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    Is MFR really much help to moulders these days ?

     Melt Flow Rate (MFR)

    Often a moulder has to change material grades. One of the first properties to be consulted on the new data sheet is usually the MFR. This is to establish if the new grade has the same melt viscosity performance.

    mfr graph

    Even if the new grade has an identical MFR, this is no guarantee it will perform the same in injection moulding.  MFR values in data sheets are derived from a relatively simple test. This is easy and inexpensive to carry out and was originally intended as a method of crudely ranking materials.  The test involves measuring the mass of melt extruded from a simple piston extruder, through a standard orifice and at standard temperatures. It is carried out at flow rates far removed from the flow rates experienced in the cavities and feed systems in injection moulding.

    The problem is that thermoplastic melts are non-Newtonian fluids, which means that the resistance to flow (apparent melt viscosity) varies with flow rate (shear strain rate).  Thermoplastic melts are shear-thinning fluids (pseudoplastic in old terminology). The melt viscosity can be several orders of magnitude lower at moulding flow rates than in the MFR test.  Melt viscosity, and hence ease of mould filling, also decreases as the melt temperature increases.

    Shear thinning is beneficial for the moulding process. However, it means that MFR is not a great predictor for what actually happens during mould filling.

    To get a better comparison of the melt flow characteristics of one grade against another, most material suppliers can provide you with more relevant viscosity data, measured over a range of shear rates and temperatures.  Much of this is accessible through databases such as CAMPUS.

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    Coping with weld lines

    In injection moulding, weld lines (knit lines) form when two melt fronts meet. If the melt fronts do not coalesce completely, at best there will be a cosmetic flaw. At worst there will be a mechanical weak-spot, with strengths of the order of 10 - 90 % of the material potential.

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    What determines friction between thermoplastic components?

    Friction is an important property for thermoplastics in bearings and gears but also has a part to play in assembly of plastic parts (snap-fit and interference-fit) and ejection during moulding.

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    Will Styrenic thermoplastics evolve further to meet design demands ?

    Styrenic thermoplastics?

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    Polypropylene: the Workhorse of the Plastics Industry

    Polypropylene. Little did Karl Zeigler or Giulio Natta realise, 60 years ago, when they were developing a catalyst system to produce a useful thermoplastic from the inexpensive monomer, propylene, that their work would have such far reaching consequences.

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    What makes medical grade plastics so special?

    Toughness and transparency are important properties for the constituents of intravenous lines.

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    The Long and the Short of Fibre Reinforcement of Thermoplastics

    Fibre Reinforcement. The advantages of adding glass fibre to thermoplastics to increase stiffness (modulus), strength, heat distortion resistance and dimensional stability are well known.

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    What causes mouldings (and moulders) to be off-colour?

    When mouldings are not the intended colour, the first thing to check is the raw material, particularly the dosing rate, if you are using masterbatch, and the quality of regrind.

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    Transparent ABS can be a clear winner

    Transparent ABS. Mentioning transparency in the context of ABS moulding materials can raise a few eyebrows. This is because ABS is normally taken to be opaque and indeed the vast majority of grades of ABS are opaque.

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    Where are Engineering Thermoplastics Blends going ?

    The timeline of appearance of materials for the plastics industry can be viewed as several overlapping phases.

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    Understanding the difference between SBS & SEBS thermoplastic elastomers?

    The difference between SBS and SEBS thermoplastic elastomers explained.

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