1. crack
Cracks are more common form of
plastic defects, the main reason is due to the stress caused by the
deformation. There are stress and deformation residual stress, external stress
and the external environment arising.
(-) Cracks caused by residual stress
Residual stress is
mainly due to the following three conditions, namely filling excess, stripping
launch and metal inserts cause. As in the case of a crack filled with surplus
produce, and its solution is mainly in the following aspects:
(1) Since the
minimum sprue pressure loss, so if a crack is generated in the vicinity of the
main sprue, you can consider using the multi-point distribution point gate,
side gate and handle-shaped gate way.
(2) ensure that the
resin does not decompose, it does not deteriorate the premise, an appropriate
increase in the temperature of the resin melt viscosity can be reduced to improve
flowability, but also can reduce the injection pressure, to reduce the stress.
(3) Under normal
circumstances, prone to stress when the mold temperature is low, the
temperature should be raised. But when the high injection speed, mold
temperature even lower, but also can reduce the stress.
(4) injection and dwell for too long can
also cause stress, which is appropriate to shorten or Th hold pressure
switching times better.
(5) a non-crystalline resin, such as AS
resin, ABS resin, PMMA resin and other more crystalline resin such as
polyethylene, POM prone to residual stresses, it should be noted.
Stripping launch, due to the small
stripping slope, the mold plastic and punch roughness, so the introduction of
excessive force, stress, and sometimes even the introduction of the rod around
produce whitening or cracking phenomenon. As long as careful observation of
crack-generating position, you can determine the cause.
In injection molding, while
embedded metal parts, most prone to stress, and prone to cracking after a
period of time before, great harm. This is mainly due to thermal expansion
coefficient differences between metal and resin stress, and over time, the
stress exceeds the strength of the gradual deterioration of the resin material
and cracks. To prevent cracks resulting, as experience, the wall thickness of 7
& quot; and metal parts embedded in an outer diameter substantially
unsuitable for general-purpose polystyrene inserts should be added with minimal
impact on the nylon insert. Since the thermal expansion coefficient of glass
fiber reinforced resin material is smaller, more suitable for the insert.
In addition, before forming the
metal inserts were preheated, also has a good effect.
Crack (ii) external stress
External stress here is mainly due to the
stress concentration caused by unreasonable design, especially in sharp corners
should pay more attention. Figure 2-2 shows that the desirability R / 7 &
quot; a 0.5 to 0.7.
Crack (iii) the external environment caused
Chemicals, moisture absorption due to water
degradation, and excessive use of recycled materials will make the physical
deterioration, cracks.
Second, less filling
Filling the main reason for
lack of the following aspects:. I insufficient capacity of the resin. ii. lack
of cavity pressure. iii. resin illiquid. iv. not exhaust effect.
As improvements, mainly from
the following aspects:
1) extended injection time, to prevent the
molding cycle is too short, causing the gate before the curing resin filled
cavity upstream difficult. 2) increase the injection rate. 3) increasing the
mold temperature.
4) increase the resin temperature. 5)
increase the injection pressure.
6) expand gate size. Height generally equal
to the gate of the wall thickness of 1/2 ~ l / 3. 7) Set the gate in the wall
thickness is maximum.
8) Set exhaust ducts (average depth of
0.03mm, width 3 ~ smm) or exhaust strokes. More important for small workpieces.
9) between the screw and the injection nozzle to leave some (about smm) buffer
distance. 10) selection of a low viscosity grade material. 11) the addition of
lubricants.
In the process of injection molding, there are still many problems. For example
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