(IV) injection molding process should pay
attention to the issues (including the requirements of the injection molding
machine)
In order to reduce internal stress and
surface quality defects in the injection molding process should pay attention
to the following aspects of the problem.
a) should be used as the special screw with
a separate temperature control nozzle of the injection molding machine;
b) injection temperature plastic resin does
not decompose under the premise, to use the high humidity injection;
c) Injection pressure: generally higher in
order to overcome the melt viscosity of the defect, but the pressure is too
high will produce stripping difficulties caused by internal stress and
deformation;
d) Injection speed: In the case of filling
meet, generally should be low, it is best to use a slow-fast and slow
multi-stage injection;
e) dwell time and molding cycle: to meet
product filling, no depression, the case of the bubble; should be kept short to
minimize melt residence time in the barrel;
f) screw speed and back pressure: Under the
premise of quality to meet the plastics should be as low as possible to prevent
degradation of the possible;
g) Mold temperature: cooling products good
or bad, a great impact on the quality, so the mold temperature must be able to
precisely control the process, if possible, some good mold temperature should
be high.
Issue (5) other aspects of
Because you want to preventing the
deterioration of the surface quality, minimize the use of general injection
molding release agent; when the use of regrind shall not exceed 20%. Except for
the outside pet, after the products should be treated to eliminate stress, pmma
should at 70-80t hot air circulation drying 4 hours; pc should be in clean air,
glycerol. Liquid paraffin and other heating 110-135 ℃, time
according to the product, the highest need more than 10 hours. The pet must
biaxial stretching step, in order to obtain good mechanical properties.
Third, transparent plastic injection
molding process
(A) process characteristic transparent
plastic: In addition to these common problems, also have some processes
transparent plastic properties, are summarized as follows:
1, process characteristics pmma the pm-ma
viscosity, somewhat less liquidity, and therefore must be high temperature
materials, high injection pressure injection job, which affect the injection
temperature is greater than the injection pressure, but the injection pressure
is increased, it will help to improve shrink products rate. Injection wide
temperature range, the melting temperature of 160 ℃,
decomposition temperature of 270 ℃, material temperature and therefore wide adjustment range, the
process better. Therefore, to improve mobility, we can proceed from the
injection temperature. Poor impact resistance, good abrasion resistance, easy
to draw flowers, easy to rattle, it should raise the mold temperature, improve
the condensation process, to overcome these deficiencies.
2, the technical characteristics of pc pc
viscosity, high melting temperature materials, poor mobility, must therefore be
at a higher temperature injection (between 270-320 ℃), the
material temperature adjustment range is relatively narrow, technology is not
as good as pmma. Injection pressure has little effect on liquidity, but the
viscosity, still larger injection pressure, corresponding to prevent stress,
dwell time should be as short as possible. Shrinkage, dimensional stability,
but a large internal stress, easy to crack, it is appropriate to raise the
temperature rather than pressure to improve fluidity, and improving the mold
temperature, mold structure and to improve the post-processing to reduce the
possibility of cracking. When the low injection speed, the gate prone to
defects such as corrugations, radiation nozzle temperature to be controlled
separately, mold temperature is higher, the flow channel, gate resistance is
smaller.
3, the technical characteristics of pet pet
molding temperature is high, and the material temperature adjustment range is
narrow (260-300 ℃), but after melting, good liquidity, so the process is poor, and
often at a nozzle extension to be added to the anti-flow devices. Mechanical
strength and performance after the injection is not high, we must in order to
improve performance and modified by stretching step. Precise mold temperature
control, to prevent warping, distortion of the important elements back to back
this proposal hot runner mold. Official high mold temperature, otherwise it
will cause poor surface gloss and stripping difficulties.
Fourth, transparent plastic parts defects
and solutions
(A)
silver streaks: from filling and condensation process, the internal stress
anisotropy effects of stress generated by the vertical direction, so that the
resin flow occurs on orientation, while non-current orientation and produce
different refractive index and raw silk flash pattern, when After its
expansion, may make the product cracks. In addition to the injection molding
process and mold note, the best products for annealing. As pc material can be
heated to above 160 ℃ holding 3-5 minutes, then natural cooling can be.
(B) bubble: Master in moisture and other
gases within the resin did not break out, (in the mold condensation process) or
because of insufficient filling, excessive condensation condensing surface and
form a "vacuum bubble."
(C) the surface gloss difference: mold
roughness, on the other hand the condensation too early, so that the resin can
not copy the state of the mold surface, all of which make the surface uneven
produce small concave strike, leaving the products lose their luster.
(d) shock pattern: refers to intensive
ripple from the center sprue formed, the reason because the melt viscosity is
too large, front-end material has been condensed in the cavity, then break
through this material and condensing surface, leaving the surface shock pattern
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(E) white, fog halo: mainly due to the fall
in the dust in the air into raw material or feedstock caused by too much
moisture.
(f) white smoke, black spots: mainly due to
the plastic barrel, due to local overheating of the machine and produce resin cylinder
formed by the decomposition or deterioration.
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