Setting Melt Flow Thresholds for Recycled Polyolefin Blends
Melt flow rate thresholds for recycled polyolefin blends must be set using multi-load shear testing to bound contamination and prevent processing scrap.
Compounding Tolerance
Meaning
Permissible limits of variation in the proportion of additives and fillers mixed into a base polymer resin represent the standard deviation of formulation accuracy. This compounding tolerance determines the consistency of the physical properties of the delivered plastic material. Moulders rely on these bounds to ensure that the batch to batch performance of raw feedstock remains within predictable limits.
When the tolerance is exceeded, the mechanical strength of the moulded parts can change. A failure to control these limits also affects the density and thermal stability of the raw material.
Formulation Variance
Additive ratios must be maintained within tight percentage bounds to prevent drift in the molecular structure of the finished compound. Plastic compounders specify the compounding tolerance based on the limits of their gravimetric feeding equipment and the sensitivity of the resin formulation. A tighter limit on reinforcing fibres like glass or carbon prevents dramatic swings in the tensile modulus of the injection moulded part.
High precision compounds demand narrow allowances to ensure stable properties.
Melt Behaviour
Rheological stability during the injection cycle depends directly on the concentration of plasticisers and lubricants. If compounding tolerance is too wide, the melt flow index of the resin fluctuates between batches, causing injection pressure to drift during production. This instability leads to a narrow processing window.
Moulding Impact
Dimensional variance in the final product often traces back to fluctuations in the filler loading of the raw material. Excessive mineral filler alters the volumetric shrinkage of the melt as it cools in the mould cavity, producing parts that fall outside of the critical dimensional limits. Scrap rates rise and tool adjustments become necessary when the incoming compound fails to hold its specified limits.
Maintaining tight control over compounding tolerance is therefore necessary for automated high volume production.