A heat seal on a packaging machine is formed by three variables acting simultaneously. Temperature melts the sealant layer of the film. Pressure pushes the two melted surfaces into intimate contact. Dwell time holds them together while the polymer chains entangle across the interface and solidify into a bond. All three variables must be correct at the same time. Fixing one while the other two are wrong does not produce a good seal. It produces a different kind of bad seal.
The frustrating reality for most packaging plants is that the seal parameter settings on the machine were established during the initial installation or line qualification, and nobody has recalculated them since. The film specification has changed twice. The seal profile has been replaced with a different thickness. The heater cartridges have been swapped out. The jaw bar surface has worn. Each of these changes shifts the effective sealing conditions, but the temperature, pressure, and dwell time settings on the HMI remain the same numbers that were entered during commissioning. The machine is running different sealing conditions than the ones displayed on the screen.
This article covers how each of the three sealing parameters works, how to determine the correct setting for your specific film, and how to diagnose which parameter is wrong when the seal quality degrades.
The Three Parameters and What Each One Controls
| Parameter | What It Does | Too Low | Too High |
|---|---|---|---|
| Temperature | Melts the sealant layer of the film to create a bondable surface. | Sealant layer does not fully melt. Seal is weak or incomplete. Film layers peel apart easily. | Film burns through. Sealant layer degrades. Seal is brittle and discolored. PTFE coating degrades faster. |
| Pressure | Pushes the two melted sealant surfaces into intimate molecular contact. | Melted surfaces do not fully contact. Seal looks formed but has low peel strength. Air channels remain in the seal zone. | Film is crushed or thinned at the seal. Sealant layer is squeezed out of the seal zone. Seal line is visible as a depression or crease. |
| Dwell time | Holds the melted, compressed film layers in contact while polymer chains entangle and solidify. | Polymer chains do not have time to entangle across the interface. Seal has low peel and burst strength even though it appears formed. | Excess heat transfers through the sealant into the structural layers of the film. Film distortion, curling, or structural weakening adjacent to the seal. |
These three parameters are interdependent. Increasing temperature reduces the dwell time needed because the sealant melts faster and reaches full entanglement sooner. Increasing pressure reduces the temperature needed because better surface contact compensates for a slightly lower melt state. Decreasing machine speed (more dwell time) compensates for lower temperature or lower pressure. Every adjustment to one parameter changes the optimal setting of the other two. This is why “just turning up the temperature” to fix a weak seal often creates a new problem: the seal is now strong but the film is burnt, curled, or the PTFE coating is degrading faster.
Temperature: How to Determine the Correct Setpoint
Start with the Film Specification
Every packaging film has a seal initiation temperature (SIT) and a seal range specified by the film manufacturer. The SIT is the minimum temperature at which the sealant layer begins to melt and form a bond. The seal range is the window between the SIT and the temperature at which the film’s structural layer begins to distort. A wider seal range means more forgiveness for temperature variation across the jaw face.
| Film Type | Typical SIT | Typical Seal Range | Notes |
|---|---|---|---|
| PE (polyethylene) | 120 to 140°C | 120 to 180°C | Wide seal range. Forgiving material. Standard VFFS film. |
| OPP (oriented polypropylene) | 130 to 145°C | 130 to 170°C | Narrower range than PE. Sensitive to over-temperature. Common snack packaging film. |
| PE/PA laminate (poly/nylon) | 140 to 160°C | 140 to 200°C | Higher SIT due to nylon barrier layer. Used for vacuum packaging, meat, cheese. |
| Metallized OPP | 135 to 150°C | 135 to 175°C | Metal layer conducts heat differently than polymer alone. Requires more uniform jaw temperature. |
| PET/PE laminate | 135 to 155°C | 135 to 190°C | PET structural layer has a high melt point. Wide seal range for the PE sealant layer. |
| Paper/PE laminate | 140 to 160°C | 140 to 190°C | Paper absorbs heat. Requires higher setpoint than the equivalent PE sealant layer on a polymer substrate. |
Why the Displayed Temperature Is Not the Seal Temperature
The temperature displayed on the machine HMI is the temperature measured by the RTD or thermocouple in the jaw bar. It is not the temperature at the film surface during the seal cycle. The actual temperature at the film surface is lower than the jaw bar temperature because heat must transfer through the seal profile (PTFE coating) and into the film during the dwell time. The temperature drop across the PTFE coating depends on the coating thickness and the dwell time. A thicker PTFE coating (from a new 3-coat seal profile) insulates more and produces a larger temperature drop. A thinner coating (from a worn profile) insulates less and delivers more heat to the film.
This is why changing to a new seal profile without adjusting the temperature setpoint can produce over-temperature seals (if the old profile was thicker and the setpoint was compensated upward) or under-temperature seals (if the old profile was thinner and the setpoint was set lower). For the full PTFE coating thickness analysis and its effect on thermal transfer, see the PTFE coating systems guide.
Pressure: How to Determine the Correct Jaw Force
Sealing pressure is the force per unit area applied to the film at the seal zone. It is controlled by the pneumatic cylinder(s) that close the jaws, and it is adjusted by the air pressure regulator on the machine’s pneumatic supply. The force delivered by the cylinder depends on the cylinder bore, the air pressure, and the number of cylinders acting on the jaw.
The Pressure Calculation
The jaw closure force from a single pneumatic cylinder is:
Force (N) = Bore area (mm²) x Air pressure (MPa)
The sealing pressure at the film surface is:
Seal pressure (N/mm²) = Total jaw force / Seal area
Where the seal area is the seal width multiplied by the seal length (the jaw face width across the film web).
What Happens When Pressure Is Wrong
Too low. The melted sealant layers do not achieve full surface contact. Microscopic air channels remain trapped in the seal zone. The seal looks formed visually but has low peel strength and fails under burst pressure testing. On a peel test, the seal peels apart cleanly instead of tearing through the film (which is the target for a strong seal). If the seal peels cleanly and the peel force is below specification, increase pressure before increasing temperature.
Too high. The melted sealant is squeezed out of the seal zone by the excess force. The film at the seal line is thinned or crushed. On filled bags, excess jaw pressure compresses the product at the seal position, which can cause product to be caught in the seal (a common defect on powder and granule fill lines). The visible symptom is a thin, hard seal line that looks over-defined compared to a normal seal. The structural symptom is a seal that fails not at the seal interface but adjacent to it, where the film has been thinned by the excess pressure.
Dwell Time: The Parameter That Gets Sacrificed for Speed
Dwell time is the duration that the heated jaws hold the film under pressure during each seal cycle. It is the time available for heat to transfer from the jaw face through the PTFE coating into the film sealant layer, and for the melted polymer chains to entangle across the interface and begin to solidify. Dwell time is directly linked to machine speed: as the machine runs faster, the cycle time decreases, and the dwell time available for each seal decreases proportionally.
The Speed vs Seal Quality Trade-Off
On a VFFS machine, the total cycle time is divided between jaw open time (bag index, fill, and film advance) and jaw closed time (the seal dwell). At 60 bags per minute, the total cycle is 1,000 milliseconds. If the jaw open time is 700ms, the dwell time is 300ms. At 100 bags per minute, the total cycle is 600ms. If the jaw open time stays at 700ms… there is no dwell time left. The machine physically cannot run at that speed with that dwell requirement.
In practice, the machine controls allow the operator to adjust the jaw timing to allocate more of the cycle to dwell and less to jaw open time. But reducing jaw open time means less time for the film to advance, the product to fill, and the bag to index. At some point, reducing jaw open time causes fill accuracy, film registration, or product feed problems. The dwell time is the first parameter that gets squeezed when the line speed is pushed above the design point.
Minimum Dwell Time by Film Type
| Film Type | Minimum Dwell at Midpoint Temp | Effect of Reducing Below Minimum |
|---|---|---|
| PE (single layer) | 150 to 250ms | Seal appears formed but peel strength drops below specification. Fails on drop test. |
| OPP | 200 to 300ms | Seal is partially formed. One side may seal while the other peels open. |
| PE/PA laminate | 250 to 400ms | Barrier layer does not reach adequate temperature for full sealant melt. Weak seal center with stronger edges. |
| Metallized OPP | 250 to 350ms | Metal layer reflects heat away from sealant. Short dwell produces cold spots in the seal zone. |
| Paper/PE laminate | 300 to 500ms | Paper absorbs heat. Insufficient dwell leaves the sealant layer under the paper still solid. |
Seal Defect Troubleshooting Matrix
When a seal station starts producing defective seals, the defect pattern tells you which parameter is wrong. The matrix below maps the most common seal defects to the specific parameter adjustment that fixes them.
| Defect | What You See | Most Likely Cause | Fix |
|---|---|---|---|
| Weak seal (peels open easily) | Seal separates cleanly at the interface. No film tearing. | Temperature too low, or dwell time too short. Sealant did not fully melt and entangle. | Increase temperature in 5°C increments. If at the top of the seal range, increase dwell time. |
| Burnt or discolored seal | Brown, yellow, or darkened seal line. Film feels brittle at the seal. | Temperature too high. Structural layers of the film are degrading. | Decrease temperature in 5°C increments until discoloration disappears. Verify RTD accuracy. |
| Seal strong on one side, weak on the other | Asymmetric peel strength across the seal width. | Temperature variation across the jaw face. Hot spot or cold spot in the heater cartridge. | Measure jaw face temperature at four points across the width. If variation exceeds 5°C, replace the heater. See the heater and RTD guide. |
| Seal looks formed but fails burst test | Seal appears normal. Bag bursts at the seal under pressure. | Pressure too low. Sealant layers melted but did not achieve full surface contact. | Increase cylinder air pressure in 0.05 MPa increments. Verify cylinder is not leaking. See the pneumatic cylinders guide. |
| Thin, over-defined seal line | Seal line appears as a sharp crease. Film is visibly thinned at the seal. | Pressure too high. Sealant layer squeezed out of the seal zone. | Decrease cylinder air pressure in 0.05 MPa increments until the seal line appears flat, not creased. |
| Product in the seal | Visible powder, granules, or liquid trapped in the seal line. | Jaw timing too early (jaws close before product clears the seal zone) or inadequate deflation. | Adjust jaw timing. Verify deflator/squeezer cylinder is functioning. Check bag length vs fill level. |
| Seal peels at one end only | Cross-seal strong in the center, weak or open at one or both ends. | Jaw bar is worn unevenly or jaw alignment is off. Pressure distribution across the jaw face is not uniform. | Inspect the jaw bar face for wear. Replace if worn. Realign the jaw if the bar is in spec. See the jaw bar replacement guide. |
| Seals degrade over the shift | Seals are good at startup. Quality drops progressively over 2 to 4 hours. | Heater resistance drift. Heater cannot maintain setpoint at production speed. See the heater and RTD guide. | Replace heater cartridge. If the heater is new, the watt density is too low for the machine speed. |
How the Three Parameters Interact on a Real Machine
On paper, each parameter operates independently. On a real machine, they interact through the mechanics of the jaw assembly. Understanding these interactions prevents the common troubleshooting mistake of adjusting one parameter and creating a new problem.
Temperature and Dwell Time
Higher temperature reduces the minimum dwell time because the sealant layer reaches full melt faster. This is the standard approach for increasing machine speed: raise the temperature to compensate for the reduced dwell time at higher speed. The limit is the film’s seal range ceiling. Above that ceiling, the structural layers of the film degrade regardless of how short the dwell time is.
Pressure and Temperature
Higher pressure improves heat transfer from the jaw face to the film by increasing the contact area between the film and the jaw surface. A film layer that sits flat against the jaw face with zero air gaps transfers heat faster than one that has microscopic air pockets between the film and the jaw. This means that increasing pressure slightly can allow a small decrease in temperature while maintaining the same seal quality. In practice, this interaction is useful when the seal is close to the film’s upper temperature limit and the seal quality is borderline: adding 0.05 MPa of pressure may improve the seal without adding temperature that would push the film into the burn zone.
Dwell Time and Pressure
Longer dwell time allows lower pressure because the melted sealant layers have more time to flow and fill the contact surface under gentle pressure. Shorter dwell time requires higher pressure to force the sealant layers into full contact before the jaws open. At very short dwell times (below 200ms), the pressure required for full contact can exceed the threshold that causes film thinning at the seal line. This is the speed ceiling for a given film and jaw geometry: the point where neither temperature, pressure, nor dwell time can be adjusted further without creating a defect.
The Seal Parameter Setup Procedure
Use this procedure whenever you change film specifications, replace a seal profile, install new heater cartridges, or set up a machine for a new product.
Why Seal Parameters Drift Over Time
Seal parameters that were correct during setup do not stay correct indefinitely. Four mechanical changes cause the effective sealing conditions to drift from the displayed settings.
PTFE coating wear. As the seal profile coating wears thinner, it insulates less and transfers more heat to the film per unit time. The effective seal temperature at the film surface increases even though the controller setpoint has not changed. Seals that were correct with a new profile become over-temperature with a worn profile. The fix is to reduce the temperature setpoint as the profile wears, or replace the profile and reset the temperature. For coating thickness measurement and wear progression, see the PTFE coating systems guide.
RTD drift. The RTD measures the jaw temperature and feeds it to the controller. As the RTD drifts (which it does over thousands of hours of thermal cycling), the controller receives an inaccurate measurement and adjusts the heater output accordingly. The displayed temperature diverges from the actual temperature. For the full RTD drift diagnostic, see the heater and RTD guide.
Cylinder seal wear. As the rod seal on the jaw closure cylinder wears, the cylinder loses air past the seal and delivers less force at the jaw face. The displayed air pressure has not changed, but the effective jaw closure force has decreased. The seal receives less pressure on every cycle. For rod seal wear and cylinder inspection, see the pneumatic cylinders guide.
Jaw bar surface wear. As the jaw bar face wears (from the repeated compression of film under heat and pressure), the contact geometry changes. The pressure distribution across the seal width becomes uneven, producing seals that are stronger in some areas than others. For jaw bar inspection and replacement criteria, see the jaw bar replacement guide.
The Components That Control Seal Quality
Every parameter discussed in this article is delivered by a physical component on the machine. Temperature is delivered by the heater cartridge and measured by the RTD. Pressure is delivered by the pneumatic cylinder and controlled by the air pressure regulator. The seal surface is defined by the jaw bar and the seal profile. Dwell time is controlled by the machine PLC and the jaw drive mechanism. When any of these components degrades, the parameter it controls drifts from the setpoint, and the seal quality follows.
For the component selection, cross-reference, and replacement guides, see the Vanguard heat sealing parts catalog, the sealing jaw selection guide, and the VFFS machine parts cross-reference.
