Capacity deviation (actual capacity differs from the labeled value, often indicating underfill) after hot filling of bag-in-box packaging is primarily related to factors such as the physical properties of the liquid in a high-temperature environment, the thermal response of the packaging material, and filling process control. The specific reasons are as follows:
1. Volumetric Contraction Effect of High-Temperature Liquids
The core of hot filling involves injecting hot liquids at 80-95°C into the bag. The volume of liquids (especially water-based liquids) varies significantly with temperature:
The inevitable consequence of thermal expansion and contraction is that hot liquids expand during filling due to the high temperature. If the volumetric measurement is strictly adhered to the “labeled capacity” during filling (e.g., if the labeled capacity is 5L, inject 5L of hot liquid during filling), the liquid will contract as the temperature drops after cooling to room temperature (around 25°C). (The coefficient of volume expansion of water is approximately 0.00021/°C; a temperature difference of 90°C to 25°C results in a volumetric contraction of approximately 1.4%). Failure to provide for shrinkage: If the filling equipment doesn’t compensate for the liquid’s temperature characteristics (i.e., overfilling at high temperatures to offset shrinkage after cooling), the actual volume after cooling will naturally be lower than the stated value, resulting in a deviation.
2. Thermal Shrinkage and Deformation of Packaging Materials
The inner flexible bag of a bag-in-box is made of a composite film (such as PET/PE, PET/aluminum foil/CPP). The physical properties of this material change at high temperatures, indirectly affecting the volume:
Composite Film Thermal Shrinkage: The inner PE or CPP film and the outer PET film have different thermal shrinkage rates (e.g., PE shrinks more than PET). When exposed to hot liquid, the bag may experience localized wrinkling and deformation due to the uneven shrinkage of the layers, reducing the actual volume within the bag. For example, if the bag’s sidewalls shrink, even if the liquid volume is normal, the bag may not be able to fully contain it, or some liquid may be squeezed out during filling.
Temporary Changes in Material Ductility: At high temperatures, the inner film (especially PE) may temporarily expand, allowing it to hold more liquid during filling. However, upon cooling, the film shrinks back to its original shape. If the liquid volume also shrinks due to cooling, this double shrinkage can further amplify the final volume deviation.
3. Inadequate Venting During Filling
During hot filling, the air inside the bag expands due to heat. Incomplete venting can directly affect the actual volume filled:
Air Occupies Space: When hot liquid is injected, the air inside the bag expands due to heat. If the valve or bag is not designed with venting channels, the expanding air will squeeze the liquid, causing some liquid to be “pushed out” of the bag during filling (especially during the sealing process), reducing the actual volume filled.
Air Contraction after Cooling: Even if no liquid overflows during filling, if air remains inside the bag, the air volume shrinks upon cooling, causing the bag to collapse, making it appear underfilled (the actual liquid volume may meet the specified value, but the visual appearance of the bag shrinking may be misinterpreted as “underfill”).
4. Metering Errors in Filling Equipment
Hot filling requires high temperature adaptability from equipment, and metering accuracy is easily affected by high temperatures:
High temperature interference with metering devices: The filling machine’s flowmeter (such as a gear pump or rotary pump) or load cell may experience accuracy deviations in high-temperature environments. For example, the viscosity of high-temperature liquids decreases (e.g., syrups and sauces become more fluid at high temperatures), which can cause the pump to “overflow” or “inaccurately meter,” resulting in discrepancies between the actual injection volume and the set value.
Lack of temperature compensation: If the equipment does not adjust metering parameters based on the liquid’s real-time temperature (e.g., liquid density decreases at high temperatures, resulting in less mass per volume), and only sets the filling volume based on normal temperature parameters, actual volume deviations will occur.
5. Secondary Impacts of the Heat Sealing Process
After hot filling, the bag opening or valve must be heat-sealed. Improper process control can indirectly lead to capacity loss:
Liquid overflow during heat sealing: If heat sealing is initiated too early (the liquid is still expanding due to the high temperature) or if the heat sealing pressure is too high, the bag may be squeezed, causing some liquid to overflow through unsealed gaps, resulting in a reduction in the final volume. “Invasion” of heat-sealed edges: At high temperatures, the heat-sealed edges of the film may become wider due to excessive softening. If the heat-sealed position deviates, the overly wide heat-sealed edges will “invade” the effective space in the bag, reducing the actual volume that can hold liquid.