Overflow and Gravity - Comparing Two Popular Filling Principles

For those just starting out in the packaging world with a new product, or even an established product that can no longer be filled by hand due to demand, the names of the different types of filling machines offered by manufacturers may mean little, if anything at all. For instance, two of the more popular liquid fillers are the overflow and gravity fillers. Yet to those not in the packaging industry, overflow and gravity probably mean very little. Below we will take a look at the differences between these two principles, while encouraging those who are new to packaging equipment to request an explanation of all of the different filling methods available for whatever products you may be packaging.


When bottles are manufactured, an attempt is made to keep the process as consistent as possible for obvious reasons. Most packagers want each and every container to look similar to help boost shelf appeal and even small inconsistencies may not be identifiable using the naked eye. However, when liquids are added to bottles with slight variations in interior volume, the visible fill level can vary, even though the same amount of product is present in each container. The overflow filling machine is used to combat these inconsistencies in volume by filling each and every bottle to the same exact level, regardless of interior volume.

Overflow fillers achieve this level fill by using a unique nozzle that not only seals over the bottle opening, but allows product to flow back through the nozzle to the holding tank or reservoir, thus eliminating product waste as well. As the nozzle seals over the bottle, the fill port is opened, allowing product to enter the bottle. Once the product reaches the specified level in the container, controlled by using spacers on each of the fill heads, the product exits the bottle via the return port, "overflowing" out of the bottle and back into the tank or reservoir. This principle can be used for free-flowing products in containers both large and small, though great variations in size may require different nozzle sizes for efficiency purposes.

As noted above, the level fill can mean that the actual volume of product will vary from container to container. While this can be a concern to industries such as distilled spirits, pharmaceuticals and others with volume requirements, almost all industries will have a target volume along with an acceptable range. In most cases, the change in interior volume will be so slight that the danger of falling outside of the acceptable range is nearly non-existent. However, packagers with this concern will want to check with their bottle manufacturers to ensure that interior volume will not vary greatly and may want to (or even be required to) include some type of quality control check in their own packaging process.


While overflow fillers will allow a packager to fill to a level, gravity filling machines let a packager fill each bottle with the same volume of product using a time based filling principle. Once again referring to the discrepancies in the interior volume of containers, a gravity filler will ensure that each bottle containers the same amount of product, even if this results in some containers looking more filled or less filled than others.

While gravity filling machines can use a number of different nozzles, generally speaking the nozzles will open for a pre-set amount of time to allow product to flow from a tank into the waiting bottles or containers. On most machines, the fill time for each individual head or nozzle can be adjusted to allow for greater accuracy. Times can also be set to fractions of a second to also improve volumetric accuracy. Once the pre-determined time has been reached, the nozzles will cut off the flow of product. For semi-automatic machinery, simple operator activated switches are used to activate the cycle, while automatic machinery will use a PLC to achieve the same.

The choice to use a fill to level system or a fill by volume system will always depend on the characteristics of the unique project at hand. For packagers using clear containers, the trend is toward the overflow principle, allowing for an increase in shelf appeal and aesthetic value as a result of the level fills. For packager not using opaque containers, the gravity filler may offer a better alternative, especially when consistent volumes are important to the industry. But even exceptions to these rules may apply where containers or product themselves are unique. The best advice for a new packager is to not only know, but understand, the different options that are available for packaging your own unique product before selecting any packaging machinery.