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Plastics failure mechanism

Discussion in 'Electronic Repair' started by N_Cook, Jun 5, 2009.

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  1. N_Cook

    N_Cook Guest

    I've never seen reference to this mechanism, so transcribed and placed
    here.

    From (UK) Hacker GP42 record player of 1966, technical bulletin, not dated
    but probably close to '66

    "Investigation over a period of time of reports of knob breakages have shown
    that the adhesive used to fix the chrome trim has affected the material and
    caused the knob to fracture after a period of time. A further investigation
    by the manufacturers disclosed that the material used in one batch of knobs
    was an incorrect type. DIAKON was the material specified and POLYSTYRENE was
    used in error. Unfortunately there is no way in which we can easily
    differentiate between these materials other than by a flame test. "

    Where I used to work , there was a flow-chart wall-chart for identifying
    plastics by flame test, smell/smoke character when burnt and other basic
    characteristics, anyone know of a WWW version somewhere?
     
  2. Sylvia Else

    Sylvia Else Guest

    I'm sure modern health and safety departments would have kittens at the
    thought of an employee deliberately igniting plastics for the purpose of
    determining the resulting smell.

    Sylvia.
     
  3. Bob Larter

    Bob Larter Guest

    Yeah, same here. The closest I can recall seeing to that was a US army
    tech guide for identifying metals based on the kind of sparks you got
    when you hit it with an angle-grinder.
     
  4. Alan Douglas

    Alan Douglas Guest

    Where I used to work , there was a flow-chart wall-chart for identifying
    My chart is from "Materials In Design Engineering", February 1959. It
    doesn't appear to be posted on the net anywhere. I could send you a
    scan.

    Alan
     
  5. N_Cook

    N_Cook Guest

  6. N_Cook

    N_Cook Guest



    Arrived safely, I wonder what HCl smells like
     
  7. Sylvia Else

    Sylvia Else Guest

    Bit like Chlorine, and just as nasty.

    Sylvia.
     
  8. Keeshond99

    Keeshond99 Guest


    I forgot to mention this in my earlier post. The causative
    mechanism is stress corrosion cracking. The solvent in the glue, in
    the presence of the stress caused by the tightening of the setscrew
    or by the stress caused by stretching the knob to fit over a push-on
    shaft, precipitates the shattering of the knob.

    While stress corrosion cracking in metals is well known, it also
    occurs in many plastics. A crack will start at the plastic surface
    and propagate perpendicular to the applied stress. In addition to
    solvents, ozone can cause stress corrosion cracking in many
    plastics. Gasoline will cause stress corrosion cracking of
    polyethylene terephthalate; this is the reason that you should
    never store gasoline in PET soft drink bottles.
     
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