Press heat control and thermocouples
The press is heated with two custom silicon heat blankets from MEI, each one 14” x 85”, 2,083 watts running on 240v. They heat very quickly and evenly, are flexible, and they can take the pressure within the press. The temperature must be ramped up from room temperature to 175F over about 10 min, then held within +/- 5 degrees of that for one hour, and these blankets coupled with a good controller get the job done well.
The heat is controlled by a custom-built controller using a solid state relay, a PID controller with ramp/soak programming features, an contactor, a few fuses, and a couple of thermocouples. We’ve not taken the time to draw a schematic, but the circuit is pretty straightforward. Power comes in to the contactor, and from there is distributed to conectors for the heat blankets. One of the hot leads is passed through a solid state relay (SSR), which is connected to the ramp/soak PID controller. A thermocouple is connected to the PID controller as well, with the sensing end placed with the heat blankets. The SSR can switch may times a second if necessary allowing pretty fine control over how much heat is delivered via the blankets.
The blankets are connected with normal 240v 15A plugs. Being able to disconnect the blankets is helpful sometimes when working on the press. The thermocouples are also connected with plugs which allows us to place the thermocouples in with the blankets before loading the laminate, and to easily replace thermocouples if they get damanged.
The SSR is a Continental RVDA rated for 25A.
The contactor with panel-mount switch is part #SD1-025-RR from automationdirect.com.
The thermocouples are from Omega, part #5SRTC-TT-K-36-36 for 5 type K thermocouples, and #RMJ-K-S for the connectors on the control box. The thermocouples are 36AWG (0.005”), and are therefore small enough to be placed directly on top of the heat blankets without harming the blanket. I place one with each of the blankets, with the sensor end in the center of the blanket. The thermocouple on the bottom is conntected to the ramp/soak PID controller, while the one on the top is used simply to monitor the temperature differential between the two sides of the laminate.