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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.