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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.

    The change of colour may make the mouldings more yellow or brown. The problem probably is thermal decomposition of the base polymer. This is due to excessive times at elevated temperatures.   Possible courses of action are as follows. Reduce the melt temperature, if possible, but certainly reduce the time the melt sits in the plasticising cylinder or in the hot runner system, by reducing the overall cycle time, reducing the cushion in the cylinder and delaying the plasticisation stage.

    Moving the job to a machine with a smaller plasticising capacity cylinder will also limit the time at high temperature.  However the decomposition may have been triggered at the drying stage. This could be as a result of selecting temperatures and dwell times beyond the recommended drying conditions.  As a rule of thumb, the rate of decomposition doubles with every 10oC rise in temperature.  Some thermoplastics are sensitive to high shear rates and the temperature rise this induces in narrow feed and cavity channels.  In this case, reducing injection speed can help.

    If the discolouration is more localised, in spots or streaks, the problem may still lie in contamination of the raw material and regrind. However, localised decomposition can result from some material being held up in dead spots in the cylinder and mould feed system.  If spots or streaks are still present after purging the system, checks should be made on the non-return valve on the screw tip and hot runner feed system. This may establish where material is being held up and exposed to multiple cycles.

    Black spots at end of flow are more likely to be caused by ‘diesel burning’….see a previous blog.

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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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    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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    Is MFR really much help to moulders these days ?

    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.

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