Melt Index

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Plastic resin pellets used in injection molding material selection

Definition of Melt Index

Melt index (MI), also called melt flow rate (MFR), describes how many grams of polymer flow through a standard die in ten minutes at a specified temperature and load. Higher values generally mean lower viscosity and easier flow.

Resin suppliers publish melt index ranges for each grade. Molders use MI together with part geometry, gate size, and fill simulation to estimate required injection pressure and cycle behavior.

Why It Matters for Plastic Injection Molding

Material selection

Matching melt index to wall thickness and flow length helps AMP avoid short shots on thin-wall parts or excessive flash on easy-flow grades. Substituting a higher-MI resin without revalidation can change shrinkage and mechanical properties.

Process setup

When troubleshooting fill issues, comparing lot-to-lot melt index certificates can explain sudden viscosity shifts. Process windows are often documented around a qualified resin grade and supplier.

Related Terms

ShrinkageGate
Higher MI improves flow but can reduce strength, heat resistance, or dimensional stability depending on polymer type. Material choice balances processability with end-use performance requirements.