Introduction
This chapter provides an introduction and overview of the injection molding machine (IMM) process. It provides text with pictorial reviews. Details on the important informa-tion pertaining to IMM and reviewed in this chapter are provided in the other chapters.
Figure 1-1 provides an overview that basi-cally summarizes what should be considered to ensure that the molded product meets per-formance requirements and provides a good return on investment to produce all types and shapes of products for all types of markets.
Injection molding is a major part of the plastics industry and is a big business world-wide, consuming approximately 32 wt% of all plastics. It is in second place to extrusion, which consumes approximately 36 wt% (1, 3, 7). In the United States alone there are about 80,000 IMMs and about 18,000 extrud-ers operating to process all the many differ-ent types of plastics. In the industry an IMM is not regarded as an extruder; however, it is basically a noncontinuous extruder and in some operations is even operated continu-ously (Chap. 15). IMMs have a screw plas-ticator, also called a screw extruder, that pre-pares the melt (3).
1
As summarized in Fig. 1-2, injection mold-ing is an important plastic processmold-ing method.
The figure shows the necessary components for the injection molder to be successful and profitable. Recognize that the first to market with a new product captures 80% of mar-ket share. The young tree cannot grow if it is in the shadow of another tree or if it does not keep up with competition. You need to be at the top of the tree looking over the other trees. Factors such as good engineer-ing and process control are very important but only represent pieces of the pie. Without proper marketing/sales you are literally out of business. This diagram is basically a philo-sophical approach to the overall industry in that it provides examples of all aspects of the technology and business that range from local to global competition. The old adage about the better mousetrap is no longer com-pletely true, since you need factors such as the support services from the "tree" to achieve commercial success and meet product design requirements (Chap. 5) (1,499).
There are many different types of IMMs that permit molding many different prod-ucts, based on factors such as quantities, sizes, shapes, product performance, or eco-nomics. These different types of IMMs are
D. V. Rosato et al. (eds.), Injection Molding Handbook
© Kluwer Academic Publishers 2000
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reviewed throughout this book, particularly in Chap. 15. Small- and large-size IMMs both have their advantages. For example, if sev-eral small machines are used rather than one large one, a machine breakdown or shutdown for routine maintenance will have less effect on production rates. However, the larger ma-chine is usually much more profitable while it is running. Because there are fewer cavities in molds for the small machines, they may per-mit closer control of the molding variables in the individual cavities.
The two most popular kinds of IMM are the single-stage and the two-stage; there are also molding units with three or more stages. The single-stage IMM is also known as the reciprocating-screw IMM. The two-stage IMM also has other names, such as the piggy-back IMM. It is comparable in some ways to a continuous extruder.
The IMM has three basic components: the injection unit, the mold, and the clamping system. The injection unit, also called the plasticator, prepares the proper plastic melt
and via the injection unit transfers the melt into the next component that is the mold.
The clamping system closes and opens the mold.
These machines all perform certain essen-tial functions: (1) plasticizing: heating and melting of the plastic in the plasticator, (2) injection: injecting from the plasticator under pressure a controlled-volume shot of melt into a closed mold, with solidification of the plastics beginning on the mold's cavity wall, (3) afterfilling: maintaining the injected material under pressure for a specified time to prevent back flow of melt and to compen-sate for the decrease in volume of melt during solidification, (4) cooling: cooling the ther-moplastic (TP) molded part in the mold until it is sufficiently rigid to be ejected, or heat-ing: heating the thermoset (TS) molded part in the mold until it is sufficiently rigid to be ejected, and (5) molded-part release: opening the mold, ejecting the part, and closing the mold so it is ready to start the next cycle with a shot of melt.
1 The Complete Injection Molding Process 3
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This cycle is more complex than that other processes such as extrusion in that it involves moving the melt into the mold and stopping it, rather than having a continuous flow of melt. The injection molding process is, how-ever, extremely useful, since it permits the manufacture of a great variety of shapes, from
simple ones to intricate three-dimensional (3-D) ones, and from extremely small to large ones. When required, these products can be molded to extremely very tight tolerances, very thin, and in weights down to fractions of a gram. The process needs to be thor-oughly understood in order to maximize its
performance and mold products at the least cost, meeting performance requirements, and with ease (see the section on Molding Toler-ances in Chap. 5).
Machine Characteristics
IMMs are characterized by their shot ca-pacity. A shot represents the maximum vol-ume of melt that is injected into the mold.
It is usually about 30 to 70% of the actual available volume in the plasticator. The dif-ference basically relates to the plastic mate-rial's melt behavior, and provides a safety factor to meet different mold packing con-ditions. Shot size capacity may be given in terms of the maximum weight that can be in-jected into one or more mold cavities, usu-ally quoted in ounces or grams of general-purpose polystyrene (GPPS). Since plastics have different densities, a better way to ex-press shot size is in terms of the volume of melt that can be injected into a mold at a spe-cific pressure. The rate of injecting the shot is related to the IMM's speed and also the process control capability for cycling the melt into the mold cavity or cavities (fast-slow-fast, slow-(fast-slow-fast, etc.).
The injection pressure in the barrel can range from 2,000 to at least 30,000 psi (14 to 205 MPa). The characteristics of the plastic being processed determine what pressure is required in the mold to obtain good products.
Given a required cavity pressure, the barrel pressure has to be high enough to meet pres-sure flow restrictions going from the plastica-tor into the mold cavity or cavities.
The clamping force on the mold halves re-quired in the IMM also depends on the plastic being processed. A specified clamping force is required to retain the pressure in the mold cavity or cavities. It also depends on the cross-sectional area of any melt located on the part-ing line of the mold, includpart-ing any cavities and mold runner(s) that are located on the parting line. (If a TP hot-melt runner is lo-cated within the mold half, its cross-sectional area is not included in the parting-line area.) By multiplying the pressure required on the melt and the melt cross-sectional area, the
clamping force required is determined. To provide a safety factor, 10 to 20% should be added.
Molding Plastics
Most of the literature on injection mold-ing processmold-ing refers entirely or primarily to TPs; very little, if any at all, refers to ther-moset TS plastics. At least 90 wt% of all injection-molded plastics are TPs. Injection-molded parts can, however, include combi-nations of TPs and TSs as well as rigid and flexible TPs, reinforced plastics, TP and TS elastomers, etc. (Chap. 6). During injection molding the TPs reach maximum tempera-ture during plastication before entering the mold. The TS plastics reach maximum tem-perature in the heated molds.
Molding Basics and Overview
The following information provides a com-plete overview of the process of 1M (Figs. 1-3 to 1-10). Continually required is better under-standing and improving the relationship of process-plastic-product and controlling the complete process.
Injection molding is a repetitive process in which melted (plasticized) plastic is injected (forced) into a mold cavity or cavities, where it is held under pressure until it is removed in a solid state, basically duplicating the cavity of the mold (Fig. 1-11). The mold may con-sist of a single cavity or a number of similar or dissimilar cavities, each connected to flow channels, or runners, which direct the flow of the melt to the individual cavities (Fig. 1-12).
Three basic operations take place: (1) heat-ing the plastic in the injection or plasticizheat-ing unit so that it will flow under pressure, (2) al-lowing the plastic melt to solidify in the mold, and (3) opening the mold to eject the molded product.
These three steps are the operations in which the mechanical and thermal inputs of the injection equipment must be co-ordinated with the fundamental properties and behavior of the plastic being processed;
different plastics tend to have different
1 The Complete Injection Molding Process 5
Fig.l-3 View of an injection molding machine.
Fig.l-4 Basic elements of injection molding.
melting characteristics, with some being ex-tremely different. They are also the prime de-terminants of the productivity of the process, since the manufacturing speed or cycle time (Fig. 1-13) will depend on how fast the ma-terial can be heated, injected, solidified, and ejected. Depending on shot size and/or wall thicknesses, cycle times range from fractions of a second to many minutes. Other impor-tant operations in the injection process in-clude feeding the IMM, usually gravimetri-cally through a hopper, and controlling the plasticator barrel's thermal profile to ensure high product quality (Fig. 1-14).
An example of complete injection molding operation is shown in Fig. 1-1. This block di-agram basically summarizes what should be considered to ensure a good return on