Using regrind in any ratio affects two things, Cost and Quality.
I agree to Tylers approach that if use of 100% regrind produces parts between your AQL's (Acceptable Quality Levels), feel free to use any percentage of regrind.
Obviously, we cannot consume more regrind than is being produced but on many occasions there are two or more parts being made in the same material. This gives use the opportunity of making Part A in 100 virgin while part B is made of 100% regrind!
Generally, RAP (Regrind As Produced) method is used for regrinding. The sprue is collected for the shift and handed over to the granulator where it is crushed and mixed with virgin material in a pre-set ratio and brought back to the machine for processing.
On some occasions, the process is automated. Conveyers are used to crush the waste throgh on-line slow speed granulators. Prorportional valve or auger feeds are used to supply virgin and regrind in a pre-fixed ration to the hopper.
If you are using RAP on continous basis, there will ALWAYS be material from the first generation, no matter how small in percentage but.....it will be there!
In contrast to this methodoligy of regind (RAP), i suggest using CASCADE method of regrind and it is best explained by an example:
Shot weight : 100 gms
Net wt of part : 75 gms
Batch size: 500,000 parts
Using RAP, the parts from the first lot will be made from virgin material. Parts from secon lot will be made of 75% virgin and 25% regrind. Third lot : 75% virgin - 18% from 1st gen and 7% from second gen (approx) ....and so on!
Using Cascading method, the first lot of (say) 300,000 parts is made from 100% virgin mateiral. This will produce a regrind of 7,500 kg.
Run this 7,500 kg (100% regrind) to produce 100,000 parts and 2,500 Kg of second regrind.
Run this 2,500 Kg (100% second regrind) to produce 33,333 parts and 833 kg of third regrind.
Run 833 kg to produce 11,106 parts and 277 kg of fourth regrind.
Sell this 277 kg of regrind and start all over again!
The benefit of this method is that you always have the regrind history of the product and you exactly know the what percentage of regrind is there since there is 100% of only ONE generation.
It is cost effective and easy to manage.
Ofcourse there are certain do's and dont's with each method and becuase the variables are so many, we cannot safely say that one method is the best one!