Injection molding is critical to the modern world. TX Mold has written several times about injection molding and the basic processes. Therefore, in this blog post, we will turn to some specific processes. The basic principles are described in the blog post “Different Injection Molding Processes”. Many people do not have a clear idea of what exactly happens in the injection molding process. However, we are surrounded by injection molded plastic products everywhere. Without injection molding, a now traditional method of processing plastics, our modern lives would be unthinkable. This is because injection molding and its various processes are part of everyday life, whether it is for products such as car headlights, ground plugs for personal computer connections, or children’s toys. Injection molding using thermoplastics or with the help of elastomers still plays the most important role where a large number of reliable parts are needed. Of course, the progress of 3D printing is something everyone is talking about. But 3D printing, as modern technology, can at best complement injection molded products, but will never replace them.
The Injection Molding Process and Its Stages
As a reminder: in principle, all injection molding processes for the production of parts are carried out in a similar way. Plastic, such as thermoplastic, is introduced in the form of pellets into a rotating screw, where it is heated and melted. The screw pushes the liquid plastic through this injection unit. The mass is then pressed at a pressure of 500-2000 bar through a nozzle into the cavity of a mold, the so-called tool. Starting from the edge of the mold, the melt cools and solidifies. The liquid plastic is pressed together by strong pressure, thus compensating for the material shrinkage caused by heat.
During the cooling of the mold, it is important to wait until the core of the melt has also solidified. The entire part of the clamping unit called the ejector side of the mold opens and the finished mold is ejected from the clamping unit with the help of a pin that is automatically inserted into the mold cavity. Today, such a plastic part can weigh between a few milligrams and 150 kilograms and can be designed using a 3D CAD system for almost any complex product.
Stages of Injection Molding
It goes without saying that a plastic injection molding company such as TX Mold must have many different injection molding machines to be able to produce products of different quality and weight classes. One must look at multi-component molded parts as being particularly complex, where the plastics produced have different properties. However, if you look at the production of an average-weight part made of no further complex plastic, for example, the process at its various stages takes approx.
- Closing the mold: 2 seconds
- Moving the injection unit forward: 1 second
- Injection of plastic into the screw: 1 second
- Pressing into the screw: 7 seconds
- Hardening, cooling: 6 – 8 seconds
- Demold and eject: 2 seconds
- Open the mold: 1 second
Therefore, the whole injection process takes about 20 to 22 seconds to produce such a finished molded part. Of course, the entire cooling process begins directly after the plastic is injected and continues for most of the injection molding machine’s life.
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Special Processes for Plastic Injection Molding
The standard procedures for plastic injection molding described above apply to the most commonly used thermoplastics, which soften when heated. Thermosetting plastics, on the other hand, harden when heat is increased. Thus, they are injected as a liquid at moderate temperatures and then harden at temperatures ranging from 266-482 °F (130-250°C). The third type of material used for injection molding includes, for example, natural rubber. These elastomers vulcanize at higher heat. Their molecules fuse together to form an elastic component. During this process, the different temperatures of the injection unit, the clamping unit, and the compound being injected must also be carefully observed.
A. Multi-component injection molding
In the case of multi-component injection molding already mentioned, the components can be produced one after another, for example in an in-mold process where the second component meets the first component already inserted in the mold. With precise temperature planning, a kind of welding process occurs at the edges of the two parts, possibly supported by an adhesion promoter. This leads to very stable structures, e.g. high-quality bicycle helmets. Multi-component injection molding can also be performed with so-called tandem molds. This type of two molds has two cavities that can be filled separately. In this way, one material can be cooled while a second material is injected at a different temperature. A variant of multi-component injection molding is the back injection of plastic with a metal foil. In this process, a very thin metal foil with a binder is inserted into a mold, allowing it to be pressed into the injected plastic and form its shape. This creates a metallic feel that is popular with consumers, while also providing some stability to the actual composition.
B. Injection molding with cavities
In another process of injection molding, gas or aqueous components are introduced into the mold and then escape or flow away after injection molding. In this way, precisely calculated cavities are created. If these gases are distributed over a large area in the plastic with a certain driving force, a foam-like structure is formed. With this thermoplastic foam molding, it is possible to obtain parts whose volume is reduced by pressing them together. Cavities in injection molding can also be created by inserting metals that have a lower melting point than the injected plastic itself. This process is called fusion core injection molding because the inserted metal is later liquefied in the reheated plastic and can flow out.
C. Extrusion injection molding
The last process described here is one of the most common special processes in injection molding, as it is used to produce PET bottles, among others. This process in plastics processing is known as injection or extrusion injection molding. Extrusion has been used for more than 200 years from the point of view of ceramics and extrusion. A liquid substance that hardens rapidly in fresh air is forced under high pressure through a narrow, usually circular nozzle or slot, thus assuming the shape of a workpiece surrounded by an opening. However, the shape it thus takes is that of a cavity. In plastic injection molding, this principle is extended to include the injection of liquid plastics. This requires a relatively complex design of the injection molding machine. The cavity, or mold, is created by a slider in which the entire geometry of the part has been prefabricated. During the injection phase, the slide moves over the injection nozzle. During this process, the movement of the slide must match the flow rate of the plastic extremely precisely. When the slide reaches the end of its stroke, the end area is filled and shaped by another injection process. As the slide moves backward, the molded part has time to cool and can be removed at the end.
TX MOLD – Over 30 Years of Injection Molding Processes
If you would like to learn more about the process of injection molding plastics or especially thermosetting plastics processing, please contact us or visit our blog. As mentioned earlier, many episodes cover these topics. We look forward to getting to know you!