After hot filling, bag-in-box valves can be difficult to open and liquid flow out smoothly. This is primarily due to the effects of high temperatures on the valve structure, pressure changes within the bag, and changes in the state of the contents. The specific reasons are as follows:
1. Valve components deform or stick due to high temperatures
During hot filling (typically 80-95°C), core valve components (such as the plastic valve body and silicone seals) can easily undergo physical changes due to the high temperatures if they lack heat resistance or are poorly designed:
Seal ring aging/sticking: Silicone seals are crucial for ensuring a tight seal. If their heat resistance is insufficient (for example, ordinary silicone seals can withstand temperatures of ≤120°C, but residual heat after hot filling may persist), high temperatures can cause the seals to harden and lose their elasticity, or they can stick to the surface of the valve body (such as PP), especially after cooling. This can increase resistance when the valve is opened and even lead to sticking.
Valve body thermal deformation: Plastic valve bodies (such as PP and PE) may deform slightly at high temperatures (such as shrinkage of the aperture and component misalignment). If this deformation does not fully recover after cooling, the internal passage of the valve can become narrow or blocked, preventing smooth liquid flow.
2. Negative pressure inside the bag causes the valve to “suck”
After hot filling, the liquid contracts during cooling, creating negative pressure inside the bag (internal pressure lower than the external atmospheric pressure).
This negative pressure can “suck” the valve’s moving parts (such as the valve core and ball valve) tightly in their sealed positions, resulting in additional pressure resistance when opening the valve, causing it to “difficult to open.”
If the valve design doesn’t account for negative pressure compensation (e.g., lacks a one-way vent), negative pressure inside the bag will persist. Even if the valve is forced open, the liquid will flow out slowly due to the insufficient pressure differential (especially for high-viscosity liquids such as syrups and sauces).
3. Cooling causes the contents to change state and clog the valve.
Hot-filled liquids (such as sauces containing particles or high-viscosity syrups) may change state during cooling, potentially blocking the valve passageway:
Increased viscosity: Liquids that are relatively fluid at high temperatures (such as jams and condensed soups) experience a significant increase in viscosity upon cooling (e.g., becoming semi-solid at room temperature). If the valve passageway is narrow, this reduced fluidity can result in poor flow. Particle Settling/Solidification: Products containing fruit pulp or spice particles may settle at the valve inlet after cooling, or partially solidify due to the drop in temperature (e.g., oily components), directly blocking the passageway and preventing liquid from flowing out.
IV. Indirect Impacts of Heat Sealing on Valves
After hot filling, the bag or valve interface must be heat-sealed. Improper process control can indirectly squeeze the valve structure:
Heat seal misalignment: If the heat seal edge is too close to the valve, the high temperature may cause the composite film around the valve to shrink, squeezing the valve body and causing the valve core to become stuck and unable to open properly.
Excessive heat seal pressure: Excessive compression can deform internal valve components (such as springs and ball valves), damaging the original opening mechanism and causing valve failure.