Roll-on forming, sizes & tamper evidence
What is actually being bought, and what is being made on your line.
A ROPP cap ships as a plain aluminum shell in 8011 alloy with a liner already seated. The thread and the pilfer-proof bridges are formed by rollers on the capping head as the cap is pressed onto the glass. That is why the same shell can succeed on one line and fail on another, and why roller setup matters more here than any figure on the cap drawing.
The shell is blank when it arrives
Threads are not pre-formed. Two thread rollers cut the aluminum against the glass thread while a third rolls the skirt under the bottle's security bead. A shell is therefore not a finished closure until it has been through a correctly set head, which is why an incoming inspection of the cap tells you very little about how the pack will seal.
Bridges are the tamper evidence
The lower band stays attached to the cap body by small aluminum bridges. Opening tears them and there is no way to close them again — unlike a shrink band, which can be replaced. That structural irreversibility is the whole reason spirits and pharmaceutical fillers choose the format.
Sizes are diameter by height
Common builds are 28×18, 30×35, 30×60 and 31.5×60 mm. The first figure has to match the glass finish; the second is a design choice, since a 60 mm skirt covers most of a spirits neck and gives a much larger decoration area than an 18 mm cap on a wine or oil bottle.
Alloy and temper set how the metal flows
8011 aluminum in H14 or H16 temper is the usual specification because it is soft enough to take a thread cleanly yet stiff enough to hold the bridges. A harder temper cracks at the bridge; a softer one deforms without holding a thread, and both failure modes appear only after a line trial.
The liner does the sealing
An EPE or comparable wad sits inside the flat top and is compressed against the glass rim as the cap is rolled down. The aluminum contributes almost nothing to sealing, so a change of liner compound is a change of closure and needs its own leak testing against the actual product.
Decoration is why brands pay for aluminum
Embossing, silkscreen printing, spray coating, frosting, gold stamping and silver plating are all available on the shell. Custom decoration typically starts around 20,000 pieces with a three-to-four-week production window, which is what usually decides whether a small run gets a printed cap or a stock one.
