Plastik Enjeksiyon Kalıplarının İncelenmesi

Abstract

Kalıplama yolu ile şekillendirme, plastiklerin üretim yöntemleri arasında en çok kullanılanıdır. Her yöntemin prensiplerine göre, gerek bu yöntem ile üretilebilecek plastik türleri açısından, gerekse yöntemin özellikleri sebebi ile, kullanılan kalıplarda bir takım farklılıklar mevcuttur. Bu çalışmada enjeksiyon ile kalıplama üzerinde durulduğundan, kalıp elemanları, kalıp tasarım kriterleri, kalıp imali gibi konular incelenirken enjeksiyon kalıpları baz alınmıştır. Çalışmaya plastik malzemelerin enjeksiyon kalıplarının tanıtımı ile başlanmış olunup, ilerleyen bölümlerde kalıp elemanları, kalıp çeşitleri, tasarım kriterleri, kalıplama sırasında ortaya çıkabilecek muhtemel hatalar ve bunların giderilmesi için alınması gereken önlemler anlatılmıştır. Yüksek yatırım maliyeti gerektiren enjeksiyon ile kalıplama metodunda, yatırımın büyük kısmını kalıpların oluşturduğu göz önüne alındığında kalıp imalatının ne kadar önemli bir safha olduğu görülmektedir. Bu sebeple kalıp tasarımının ve imalatının ne kadar özen gerektiren bir uğraş olduğu daha iyi anlaşılmaktadır. Bu çalışmada her ne kadar enjeksiyon kalıpları konusunda kapsamlı bir kaynak oluşturulmaya ve kalıp ile ilgili tüm konulara yer verilmeye çalışılmışsa da plastik malzeme ve üretim teknolojisinin gelişimi ile devamlı oluşan yeni bilgilerin daima izlenmesi ve göz önüne alınması gereklidir; Çünkü metal malzemelere alternatif olarak geliştirilen suni malzemeler arasında plastikler, devamlı olarak yeni araştırmalarla, kendini geliştirecektir.

The necessity of materials for industry in 20th century has brought different materials except for the metal materials. There were a lot of materials for daily needs like ceramics, glass, wood, but the most important materials of these were plastics. Although new materials caused some limitation in both designs and using in view of functional or aesthetics, when the plastic materials had invented in 20th century, these problems have been solved mostly. There are some reasons to prefer using plastic materials. The most important characteristics of plastic materials can be counted as follow: -Lightness, -Low costs, -Easy treating, -Resist to chemical agent, -Inductively, -Aesthetic properties, -Changeable their physical and chemical properties. With development of using plastic materials, methods of manufacturing became under developed. Now plastics can be processed by all kind of moldings (like metals). Injection moldings, extrusion moldings, compression moldings and transfer moldings are important kinds of manufacturing methods. There are different equipment, criteria of product and differences for in each ix these different manufacturing methods. There is one thing that is very important and accomplice for all plastic moldings: Molds. Molds are very expensive equipment. The most important subject apart from production of plastics is how it will be formed. It is necessary to realize some criteria to chose the forming method of plastics. Forming process of plastics are generally final process and after forming if there is a failure in process, to correct it will be very expensive and difficult. Therefore it is very important to choose best forming method. Below, some criteria that effect to determine forming method are counted:. Construction of part,. Kind of plastic material that is projected,. Number of product,. Rate of production,. Quantity of production,. Investment cost of production,. How forming method effects properties of part,. Characteristics of product after production. When these factors are examined, that is realized: injection molding is the best forming method in either. Injection molding gives very good results in mass productions. Injection molding consist of two investment.: Injection machine and injection mold. In operation of plastic injection, plastic that is powder or granule, is heated. Than material that is changed in to liquid state by the effect of heat, is forced in to injection mold. After molten plastics enter the mold, it becomes cool and solid. After solidify is completed, mold is opened and as a result, product is removed from mold. Injection machine is consist of a hopper, a rotating extruder screw and an injection chamber. Plastic material in granular form is loaded into the hopper. From there, plastic granular drops onto the feeding portion of the rotating extruder screw. The screw moves the material through a heated extruder barrel. In this place, plastic is heated and than advanced to the exit of injection machine. Mold that has configuration of the product consist of two parts. They are cavity and core. Mold is closed by the clamping plates in mold base. Than molten plastic material is forced by the injection machine in to the mold. Injection mold has two functions. It forms the air space into which the hot plasticized material is injected under pressure, and it acts as a heat exchanger removing heat from the material in the mold until it is rigid enough to be ejected so that the final part will conform to all its specifications. The duality of the part and its cost of manufacturing are largely depend on mold design, construction and excellence of workmanship. Over the years, as the size of the molding machine increased, so did the cost of molds. Two critical areas in ensuring the right mold are the design of the piece and the design of the mold. An acceptable product is not possible with an incorrect part design. An acceptable may be possible with an incorrect mold design, but always under extremely difficult and uneconomical conditions. The mold making industry is composed largely of shops that have developed special services. Custom mold makers are specialist since they provide services most frequently required by their costumes in a particular area or market. Metal-working machines are available in great variety and new processes and equipment is developed every year. Few shops can afford the investment in each type of equipment that is most efficient in building a specific mold. XI The success that an injection molding operation depends on the care and ingenuity reflected in the design of its molds. First, the prime function of injection molding equipment is to operate the mold efficiently and expeditiously though no degree of retirements in the equipment can compensate for poorly designed molds. In designing an injection mold, the designer must have knowledge of the following physical of the molding process.. Provisions for mounting the mold; Size of the mold which may be accommodated by an injection molding machine is determined by the size of the movable and stationary platen and by the opening between the platen less the stroke required for ejection of molded parts.. Available clamping forces and injection pressure; Clamping force and injection pressure is depend on each details of the mold, especially total projected area of the mold cavities and runners.. Provisions for ejection; The mold design makes provisions for an injection plate that moves pins or other components for forcing parts out of the mold cavities.. Connections from mold cavities to the plasticizing chamber; The hot plastic material is fed to the molds from a nozzle, through a sprue bushing, runners and gate. The sprue bushing and runners are included in the design of the mold.. Parting lines of the molds; Before the injection mold is laid out on the drafting board, the designer must study the part to be produced and decide where the mold parting lines are to be located when this decision is given, the actual mold design will proceed rapidly. xu The other most important subject in design of injection molds is the heating and cooling of molds. Injection molds are generally heated by using hot hydraulic oil, and they are sometimes heated by electric cartridge or steam. Mold makers must adjust in the best way the location of heating channels by considering part construction, mold type, type of plastic. Unless mold is not heated correctly and sufficiently, it is very difficult to obtain an acceptable product from mold. Therefore, heating and cooling system are important subjects in mold designs. While the product is design, some criteria of the molding that cause much failure after molding, must be established very certainly. Thickness of the product is generally very changeable size, but the shrinkage is the worst problem that is effect from all molding parameters. From this view point, cross-section of the product is same size and does not have big difference as far as possible. Another subject of the product design is taper addition. After mold is full with molten plastic, result of the cooling and shrinkage, products in mold become stick on the mold wall. In this stage, removing product from mold is very important. In the early years of injection molding, moldmakers had to built their own mold bases, a costly and inefficient process, as most mold makers do not have the large equipment for such machining. The manufacture of standard mold bases, an important factor in the development of more efficient mold making, was started in 1942. The aim of this study is to get a good resource about injection molds in view of designs and manufacturing.