Most plastics are recycled mechanically though they can also be recycled other ways.
Compared with lucrative recycling of materials, such as glass and metal, plastic polymers recycling is often more challenging because of low density and low value. There are also numerous technical hurdles to overcome when recycling plastic. Wikkipedia
Mechanical Recycling
Mechanical recycling, very simply, consists of melting down the old plastic and using it to make new products
Mechanical recycling can only be done a few times before the polymers break down and the quality of the plastic degrades. So, unlike glass for instance, you cannot necessarily use mechanically recycled polymers to remake the same product. You have to use them to make something different. The chain of recycling might go as follows
- Virgin PET bottle to fleece or carpet
- Carpet fibers to plastic lumber.
- Plastic lumber to waste disposal plant – though manufacturers claim that plastic lumber can be recycled again.
This process is sometimes called DOWNCYCLING.
Sadly it is not quite as easy as that. There are hundreds of kinds of plastic – some easy to recycle, others not (plastic film for example ). Consequently is not cost effective to recycle all plastics. While it is theoretically possible to recycle nearly all plastics. generally it is the simpler plastics that actually are recycled.
ALL the above is open to change and may have changed by the time you read this. It will also depend where you are reading it.
Sorting Plastics for recycling
Plastics must often be of nearly identical composition to mix efficiently.Wikkipedia
When different types of plastics are melted together, they tend to phase- separate, like oil and water, and set in these layers. The phase boundaries cause structural weakness in the resulting material, meaning that polymer blends are useful in only limited applications. Wikkipedia
This means that plastics have to be identified and sorted into types. In the olden days recyclers had to rely on the plastic code stamped onto the plastic product and sort by that. These days most use automatic sort systems to identify the resin, such as near infrared (NIR) technology. “NIR sorting is the industry’s preferred plastics sorting technology because it can accurately identity the many different polymers already in use today (different polymers reflect an identifiable light spectrum).” Inez
NB PLA plastic manufacturers claim that NIR technology can recognise PLA plastics (compostatble plastics) and so refute the oft stated claim that PLA plastics bugger up the recycling system. You can see some test results here.
Additives, fillers, and reinforcements are used to change and improve the physical and mechanical properties of plastics. However the widespread use of dyes, fillers, and other additives in plastics can also affect the ease with which they are recycled. You can read about plastic additives here
“The polymer is generally too viscous to economically remove fillers, and would be damaged by many of the processes that could cheaply remove the added dyes. Additives are less widely used in beverage containers and plastic bags, allowing them to be recycled more often.
The percentage of plastic that can be fully recycled, rather than downcycled or go to waste can be increased when manufacturers of packaged goods minimize mixing of packaging materials and eliminate contaminants. The Association of Plastics Recyclers have issued a Design Guide for Recyclability. Wikkipedia
You can read the Design Guide for Recyclability. here
Mechanical Recycling Case Studies
Melt & Reform my visit to a mechanical plastic recycling plant 
Melt & Press – making innovative plastic sheeting
Plastic lumber sturdy, longlasting and functional? Or tacky as anything!
Make your own plastic recycling machine.
More on recycling here
Other ways to recycle and reuse plastic trash here
Recycling and waste plastic – a discussion
Related recycling posts