How Much Fill Is Enough?

Variations in container dimensions, differences in the amount of product filled due to filling machine tolerances, allowance for thermal expansion and aesthetic considerations are all factors in determining how much bulk is enough to fill a container.

Anyone who has spent time in the personal products industry and who has read consumers letters, has seen at least one complaint stating that upon opening a newly purchased product, the writer found the container was not what they considered to be ‘full.’ Such complaints usually go on to state that the customer felt cheated, wanted a refund, and would not purchase the company’s products ever again.

Most consumers have no idea of the complexity of deciding and specifying the amount of product that can be safely filled in various packages.

Manufacturing Variations to Consider

In preparing a filling specification, the first item to be considered is the fact that there are variations in the manufacturing processes. Tools for injection molding usually have several cavities connected by runners. All the cavities in the tool are supposed to be exactly the same size. However, as a result of machining tolerances, each one may differ from the others by a small amount. Although the runners are designed so that each cavity receives exactly the same amount of molten resin, there are often slight variations in the amount of resin actually reaching each cavity. Also, the tool cycle times may vary from one shot to the next, and so there may be slightly different amounts of resin entering each of the cavities from one shot to the next.

Molding: A Complicated Process

The injection molding process transforms a thermoplastic resin such as polyethylene, polypropylene, etc., into a desired component. Most thermoplastic materials can be softened or melted to form a useful item. This class of materials can then be softened or melted again, and formed into another, possibly different item. For injection molding applications, the resin is generally received in pellet form. Under heat and pressure, the pellets are melted and the molten plastic is forced by a reciprocating screw mechanism through a sprue bushing. After the sprue bushing, the resin flows through a series of channels, past a gate into the cavity. The cavity has the female shape of the finished part.

To enable the configuration of internal features, such as the pan well of a compact, there is a core associated with each cavity. The core is a male shape that has the configuration of the internal features. The cores may vary slightly from each other in the same way that the actual molds vary from each other.

The blow molding process that is used in the manufacture of bottles and jars can vary in a manner similar to that inherent in the injection molding process. Basically there are three blow-molding processes commonly utilized to produce hollow containers. The three methods are injection blow molding, extrusion blow molding and stretch blow molding.

Injection blow molding uses an injection molded parison or pre-form shaped like a test tube. This process is capable of accurate part weight, and very precise neck finish detail. In this process, the resin is injection molded into a parison cavity around a core rod. While still hot and on the core rod, the parison is transferred to the blow mold (female) cavity. Once inside the blow mold cavity, air or nitrogen is injected through the core rod causing the parison to expand until it takes the shape of the cavity. Once in contact with the cavity, the plastic cools, solidifies and has the intended shape.

In the extrusion blow molding process the molten resin is extruded as a tube. Once extruded, the tube or parison is enclosed by the two halves of the bottle mold. A hollow blow pin is inserted into the parison. Air or nitrogen is injected through the blow pin causing the parison to expand to conform to the shape of the mold and cooling the resin at the same time.

In the stretch blow process the parison is conditioned at a temperature slightly higher than the glass transition temperature. Then the parison is rapidly cooled and stretched to conform to the mold’s contours. This process has the advantages of improving the bottles’ impact strength, gloss, stiffness, and barrier properties.

In each of these processes there can be slight variations in dimensions from one cavity to another and slight differences in the amount of resin utilized in the formation of the perform.

Tolerances Establish Parameters

Container manufacturers recognize the potential for variations in bottle weights and differences in dimensions. As a result, when preparing the specification for a particular container, manufacturers list tolerances for such parameters as container weight, height, diameter, wall thickness, etc. Tolerances for each of these dimensions are the amount by which each dimension can vary and still yield an acceptable container.

All these parameters are related and, when taken together, they determine the volume of product that the container can hold. For example, a container that weighs slightly less than the optimum specified weight, would have thinner walls, and consequently be able to contain a greater volume of the product. Conversely, a container that is slightly heavier than the specified weight would have thicker walls, and would hold a slightly smaller volume of product.

Glass containers are subject to the same variations in dimensions as plastic containers. Again, this can be due to slight variations in the weight of the gob as well as to slight variations in the dimensions of the molds themselves.

Variations such as those described above do occur. They occur between successive shots from the same cavity of any given tool and they occur due to variations between cavities of the same tool.

Variations Affect Filling

Companies that fill product into bottles or jars need to consider the dimensional variations in the containers as well as the filling tolerances of their machines when developing filling specifications and label claims. Label claims are the quantities of product stated on the packages’ labels. Filling specifications, on the other hand, tell the manufacturing operation how much product can be safely filled into the container.

The state and federal governments have specific requirements for label claims. Basically, the regulations state that if there are a certain number of packages that contain less of the product than the amount stated on the label, then there must be an equal number of packages that contain more product than shown on the label. Of course, all packages can have at least the amount of product stated on the label.

How to Know What’s Enough

With all these considerations in mind, the following useful method for developing volumetric filling specifications has been developed.

For plastic containers especially, it is best to have at least two specimens of the container. Because the possibility exists that the plastic can expand under the internal weight of the water that will be used, it is necessary to work as quickly, yet accurately as possible. Expansion of the plastic can yield incorrect results. Further, if the surface of plastic containers is to be decorated, the surface needs to be flame treated to enable adhesion of any inks or other surface treatment. Flaming causes some shrinkage of the plastic and so an allowance for shrinkage must be provided for accurate results.

The first step in developing a volumetric fill is to fill one of the specimen containers to the highest level possible with water. At this level, the meniscus (the curved upper surface of the liquid in a container) should be slightly above the sealing land, the flat surface at the very top of the container (above the threads). When a cap is applied, this surface makes an indentation in the liner forming a sort of ‘baffle’ seal.

Utilizing a glass rod, or a plastic plate with a countersunk hole, make the liquid level the same as the level of the sealing land. When this step has been completed, it is likely that some water will have trickled down the side of the container. It will be necessary to completely dry the outer surface of the container before continuing to the next step.

All materials expand and contract in response to temperature changes in the environment. To prevent product leakage during storage or transit, it is necessary to allow for such expansion. Most companies have rules of thumb regarding the allowance needed for thermal expansion, which must be taken into account when specifying fill levels. The allowance for thermal expansion is usually referred to as the headspace requirement.

For purposes of this work, one gram of water (a unit of mass) is considered to be equal to one milliliter (ml) or one cubic centimeter (cc) (a unit of volume).

Adjust the scale of a triple beam balance of the type commonly found in laboratories to be equal to the minimum overflow capacity specified for the container. After drying the container, place it on the platform of the triple beam balance. Adjust the tare counterweight (usually the center beam of the triple beam balance) so that the filled container is balanced.

When determining the fill level of jars, remove enough water so that the level is about 0.1 inch below the container’s sealing land.The amount of water removed should be at least equal to the allowance required for thermal expansion. If necessary, remove additional water to compensate for thermal expansion.

Adjust the scale again to balance the container. This scale reading will correspond to the maximum fill in the jar with the minimum specified overflow capacity.

Utilizing a previously untested jar, repeat the steps outlined above with regard to filling the container to its maximum level, leveling the meniscus, and drying the jar’s surface. This time, however, adjust the balances scale to equal the maximum overflow capacity shown on the specification.

Place the filled container on the balances platform and adjust the tare counterweight to balance the jar. Reset the scale to the pre-determined minimum fill, which is usually equal to the label claim. Remove enough water to so that the jar is again balanced. The scale reading is the minimum fill in a maximum capacity jar.

In both examples, it is possible to see the level to which the jars might be filled. The volume of product corresponding to the weights determined in these steps can be calculated by multiplying the specific gravity of the product by the weight of the water.

Keep Function in Mind

Sometimes the fill level of a package is dictated by aesthetics. For example, many mascara vials are purposely made substantially larger than is required to fulfill the label claim. If the level of the mascara in the vial is too near the top, the brush would act as a piston as it is withdrawn from the package. Thus some of the mascara would come out of the package at the top of the brush. Some of this excess mascara would be deposited on the sealing land when the brush is re-inserted. The act of replacing the cap on the vial would force the mascara down onto the threads of the neck finish. The end result of this would be a very messy package.

To keep this from happening, mascara vials are made over-size so that the fill level can be kept low enough that the brush will not act as a piston. There are other products with similar characteristics that also need to be filled into over-size containers.