The injection molding process is one of the four major plastic molding processes. The barrels, basins, toys, and other daily products we use in our lives are all formed by the injection molding process. So what are the important factors to pay attention to in the plastic injection molding process? The completion of the injection molding process generally requires six processes of clamping, filling, holding pressure, cooling, demolding, and opening the mold. In these six processes, we must remember these five elements, namely pressure, speed, position, temperature, and time.

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Five Elements of Injection Molding Process Parameters!
The injection molding process is one of the four major plastic molding processes. The barrels,
basins, toys, and other daily products we use in our lives are all formed by the injection molding
process. So what are the important factors to pay attention to in the plastic injection molding
process?
The completion of the injection molding process generally requires six processes of clamping,
filling, holding pressure, cooling, demolding, and opening the mold. In these six processes, we
must remember these five elements, namely pressure, speed, position, temperature, and time.
1. Injection pressure
The injection pressure is provided by the hydraulic system of the injection system.
The pressure of the hydraulic cylinder is transferred to the plastic melt through the screw of the
injection molding machine. The plastic melt is pushed by the pressure and enters the vertical
flow channel of the mold (or the main flow channel for some molds), the main flow channel, and
the split flow through the nozzle of the injection molding machine. And enter the mold cavity
through the gate. This process is the injection molding process or the filling process.
The existence of pressure is to overcome the resistance in the melt flow process, or conversely,
the resistance in the flow process needs to be offset by the pressure of the injection molding
machine to ensure the smooth progress of the filling process.
In the injection molding process, the pressure at the nozzle of the injection molding machine is
the highest to overcome the flow resistance of the melt throughout the process. After that, the

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pressure gradually decreases along the flow length to the wavefront of the front end of the melt.
If the cavity is well vented, the final pressure at the front end of the melt is atmospheric pressure.
There are many factors that affect the melt filling pressure, which can be summarized into three
categories:
(1) Material factors, such as plastic-type, viscosity, etc.;
(2) Structural factors, such as the type, number, and location of the pouring system, the shape of
the mold cavity and the thickness of the product, etc.;
(3) Process elements of molding.
2. Injection time
The injection time mentioned here refers to the time required for the plastic melt to fill the cavity,
excluding auxiliary time such as mold opening and closing.
Although the injection time is short and the impact on the molding cycle is small, the adjustment
of the injection time has a great effect on the pressure control of the gate, runner, and cavity.
Reasonable injection time is helpful for the ideal filling of the melt, and it is very important for
improving the surface quality of the product and reducing the dimensional tolerance.
The injection time is much lower than the cooling time, about 1/10 to 1/15 of the cooling time.
This rule can be used as a basis for predicting the total molding time of plastic parts.
In the mold flow analysis, only when the melt is completely driven by the screw rotation to fill the
cavity, the injection time in the analysis result is equal to the injection time set in the process
conditions.
If the holding pressure switch of the screw occurs before the cavity is filled, the analysis result
will be greater than the setting of the process conditions.
3. Injection temperature
Injection temperature is an important factor affecting injection pressure. The barrel of the
injection molding machine has 5-6 heating sections, and each raw material has its proper
processing temperature (for detailed processing temperature, please refer to the data provided
by the material supplier).
The injection temperature must be controlled within a certain range. If the temperature is too
low, the melt will not be plasticized, which will affect the quality of the molded parts and increase
the difficulty of the process; if the temperature is too high, the raw materials are easy to
decompose.
In the actual injection molding process, the injection temperature is often higher than the barrel
temperature, and the higher value is related to the injection rate and the properties of the
material, up to 30°C.
This is due to the high heat generated by the shear when the melt passes through the injection
port. There are two ways to compensate for this difference in mold flow analysis. One is to try to
measure the temperature of the melt during air injection, and the other is to include the nozzle
when modeling.
4. Holding pressure and time

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At the end of the injection molding process, the screw stops rotating and only advances. At this
time, the injection molding enters the pressure holding stage. During the pressure holding
process, the nozzle of the injection molding machine continuously feeds the cavity to fill the
volume vacated by the shrinkage of the part.
If the pressure is not maintained after the cavity is filled, the part will shrink by about 25%,
especially the shrinkage marks are formed at the ribs due to excessive shrinkage. The holding
pressure is generally about 85% of the maximum filling pressure, of course, it should be
determined according to the actual situation.
5. Backpressure
Backpressure refers to the pressure that the screw needs to overcome when reversing and
retracting the material. The use of high back pressure is conducive to the dispersion of the color
material and the melting of the plastic, but it also prolongs the screw retraction time, reduces the
length of the plastic fiber, and increases the pressure of the injection molding machine.
Therefore, the back pressure should be lower, generally not more than the injection molding 20%
of stress. When injecting foam, the back pressure should be higher than the pressure formed by
the gas, otherwise, the screw will be pushed out of the barrel. Some injection molding machines
can program the back pressure to compensate for the reduction in screw length during melting,
which reduces the heat input and lowers the temperature. However, as the result of this change
is difficult to estimate, it is not easy to adjust the machine accordingly.