material for fluctuating temperatures 3

[metalfinder]

I received the following question from one of my customers. I would have suggested the Invar 36 if they had not already been using it. Any suggestions?

"We have a machine – short description:

· We apply electrical current (36 Volts, 40 amps max, total power is 1440 watts) to a piece of Invar A36 and / or Titanium Grade 2 ( a picture is shown below). We have used both the Invar and Titanium with very little success.

· The metal (from this point called a Knife) gets heated up to a constant temperature of 310 degrees F. The heating of the knife is based on the resistance of the material with power running through it.

· During the running of the machine we cycle a spike program to increase the temperature on the knife up to 600 degrees F. The spiked current is in milli seconds.

· This cycling happens every 1 seconds.

· What we are doing is cutting through a plastic bag and sealing the cut edges.

· The knife is buried in Macor Ceramic and has 0.010 inch exposed. The knife radiates the heat to the ceramics and the ceramics actually does the sealing, while spiking the knife allows the cut to take place.



· Here is the issue. The bag have liquid in them that is 180 degrees F. Every once in a while we get a leaker happening and it leaks liquid onto this knife. What we believe is happening is because we are constantly heating and cooling the knife that we maybe tempering the metal and ultimately fatiguing it because after so many the knives break – they actually burn out like a light bulb.



· So what I am looking for is a metal material that I can apply a constant voltage to it, bringing it temperature to roughly 310 F, spike it 600 degrees F for milli seconds that would not be affected if we rapidly cooled it down to 180 degrees F by having fluid leak on it.



If you can offer help please do! This is a real issue for us and I need solutions now. I realize that you (who is receiving this email) might not be able to help, but if you have a material expert in house or someone you know I would appreciate the help.

 

[Compositepro]

It is a impulse heat sealing machine and the "knife" is a metal strip heating element. These elements are very prone to burning out because any small flaw, corrosion or irregularity will increase the resistance in that spot which cause the temperature spikes to get much higher, until eventually you have a burn-out.

The temperature at any point is a balance between heat generated and heat flow out. Any dirt or flaw in the rubber that presses the bag to the heater strip or "knife" will lead to a hot spot. They may have more of a maintenance or housekeeping problem more than a material problem. "Leakers" are already and indication that something needs to be fixed.

 

[davefitz]

a 300F increase in temperature will likely cause some areas of the knife to expereience high thermal stresses above 1.5* yield. ( E*alpha*DT/(1-poisons) = rough idea of stress magnitude, 300F= DT

you need to model this ina finite element model, including contact conductance to the adjoining cooler ceramic, to confirm the stress distribution during 1 complete cycle. Then hypothesize a modified part geometry which has thermal stress reliefs included.

 

[EdStainless]

I would suggest that they stop relying on resistance heating.
Then they should consider moving to heating by induction and using a knife alloy that is meant for thermal shock.
Perhaps one of the controlled expansion Ni alloys, such as 903 or 909. These alloys are designed to resist thermal fatigue.
Another alloy option (but an exotic one) would be beryllium nickel.

The back edge of the knife would have the induction coil. These are usually water cooled copper. This would allow rapid uniform heating of the knife. The induction power

The problem with resistance heating is that the hot places have a higher resistance, so they get even hotter. I'll wager that they are way over heating some locations on the knife.

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Plymouth Tube

 

[unclesyd]

We used numerous hot knives of the Handheld type to cut fiber tow. Some of these guns go on and off a couple hundred times a shift.
You might want to reverse engineer one.

We also had several of the bench mount boxes to cut the heavier fiber tow. The blades on these machines are some type carbide. I don't know which one as we were never called about a problem. Buy some blades and adapt them.

https://www.wirecare.com/land_hotknife.asp