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    A Polymer Data Sheet – Just how useful are they?

    A Polymer Data Sheet - Just how useful are they?

    Every polymer supplier will provide you with a basic data sheet describing the properties of a particular grade.  The good news is that each property value relates to a well understood, repeatable, reliable test related to international standards. Therefore, this makes comparison with other grades of similar plastics from a range of suppliers much easier.

    Data Sheet – bad news

    The bad news is that the standard test methods refer to a highly stylised test under a specific set of conditions (temperature, load, time, etc). Most likely these conditions do not correspond to the product service conditions you have in mind. As a result, materials data sheets are at best a snapshot of the potential of a particular grade.  Very few of the values can be used directly by design engineers in their development calculations.

    Data Sheet – good news

    The good news is that most suppliers, on request or through websites such as UL IDES Prospector, or software such as Campus can provide additional and more relevant property data.

    For calculations of mechanical functionality and also section thickness in a particular design, the modulus of rigidity data accessed from basic data sheets will refer to short term loading and low strain at room temperature.   For load/deformation data over longer periods and higher strains, designers need to access creep data derived from measuring the change in dimensions under a range of loads and at different temperatures and humidity conditions.  Such data may not be easy to come by or has to be generated at a significant cost.

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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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    Creepy things can happen with long term loading of thermoplastics.

    Deformation is defined as the change in the shape of a body caused by the application of a force (stress). It is proportional to the stress applied within the elastic limits of the material.

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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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    Do your mouldings suffer from jet lag?

    When explaining the moulding fault commonly known as ‘jetting’ to part-time students from the plastics moulding sector, I usually prefaced my remarks by saying that ‘jetting’ was not a fault that you should see these days. Then one of the students appeared the next week with a classic example of ‘jetting’.

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    Do Izod and Charpy make the appropriate impact?

    Cracking of plastics components under impact was a major blow to the plastics industry in the early days.

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