When selecting a liquid filling machine, filling accuracy is usually a key consideration. It directly affects quality. A small filling difference can become a large problem during production. It may cause inconsistent dosage, increased product waste, or extra quality checks. In this article, let’s take a deep dive into liquid filling accuracy for medicines and help you make a wise choice.
What is filling accuracy in liquid fillers?
Liquid filling accuracy is usually expressed as a number to show the variation between the actual liquid amount and the target filling volume. It is used to check if the actual filling amount meet the requirement. If there is a large difference, you need to find problems and take measures.
Here is a formula to help you calculate the number of accuracy.
Filling deviation (%) = [(Actual fill volume − Target fill volume) / Target fill volume] × 100
Example:
Target fill volume: 100 mL
Actual fill volume: 99.5 mL
Accuracy deviation: (99.5 − 100) / 100 × 100 = −0.5%
The result means the container is underfilled by 0.5% compared with the target volume.
The deviation number could be usually listed in the specification of a liquid filling machine, such as ±0.5%. it means the actual fill amount may be 0.5% above or below the target volume. Take the required fill amount of 100 mL for example. A machine with ±0.5% of filling accuracy may complete the filling of 99.5 mL – 100.5 mL. For 200 mL of the required amount with the same ±0.5% accuracy, the measured filling range may be 199 mL – 201 mL. If you have a higher accuracy demand, you may contact with your supplier to see if it is available.
Why should you consider filling accuracy?
Filling accuracy helps you know whether the filling process is stable. It could be also used to
- Confirm that containers have the same amount of product as required.
- Reduce the risk of underfilling or extra product loss caused by overfilling.
- Support process control and quality.
Sometimes, small volume differences may have a big effect. In worse cases, it may cause product recalls. For example, an FDA-posted recall of B. Braun 0.9% Sodium Chloride Injection USP 250 mL involved low fill volume issues.
Is there a standard filling accuracy requirement?
There is no one filling accuracy number that applies to all liquid products. The accuracy depends on the product type, filling volume, regulatory and requirements.
In real production, filling accuracy involves three different levels.
- Legal demand. These requirements depend on the market and product type. They may include minimum fill, average net content, individual container limits, other local regulatory, etc.
- Machine operation. It controls if the filling system can dispense the target volume consistently.
- Product requirement. The accuracy needed to maintain product quality, dosage consistency, and regulatory compliance.
A filling machine may have a specified accuracy, but the actual performance depends on factors, such as product properties, filling volume and speed. Pharmaceutical products usually require tighter control because filling errors can directly affect dose accuracy. For example, a small-volume injectable product may require stricter control than a large-volume oral liquid. Because the same absolute volume difference creates a larger percentage variation in the delivered dose.
In short, there is no universal “±X%” filling accuracy standard. The suitable accuracy level depends on the product, industry, and production conditions.
What factors affect liquid filling accuracy?
Liquid filling accuracy depends on how consistently the product flows and how well the filling process controls each dose. Changes in liquid properties or process conditions can cause volume variation between containers.
1. Liquid viscosity
Viscosity affects how easily a liquid moves through the filling system. Low-viscosity liquids often have good flowing ability. High-viscosity liquids may flow slower and require more time to reach the target volume. Changes in viscosity between batches or during production can cause differences in filling performance. You need to consider the following points for different properties.
| Liquid type | Common considerations | Typical products |
|---|---|---|
| Low-viscosity liquids | They usually flow easily, but they may create problems such as dripping, splashing, or foaming. They can be filled by peristaltic pumps or other dosing systems. | Oral solutions, drops, some injectable solutions |
| Medium-viscosity liquids | They require stronger pumping force and better flow control. | Syrups, concentrated solutions, some suspensions |
| High-viscosity liquids | They may require positive displacement pumps, larger flow paths, or temperature control. | Thick suspensions, gels, viscous formulations |
2. Density
Density affects the relationship between liquid weight and volume. When manufacturers use weight-based measurement to verify filling performance, density changes have important influences. For example, the same filled weight may represent different liquid volumes if the product density changes. This may lead to incorrect evaluation of fill volume consistency. Density variation may occur because of
- Formulation differences.
- Concentration changes.
- Temperature changes.
Therefore, manufacturers should control product density within the defined range and use accurate density data during filling verification.
3. Suspension stability
Suspensions contain solid particles that can settle over time. If the product is not uniform before filling, different containers may receive different concentrations or particle levels. This can affect both product consistency and filling results. So, you need to control
- Mixing conditions.
- Holding time.
- Product circulation before filling.
4. Temperature changes
Temperature can change liquid flow during filling. For many liquids, higher temperature lowers viscosity, while lower temperature increases resistance to flow. These changes can affect filling speed and volume consistency. Stable temperature control is needed to maintain better filling performance.
5. Foaming
Foam is commonly caused by high filling speed, agitation during mixing, or pumping conditions. It can change the liquid flow behavior during filling and may affect the accuracy of fill level detection. During the filling process, the system needs to stop liquid delivery based on the target fill amount. However, when excessive foam forms on the liquid surface, it may cause
- The liquid level to no longer represent the actual liquid volume.
- Visual inspection or level detection systems to make inaccurate judgments.
- Different foam amounts between containers, leading to measurement variation.
6. Machine setting
Even with a stable formulation, process settings can influence filling accuracy. Important parameters include
- Filling speed.
- Filling time.
- Dosing settings.
- Adjustment frequency.
Improper settings may cause unstable filling results, especially during long production runs.
How to evaluate liquid filling accuracy?
Liquid filling accuracy should be evaluated through both individual fill checks and overall process performance. One single container meeting the fill requirement does not mean that the entire batch process is stable. A reliable evaluation also checks whether the process remains stable during production.
Fill weight/volume verification
During process evaluation or routine quality checks, you need to measure selected filled containers and compare the actual fill amount with the target volume. The evaluation focuses on average fill amount, variation between containers, and whether results meet defined limits. The evaluation process is different in various stages.
1. Development/validation stage
- Collect some filled samples.
- Measure the fill weight or volume.
- Analyze fill variation
Purpose: To confirm whether the filling process meets the established requirements.
2. Routine production
- Perform regular sampling checks.
- Use online monitoring systems if available.
- Analyze process data trends.
Purpose: To confirm that the filling process remains stable and consistent during routine production.
Statistical process monitoring
A single measurement cannot represent the whole filling process. You should analyze filling data over time to monitor
- Average fill amount.
- Variation level.
- Changes in filling trends.
They help you identify process variation and unstable filling performance. Then, you can take measures in time to optimize the filling process.
In-process checks
In-process checks verify that filling accuracy remains stable throughout a production batch. You may collect samples at different stages, including at the beginning, middle and end of batch. Then you can know whether filling performance stay stable over time. Without in-process checks, the variation may only be discovered after production completed.
How to improve accuracy for liquid filling?
The complete filling process refers to many stages. Manufacturers need to control them to reduce filling variation.
Control liquid properties before filling
A stable liquid condition makes the filling process more consistent. Before filling, you should control
- Formulation consistency: Keep raw materials, concentration, and batch preparation conditions within preset limits.
- Temperature: Maintain suitable temperature conditions to keep liquid properties stable during filling.
- Sedimentation: For suspensions, define suitable mixing conditions and holding time to maintain product uniformity.
Optimize filling parameters
Even with a stable product, improper operation can increase fill variation. You should pay attention to
- Filling speed: Excessive speed may cause unstable flow or foaming.
- Filling time and dosing settings: Match operating parameters with product flow properties to maintain consistent volume.
- Nozzle movement and position: Keep the filling path consistent to reduce differences between containers.
- Foam control: Adjust operating conditions to reduce air during filling.
Conduct regular calibration and maintenance
Liquid filling accuracy is also related to machine components. It they wear out or get broken, the accuracy would be affected. Thus, regular maintenance is necessary. You can
- Calibrate dosing components.
- Check measurement systems.
- Replace worn parts.
- Clean product-contact areas.
The maintenance schedule could be set based on your production, such as checking the equipment before each batch or performing calibration monthly or quarterly. For key component maintenance, a weekly inspection is generally recommended.
Adopt process validation and qualification
For pharmaceutical production, filling accuracy should be supported by equipment qualification and process validation. This may include IQ, OQ and PQ, as well as routine monitoring, in-process checks, deviation handling, etc. They help confirm that the equipment and process can operate as required. Validation does not mean the filling process will remain perfect forever. It demonstrates that, under preset conditions, the machien can consistently produce products that meet requirements.
