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