Modern Plastics Handbook 2011 Part 9 pps

The chief disadvantage of mobile drying is ■ Floor space. Space around the processing machine must be available for the dryer. However, the mobility of these units means that they can be stored out of the way when not in use. 7.18.3. Central drying systems For a processor that uses a lot of hygroscopic material, a central system (Fig. 7.48) may offer significant advantages. In fact, it is easy to see how a large-volume, continuous run processor could justify a central system. Generally, these processors have the need to handle large amounts of similar materials, fed to machines that may make the same product day after day for long periods of time. Custom processors, however, usually make frequent material changes and are less likely to need a central drying system. And yet, even these short-run processors tend to specialize, running lots of similar parts using similar materials. Even if they cannot standardize production plantwide, it may be possible to create discrete manufacturing cells within their plant and centralize drying with each cell. Thus almost any processor can achieve the following: 7.92 Chapter Seven, Section Two Figure 7.48 A central material drying-distribution system includes one or more dryers, serving multiple hoppers, dedicated to drying different materials. A manifold system allows dried material to be conveyed wherever it is needed. 0267146_Ch07_Harper 2/24/00 4:48 PM Page 92 ■ Energy efficiency. One large dryer can efficiently serve multiple hoppers. Booster heaters located at the inlet to each hopper make it possible to exactly match the temperature requirements of each resin and avoid heat losses between the dryer and the hopper. ■ Safety. Drying takes place in one area, away from the processing machine. ■ Easy material changes. Switching from one material to another may be as simple as switching conveying tubes at a central materials distribution manifold. New materials can be loaded and predried with- out affecting on-going production. ■ Floor space and manpower. No machine-side floor space is required. A central system requires less floor space per machine because one dryer can serve several hoppers and one hopper can serve several machines. ■ Material control. With a well-organized, well-controlled central system there is less chance of contamination and improper blending. Pocket conveying meters material into the distribution box under the drying hopper so that only small amounts of material are conveyed and no extra material remains in the conveying lines to absorb moisture or contaminate subsequent material lots. The only real disadvantages to a central system are ■ Capital cost. A central system will always require more up-front expenditures and costs associated with central material conveying systems required to get material from the dryers to the processing machines. System expansion can be expensive too. ■ Material control. This is both an advantage and a disadvantage. An error or miscalculation in material control can be extremely costly because of the amount of material involved and the number of processing machines served by a central system. 7.19 Gas or Electric? Plastics processors, historically, have been very dependent on electric power for most of their processing needs. And, because of its depen- dence on heat, the resin drying process can be a heavy consumer of electrical power. It’s not surprising, then, that processors are showing increasing interest in natural gas as an alternative energy source for drying. In fact, a gas dryer can provide energy cost savings of up to 70% (see Fig. 7.49). Auxiliary Equipment: Drying and Dryers 7.93 0267146_Ch07_Harper 2/24/00 4:48 PM Page 93 Today’s gas dryers incorporate the most advanced gas burners available, featuring a ceramic-metal fiber matrix firing surface that provides flameless, efficient radiant gas heat, with low emissions. Gas-fired dryers are available in both large central systems and machine-side portable units. In addition, process air heaters are available to convert installed dryers from electrical heating to gas (Fig. 7.50). As an alternate heat source, natural gas has these advantages: 1. The cost per btu is approximately one-quarter of the cost of electric per Btu. 2. Natural gas appliances have a long and proven track record as a safe heat source. 3. By reducing electric consumption during peak hours, companies whose rates are determined by peak usage can qualify for a lower overall electric rate. 4. Retrofitting existing dryers with a gas process air heater can free up existing electric switch gear for use on other new machinery. 5. Gas process heaters require less maintenance than comparable electrical units. 7.20 Handling Dried Material Once plastic materials have been properly dried, it is imperative that they be protected from moisture regain prior to molding. As noted in Sec. 7.18, each drying system will approach the problem in a slightly different way. A drying hopper that is mounted directly to the machine throat, for instance, will require no special accommodations because the resin 7.94 Chapter Seven, Section Two 0 10 20 30 40 50 60 70 80 ELECTRIC COSTS GAS Figure 7.49 Bar chart illustrates the savings that are possible when using gas instead of electricity for drying. 0267146_Ch07_Harper 2/24/00 4:48 PM Page 94 goes directly from the hot, dry environment of the hopper to the processing machine, eliminating the possibility of moisture regain. When dried material needs to be conveyed from hopper to machine, however, precautions need to be taken. The key is to prevent material from coming into contact with moisture-laden ambient air for any appreciable length of time. Some materials regain moisture slowly and will stay dry enough to process for 2 or 3 h after exposure to ambient air, while others will regain an unac- ceptable amount of moisture in a matter of minutes. Many processors choose to use a dry air generator to produce conveying air, thus avoiding exposing dried resin to ambient air. However, the cost associated with adding another piece of equipment to the system may be completely avoidable if precautions can be taken to limit the amount of time the material is exposed to ambient air. A better approach is to keep material out of the conveying lines as Auxiliary Equipment: Drying and Dryers 7.95 Figure 7.50 Here, a gas-fire process air heater has been used to convert an existing electric dryer to clean, economical, and safe natural gas. 0267146_Ch07_Harper 2/24/00 4:48 PM Page 95 much as possible. That calls for pocket conveying, a technique in which a discrete amount of dried material is dispensed into a closed chamber (see Fig. 7.51) under the drying hopper. That small “pocket” of material is then vacuum conveyed, using ambient air, to a small hopper loader (Fig. 7.52) on the processing machine. The quantities being conveyed are so small that they can be processed within minutes of leav- ing the drying hopper. In addition, the supply lines are constantly purged of material, so nothing is left behind to become separated or to pick up moisture. For the ultimate protection against moisture regain, processors can combine pocket conveying with dry-air conveying. 7.96 Chapter Seven, Section Two Figure 7.51 A “pocket” conveying valve avoids moisture regain problems by moving only small amounts of dried material and preventing material from remaining in conveying lines. 0267146_Ch07_Harper 2/24/00 4:48 PM Page 96 Auxiliary Equipment: Drying and Dryers 7.97 Figure 7.52 To minimize the time dried material is outside the drying hopper before molding, and thus prevent moisture regain, mini hoppers, like the one in the foreground, are used. 0267146_Ch07_Harper 2/24/00 4:48 PM Page 97 7.99 7.21 Introduction We begin with some definitions of the acronyms used in this section. The term computer-aided drafting (CAD) is part of common language today and refers to the use of computers for drafting and modeling of product designs. In a sense, CAD is the technological backbone that provides opportunities for concurrent engineering and for the subsequent use of CAE and CAM. The CAD industry has seen tremendous growth since its inception in the 1970s and continues to grow with advances in both tech- nology and integration of CAE and CAM products. We will not discuss CAD specifically in this section but will mention how trends in the CAD industry are impacting plastics CAE. CAD has been embraced by many companies and plays a central role in CAM and CAE. Using a CAD system, the designer creates a representation of the part to be manufactured. An application is a component to be made from plastic or the mold to produce the part. The CAD system creates a representation of the component’s geometry. This representation may be used for a variety of downstream operations including rapid prototyping, CAE analysis, numerical machining, mold building, or tolerance and assembly checking. Computer-aided manufacturing (CAM) refers to the production or alteration of control data for manufacturing. Often the term is used to specifically refer to computer numerically controlled (CNC) machine tooling. With regard to the plastics industry, CAM generally refers to the generation of CNC cutter paths for the production of molds and dies. More recently, plastics CAM has been extended by the availabil- ity of “smart” controllers for injection-molding machines. This is an important development and will be discussed in detail subsequently. The term computer-aided engineering (CAE) describes the use of computers for analysis of a particular design. Often the design is a new product, but in the context of plastics it could be a cooling circuit lay- out for an injection mold or even the mold itself. Frequently, the term CAE is used to embrace both CAM and CAD though, strictly speaking, it refers to the analysis stage only. There are many types of analyses available these days. Typical examples include ■ Structural analysis for determination of deflections and stresses in a design subject to applied load. ■ Thermal analysis in which the temperature distribution is calculated. Section 3 CAD, CAM, and CAE Peter Kennedy Moldflow Corporation, Lexington, Massachusetts 0267146_Ch07_Harper 2/24/00 4:48 PM Page 99 ■ Flow analysis in which the flow of a material through a defined region is calculated. ■ Mechanical analysis where motion of a linkage or mechanical system is determined. Regardless of the type of analysis, all CAE involves the use of a mathematical model that simulates the physical process or conditions to which the design is exposed. 1 The mathematical model is typically a set of equations, usually involving partial derivatives and suitable boundary conditions to ensure a unique solution. In order to imple- ment the mathematical model in computer software, we need to use appropriate numerical methods for the solutions of the equations forming the mathematical model. One of the features of all numerical methods is that the problem must be discretized. For CAE this means creating a set of points, which are called nodes, at which the quantities (e.g., temperature, pressure, stress) of interest will be calculated. One of the most popular methods is the finite element method. 2 In addition to the generation of points, the finite element method requires that the points be arranged to form geometrical entities called elements. The combination of nodes and elements is called a mesh. For two-dimensional problems, the mesh generally consists of triangular or quadrilateral elements. In three dimensions, the elements are usually tetrahedral or hexahedral in shape. Thus, the discretization step in finite element analysis involves the generation of a mesh which rep- resents the region in which a solution to the problem will be sought. These ideas are illustrated in Fig. 7.53. It is common to refer to meshed objects as models for analysis. These models, not to be con- fused with the mathematical model mentioned earlier, are abstrac- tions of the component under consideration and provide information for the analysis in a form understood by the computer. In the plastics industry, and for the purposes of this section, CAE describes the simulation of a particular process, e.g., extrusion, injection molding, film blowing, etc. Generally, this will involve use of a computer code, input of material properties, definition of the region in which calculations are to be carried out, and input of processing conditions. We will see later that generation of a mesh is an important part of the process and can represent a significant part of the total time involved in CAE. In this section we focus on injection molding. For us, CAE will involve the simulation of the injection-molding process. Injection molding is the most mature area of the plastics industry with respect to utilization of CAD/CAM and CAE. Moreover, developments in the injection-molding field represent the state of the art. Of course, many of the general ideas of simulation of injection molding may also be applied to other plastic part production methods. 7.100 Chapter Seven, Section Three 0267146_Ch07_Harper 2/24/00 4:48 PM Page 100 7.22 Simulation and Polymer Processing All major manufacturing processes for plastic products have a common feature, namely, the melting and subsequent solidification of the material. The notion that processing has a dramatic effect on the properties of the manufactured article has been known since plastic processing began. In practice, the relationship between process variables and article quality is extremely complex. It is very difficult to gain an understanding of the relationship between processing and part quality by experience alone. It is for this reason that simulation was born, but it is interesting to note that CAE has been much more successful in injection molding than in other areas. In this section we review why this is the case and discuss some aspects of simulation for other processes. The major processes encountered today are ■ Extrusion ■ Blown film extrusion ■ Blow molding ■ Vacuum forming ■ Injection molding and its variants such as injection compression molding, gas injection, and co-injection. 7.22.1 Extrusion Much effort has been spent on studying extrusion, as the plastication stage is applicable to other processes. Plastication models were given in Ref. 3. However, the plastication process is still actively researched today, particularly with regard to mixing and screw efficiency. 4,5 Auxiliary Equipment: CAD, CAM, and CAE 7.101 Figure 7.53 Solid model geometry on left and meshed model on right. 0267146_Ch07_Harper 2/24/00 4:48 PM Page 101 [...]... created in a form that resembled the part geometry and that results could be shown on the part representation In 198 3, Moldflow introduced a finite element filling analysis program that found ready acceptance in the market For the first time, analysts could model the part under consideration in a form that resembled the component and display the results of analysis on the part 0267146_Ch07_Harper 7.108... Three connector elements In particular, at any rib a connector element must be introduced (Fig 7.63) This technique is called DD/FEA The name derives from the fact that you are, in fact, doing two FEA analyses—one on each side of the component DD/FEA has had a striking impact on plastics CAE since its introduction by Moldflow in 199 7 We return to this in a later section 7.25 .9 Three-dimensional FEA All... material, and machine combination In October 199 8, Moldflow released a family of products called MPX (Moldflow Plastics Xpert) The Moldflow Xpert Series is the downstream realization of Moldflow’s Process Wide Plastics Simulation Strategy to link design, CAE, and the shop floor Resulting from several years’ research and development with industry and academia, the Plastics Xpert has been designed to assist... to have a dramatic affect on part quality—particularly part weight, dimensional tolerance, and warpage In the packing phase, material is subjected to high pressure while it cools in the mold The pressure temperature history determines the ultimate density of the molded material A common goal of the optimization is, therefore, to minimize density variation throughout the part Alternatively, it is possible... injection-molding simulation 0267146_Ch07_Harper 2/24/00 4:48 PM Page 1 09 Auxiliary Equipment: CAD, CAM, and CAE 7.1 09 Figure 7.57 Layflat of box By varying thickness along the lines, the flow pattern can be modified to remove the gas trap 7.24.3 Simulation of warpage Moldflow introduced software for prediction of shrinkage and warpage in 199 0 Since this time, analysis of fiber-reinforced materials has been... the plastic part is thin walled and makes use of the Hele-Shaw approximation Three-dimensional finite element analysis (3D/FEA) eliminates this requirement In so doing, it introduces a new class of components to simulation With 3D/FEA it is possible to simulate the molding of parts for which a midplane is not available Typically, such parts are chunky—some examples are given in Fig 7.64 Many parts (a)... 0267146_Ch07_Harper 2/24/00 4: 49 PM Page 123 Auxiliary Equipment: CAD, CAM, and CAE 7.123 Figure 7.64 Parts such as these do not possess a midplane and so cannot be analyzed with conventional CAE Such parts require full 3D FEA analyses contain inserts, either metal or some other material, and it can be difficult to analyze these using conventional shell-based analysis These parts are also amenable to 3D/FEA... the part; part shrinkage; deflections in the x, y, and z directions; and residual stresses and strains The deformed shape and its accompanying stress distribution may be subsequently used for structural analysis 7.25.5 Optimization One of the most exciting possibilities for simulation is the potential to optimize part designs and manufacturing processes Simulation has always been used to improve part. .. cooling time tcool Such a profile is capable of improving part quality dramatically It is almost impossible for a molder to define an optimum profile such as this since there is no way of quantifying the effect of changing pressures and rates of pressure decay 0267146_Ch07_Harper 2/24/00 4:48 PM Page 1 19 Auxiliary Equipment: CAD, CAM, and CAE 7.1 19 P Pc Pd tc tcl1 tcl2 tcool Figure 7.60 Optimized packing... overview of this approach is given in Ref 16 The bibliography of Ref 16 cites hundreds of empirical studies, each contributing to the relationship between processing and part quality Demand for increased quality of molded parts in the 197 0s saw an increased interest in mathematical modeling of the injection-molding process During this time, many pioneering studies were published (see, for example, Refs . used. 0267146_Ch07_Harper 2/24/00 4:48 PM Page 97 7 .99 7.21 Introduction We begin with some definitions of the acronyms used in this section. The term computer-aided drafting (CAD) is part of common language today and. provide energy cost savings of up to 70% (see Fig. 7. 49) . Auxiliary Equipment: Drying and Dryers 7 .93 0267146_Ch07_Harper 2/24/00 4:48 PM Page 93 Today’s gas dryers incorporate the most advanced. each contributing to the relationship between processing and part quality. Demand for increased quality of molded parts in the 197 0s saw an increased interest in mathematical modeling of the

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