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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.


    It is easy to say that the cause is differential shrinkage. However, the non-uniform shrinkage arising from residual stresses has two components: thermally-induced stresses and flow-induced stresses.

    Thermally-induced stresses arise from non-uniform cooling, either related to thick and thin sections or from unbalanced mould cooling.

    When melt is injected into a mould cavity, the polymer chains become elongated in the direction of flow and will do their best to recover before the moulding is completely cooled, causing greater shrinkage in the flow direction than the transverse direction, which will twist a centrally gated disc out of plane.

    To complicate matters, shrinkage and warping are materials related.  Semi-crystalline thermoplastics have a higher mould shrinkage than amorphous thermoplastics and consequently a greater tendency to warp.  Warping is also a problem with fibre-filled materials because of alignment of fibres in the flow direction, giving non-isotropic properties.

    As with many moulding faults, prevention requires action at different stages.  Designers should avoid geometric asymmetry if possible.  Toolmakers need to be careful about gate positions and concentrate on effective, balanced mould cooling.   Processors have to balance the holy grail of short cycle times with the need to anneal out residual stress, either in the mould or after ejection.  I am old enough to remember mould shops with areas of floor space occupied by mouldings cooling on jigs to prevent warping.  In some cases warping does not show up until well into service.  Today processors have to juggle injection rates, pressure dwell times and mould temperatures if designers and toolmakers have failed to do their bit.

    At the risk of repeating myself, the best advice is to carry out a full simulation analysis at the design stage.

    Finally, more information on this topic may be available on the excellent British Plastics Federation website.

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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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