Plastik ürün tasarım ilkeleri

Abstract

Bu tez plastik parçaların tasarımında kullanılabilecek ilkeleri ortaya koymaktadır. İkinci bölümde plastiklerin mekanik davranışlarını etkileyen fiziksel ve kimyasal özellikler (zincir şekilleri, stereodüzen, amorf ve kristalin yapı vb.) anlatılmıştır. Üçüncü bölümde çok kısa olarak, plastiklerin mekanik özellikleri bakımından metallerden farkları belirtilmiş ve sonra tasarımda büyük önemi olan viskoelastik davranış incelenmiştir. Dördüncü bölümde plastiklerin pratik hesap yöntemi ve örnekler verilmiştir. Beşinci bölümde elastiklik modülleri düşük olan plastiklerin çeşitli geometrik düzenlemelerle rijtillerinin nasıl artırılabileceği örneklerle gösterilmiştir. Altıncı bölümde plastiklerin değişken yüklere karşı tepkileri ve değişken yüklere maruz parçalarda ne gibi hususlara dikkat edilmesi gerektiği anlatılmıştır. Sonraki bölümde plastik ürün tasarımında izlenebilecek adımlar verilmiştir. Son bölümde ise kalıplanarak üretilecek parçalarda tasarım sırasında dikkat edilmesi gereken geometrik özellikler (çeper kalınlıkları, kaburgalar, yuvarlatmalar vb.) verilmiştir.

Early attempts to design with plastics by using material properties generated with the same testing procedures and the same design rules that were commonly used for metals produced products of poor quality. The reputation of cheap plastic parts become a stigma that still must be overcome in many product lines. The first uses of plastics tended to be on a substitution basis, often involving components that were not intended to carry significant loads. The motivations were that plastics were less expensive, lighter in weight, did not corrode, and could be coloured throughout. Unfortunately these applications frequently involved moderate heat, sunlight or chemical exposures that led to unsightly apperance if not outright cracking and failure. Distortion of parts due to creep and the much lower modulus of most plastics also produced unsatisfactory performance. Gradually engineers and designers who were serious about using plastics and could see the many advantages they had to offer as true engineering materials, began to take a more careful look at how to actually-design with plastics. They also began to demand a more consistent product from the plastics suppliers. Batch to batch variation of early polymers was quite unacceptable product life. Designing with plastics was thus on an evolutionary track. Designers began to learn more about how to work with materials that had low modulus, were significantly lower in strength, were much more temperature-sensitive, and whose properties were time dependent. At the same time raw stock producers were learning how to build consistency into their products. And, of course, new and improved plastics with enhanced properties were being developed at an increasing rate [1, p.3.1-3.2] Today plastics have become increasingly important in industry. Plastics are used in many applications, ranging from food packaging to aerospace industry. Because plastics are different from other materials, different design approach must be applied. This thesis is written to show the principles of plastics product design. The text is divided into eight chapters. The first chapter is introduction. The second chapter covers the physical structure and molecular configuration of the polymer materials. Because mechanical properties of plastics depend on chemical and physical structure. For example thermoplastics have linear molecules. In linear molecules, there are strong covalent bonds between molecules that form the chain. And there are Van der Waals force between chains. This force is weak and XI temperature - sensitive. When plastic is heated, it becomes weak. Linear chains can form crystalline structure which has high tensile strength. Thermoset plastics have cross linked molecules. The molecules which form the structure are bonded to each other strong bonds. These bonds break at only high temperatures. Polymers may also have chain extensions or branches perpendicular to the linear polymer chain. Branched polymers, which occupy more volume than linear polymers, have low density and low specific gravity. However, because the branch chains are not attached to other chains, they are considered to be thermoplastics. Thermoplastics usually consist of mixtures of long - chain molecules with different molecular weights. Proteins, for example, consist of polymers with identical molecular weights and are known as monodisperse structures. Commonly occuring mixtures of polymers are known as polydisperse structures. Therefore, most moleculer weight values for synthetic thermoplastics are avarage values. Very low molecular weight polymers are called oligomers, and high molecular weight polymers consisting of multiples of the same repeat unit are called homopolymers. The term homopolymer is used to_ differentiate between macromolecules with more than one repeat unit (copolymers) and those with multiples of the same repeat unit. However, the term polymer usually indicates a homopolymer. The size of an avarage thermoplastic homopolymer can be designated by its degree of polymerization or number of repeat units. The physical structure is examined in two ways: stereo regularity and arrangement of molecul chains. If a polymer chain is to crystallise it must have a regular molecular structure. By regular is meant that the shape repeats itself at regular intervals. Symmetric chains (linear, isotactic, syndiotactic) form crystalline structure and non- symmetric chain form amorphous structure [2,p27,29] In chapter three differences between ideal elastic solid and plastics are given briefly. And then the viscoelastic behaviour which is important for design is explained. The major difference between metals and plastics design lies in the choice of structural properties to be used in standard design theory and formulas. For metals, these properties are relatively constant over wide ranges of temperature, time, and other measures of the environment. For plastics, structural properties are more sensitive to comparable changes in environment [3,4]. Another design consideration which is important in many design cases like automobile bumbers is toughness of plastics. If a polymer with no secondary transitions is struck a blow at some temperature well below its glass transition temperature, deformation will be very limited before fracture occurs. Nevertheless because of the high modulus quite high tensile strengths will be recorded, of the order of 55 MPa. The energy to break will be given by area under the stress - strain curve and will not be very large. xn On the other hand, ifan amorphous polymer is struck above the Tg, i.e. in the rubbery state, large extensions are possible before fracture occurs and, although the tensile strength will be much lower, the energy to break will be much more, so that for many purposes the material will be regardes as tough. Toughness is not simply a function of polymer structure or the mode of stressing. It clearly will also depend on the temperature and the rate of striking but more important still it will depend on the product design and method of manufacture [5,pl81-183] The next chapter is concerned with practical assessment of long term behaviour. To achieve this a practical way, pseudo-elastic design approach, is introduced. This approach uses classical elastic analysis but employs time and temperature dependent data obtained from creep curves and their derivatives [5,pl92]. The fifth chapter covers the geometrical considerations in design, with particular emphasis on the methods of improving the efective stiffness of plastics structures and of compensating for the non-linear stress-strain behaviour of the materials. To achieve stiffening, sandwich structures, ribs and corrugated panels can be used [6,7]. Structural shapes which are applicable to all materials are discussed such as sandwich structures, ribs and corrugated panels. In the case of plastics, emphasis is on the way plastics can be used in these structures and why they are preferred over other materials. In many cases plastics can lend themselves to a particular field of application only in the form of a sophisticated lightweight stiff structure and the requirements are such that the structure most be of plastics, e.g. in a radome. In the other instances^ the economics of fabrication and erection of a plastics lightweight structure and the intrinsic apparance and other desirable properties make it preferable to other materials. The sixth chapter relates to dynamic loads applied to plastics. Since plastics materials exhibit a complicated stress response, it would be expected that the behaviour under this type of load would differ substantially from the behaviour under static loading. Plastics exhibit hysteresis effect. Because of this effect some amount of heat is produced in the part. Heat can cause failure [6,p90]. The designer can use several approaches to prevent hysteresis failure. The first is material selection. The suffer the material is, the smaller the strain is for a given stress level and the lower the hysteresis loss per cycle. The second approach the designer can use is to improve the heat transfer conditions from the part. This can be accomplished in several ways: To operate the part in coolant medium, increasing the surface area of the part by using fins etc. In the next chapter, design procedure for plastics part is given. This chapter is a synthesis of previous discussions. The steps above are summary of the steps in the design of a plastic part [6, p230]: 1. Define the function of the part with life requirements. Xlll 2. State space and load limitations of the part. 3. Define all of the environmental stresses that the part will be exposed to in its intended function. 4. Select several materials that appear to meet the required environmental stresses that the part will be exposed to in its intended function. 5. Do several trial designs using different materials and geometries to perform the required function. 6. Evaluate the trial designs on a cost effectiveness basis. Determine several levels of performance and the specific costs associated with each to the extent that it can be done with available data. 7. Determine the appropriate manufacturing process for each design. 8. Based on the preliminary evaluation select the best apparent choices and do a detailed design of the part. 9. Based on the detailed design select the probable part design, material and process. 10. Make model if necessary to test the effetiveness of the part. 1 1. Boild prototype tooling. 12. Make prototype parts and test parts to determine if they meet the required function. 13. Redesign the part if necessary based on the prototype testing. 14. Retest. 15. Make field tests. 16. Add instructions for use. In the last chapter, individual design criteria relating to the placement of ribs, bosses, radii and so on, are discussed in detail. Designers should be aware, however, of the uniqueness of plastic materials as used in molding processes. Chemically, the molecules of thermoplastics consist of long chins of repeating units. When melted and injected under high pressures into a closed mold, the polymer withstands forces and undergoes changes. Injection molding has been compared by some to stuffing coil springs into a cavity. If the cavity has generally rounded and uniform contours, it is relatively easy to fill. If, however, it has sharp corners and thick-and^thin areas, it not only will be more diffucult to fill, but, when filling is completed, the springs will be more compressed, stretched and distorted. When the analogue of this happens in molding plastics, a part is said to contain molding strains; though undesirable, such strains are present to some degree in every plastic part. When a molding is heated, it warps because it is pulled in different directions by the strains within it and is less able, because of softening (when the temperature is raised sufficiently), to maintain its shape. When the polymer is injected into every portion of the mold cavity with about equal force and uniformly cooled, the distribution of internal stresses will tend to reach a balance and yield a part with less tendency to warp. High levels of mold strain, on the other hand, are especially detrimental when the part is subjected to further external strain or stress, whether it be physical force, heat, or a stress crack agent [8, p3 1 8]. Under favorable conditions, the design of wall thickness normally depends upon the selection of the material. Occasionally, however,limitation of space precludes this, and the selection of material becomes predicated partly upon the wall thickness xiv available plastic parts should be designed with the minimum wall thickness that will provide the specific structural requirement. It should also be remembered that molding phenomena, such as flow and cure, can influence the choice of wall thickness. Basically, wall thickness should b.e made as uniform as possible to eliminate part distortion, internal stresses and cracking. Sharp corners in plastic parts are perhaps the greatest contributors to part failure. Elimination of sharp corners reduces the stress concentration at these points and produces a molding with greater structural strength. Fillets provide streamlined flow paths for the molten polymer in filling the mold and permit easier ejection of the part from the mold. The function of ribs is to increase the rigidity and strength of a molded piece without increasing wall thickness. The use of ribs usually will prevent warpage during cooling, and, in some cases, they facilitate flow during molding. Modern molding practice dictates certain principles of design, which should be observed if molded articles are to be produced successfully. Most elemantary is the fact that the piece must be easily removed from the mold after it is formed. This point frequently is overlooked, and many products are designed with undercuts that make it impossible to eject them directly from the mold cavity. If undercuts are essential, then split molds or removable mold sections are required, which increase the cost of molds and of the molded articles. Typical devices are the side -pull core used for external undercut molding and split - pin molding for internal undercuts. In the design of articles produced from moldable rigid and -elastomeric plastics, it is important that consideration be given to the easy removal of the piece from the mold cavity. Draft or taper should be provided, both inside and outside. Bosses are protruding studs or pads that are used in design for the reinforcement of holes of for mounting an assembly. The same general precautions to be considered in the design and use of ribs may apply to the utilization and design of bosses. For a variety of reasons, holes are required in a molded piece. They should be designed and located so as to introduce a minimum of weakness and to avoid complication in production. Inserts in plastic parts can act as fasteners or load supports, or may simplify handling or facilitate assembly. Inserts may be functional or purely decorative, but they should be used sparingly because they increase costs. Inserts derive a good deal of their holding power from the fact that plastic materials, when cooling, shrink around a metal insert. Parting lines on the surface of a molded object, produced by the parting line of the mold often can be concealed on a thin inconspicuous edge of the part. This preserves the good appearance of the molding and, in most cases, eliminates the need for any finishing. Close coordination between the designer and the molder in the early stages of design usually will determine the best location of the parting line. xv Wide, sweeping curves and domed rather than flat surfaces should be employed. Improved flow and distribution of material, during the molding operation, will result and the tendency to warp will be greatly reduced. As a result, the appearance of the molded piece imporves. Names, monograms, dial numbers, instructional information, and the like, are frequently required on molded articles. The lettering must be applied in such a manner as not to complicate the removal of the article from the mold. This is accomplished by locating it perpendicular to the parting line and providing adequate draft. XVI BOLUM 1 GİRİŞ Plastiklerle tasarımdaki ilk girişimlerde, metallere uygulanan test usulleriyle elde edilmiş malzeme özellikleri ve metaller için geçerli olan tasarım kuralları kullanılıyordu. Plastiklerle metaller arasında çok büyük farklar bulunduğundan, bu durum mamullerin düşük kalitede olmasına yol açıyordu. Plastikler ilk olarak daha çok, dekoratif parçalarda ve önemli miktarda yük taşımayan parçalarda kullamlmıştır. Plastik kullanınuna yol açan faktörler, plastiklerin ucuz ve hafif olması, korozyona uğramaması ve kolayca renklendirilebilmeleri olmuştur. Fakat plastikler, sıcaklıktan, güneş ışığından ve kimyasal maddelerden etkileniyor, dış yüzeylerde çatlaklar ve bozulmalar görülüyordu. Bunun yanında, sürünme nedeniyle ortaya çıkan şekil değiştirmeler ve düşük elastiklik modülleri nedeniyle, plastik parçalardan istenen performans alınamryordu. Bütün bunlara rağmen, plastik kullanımında ciddi olan mühendis ve tasarımcılar, plastiklerle gerçekten nasıl tasarım yapılacağı konusuna daha dikkatli bakmaya başladılar. Aynı zamanda, hammadde üreticilerinden daha kaliteli ve partiden partiye özellikleri değişmeyen ürünler talep etmeye başladılar. Çünkü ilk polimerlerde özellikler partiden partiye değişiyordu. Bu da nihai mamulün kalitesini ve ömrünü düşürmekteydi. Bu nedenlerden dolayı plastiklerle tasarım aşama aşama gelişmiştir. Zamanla tasarımcılar, elastiklik modülleri ve dayanımları düşük, sıcaklığa daha fazla duyarlı ve özellikleri zamana bağlı olan bu malzemelerle nasıl çalışılacağı hakkında daha