In our little series on materials used in injection molding, we finally come to the best-known and most frequently used plastics. A thermoplastic or elastomer is a plastic that first becomes soft and moldable under the influence of heat and then liquefies.
When the thermoplastic cools again, it retains the shape it was given. This makes it an ideal molding process for use in injection molding. One of the most important properties of thermoplastics is, of course, that the process can be repeated as often as needed, as long as the specific temperature range of the thermoplastic in question is not exceeded. In this case, as thermal decomposition occurs, the molecular chains disintegrate and the thermoplastic loses its properties. A large number of thermoplastics in use today also have different properties that can be distinguished on the basis of mechanical, thermal, and chemical capabilities.
This means how well a thermoplastic can be processed after molding, in which temperature ranges it can retain its properties, and under which chemical influences it will not react or react in some desirable way. For example, the simplest thermoplastics, such as polyethylene, are used as simple packaging materials in quantities of millions of tons. On the other hand, the more complex PMMA is processed as acrylic glass into sometimes very complex products.
At TX Mold, we mold every common elastomer, including ABS (acrylonitrile-butadiene-styrene), PA (polyamide), PA6.0, PA6.6, PC (polycarbonate), PE (polyethylene), PMMA (polymethyl methacrylate, acrylic glass), POM (polyoxymethylene), PP (polypropylene), and PS (polystyrene). Thermoplastics are mostly traded in the form of pellets and have long gained importance on stock exchanges due to their importance for global production. Market prices can be determined through raw material indices for plastics, the so-called “plastics”, and even commodity futures trading for the most important plastics, such as PE.
Injection Molding of Thermoplastics
Although thermosets and elastomers require a more complex design of temperature distribution, injection process, and mold tempering stages, injection molding with thermoplastics as a process is arguably the basis of molding injection. This principle is easy to explain. Thermoplastics are heated into granular form in order to be injected under pressure as a liquid into a hollow mold, known as a mold. This cavity or hollow consists of two parts, one of which is movable. When the molten pellets form and hold the desired shape after cooling, the moving part opens and the workpiece or molded part is ejected. Then the process starts again. We will explain this process in more detail below because it is not so simple in the industrial production of plastic molds.
After the invention of the injection molding machine in 1926 and the first mass production of injection molding machines, it was not until the mid-1950s that significant changes occurred. The original injection method used a piston or cylinder to deliver heated material into the mold. The important change was the screw, which is still used in a similar manner today. By virtue of their characteristics, the melting and processing of plastics can be further automated and accelerated. In principle, the so-called screw-type plunger injection molding machines are used in exactly the same way today.
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Modern Injection Molding of Thermoplastics
The pellets are fed through a hopper into a heated screw inside a barrel. As the screw rotates, the plastic granules are sheared and ground, so to speak. This generates additional frictional heat, which is used to initiate the liquefaction process.
As the liquefied granulate reaches the injection nozzle, the screw retracts little by little. As it does so, the measuring instrument gives feedback on the filling level so that the filling phase is interrupted in time. The plastic pellets in front of the screw are now liquefied into a melt that is ready to fill the mold. For this injection stage, the uniformity of the melt is a decisive factor, otherwise, defects or voids will appear in the molded part. Therefore, a higher mechanically or hydraulically generated pressure is used, between 500 and 2000 Bar, depending on the material.
The liquid thermoplastics pass through a reflux barrier and through a nozzle. From there, they pass through a heated channel into the mold. A certain residual pressure is maintained even after injection so that subsequent warpage, but above all the volume reduction generated by the cooling process, can be filled and compensated.
Cooling and Ejection During Injection Molding
When the retracted screw starts rotating again for the next batch of plastic pellets, the material in the cavity can be cooled down until everything is solidified. To do this, the temperature of the mold is kept around 68-212 °F (20-100°C), while the temperature of the liquid plastic is 392-572 °F (200-300°C). These values vary depending on the properties of the thermoplastic and must be matched exactly.
A mold that is too cold will interfere with the uniform distribution of the plastic, while a mold that is too hot will prolong the cooling process. Often, the nozzles and cavities are also separated from each other in order to better maintain their respective temperatures. In this way, cooling can take place evenly until even the core, the “soul” of the molded part, has solidified. At the end of the cooling process, the molded part is ejected and can be used or shipped away for further processing.
Advantages of Modern Injection Molding
The advantages of this form of injection molding are obvious. Millions of similar and equivalent elements can be produced at low unit costs. Using advanced molds or multi-mold injection molding machines, different types of plastics can be joined together without welding or screwing. Today, as many as five or even six parts can be combined in an instant, with artificial intelligence taking over complete process control, along with quality control.
However, this combination of different plastics also has the disadvantage of being more difficult to reprocess. Plastics consisting of the same material, such as polyethylene, can be recycled relatively easily. To do this, however, they must be collected by type, as it is still very difficult to separate the different types of plastics by machine. What is almost completely impossible is to recycle workpieces or parts made of different plastics together.
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 Thermoplastic processing, please contact us or visit our blog. As mentioned earlier, many episodes cover these topics. We look forward to getting to know you!
