Aligning Four-Side-Seal Machines for Crisp Square Pouch Corners
A four-side-seal machine produces a pillow of film that is sealed on all four edges, then folded so the seal lines meet at clean 90-degree corners. When the geometry drifts, those corners turn into rounded bubbles, slanted edges, or worse, pinched seals that compromise barrier integrity. For Australian converters running stick packs, single-serve powders, and pharmaceutical sachets, the look of the corner is also the look of the brand. A wrinkled or skewed pouch suggests a careless line, even when the contents inside are perfectly within spec.
The Therapeutic Goods Administration scrutinises primary packaging for medicines, and the major supermarket buyers in Sydney and Melbourne apply their own quality gates to nutraceutical and food sachets. Squared, uniform corners read as a sign that the seal temperatures were stable and the web tension was under control. That is why tightening up on corner alignment pays back in audit outcomes as well as on-shelf appeal.
This walk-through covers the mechanical, thermal, and web-handling adjustments that determine how square those corners stay, from the first jaw set-up through to the last check before a production run.
The Geometry Behind a Square Pouch Corner
A square pouch is a trick of folded film. After the vertical seal joins the two webs and the horizontal jaws make the top and bottom seals, the resulting tube is creased along diagonal fold lines that meet at a point. If the horizontal jaws are not parallel to each other and to the centreline of the film, those fold lines wander away from the corner. The film has nowhere to go, so it bunches into a rounded bubble, or worse, the seal itself tracks sideways and breaches the fold.
The corner radius is a function of three things: jaw parallelism, the distance between the cross-seal jaws, and the dwell time during which heat and pressure are applied. Any of those slipping out of tolerance shows up first at the corner, before it shows up anywhere else on the pouch. That is why experienced operators treat the corner as the first place to look when a sealing job starts to drift.
For Australian lines that switch frequently between SKUs, a 60-gram sports nutrition sachet in the morning and a 30-gram herbal extract in the afternoon, the corner is also a quick proxy for whether the changeover has been completed properly.
Pre-Alignment Checks Before You Touch the Jaws
Before you reach for an Allen key, walk the machine while it is cold. Look at the film path from unwind to take-up. Confirm that the edge guides are centred on the printed eye mark, that the forming collar is clean, and that the cross-seal jaws are free of cured polymer build-up from the previous run. A speck of residue the thickness of a sheet of copy paper is enough to tilt one jaw face and throw the corner off square.
Facilities that run capsule filling machinery alongside their sachet lines often share the same calibration discipline between changeovers, simply because the same care that prevents a capsule from being scored also prevents a film from being pinched. The principle holds even when the equipment is not.
Allow the sealing block to reach a stable temperature before any adjustment. Cold jaws give a false reading because the metal contracts unevenly as it warms up. A half-hour soak at the setpoint temperature is normal on a busy floor in Dandenong or Huntingwood, where ambient swings of ten degrees between a refrigerated truck dock and the production hall are common through the day.
The reference below summarises the most common corner defects, their usual root causes, and the adjustment that clears them.
| Defect at the corner | Likely root cause | Adjustment to try |
|---|---|---|
| Rounded, puffy corner | Cross-seal jaw gap too wide | Reduce jaw gap by 0.05–0.10 mm per side |
| Slanted corner tracking one way | Edge guide offset or misaligned film | Re-centre edge guide on printed register mark |
| Pinched corner with seal creep | Excessive dwell time or pressure | Reduce dwell by 0.1 s and back off pressure 5–10 % |
| Soft, leaking corner | Seal temperature low at one jaw | Check heater cartridge and thermocouple seating |
| Diagonal wrinkle from corner | Film tension imbalance between unwind and forming | Balance draw rollers and check brake torque |
Keep that grid taped inside the guard door. It saves twenty minutes of guessing when a corner defect shows up at the start of a shift.
Setting Cross-Seal Jaw Parallelism
Parallelism is the foundation. With the machine cold and isolated, fit a feeler gauge between the upper and lower jaw faces at each end of the seal bar. The gauge should slide through with the same light drag on both ends. If one end is looser, the jaw is tilted and the corner on that side will always sit higher than the opposite one.
Most four-side-seal machines have a single set screw at each end of the jaw carrier that lifts or drops the sealing block against a fixed reference plate. Loosen both screws, shim until the feeler reads evenly, and re-torque to the manufacturer's figure. On equipment built in Songjiang District workshops, that figure is typically 18 to 22 N·m for M8 studs, worth confirming against the manual because over-torquing distorts the jaw face and creates a different problem on the next run.
Run three cold pouches through after the adjustment. They will not seal properly, but the impression left on the film shows whether the two jaw faces are mating evenly. A clean, identical mark on both faces confirms parallelism. A heavier mark on one end tells you to shim again.
Balancing Heat, Pressure and Dwell
Once the jaws are physically square, the next layer is thermal. A jaw that runs two degrees hotter than its partner will burn the film on one side of the corner and leave the other side under-sealed. Start by checking the thermocouple seating. A thermocouple that has worked loose from its pocket reads low, and the controller compensates by over-driving the heater. That is the most common cause of one-sided corner distortion on machines that have been in service for a few years.
Set the seal temperature to the middle of the film supplier's recommended window, then adjust dwell in 0.1-second steps. Faster cycles favour throughput, which matters on a high-volume mineral supplement line in Carole Park, but only if the corner holds. If the corner starts to round, lengthen the dwell slightly before you turn up the heat. Heat is the easier knob to turn, but dwell gives a more stable result over a long run.
Pressure follows heat. Too little pressure and the corner stays soft. Too much and the film squeezes out of the seal area, leaving a thin line that fails a dye-pen test. A useful starting point is the figure listed in the machine's commissioning sheet, then fine-tune by feel of the finished corner.
Film Tension and Registration
Web tension is the silent partner of jaw alignment. If the unwind is too loose, the film balloons into the sealing area and the corner folds unevenly. If it is too tight, the film stretches before the jaws close and the corner recovers into a wavy edge after the seal is made. The target tension varies with film construction, but a metallised PET/PE laminate typically wants around 1.5 to 2.5 N per centimetre of web width at the forming collar.
Common symptoms to watch for:
- Side seal drifting sideways across the pouch — edge guide is hunting; check the photo-eye alignment
- Corner creasing diagonally inward — unwind brake is set too light, web is feeding in slack
- Pouch length growing as the run continues — draw rollers are slipping; check pneumatic pressure on the nip
- Pouch length shortening — take-up is over-driving; reduce torque on the winder
Australian converters running tropical-climate warehouses from Cairns to Darwin know that humidity drifts tension over the course of a day. A quick tension check at the start of each shift catches the drift before it shows on the corner.
Verifying the Setup With Test Pouches
Before committing to a full production run, pull ten pouches at the start of every new SKU and lay them out on a clean bench. The corners should sit flat against the surface, with no lift at the fold. Hold each pouch up to a strong light and look for seal continuity along the diagonal fold lines. Any pinhole of light is a sign that the corner seal is not being made.
What to inspect on those first ten pouches:
- Corner squareness against a 90-degree reference block
- Seal width at the corner versus the centre of the side seal, which should be within 10 %
- Print registration on both faces, especially around the diagonal fold
- No die-cut dust or fibre inside the seal area
If anything is off, adjust one variable at a time and re-run a small batch. Changing two things at once means you cannot tell which one fixed the issue, and the next shift will be guessing too.
Keeping Alignment Stable Through the Run
Alignment is not a set-and-forget task. Heater cartridges age, pneumatic seals leak, and operators adapt the machine to whatever film is in front of them without recording the change. A short routine at the start of each shift keeps everything honest and is cheaper than scrapping a pallet of branded sachets.
A practical routine for a small Australian contract packer:
- Five-minute visual check of jaw faces at the start of every shift
- Thermocouple reading logged against the setpoint before the first production pouch
- Edge guide centration confirmed after every film roll change
- A written entry in the shift log whenever an adjustment is made, with the original value and the new one
Train every operator on the same routine. The strongest lines in the country, whether they are running paracetamol sticks in Sydney or sports-nutrition sachets in Melbourne, are the ones where the morning shift and the afternoon shift produce identical-looking pouches. That consistency is built, not bought, and it starts with how carefully the four-side-seal machine is aligned in the first place.