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.
Sad to admit, but in fact, most folks that are quoting probably don’t realize the write UP in material “cost” that should be considered when significantly less percentages of regrind are incorporated. If you DON’T use it up and end up selling it for a few cents on the dollar, the WRITE DOWN is actually a part of the COST of the product you originally bought the material for. And most don’t have any idea of what the “true” cost to own of an 80:20 mixture is considering handling, size reduction, inventory holding and shrinkage costs etc are over the life of a business project. This should be part of the ‘living business plan’ for every product life cycle in your factory.