We are covering the injection molding process and the corresponding plastics more frequently in our blog because we want to give our readers, friends and, of course, customers a closer look at the injection molding process and the corresponding plastics.
The various basic materials used in injection molding, i.e. the very different types of plastics, can be classified according to different criteria. For practitioners, injection molders, and users, it is of course the properties that matter most, which show the different grades of plastics in processing and later use. Thermoset plastics have proven themselves in injection molding and can withstand good mechanical stress, even at temperatures ranging from -22 °F (-30 °C) to over 212 °F (100 °C) without immediately melting or breaking hard molded plastic parts.
Pot handles, headlamp covers, and especially the housings of many appliances are molded from thermoset plastics. After processing, the thermoset is such a rigid material that it can easily be machined into its final shape after injection molding. Drilling, sawing, or grinding can be done with thermoset processing. However, the planned molding of a thermoset part must already be achieved and completed after the first injection molding. Once cured, the plastic cannot be reshaped like a thermoplastic but will disintegrate if the temperature is too high. In terms of its hardness and brittleness, thermoset, unlike thermoplastic, is more susceptible to sharp impacts, in which case it may break. What is the reason for this? Let’s take a closer look at the properties of its molecules.
The Molecular Composition of Thermosets
Compared to thermoplastics, thermosets have different arrangements of molecules. Thermosets include very well-known materials such as polyesters, epoxy resins, and polyurethanes. Casting resins and often glass fibers are also composed of thermoset resins. However, fiberglass, a so-called glass fiber reinforced plastic, is a mixture of thermoset and, for example, epoxy resins. In mixtures, however, thermosetting plastics show outstanding capabilities. Whether as a roof rack or swimming pool, it proves its worth whenever reliable impermeability and fracture resistance are required.
What all thermoset plastics have in common is that their molecular structure is very different from that of other plastics used in injection molding. During the chemical production of plastics, polymers are formed as more and more monomers attach to the starter molecule to compensate for chemical imbalances. In thermoplastics, they form long, flexible chains. However, in thermoset plastics, monomers are trifunctional, i.e. they have three functional groups that act as molecules. As a result, these molecules are linked in a three-dimensional manner to form networks with tight lattices.
There are various methods to produce these molecules, such as so-called polycondensation or polyaddition. However, in all processes, the specificity of thermosets is explained by their three-dimensional connections. On the one hand, their stability is due to their tight mesh structure and the rapid distribution of compressive or tensile forces over the entire part. On the other hand, this means that their molecules are so close that they are no longer able to slip past each other during the movements that occur during reheating, as is the case with the molecules of thermoplastics. The increase in motion or oscillation, while the molecules cannot migrate, prevents the softening, which ends with the disintegration of the whole element.
However, three-dimensional molecular connections are responsible for another important property of thermosets. Electrical energy or voltage can only propagate in a very limited range. This is why thermoset plastics are the perfect shield for cables and devices. The first known thermoset material is also the oldest industrially produced plastic: Bakelite conquered the market in 1905 and soon all sockets, electrical housings, and even telephones were made of Bakelite.
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How Does Thermoset Injection Molding Work?
Due to the nature of the three-dimensional structure of the molecular chains of thermoset plastics, this type of plastic liquefies into a trickle at relatively low temperatures above 86 °F (30 °C) and solidifies again above 266 °F (130 °C). Thermoset plastics with little flow are pressed into a heated mold at a high pressure of 2500 Bar. After solidification, the thermoset plastic cannot be liquefied again.
A. Detailed process
The injection molding machine that initially molds the thermoset plastic requires a different temperature and pressure than the thermoplastic. Depending on the starting material, thermoset plastics liquefy from the initial form, from about 86 °F (30 °C), and solidify from about 266-482 °F (130-250 °C). The screw conveyor that conveys the pebbles to the machine and into the mold must accommodate this temperature and is heated to between 104-230 °F (40-110 °C). Since the filler makes the molten thermoset material not flow easily, the pressure of the melt must be adjusted to a range of up to 2500 Bar.
The mold, i.e. the cavity or hollow mold in which the thermoset resin is to be formed, must first be heated to the hardening temperature of the corresponding thermoset resin, rather than immediately entering the cooling phase. As mentioned earlier, this temperature is between 266-482 °F (130-250 °C). In thermoset injection molding, curing is so reliable and dimensionally stable that wall thicknesses of up to 50 mm can be achieved. Hardening takes place while the mold is still hot, and the finished molded part can still be thermally sprayed after cross-linking.
Because of these conditions, injection molding of thermosets is delayed each time, longer than injection molding of thermoplastics with wall thicknesses of about 4 mm. This has a significant impact on production speed and thus on the economics of injection molding, especially for high volume production. Although thermosets are generally cheaper than thermoplastics in terms of pure material cost, this advantage is offset by the cost of injection molding.
Therefore, the decisive parameters for thermoset injection molding include maintaining a precisely defined temperature range, increasing the pressure on the melt when injecting through a screw conveyor, and extending the curing time in the mold.
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