Melt flow index (MFI / MFR)
The mass of molten polymer that flows through a die in 10 minutes at a set temperature and load – 230 °C/2.16 kg for PP, 190 °C/2.16 kg or 5 kg for PE – expressed in g/10 min (ISO 1133, ASTM D1238). A low MFI means high molecular weight and strength, a high MFI easy flow: pipe and blow-moulding grades are about 0.1–1, film 1–4, injection 10–60, and fibre and meltblown 25–1,500 g/10 min. When comparing offers, always check the test conditions.
Related terms
Homopolymer, random copolymer and block (impact) copolymer
A homopolymer is made from one monomer (e.g. propylene only); it is hard, stiff and cheap, used for raffia, BOPP film and general injection moulding. A random copolymer has a little ethylene randomly built into the chain, making it clearer and more flexible – preferred for transparent containers and hot-water pipe (PP-R). A block or impact copolymer contains a rubbery phase giving high impact strength at low temperatures, used for automotive parts, pails and crates. Because the wrong type causes cracking or clarity problems, the order must state the type clearly.
Prime, wide-spec and off-grade polymer
Prime material is original product sold under the producer’s brand and grade number, meeting every value in the TDS. Wide-spec is production in which one or more values – e.g. MFI or colour – fall outside the narrow range but remain usable; off-grade may be transition production, sweepings or contaminated material and is usually sold unbranded and without a COA. Off-grade is cheaper but varies from lot to lot, so samples must be tested and “as is” terms and claim rights made clear.
Technical data sheet (TDS)
A product description sheet showing typical physical and chemical properties, test methods, applications, processing conditions and storage advice. For a plastic resin it lists MFI, density, tensile strength, impact strength and recommended melt temperature. Because the values are “typical” they are not guaranteed; binding values should be written into the contract as a specification and confirmed for each lot by a COA.