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    Low cost Identification of plastics

    Low cost identification of plastics is certainly desirable. Identifying an unknown plastics material can be an expensive exercise if you submit it for a full chemical analysis.

    flame-resized-600 identification of plastics

    Most of the time we just need to know if it is high density polyethylene (HDPE) or polypropylene (PP). Or maybe we are wondering if it is polystyrene or polycarbonate.

    By methodically applying a battery of low cost tests (density, cutting, heating, burning), allied to simple observations (transparency, hardness, stiffness) and some basic knowledge of plastics materials and associated manufacturing processes, it is possible to distinguish between different families of plastics. We can even make educated guesses about particular grades (eg filler content).  Heating and burning tests are carried out on tiny slivers generated during the cutting test.  The key steps are establishing whether the material is self-extinguishing or free burning, observing the colour of the flame and identifying the degradation volatiles by smell.

    Using this procedure with students, some with a non-chemistry background, I found that, after 30 minutes demonstration of the tests, with appropriate attention to hazards and safe working practices, and 30 minutes practice on a set of ‘known’ samples, students could, in a further 60 minutes, identify most of a set of 16 ‘unknown’ samples, some with a high degree of confidence.

    The key to the procedure is a set of ‘known’ samples. Therefore, all tests on ‘unknown’ materials are carried out in parallel with ‘known’ samples until the nearest match is obtained.

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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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    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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    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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    What gives Moulders a Warped View on Life?

    Warping is one of the most frustrating injection moulding faults. This is because it does not always appear immediately, the causes are complex and prevention is difficult.

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