Injection molding process flow

Injection molding process flow

The most important process today for producing a wide variety of molded parts from thermoplastics is injection molding. However, the injection molding manufacturing process can also be used to process elastomers and thermosets. Because of the widely automated injection molding process uses and its variability, due to this it’s possible to produce any kind of molded parts of almost all shape and size using the injection molding process flow.

Injection molding is a so-called discontinuous process. While the tool is usually cooled when processing thermoplastics, elastomer and thermoset processing requires the tool to be heated. This is essential for hardening or vulcanization of the injected material.

Detailed injection molding process flow
Plasticizing and dosing

First, the thermoplastic to be sprayed is poured into a rotating screw via a funnel in the form of granules or powder. The granules are conveyed by rotation towards the tip of the screw. By cutting and shearing the granules, the so-called friction heat is created, which, together with the heating of the cylinder in which the screw rotates, ensures that the plastic granules melt.

As the process continues, the plastic melt accumulates at the tip of the screw, where the outlet nozzle is located, which is closed at this point. This creates pressure on the snail. Since this is axially movable, it screws backwards out of the molten mass under this pressure, similar to a corkscrew. The backward movement of the screw is braked by a hydraulic cylinder or electrically controlled. This causes dynamic pressure to build up in the melt. This dynamic pressure in conjunction with the rotation of the screw compresses and homogenizes the melt.

The position of the screw is continuously measured throughout the entire process. As soon as an amount of material sufficient for the volume of the workpiece to be manufactured has accumulated in front of the nozzle, the rotation of the screw is stopped and the dosing process is completed. At the same time, the screw is actively or passively relieved in order to decompress the melt.

The injection

Injection represents the next step in the injection molding process. During this injection phase, the injection unit moves to the clamping unit. It is pressed on with the nozzle and the screw is simultaneously put under pressure on the back. This creates pressures between 500 and 2,000 bar, with the help of which the melt is pressed through the nozzle and the sprue or the sprue system of the tool into its cavity. This is responsible for the shape of the later workpiece. During this process, a so-called non-return valve prevents the melt from flowing back towards the filling funnel.

The primary goal during injection in the injection molding process is to ensure the melt’s flow behavior is as laminar as possible. Where the melt touches the cooled tool wall in the injection molding tool , it is immediately cooled down and solidifies in place. This increases the pressure on the advancing melt as the melt channel narrows. The spraying speed and the shear deformation then also increase, as a result of which the melt is strain-deformed towards the edge at the front of the melt. Here the heat dissipation through the cooled tool wall and the heat supply through the shear heating overlap.

The high injection speed creates a shearing speed in the melt, which allows the melt to flow more easily into the tool. However, a consistently high injection speed should be avoided. Because an increased shear rate is also accompanied by increased molecular breakdown. The surface, appearance and orientation of the finished workpiece are decisively influenced in the injection phase.

The pressing and cooling down

The next steps in the injection molding process are pressing and cooling. At 20 to 120°C, the tool is significantly cooler than the melt at 200 to 300°C. Therefore, the melt cools in the mold and finally solidifies at the freezing point of the mass. During cooling, volume shrinkage occurs, which has a negative impact on both the dimensional accuracy and the surface structure and quality of the workpiece. This shrinkage is at least partially compensated for by maintaining a reduced pressure even after the tool has been filled. This allows material to flow into the mold and compensate for the shrinkage that occurs there. Pressing can continue until the so-called sealing point is reached, at which the sprue solidifies.

Once the pressing is finished, the nozzle of the screw cylinder is closed and metering and plasticizing of the mass for the next workpiece can begin. Meanwhile, the material in the mold continues to cool until the liquid core of the molded part, the so-called soul, has solidified. In most cases, the plastic will then have sufficient rigidity for the workpiece to be removed from the mold.

The injection unit can now be lifted off the clamping unit, as liquid plastic can no longer escape from the sprue. This lifting is important in order to prevent the nozzle on the screw barrel from “freezing” due to excessive heat transfer from the nozzle to the significantly cooler tool.

The Demoulding

Demoulding is the last step in the injection molding process. The ejector side of the tool opens. Pins penetrate the cavity of the tool and push the molded part out of the mold. It now either falls into a waiting container (bulk material) or is removed from the tool by suitable handling devices. These either store the molded part in an orderly manner or make it available directly for further processing.

The sprue must be removed from almost all molded parts. This happens either in a separate processing step or automatically during demolding. Sprueless injection molding is also possible. This requires working with hot runner systems that ensure that the gating system always remains above the solidification temperature of the processed mass. The material in the sprue can then be used for the next molded part.
After demolding is complete, the tool closes and the process begins again.