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    What does a moulder understand by the term ‘diesel burning’

    In the early days plastics moulders tended to apply descriptive terms to moulding faults, such ‘gate splay’, ‘mica marks’, ‘orange peel’, ‘flash’ and ‘record grooves’. (Explain that to the younger generation).  These terms do not really indicate the root cause. Nor do they suggest a possible solution, but diesel burning is an accurate description of what has happened. 

    scorch-mark-poor-mold-venting-end-flow-path diesel burning

    When a rapidly approaching thermoplastic melt front chases air in the mould into a corner, the highly compressed air reacts with the hot melt, which is essentially a hydrocarbon fuel, to give a high temperature oxidation reaction. In other words this is combustion as in a diesel engine.  If the combustion is inefficient the polymer will be converted to carbon. This gives a black spot at the extremity of the moulding.

    To avoid diesel burning, or the associated fault, gas spotting, mould venting needs to be improved to release the compressed air.  Reducing the clamp force can be a quick fix . I have even seen moulders adding a dummy ejector pin near the offending area.  Prevention is always better than cure and injection moulding simulation during the part design can quickly identify possible problems and avert them by changing the gate position.  Reducing the injection pressure can also help.

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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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