Crowning glory The humble crown – it merits a degree course in its own right

The crown is by definition a flat metal closure with a corrugated skirt, which serves to seal bottles mechanically by crimping the skirt over the bottle’s neck from outside. That’s it. The crown – a simple sheet-metal seal for beer bottles? Far from it. A high degree of technical precision and a great amount of R&D efforts are concealed in this seemingly simple, tiny thing, offering as it does well-nigh infinite opportunities for product marketing.

Drama at the Warsteiner Brewery. In 2012, for its annual “Crown Championship”, Warsteiner organised a lottery where you could win one of 90 (yes, really!) Mercedes C-Class cars with an AMG sports package, plus more than 7 million instant prizes. Five of the main prize-winners were told in Warstein that their winning code had been issued twice, and that they had to share their car with a second winner. Sheer horror in the eyes of the winners, Had the supplier of the crowns made a mistake? He was responsible for ensuring that each winning number appeared only once, that the umpteen millions of crowns with winners and blanks were homogeneously mixed, that the prizes were evenly distributed over a defined time period, that traceability was guaranteed.

What had gone wrong, despite all the quality control? Nothing. Warsteiner was having a joke, and confronted the five main prize-winners in front of a concealed camera with a second “winner” and a notary, both of whom were merely actors (you can see this on YouTube). In the end, the drama was resolved to the palpable relief and satisfaction of all concerned.

Around 12 billion crowns a year

Today’s Rauh Group, originally founded in 1947 as a sole-proprietorship family firm, ranks among the leading vendors of closures in Europe. Its production operations at its three facilities in Küps, Lohne and Ljubljana (Slovenia) manufacture around 12 billion crowns a year with a workforce of 190. In addition, four billion aluminium roll-on closures, 60 million swing-stoppers and 50 million natural and plastic-capped corks are dispatched to breweries and to bottlers of mineral water and wine all over the world. The Rauh Group is managed by Roland Rauh, together with his children Christel and Florian Rauh. The daily agenda for Michael Blüchel as Head of Production and Nico Engelhardt, who is in charge of quality assurance at the Küps facility, consists of making top-quality closures and getting them to the bottlers just in time.

The plants are equipped with machines to the latest state of the art. A separate printshop coats the tinplate sheets and applies the appropriate design to them in an offset-printing process. Later on, exactly 729 crowns are punched from each sheet. A special machine inserts into the punched blank a sphere of extruded plastic compound, which downstream is shaped to form a seal. Camera-based inspections of each and every single crown, scrutinising it both inside and out, backed up by comprehensive in-process monitoring, serve to assure the stringent quality requirements applying for the closures.

Instant prizes or playing draughts at your leisure

In response to the ever-increasing scope of promotions, and with a view to the rapidly growing diversification of prize distributions, to the type and number of the various winning codes involved, and their legibility, the Rauh company has developed a process that is different from previous identification options like inkjet printing or lasering on the laser-sensitive compound. With this new process, the crown’s inside is printed with a coloured mirror, which the desired code is lasered into. The crown is then fitted with a transparent seal. Thus direct contact with the beverage – as was the case with previous processes – has been eliminated. “We have meanwhile put several billion of these lasered codes on the market”, explains Michael Blüchel.

A question of sealing

The American inventor William Painter had first filed a patent application for the crown closure in 1892. This, however, also required a sealing material. Thanks to its natural properties, cork offered the best preconditions in this context, so that initially thin slices of natural cork were used for this purpose. Later on, these natural-cork slices were replaced by ones made of composition cork, enabling the bottles to be sealed faster and more efficiently. What persisted, though, was the disadvantage of oxygen permeability, plus a risk of taste impairment to the beverage caused by cork and glue. Attempts were made to minimise these drawbacks by inserting a piece of tin foil. As from 1970, manufacturers started to develop seals made of expanded PVC, which exhibited very good sealing properties even in the case of minor to medium-serious damage to the neck finish, and also enhanced taste stability as compared to cork. But PVC contains plasticisers and phthalates, rendering it unsuitable for coming into contact with beverages. What’s more, the relatively high level of gas exchange easily led to an impaired ageing stability.

This was the reason why as from 1985 PVC-free seals were launched on the market. In these, PE (polyethylene) granules are inserted into the crown using an extruder. A punch embosses a profile into the seal. “The PVC- and phthalate-free thermoplastic elastomers offer a significantly reduced gas exchange, do not affect the taste so much, and extend the product’s shelf-life. However, they tend to have poorer sealing properties in the case of medium-serious damage to the neck finish and are more sensitive as far as crowning errors are concerned”, explains Nico Engelhardt.

Options for improvement, thanks to barrier compounds

Depending on the product, the temperature treatments and the bottle quality involved, the choice of the right sealing compound assumes major importance. Oxygen-related problems for beer, such as taste impairment, a change in colour or also cloudiness, are caused by oxygen pick-up during filling, with this oxygen permeating into the beer after the bottle has been crowned. “Choosing the right sealing compound offers options for improvement”, says Nico Engelhardt sagely. The manufacturers have come up with three kinds of solution here: firstly passive barrier products, secondly active ones also referred to as scavengers, and thirdly a combination of passive and active protection. All three sealing-compound variants perceptibly reduce the amount of dissolved oxygen in the beer. Needless to say that the combination product provides the most marked reduction. “The main criterion of quality control we’re concentrating on is always the sensor technology involved, this we check using the most sensitive beverage of all, which is pure water”, explains Nico Engelhardt.

There are a whole lot of parameters that must be given due consideration if an optimum result is to be achieved, e.g. the use of paints and inks, the crowning function at the filler and – last but not least – also the conditions prevailing during storage on the bottler’s premises. The crown, as insignificant as it might seem at first glance, merits a degree course in its own right.

21 instead of 24 teeth

The inventor William Painter had given his crown 24 teeth. Little did he know then what modern high-speed lines would look like. During automatic feed to the sealer in a crown chute, these crowns would quite easily get snagged. “This is due to the fact that with an even number of teeth two on each side are flush against the metal transfer rails, which may decelerate the crown flow, with the result of the crowns jamming in the feed chute. With an odd number of teeth, however, there are two teeth flush against one side and only one tooth against the other, which means the crown can turn and will slide towards the sealer without any problems”, explains Michael Blüchel. When people came to realise this, the manufacturers reduced not only the number of teeth from 24 to 21 but also the crown’s height from 6.75 millimetres to 6.0 plus/minus 0.15 millimetres. Even today, some bottlers are learning the hard way that every tenth of a millimetre in the crown’s diameter is crucial.

A turn for the better

Crowns can be manufactured from a variety of sheet-metal types, from chrome-plated, tinned or stainless-steel sheets. Whereas chrome-plated or tinned sheets differ only in terms of gloss, stainless steel is the preferred material in cases where corrosion problems have to be expected. With trendy new drinks, for example, where it needs to be assumed that they will be stored in iced water, or for beverages intended to be sold in the world’s tropical regions.

Internationally popular variants include twist-off crowns, which are not themselves fitted with a thread. Rather, the flights are not produced until the crown is applied to the bottle, into which a thread has been moulded. Then, the bottle can be opened by turning the crown. It has to be said, though, that the bottles’ flights are rather susceptible to damage, which is why bottles of this kind are predominantly non-returnable ones. They are handled on standard crowning equipment.

Comeback for swing-stoppers

As far as beer bottles are concerned, the crown has over recent years acquired some competition from the swing-stopper reborn. Swing-stoppers have the big advantage that consumers can reclose the bottle, as well as lending the product upmarket appeal and a traditional image. The use of a variety of wire qualities, ranging from galvanised material right through to stainless steel, means that swing-stoppers have quite a lengthy useful lifetime. Usually, plastic stopper heads are nowadays employed as an alternative to the traditional porcelain version.

Sheet thicknesses and costs

When it comes to crowns, too, costs are obviously a major factor. This is why bottlers have been requesting studies examining the possibilities of reducing sheet thicknesses used for crowns. The standard material is a mere 0.235 millimetres thick; substantial reductions, down to 0.20 to 0.18 millimetres, have already been achieved.

On an international scale, there are also trials ongoing with sheets whose thickness has been reduced to 0.16 and 0.15 millimetres. “Economies like this may well come at the expense of the lateral runout characteristics and the sealing properties, as well as leading to CO2 losses”, warns Nico Engelhardt. “This kind of reduction mania runs the risk of ruining that type of closure sooner or later.” Wouldn’t that be a pity, and a totally undeserved end for “the crowning glory” that has meanwhile endured for more than 120 years?