Best way to add PID to thermoform machine?
Vacuum forming is a bit of a special plastics application. Usually you're using IR heating of the plastic (that calrod in the lid) and it's difficult to get a temperature reading of the plastic directly.
You might do better to measure the droop in the plastic or something of that ilk.
If your mains voltage is steady the power output of the heater will be steady.
Edit:
I'm not too sure about your original wiring diagram. I guess the switches are illuminated and there are some lamp pins, but it looks like they're switching the line on one switch and the neutral on the other which isn't kosher.
But anyway, I think what you want to do is take the two wires that go to the heater and connect the temperature controller, SSR and heater to those two wires only (plus ground if there's a spot on the controller for it, and ground on the SSR heatsink).
So the two dark gray wires power the controller, one side of it goes through the SSR to one side of the heater, the other side directly to the heater. The +/- drive for the SSR and the sensor also need to be connected (both with the correct polarity).
Remember that the controller can only reduce the heat from the heater element, so the sizing of the heater will be considerably underpowered most likely. You probably would do better to replace the heater with a more powerful one, and add an overtemperature cutoff thermal fuse so when (not if) the SSR fails on it won't cause a fire.



simulate this circuit &ndash; Schematic created using CircuitLab