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Under normal circumstances, compressing mold whithstands a larger force, easyly produce fatigue
cracking; injection mold temperature changes more rapidly, easyly produce thermal fatigue cracks.
4 The performance requirements of Plastic mold material
The main forms of plastic mold failure are wear, corrosion, deformation and fracture. Material
selection and heat treatment of plastic mold should be based on the performance requirements of the size
of the mold, shape, working conditions and failure, including the use of both the material properties and
process performance.
4.1 Requirements on the using performance of materials
These material performance requirements on mold depend on the nature of the processed plastic and
surface quality that plastic products required:
1) When the processed plastics contains hard filler, the hardness and wear resistance requirements of
material are high.
2) When corrosive substances is precipitated in the plastic molding process, material corrosion
resistance is required.
In general, thermosetting plastics contains solid fill, and the cross-linking reaction, often release
chemical gases and other substances, which requires that material also has the performence of high wear
and corrosion resistance. 3) When the plastic products have a high surface quality requirement, the light and small damage on
the mold surface is enough to cause mold failure, the mold need to be re-polished and can continue to use,
which makes the higher requirements of wear and corrosion resistance on mold material.
These performance requirements of mold material depend on die pressure, work frequency, impact
load and size and complexity of mold.
When the work temperature of mold is high, the local surface of the mold cavity is under the combined
effect of pressure and temperature ,which may produces temper softening and plastic deformation, or
produces surface tensile stress due to annealing changes.In addition ,the result of alternating heat load
generates thermal fatigue cracks. At this time, requires material have the thermal conductivity, a certain
degree of hot strength and thermal fatigue resistance .
4.2 Processing performance requirements on material
Most of the plastic mold requires high precision, surface quality, process performance requirements of
material is very prominent. Which makes it difficulty to manufacture mold and causes the cost of mold
increase, thus:
1)Good cutting processing and surface polishing When the plastic products is complex in shape, and
has high surface quality requirements, or has fine pattern on the surface, which requires the mold
material facilitate cutting, suitable polishing, and a good light etching . In particular, plastic mold needed
to be pre-hardened, the pre-heat treatment meets 35～45HRC hardness requirements, no heat treatment is
needed after machining,in order to ensure dimensional accuracy and surface roughness. This requires
material in a state of high hardness is still a good machinability. The material properties of Polishing and
light etching requires high metallurgy quality, less non-metallic inclusions, homogeneous fine, high
hardness and uniform.
2)Good processing of heat treatment and surface treatment High precision plastic molds,requires the
heat treatment process of material simple and deformation small. The mold requires wear and corrosion
resistance, requires that materials can be improve the corresponding properties of the surface by surface
treatment and will not adversely affect the overall performance.
3)Other manufacturing processing For the mold using cold extrusion to forming cavity, requires that