will happen.

    Thank you very much and I will make sure to get back for the update.

    Sammy

  13. #13
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    Sep 2015
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    Re: Hot Runner-Nozzle Heater

    Jendalf,

    Sorry but I dot quote understand your second instruction, are your saying that I should increase the HR temp than of the barrel with +5/-10 deg. C variance?
    I appreciate if you make it clearer.

    I am not sure of the system for closing and opening. I will get back to you on this.

    Thank you so much!

    Sammy

  14. #14
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    Re: Hot Runner-Nozzle Heater

    If you setup the temperature on the barrel 240 °C the hot runners temperature should be around 250°C generally for PP. But def. not 380!

  15. #15
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    Sep 2015
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    Re: Hot Runner-Nozzle Heater

    Jendalf,

    Yes. I believe that also. Perhaps the heater actual power output is already below the input value. I will consider also checking the calibration status of the heater.

    Thank you!

    Sammy

  16. #16
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    Re: Hot Runner-Nozzle Heater

    There is another issue that may be at work here. The hot tips grow as they are heated and on occasion they grow too tall and create thin areas in the part, which makes fill the part difficult. Also some drops have tips inside them to control freeze off and stringing/gate vestige. At times the tip inside grows too much restricting the resin flow path. This can usually be determined by measuring the assembly at temp or by visual inspection while in the mold.
    Rick.

  17. #17
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    Re: Hot Runner-Nozzle Heater

    Both are possible, but heat influence on thermal behavior of the functional units of the hot runners is more probable.

    note (out of theme):
    I´v challenged simmilar effect. Problem here was the pneumatic controled needle malfunction due to the low temp of cooling water. The cooling water that cool down the base of the needle (that is connected to the manifold) caused the tightening of its diameter around the needle. The pneumatic wasnt able to moove with the needle (due to the friction) and that casued me short shots. Quite interesting

    J

  18. #18
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    Sep 2015
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    Re: Hot Runner-Nozzle Heater

    Rick and Jendalf,

    Please correct me if my understanding is not inline with what you wanted to explain, but this tip expansion basically restrict flows, correct?
    Does it really causes the nozzle heater got shorted easily? Does build up pressure inside the hot runner passage affects the heater itself?

    Sammy

  19. #19
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    Re: Hot Runner-Nozzle Heater

    Sammy,
    nothing is 100% sure here, but the heat influence is one of the most probable option. Especially because of the high temperatures set on HR.
    Another thing is that if the nozzle is selfclose the function unit inside can be weared or broken or it can contain the forreign element inside (like metal).

    J

  20. #20
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    Re: Hot Runner-Nozzle Heater

    Sammy, the tip growth from heat restricts melt flow. To compensate for the loss, the heat has to be cranked up really high to get the resin to start flowing again.
    Obviously there's an issue with the hot runner and tooling due to the fact that you know which zones the failure occurs. Be that too much cooling or what have you. The hot runner is an extension of the barrel so it should be close to the same heat range. Then only slight differences should be required to balance the cavities. It could be that the gate inserts are wrong and that is driving the crazy high heats.
    Rick.

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