Rheological Variance and Gel Contamination in House Blended Compounds
House blending recycled resins causes melt viscosity drift and crosslinked gel contamination that disrupts mold filling and degrades part mechanical strength.
House Blend
Meaning
Custom mixtures of virgin resin and in-house regrind materials are prepared by plastics processors to reduce raw material costs and minimize production waste. Utilizing a house blend allows a moulder to reclaim runners, sprues, and rejected parts by granulating them and mixing them back into the primary feed hopper. This practice requires tight control to avoid altering the mechanical properties of the moulded part.
The ratio of regrind to virgin resin must be carefully monitored to maintain a steady melt viscosity.
Economic Incentive
Reclaiming production scrap directly at the press lowers the overall cost per kilogram of the manufactured part. Extrusion and moulding trials run with a house blend can achieve savings of up to twenty percent on material purchases. This reclamation prevents the accumulation of bulky scrap in the warehouse.
It also eliminates the freight fees associated with shipping scrap to external recycling facilities.
Processing Risk
Additive depletion and thermal history degrade the molecular weight of the recycled plastic fraction with each heat cycle. If a house blend contains too many generations of regrind, the polymer chains shorten and the part becomes brittle. Contamination from dust or mismatched polymers can also occur in the granulator, causing cosmetic defects or mechanical failures in the finished mouldings.
Processors must establish a maximum limit on the percentage of regrind allowed in the hopper.
Rheological Variability
Fluctuating proportions of virgin and recycled material alter the melt flow behaviour in the injection barrel. The house blend often exhibits a higher melt flow index than pure virgin resin, which can cause flash or uneven cavity filling. This instability forces the machine operator to adjust injection pressure and cushion size frequently to prevent out-of-specification parts.
Automated dosing systems help mitigate this risk by precisely metering the regrind fraction into the virgin stream before it enters the throat of the extruder.