Incorrect filling volume in syringe filling machines: causes, warning signs, and solutions
In syringe filling and packaging lines, incorrect filling volume not only leads to product loss but can also directly affect batch quality, dose uniformity, and quality control requirements in pharmaceutical, cosmetic, biotech, or other high-precision liquid production processes.
Incorrect filling volume is one of the most common and critical issues in syringe filling machines. This issue can appear in many forms: underfilled syringes, overfilled syringes, inconsistent filling between cycles, some filling heads deviating more than others, or filling errors occurring only at high operating speeds.
Why should incorrect filling volume be addressed early?
Affects dosing accuracy
For products requiring strict dosage control, even a small deviation may cause the batch to fail internal inspection or quality requirements.
Causes product loss and higher reject rates
Overfilling wastes product, while underfilling increases rejection rates. If the issue repeats cyclically, production losses can become significant.
Risk of contamination and unstable process
When filling nozzles drip, splash liquid, or generate air bubbles, the filling area becomes contaminated more easily, increasing cleaning time and machine downtime.
Signs that a syringe filling machine is experiencing filling inaccuracies
Filling inaccuracies do not always appear clearly at the beginning of production. In some cases, the machine runs normally during slow testing, but errors increase once continuous production starts or speed increases. Common signs include:
Product-related signs
- Uneven liquid levels between syringes.
- Some syringes contain less or more liquid than the reference sample.
- Air bubbles appear inside the syringes after filling.
- Liquid residue around syringe tips, bodies, or trays.
- Large fluctuations in sample weighing results.
Machine-related signs
- Dosing pump operates inconsistently or makes unusual noise.
- Filling nozzle drips after cycle completion.
- Some filling heads dispense more or less than others.
- Errors become worse when operating speed increases.
- HMI displays pressure, sensor, servo, or positioning alarms.
Common causes of incorrect filling volume in syringe filling machines
1. Dosing pump or peristaltic pump calibration drift
The pump directly determines the amount of liquid delivered into the syringe. When piston pumps, dosing pumps, or peristaltic pumps become worn, lose stroke accuracy, rotate at incorrect speeds, or are not recalibrated after long-term operation, the actual filling volume will deviate from the set value.
In peristaltic pumps, hardened, collapsed, stretched, or aged silicone tubing can also change the dispensed volume. The machine may still operate normally, but filling volume will fluctuate between cycles.
2. Air bubbles or air leakage in the supply line
Air bubbles are one of the most common causes of underfilling. When air enters the tubing, part of the pump stroke compresses air instead of pushing liquid. As a result, less liquid enters the syringe, liquid levels become inconsistent, and air bubbles may appear in the final product.
Air bubbles often occur due to loose fittings, aged tubing, leaking suction connections, low feed tank level, foaming products, or insufficient priming procedures.
3. Clogged or worn filling nozzles, needles, or valves
If the filling nozzle becomes partially clogged, flow decreases and the actual filling volume becomes lower than the set value. Conversely, if the valve does not close properly, dripping may occur after each cycle, causing uncontrolled excess liquid or contamination around the filling area.
For viscous products, products that dry easily, crystallize, or contain sticky components, filling needles should be inspected more frequently. Even a small deposit on one filling head can create inconsistent filling results between syringes.
4. Product viscosity changes
Even with the same settings, if the viscosity of the liquid changes due to temperature, mixing time, formulation composition, or storage conditions, filling volume may vary. Higher viscosity makes the flow slower, causes stringing, dripping, or incomplete cut-off.
Therefore, when troubleshooting filling inaccuracies, not only the machine but also the product condition should be checked: viscosity, temperature, holding time in the tank, mixing condition, and feeding stability.
5. Sensors, servo systems, or timing synchronization issues
Syringe filling machines require multiple coordinated movements: tray/nest positioning, nozzle lowering, liquid dispensing, nozzle retraction, and stopper insertion. If position sensors, servo systems, lead screws, stepper motors, or valve timing become misaligned, filling stability may be affected.
Even a small timing issue can cause filling to start before the nozzle reaches the correct position, or the nozzle to retract before dispensing finishes. This may result in splashing, air bubbles, dripping, or underfilling.
6. Incorrect operating parameters
After changing products, syringe sizes, pumps, silicone tubing, or performing mechanical maintenance, HMI settings should be rechecked. Parameters such as filling speed, delay time, pump stroke, suck-back, filling needle depth, and nozzle lifting/lowering speed can all affect filling accuracy.