Insights / Packaging Resources / Packaging Fundamentals

Answers to Questions about Filling, Capping, & Finishing Glass Products

By: Berlin Packaging Specialist
Date: January 26, 2020

Filling, Capping, & Finishing Glass

Should a glass container be washed before filling it for the first time? When a glass container reaches the warehouse after production, it is clean because all the production phases guarantee this feature. Nevertheless, since the openings of the jars and bottles sometimes remain in contact with the packaging materials for quite a long time and condensation may form, the operator responsible for the final packing of the product before it is actually filled generally carries out an inspection using special equipment. The container is often turned upside down and blown with compressed air, or in other cases a precautionary wash is carried out with steam that is then powerfully aspirated in order not to leave any residue.

What is the “head space” in a bottle or jar? Why can’t jars be filled to the brim? Independently from the filling method, head space is necessary to allow the expansion of the liquid contained, caused by variations in temperature. The percentage of head space compared to the volume of the container must be calculated according to the filling method and also according to the type of content. For example, alcoholic drinks require a head space of about 4–5% of the volume to allow for expansion of the product, while maple syrup — which is generally packed hot — requires a head space of 7% to ensure the nominal quantity after cooling. The filling level for bottles is indicated on the bottom, and it is expressed in the distance from the brim (in millimeters). The nominal capacity for jars indicates the milliliters of water that can be contained by filling them to the brim while the product content is generally expressed in grams. Jars that contain several different products (each product has different volume variations) require careful attention to the head space during the packaging process and the heat treatment which follows the filling and sealing operations.

Can glass containers be filled with hot products? Yes, they can. For instance, fruit jams are poured into glass pots at about 85°C. The problem is the difference between the temperature of the glass and the temperature of the product.

Glass containers should be adequately “acclimatized”: it is recommended that they are not moved from the warehouses (that are generally unheated) directly to the filling plant. Careful attention should also be paid to the subsequent heat treatments when the containers are sealed (pasteurization and sterilization), since these treatments influence the internal pressure.

What is the significance of the “minimum through bore" and the internal profile of the neck? These elements are of crucial importance because the glass container is filled using a steel tube. Failure to take account of the minimum through bore (internal diameter of the opening) can lead to breakage of the container or tube, damage to the filling process, failure to complete the filling phase, etc. The internal profile of the neck refers to the stoppering profile inside the neck down to below the finish, while the minimum through hole applies to the whole length of the bottle neck.

What is the difference between natural cork and synthetic cork? With current technology, there are some fundamental differences between the two closure systems. A natural cork adapts better to the inside of the neck, allowing the appropriate, slow transmission of air, which is ideal for the natural aging of wine. It also has no expiry date, unlike synthetic corks whose already low elasticity gradually disappears with time when not in use. When a synthetic cork is used, particular attention is required during the bottling process because it is necessary to adopt an air pre-vacuum system while introducing the cork (in order to prevent the cork rising again) and the jaws should be perfectly closed. Natural cork, on the other hand, is much more flexible and even private users can insert them using a very simple machine with no other tools necessary. Obviously, using synthetic corks for short periods avoids the problem of cork taint. Moreover, synthetic corks do not release any particles into the liquid and they are therefore often used for spirits, in the form of the so-called “mushroom cork”.

How is the size of a synthetic cork determined? The answer is simple for synthetic corks since there are only two standard sizes for diameter and two for length, that is to say ø 22 x h 38 or h 42 for still wines and ø 23 x h 38 or h 42 for sparkling wines. Unfortunately, at present, few manufacturers have a synthetic cork suitable for containing pressure from gases.

Do different types of closures on bottles affect the contents? Closures for jars depend on the type of product contained and on the heat treatment used. Over time, the need for products that could last longer while retaining their principal features, thus extending their distribution, has encouraged the development of new sealing and packing systems.

Why do labels on bottles come off quite easily or have wrinkles? The reason can range from incorrect application methods or a lack of compatibility between the label glue and the surface treatment of the container to the glass shape that may have only one radius of curvature in the label application area. It often depends also on the label material, whether it is paper, PVC, or something else, and on the ability of the material to adapt its shape on application to glass. All treatment processes can be carried out well or badly and labels also fall into this category of quality control issues, which include the features of the glass, the selection of the most appropriate material, the quality of the labeling system, the overall quality of the production process, and the ability of the worker to adjust all the components so that the work may be carried out appropriately.

What causes the “waves” that are visible around the lower circumference of a clear glass bottle? These waves are left by the “parison” and caused by the different temperature of the blank mold at the points where the glass ends after the gob is loaded. Careful adjustment of the production equipment helps to prevent this optical effect (that is even more visible on clear glass bottles). Progressive pressure valves are used to limit this inconvenience. Once the bottle is filled, 90% of the optical effect disappears and the mechanical strength of the bottle is not affected. Other shadows on the body of the bottle may be caused by the incorrect loading of the glass gob.

Why does a white film sometimes appear on glasses and bottles? The cause of this white film is the humidity that is present in many environments, which creates a thin film on the surface of the containers. This film tends to draw out alkalis (sodium and calcium) from the surface of the glass, increasingly over time, until the contact liquid has completely evaporated. The resulting cloudy look is caused by the formation of carbonates due to the presence of CO2 in the air. Usually, washing or filling the glass will remove the film. It is important to note that this is just an “aesthetic” defect that doesn’t affect the use of the container and doesn’t cause any damage to the product inside. If glass is unused for a long time and kept in damp environments (in holiday homes for example) or after prolonged use, this process of extraction can become almost irreversible, with the accumulation of large quantities of insoluble calcium carbonate. This may result in a cloudy, rough surface caused by micro-abrasions and insoluble calcium deposits — a defect that is visible but harmless in terms of health.

Are bottle caps suitable for all types of bottled products? It depends on the product characteristics and on operational requirements in order to achieve the longest and best shelf life for the products. From the old methods of sealing with oil and natural corks, we have now arrived at a host of increasingly specialized and specific closure solutions to deal with all possible problems involving protecting the packed product from the external environment. These solutions also seek to offer the consumer ways to detect any chance or intentional interference with the original packaging (tamper-proof seals and flip caps, etc.).

How are screw caps manufactured and why are they categorized as pasteurizable or sterilizable lids? Screw caps are manufactured from tinplate sheets: they are cut into disks which are then deformed at their edges using special chucks which form the traditional “skirt” and the threads that allow the cap to fasten onto the glass jar. In order to ensure a perfect coupling between the jar and the lid, some “soft materials” called mastics are used, which need to be resistant to heat treatments to enable the preservation of the contents of the jar. More specifically, pasteurization at a temperature below 90°C; sterilization between 120 – 125°C.

How is the height of a screw cap determined? These parameters are fixed by international standards, which establish diameter and height along with other data, e.g., ø 28 top and bottom.

Are metal and plastic screw caps interchangeable? Some kinds of caps have the same gauge for the screw thread and therefore they are interchangeable as long as there are no specific requirements deriving from the contents or the filling procedures.

What is the most suitable way to fit the caps and lids manually or automatically? Both methods are suitable provided that the entire packaging process is correct. The automatic method ensures a more consistent and uniform application of the closure to the glass container.

Why are there different tops for bottles? The type of closure system used must be selected according to the contents of the bottle/jar, but the traditional mode of consumption is often the main factor influencing this decision. Closures themselves, whether made of cork or synthetic material, have no technical limitations in their use. Aluminum screw caps — very useful because they can be reclosed — are used mainly for carbonated drinks, spirits, or oil.

Why do some bottles on the market use non-refillable caps? Non-refillable caps are required by law in accordance with art. 18 of Law 161/2014, which regulates the distribution chain for virgin olive oils to prevent false imitations. The same principle is applied to alcoholic drinks, for marketing purposes, although it is not currently required by law. In this case, the closure also serves to control the flow when pouring.

How can we be sure that supplemental decoration will dot “detach” from the bottle? The term glass decoration means modifying the appearance of the container with a second processing. There are different ways to decorate glass. The most commonly used methods on bottles and jars are silk printing, sandblasting, acid frosting, and painting.

Can the decoration or application of a frosted finish on a bottle be harmful for its contents? These operations are only harmful if the interior of the bottle mouth is not kept adequately protected during the procedure.

Are there any size limitations for glass containers in countries inside or outside the EU? Containers are divided into two categories: measured containers (mainly bottles) and non-measured containers (pots/jars). The first category shows the capacity of the container on the bottom and the symbol 3 (reversed epsilon). The capacity is pre-determined, depending on the type of liquid contained.

For instance, in EU countries, alcoholic drinks can be in bottles of 700 ml or 1500 ml, while in the U.S, bottles can be 750 ml or 1750 ml. Other capacities are used in both geographic areas (250 ml, 375 ml, 500 ml, 1000 ml). Non-measured containers are generally pots or jars, where the volume of the container does not correspond to the volume of the content. These containers are produced to give the user the ideal volume for packaging a certain quantity of the product. In this case, the label provides the necessary information, stating the product weight, the amount of preserving liquid and the symbol “e”, which means that the filler is responsible for providing the content details. There are no general rules that apply to these containers because they differ according to market sector, national units of measurement, normal market practice etc.

Who decides the weight of a bottle and on what basis? There are a number of factors that influence the weight of a bottle: the bottle volume; the minimum thickness required for the sides and neck of the bottle to make it possible to use; the need to provide specific axis loads or to ensure a certain resistance to internal pressure (e.g., champagne bottles); and finally aesthetic requirements.

Why do bottles have the same capacity but different glass weights? This is due to different manufacturing requirements, for example the shape of a bottle may require a higher quantity of glass (e.g., for edges) or the use of the bottle may require a higher mechanical strength (for carbonated drinks or champagne), or for marketing purposes, because a heavier bottle appears to be more valuable or because the thickness of the glass can result in different shades of color.

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