Plastik enjeksiyon kalıplarının tasarımı ve üretimi
Başlık çevirisi mevcut değil.
- Tez No: 75571
- Danışmanlar: PROF. DR. TEOMAN KURTAY
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1998
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Makine Ana Bilim Dalı
- Bilim Dalı: İmalat Bilim Dalı
- Sayfa Sayısı: 212
Özet
ÖZET Plastik enjeksiyon yöntemi özellikle termoplastiklerin kalıplama yolu ile şekillendirilmelerinde kullanılan en önemli yöntemlerden biridir. Özellikle yüksek üretim adetlerine uygun bir yöntem olarak karşımıza çıkmaktadır. Yöntem hammaddeyi bir operasyonda bir çok uygulamalar için mamul haline getirdiği için oldukça etkin bir üretim yötemidir. Yöntemi en önemli bileşenleri kalıp ve enjeksiyon makinasıdır Bu çalışmada plastik enjeksiyon kalıplarının tasarımları ve üretimleri konusu üzerinde durulmuştur. Yöntemin ilk yatırım maliyetleri yüksek olduğu ve kalıp yapımıda uzun sayılabilecek sürelerde gerçekleştirildiğinden taşarımda ve üretimde her adımda dikkatli olunması zorunluluğu vardır. Bu nedenle konular mümkün olduğu kadar bölünüp her bir bölüm kendi içinde yeteri kadar detaylandırılmaya çalışılmıştır. Çalışmanın başlangıcında yöntem ve bileşenleri tanıtılmıştır. Daha sonra yöntemin başarısı üzerinde önemli etkileri olan malzeme özellikleri üzerinde durulmuştur. Kalıp tasannu konunun en can alıcı noktalarından bir olduğu ve kalıp üretimini de doğrudan etkilediği için mümkün olduğu kadar geniş bir şekilde ele alınmaya çalışılmış ve bazı tasarım detaylarının üretimle ilgisi zaman zaman vurgulanmıştır. En son bölümde ise kalıp üretimi üzerinde durulmaya çalışılmış, kalıp üretiminde kullanılan malzemeler ve üretim teknikleri anlatılmıştır. Çalışma, özellikle konu ile yeni ilgilenmeye başlayanlar için sınırlı da olsa bir kaynak teşkil edebilmek üzere hazırlanmıştır. Bu nedenle daha çok bir tanıtım niteliğine sahiptir. İçerisinde bulunan bölümlerin hemen hemen hepsi daha derin teorik ve pratik birikimler üzerinde kurulmuşlardır. Konu ile ilgilenenlerin daha kapsamlı bir inceleme yapabilmeleri için çalışmanın sonunda verilmiş olan kaynaklar yararlı olacaklardır. XV
Özet (Çeviri)
SUMMARY DESIGN AND MANUFACTURING OF PLASTIC INJECTION MOLDS Injection molding represents the most important process for manufacturing plastics. It is suitable for mass producing articles, since raw material can be converted into a molding by a single procedure. In most cases finishing operations are not necessary. An important advantage of injection molding is that with it one can make complex geometries in one production step in an automated or semi-automated process. Typical injection moldings can be found everywhere in daily life; examples include toys, automotive parts, household articles, and consumer electronics goods. The great economic significance of plastics is intimately tied to their properties. A fundamental feature of plastics is their variety. The properties of plastics can be changed within wide limits, and combined to give a diversity a unknown in other groups of materials. The main properties of plastics are summarized as follows: - Range of densities (0.8 g/cm3 - 2.2 g/cm3 ) - Wide range of mechanical properties - Easy processabilty - Modifiability by additives - Low thermal and electrical conductivity - Transparency - High chemical resistance - Recyclability - Low energy consumption for raw material production XVIPlastics can be classified according to several criteria. Generally an initial rough classification can be made to their chemical structure. This initial differention is between cross-linked and non-cross-linked materials. Thermoplastics are not cross- linked; elastomers and thermosetting plastics are cross-linked materials. Plastics are made of linear or branched molecules. In thermoplastics materials there is no chemical connection between individual macromolecules. Therefore they can be reused several times. With thermoplastics one further differentiates between those plastics in which macromolecules are arranged at random and those materials with some areas arranged in a regular way. If the arrangement of macromolecules is random, the materials are termed amorphous. They can be easily identified by their transparency, if no color pigments are admixed. Materials with molecules arranged regularly in some areas are termed semi-crystalline. They are not transparent even if pigments are not admixed. There are also plastics that can be produced in either an amorphous or a semi-crystalline state depending on the processing parameters. Amorphous and semi- crystalline thermoplastics have different properties with regard to processing. They also show different performance. Determining the flow behavior and knowing the rheological properties are important for plastics processors. Plastic processors use rheological properties for initial control of the material as well as for checking the process. The rheological behavior of plastic melt is termed viscoelastic. This means that molten plastics behave viscously (like liquid), but also elastically (like an elastic solid). In most cases the viscous properties dominate in the molten state. The viscous properties can be characterized by the viscosity, which is measure of melt's inner resistance to flow processes. To maintain a flow process, a force is necessary ; the force depends on the size of the macromolecule, especially on the molecular weight, as well as on other parameters. During the process, this force is supplied by injection unit of injection machine. Injection molding consists of two main elements, the injection molding machine and the injection mold. An injection machine can be broken down into following components. - Plasticating / injection unit XV'U- Clamping unit - Control system - Tempering devices for the mold. The mold is the key element in an successful injection molding process. A mold with one or more cavities has to be manufactured individually for each part geometry. A mold must : distribute the melt, form the melt into a final geometry, cool the melt, eject the finished part. The functional elements of a mold accordingly fall into following groups. - Runner system (takes up and distribute the melt) - Cavity (forms the melt) - Tempering system (cools the melt) - Ejector system (ejects the molded part) Apart from these processing elements there are additional requirements that the mold must fulfill. The mold must be able to be mounted to the plates of the injection molding machine. To simplify clamping the mold onto the mold carrier plates of the clamping unit and to align the feed bushing with the nozzle of the plasticating cylinder, molds are provided with alignments that fit into a corresponding hole in the mold carrier plate, on either the nozzle or the ejector system. In addition to forming the injection-molded article, the mold has to another important task, namely removing the injection-molded product. This is possible only if the mold comprises at least two parts, which can be separated without difficulty and fitted together again precisely. For this, the mold pieces must be guided with respect to each other. The function of the runner system is to take up the hot melt coming from the nozzle of the plasticating unit and the transport the melt to the cavity or to distribute it to several cavities. During injection the nozzle of the plasticating unit is in close contact with the sprue bushing of the mold and presses hot melt into the sprue. In the case of multicavity mold, the melt then reaches the sprue channel and distributed via runners Willand gates to the various cavities. The gate is connection with a very small section is to reduce visible marks on the molding when the runner system is removed; another is to add additional frictional heat because the melt has already cooled while flowing through the runner system. For a multicavity mold, the runner system must be designed in such a way that melt of the same temperature and pressure fills the cavities simultaneously and uniformly, otherwise moldings of different qualities and properties would be produced during one shot. The gate should be located so that weld lines are avoided or minimized. Weld lines occur when melt streams come from different directions and meet, such as when a cavity is filled from two or more gates or when the melt must flow around hindrances, (for example cores). If welding inadequate, the result may be visible marking and reduced mechanical strength. Care should be taken to ensure that the gate is placed, if possible, in the region of the molded piece with the greatest wall thickness, because the material shrinks during cooling. To compensate for the shrinkage, sufficient additional melt must be conveyed to all areas of the cavity after injection, during the holding pressure phase. However, this can be accomplished only as long as the material has not completely solidified. Because the thickest areas solidify last and, therefore, can be provided with melt during the holding pressure phase for the longest time, the gate should be positioned there. The cavity distributes the melt, forms it, and thus gives it the final shape of the molding. The cavity represents the negative shape of molding walls. Injection moldings are very often complex geometries with undercuts. In such cases the cavity must be formed by movable mold walls that slide into their final position to build up the cavity walls as the mold is closed. These sliding or turning mold parts are necessary for easy ejection of the part. The part properties depend on both mold design and the processing conditions. Flow processes in the runner system and the cavity give rise to preferred orientations of the polymer macromolecules as well as to internal stresses in the molding. All these internal characteristics generated in the cavity eventually influence the part properties and thus the part quality. XIXThe mold consists of at least to parts, so that the finished piece can come out. For this, the mold is opened at the parting line. The finished molded part can be removed manually from the open mold, or it can be pushed out by an ejector system as the mold is being opened. Depending on the part geometry, such ejectors may consist of pins or rings, embedded in the mold, that can be pushed forward when the mold is open. The two halves of an injection mold must be clamped so that they fit together precisely. To ensure the correct position of the mold halves, the mold is provided with one or several alignment devices. The mold halves close tightly enough so that the melt under pressure can not leave the mold cavity; on the other hand, the air inside the cavity must be able to escape while the melt is flowing in. Removal of the finished part is more difficult if it has undercuts. In this case the molded part can be removed only if the mold has more than two movable parts. Another functional element of a injection molds is the tempering system. The duty of the tempering system of the mold is to cool the melt so that it can solidify and subsequently be removed. Tempering is very important, since it affects both molded part quality and cooling time. In thermoplastic processing, hot melt must be cooled from a melt temperature between 200 - 300 °C to the ejection temperature (between 50 - 110 °C). The cooling time, that the time required to cool the melt temperature down to removal (ejection, demolding) temperature, makes up the bulk of cycle time and thus has a direct influence on the economic aspects of injection molding process. Therefore an estimate of cooling time is of paramount importance to an estimate of the production costs. For thermoplastics, short cooling times are obtained by low melt and mold wall temperatures as well as by demolding temperatures as high as possible. However, limits to these temperatures are set by the processed quality of the molded part: -Low melt temperature increase the pressure loss during cavity filling, and also lower the quality of the weld line. -Low mold wall temperature reduces the surface quality of the molding. -If the demolding temperature is to high, the ejector pins cause plastic deformation of the molded part. XXThe injection molding technique has to meet the ever increasing demands for a high quality product, which should still be economically priced. This is feasible only if the molder can adequately control the molding process, if the configuration of the part is adapted to the characteristics of the molding material and the respective conversion technique, and a mold is available which satisfies the requirements for reproducible dimensional accuracy and surface quality. Therefore injection molds have to be made with the highest precision. They are expected to provide reliable and fully repeatable function in spite of being under extreme loads during the molding process, and a long service life to offset the high capital investment. Besides initial design and maintenance while in service, reliability and service life are primarily determined by mold material used, its heat treatment and the machining operations during mold making. Injection molding uses almost exclusively high-strength molds made of metals, primarily steel, because costs of mold materials are only a small part of the total costs of manufacturing. Cavities are frequently made of a different high-grade steel or, in certain cases, of other metals and inserted into the cavity retainer plate. Inserts made of materials other than steel are preferably used for cavities that are difficult to shape. Several factors determine the selection of materials for cavity and core. They result from economic considerations, nature and shape of the molding and its application, and from specific properties of the mold material. Details about the molded part should provide information concerning the plastic material to be employed. Probably 90% of all molds are made by machining operations, primarily turning, milling, drilling and grinding. The machinery, frequently special equipment, has to finish the object to extent that only little postoperation, mostly manual in nature (polishing, lapping and finishing), is left. Up-to-date tool shops are equipped with modern numerically-controlled (NC) machines, which deliver high accuracy and provide greater efficiency against rejected. The result of an inquiry shows only share of 25% NC-machining against 75% for the copying technique but this does not hold true for modern mold shops and the fabrication of large molds. XXIMachining frees existing residual stresses. This can cause distortion either immediately or during a later heat treatment. It is advisable, therefore, to relieve stresses by annealing after roughing, any occurring distortion can be compensated by ensuring finishing which usually does not generate any further stresses. After heat treatment the machined inserts are smoothened, ground and polished to obtain a good surface quality, because the surface conditions of a cavity are, in the and, responsible for the surface quality of a molding and its ease of release. Defects in the surface of the cavity are reproduced more or less pronounced depending on the molding material and processing conditions. Deviations from the ideal geometrical contour of the cavity surface such as ripples or roughness diminish the cosmetics appearance in particular and form undercuts, which increase the necessary release forces. XXll
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