If your side sealing bags “look sealed” but open with a light pull, leak at the corners, or vary from roll to roll, don’t start by turning the temperature up blindly. A weak seal is usually a heat + pressure + time problem plus one more thing side sealing machines are sensitive to: web feeding and tension stability.
Use the quick table below first, then follow the sections in order. You’ll fix faster and waste less film.
Weak Side Seals Quick Diagnosis Table
| Symptom you see | Most likely cause | Fast check (1–3 min) | Practical fix |
|---|---|---|---|
| Seal opens with a light pull, no obvious burn | Low effective heat at film interface OR short dwell time | Slow the line 10–20% and test again | Increase dwell time first, then adjust temperature in small steps |
| Seal looks “white” or hazy, peels easily | Overheating surface / poor pressure contact / dirty seal face | Wipe sealing area and bar, check PTFE tape condition | Replace PTFE/Teflon tape, improve pressure uniformity, reduce temp slightly |
| Seal has wrinkles or “channels” (tiny paths) | Film not flat, tension unstable, web misalignment | Stop and inspect film path + rollers near seal | Fix feeding flatness first, then re-tune seal settings |
| Seal strong in middle but weak at top/bottom edges | Pressure not even, bar not parallel, pad worn | Paper-strip test across bar width | Re-level sealing bar, replace silicone pad / pressure strip |
| Seal strong sometimes, weak sometimes | Temperature control drift / sensor issue / speed fluctuation | Compare “display temp” vs actual behavior over time | Check heater, thermocouple placement, stabilize speed and tension |
| Pinholes or leaks at start/end of bag | Dwell/pressure timing mismatch, film slipping, contamination | Watch seal timing, inspect cut/seal overlap | Adjust timing, reduce slip, clean seal area and film dust |
Rule for troubleshooting: change one variable at a time (speed OR temperature OR pressure OR dwell). If you change three at once, you’ll never know what fixed it.
What “Not Strong Seal” Means in Side Sealing Bags
When people say “seal not strong,” they usually mean one of these:
- Low peel strength: you can peel the seal apart with little force.
- Partial bonding: the outside looks fused, but inside layers didn’t truly melt together.
- Micro-channels: tiny unsealed paths caused by wrinkles, dust, or uneven pressure—bags may leak even if the seal “looks OK.”
Side sealing bags are also more sensitive than many people expect because the seal quality is linked to how flat and stable the film web travels. A perfect temperature setting won’t help if the film arrives at the sealing bar with wrinkles, drifting tension, or misalignment.
Why Temperature Is Correct but the Seal Is Still Weak
It’s very common to hear: “We already increased temperature. Still weak.” That usually means your display temperature isn’t the same as effective heat delivered to the film interface.
Heat is not reaching the film interface
Common reasons:
- Worn PTFE/Teflon tape reduces real heat transfer and creates uneven contact.
- Sealing bar face contamination (dust, powder, degraded film residue) acts like insulation.
- Poor contact pressure means heat touches only parts of the web.
- Hot knife / sealing bar alignment is slightly off, so the film touches unevenly.
Fast check: slow the machine down briefly.
If the seal improves immediately, your problem is often time (dwell) or effective heat transfer, not just the temperature number.
Cooling too early or too fast
If the film is pulled, stretched, or cooled immediately after sealing, the bond can “freeze” before it’s fully formed.
Signs: seal looks formed, but peels open like a zipper.
Fix approach:
- Reduce pulling tension right after sealing.
- Ensure the film is not being forced away from the sealing area too quickly.
- If you have cooling air near the seal, test with reduced airflow for a short run.
How Pressure and Sealing Bar Alignment Create Real Bonding
Side seals are not only “heat bonding.” They are heat + uniform pressure bonding.
If pressure is low or uneven, you get:
- weak edges,
- inconsistent peel strength,
- channels (especially when the film has slight wrinkles).
Key items to check:
- Sealing bar parallelism: if the bar is not parallel to the pressure pad, one side will be stronger.
- Pressure pad condition: silicone/pressure strip wears, hardens, or gets grooves.
- Pressure distribution: pneumatic pressure may be fine, but contact pressure can still be uneven.
Fast check: paper-strip test
- Cut several strips of thin paper.
- Place them across the sealing width (left/middle/right).
- Close the sealing bar at normal pressure (no heat needed for this test).
- Pull each strip: resistance should feel similar.
If left is tight and right is loose, your alignment or pad wear is the real issue.
What Dwell Time Really Controls in a Side Sealing Machine
Dwell time is the time the sealing bar applies heat and pressure to the film.
In simple terms:
- If you run faster, you reduce dwell time.
- If dwell time is too short, the surface may soften but the interface won’t fuse.
Practical tuning order (recommended):
- Lower speed slightly to increase dwell time and stabilize the process.
- Tune temperature in small steps (avoid huge jumps).
- Tune pressure last (after feeding is stable), because pressure changes can also create wrinkles.
Good habit: Write down a baseline setting that produces a stable seal at a moderate speed. Then increase speed step-by-step while keeping seal strength acceptable.
Where Film Feeding and Web Tension Break the Side Seal
This is the big difference between “a seal theory article” and what actually happens on a side sealing bag making machine.
If the film feeding is unstable, you’ll see:
- wrinkles entering the sealing area,
- seal line drifting,
- one side stronger than the other,
- random weak seals even with “correct settings.”
Common feeding/tension issues:
- Unwinder brake not smooth → tension waves
- Dirty or worn rollers → slipping, uneven web travel
- Static electricity → film clings and travels unevenly
- EPC/guide misalignment → seal line moves and pressure is inconsistent
- Printed film tracking (photoelectric eye) drifting → timing offsets affect seal and cut consistency
Fast checks you can do immediately:
- Watch the film right before the sealing zone. Is it perfectly flat?
- Check for film dust, powder, or residue on guide rollers.
- Mark the web edge and see if it “walks” left/right during operation.
Fix order (saves time):
- Clean rollers + sealing bar face
- Stabilize tension and web guiding
- Then tune temperature/pressure/dwell
How to Fix Wrinkles and Micro-Channels in the Seal Area
Wrinkles are not cosmetic. A wrinkle inside the seal becomes a micro-channel that leaks.
Common sources:
- Film is not flat due to tension waves
- Roller misalignment
- Web guiding drift
- Pressure too high, squeezing the film and creating folds
- Incorrect sealing bar timing causing “drag”
Fix sequence that works in real production:
- Get the web flat (tension stable, rollers clean, guiding stable)
- Reduce pressure slightly if pressure is creating folds
- Increase dwell time slightly
- Fine-tune temperature
If wrinkles remain, don’t keep increasing temperature—it often makes the seal look worse and still leak.
What Parts Wear Out First When Side Seals Become Weak
When seal strength suddenly becomes inconsistent, these are the first wear items to suspect:
- PTFE/Teflon tape: becomes thin, damaged, or contaminated
- Silicone rubber strip / pressure pad: hardens, grooves, loses resilience
- Heater cartridge / heating element: partial failure causes uneven heating
- Thermocouple / temperature sensor: reading “stable” while real contact heat drifts
- Sealing bar face: scratches and residue create uneven contact
Simple rule: if adjustments used to work and now they don’t, it’s often a wear item, not “wrong settings.”
How to Test Seal Strength Without Lab Equipment
You don’t need a lab to improve consistency—you need a repeatable method.
Simple peel test (shop-floor version)
- Cut a strip with a consistent width (for example, 15–20 mm).
- Peel at the same angle each time.
- Compare “before vs after” changes.
Tip: Always test after the machine runs stable for a few minutes. Don’t test during start-up fluctuations.
Leak check (for packaging-style bags)
- Inflate with air gently and press the seal area.
- Or use a quick water test for a small batch.
Record like a professional (takes 30 seconds)
Write down:
- Film type and thickness
- Line speed
- Temperature setting
- Pressure setting
- Dwell time / sealing time setting
- Result: strong/weak/leak rate
After one shift, you’ll see patterns clearly.
Are You Looking for a Reliable Side Sealing Bag Making Machine Manufacturer
If you’re selecting a side sealing bag making machine (or trying to solve recurring sealing issues), the fastest way to get a correct machine configuration is to provide the production details up front.
At Wilsonmachines, we support bag making machine selection and setup based on your real job requirements, such as:
- bag type (side sealing, bottom sealing, T-shirt, etc.)
- film type (HDPE/LDPE/PP, printed or unprinted)
- thickness range and bag width/length
- target speed and output
- options like photoelectric tracking for printed film, web guiding, and stability-focused configurations
To get an accurate recommendation or quote, send:
- bag type + bag size range
- film type + thickness range
- is it printed film
- target bags/min or production per shift
- any special functions (punch, easy tear, embossing, gusset, etc.)
Conclusion and Next-Step Troubleshooting Checklist
If your side seal is not strong, follow this checklist in order:
- Clean first: sealing bar face, PTFE tape area, nearby rollers
- Check web flatness: is the film entering the seal zone flat and stable
- Stabilize tension: unwinder braking, roller slip, guiding drift
- Verify pressure uniformity: paper-strip test across sealing width
- Tune dwell time: slow down briefly; if seal improves, dwell is the key lever
- Tune temperature carefully: small steps, avoid chasing with big jumps
- Inspect wear items: PTFE tape, silicone pad, heater/sensor health
- Use a repeatable test: peel test + simple log sheet
If you want, you can paste your current settings (film type/thickness, speed, temperature, pressure, dwell) and the symptom you see from the table, and I’ll help you pinpoint the most likely cause in 2–3 steps.
FAQ
1) Why is my side seal weak even when the temperature setting is “high enough”?
A high display temperature doesn’t guarantee high interface temperature at the film layers. In real production, weak seals with “normal temperature” are often caused by poor heat transfer (worn PTFE/Teflon tape, residue on the sealing face, uneven contact pressure, or a bar that isn’t perfectly parallel). A fast shop-floor test is to slow line speed by 10–20% for a short run: if seal strength improves immediately, you’re typically limited by dwell time or contact/heat transfer, not just the temperature number. When adjusting temperature, operators usually move in small steps (about 5°C at a time) to avoid chasing the problem and creating haze, distortion, or sticking.
2) How can I tell whether to change dwell time, pressure, or temperature first on a side sealing bag making machine?
Use a “one change at a time” rule and start with the variable that gives the cleanest signal. Most plants test dwell time first by slowing the machine slightly; it’s the quickest way to see whether the film simply needs more time under heat + pressure. Next, verify pressure uniformity (one side weak often means alignment/pad wear, not temperature). Only after feeding is stable and pressure is even should you fine-tune temperature. This sequence reduces scrap because temperature changes can introduce new defects (wrinkles, whitening, sticking) that hide the original root cause.
3) What are the most common wear parts that suddenly make side seals “not strong”?
The top repeat offenders are PTFE/Teflon tape, the silicone pressure pad/strip, and the heater + thermocouple (sensor) system. PTFE tape and pads don’t always “look bad,” but once they glaze, groove, or harden, the sealing contact becomes uneven and the effective heat transfer drops. In many factories, PTFE tape is treated as a consumable and replaced whenever seal quality starts drifting (frequency depends on film type, temperature, and run hours). If the issue appears randomly across a shift, also suspect sensor/heater stability—temperature can drift in real contact even if the controller display looks steady.
