18 basic injection molding technologies

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18 Basic Injection Molding Technologies
Below are the 18 basic injection molding technologies that injection molding technicians must
know.
01. Nozzle in the injection molding technologies
The melt usually flows from the nozzle into the nozzle, but in some injection molds, the nozzle is
part of the mold because it extends to the bottom of the mold. There are also two main types of
nozzles: open nozzles and closed nozzles. In injection molding, open nozzles should be used more
because they are cheaper and less likely to stay.
If the injection molding machine is equipped with a pressure-removing device, this nozzle can be
used even for low-viscosity melts. Sometimes a closed nozzle must be used. This nozzle acts as a
stop valve to block the plastic in the injection cylinder.
Make sure that the nozzle is correctly connected to the nozzle bushing, and the top hole is
slightly smaller than the nozzle sprue bushing, which makes the nozzle easier to withdraw from
the injection mold. The hole of the nozzle sleeve is 1mm larger than the nozzle, that is, the radius
of the nozzle is 0.5mm smaller than the radius of the nozzle sleeve.
02. Filter and combined nozzle in the injection molding technologies
Impurities in the plastic can be removed by the filter of the extensible nozzle, that is, the melt
and the plastic flow through a channel, which is divided into a narrow space by the insert. These
narrows and gaps can remove impurities and improve the mixing of plastics.
Therefore, it can be extended, and a fixed mixer can be used to achieve a better mixing effect.

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These devices can be installed between the injection cylinder and the nozzle to separate and
remix the melt, most of which are to make the melt flow through the stainless steel channel.
03. Exhaust in the injection molding technologies
Some plastics need to be vented in the injection cylinder during injection to allow the gas to
escape. In most cases, these gases are just air, but they may be water or monomolecular gases
released by melting.
If these gases cannot be released, the gas will be compressed by the melt and carried into the
mold, and it will expand and form bubbles in the product. To get rid of the gas before it reaches
the nozzle or mold reduces or reduces the diameter of the screw root to depressurize the melt in
the injection cylinder.
Here, the gas can be discharged from the holes or holes in the shooting cylinder. Then the
diameter of the screw root is increased, and the melt glue of the devolatile matter is adapted to
the nozzle. An injection molding machine equipped with this facility is called an exhaust injection
molding machine.
There should be a good fume exhauster with a catalytic burner above the exhaust injection
molding machine to remove potentially harmful gases.
04. Increase the role of backpressure in the injection molding technologies
In order to obtain a high-quality melt, the plastic must be heated or melted consistently and
mixed thoroughly. Only use the correct screw to melt and mix properly, and there is enough
pressure (or back pressure) in the injection cylinder to achieve the consistency of mixing and
heat.
Increasing the resistance of the oil return can generate backpressure in the injection cylinder. But
the screw takes longer to reset, so there is more wear and consumption in the drive system of
the injection molding machine. Keep back pressure as much as possible, isolate it from the air,
and also need the same melting temperature and mixing degree.
05. The check valve in the injection molding technologies
No matter what kind of screw is used, its tip is usually equipped with a stop valve. In order to
prevent the plastic from flowing out of the nozzle, it is also equipped with a pressure-reducing
(reverse cable) device or a special shooting nozzle.
If a check valve is used, it must be checked regularly because it is an important part of the
injection cylinder. At present, the switch-type nozzle is not commonly used because the plastic
inside the nozzle is easy to leak and decompose. At present, each type of plastic has a suitable
nozzle type.
06. Screw back (reverse cable) in the injection molding technologies
Many injection molding machines are equipped with screw retreat or suck-back devices. When
the screw stops, it is withdrawn by hydraulic pressure to suck back the plastic at the tip of the
nozzle. This device allows the use of an open nozzle. Keep the amount of suck back as low as
possible, because the ingress of air can cause problems for some plastics.
07. Screw gasket in the injection molding technologies

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In most injection molding cycles, it is necessary to adjust the rotation amount of the screw, so
that after the injection of the screw, there will be a small amount of soft cushion plastic, which
can ensure that the screw achieves an effective advancing time and maintains a fixed shooting
pressure.
The padding for small injection molding machines is about 3mm; for large injection molding
machines, it is 9mm. No matter how large the screw gasket value is used, it must remain the
same. Now the screw pad material is controlled within 0.11mm.
08. The rotation speed of the screw in the injection molding technologies
The rotation speed of the screw significantly affects the stability of the injection molding process
and the heat acting on the plastic. The faster the screw rotates, the higher the temperature.
When the screw rotates at a high speed, the friction (shear) energy transferred to the plastic
improves the plasticization efficiency but also increases the unevenness of the melt temperature.
Due to the importance of the screw surface speed, the screw rotation speed of a large injection
molding machine should be less than that of a smaller injection molding machine. The reason is
that the sheer heat generated by the large screw is much higher than that of the small screw at
the same rotation speed. Due to different plastics, the speed of the screw rotation is also
different.
09. Injection volume in the injection molding technologies
The evaluation of the syringe molding machine is usually based on the amount of PS that can be
injected in each injection and may be measured in ounces or grams. The other sorting system is
based on the volume of melt that the injection molding machine can inject.
10. Plasticizing ability in the injection molding technologies
The evaluation of an injection molding machine is usually based on the amount of PS that can be
uniformly melted or heated to a uniform melt temperature within 1 hour (in pounds and
kilograms), which is called the plasticizing ability
11. Estimation of plasticizing capacity in the injection molding technologies
To determine whether plasticization can be maintained throughout the production process, a
simple formula about output and plasticization capacity can be combined, as shown below:
t=( total injection volume g X 3600 ) ÷ ( plasticization volume of injection molding machine
kg/hX1000 )
t is the minimum cycle time. If the cycle time of the mold is lower than the value of t, the
injection molding machine cannot fully plasticize the plastic to achieve a uniform melt viscosity,
so the injection molded parts often have deviations. Especially when paying attention to the
quality of injection molding with thin walls or close tolerances, the injection volume and the
plasticization volume must match each other.
12. Residence time of the shooting cylinder in the injection molding technologies
The rate of decomposition of plastic is dependent on temperature and time. For example, plastics
will decompose after being exposed to a high temperature for a period of time; but in a relatively
low-temperature environment, it will take a long time to decompose. Therefore, the residence

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time of the plastic in the injection cylinder is very important.
The actual residence time can be determined through experiments. The method is to measure
the time required for the colored plastic to pass through the injection cylinder. It can be roughly
calculated by the following formula: t=( rated material volume of the injection cylinder gX cycle
time S ) ÷ ( material injection G X 300)
Please note that some plastics stay in the shooting tank longer than the calculated time, because
they can agglomerate in the shooting tank.
13. Calculate the residence time and importance in the injection molding technologies
In general, the residence time of a certain plastic on a specific injection molding machine should
be calculated. Especially when the large injection molding machine uses a small amount of
injection, the plastic is easy to decompose, and this is not detectable from observation.
If the residence time is short, the plastic will not be plasticized uniformly; if the residence time is
too long, the properties of the plastic will be attenuated. Therefore, the stay time must be
consistent.
Method:
Ensure that the plastic input to the injection molding machine has a stable composition,
consistent size, and shape. If there is any abnormality or loss of the parts of the injection molding
machine, it must be reported to the maintenance department.
14. The temperature environment of the injection cylinder in the injection molding technologies
It should be noted that the melt temperature is very important, and any injection cylinder
temperature used is only indicative. If you have no experience in processing a particular plastic,
please start with the lowest setting. Usually, the temperature in the first zone is set to the lowest
value to prevent premature melting and sticking of the plastic in the feed port.
The temperature in other areas then gradually rises until it reaches the nozzle. In order to
prevent dripping, the temperature at the tip of the nozzle is often slightly lower. The mold is also
heated and cooled. Due to the size relationship of many molds, the molds are also distinguished,
but unless otherwise specified, each area should be set to the same size.
15. Melting temperature in the injection molding technologies
The nozzle can be measured or measured by the air jet method. When using the latter for
measurement, care must be taken to ensure that there will be no accidents when cleaning the
hot-melt plastic, because the high temperature of the hot-melt plastic will burn the skin and even
corrode the skin. In the injection molding plant, burn accidents occur from time to time.
Therefore, you should wear gloves and face shields when dealing with hot plastics or when you
encounter the danger of splashing hot-melt plastics. To ensure safety, the tip of the heat control
needle should be pre-heated to the temperature to be measured.
Each plastic has a specific melt temperature. To reach this temperature, the actual injection
cylinder adjustment value of injection molding depends on the screw rotation speed, back
pressure, injection volume, and injection cycle.
16. Mold temperature in the injection molding technologies

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Always check whether the injection molding machine is set and running at the temperature
specified on the record sheet. It is very important. Because temperature will affect the surface
finish and output of injection molded parts. All measured values must be recorded, and the
injection molding machine must be inspected at the specified time.
17. Uniform cooling in the injection molding technologies
The finished injection molded parts must be cooled uniformly, that is, different parts of the mold
must be cooled at different rates so that the entire product will be uniformly cooled.
Injection-molded parts must be cooled as fast as possible while ensuring that no defects such as
uneven surfaces and changes in physical properties will occur.
The cooling rate of each part of the injection molded part must be equal, but it means that the
mold is cooled in an uneven way, for example, cold water is injected into the inner core of the
mold, and warmer water is used to cool the outside of the mold. This technique should be used
for injection molded parts with precision tolerances or large-scale products with long gates.
18. Temperature and cooling check in the injection molding technologies
Always check whether the injection molding machine is set and running at the temperature
specified on the record sheet. It is very important. Because temperature will affect the surface
finish and output of injection molded parts. All measured values must be recorded, and the
injection molding machine must be inspected at the specified time.
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