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    requirements of the surface roughness  and precision of plastic products are high, wear and tear on the
    mold surface take up a larger proportion of failures.
      3.1 Surface wear failure and the main failure reasons
    Thermosetting plastics flows on the surface of the mold serious friction, causing the surface roughness
    of the cavity or core surface increase. This will definitely affect  the appearance quality of suppression
    parts, which needed to be removed and regular polished. After repeatly polished, the cavity comes to
    failure due to size tolerance.
    When  plastics contains mica, silica sand, glass  fibers and other solid inorganic filler, it will
    significantly increase die wear. This not only causes rapid the deterioration of cavity surface roughness,
    but also causes dramatic changes in the mold cavity size.
    If plastic contains elements such as chlorine, fluorine, it will precipitation HC1, HF and other strong
    corrosive gases when heated, corrode
     mold surface,results in mold failure. If corrosion  comes with  wear ,the  two cross-injury will
    aggravate the failure of mold.
      3.2 The main reason of the plastic deformation failure
    Bakelite mold made in carburization of steel or carbon tool steel, especially small mold which is used
    overloaded in large-tonnage compressors, is more susceptible to produce overload plastic deformation.
    There may be sag, pitting, surface wrinkling in other parts of the cavity. Especially plastic deformation is
    more prone to produce in the edges and corners.  
    The main reasons of the plastic deformation failure  are that the hardened layer in the surface of the
    mold cavity is too thin,and lack of deformation resistance,or working temperature is above tempering
    temperature and lead to the phase transition and soften, leaving the mold failure early.    3.3 Fracture and Deformation failure and the main failure reasons  
    When the plastic cavity structure is complicated, while pressure it withstands is large, parts of cavity
    may be in a complex stress state, coupled with the stress concentration caused by structural factors, which
    may produce mold fracture.  
    The main failure reasons are due to structural  faults, temperature stress resulting from structure,
    thermal stress or insufficient tempering, at working temperature, residual austenite changes into
    martensite, which induce partial volume expansion and internal organization stress in  mold.  
    So,much attention should be put on the mold design, fully temper measure should be taken in heat
    treatment, the more important thing is using toughness steel to make  plastic mold, using high toughness
    steel (penetration carbon steel or hot work die steel)to make large and medium-sized complex bakelite
    mold cavity.  
      3.4 Fatigue and thermal fatigue failure and the main failure reasons
    The mechanical load of plastic mold is cyclical change. For example, in the filling and packing period
    of injection mold, cavity withstands the high-pressure of plastic melt tension, and in the cooling and
    stripping stages, outer load is completely removed. Repeatly working one after another, the cavity surface
    withstands pulse tensile stress, which may cause fatigue failure.  
    Heat load of Plastic mold is also cyclely change. For example, the melt material temperature injected
    into the cavity is generally around 200℃ or higher, while the cool temperature after hardening is about
    50℃. Repeatly heating and cooling , stress concentration may occurs in cavity surface which can lead to
    thermal fatigue crack. Coupled with the pulse tensile stress of cavity surface, the thermal fatigue crack
    may expand depth, which are the crack sources of fracture or fatigue fracture.  
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