Generating the process tree

The process tree consists of inputs, outputs, waste and emissions of the set of economic and physical processes which contribute to the service provided by the previously-determined functional unit. It is drawn up based on the functional unit itself and the scope of the study. There is agreement at large on applying a cradle-to-grave approach, to cover the steps from the extraction of natural resources to waste management, including reuse, recycling, incineration, and disposal. 

Once the process tree is complete the deUneation of the system boundaries can take place. There are three boundaries to consider  

To delineate, all economic inputs and outputs need to be traced back to environmental inputs and outputs starting from the functional unit. This creates a natural border between the product system and the environmental system. The chain of processes is only broken when there is recycling to or from other product systems (open-loop recycling). The treatment of emissions in the environmental system is not a part of the product system that produced them it must be considered as an individual system. 

Main process tree with different product system.s highlighted. 

With polymer products, most processes generate more than one marketable product. In-plant trimmings, runners, and off-specification products can be used as input to other product systems. To avoid an overwhelmingly large product system, most practitioners propose that the process tree be cut at the input of the recycled material. Those processes that are excluded must nevertheless be clearly described so as to preserve transparency. 

---