Plastic Injection Molding Process
Injection molding is sometimes referred to as a “net shape” manufacturing process because the molded parts emerge from the molding process in their final form with no or minimal post-processing required to further shape the product. An operating injection molding machine is depicted in Fig. 1.1. The mould is inserted and clamped between a stationary and moving platen. The mould typically is connected to and moves with the machine platens, so that the molded parts are formed within a closed mould, after which the mould is opened so that the molded parts can be removed.
The plastic mould cavity is the “heart” of the mold where the polymer is injected and solidified to produce the molded part(s) with each molding cycle. While molding processes can differ substantially in design and operation, most injection molding processes generally include plastication, injection, packing, cooling, and ejection stages. During the plastication stage, a screw within the barrel rotates to convey plastic pellets and form a “shot” of polymer melt. The polymer melt is plasticized from solid granules or pellets through the combined effect of heat conduction from the heated barrel as well as the internal viscous heating caused by molecular deformation as the polymer is forced along the screw flights. Afterwards, during the filling stage, the plasticated shot of polymer melt is forced from the barrel of the molding machine through the nozzle and into the plastic mould. The molten resin travels down a feed system, through one or more gates, and throughout one or more mould cavities where it forms the molded product(s).
Plastic Mould Structure
An plastic injection mould has many structures to accomplish the functions required by the injection molding process. Since there are many different types of plastic moulds, the structure of a simple “two-plate” mould is first discussed. It is important for the mould designer to know the names and functions of the mould components, since later chapters will assume this knowledge. The design of these components and more complex moulds will be analyzed and designed in subsequent chapters.
An isometric view of a two-plate mould is provided in Fig. 1.4. From this view, it is observed that a mould is constructed of a number of plates bolted together with socket head cap screws. These plates commonly include the top clamp plate, the cavity insert retainer plate or “A” plate, the core insert retainer plate or “B” plate, a support plate, and a rear clamp plate or ejector housing. Some plastic mould components are referred to with multiple names. For instance, the “A” plate is sometimes referred to as the cavity insert retainer plate, since this plate retains the cavity inserts. As another example, the ejector housing is also sometimes referred to as the rear clamp plate, since it clamps to the moving platen located towards the rear of the molding machine. In some plastic mould designs, the ejector housing is replaced with a separable rear clamp plate of uniform thickness and two parallel ejector “rails” that replace the side walls of the integral “U”-shaped ejector housing. This alternative rear clamp plate design requires more components and mold-making steps, but can provide material cost savings as well as mold design flexibility. The mold depicted in Fig. 1.4 is referred to as a “two-plate mold” since it uses only two plates to contain the polymer melt. Mould designs may vary significantly while performing the same functions. For example, some mold designs integrate the “B” plate and the support plate into one extra-thick plate, while other mold designs may integrate the “A” plate and the top clamp plate. As previously mentioned, some mold designs may split up the ejector housing, which has a “U”-shaped profile to house the ejection mechanism and clamping slots, into a rear clamp plate and tall rails (also known as risers). The use of an integrated ejector housing as shown in Fig. 1.4 provides for a compact mold design, while the use of separate rear clamp plate and rails provides for greater design flexibility.
To hold the mould in the injection molding machine, toe clamps are inserted in slots adjacent to the top and rear clamp plates and subsequently bolted to the stationary and moving platens of the molding machine. A locating ring, usually found at the center of the mold, closely mates with an opening in the molding machine’s stationary platen to align the inlet of the mold to the molding machine’s nozzle. The opening in the molding machine’s stationary platen can be viewed in Fig. 1.1 around the molding machine’s nozzle. The use of the locating ring is necessary for at least two reasons. First, the inlet of the melt to the mold at the mold’s sprue bushing must mate with the outlet of the melt from the nozzle of the molding machine. Second, the ejector knockout bar(s) actuated from behind the moving platen of the molding machine must mate with the ejector system of the mold. Molding machine and mold suppliers have developed standard locating ring specifications to facilitate mold-to-machine compatibility, with the most common locating ring diameter being 100 mm (4 in).
Another isometric view of the plastic mould is shown in Fig. 1.5, oriented horizontally for operation with a horizontal injection molding machine. In this depiction, the plastic melt has been injected and cooled in the mold, such that the moldings are now ready for ejection. To perform ejection, the mold is opened by at least the height of the moldings. Then, the ejector plate and associated pins are moved forward to push the moldings off the core. From this view, many of the mold components are observed, including the “B” or core insert retainer plate, two different core inserts, feed system, ejector pins, and guide pins and bushings.
The three-plate mould is so named since it provides a third plate that floats between the mold cavities and the top clamp plate. Figure 1.7 shows a section of a three-plate mold that is fully open with the moldings still on the core inserts. As shown in Fig. 1.7, the addition of the third plate provides a second parting plane between the “A” plate assembly and the top clamp plate for the provision of a feed system that traverses parallel to the parting plane. During molding, the plastic melt flows out the nozzle of the molding machine, down the sprue bushing, across the primary runners, down the sprues, through the gates, and into the mold cavities. The feed system then freezes in place with the moldings. When the mold is opened, the molded cold runner will stay on the stripper plate due to the inclusion of sprue pullers that protrude into the primary runner. As the mold continues to open, the stripper bolt connected to the “B” plate assembly will pull the “A” plate assembly away from the top clamp plate. Another set of stripper bolts will then pull the stripper plate away from the top clamp plate, stripping the molded cold runner off the sprue pullers. The ejector plate may be designed and actuated as in a traditional two-plate mold to force the moldings off the core. The three-plate mold eliminates two significant limitations of two-plate molds. First, the three-plate mold allows for primary and secondary runners to be located in a plane above the mold cavities so that the plastic melt in the cavities can be gated at any location. Such gating flexibility is vital to improving the cost and quality of the moldings, especially for molds with a high number of cavities. Second, the three-plate mold provides for the automatic separation of the feed system from the mold cavities. Automatic de-gating facilitates the operation of the molding machine with a fully automatic molding cycle to reduce molding cycle times.
There are at least three significant potential issues with three-plate molds, however. First and most significantly, the cold runner is molded and ejected with each molding cycle. If the cold runner is large compared to the molded parts, then the molding of the cold runner may increase the material consumption and cycle time, thereby increasing the total molded part cost. Second, the three-plate mold requires additional plates and components for the formation and ejection of the cold runner, which increases the cost of the mold. Third, a large mold-opening stroke is needed to eject the cold runner. The large mold-opening height (from the top of the top clamp plate to the back of the rear clamp) may be problematic and require a molding machine with greater “daylight” between the machine’s stationary and moving platens than would otherwise be required for a two-plate or hot runner mold.