The seal station on a VFFS machine has two jobs: heat the film to sealing temperature and hold it there for the correct dwell time. The heater cartridge provides the heat. The RTD measures the temperature. Together, they form a closed-loop control system that keeps the jaw face at the exact temperature required to fuse two layers of thermoplastic film into a hermetic seal. When either component degrades, the temperature at the jaw face drifts from the setpoint, and the seal quality changes in ways that look like a film problem, a pressure problem, or a jaw alignment problem before anyone suspects the heater or sensor.

The diagnostic trail is predictable. Seal peel tests start showing inconsistent results. One side of the seal is strong. The other side is weak. Maintenance checks the jaw alignment, adjusts the pressure, and verifies the dwell timer. Everything checks out. The machine keeps producing inconsistent seals. Three shifts later, someone finally pulls the heater cartridge and finds a discolored section where the heating element has started to fail internally, creating a cold zone on the jaw face that no amount of pressure or dwell time adjustment can compensate for.

This article covers how heater cartridges and RTDs work together to control seal quality, how each component fails, how to diagnose temperature-related seal problems, and how to source the correct replacement when the OEM part number is no longer available.

How the Heater and RTD Work Together

The heater cartridge is an electrical resistance element that converts electrical energy into heat. It slides into a bore machined into the jaw bar and transfers heat from its outer surface through the jaw bar body to the sealing face. The RTD (Resistance Temperature Detector) is a precision temperature sensor that sits in a separate bore in the jaw bar, close to the sealing face. It measures the actual jaw temperature and sends that measurement back to the temperature controller, which adjusts the power to the heater to maintain the setpoint.

This is a closed-loop system. The controller tells the heater to produce heat. The RTD tells the controller what temperature the jaw has reached. The controller compares the measured temperature to the setpoint and increases or decreases heater power accordingly. If either half of this loop fails, the temperature control fails, and the seal quality follows.

What the Heater Controls

Watt density determines how fast the jaw reaches sealing temperature from cold start and how quickly it recovers temperature after each seal cycle draws heat out of the jaw face. Higher watt density means faster heat-up and faster recovery. The trade-off is that higher watt density concentrates more heat per square inch of heater surface, which increases the thermal stress on the heater element and accelerates its degradation over time.

Sheath material determines the heater’s corrosion resistance and how easily it slides in and out of the jaw bar bore. The Hayssen and Triangle heater cartridges stocked by Vanguard use 304 stainless steel sheaths for corrosion resistance and a PTFE coating for ease of removal. Without the PTFE coating, the heater seizes into the jaw bar bore from thermal expansion, oxidation, and micro-welding between the heater sheath and the bore wall. Extracting a seized heater without damaging the jaw bar bore is difficult, time-consuming, and sometimes impossible.

What the RTD Controls

The RTD does not control temperature. It measures temperature, and the controller uses that measurement to control the heater output. The accuracy of the temperature control is only as good as the accuracy of the RTD measurement. An RTD that has drifted 5°C high tells the controller the jaw is hotter than it actually is. The controller reduces heater power. The jaw runs 5°C below setpoint. The seals are under-temperature and weak. The operator sees weak seals, increases the temperature setpoint by 5°C to compensate, and the machine runs correctly again, but the setpoint is now 5°C above the true optimum. When the RTD drifts further, the operator increases the setpoint again. Eventually the setpoint is so far above the true process temperature that a new RTD installed without adjusting the setpoint back produces seals at 10 to 15°C above the correct temperature, which burns the film and damages the PTFE coating on the seal profile.

How Heater Cartridges Fail

Heater cartridges in packaging machine jaw bars fail through three mechanisms, all of which produce different symptoms on the machine.

Internal Hot Spots

The heating element inside the cartridge is a coiled resistance wire surrounded by compacted magnesium oxide (MgO) insulation. Over millions of thermal cycles, the MgO insulation shifts, cracks, or compacts unevenly. Sections of the resistance wire that lose insulation coverage overheat locally, creating hot spots on the heater sheath. Those hot spots transfer to the jaw bar, producing temperature variation across the sealing face. The hot zone over-seals the film (burn marks, film thinning, coating damage). The cold zone under-seals the film (weak peel strength, partial seal).

Symptom on the machine: Inconsistent seal strength along the width of the cross-seal. One side passes peel testing, the other side fails. The problem does not respond to pressure, dwell, or temperature adjustment because the temperature across the jaw face is physically uneven.

Open Circuit (Complete Failure)

The resistance wire breaks. The heater stops producing heat entirely. The temperature controller detects the open circuit and triggers a temperature alarm or fault. This is the failure mode that gets immediate attention because the machine stops. It is the easiest failure to diagnose and the most expensive if no replacement heater is on the shelf because every minute without a heater is a minute of zero production.

Resistance Drift

The resistance of the heating element changes over its service life as the wire oxidizes and thins from thermal cycling. A heater that originally measured 45 ohms may measure 52 ohms after two years of service. Higher resistance at the same voltage means lower wattage output. The heater produces less heat. The jaw takes longer to reach setpoint on startup and recovers more slowly between seal cycles. At high machine speeds, the recovery time exceeds the cycle time, and the jaw temperature drops progressively over the production run.

Symptom on the machine: Seals are good at startup but degrade progressively over the first hour of production. By mid-shift, the seal temperature is consistently below setpoint at rated machine speed. Slowing the machine down (giving the heater more recovery time) fixes the seal quality temporarily but reduces throughput.

How RTDs Fail

Drift

The platinum element inside a PT100 RTD changes resistance with temperature at a precisely calibrated rate (0.385 ohms per degree Celsius). Over time, the platinum element degrades from thermal cycling, mechanical vibration, and contamination from moisture ingress. The resistance-temperature relationship shifts, and the RTD reports a temperature that is higher or lower than the actual jaw temperature. This is the most common RTD failure mode and the most insidious because the machine keeps running with no fault or alarm. The controller simply adjusts the heater output based on the wrong measurement.

Open Circuit

The RTD wire or its connection to the lead wires breaks. The controller reads an infinite resistance, which it interprets as an extremely high temperature. The controller cuts power to the heater. The jaw goes cold. The machine faults. This is a clear, immediate failure that gets diagnosed and fixed quickly.

Short Circuit

Moisture ingress into the RTD probe causes a partial short between the platinum element and the sheath. The controller reads a lower resistance than actual, which it interprets as a lower temperature. The controller increases heater power. The jaw runs above setpoint. The film burns, the PTFE coating on the seal profile degrades, and the seal quality deteriorates from over-temperature. This failure mode is common in washdown environments where moisture penetrates the RTD cable entry point.

Diagnosing Temperature-Related Seal Problems

When a seal station produces inconsistent or defective seals and the pressure, dwell, and film specification all check out, the problem is almost always in the heater or RTD. The diagnostic process below isolates which component has failed.

Why Heaters and RTDs Should Be Replaced as Matched Pairs

Replacing the heater without replacing the RTD (or vice versa) solves half the problem and creates a diagnostic trap. A new heater installed with a drifted RTD will run at the wrong temperature because the controller is still getting the wrong measurement. A new RTD installed with a degraded heater will accurately measure the wrong temperature (the temperature the degraded heater produces, not the temperature a healthy heater would produce). The controller will show a stable reading, the operator will assume the system is working correctly, and the machine will produce marginal seals until someone realizes that both components needed replacement.

Best practice is to replace the heater cartridge and RTD as a matched pair at the same PM interval. When both components are new, reset the temperature setpoint to the film manufacturer’s recommended sealing temperature (not whatever the previous setpoint was, which may have been inflated to compensate for drift).

Vanguard Heater and RTD Cross-Reference: Hayssen and Triangle

All heater cartridges and RTDs listed above are manufactured to OEM-equivalent specifications with 304 stainless steel sheaths, PTFE coating for removal ease, and high-temperature wire sleeving rated above 1000°F. For the complete range of heat sealing components including jaw bars, seal profiles, and bushings, browse the heat sealing parts catalog and the heaters and RTDs catalog.

When the OEM Heater or RTD Is Discontinued

Heater cartridges and RTDs for packaging machines are specified by a combination of diameter, insertion length, wattage, voltage, sheath material, lead wire type, and lead wire length. Changing any of these parameters produces a heater or sensor that does not fit the jaw bar bore or does not deliver the correct thermal output for the application. Generic industrial heater suppliers stock standard diameters and wattages, but they do not stock a 12mm diameter, 180mm insertion length, 400W/240V cartridge heater with a PTFE-coated 304 SS sheath and 600mm high-temperature leads for a specific Hayssen jaw bar from 2008.

Vanguard’s custom fabrication process manufactures heater cartridges and RTDs to exact specification. Provide the worn heater or RTD, the OEM part number, or a dimensional specification. Vanguard confirms fabrication feasibility within one business day.

For the full technical context on how heaters and RTDs interact with jaw bars and seal profiles, see the heat sealing machine maintenance guide, the Teflon coating guide, and the jaw bar replacement guide. If your OEM heater or RTD has been discontinued, submit your part details through the custom parts request. Vanguard ships across the US, Canada, and Mexico.