Basınçlı kalıplama yönteminin incelenmesi

Abstract

Compression molding is a process used (mainly) for the molding of thermoset materials. It requires a matched set of male and female dies. Molding material in the form of powder or precompressed, mostly cylindrical pellet and in a partially polymerized state is placed between the two halves of the hot mold which are then closed together by hydraulic means. Under the action of both heat and pressure, the molding material becomes plasticized, fills the mold to the shape of the cavity, cures, and hardens. Thus the product can be taken out of the mold and utilized after flash trimming or polishing. The described technology itself determines the most important parts of the compression molds. A mold cavity formed in some kind of material (metal), that suits the shape of the required piece, is needed. Furthermore, there is need for a heating system to ensure the quantity of heat necessary for the chemical reaction. Other tool parts might be needed to compress the molded piece and force into every part of the mold cavity. Naturally, these tool parts, by which the hardened product can be removed from the mold should be provided. Furthermore the tool parts by which the mold is fastened to the pressing machine should also be provided. The mold cavity is formed in the mold body or mold cup. This-apart from the shrinkage taking place at cooling-corresponds to the geometrical shape of the required product. Since the plastic powder or precompressed pellet fed into the mold is still much looser than the finished product will be, the mold cavity is to be expanded with the loading chamber. The punch (upper part of the mold) enters this loading chamber, closing the mold cavity, and transmitting the pressure necessary for the molding from the machine to the plastic material. In order to ensure uniform heating, both mold parts have to be heated with a lower and upper heating system. If the lower part of the product is hollow, a core is placed into the cup. The piece is removed from the cup by the ejector, or by a stripper if it remained on the punch.The simultaneous operation of several ejector or stripper pins in ensured by the ejector plate or strpper plate. The accurate fitting of the mold punch and loading chamber is accomplished by the application of the guide pins and bushing. The space necessary for the movement of the ejector or stripper plate is ensured by the lower and upper bearing plates or clinders. If there are undercuts on the product, then the mold cavity is not made directly into the mold body, but into a split insert of a truncated cone or truncated pyramid shape. Compression molds may be classified into three general types. These are positive-type mold, semipositive-type mold and flash-type mold. If the flash is in the direction of the pressure, cross section of the loading chamber concurs with the outline of the product. Such molds are called positive-type molds. Thickness of the flash depends on the fitting accuracg of the punch and loading chamber. It is necessary to make sure that the punch or punch holding plate is seated directly on the lower standard frame or on a thrust strip. In case of the positive molds, pressure of the machine is taken up directly by the products; thus, the compressive force of the machine is fully utilizable. This type of mold fully confines the molding material and full mold pressure is exerted at all times. There is insuficient clearance between the punch and loading chamber for the molding material to escape, and there is no device limiting the closed height of the mold. The molding material must therefore be weighed accurately. This mold type permits the production of products with a uniform thickness in multicavity construction, because even if one of the cavities were overfed, it receives higher pressure than the others; thus, the excessive material is squeezed out. This type of mold is desirable when the part must be dense, and is appropriate for the molding of high-impact materials. However, positive mold fitting is not always applicable. For instance, a "razor edge" would develop on the edge of the punch, which is naturally inadmissible. For such products, a semipositive-type mold is designed, with the flash at a right angle to the direction of pressure. They differ from the positive-type molds in that the male punch part only "telescopes" into the female part of the mold to exert positive pressure on the molding material at final closing. Considerably higher compression is required for the semipositive-type molds because very high pressure is necessary to squeeze out of material from the thinning (and thus faster hardening) flange part between the two mold parts when the mold is closed. This type of mold is suited for quality production molding and widely used. Immidiately before closing the mold, the gap is already so narrow that the excess material cannot flaw out of the mold; consequently, a thick flash or an oversized piece is obtained even in case of high specific pressure. This fault canbe corrected by using discharge channels. It may occure in practise that both the possitive and semipassitive molds can be used, and it is up to the designer to select the most suitable on in the given case. In this case besides the already mentioned molding pressure difference, it is necessary to consider the problems of ejection of the product, and removel of the flash. In certain cases, it is practicable to use an inclined semipasitive mold. Here the excess material runs out of the mold more easily, than at the internal vni semipositive mold, and the inclined flash is easy to remove. The disadvantage is that fitting of the mold cup and punch requires more careful work. The flange of these molds has an inclination of about 30-40°. Although the flash-type mold is not commenly used for production purposes, it has the advantage of low cost. In operation, the mold is loaded with an excess weight of molding material which, when pressure is applied, will squeeze out of the cavity over the land area. Due to this flow, which cannot successfully be limited, it is not possible to obtain a molding of high density. Only flat and shallow parts should be made in a flash-type mold. It is not suitable for deep draw parts because the molding pressure exerted is not sufficient to make the plastic material flow any great distance. The correct selection of the material for the compression and injection molds is not an easy task. According to experience, the designers do not pay sufficient attention in selecting the most suitable material. True, the matter requires a manysided consideration; however, the time spent on it will amply compensate in the course of construction and use of the mold. There are recommendations in the standards and technical books, and a certain practice has develeoped which is successful in many cases, but it sometimes fails (specifically in the more delicate cases). As a result of erroneous material selection, various extra-works will be necessary, e.g., distorted mold parts to ground to size or hardening craks, or chippings occur as a result of coarse grain or glass-hard surface. Most of the mold constructors have their own pattern, prescribing the accustomed material for the mold parts automatically. Use of the pattern, regrettably, is also supported in most cases by the limited stock. Certain steel types in certain size are stocked, the acquisition of others, less frequently used is difficult. A few aspects will be summarized in the following for seletion of the optimal steel quality. First, a survey is made concernining the characteristics required for the steel material of the molds. It is noted that the order of the following enumeration is not identical with the order of their importance, because their importance varies from case to case. The molds must be wear-resistant. The serial magnitude of the plastic products (in the range of hundred thousands, or millions) has to be endured by the molds without significant wear. The wear-resistance is concomitant with the surface hardness. Hence, especially the mold cavity and mold parts sliding on each other must have hard surfaces, and according to the practical experience, they should be of different hardness. Steel has to be very tough and resistant against stresses and fatigue, because as it is well known, a 1000-2000-kp/cm2 compression is necessary at the molding of certain duroplast types. It is essential that the steel should not tend towards distortion, because subsequent grinding of the hardened mold parts to the correct size - if required by the shape and size of the product - is very expensive and time-consuming work. IX The mold must be free-cutting. If the material is too soft, it spreads, is difficult to cut, and the surface is difficult to polish. On the other hand, if it is too hard, only a low cutting speed and feeding can be applied. Measurements and calculations have proven that under identical conditions (i.e., in case of an identical life span of the knife, feed, depth of cut, knife material, etc.N approximately half of the cutting speed is applicable for the cutting of steel of 100-110 kp/mm2 tensile strength, as for the machining of steel with 60-70 kp/mm2 strength. The time difference of machining is even more significant with manual fitting, filling, and polishing. The hard material considerably increases the time of producing the mold, and thereby its production cost as well. The mold must be well-polishable, especially for the injection molding of transparent, thermoplastic materials. Certain highly alloyed steel types have a softer dendritic structure at the grains, considerably reducing the polishability. Only careffully produced, completely homogeneous "inclusion-free" steel can be used for plastic wolds. Careful material acceptance, possibly ultrasonic test, may save a great deal of annoyance and working time. It is well known that the high cost of the mold prevents the spread of plastics. In the interest of reducing this cost, the necessity of applying the most modern technology must be emphasized. Spark erosion macliining and cold hobbing are modern production methods of the compression molds. When hardening the molds for complicated, high precision products of uneven mass distribution, even in case of the most careful and most competent work, the risk of distortion or cracking exists. In such cases, it is advisable to make the mold with spark erosion machining. It is based on the recognition, that the are generating at the opening of the electric circuits, seizes the hot metal particles. This phenomenon is very inconvenient for electric apparatuses, but is utilized by the spark erosion machining, at which a permanent arc is maintaned between the socalled electrode and product. As a result of the electroerosion process, a cavity conforming to the shape of the electrode is brought about in the product, which may be of hardened steel or even a sintered carbid metal. The attainable mean surface roughness Ra = 0,5-1 pm. The cavities of compression molds can be produced by the up-to-date method of cold hobbing. Three varieties of the cold hobbing are used. Sink-Hobbing It is used to produce relatively large, but not too deep mold cavities. A hardened hob corresponding to the shape of the product to be made is pressed with very high pressure in cold condition into a softer steel, into the so-called matrix. Reducing Hobbing It is used for the production of cavities for longer, thinner, shaped (a.g. octagonal) products, so that the hardened hob corresponding to the cross section of the products is placed into the cylindrical hole of the soft die. Then the disk is pressed through a reducing hole. This way the soft steel takes up the shape of the hardened hob. Two - Directional Hobbing It is essentially the combination of the two previous process, i.e., such as sinking, in the course of which the wall of the cavity made at pressing is ironed with a deflector insert onto the hob. Compression molds are heated with electricity, steam, or hot liquid (e.g., hot water). Electric heating is the most frequently used method. Ensuring the necessary output is possible, in most cases, without any difficulty. The main's loss is minimal. However, its drawback is that the difference between the temperature of the heating wire (600-800 °C) and the necessary temperature of the mold cavity (160-180 °C) is too much. Hence, the uniform temperature on the surface of the product is difficult to install the heat sensors and controls in such a way, that after switching off the heater, the heat quantity accumulated in the mold body should not cause a further temperature ise "after heating". Steam heating with steam of suitable pressure and temperature. Its drawback is low efficiency, significant network losses, cost of investment and operation of the high pressure boiler and network, difficulties and expenses of fuel delivery, storage and stocking. With regard to all these conditions, the prime cost related to the calory unit generally higher, than the cost of electricity. The most up-to-date heating method is the combination of the two: i.e., equipment, that produces liquid of suitable temperature at the location (in most cases not steam) with electric heating in the vicinity of the molding machines. There are a host of decisions the mold designer will make concerning how dhe designed part is to be molded. The product designer should be sensitive to these considerations, and anticipate them in his conversations with whoever is to build the mold. In general, uniform wall thickness of material is desirable. Irregularity in thickness will probably cause irregularity in setting and contraction, thus creating internal part stresses. Such stresses will always try to relieve themselves and may cause concave depressions known as "sink mark" on the thick sections, or may cause warping. If the part cannot be redesigned, a low- shrinkage material should be used with gradual wall section changes. In general, an undercut which has the effect of making difficult or impossible to eject the part from a simple, rigid, two part mold and a sharp corner that causes stress consentration are to be avoided.