- Injection Molding and Injection Molding Process
- 1. What is injection molding?
- 2. Construction of injection molding machine
- 3. What are the injection molding processes?
Injection Molding and Injection Molding Process
1. What is injection molding?
Injection molding is used primarily for plastic processing, producing small parts and prototypes almost exclusively from plastic. Injection molding is the most common manufacturing method for prototypes and parts produced in small batches for end users. Injection molding involves a mechanism whereby a thermoplastic or thermoset polymer is injected into a cavity and then solidified into a finished part. The solidified material is then ejected from the mold. This injection molding process can produce a large number of economically ready-to-use molded parts. Injection molding machines are also capable of producing these items with high precision in a short period of time. The respective material, usually plastic pellets, is liquefied (plasticized) and injected under high pressure into the cavity of the injection mold. This cavity determines the shape and surface structure of the finished part. Nowadays, parts ranging from a few tenths of a gram to double-digit kilograms can be produced. In addition, by injection molding, the surface of the part can be chosen almost freely. The injection molding process is used to produce smooth surfaces, textures, patterns and engravings. After the plastic block in the injection mold has cooled, the mold can be opened and the plastic part removed. By the time it cools down, the material has returned to a solid state.
The injection molding process is economically viable for high-volume products. The cost of building the mold represents a significant portion of the necessary investment. This means that even a simple mold is only worth the investment after several thousand parts have been produced. The injection mold can be used to produce up to several million parts. This makes injection molding the most widely used process for mass-producing plastic items. Injection molding is preceded by the use of molds to produce injection molds for various injection molding processes.
Plastic molds produced by the injection molding method can be divided into the following categories. category A parts are very precise parts that require great precision, category B parts are very demanding technical plastic parts, and category C parts are less demanding geometrically simple molded plastic parts.
All categories have, among others, the following basic quality characteristics. Size and weight of molded parts, surface structure, strength and warpage phenomena.
1.1. Overview and characteristics
Injection molding is a molding process that uses molds (casting dies). Materials such as synthetic resins (plastics) are heated, melted, and then placed into a mold where they are cooled to obtain the desired shape. Since it is similar to the process of injecting a liquid with a syringe, this process is called injection molding. The process flow is as follows. The materials are melted and poured into a mold where they harden, and then the product is removed and finished.
With injection molding, parts of different shapes, even complex shapes, can be produced in large quantities continuously and quickly. Therefore, injection molding is used to manufacture consumer products and products for various industries.
1.2. Injection Molding Machines
There are various types of injection molding machines, such as electric motors driven by servo motors, hydraulic machines driven by hydraulic motors, and hybrid machines driven by a combination of servo motors and hydraulic motors. An injection molding machine basically consists of an injection unit that injects molten material into the mold and a clamping unit that controls the mold.
In recent years, the use of CNC systems in injection molding machines has become more common, leading to the popularity of those injection molding machine models that can be controlled at high speed and programmed. On the other hand, special machines have also been used, such as models used to manufacture light guides for LCDs.
Basic structure of an injection molding machine
A Cylinder (heated material)
B Nozzle (injects molten material)
C Funnel (feeds material)
D Mold (material is poured into the mold cavity between two plates)
1.3. Injection Molding
Injection molding begins when resin pellets (granules) are poured into the hopper, which is the entrance to the material. The pellets are then heated and melted in the cylinder in preparation for injection. The material is then forced through the nozzle of the injection unit and is then conveyed to a channel in the mold called the gate, which then enters the mold cavity through the gate system. After the material has cooled and hardened, the mold is opened and the molded part is ejected from the mold. To complete the mold, the sprues are cut from the mold.
It is important that the molten material is delivered evenly into the mold, as there is often more than one cavity in the mold, so more than one mold can be made at a time. Therefore, the mold should be designed so that multiple molded parts can be produced simultaneously, for example through equally sized gates.
Although injection molding is suitable for mass production, it is important to understand the various conditions for producing high-precision products, including the choice of resin material, the machining accuracy of the mold, and the temperature and speed of melt injection.
Nozzle for injection of molten material
*After molding, the gate is removed from the mold to complete the process.
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2. Construction of injection molding machine
A screw-type plunger injection molding machine basically consists of two parts. The injection unit or plasticizing unit plasticizes, prepares and doses the plastic, while the clamping unit closes, holds and opens the mold.
The injection unit consists mainly of a plasticizing barrel, a horizontal cylinder and a screw located inside it. The plasticizing cylinder has at one end a hopper for filling the raw material and at the other end a nozzle, which can be either closable or non-closable. It is located between the plasticizing cylinder and the clamping unit and acts as a transition.
3. What are the injection molding processes?
The materials processed by injection molding can be classified as thermoplastics, thermosets and elastomers. All three types of plastics can be used for injection molding. However, the most commonly used process for plastics processing is injection molding of thermoplastics, as it makes the most economic sense. The injection molding processes for thermosets and elastomers are in principle similar to those for thermoplastics, the main difference being in the parameters, such as temperature.
Other injection molding processes are listed below.
Thermoplastic injection molding
Elastomer injection molding
Thermoset injection molding
Thermoplastic foam molding (TSG)
Multi-component injection molding
Powder Injection Molding (PIM)
Water Injection Molding (WID)
Melt core injection molding
Gas Injection Molding(GID)
Gas Injection Molding(GHD)
3.1. Thermoplastic injection molding
Thermoplastic material flows into a rotating screw in pellet form. The pellets are heated and melted by the heat in the mold. As the nozzle remains closed, the melted pellets accumulate in front of the screw tip.
During the injection phase, the plasticizing unit is moved to the clamping unit, which is pressed together with the nozzle and the screw is pressurized. During this process, the molten plastic is forced under high pressure through the open nozzle molding cavity. During this process, the backflow of melt is stopped by a check valve.
During the injection process, the melt in the mold is immediately cooled upon contact with the cooled mold walls, where it solidifies. The subsequent plastic is pressed through such conical melt channels at a higher velocity. The high injection speed creates a shear rate in the melt, which makes it easier for the melt to flow. However, it is not always recommended to inject at high speeds because high shear rates also increase molecular degradation. The surface, appearance, and orientation state are also influenced by the injection stage.
Since the mold is colder than the plastic block, the melt cools in the mold and solidifies after reaching the freezing point. Cooling also reduces the volume, which has a detrimental effect on the dimensional accuracy and surface quality of the part. To partially compensate for this shrinkage, it is important to keep the pressure low even after the mold is filled so that the material can flow in and compensate for the shrinkage.
After the compression is completed, the nozzle can be closed and the plasticizing and metering process for the next molded part can already start in the injection unit. During the remaining cooling time, the material in the mold continues to cool until the liquid core of the part has also solidified and has reached sufficient rigidity to allow demolding.
The injection unit is then moved (lifted) away from the clamping unit, as no more plastic can flow out of the gate. This helps prevent excessive heat transfer from the hotter nozzle to the cooler mold, thus avoiding excessive cooling (freezing) of the nozzle.
For demolding, the clamping unit opens, and the part is ejected by the pin. It then either falls out or is removed from the mold by some device. After demolding, the mold is closed again and the cycle starts again.
3.2. Thermoset injection molding
In contrast to thermoplastic injection molding, thermoset injection molding is hardened primarily by the action of heat. After the thermoset resin has hardened, remelting is no longer possible. However, thermoset plastics can be recycled. Therefore, the mass that has not yet hardened must be injected into the mold at a lower temperature and then hardened there at a higher temperature. With thermoset injection molding, very large parts with wall thicknesses of up to 50 mm can be formed.
Among other things, thermoset plastics are commonly used for automotive headlights. Although thermoset plastics are usually less expensive materials, they usually lose out to thermoplastics in economic comparisons. For wall thicknesses up to about 4 mm, the processing cycle time for thermoset materials is usually longer than that for thermoplastics.
3.3. elastomer injection molding
Elastomers, like thermosets, are vulcanized by the influence of heat. Therefore, the temperature of the mold must also be higher than the molded plastic during the injection process. Thermoplastic elastomers are an exception to this rule.
Elastomers can be drawn in as a free-flowing powder or as a special screw conveyor in the form of a belt. The barrel is usually heated by water tempering to about 176 °F（80°C） to avoid overheating. Because of the low viscosity of molten elastomers, mold design requires more effort than thermoplastic molds. Other than these special features, the injection molding process is essentially similar to thermoplastic injection molding.
Elastomer injection molding typically uses injection molding machines with several clamping units for several molds and different parts, because the vulcanization time is much longer than the lead time of the injection unit.
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