The important consideration should be end product and what your definition of "REGRIND" is. Reprocessing your own internal scrap (if kept clean and uncontaminated with anything else) is a significantly different question basis than using unknown or unclassified "regrind" from outside sources. As to the end applications – many custom processors adopt a simple program internally like the 80:20 approach above simply because it generally insures inventory 'velocity' versus turning cash into useless warehouse ballast by using start-up scrap, sprues etc back into the product as the run progresses. There is no written down /prescribed guidelines for use of Regrind or process waste back into to Processing machines (Injection molding/Extrusion/Blow etc). For non-engineering products & commodity products in India, there are processors who use only Recycled/Regrind material for cost effectiveness. Generally, most of processes 80:20 virgin to regrind is preferred, but it's not a guideline. All depends upon type of product, property requirement & application of products etc.