Injection
Molding Basic & Systematic Mold Proving ②
3. Basic Principles of Injection Molding
Conditions
1) Cushion and Supplementary pressure
Switchover Point
The screw diameter(mm) of an injection
device is most important in the injection molding of an injection molding
machine. In other words, the theoretical injection volume is calculated by
screw diameter (D) and screw(measurement) stroke (S).
- Theoretical Injection Volume(㎤) = cross sectional areas of screw(㎠) x
stroke of screw(㎝)
- Cross sectional areas of screw(㎠) = π(3.1416) x radius of screw(㎝) x radius
of screw(㎝)
- Injection Weight(g) = theoretical
injection volume(㎤) x injection rate(g/㎤)
The injection pressure and injection rate
are more important in injection molding, and these are also determined and
changed by the screw diameter. This injection pressure refers to the
theoretical injection pressure applied at the screw end, which is changed
according to the diameters of the injection cylinder and screw as well as
hydraulic pressure, not a hydraulic pressure that is typically set. However,
this screw diameter is not limited to the above-mentioned contents that
everyone knows.
Cushion refers to the final position of
the screw at the time when the injection filling is progressed and
supplementary pressure is completed. In short, this cushion location is also
determined by screw diameter (D). In other words, it is advised to locate the
cushion between 10 and 20% of the screw diameter. As an example, it would be
better to locate the cushion between 5mm and 10mm if the screw diameter is
50mm.
However, there is something more important.
It is the supplementary pressure switchover point(position) that is marked as the screw position. It is a key to identify at
what point(position) of a screw the supplementary pressure switchover starts
after beginning an injection filling, and at what point(position) the cushion
is completed after completing the filling. In translating the measured
volume(injection volume) into weight, 98% of the weight is assigned to the
injection filling and remained 2% to supplementary pressure, respectively,
while taking dimension, weight, and contraction of the products in order to
achieve the desired quality and maintain the reproducibility. In other words,
98% of the injection weight should be filled with injection, then the
supplementary pressure id progressed with the remaining 2% of the injection
weight after completing the supplementary pressure switchover by the supplementary pressure
switchover point(position).
There are two methods to identify the
supplementary pressure switchover point that corresponds to 98% of the
injection volume, for instance, a method of measuring the weight of the
injection product and converting it to the screw distance and a partial
injection that is a method of injecting while gradually reducing the
supplementary pressure switchover point with no supplementary pressure motion
at the early stage of the test. The supplementary pressure switchover point
that corresponds to 98% of the injection volume is determined by conducting the
two methods mentioned above in parallel.
For instance, in the case of the screw with
50 mm in diameter, if the shot size of the injection weight is 100 mm and the
cushion is maintained at 8mm of the screw position, the measurement completion
position must be 8 + 100 = 108mm in order to inject the volume of 100mm. Here,
the percentages of both injection weight and screw distance are matched
together.
In other words, when the total shot size is
100mm, the 98% is 98mm, and 2% is 2mm, therefore, it adding 2% weight, which is
2mm, to the desired cushion volume of 8mm, then the optimal supplementary
pressure switchover position results in 10mm.
That is, if the supplementary pressure
switchover position is set at 10mm, an injection starts at 108mm and fills
98mm, which is 98% of the supplementary pressure switchover position 10mm.
Afterward, it is switched over to the supplementary pressure and the remaining
2% of the injection weight, 2mm, which was set as the supplementary pressure
time and pressure, is supplemented to the injection weight, thereby completing
the injection supplementary pressure process that finally supplements the
quality of appearance shape through the weight, dimension, and contraction of
product.
Cushion, as it implies a sitting mat,
indicates that there is a change. The change in this cushion is the change in
the injection pressure of the injection molding machine to the temperature
changes in materials, molds and heating cylinders and it is used for making the
reproducible products in terms of weight and dimension. Therefore, it is
natural that the change in cushion increases as the weight of a molding product
increases.
(If the allowable deviation is 1.0%
compared to the weight of a molding product, based on the PC (the injection
rate of 1.04(g/cm3)) of the molding product with a screw diameter of 50mm and a
measured stroke(shot size) of 100mm, the weight of the molding product is
calculated to be a 204g(=π(3.1416) x 2.5(㎝) x 2.5(㎝) x 10(㎝) x 1.04(g/㎤)).
Therefore, the tolerance of 1.0% shall be subject to deviations of
approximately 2g.)
2) Importance of Suck Back Volume and Back
Pressure
Finally, the Suck Back Volume after measurement, which is the most important, must be set at 10%
of the screw diameter to ensure that the operation and position of the NRV
(Screw Head Assembly) check ring become identical for every shot, thereby
maintaining the measured volume and injection volume constant to reduce the
deviation of the cushion volume.
For example, if the screw diameter is 50mm,
the suck back volume should be set to at least 5mm after completing the
measurement. The final position of the screw after measurement and suck back
becomes 113mm, stemming from 108mm of measurement volume plus 5mm of suck back
volume, and injection begins from here with no impact on the weight of a
product at all.
In the case of a significant change in
cushion volume due to various factors such as specific plastic materials (in
particular, recycled products or those using the additives such as GF), molds
that use hot runners and valve gates, and related temperatures, if the suck
back volume after measurement mentioned above is set to a 10% of the screw
diameter, it can have an excellent effect. However, although it is known that
the suck back is not usually used when using the valve gates, the suck back
volume must be set to a certain level in order to reduce the weight of a
molding product and the changes of the cushion.
Finally, it would be better to use the back
pressure to some extent(i.e. 3 - 5 kg/㎠(bar)) that is
operating simultaneously during measurement compared to not use it at all
because it can play important roles, such as keeping the measurement volume
constant and releasing the gas, to some extent, even if no back pressure is required. Of course, in order to increase the melting and mixing
effects of materials, which are the most important factors, it is necessary to
optimally set the back pressure.
The descriptions thus far are related to
the principle of the most basic condition settings in injection molding under
the optimal state of the performance and quality of the injection machine.
However, with regard to the significant
change in the cushion volume, it cannot be ruled out the possibility of the
root causes in unbalance and control of many hot runners and valve gates
installed in the mold of a specific complex molding product.
3) Importance of Measuring Stroke
The top priority for the quality of
injection molding products is to maintain the weight and dimensions of the
molding products constantly.
In other words, first of all, the measured
volume (weight) should be constant. This indicates that plastic materials
should be supplied consistently with the heating cylinders. To this end, the
most fundamental factor is to maintain consistency by correctly setting the
state of the plastic material supplied, the temperature of the resin supply
section, cylinders, as well as the molds, and its profile.
In addition, the fundamental principle of
an injection molding process involves the correlation between temperature and
pressure, and it is very important to set the optimal back pressure and
injection pressure that are required for measurement and injection under the
materials mentioned above and constant temperarure conditions. It is also
necessary to reduce the shear heat due to a rapid change of each condition such
as speed and pressure in the injection and measurement processes.
In conclusion, it is required to make the
measurement and injection volumes constant in every shot and the
reproducibility of the operation and location of the Screw Head Assembly (NRV)
CHECK RING plays the most important role here. A constant volume can be
measured and injected only if the position of the NRV CHECK RING is in the same
position at injection and measurement in every shot, which is a basic
condition.
Moreover, in order to secure the
reproducibility in the weight and dimension of the injection molding products,
the minimum volumes of measurement(injection) must be over 1D of the screw
diameter and the maximum volumes of measurement(injection) must be below 3D of
the screw diameter. It should be noted that these are the principles to secure
the reproducibility of the molding products.
Metering refers to the entire stroke of a
screw within one cycle. Thermal problems or surface defects may occur if this
measurement stroke selected is relatively small or too large compared with the
screw diameter.
Disadvantages in case that Measurement
Strokes is smaller than Screw Diameter(1D)
* Providing a long residence time in
thermally sensitive plastics.
* Requiring a relatively long response
time, and occurrence of the response time deviation of CHECK RING
Disadvantages in case that Measurement
Strokes is over 3-fold of Screw Diameter(3D)
* Occurrence of stripes due to a material
that is not completedly melted.
* Occurrence of bubbles
* Thermally nonumiform melting state
4) Method to set the temperature of a
heating cylinder
The temperature of a heating cylinder,
which is an important factor with regard to this measurement stroke, is a
temperature measured near the hole in the longitudinal direction of a heating
cylinder (in a nozzle near the runner). Depending on the operation, the driving
energy generates 60% to 85% of the energy needed to melt materials, however,
this melting temperature can be strongly influenced by the temperature of
cylinder walls, especially by 2 zones on the rear side.
Although the temperature of a heating
cylinder should start with a mean recommendation value in the beginning, 3
different ways of setting exist as shown below.
In case of the thermally sensitive plastic
materials, the temperature profile that increases toward the nozzle direction
is employed to apply less heat to the melting material. This profile is
advantageous if the melting material has a long residual time.
* In case of very long cooling time
* In case of very small measurement stroke
* In case of having big material volume
within screw channel or hot runner.
In case of the temperature profile that
increases toward the nozzle direction and then, decreases, an open nozzle is
used to prevent the the phenomena described below.
* In case of preventing the stringing
phenomenon
* A decreasing temperature profile in case
of preventing a large volume of leakage loss
The followings are the cases of using the
temperature profile slightly decreasing in the nozzle direction of at the
hopper section in order to transfer more heat to the melting materials.
* In case that a significant amount of
resin is filled by a big measurement stroke and has a short colling time
* In case of using a deep-bladed screw or a
barrier screw
In addition, the temperature control of the
resin supply section is essential for the supply performance as well as the
stability of molding material transfer. The temperature control in this section
should be suitable for the relevant operating conditions and the environment
for friction. However, if the friction behaviors are not identified, the
optimal temperature of the resin supply section must be determined when setting
up the machine. Constant measurement volume can be considered in this case as
guidance of supply performance by screw strokes.
It is also recommended to start with a mean
recommendation value of this resin supply section and to check if the
plasticization time is constant for each shot. If not, a gradual change in the
temperature of the available resin supply section leads to a better machine
setup.
* Source: www.lsinjection.com