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What is injection molding

Spritzguss Kleinserie Schweiz

S injection molded is a method which is used in the plastic processing.

D abei is liquefied with an injection molding machine, the respective material and injected into a mold, the injection mold under high pressure. In the tool, the material returns to its solid state through cooling or a crosslinking reaction and is removed as a finished part after the tool is opened.

The cavity, the cavity, of the tool determines the shape and the surface structure of the finished part. Almoplast is able to manufacture parts in the weight range from a few tenths of a gram up to a size of 1 kg.

With this method, molded parts that can be used directly can be produced inexpensively in large numbers. The cost of the tool makes up a large part of the investment required. As a result, even with simple tools, the economic threshold is only reached with a few thousand parts. On the other hand, the tools can be used to manufacture up to a few million parts.

Injection molding, in particular advanced special processes, allows an almost free choice of shape and surface structure, such as B. smooth surfaces, grains for touch-friendly areas, patterns, engravings and color effects. Together with its economic efficiency, this makes injection molding the most widespread method for mass production of plastic parts in practically all areas.

Subdivision of the injection molding process.

In injection molding, plastics are almost exclusively used. These can be divided into thermoplastics, thermosets and elastomers. All three types of material can be used in the injection molding process, with thermoplastic injection molding having the greatest economic importance. It is the most commonly used plastics processing method of all.

Therefore, in the following, the process will first be explained for thermoplastics. In principle, the injection molding of thermosets and elastomers works in the same way and differs primarily only in the operating parameters (e.g. temperatures).

The special procedures described below represent extensions or modifications of the basic process for certain applications.

Thermoplastic injection molding

D today usual ie Schneckenkolbenspritzgiessmaschine consists of two units: the injection unit or plasticizing unit which plasticizes the resin, processed and metered, and the closing unit, which closes the mold, shuts and opens again.

The injection unit essentially consists of a horizontal cylinder, the plasticizing cylinder and a screw located in it. The screw rotates and is also axially movable in the cylinder. At one end of the plasticizing cylinder there is the feed hopper for loading the raw material, at the other there is the nozzle, which can either be closed or not.This represents the transition to the closing unit.

The clamping unit consists of the mold itself, which can be separated into two halves (mold parting plane). The halves are mounted on two clamping plates, one of which, the nozzle side, is rigid and faces the nozzle of the injection unit. The other, the ejector side, is movable. It can be moved hydraulically or electromechanically (toggle mechanism) away from the nozzle side or pressed onto it with force. It contains the eponymous mechanism, which demolds (ejects) the injection-molded part.

Injection and clamping units must be tempered according to the material, component and process (the injection mold may be temperature-controlled). Since the two units have different temperatures, they can be moved away from one another for thermal separation, except in the case of hot runner systems.

Piston injection molding machines were used until 1956.


Principle: 1. Screw 2. Filling funnel 3. Granulate 4. Plasticizing cylinder 5. Heating elements 6. Tool

Plasticizing and dosing

The thermoplastic material trickles into the threads of the rotating screw in the form of granules. The granulate is conveyed towards the tip of the screw and heated and melted by the heat of the cylinder and the frictional heat generated when the material is cut and sheared. The melt collects in front of the screw tip because the nozzle is closed at this point. Since the screw can move axially, it retreats from the pressure and it also unscrews itself from the mass like a corkscrew. The backward movement is braked by a hydraulic cylinder or electrically so that dynamic pressure builds up in the melt. This dynamic pressure in connection with the screw rotation compresses and homogenizes the material.

The screw position is measured and as soon as an amount of material sufficient for the workpiece volume has accumulated, the dosing process is ended and the screw rotation is stopped. The screw is also actively or passively relieved so that the melt is decompressed.


In the injection phase, the injection unit is moved to the clamping unit, pressed down with the nozzle and the screw is put under pressure on the back. The melt is pressed under high pressure (usually between 500 and 2000 bar) through the open nozzle and the sprue or the sprue system of the injection molding tool into the forming cavity. The non-return valve prevents the melt from flowing back towards the feed hopper.

During injection, attempts are made to achieve the most laminar flow behavior of the melt. This means that the melt is immediately cooled down in the mold where it touches the cooled mold wall and remains solidified and “stuck”. The advancing melt is pressed through the melt channel, which is narrowed as a result, at an even higher speed and with even more shear deformation and is deformed towards the edge at the front of the melt. Heat dissipation via the mold wall and heat supply through shear heating are superimposed. The high injection speed creates a shear rate in the melt that allows the melt to flow more easily. Nevertheless, rapid injection is not always desirable, because the high shear rate also increases the breakdown of molecules. The surface, appearance and orientation are also influenced by the injection phase.

Press down and cool down

Since the tool is colder (typically 20 to 120 ° C) than the plastic compound (typically 200 to 300 ° C), the melt cools down in the mold and solidifies when it reaches freezing point. The cooling goes hand in hand with a volume shrinkage, which has a negative effect on the dimensional accuracy and surface quality of the workpiece. In order to partially compensate for this shrinkage, a reduced pressure is maintained even after the mold has been filled, so that material can flow in and compensate for the shrinkage. This pressing can continue until the sealing point is reached, i.e. the sprue has solidified.

After the pressing has been completed, the nozzle can be closed and the plasticizing and metering process for the next molded part can already begin in the injection unit. The material in the mold continues to cool during the remaining cooling time until the core, the liquid core of the workpiece, has solidified and a rigidity sufficient for removal from the mold has been achieved.

The injection unit is then moved away from the clamping unit (lifted off), since no more plastic can escape from the sprue. This serves to prevent excessive heat transfer from the warmer nozzle to the colder tool and thus prevents the nozzle from cooling down too much (freezing).


For demoulding, the ejector side of the clamping unit opens and the workpiece is ejected by pins penetrating the cavity and either falls down (bulk material) or is removed from the tool by handling devices and stored in an orderly manner or immediately sent for further processing.

The sprue must either be removed by separate processing or is automatically cut off during demolding. Injection molding without sprue is also possible with hot runner systems, in which the sprue system always remains above the solidification temperature and the material contained can thus be used for the next shot.

After demolding, the tool closes again and the cycle starts again.

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