Secrets of injection molding process adjustment

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Secrets Of Injection Molding Process Adjustment
Proportional control of injection speed has been widely adopted by injection molding machine
manufacturers. Although the computer-controlled injection speed segmented control system has
long existed, the advantages of this machine sets are rarely used due to the limited information.
This article will systematically explain the advantages of applying multi-speed injection molding,
and briefly introduce its use in eliminating short shots, trapped air, shrinkage, and other product
defects.
The close relationship between injection speed and product quality makes it a key parameter of
injection molding. By determining the start, middle, and end of the filling speed segment, And to
achieve a smooth transition from one set point to another set point, a stable melt surface speed
can be ensured to produce the desired molecule and the smallest internal stress.
We recommend the following speed segmentation principle:
1) The velocity of the fluid surface should be constant.
2) The fast injection should be used to prevent the melt from freezing during the injection
process.
3) The injection speed setting should take into account that the critical area (such as the runner)
is filled quickly while slowing down at the water inlet.
4) The injection speed should be guaranteed to stop immediately after the cavity is filled to

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prevent overfilling, flashing, and residual stress.
The basis for setting the speed segmentation must take into account the geometry of the mold,
other flow restrictions, and unstable factors. The speed setting must have a clear understanding
of the injection molding process and material knowledge, otherwise, the product quality will be
difficult to control. Because the melt flow rate is difficult to directly measure, it can be calculated
indirectly by measuring the screw forward speed or cavity pressure (make sure that the check
valve is not leaking).
Material characteristics are very important, because polymers may degrade due to different
stresses. Increasing the molding temperature may lead to severe oxidation and degradation of
chemical structure, but at the same time, the degradation caused by shear becomes smaller,
because high temperature reduces the viscosity of the material. Reduced shear stress.
Undoubtedly, the multi-stage glue injection speed is very helpful for forming heat-sensitive
materials such as PC, POM, UPVC, and their blending ingredients.
The geometry of the mold is also a determining factor: thin-walled parts need the maximum
injection speed; thick-walled parts need a slow-fast-slow speed curve to avoid defects; in order to
ensure that the quality of the parts meets the standard, the injection speed setting should ensure
the melt forward flow rate constant.
The melt flow speed is very important because it will affect the molecular arrangement direction
and surface state of the part; when the front of the melt reaches the cross-region structure, it
should be slowed down; for complex molds with radial diffusion, the melt throughput should be
guaranteed Increase in a balanced manner; long runners must be filled quickly to reduce the
cooling of the melt front, but the injection of high-viscosity materials, such as PC, is an exception
because too fast speed will bring cold material into the cavity through the water inlet.
Adjusting the injection speed can help eliminate defects caused by the slowing of the flow at the
water inlet. When the melt reaches the water inlet through the nozzle and the runner, the
surface of the melt front may have cooled and solidified, or the melt stagnated due to the sudden
narrowing of the runner until sufficient pressure is established to push the melt through the inlet.
Nozzle, this will cause the pressure through the inlet to the peak.
High pressure will damage the material and cause surface defects such as flow marks and
scorched water inlet. This situation can be overcome by decelerating just before the water inlet.
This deceleration can prevent Stop the excessive shear at the water inlet, and then increase the
rate of fire to the original value. Because it is very difficult to precisely control the rate of fire to
slow down at the water inlet, it is a better plan to slow down at the end of the runner.

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We can avoid or reduce defects such as flash, scorch, trapped air, etc. by controlling the injection
speed at the end. The deceleration at the end of the filling can prevent overfilling of the cavity,
avoid flash, and reduce residual stress. Air traps caused by poor exhaust or filling problems at the
end of the mold flow path can also be solved by reducing the exhaust speed, especially the
exhaust speed at the end of the injection.
The short shot is caused by the slow speed at the water inlet or the blockage of local flow caused
by the solidification of the melt. This problem can be solved by increasing the injection speed just
after passing the water inlet or local flow obstruction.
Flow marks, scorching of the water inlet, molecular rupture, delamination, peeling, and other
defects that occur on the heat-sensitive material are caused by excessive shear when passing
through the water inlet.
Smooth parts depend on injection speed, glass fiber filling materials are particularly sensitive,
especially nylon. Dark spots (wavy lines) are caused by unstable flow caused by viscosity changes.
Distorted flow can cause wavy lines or uneven fog. What kind of defects are produced depends
on the degree of instability of the flow?
When the melt passes through the water inlet, the high-speed injection will cause high shear, and
the heat-sensitive plastic will be burnt. This burnt material will pass through the cavity to reach
the flow front and appear on the surface of the part.
In order to prevent injection patterns, the injection speed must be set to fill the runner area
quickly and then pass through the water inlet slowly. Finding this speed conversion point is the
essence of the problem. If it is too early, the filling time will increase excessively, and if it is too
late, excessive flow inertia will cause the appearance of jet patterns. The lower the melt viscosity
and the higher the barrel temperature, the more obvious the tendency of this kind of pattern to
appear. Since the small water inlet requires high-speed and high-pressure injection, it is also an
important factor leading to flow defects.
Shrinkage can be improved by more effective pressure transmission and a smaller pressure drop.
Low mold temperature and too slow screw advancement speed greatly shorten the flow length,
which must be compensated by high shooting speed. The high-speed flow will reduce heat loss,
and due to frictional heat generated by high shear heat, it will increase the melt temperature and
slow down the thickness of the outer layer of the part. The intersection of the cavity must be
thick enough to avoid too much pressure drop, otherwise shrinkage will occur.

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Most injection molding defects can be solved by adjusting the injection speed, so the skill to
adjust the injection process is to set the injection speed and its segmentation reasonably.

Secrets of injection molding process adjustment

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