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A screw cap is a metal cap that screws onto threads on the neck of a bottle, generally with a metal skirt down the neck to resemble the traditional wine capsule ("foil"). A layer of plastic (often PVDC), cork, rubber, or other soft material is used as wad to make a seal with the mouth of the bottle. Its use as an alternative wine closure is gaining increasing support as an alternative to cork for sealing wine bottles.
Benefits and concerns
In brief, compared to cork:
Traditionally associated in the US with extremely inexpensive jug wines or even "skid row" wines, the screwcap is making a comeback due to concern about premature (or sporadic) oxidation and cork taint. Screwcaps have a much lower failure rate than cork, and in theory will allow a wine to reach the customer in perfect condition, with a minimum of bottle variation. Cork, of course, has a centuries-old tradition behind it, and there are also concerns about the impact of screwcaps on the aging of those few wines that require decades to be at their best. Some argue that the slow ingress of oxygen plays a vital role in aging a wine, while others argue that this amount is almost zero in a sound cork and that any admitted oxygen is harmful. Various studies are underway, although one data point is that producers in Champagne have aged their wines under crown cap for quite some time with no apparent outcry. No manner of removing the cap with the grace of traditional cork pulling has yet emerged. 
1. How to remove a screwcap - Sediment
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3. Amcor Closure Systems Pty Ltd
4. / Screwcap now "the norm": new study - Decanter
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6. Screwing around with caps - Napa Valley Register
7. The Search for the Perfect Seal - Vintage Direct
8. Dr Vino - Bringing closure? A screwcap-cork showdown
9. Food production daily - Largest wine producer in the US adopts screw tops
This article uses material from the Wikipedia article "Screw_cap_(wine)", which is released under the Creative Commons Attribution-Share-Alike License 3.0.