Third, the crystalline thermoplastics no
crystallization phenomenon can be divided into crystalline and non-crystalline
plastics (also known as amorphous) plastic according condensation occurs when
two categories. The so-called phenomenon of crystallization from the molten
state to the plastic is condensed, the molecules move independent, completely
unordered state, stops the free movement of molecules into, according to a
fixed position slightly, and there is a tendency to make the molecular
arrangement becomes a formal model phenomenon. As discriminating these two
types of standard visual appearance of plastic transparent plastic thick
plastic parts, generally crystalline material is opaque or translucent (such as
POM, etc.), the amorphous material is transparent (such as PMMA, etc.). But
there are exceptions, such as poly (4) methyl pentene to crystalline plastics
there is a high transparency, ABS is an amorphous material but not transparent.
Design and selection of the injection molding machine mold should pay attention
to crystalline plastics have the following requirements and considerations:
① temperature rises to more heat molding temperature required, use
the plasticizing capacity of the device.
② release when cooled back of the heat, to cool sufficiently.
③ molten and solid big difference in specific gravity, forming large
shrinkage, prone to shrinkage, porosity.
④ cooling fast, low crystallinity, shrinkage, high transparency. The
degree of crystallinity and related plastic wall thickness, thickness is slow
cooling, high crystallinity, shrink large, good physical properties. So the
crystalline material should be required to mold temperature must be controlled.
⑤ significant anisotropy, internal stress. After the release of the
molecule has not crystallized continued tendency to crystallize, in energy
imbalance, prone to distortion, warping.
⑥ crystallization temperature range is narrow, prone to not melt the
end of the injection mold or clogged inlet. Fourth, the heat-sensitive plastic
and susceptible to hydrolysis plastic
4.1 heat-sensitive means some plastics are
more sensitive to heat, long time at a high temperature heating or inlet
cross-section is too small, large shear, solid color temperature increased
prone to degradation, decomposition tendency, with this feature plastic called
heat-sensitive plastic. Such as hard PVC, polyvinylidene chloride, vinyl
acetate copolymer, POM, polychlorotrifluoroethylene and the like.
Heat-sensitive plastic decomposition by-products generated when monomers,
gaseous, or solid, in particular, some decomposition of the gas on the human
body, equipment, molds are stimulated, corrosive or toxic effects. Therefore,
mold design, injection molding machines and molding the choice should be noted,
should use screw injection molding machine, injection system should be large
cross-section, molds and chrome barrel should not be there * angle hysteresis
material, must strictly control the molding temperature, plastic the addition
of stabilizers, weaken its thermal performance.
4.2 Some plastic (e.g. PC) containing a
small amount of water, even, but at a high temperature, high pressure will be
decomposed, this performance is called easily hydrolyzable, which must be
pre-heated and dried. Fifth, stress cracking and melt fracture
5.1 Some plastic to stress sensitivity,
easy to produce internal stress during molding and crisp and easy to crack,
plastic parts in the external force or cracking which occurred in a solvent. To
this end, in addition to improving additives in the feed opening crack
resistance, the raw material should note dried, reasonable choice molding
conditions to reduce stress and increase resistance to cracking. And should
choose the right shape of plastic parts, inserts and other measures should not
be set to minimize the stress concentration. Mold design should increase the
stripping slope, choose a reasonable inlet and ejector mechanism, molding
material temperature should be appropriately adjusted, mold temperature,
injection pressure and cooling time, try to avoid too cold brittle plastic
stripping , molded plastic parts also should be treated after improving crack
resistance, eliminate stress and prohibiting contact with the solvent.
5.2 When some melting of the polymer melt
flow rate at constant flow rate through the nozzle orifice exceeds a certain
value, the apparent melt surface transverse cracks called melt fracture,
undermine appearance and physical properties of plastic parts. Therefore, in
the selection of high melt flow rate polymers and the like, should be increased
nozzle, runner, inlet cross-section to reduce the injection speed, material
temperature increase. Sixth, thermal properties and cooling rate
6.1 There are different kinds of plastic
specific heat, thermal conductivity, heat distortion temperature thermal
properties. High specific heat of plastics requires heat, should be used
plasticizing capacity of the injection molding machine. High heat distortion
temperature plastic cooling time can be as short, releasing early, but after
stripping to prevent deformation during cooling. Low thermal conductivity of
plastic cooling rate (such as ionic polymers slow cooling rate), it must be
cooled sufficiently to strengthen the mold cooling effect. Hot runner mold for
low specific heat, high thermal conductivity of plastics. Large specific heat,
thermal conductivity, low thermal deformation temperature is low, slow cooling
is not conducive to high-speed plastic molding, injection molding machine must
use the appropriate and strengthen the mold cooling.
6.2 All kinds of plastic and plastic parts
according to their shape characteristics of the types, requirements must
maintain proper cooling rate. So mold heating and cooling systems must be set
according to molding requirements to maintain a certain mold temperature. When
the material temperature should be raised when the mold temperature is cooled
to prevent deformation of plastic parts after stripping, reduce cycle time,
reduce the crystallinity. When the plastic is not sufficient to heat the mold
to maintain a certain temperature, the mold should be provided with a heating
system, the mold is maintained at a constant temperature to control the cooling
rate, ensure liquidity, improve conditions or to control the filling plastic
allowed to cool slowly, thick-walled plastic parts inside and outside to
prevent uneven cooling and improve the crystallinity degree. Good liquidity,
large forming area, press the uneven temperature materials molding plastic case
is sometimes used interchangeably to be heated or cooled or local heating and
cooling and use. For this reason the mold should be provided with appropriate
cooling or heating system.
Seven, hygroscopic
Plastics additives due to different degrees
of water affinities, so the plastic can be divided into moisture, water and non-absorbent
adhesion is not easy to adhere to the two water and feed water content must be
controlled within the allowable range within, or in high temperature, high
pressure water into the gas or the occurrence of hydrolysis, the resin foaming,
poor decreased mobility, appearance and mechanical properties. So hygroscopic
plastic must require appropriate heating methods and specifications for
preheating, when using to prevent further moisture.
more information www.cnmoulding.com
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