Question regarding recycle content in Extruded Foam Plastics:

I am looking for references (citations) that show the maximum percentage of
recycle foam polymer that may be used during foam sheet or shape
extrusion.  In other words if a foam extrusion and thermoforming
process line produces say 50% trim and waste (typical for a
thermoforming foam sheet line), how much of that waste can be used
over by that same extrusion line?  I need several(3 or more) bibliographic
references with sources (either web sites, books, or papers will be
fine.)

I think it should be depend on the kind of polymer (PP,PE,ABS or else)

You are right: it will depend on the kind of polymer.  I am looking
for examples say of 2 or 3 or more kinds of polymers that show a
practical limit when using a recycled foamed polymer into the input of
an extruder making that foamed polymer again.

100% recyled content should be practical some days, if you
occasionally trash a shifts worth of scrap. The scrap will pick up
foreign matierial from the machinery, room and employees. Excessive
heat will occasionally damage a batch. Intense clean room conditions
will help, but the limit is when clean costs exceed the value of a
shift of scrap. It likely is not cost effective to trash only the
dirty and heat damaged scrap, using humans to make the decision/sort.
When convenient, the smallest peices of scrap should be scrapped,
especially peices with all dimentions less than one millimeter = dust.
The allowable recyle content depends considerably on the degree of
perfection expected by your customer. Please forgive my comment if I
missed the big picture.   Neil

Neil:
You are right about the some day 100% will be economic...  That is the
point of my question-- today many foam producers use some limited
regrind.  I am looking for indications of the present limiting percent
regrind cosistent with acceptable foam products.