Injection Molding And How It Works

Injection Molding and How It Works

Injection Molding And How It Works

Of course, we have already introduced the injection molding process on our website, but when talking to visitors, we always find that they are interested in the basics of injection molding machines. Therefore, we would like to summarize here again some basic terms and the rough process of plastic injection molding.

Injection Molding Machine

By definition, an injection molding machine is a system that discontinuously produces molded parts consisting of different types of plastics. These products are usually supplied in pellet form. The most commonly used plastics come from the thermoplastic group. Thermoplastics become liquid when heated and solidify in a presumed form when cooled. This process is repeatable. Thermosets and elastomers are used less frequently. These two types of plastics crosslink so much during solidification that they cannot be softened afterward. In addition, injection molding is very different in terms of pressure and temperature distribution. For the description of the injection molding machine, we use a simple elastomer

In principle, the plastic is heated until it is liquefied. Then, the thermoplastic is filled by screws into a place called a mold, pressed together, and cooled. Afterward, the desired shape has solidified and can be removed. The whole process is timed very tightly so that while the cooling process is still going on, the pellets of the next component are already being filled into the screw. In this way, thousands of components can be produced in a relatively short period of time. The injection molding machine consists of two components, namely the plasticizing unit and the clamping unit.

Injection Unit or Plasticizing Unit

In the injection unit or plasticizing unit, plastics are usually introduced in pellet form through the feed hopper and prepared for injection molding. To do this, they must be heated, liquefied, and homogenized. The injection unit of the machine, which is shaped like a cylinder, is heated from the outside by means of a heating coil or belt. Inside, there is a rotating screw that is exactly the right size for the barrel. As the pellets are conveyed forward by the screw to the nozzle, they are also heated, ground, and mixed or distributed by friction. The plastic, liquefied uniformly in this way, is stopped from flowing back by a check valve as it reaches the nozzle.

This creates a certain pressure that compresses the plastic. At the same time, the screw is pushed back so that the plastic can be pressed in again through the nozzle and move forward again. At the same time, the nozzle is metered by closing in a precisely calculated manner, thus interrupting the injection. The screw is then retracted to process the next batch of granular material. During this injection phase, the material is pressed into the clamping unit at extremely high pressures of up to 2000 Bar. Of course, the speed of the injection process must be calculated extremely precisely so that no voids or cracks are formed in the workpiece.

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Locking Unit

The clamping unit contains, among other things, the molding tool. The tool is mounted on two vertical presses. The mold forms more or less complex cavities, i.e. molds, in which the new shape of the plastic is formed. A holding plate with half of the mold is screwed to the plasticizing unit. The mold is inserted through a hole in the middle of this so-called clamping plate. Before injecting the plastic, a second moving platen usually moves to the first platen on a horizontal tie bar. It carries the second half of the mold. The third platen provides pressure for the two clamping platens and is connected to the moving platen by means of a hydraulic cylinder, an electric motor, or a mechanical toggle bar. The two clamping plates are initially heated to a temperature corresponding to the mass of the liquid. During the injection process, additional pressure is generated to give shape to the liquid plastic in the mold. This requires a certain holding pressure to compensate for the volume loss that occurs as the plastic cools or hardens.

Therefore, after the actual filling, some liquid plastic must still be added until the part is perfectly formed. After a short cooling phase, the moving platen is removed from the nozzle side and becomes the injector side. The injection molded part contained in it is then ejected.

As mentioned earlier, the mold represents the one that gives shape to the later injection molded part. Depending on the order, this can be a relatively simple Lego brick or a highly complex functional part, such as a machine. In any case, the mold must be made of the best metal to withstand these processes over time. Hardened steel is usually used for this purpose. Only in special cases and in small batches, for example, do some manufacturers use aluminum tools. The tool is manufactured in the most precise way, today mainly through 3D CAD specifications. It is the production of such tools that requires most of the costs in injection molding.

At TX MOLD, We Work with Different Injection Molding Machines.

Depending on the workpiece, TX MOLD choose one of nine different injection molding machines. These state-of-the-art machines differ in terms of the specifications of the various technologies, the closing pressures that can be generated (25 to 100 tons) and the size of the parts to be produced. These can be small pieces weighing no more than 0.01 grams, but they can also be really “big” pieces weighing 250 grams. It goes without saying that the design of the machine and the screw piston must be different to produce such dimensional differences.

Would you like to learn more about TX MOLD injection molding? Please contact us. We would be happy to inform you about our products and possibilities.

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