Startup and operation
Version 1
RES-408
Page 27
10.3
Heatsealing band
10.3.1 General
The heatsealing band is a key component in the control loop, since it is both a heating element and a sensor. The
geometry of the heatsealing band is too complex to be discussed at length here. We shall therefore only refer to
a few of the most important physical and electrical properties.
The measuring principle applied for this system necessitates a heatsealing band alloy with a suitable temperature
coefficient TCR, i.e. one whose resistance increases as the temperature rises.
Too low a TCR leads to oscillation or uncontrolled heating.
If a heatsealing band with a higher TCR is used, the controller must be specially calibrated.
The first time the heatsealing band is heated to approximately 200…250°C, the standard alloy undergoes a once-
only resistance change (burn-in effect). The cold resistance of the heatsealing band is reduced by approximately
2…3%. However, this at first glance slight resistance change results in a zero point error of 20…30°C. The zero
point must therefore be corrected after a few heating cycles (
section 10.3.2 "Burning in the heatsealing band"
on page 27).
One very important design feature is the copper or silver-plating of the heatsealing band ends. Cold ends allow
the temperature to be controlled precisely and increase the life of the teflon coating and the heatsealing band.
An overheated or burned-out heatsealing band must no longer be used because the TCR has been
altered irreversibly.
10.3.2 Burning in the heatsealing band
If a new heatsealing band has been used, the zero point is first of all calibrated while the band is still cold by acti-
vating the "AUTOCAL" function on the controller. When the "AUTOCAL" function has finished, the controller out-
puts a voltage equivalent to a temperature of 20°C at the actual value output. Adjust the set point to approximately
250°C and activate the "START" signal to heat for approximately 1 second. After recooling, the controller usually
outputs an actual value of less than 20°C. Repeat the "AUTOCAL" function. The heatsealing band has now been
burned in and the change in the alloying properties stabilized.
The burn-in effect described here does not occur if the heatsealing band has already been thermally pretreated
by the manufacturer.
10.3.3 Replacing the heatsealing band
All power supply leads must be disconnected from the RESISTRON temperature controller in order to replace the
heatsealing band.
The heatsealing band must be replaced in accordance with the instructions provided by the manu-
facturer.
Each time the heatsealing band is replaced, the zero point must be calibrated with the AUTOCAL function while
the band is still cold, in order to compensate production-related resistance tolerances. The burn-in procedure
described above must be performed for all new heatsealing bands.
10.4
Startup procedure
Please also refer to section 1 "General information" on page 3 and section 2 "Application" on page 7.
Installation and startup may only be performed by technically trained, skilled persons who are
familiar with the associated risks and warranty provisions.
!
!
!