Hello,
Heat shock is generally thought of as a test of cure (or over cure). As you say the enamels you are using are from reputable companies. You take a sample, elongate it some, wrap it around a mandrel, and then place in a lab oven for a period of time at a temperature that is 20 degrees hotter than the thermal rating of the wire.
Over the years I have learned a few things:
1) Get input from the enamel supplier as to suggested temperature profile in the oven.
2) Get input from the oven manufacturer regarding what they think are good temperature speeds.
The best performance occurs when you set the temperature profile, air flow, fan speed, and wire speed for each type enamel and wire size. This means you are operating at optimal conditions.
If you try to run all products and enamels and only vary the speed, you compromise production and quality.
There is no normal reason that I can think of that all of your enamels would fail heat shock. My first thought is that I would check the accuracy of my test oven. Normally heat shock tests are 20 degrees over the thermal rating of the wire for a specific time. If your oven is off even a little it would make a difference.
If the lab oven temperatures are good, then you need to look at your oven. Maybe you have a hot spot or the wire is dragging on the oven or touching a heating element (assuming they are in the wire chamber).
- Is the wire centered in the wire chamber?
- Is it too close to one wall?
- Are you running the oven too hot?
- What kind of oven are you using? Electric? Radiant? Recirculating? Single direction air flow?
- Does the air flow with the direction of the wire or against the movement of the wire?
There are a lot of questions that perhaps you can answer.
Richard
