How to Maintain Filling Accuracy When Switching from PET Bottles to Pouches

Switching PET bottles to pouches changes more than your packaging design. It changes how your filling system behaves. Bottles are rigid. Their neck finish and wall geometry give the filler a consistent reference points every cycle. Pouches have none of that. They’re soft, compressible, and often shift on the conveyor. If you run the same fill parameters used for bottles, you’ll see weight variation of 3% to 5% sometimes more. That’s not acceptable for regulated products like food, chemicals, or pharmaceuticals.

The root cause isn’t your machine. It’s the physics of filling a collapsible container. Without a rigid wall, back pressure drops as the pouch inflates. The air gets trapped in corners. Viscous products cling to film surfaces. Nozzle retraction can pull the product back out. All of this throws off volumetric or time-based dosing.

But you don’t need to buy new equipment. You just need to adjust your process around how pouches actually behave. Here’s what works in real production.

Why Pouch Filling Demands a Different Approach

PET bottles have consistent internal volume, straight sidewalls, and a fixed neck ID. Your nozzle seats the same way every time. Pouches? They’re formed from roll stock. Even minor wrinkles from the pouch-making stage can change how the opening aligns with the nozzle. Film gauge variation common across rolls means one batch of pouches might hold 5% more volume than another at the same fill level.

Thermal properties matter too. Polyethylene-based films expand slightly when warm. In a hot plant, that changes fill dynamics. Foil laminates resist expansion but can crack under pressure if the nozzle pushes too hard.

Then there’s product of rheology. Newtonian fluids like water fill predictably. But shear-thinning fluids like sauces or lotions behave differently under pressure. At high speed, they thin out and overfill. At low speed, they thicken and underfill. Bottles absorb this variation. Pouches amplify it.

Most importantly, pouches lack structural memory. Once filled, they conform to product weight and ambient pressure. A pouch that looks full at the filler might settle and appear underfilled an hour later. That’s not a short fill. It’s post-fill relaxation and it fools visual inspection every time.

How to Achieve Consistent Fill Accuracy on Pouch Lines

Success comes from controlling three physical variables: pouch stability, air management, and real-time feedback. Ignore any one of them, and accuracy drifts.

1. Provide Positive Pouch Support at the Fill Station
Passive nests aren’t enough. Use an active clamping system that holds the pouch body and neck independently. The neck must be held in X Y and Z planes within ±1 mm tolerance. Many high-accuracy lines use servo-driven jaws synchronized with the filler stroke. For stand-up pouches, include bottom support to prevent sag during fill. Test with your worst-case pouch thin film, tall profile, wide opening. If the neck moves during nozzle insertion, you’ll get inconsistent seating and variable void volume.

2. Choose the Right Nozzle Technology for Your Product Viscosity
Top-down filling with a standard straight nozzle creates air entrapment in pouch corners, especially with non-Newtonian fluids. For water-thin products, a vented fill nozzle works well it evacuates headspace air before or during dosing. For sauces, gels, or pastes (viscosity >500 cP), switch to bottom-up filling. The nozzle extends to the base of the pouch and fills upward, forcing air out through the open top. Some advanced systems use dual-stage nozzles: vacuum first, then fill, then gentle blow-off to clear residue. Nozzle tip geometry also matters rounded tips reduce film puncture risk versus sharp edges.

3. Implement Multi-Stage Fill Profiles Based on Product Rheology
Don’t use single-speed filling. Program your piston or pump system for at least two phases:

• Phase 1: 80 to 90% of target volume at high speed (e.g., 200 mL/sec for liquids)
• Phase 2: Final 10 to 20% at low speed (e.g., 30 to 50 mL/sec) to reduce turbulence and meniscus bounce

For high-viscosity products, add a dwell time of 0.3 to 0.8 seconds between phases to let air escape. Systems with servo-driven dosing pumps can even taper speed continuously during the final 5%, which cuts variation to under ±1%.

4. Use Mass-Based Feedback, Not Volumetric Assumptions
Time-pressure fillers to assume constant viscosity and temperature. That fails with pouches. Instead, integrate an in-line checkweigher right after the sealing station. Use that data to close the loop. Better yet, install a load cell on the filler frame or torque sensor on the pump motor to infer fill weight in real time. Modern PLCs can adjust fill volume every 20 to 50 cycles based on running averages. This compensates for film stretch, temperature drift, and product batch changes without stopping the line.

5. Control Product Temperature and Ambient Conditions
Viscosity changes by 2 to 5% per 1°C shift for many oils and emulsions. Install a PT100 sensor in your product hopper and feed that data to your filler controller. Set a tolerance band e.g., ±0.5°C and auto-adjust fill time if temperature drifts. Also monitor plant humidity. High humidity causes static cling in thin films, leading to misaligned openings. Some lines use ionizing bars at the pouch infeed to neutralize charge.

6. Qualify Pouches as a Process Input Not Just Packaging

Don’t treat pouches like passive containers. They’re an active part of your fill system. Before full production:

• Measure film thickness at 10 points across the roll (use micrometer, not caliper)
• Test seal integrity on 50 pouches from the first 100 meters of roll stock
• Run fill trials at min/max product temperatures
• Record fill weight distribution over 200 pouches

If CV (coefficient of variation) exceeds 1.5%, go back to support or nozzle design. Never assume vendor specs match reality.

7. Train Operators to Diagnose Fill Errors by Symptom

• Dripping after retraction: Nozzle orifice too large or retraction speed too fast
• Pouch leaning or buckling: Insufficient body support or overfill pressure
• Consistent low weight with thick product: Air trapped in upper corners switch to bottom fill
• Weight drift over time: Product warming in hopper or film static buildup

Empower your line crew to make small adjustments within defined limits. A 0.1-second change in fill time often fixes what looks like a “machine problem.

Pre-Production Checklist for Pouch Fill Accuracy

Before you commit to full runs:

• Pouch neck alignment repeatability: ≤2 mm deviation
• Nozzle type matched to product viscosity and pouch geometry
• Fill profile split into at least two speed stages
• In-line weighing with automatic feedback loop active
• Product temperature monitored and logged
• First 100 pouches from production roll tested for weight CV
• Operators trained on fill fault recognition and response

Skip any of these, and you’ll chase variation for weeks.

Final Thoughts

Switching to pouches can reduce material cost and shipping weight but only if your fill process keeps up. The best operations don’t force bottle logic onto flexible packaging. They treat pouch filling as its own discipline, with its own physics and control points.

They measure weight, not just time. They support the pouch like it’s part of the machine. And they let real data, not assumptions, drive adjustments.

Because in regulated or high-volume production, ±3% isn’t “close enough.” It’s scrap.

Need help tuning your filler for pouch accuracy without reengineering your line?
Contact our experts at [email protected]. We’ve helped food, chemical, and personal care manufacturers achieve ±1% fill accuracy on stand-up, spouted, and flat pouches, using their existing equipment. Let’s review your product, pouch, and line before your first trial run.