We crowns for bottles were invented in America by William Painter in 1892. Since then we have dominated the closure market for glass beer bottles and many soft drinks as well. To fit around the bottle top the outer edge has 21 serrated “teeth” and the underside is coated with a flowed-in plastisol liner. Early seals used glued cork or multi-layered paper. All materials are thoroughly tested for flavor taints and food compatibility. A crown cork works by simply being pressed onto the bottle top and the “teeth” are crimped around the bulge in the bottleneck and are wrapped underneath it, locking it in position. The sealing compound inside the crown is compressed against the bottle top forming a complete gas-tight seal. Nowadays we crowns are manufactured by the billion all over the world and are regarded as the standard closure for beer and soft drinks bottles. We have the benefit of low cost, reliability, convenience and pressure resistance. We have been around for so long that almost all the problems are known and understood and the failure rate during production and use is very low.
We Crowns are made of low carbon steel that is supplied as sheets usually with a thickness of 0.24 mm although other sizes are used. This steel comes in 2 forms:
- Tin plate.
- Tin-free steel.
The tin plate is formed by coating the steel electrolytically with tin to a standard thickness. The alternative tin-free steel is protected by electrolytic coating on both sides using chromium and chromium oxide. In the first stage in the production process, the sheets are varnished on both sides. The two sides are slightly different. On the internal side, a varnish is used which will protect the sheet from corrosion and also facilitate anchoring of the lining to the shell. The external side may be colored and then varnished with a tough, abrasion resistant varnish that also has good slip qualities. After application of the varnishes, inks are applied to give the customer’s required design, often followed by a transparent finish varnish. We are then formed by passing the printed sheets through molding and blanking presses. The standard dimensions of a crown are:
External diameter – 32.1 mm
Internal diameter – 26.75 mm
Depth – 6.0 mm
No. of teeth – 21
After being pressed out the crowns have a soft lining compound applied to the inside surface. There is a range of materials used for this, the main one being PVC although alternative thermoplastics are available. These plastics are usually blended with stabilizers and adhesives to assist in binding the plastisol to the metal. The liner is applied under pressure and then it is hot moulded to the inner crown surface to give it a predetermined profile. The weight of liner used is usually around 200 mg, and it is shaped to be relatively thick on the outer edge, where it is in contact with the bottle top. After moulding we are baked to modify the liner material and ensure adhesion to the metal. Once we are cooled the we are inspected, nowadays using high-speed cameras, then packed for shipment. Various dimensional checks and others such as hardness, corrosion resistance, and pressure resistance are carried out. We usually are designed to fit a bottle with an external neck diameter of 26.85 mm and they have an external diameter of 28.5 mm when crimped. They should be able to withstand a bottle pressure of 10 bar (150 psi) and the minimum acceptable is usually 8 bar.
Schematic representation of a crown cork The majority of us produced are applied simply by pressure and removed with a bottle opener. Another type is the twist-off crown. These are applied to bottles with a modified neck with external screw threads. They can be made from slightly thinner steel (0.22 mm.) and are not crimped on to the same diameter as normal crowns (28.7 mm.).
Nice to meet you see you soon on your next pint ….