Packaging is sorted according to which material weights the most.
If the packaging does not specify whether it is metal or plastic, try scrunching it: if it does not return to its original shape, it is probably metal. If it does, it is plastic. If you are still not sure which material it is, put it into the mixed waste.
Coffee and crisp packets as well as some dog and cat food packaging are usually mainly plastic, although they may have a thin layer of aluminium foil. These packets are sorted into plastic packaging, but due to their metal composition, they are generally not recyclable but are used in energy production. Some coffee packets are made of plastic only.
Twist ties for bags may contain metal wire in addition to plastic. Perform the scrunch test mentioned above: if the tie does not return to its original shape, it contains more metal than plastic and should be put into a metal recycling container.
Milk and juice cartons are recycled as carton packaging even though they contain carton and plastic and often also metal.