rotationalmoulding_conference_workshop_problem two

Rotomould WorkshopProblem 2 Why do moulders use difficult to mould materials – low MI instead of high MI

Materials, by Ian Hansen, Rotoconsult

2

Why would moulders of large tanks prefer low Melt Flow materials? (2MI or 3MI)

Slow or long heat cycles, typically difficult to mould, compared to 5MI or 6MI materials

Lower MFI

Higher Viscosit

yDifficult

flow

Higher MFI

Lower Viscosit

yEasier flow

Flowability of the polymer once melted

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Melt Flow Index ASTM D1238 or ISO 1133

As MFI increases, faster & easier to mouldAs MFI decreases, tensile strength improves

Melt Flow Increase

Recall from discussions on Problem 1 that the Melt Flow Index, or Melt Index ,is a relative indication of the ease of flow of a polymer melt.

Melt Flow Decrease


Source : ARMA Materials Properties Guide

Why use low MI?Lower Melt Index = Higher toughness

Longer polymer chains

More Entanglement

Cracking Resisted

HIGHER TOUGHNES

S

toughness is the ability of a material to absorb energy and plastically deform without fracturing.


Large above ground water or chemical storage tanks are subject to substantial hydrostatic loads

An increase in toughness typically equals … Increased tensile strength Increased flexural strength Increased impact strength

Why use low MI?

Units LL705UV LL711UVMFI g/10min 5.0 3.0Density g/cm3 0.935 0.938Tensile Strength at Yield Mpa 17.5 19.5Flex Modulus 1% Secant Mpa 700 800Hydrostatic Design Basis Mpa 6.89 8.62

As MI decreases

Toughness

increases

Lower Melt Index, typically combined with higher density, results in greater toughness

ianhansenconsult.com

19.5Mpa

17.5Mpa

Higher MI better for toys

Lower MI better for large tanks

Select the right MI for the right product


Ian Hansen Consult

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