Plastic injection molding

In the case of plastic injection molding, molten plastic is injected into the shaping cavity (cavity) of a tool, compressed under pressure, cross-linked or brought to cooling and then ejected as a molded part.
The injection molding is the most used method for the highly automated production of workpieces from plastic by the processing of powder or granules. Complexly shaped workpieces can be produced in almost any size. The high accuracy and short cycle times predestine the method for economical series production.

While injection molding is a cyclic process for producing workpieces with a specific geometry, extrusion is a continuous process for producing continuous profiles. During extrusion, the plasticized plastic is not injected into a closed mold, but is pressed through a die.

While injection molding is a cyclic process for producing workpieces with a specific geometry, extrusion is a continuous process for producing continuous profiles. During extrusion, the plasticized plastic is not injected into a closed mold, but is pressed through a die.

In plastic injection molding, the raw material – usually as granules – is first heated and plasticized in a screw cylinder. The plastic mass is then pressed into the tool with high pressure. This is then cooled to solidify the melt and finally remove the finished workpiece.

The most common processes for processing plastics include:

1. Plastifying and dosing
At the beginning of the injection molding cycle, the tool is opened and a metered mass volume is present. The molding material is produced by drawing the plastic granulate from the funnel by a screw and melting it. The material is plasticized by the action of heat in the heated screw cylinder. The screw transports the material into the front cylinder part where it collects behind the nozzle. This results in a dynamic pressure which causes a reverse movement of the worm.

2. Injection
If the tool is now closed and a melt quantity sufficient for the tool filling between the nozzle and the screw tip, the injection can take place. For this purpose, the axially displaceable screw presses the plastic material into the mold. The pressure rises to a maximum value during filling and then decreases gradually.

3. Reproduction and cooling
In order to be able to compensate for the shrinkage of the solidifying melt, a pressure is maintained. The mass cushion remaining in front of the screw tip can thus flow into the mold. This serves to avoid frequent errors in injection molding: incidence points and lunkers. The tool is cooled to accelerate the cooling process of the injection molding.

4. Deforming and evaluating
The tool for ejecting the solidified injection can now be opened. In this case, the workpiece falls out of the mold or is removed from a handling device. Thereafter, the casting which is not part of the molding can be removed manually or automatically.

What materials can be processed in plastic injection molding?

Advantages of plastic injection molding are:

• High reproducibility of production
• No or only slight reworking of the molded part is necessary
• Fully automated method

Often the workpieces produced during injection molding can be used without further reworking. Accordingly, the direct path from the raw material to the finished part contributes to the attractiveness of the process.

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