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  1. #1
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    Flat NiCr resistance tape temperature controller using pulse methode

    I need a temperature controller for low density linear polyethylene plastic bag sealing system using flat nickel chromium resistors with pulse method.

    Regards,

    Fernando

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    Re: Flat NiCr resistance tape temperature controller using pulse methode

    This sounds like two long clamps which are heated almost red hot, then are clamped onto a wide bag. The temperature, pressure, and clamp time must be exactly right, so as to melt the two layers of plastic just hot enough so they adhere, yet not overheat which might create blobs and holes at some spots.

    Do you have a temperature sensor in mind? Do you need to send different Ampere levels through the nichrome?



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    Re: Flat NiCr resistance tape temperature controller using pulse methode

    I need a temperature controller for...
    The statement ignores that these sealing system usually work without temperature control, they just adjust the pulse energy, either by varying the voltage or preferably the pulse duration.

    Attaching a temperature probe to the Nichrome tape isn't easy and won't necessarily provide reliable by-pulse temperature control due to the measurement dead time.



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    Re: Flat NiCr resistance tape temperature controller using pulse methode

    Hi,

    I wonder why it is NiCr.
    If the characteristic is NTC style, then you may use current control.. and temperature is relatively stable over the total length.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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    Re: Flat NiCr resistance tape temperature controller using pulse methode

    I presume there's no suitable NTC heater material available working in this situation.

    The bag sealing machines are industry standard products since at least 50 years. As most production equipment, it needs an experienced technician for setup.

    I did some experiments with temperature control of a similar machine as student. If I remember right, it didn't work well.



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    Re: Flat NiCr resistance tape temperature controller using pulse methode

    Hi,

    for sure you know..the problem with metal (= PTC) is that the coldest spot generates the least power but ideally it should be the other way round.
    So maybe NiCr is used because it has a better behaviour than most pure metals...


    Klaus
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    Re: Flat NiCr resistance tape temperature controller using pulse methode

    Nichrome (any used alloy, e.g. Ni80Cr20 or Ni90Cr10 has relative low T.C., I won't consider it as PTC.



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    Re: Flat NiCr resistance tape temperature controller using pulse methode

    Hi,

    yes, I know.

    Ideally one wants NTC for good thermal performance = equal temperature across that whole length..
    Pure metal are PTC.

    NiCr is somewhere inbetween. Thus - I thought - NiCr is better than pure metal.

    But I just read about NiCr. Benefits are: No/less corrosion, high temperature...

    Klaus
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    Re: Flat NiCr resistance tape temperature controller using pulse methode

    Nichrome has low temp coefficient and hence not suitable for reliable temp measurements.

    It is better consider a fixed power device. Consider the following steps:

    1. Some energy is dumped onto the resistor. The pulse is turned off before the sealing takes place.

    2. The temp of the heater increases - also the sealing edge is heated up to the required temp - perhaps a little bit higher than the required.

    3. The sealing edge is pressed against the plastic and the seal is made.

    Between seals the sealing knife does not cool down to the ambient temp and hence smaller energy pulses may be needed for subsequent seals.

    The ambient temp need to be considered so that the correct temp on the sealing edge may be maintained.

    The frequency of sealing (when the major energy is dissipated) is also need to be considered.

    The shape and the heat capacity of the sealing edge (thermometric conductivity) is most important so that the max heat is transferred to the plastic to be sealed.

    If the NiCr strip is THE sealing knife itself, you need to consider covering it (say PTFE) so that sufficient energy is transferred to the plastic (bare metal will not be suitable).



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    Re: Flat NiCr resistance tape temperature controller using pulse methode

    Very thank you everyone for your tips and information. I'm in contact with an Italian manufacturer of this type of sealing machine and seems that he will give me all info I need.

    Regards,

    Fernando.



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