In short: In injection molding, three-dimensional bodies are produced with the help of a two-part die that gives shape to the injected liquid plastic before it cools. During extrusion, the liquid plastic is pressed through the die by an extruder and then solidifies into a rather slender two-dimensional, i.e., flat shape, such as a profile.
Production of Spatial and Flat Parts Made of Plastic
This issue comes up more often when we provide guided tours or engage with young people at TX Mold. In fact, what is the difference between plastic injection molding and plastic extrusion? Both are very successful primary plastic molding processes. Primary molding is the production of solid geometric molded parts or bodies from plastic, and it plays an extremely important role in almost all areas of life today. Modern life would be unthinkable without an endless supply of components made of plastic. But there is an important distinction between these components. Some form spatial shapes, while others are quite flat. You can also call it three-dimensional and two-dimensional. Plastic injection molding is best suited for producing three-dimensional components.
On the other hand, flat sheets and most slender components, such as plates and profiles, are produced by extrusion through an extruder. For both processes, thermoplastics are the most commonly used plastics. A brief reminder: After production by polymerization, thermoplastics usually arrive at the plastics processing company in pellet form. Here, they are intensely heated and liquefied in the process. They are now placed in a molding process and shaped into the desired shape. During this process, they retain this shape as they cool down. This process can be repeated to some extent in thermoplastics as opposed to thermosets, which will soften again by reheating and thus can be molded. In this case, however, we are no longer talking about the original cast mold, but about molding, since the original mold has already been created.
Injection Molding
In injection molding, particles of thermoplastic are heated, then liquefied by a screw and placed in a mold. In injection molding, this mold is called a die and is a cavity that acts as a mirror image. Depending on the parts to be molded, simple or highly complex molds are usually made of hardened steel and consist of two parts. The injection unit and the clamping unit are filled with liquid plastic during the injection process. Once the cooled plastic has sufficiently solidified, the two halves open and the molded part is ejected. The part has taken on the shape given to it by the mold. After ejection, the mold closes again and the next batch of liquid plastic is poured in.
The two halves of the mold must be pressed together with great force until the machine pulls them apart again. The whole process is not very long, usually about 20-30 seconds from filling to ejection. In this way, thousands of similar parts can be produced. However, one of the main challenges of injection molding is the construction of the mold. Today, the production of injection molds is done through 3D CAD, however, it can take several weeks due to the sometimes demanding requirements of the molded part. The construction of the mold is considered to be the most expensive part of the process. Therefore, in most cases, injection molding is only worthwhile if a sufficient number of parts are produced.
TX Molds Injection Molding & Tooling
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Extrusion Molding
Extrusion, on the other hand, does not require a mold to give a liquid plastic its shape. The liquid plastic is homogenized and compressed in a hopper or extruder and pressed through a screw in a nozzle. The shape of the nozzle determines the shape of the part. Nozzles can produce any complex cross-section. For example, a circular or ring-shaped nozzle can also be used to produce tubing. If you want to produce cables in this way, you can inject plastic by feeding the wire through a hole in the center of the nozzle, which is then encased by the plastic that emerges. Narrow-shaped nozzles can be used to extrude sheets which, after leaving the nozzle, are further molded during the cooling process, for example, to form window profiles. Forming can be added to this primary molding process. These sheet sections are then heated again and shaped with the help of another track or vacuum.
Thus, just like injection molding, extrusion molding requires the creation of a master mold, i.e. a die. Although extremely precise profiles, rails, and even hulls can be produced with the help of modern processes using extruders, the nozzles are far less complex than the tools because of their two-dimensional shape. As a result, it can not only be produced much faster but also cheaper. The two processes are broadly comparable in terms of part life or yield. In the injection molding process, one is talking about the quantity that can be produced from a single tool, whereas in the extrusion process, the yield is measured in meters. Depending on the complexity, a mold can produce, for example, 100,000 parts, while the lifetime of a mold is, for example, one million meters.
Which Process is Used?
Of course, the final choice of process depends on the type of part to be produced and the application. For flat profiles, especially of different lengths, extrusion is always the more economical method. On the other hand, if the parts to be molded are always of the same dimensions, have more three-dimensional shapes, and form a closed body, as in the case of housing, injection molding is more suitable. Both processes emerged in their current form only in the 1930s and 1950s with the invention of modern plastics.
However, as a principle and using different materials, extrusion and injection molding have a much longer history. For example, the first patent for extrusion was granted as early as 1797 and used molten copper as the method. The beginnings of injection molding date back to the first half of the 19th century, when the method was used to produce metal type for letterpress printing. The first plastic, celluloid, was injection molded into billiard balls in 1872.
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