1 Introduction to Plastics Processing

1 Introduction to Plastics Processing 1.0 Introduction Material characteristics and the injection molding process interactively affect the quality of the resulting molded part Fortunately, the overall process is governed by thermodynamic principles, making it possible to manage the characteristics of the final product In this chapter, the variables that influence final part size are identified and explained in the context of the fundamental notions of interactivity and thermodynamics 1.1 Interactivity Basics Two general types of plastic materials are commonly used in injection molding These are referred to as thermoset and thermoplastic, reflecting their manufactured part properties A thermoset plastic is one in which cross-linking is stopped early in the reaction The reaction either will not continue, or will continue at a very slow rate, under normal conditions At temperatures above about 93°C (200°F), the material is a viscous fluid that can be forced into a mold At temperatures of 150°–175°C (300° to 350°F), the cross-linking reaction proceeds at a rapid rate until the reaction is complete and essentially all possible cross-links are established (For thin parts, the reaction is complete in a matter of a few seconds Thicker wall parts can require several minutes or more.) Once the reaction is complete, the material will not again soften to allow molding This is a onetime process and is irreversible (although some molders are grinding up sprues, runners, and bad parts and are adding a small percentage of the resulting particles as filler to the unprocessed raw material for subsequent molding of additional parts) Thermoset materials include many types of rubber, alkyds, phenolic (panhandles and many electrical products), diallyl phthalate (mostly electrical parts) and melamine (commonly used in dinnerware) A thermoplastic material also softens to a viscous fluid when heated; however, few, if any, cross-links are established during processing A thermoplastic hardens to a useful condition when cooled While soft, the material can be forced into a mold to assume the © Plastics Design Library shape of the mold This cycle can be repeated many times because the finished part can be ground up and reprocessed There is no significant chemical reaction during the processing of thermoplastics other than some degradation of the physical properties When properly processed there is little degradation, but if the material requires drying and is not properly dried before processing, or if excessive heat is used during processing, significant degradation will occur The processing temperatures required for thermoplastics differ according to their melt temperature Some plastics can be processed at 205°C (400°F) or less Other high performance thermoplastics can require processing temperatures of 315°C (600°F) or more Common thermoplastics are seen in everyday life Most jars and bottles containing liquids or medicines are thermoplastics such as polyethylene or polystyrene Furniture, carpets, and floors are usually partially or wholly made of one of several thermoplastic resins such as vinyl or thermoplastic polyester Electrical wall outlets can be either an engineering grade of thermoplastic or thermoset Telephones, computers, television housings, and other electronic devices are molded of one or more thermoplastic resins, most commonly the thermoplastic alloy known as acrylonitrile-butadienestyrene (ABS) The foam cushions in chairs and beds are often thermoset polyurethane Many shoes are thermoplastic, including the soles Most of the storage containers used in the kitchen are thermoplastic, usually polyethylene Much of a car’s interior is made of thermoplastic, as are many external surfaces Medicine has undergone a revolution by virtue of plastics Most medical devices are now discarded rather than being (imperfectly) sterilized Many devices that are being implanted in the human body are made of, or contain some form of, thermoplastic There are thermoplastics that act like rubber, others that act like glass, and still others that mimic some metals Practically all toys are molded of thermoplastics Milk and soda containers are made of thermoplastic resins Most tools have housings or handles comprised of thermoplastics Plastics in general, and thermoplastics in particular, have become so pervasive in our world that it is unlikely that an individual can anything without direct or indirect contact with plastic Ch 1: Introduction to Plastics Processing There are many more molders processing thermoplastics than are processing thermosets Thermosets seem to be more stable than thermoplastics, and the challenge of controlling shrink and warp is less Therefore, this book focuses on the injection molding of two main types of thermoplastics: crystalline and amorphous Crystalline plastics form crystals when they cool but not totally crystallize They form islands of crystals surrounded by amorphous material; see Fig 1.1.[1] The crystalline areas are true solids; thus they tend to be more rigid than amorphous plastics Amorphous plastics, on the other hand, never form crystals and really never solidify Amorphous materials are more subject to creep (For comparison, consider glass, which is also an amorphous material Glass that has been in a window for many years is measurably thicker at the bottom than at the top.) Crystals lock portions of a molecule relative to other molecules The amorphous areas around the islands of crystals are the only areas that allow limited creep and only until the molecules between the crystals are fully extended In general, it can be said that polymers with molecules having very uniform linear shapes can crystallize, while polymers with molecules having irregular shapes, with perhaps many branches, are not likely to crystallize They form amorphous masses when they cool from the molten condition It can be argued that, although the polymers used in commodity and engineering materials creep, they not creep indefinitely and they gradually recover completely when the stress is removed.[2] From a theoretical standpoint, this contention may be true for semicrystalline materials; however, it is extremely rare in practice that a stress is removed More often than not, loaded plastic deforms under excessive stress until other factors reduce the stress to a “bearable” level or until the application fails Herein is an essential challenge to part designers and molders working with plastics The injection molding of thermoplastics is a form of processing in which highly complex physical processes take place Each molding compound reacts differently as it is heated to a temperature suitable for molding and as it cools within the mold The molding compound first has to be melted, then injected at high pressure into a “cold” mold Since the mold is cooler than the compound, the shaped plastic part solidifies rapidly and can then be removed from the mold Each step of the injecting and cooling process affects the quality of the subsequent molded part, as shown in Fig 1.2 Each of the factors on the left affects filling and/or cooling to varying degrees Note that the temperature control system of the mold plays a central role in the quality and cost-efficiency of injection-molded parts It decisively influences quality features such as surface appearance and warpage Efficient mold-temperature control also helps to save costs, since the cooling time, and hence the cycle time, can be optimized Cooling that is too aggressive can cause post-molding problems such as excessive size change; these can occur days, weeks, or even months later Control systems are examined in more detail in Chs and The thermodynamic processes that prevail during each step of the injection molding process are described in the following section Figure 1.1 A representation of a semicrystalline plastic at room temperature Area A represents a crystalline area while area B represents an amorphous area.[1] (Courtesy of Quantum Chemical Corporation.) Ch 1: Introduction to Plastics Processing © Plastics Design Library Figure 1.2 The diagram shows the primary variables that affect the final size of a molded part These variables are the focus of discussion in the chapter sections noted 1.2 Thermodynamic Principles Governing Injection Molding When thermoplastics are injection-molded in a machine resembling the one depicted in Fig 1.3, granules of plastic are melted inside a heated barrel (tube) In the barrel, a screw conveys the plastic forward along the screw into a holding space while the previously injected part cools The plastic granules are brought from room temperature to a molten state in a matter of a few seconds The molten material is then stored and develops a “heat history” until the previously molded part cools and is removed from the mold Figure 1.3 A typical injection-molding machine The raw material hopper is at the far right end over the heating cylinder and injection unit.[60] (Courtesy of Toyo Machinery & Metal Co., Ltd.) © Plastics Design Library Ch 1: Introduction to Plastics Processing 1.2.1 Filling Figure 1.4 shows a schematic diagram of the actions taking place in the injection-molding machine.[3] Before the material is injected, the mold is closed with pressure adequate to resist opening under the injection pressure Once the injection signal has been given, the screw moves forward and presses the molten plastic through both the machine nozzle and the runner/gate system into the cavity At this point, the plastic melt and mold may see pressures in the range of 1360 bar (20,000 psi) The filling process frequently imposes a high level of mechanical and thermal stress on the melt The chief parameters affecting this stress are the nozzle/ runner/gate geometry, wall thickness of the molded part, filling rate, molding compound temperature, and moldwall temperature The pressure acting on the melt as it moves through the system causes internal friction as the material flows through restrictions and around corners This friction adds heat to the molten mass Experimental tests have shown that the mean temperature increase of the plastic material due to friction is approximately equal to the energy given up in pressure loss as the plastic flows into the mold Eq (1.1) ∆vM = ∆p/(r×c p) ∆vM = Mean temperature increase in the melt ∆p = Pressure differential in a flow section of the distribution system r = Melt density cp = Specific heat capacity Note that this equation does not allow for the exchange of heat with the cooler mold cavity It does describe the process independently of the part geometry.[4] As the plastic flows into the mold, it comes in contact with the walls of the mold and starts to cool immediately The thickness of the cooled and relatively stable plastic against the wall depends on the rate of flow of the plastic past the wall and the temperature of the wall The faster the plastic flows and the hotter the wall, the thinner the solidified plastic wall and the more friction heat generated There can actually be a rise in the plastic temperature next to the stable wall due to frictional heating, so that there is a temperature peak adjacent to the stable solidified wall of plastic that is higher than the core temperature of the flowing plastic See Fig 1.5.[4] Another phenomenon taking place during the moldfilling process is that the polymer molecules are partially oriented and stretched in the direction of flow The molecules try to relax from this stretched condition (Their natural condition is to be more randomly oriented, like a length of string stuffed in a cup.) This orientation can cause greater shrinkage of unfilled materials in the direction of flow Amorphous materials shrink slightly less when cooled rapidly than when cooled more slowly However, time and exposure to heat will encourage additional shrinkage Over time, especially at elevated temperatures, the ultimate size change is nearly the same Amorphous materials behave like a box full of corn chips Their shape is such that they will not closely nestle together When shaken violently and suddenly stilled, their apparent volume in the box is greater than if the shaking is gradually diminished, allowing the chips to nestle more closely Amorphous materials, of course, not “nestle” as intimately as crystalline structures Figure 1.4 Schematic of a typical injection-molding machine.[3] (Reprinted with permission of Voridian, Division of Eastman Chemical Company.) Ch 1: Introduction to Plastics Processing © Plastics Design Library begins to drop This mixture of amorphous and crystalline volumes (semicrystalline) results in much more shrinkage than pure amorphous materials because the crystalline structure is much denser The amount of time available to create the crystalline structures also affects the percentage of the volume that is crystalline Thus, hotter mold surfaces or thicker sections tend to allow a greater percentage of crystalline formation than cold molds and thin-walled parts Semicrystalline materials act like toothpicks If you have a container with a large quantity of toothpicks inside and shake it violently (equivalent to heating the plastic), the toothpicks are randomly oriented If you suddenly stop shaking (rapidly cooling) the container, the toothpicks are mostly still randomized, but if the magnitude of shaking diminishes gradually, the toothpicks will nestle together in clusters and become more organized (crystallized) The degree of organization depends in part on the rate of reduction of the shaking (rate of cooling) 1.2.2 Figure 1.5 A typical temperature profile of a flowing plastic melt The shape and magnitude of the temperature variation will differ depending on material and flow rates In the case of semicrystalline plastics, the molecular chain is often folded back upon itself in a nested or layered condition as it attempts to form crystals Other molecular chains are normally incorporated so that any one crystal contains fragments of many different molecules The crystallization process tends to pack the long-chain molecules side by side, causing a more compact structure across the direction of flow than along it This sometimes results in greater cross-flow shrinkage than longitudinal shrinkage Crystalline molecules cool to a certain point, then begin to consolidate into crystals Since the formation of crystals starts in a multitude of places more or less simultaneously, the various crystals cannot mesh to form a single large crystal the shape of the part As crystals form, they give up a lot of heat; therefore, the material temperature changes very little as the crystals are forming When the crystals begin to abut one another, the remaining volume within the part forms an amorphous mass as the material temperature again © Plastics Design Library Holding Once the mold is completely filled with plastic, the holding phase of the cycle begins Pressure is maintained on the plastic in the cavity until the gate freezes or until pressure is released on the plastic still in the barrel of the machine During this phase, a small amount of plastic will flow into the mold as the plastic in the mold cools and shrinks This holding time and holding pressure have a significant effect on in-mold shrinkage Figure 1.6 represents the volume-versus-temperature relationship of amorphous and crystalline materials.[4] The crosshatched area represents the space between molecules When amorphous or crystalline materials are in the fluid state, there is no crystalline structure and a significant amount of free space exists around the molecules The hotter the temperature, the more space there is and the more space the individual molecules occupy As the fluid cools, the amorphous material cools and contracts along the solid line above the hatched area Because amorphous materials not form crystals, free spaces always remain between the molecules Crystalline materials rarely form single crystal structures so there is usually some free space between the crystals The lower solid line in the figure represents the volume occupied by a fully crystallized material Ch 1: Introduction to Plastics Processing Figure 1.6 A volume vs temperature chart.[4] (Courtesy of Bayer.) Semicrystalline materials fall somewhere in between the two lines There are amorphous regions between small crystalline regions The amorphous regions contain some free space so these types of materials never reach their theoretical maximum density The faster semicrystalline plastics cool, the smaller the crystalline regions and the larger the amorphous regions This uncertainty accounts for a significant amount of the unpredictability of plastic shrinkage Unfortunately, even after semicrystalline materials cool to room temperature, they may continue to slowly increase their percent of crystallization and thus continue to shrink The solid line within the hatched area of the figure represents one possible temperature-versus-volume curve for a semicrystalline material 1.2.3 Cooling After the holding phase, the plastic continues to cool until it reaches a temperature at which it is rigid enough to be removed from the mold and remain adequately stable Too short a cooling time results in a part with excessive shrinkage or warpage Too long a cooling time results in excessive molded-in stresses (and possible breakage), as well as an uneconomical cycle time Ch 1: Introduction to Plastics Processing The temperature of the plastic is not uniform when it is removed from the mold The temperature profile across the wall of the molded part is represented in Fig 1.7 as a function of time after the mold fills.[4] Plastic is a poor conductor of heat The temperature of the core of the plastic part when it is removed from the mold is higher than the surface temperature The core takes longer to cool and shrink than the surface There are always some molded-in stresses as a result of this differential cooling The greater the part wall-thickness, the greater this differential cooling and stress For very thick walls, the core temperature can be so high that even though the part looks all right when it is removed from the mold, the heat from the core material can remelt the surface and cause all sorts of difficulties For this reason it is sometimes appropriate to place thick-walled parts into a cooling fluid to keep the surfaces rigid until the core is fully cooled It should be apparent, then, that a mold has several functions It provides an appropriate shape for the plastic part and necessary strength to resist the extremely high injection pressures (which can be over 1350 bar, 20,000 psi) A mold also functions to efficiently and uniformly remove heat from the plastic part, and therefore serves as a heat exchanger © Plastics Design Library Figure 1.7 Temperature profiles through the part wall at different times after the cavity fills.[4] (Courtesy of Bayer.) While the part is cooling, amorphous materials behave differently than crystalline materials Amorphous molecules gradually form friction bonds, as opposed to crystalline bonds, with adjacent molecules, and the mass becomes progressively more viscous until it is rigid enough to retain the desired shape At this point it is removed from the mold Continued cooling causes it to become more rigid until it reaches its maximum strength and rigidity Plastic shrinkage after the part is removed from the mold is more complicated than simple thermal contraction Simple thermal contraction does not include excursions into the molten condition, as does the molding process Furthermore, most materials not have the long molecular chain structure that plastics have This structure encourages additional stress relaxation and crystallization at the temperature at which the molded parts are normally used © Plastics Design Library Amorphous materials change very little beyond gradual stress relaxation after they are cooled to room temperature Semicrystalline materials, on the other hand, continue to build the crystalline structure for a while after the part is removed from the mold The change in structure from room temperature out of the mold to forty-eight hours later is not nearly as great as the change that occurs during the molding process, but must be considered Some semicrystalline materials such as nylon are hygroscopic and must be thoroughly dry before molding After molding they will absorb moisture from their surroundings until they are “saturated.” This changes the size and physical characteristics of the material For example, dry nylon is much more brittle than saturated nylon, while nylon with a modest amount of absorbed moisture is quite tough Ch 1: Introduction to Plastics Processing