Heat transfer issue

[fcesar]

On our equipment, we have a tool that closes on a film and welds the 2 parts on PP film. Like closing the top (or bottom) of a cereal bag, potatoe chips, Doritos, etc.
We are having problems maintaning the temperature on this tool when the machine stops and starts. On some of the tool, the temperature goes up 10 deg when the equipment stops and down 10 when it starts. The temperature stabilizes quickly and there is little to no variation when the machine is running.
When changing the PI&D controler parameters, all it changes is the amount of time it takes for the temperature to stabilize, but it changes nothing on the over/under shooting when the machine stops /starts.
I'm not an expert on TD but logic tells me this could be the fit between the heater unit and the tooling. If the fit is not tight, I will not have a good heat transfer.
Does anyone have any experience with something like this or any other possible cause?
The heater is a 500W cylinder the fits inside at the same length as the tool. The RTD (a PT100)is on the center of the tool between the heater and the face of the tool where it contacts the film.
Thanks

 

[Compositepro]

It is not likely that the problem is heater fit. First questions are how accurate or consistent must the temperature be for the process to work and what problems are you having?

When you have heat flow there must be temperature differences. Having the RTD between the heat source and the load is the typical arrangement but it only measures the average temperature between heat source and controlled surface. If the load changes there will still be a change in the sealing temperature even though the thermocouple reads the same.

The issues you must address are thermal resistance and time delay between load change and actually measuring the temperature change. You can get rid of some of the time delay by using two thermocouples wired in parallel. One placed very close to your tool surface and the other very close to the heater. This way they will respond to load changes much faster (also, RTD's are typically not designed for fast response). There will still be temperature gradient from heater to tool surface.

Heatpipe technology can also reduce time delay and temperature gradients.

Yet another approach is feed forward control where you control heater power proportional to line speed and have a temperature feedback loop to fine tune the temperature. Given your process, I would guess that for a proper seal you actually want the tool temperature hotter at higher line speeds.

Your problem is probably more complicated than you realize but is a common one. I see many machines that are designed only for steady state operation and thus handle starts and stops poorly.

 

[fcesar]

The temperature we are keeping is 215 C. Trip point + - 7.
While running, the system maintain dead on 215. If the PI&D is set correct, it goes up and down a couple degrees before stabilizing.
The variation can be as high as 12-15 deg up and 10 deg down.
All the tools we have show variation when start and stop but they vary at most 5 deg staying between the trip points. Some of them just "refuses to cooperate".
You gave me some idea on where to look. There might be some slight differences between the tools and the assembly of the heater and RTD.
Thank you very much

 

[MintJulep]

As my guess I'll suggest that you need to change the derivative gain on the controller.