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1、The Injection MoldingThe Introduction of MoldsThe mold is at the core of a plastic manufacturing process because its cavity gives a part its shape. This makes the mold at least as critical-and many cases more so-for the quality of the end product as, for example, the plasticiting unit or other compo

2、nents of the processing equipment.Mold MaterialDepending on the processing parameters for the various processing methods as well as the length of the production run, the number of finished products to be produced, molds for plastics processing must satisfy a great variety of requirements. It is ther

3、efore not surprising that molds can be made from a very broad spectrum of materials, including-from a technical standpoint-such exotic materials as paper matched and plaster. However, because most processes require high pressures, often combined with high temperatures, metals still represent by far

4、the most important material group, with steel being the predominant metal. It is interesting in this regard that, in many cases, the selection of the mold material is not only a question of material properties and an optimum price-to-performance ratio but also that the methods used to produce the mo

5、ld, and thus the entire design, can be influenced.A typical example can be seen in the choice between cast metal molds, with their very different cooling systems, compared to machined molds. In addition, the production technique can also have an effect; for instance, it is often reported that, for t

6、he sake of simplicity, a prototype mold is frequently machined from solid stock with the aid of the latest technology such as computer-aided (CAD) and computer-integrated manufacturing (CIIS). In contrast to the previously used methods based on the use of patterns, the use of CAD and CAM often repre

7、sents the more economical solution today, not only because this production capability is available pin-house but also becausewith any other technique an order would have to be placed with an outside supplier.Overall, although high-grade materials are often used, as a rule standard materials are used

8、 in mold making. New, state-of-the art (high-performance) materials, such as ceramics, for instance, are almost completely absent. This may be related to the fact that their desirable characteristics, such as constant properties up to very high temperatures, are not required on molds, whereas their

9、negative characteristics, e. g. low tensile strength and poor thermal conductivity, have a clearly related to ceramics, such as sintered material, is found in mild making only to a limited degree. This refers less to the modern materials and components produced by powder metallurgy, and possibly by

10、hot isocratic pressing, than to sintered metals in the sense of porous, air-permeable materials.Removal of air from the cavity of a mold is necessary with many different processing methods, and it has been proposed many times that this can be accomplished using porous metallic materials. The advanta

11、ges over specially fabricated venting devices, particularly in areas where melt flow fronts meet, I, e, at weld lines, are as obvious as the potential problem areas: on one hand, preventing the texture of such surfaces from becoming visible on the finished product, and on the other hand, preventing

12、the microspores from quickly becoming clogged with residues (broken off flash, deposits from the molding material, so-called plate out, etc.). It is also interesting in this case that completely new possibilities with regard to mold design and processing technique result from the use of such materia

13、ls.A. Design rulesThere are many rules for designing molds. These rules and standard practices are based on logic, past experience, convenience, and economy. For designing, mold making, and molding, it is usually of advantage to follow the rules. But occasionally, it may work out better if a rule is

14、 ignored and an alternative way is selected. In this text, the most common rules are noted, but the designer will learn only from experience which way to go. The designer must ever be open to new ideas and methods, to new molding and mold materials that may affect these rules.B. The basic mold1. Mol

15、d cavity spaceThe mold cavity space is a shapeinside the mold, “excavatedirf such a manner that when the molding material is forced into this spaceit will take on the shape of the cavity space and, therefore, the desired product. The principle of a mold is almost as old as human civilization. Molds

16、have metals into sand forms. Such molds, which are still used today in foundries, can be used only once because the mold is destroyed to release the product after it has solidified. Today, we are looking for permanent molds that can be used over and over. Now molds are made from strong, durable mate

17、rials, such as steel, or from softer aluminum or metal alloys and even from certain plastics where a long mold life is not required because the planned production is small. In injection molding the plastic is injected into the cavity space with high pressure, so the mold must be strong enough to res

18、ist the injection pressure without deforming.2. Number of cavitiesMany molds, particularly molds for larger products, are built for only cavity space, but many molds, especially large production molds, are built with 2 or more cavities. The reason for this is purely economical. It takes only little

19、more time to inject several cavities than to inject one. For example, a 4-cavity mold requires only one-fourth of the machine time of a single-cavity mold. Conversely, the production increases in proportion to the number of cavities. A mold with more cavities is more expensive to build than a single

20、-cavity mold, but not necessarily 4 times as much as a single-cavity mold. But it may also require a larger machine with larger platen area and more clamping capacity, and because it will use 4 times the amount of plastic, it may need a large injection unit, so the machine hour cost will be higher t

21、han for a machine large enough for the smaller mold.3. Cavity shape and shrinkageThe shape of the cavity is essentally the “ negative ” of the shape of the desired product, with dimensional allowance added to allow for shrinking of the plastic. The shape of the cavity is usually created with chip-re

22、moving machine tools, or with electric discharge machining, with chemical etching, or by any new method that may be available to remove metal or build it up, such as galvanic processes.It may also be created by casting certain metals in plaster molds created from models of the product to be made, or

23、 by casting some suitable hard plastics. The cavity shape can be either cut directly into the mold plates or formed by putting inserts into the plates.C. Cavity and coreBy convention, the hollow portion of the cavity space is called the cavity. The matching, often raised portion of the cavity space

24、is called the core. Most plastic products are cup-shaped. This does not mean that they look like a cup, but they do have an inside and an outside. The outside of the product is formed by the cavity, the inside by the core. The alternative to the cup shape is the flat shape. In this case, there is no

25、 specific convex portion, and sometimes, the core looks like a mirror image of the cavity. Typical examples for this are plastic knives, game chips, or round disks such as records. While these items are simple in appearance,they often present serious molding problems for ejection of the product. The

26、 reason for this is that all injection molding machines provide an ejection mechanism on the moving platen and the products tend to shrink onto and cling to the core, from where they are then ejected. Most injection molding machines do not provide ejection mechanisms on the injection side.Polymer Pr

27、ocessingPolymer processing, in its most general context, involves the transformation of a solid (sometimes liquid) polymeric resin, which is in a random form (e.g., powder, pellets, beads), to a solid plastics product of specified shape, dimensions, and properties. This is achieved by means of a tra

28、nsformation process: extrusion, molding, calendaring, coating, thermoforming, etc. The process, in order to achieve the above objective, usually involves the following operations: solid transport, compression, heating, melting, mixing, shaping, cooling, solidification, and finishing. Obviously, thes

29、e operations do not necessarily occur in sequence, and many of them take place simultaneously.Shaping is required in order to impart to the material the desired geometry and dimensions. It involves combinations of viscoelastic deformations and heat transfer, which are generally associated with solid

30、ification of the product from the melt.Shaping includes: two-dimensional operations, e.g. die forming, calendaring and coating; three-dimensional molding and forming operations. Two-dimensional processes are either of the continuous, steady state type (e.g. film and sheet extrusion, wire coating, pa

31、per and sheet coating, calendaring, fiber spinning, pipe and profile extrusion, etc.) or intermittent as in the case of extrusions associatedwith intermittent extrusion blow molding. Generally, molding operations are intermittent, and, thus, they tend to involve unsteady state conditions. Thermoform

32、ing, vacuum forming, and similar processesmay be considered as secondary shaping operations, since they usually involve the reshaping of an already shapedform. In some cases, like blow molding, the process involves primary shaping (pair-son formation) and secondary shaping (pair son inflation).Shapi

33、ng operations involve simultaneous or staggered fluid flow and heat transfer. In two-dimensional processes, solidification usually follows the shaping process, whereas solidification and shaping tend to take place simultaneously inside the mold in three dimensional processes. Flow regimes, depending

34、 on the nature of the material, the equipment, and the processing conditions, usually involve combinations of shear, extensional, and squeezing flows in conjunction with enclosed (contained) or free surface flows.The thermo-mechanical history experienced by the polymer during flow and solidification

35、 results in the development of microstructure (morphology, crystallinity, and orientation distributions) in the manufactured article. The ultimate properties of the article are closely related to the microstructure. Therefore, the control of the process and product quality must be based on an unders

36、tanding of the interactions between resin properties, equipment design, operating conditions, thermo-mechanical history, microstructure, and ultimate product properties. Mathematical modeling and computer simulation have been employed to obtain an understanding of these interactions. Such an approac

37、h has gained more importance in view of the expanding utilization of computer design/computer assisted manufacturing/computer aided engineering (CAD/CAM/CAE) systems in conjunction with plastics processing.It will emphasize recent developments relating to the analysis and simulation of some importan

38、t commercial process, with due consideration to elucidation of both thermo-mechanical history and microstructure development.As mentioned above, shaping operations involve combinations of fluid flow and heat transfer, with phase change, of a visco-elastic polymer melt. Both steady and unsteady state

39、 processes are encountered. A scientific analysis of operations of this type requires solving the relevant equations of continuity, motion, and energy (I. e. conservation equations).Injection MoldingMany different processes are used to transform plastic granules, powders, and liquids into final prod

40、uct. The plastic material is in moldable form, and is adaptable to various forming methods. In most cases thermoplastic materials are suitable for certain processes while thermosetting materials require other methods of forming. This is recognized by the fact that thermoplastics are usually heated t

41、o a soft state and then reshaped before cooling. Theromosets, on the other hand have not yet been polymerized before processing, and the chemical reaction takes place during the process, usually through heat, a catalyst, or pressure. It is important to remember this concept while studying the plasti

42、cs manufacturing processes and the polymers used.Injection molding is by far the most widely used process of forming thermoplastic materials. It is also one of the oldest. Currently injection molding accounts for 30% of all plastics resin consumption. Since raw material can be converted by a single

43、procedure, injection molding is suitable for mass production of plastics articles and automated one-step production of complex geometries. In most cases, finishing is not necessary. Typical products include toys, automotive parts, household articles, and consumer electronics goods,Since injection mo

44、lding has a number of interdependent variables, it is a process of considerable complexity. The success of the injection molding operation is dependent not only in the proper setup of the machine variables, but also on eliminating shot-to-shot variations that are caused by the machine hydraulics, ba

45、rrel temperature variations, and changes in material viscosity. Increasing shot-to-shot repeatability of machine variables helps produce parts with tighter tolerance, lowers the level of rejects, and increasesproduct quality ( i.e., appearance and serviceability).The principal objective of any moldi

46、ng operation is the manufacture of products: to a specific quality level, in the shortest time, and using a repeatable and fully automatic cycle. Molders strive to reduce or eliminate rejected parts, or parts with a high added value such as appliance cases, the payoff of reduced rejects is high.A ty

47、pical injection molding cycle or sequence consists of five phases:1 Injection or mold filling2 Packing or compression3 Holding4 Cooling5 Part ejectionInjection Molding OverviewProcessInjection molding is a cyclic process of forming plastic into a desired shape by forcing the material under pressure

48、into a cavity. The shaping is achieved by cooling (thermoplastics) or by a chemical reaction (thermosets). It is one of the most common and versatile operations for mass production of complex plastics parts with excellent dimensional tolerance. It requires minimal or no finishing or assembly operati

49、ons. In addition to thermoplastics and thermosets, the process is being extended to such materials as fibers, ceramics, and powdered metals, with polymers as binders.ApplicationsApproximately 32 percent by weight of all plastics processed go through injection molding machines. Historically, the majo

50、r milestones of injection molding include the invention of the reciprocating screw machine and various new alternative processes,and the application of computersimulation to the design and manufacture of plastics parts.Development of the injection molding machineSince its introduction in the early 1

51、870s, the injection molding machine has undergone significant modifications and improvements. In particular, the invention of the reciprocating screw machine hasrevolutionized the versatility and productivity of the thermoplastic injection molding process.Benefits of the reciprocating screwApart fro

52、m obvious improvements in machine control and machine functions, the major development for the injection molding machine is the change from a plunger mechanism to a reciprocating screw. Although the plunger-type machine is inherently simple, its popularity was limited due to the slow heating rate th

53、rough pure conduction only. The reciprocating screw can plasticize the material more quickly and uniformly with its rotating motion, as shown in Figure 1. Inaddition, it is able to inject the molten polymer in a forward direction, as a plunger.:Molding OverviewMcvfitilsSt 牌 i?naryPlaten MHd 印曲哼nPlal

54、e?HupperRotating andR BciprccotingrcFIGURE 1. The rec ip rocat ing -screw injectton machineDevelopment of the injection molding processThe injection molding process was first used only with thermoplastic polymers. Advances in the understanding of materials, improvements in molding equipment, and the

55、 needs of specific industrysegments have expanded the use of the process to areas beyond its original scope.Alternative injection molding processesDuring the past two decades, numerous attempts have been made to develop injection moldingprocesses to produce parts with special design features and pro

56、perties. Alternative processes derivedfrom conventional injection molding have created a new era for additional applications, more designfreedom, and special structural features. These efforts have resulted in a number of processes,including:Co-injection (sandwich) moldingFusible core injection mold

57、ing)Gas-assisted injection moldingInjection-compression moldingLamellar (microlayer) injection moldinLive-feed injection moldingLow-pressure injection moldingPush-pull injection moldingReactive moldingStructural foam injection moldingThin-wall moldingComputer simulation of injection molding processe

58、sBecause of these extensions and their promising future, computer simulation of the process has alsoexpanded beyond the early "lay-flat," empirical cavity-filling estimates. Now, complex programs simulate post-filling behavior, reaction kinetics, and the use of two materials with different

59、 properties, or two distinct phases, during the process.The Simulation section provides information on using C-MOLD products.Among the Design topicsare several examples that illustrate how you can use CAE tools to improve your part and molddesign and optimize processing conditions.Co-injection (sandwich) moldingOverviewCo-injection molding involves sequential or concurrent injection of two different but compatible polymer me