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Plastik kalıp tasarım esasları ve uygulamalı kalıp tasarım örneği

Principle of plastic injection mold design and a sample of applied mold design

  1. Tez No: 83031
  2. Yazar: CANKUT BUCAKLIGİL
  3. Danışmanlar: PROF. DR. TEOMAN KURTAY
  4. Tez Türü: Yüksek Lisans
  5. Konular: Makine Mühendisliği, Mechanical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1999
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 200

Özet

PLASTİK KALIP TASARIM ESASLARI VE UYGULAMALI KALIP TASARIM ÖRNEĞİ ÖZET Plastik enjeksiyon kalıpçılığı günümüzde plastik ürünlere gösterilen ve gittikçe artan bir taleple orantılı olarak önem kazanmaktadır. Dünyada bir çok ülkede enjeksiyon kalıpçılığı ayrı bir sanayii dalı haline gelmiştir. Hatta bazı ülkelerde kalıp tasarım ve imalat okullarında kalıp tasarımcıları yetiştirilmektedir. Portekiz, Tayvan, Çin, Güney Kore gibi ülkeler günümüzde kalıp imalatında dünya lideri konumundadırlar. Plastik malzemeler oldukça çeşitlilik gösterirler. Her plastik malzemenin fiziksel, kimyasal özellikleri farklılık gösterirken kalıp tasarımı açısından da plastik malzeme önem kazanır. Kalıp tasarımcısı tasarımın başında kullanacağı plastiğin tüm özelliklerini göz önünde bulundurmalıdır. Plastiğin çekme payı, akıcılığı, kırılganlık özelliği vs. gibi özellikleri tasarım aşamasında önemle dikkat edilecek hususlardır. Plastik ürünü seri olarak üretildiği makinalar enjeksiyon makinalandır. Enjeksiyon makinası, plastik hammaddenin kademeli olarak ısıtılıp eriyik hale getirildikten sonra bir vida vasıtasıyla basınç uygulayarak kalıp boşluğuna basan, kalıptaki ürüne soğutma sağlayan ve ürünün kalıptan çıkması için itici mekanizmasını aktif hale getiren elektronik ve hidro-mekanik donanımlı bir makinadır. Bir enjeksiyon kalıbı temel olarak dişi taraf ( A bölümü ) ve erkek taraf ( B bölümü ) olmak üzere iki kısımdan oluşur. Dişi taraf enjeksiyon memesine bağlanır ve genelde ürünün kozmetik tarafını oluşturduğu için hassas bir yüzey kalitesine sahiptir. Erkek taraf ise erkek çekirdeklerin, itici plaka ve pimlerin olduğu taraftır. Kalıp ana plakalar içerisine yerleştirilmiş bir çok sayıda daha küçük ebatlı parçalardan (lokmalar) oluşur. Enjeksiyon kalıpları maçasız olabileceği gibi kompleks maça sistemlerini de içeren bir yapıda olabilir. Maçalı kalıplar pimli mekanik veya hidrolik mekanizmalı da olabilir. Bir kalıbın sağlıklı çalışabilmesi ve doğru ürün basabilmesi için tasarım aşamasında bir çok faktörün göz önünde bulundurulması ve tasarımda en uygun optimazisyona gidilmesi gerekmektedir. Parçanın yolluk detayı, itici pimlerin yeri, sayısı, şekli, soğutma veya ısıtmanın fazla veya yetersiz olması, kalıp ayrılma çizgisinin doğru yerden geçirilmesi, yüzey parlatma işlemlerinin yeterliliği, lokma ve genel kalıp plakası boyutlarının ölçüleri, uygun kalıp malzemesi seçimi, lokma konfigürasyonlan ve daha bir çok faktör bir kalıbın istenen şekilde çalışabilmesi için doğru tasarım şartını gerektirmektedir. XII

Özet (Çeviri)

PRINCIPLE OF PLASTIC INJECTION MOLD DESIGN AND A SAMPLE OF APPLIED MOLD DESIGN SUMMARY The function of the mold is two : imparting the desired shape to the plasticized plastic and solidifying ( cooling for thermoplastic and heating for thermosets ) the injection molding part. It basically has two sets of components : (1) the cavities and cores and (2) the base in which the cavities and cores are mounted. The mold, which contains one or more cavities, consists of two basic parts : a stationary mold half on the side where the plastic is injected, and a moving half on the closing or ejector side of the machine. The separation between the two mold halves is called the parting line. In some cases, the cavity is partly in the stationary and partly in the moving sections. These factors are interrelated, but the size and weight of the molded parts limit the number of cavities in the mold and also determine the machinery capacity required. In the case of large molded parts, such as an auto radiator grille or a one- piece bucket chair, the large exterior dimensions of a single-cavity mold require a correspondingly large clearance between the machine tie-rods. Similarly, the machine tie-rod clearances basically limit the number of cavities that can be installed in a multicavity old. From the general concept of the molding operation, it is important to design a mold that will safely absorb the forces of clamping, injection, and ejection. Furthermore, the flow conditions of the plastic path must be adequately proportioned in order to obtain uniformity of product quality in cycle after cycle. Finally, effective heat absorption from the plastic by the mold has to be incorporated for a controlled rate of solidification prior to removal from the molds. The mold designer should become thoroughly familiar with the processing information of the plastic material for which the mold is being built, so that the molding factors that enter into the design are fully taken into account. The mold determines the size, shape, dimensions, finish and often the physical properties of the final product. It is filled through a central feed channel called the sprue. This sprue, which is located in the sprue bushing, is tapered to facilitate mold release. In single-cavity molds, the sprue usually feeds the polymer directly into the mold cavity., whereas in multicavity molds it feeds the polymer melt to a runner system ( cold or hot ), which leads into each mold cavity through a gate. XlllThe mold is aligned with the injection cylinder by means of a ring in the stationary mold half., into which the cylinder nozzle seats. The locating ring surrounds the sprue bushing and is used for locating the mold in press platen concentrically with the machine nozzle. The opening into which the ring fits is made to a tolerance of 0.00 and +0.05 mm. The ring itself is made 0.25 mm smaller than the opening, providing a clearance of 0.13 mm Per side. A clearance above this amount may cause misalignment with the nozzle, which in turn would entrap part of the sprue, causing sprue sticking on the wrong side. The sprue bushing on the locating ring end has a spherical radius 0.12 or 0.19 mm to fit the machine nozzle radius. The hole through the length of the sprue has a 0.40 mm/cm taper of 1 degree, 1 1.5 minute Per side. This hole must have a good reamed and polished finish to prevent sprue sticking. The parting line is formed by cavity plates A and B. Cavity plate A retains the cavity inserts and supports the leader pins, which maintain the alignment of cavity halves during operation. These guide pins are preferably mounted in the stationary mold half to ensure that the molded products will fall out of the mold during ejection without being fouled. One of the four leader pins is offset by an amount of about 4.8 cm to eliminate the chance of improper assembly of the two halves. The alignment of mold halves is usually accomplished using leader pins. Many moldmakers use tolerances of ± 0.033 mm per side pin to bushing. Tighter tolerances of ± 0.01 to ± 0.02 mm provide more accurate alignment and less wear. On ejector systems, a minimum of four leader pins and bushings are used to prevent cocking of the platen, which reduces wear and prevents seizing. Mating with A plate is B plate, which holds the opposite half of the cavity or the core and contains the leader pin bushings for guiding the leader pins. The core establishes the inside configuration of a part. B plate has its own backup or support plate. The backup B plate is frequently supported by pillars against the U-shaped structure known as the ejector housing. The U-shaped structure provides the space for the moving ejector plate or ejection stroke, also known as the stripper stroke. The ejector plate, ejector retainer, and pins are supported by the return pins. When in an inactivated position, the ejection plate rests on stop pins. When the ejection system becomes heavy because of required high injection forces, additional supporting means are provided by mounting added leader pins in the rear clamping plate and bushing in the ejector plate. The overall height of the mold should correspond to the open space in between the machine platens. In the moving mold half, spacers are used to create space for the ejector system., which consist of two ejector plates with ejector pins. The open space should be such as to permit the ejector pins to complete their ejection stroke. It must be noted that the mold height, or dieheight, in the usual horizontal operating machine is the horizontal dimension of the mold. When the mold is removed and placed“upright”on a workbench, its mold height is vertical. All the mold plates ( excluding the ejector parts ) and spacer blocks are ground to a thickness tolerance of ± 0.025 mm. Conceivably, a combination of tolerance could build up to cause an unevenness at the four corners. If great enough, such a condition would damage a platen when under full ram pressure. It is advisable to check the uniformity of all four corners prior to preparing the base to receive cavities. xivBoth mold halves are provided with cooling channels filled with coolant to carry away the heat delivered to the mold by the hot thermoplastic polymer melt. With thermosets, electric heaters are located in the mold. When the mold opens, molding and sprue are carried on the moving mold half; subsequently, the central ejector is activated, causing the ejector plates to move forward, so that the ejector pins push the article out of the mold. Ejector pins have a tendency to produce a very slight flash line, which in some areas of a part may be objectionable ; therefore, their location and the amount of recess formed by them in the part should be agreed on with product designer. In the smallest injection molding machine, the mold may be completely demountable, and while being filled is held in a simple vise. This can be vertically or horizontally acting to suit the cylinder ; some cylinders are down-stroking and some horizontally acting. With a horizontally acting cylinder and vertical clamp, the runners and sprue bushing are in the same plane ; and often because the pressures involved are not very great, the hardened sprue bushing is replaced by a simple runner cut into one or both halves of the mold. With the larger horizontal clamping, though should always be given to whether a horizontal or vertical flash line is either possible or desirable. With the vertical flash line, sometimes called a positive mold, it can be seen that material cannot escape from the parting line of the mold until considerable opening movement has taken place. If an over-full shot were to be made, the mold would not flash although the shot weight might be too great. This design also has the advantage that oversize moldings can be made with a relatively small mold locking force. If the correct amount of material is injected into the mold, it may open slightly ( a few hundreds of an mm ) but no material escapes; as the mold cools, the mold closes again, thus“compression molding”the part. This positive mold is the principal type used in compression molding. It is also the design used with certain“structural foam”molding and in“combing”injection molding with compression. Some of general mold descriptions and terminology are below. Mold : A mold is one of the most important pieces of production equipment in the plant. It is controllable complex device that must be an efficient heat exchanger. If not properly handled and maintained, it will not be an efficient operating device. Mold Backing Plate : In injection molding equipment, this is a heavy steel plate that is used as a support for the cavity blocks, guide pins, bushing, etc. Mold Base : By this term, it means an assembly of precision steel plates that holds or retains the cavities in a mold. Provides a means for melt to be injected into the cavities and for solidified parts to be ejected from the mold. It is the assembly of all parts in the mold, other than the cavity, core, and pins. Also called mold frame, mold set, die base, die shoe, or shoe. xvMold Bottom Plate : Refers to the part of the mold that contains the heel radius and pushup ( ejection mechanism ). It is used to join the lower section of the mold to the platen of the press. Mold Cavity : This is depression in the mold ; the space inside a mold where the plastic produces the product ; the female portion of the mold ; that portion of the mold that encloses the molded product ( also referred to as the die or tool ) ; also the space between matched molds. Inserted cavities can be used or a depression in the mold is made by casting, machining, hobbing, or a combination of these methods. Depending on the number of cavities, molds are designated as single-cavity, double- cavity, 32-cavity, multi-cavity, etc. Mold Cavity Coating : This is a coat of plastic over the. bare mold. It is used to seal the mold and make a smooth surface on which to mold parts. This is often referred to as a tooling gel coat. Mold Cavity, Double : A double-cavity mold is a mold possessing two cavities for the simultaneous fabrication of two parts. Mold Cavity, Duplicate plate : By this term, it means a removable plate that retains cavities, used when two-plate operation is necessary for loading inserts, etc. Mold Cavity Fabricating Equipment : Tool-room equipment is used for machining mold bases, cores, cavities, pins, blocks, and other parts. Fabrication can be assisted by electronically punched tape and CAM. Mold Cavity, Female : In molding practice, this is the intended cavity half of a mold designed to receive the male half. The term half is only used as one part of the conventional two-part mold ; it does not represent a measurement of half. Mold Cavity Impression : Molds may be designated as single or multiimpression. The term cavity in place of impression has more general use- thus, it is said multicavity. Mold Cavity Packing : Plastic is a compressible fluid. Therefore, it holds pressure and shrinks as it cools, requiring decisions to be made about the amount of overpacking necessary to minimize problems that occur when the plastic cools and shrinks, such as developing undue frozen stresses or causing flash ( even with thermoset plastics, controlling the amount of flash that will occur ). There is a tradeoff between overpack and shrink arrived at with a certain amount of guesswork, that is based on experience. However, computer software provides greater insight into the compressibility of plastic materials, so one is able to make better decisions. Mold Cavity Register : Refers to the angled faces on parts of the mold that match when the mold is closed and thus ensure correct alignment of the parts. Mold Cavity Retainer Plate : These are plates in the mold that hold the cavities and forces. Such plates are at the mold parting line and usually contain the guide pins and bushings. They are also called force retainer plates. XVIMold Cavity, Split : By this term, it means a cavity of a mold that has been designed in sections to permit performing different actions. These are blocks that, when assembled, contain a cavity for molding products having undercuts. Mold Cavity Surface Finish : The surface of a cavity affects appearance, ejectability, and cost. It can be specified by comparing it with six different finishes using the standard. Companies that provide the service of surfacing generally have more detailed information. Surface finishes include chrome-plated, electroless nickel, etched, sand-blasted, and EDM (Electro Discharging Machine ). Mold Chase : This is the main body of the mold ( usually steel ) that contains the molding cavity(s), cores, pins, guide pins, or bushing. More specifically, it is an enclosure of any shape used to (1) shrink-fit parts of a mold cavity in place, (2) prevent spreading or distortion in hobbing, and (3) enclose an assembly of two or more parts of a split cavity block. It is also called a spacer and bolster. Mold Chase, Floating : Refers to the mold member, free to move vertically, that fits over a lower plug or cavity, and into which an upper plug telescopes. Mold Classification by Operation : There are basically three modes of operation, namely automatic, semi-automatic, and manuel. Mold Closed Process : This is a family of techniques for reinforced thermoset plastics fabrication utilizing a two-piece male and female mold ; the processes are usually extensively automated. Mold, Cold Slug : By this term, it means the first thermoplastic melt to enter an injection cold runner mold, so-called because in passing through the sprue orifice, it is cooled below the effective molding temperature. Mold, Cold Slug Well : This is space provided directly opposite the sprue opening in an injection mold to trap the cold slug. Mold, Combination : Refers to a mold that has both positive portions or ridges, and cavity portions such as a refrigerator door liner. Mold Cored : By this term, it means a mold incorporating passages for electrical heating elements, water, steam, etc. Mold Core Pin : (1) This is a pin used to produce a hole in a mold. (2) In injection blow molding, it is the internal rod used to hold the inside of the preform. This rod also retains the plastic melt during the injection molding steps as it is transferred through the cycle. The core is also the blowing pin where air or a blowing medium blows through the channels cut in the center of this core rod to expand the perform in the blowing mold. Mold, Duplicating : Refers to a mold made by casting over or duplicating another product by mechanical reproduction using cutting tools that are guided by a master, proportional in size to the desired finished products. XVIIMolded Edge : This is an edge that is not physically altered after molding ( with fiber reinforcements ) for use in final form, particularly one that does not have fiber ends along its length. Molded Net : Refers to a description of a molded part that requires no additional processing to meet dimensional requirements. Mold Efficiency : In a multimold blowing system, this is the percentage of the total a turnaround time of the mold actually required for forming, cooling, and ejection of the blown part. Mold Ejection : By this term, it means a device or system fitted to usually the moving platen of a machine for operating the molding ejector(s) to remove molded parts. It may be operated mechanically ( including springs ), hydraulically, pneumatically, or electrically. It operates in sequence with the clamping close preposition, a provision in the clamping unit that actuates the ejection action. It is available in various designs and qualities, or by systems such as knockout pins, stripper plate or ring, unscrewing, cam, removable insert, or bushing. The choice of ejector system is largely governed by article shape, and by rigidity of flexibility of the plastic used. The mold should preferably be fitted with ejectors at those spots around which the molding is expected to shrink ( e.g. around cores ). At high mold temperatures, allowance must be made for thermal expansion of the mold platens. These platens will expand more than the plates of the ejector mechanism. It is, therefore, recommended that the ejectors be provided with a cylindrical head and mounted with some clearance to allow the correction of possible variations in center distances during machine operation. The ejection of articles with large cylindrical or flat surfaces tend to create a vacuum between the article and cavity wall. In such cases, release may be improved and the vacuum broken by an air ejection system. xvin

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