Reçine oranı, reçine özellikleri ve kabuk kalınlığının kabuk kalıp özelliklerine etkisi

Abstract

REÇİNE ORANI- REÇİNE ÖZELLİKLERİ VE KABUK KALINLI?ININ KABUK KALIP ÖZELLİKLERİNE ETKİSİ ÖZET Bu çalışmada kabuk kalıplama (shell molding) yönteminde; kullanılan reçine kaplı kumun reçine oranının ve kabuk kalıp kalınlığının, ayrı ayrı ve birlikte değişiminin, kabuk kalıp özelliklerine ve kabuk kalıp maliyetine olan etkileri incelenmiştir. İki değişik katı reçine sıcak kaplama yöntemiyle Şile yöre si silis kumuna kaplanmıştır. Kaplama işlemi optimum kaplama sıcaklığı, karıştırma süresi ve heksamin, kalsiyum stearat oranları bulunarak yapılmış, önce % 1,5-7,0 oranları arasında çeşitli oran larda reçine kaplı kumla sabit kalınlıkta numuneler, daha sonra % 4,5 oranında reçine kaplı kumla 7,0-16,5 mm arasında değişen ka lınlıklarda numunelerle reçine oranı ve kabuk kalınlığı değişiminin etkileri birbirinden bağımsız olarak araştırılmıştır. İki ayrı reçinenin değişik fiziksel ve kimyasal özellikleri ile bu reçinelerle yapılan kabuk kalıp özellikleri arasında iliş ki kurularak, reçine özelliklerindeki değişimin kabuk özelliklerini nasıl etkilediği incelenmiştir. Kabuk kalıbın yük taşıma kapasitesi (eğme momenti) sabit kalacak şekilde reçine oranı ve kabuk kalınlığı aynı anda ve ters yönde değiştirilerek, bu değişimin kabuk kalıp özelliklerine ve kabuk kalıp maliyetine etkileri incelenmiştir. Kabuk kalıbın ısıl şok direnci, sıcak deformasyon ve gaz geçirgenliği özellikleri ile kabuk maliyeti arasındaki ilişkiler incelenerek optimum kabuk özelliği ve maliyet bölgeleri araştırılmıştır. Sabit eğme momentinde reçine oranı artıp kabuk kalınlığı azaldıkça, kabuk kalıbın gaz geçirgenliği, döküm sıcaklığında yük le birlikte deformasyonu ve ikincil genleşmesi artmış, toplam da yanma süresi ise azalmıştır. Döküm sıcaklığında ısıl şok direnci ve yüke karşı deformasyonu önce azalmış, % 3,0-3,4 reçine oranı civarında en düşük değerlerine ulaştıktan sonra yeniden artmıştır. Kabuk kalıp malzeme maliyeti de reçine oranı artıp, kabuk kalınlığı azaldıkça önce azalmış, % 3,0 reçine oranında en düşük değerine ulaştıktan sonra yeniden artmıştır. En düşük kabuk kalıp maliyeti noktası, değişik eğme momentine sahip kabuk kalıplar için değişik kalınlıklarda, ancak aynı reçine oranında (% 3,0) gerçekleşmiştir. İşçilik maliyetinin çok yüksek olduğu işletmeler için bu nokta % 3,4 reçine oranına kaymıştır.

EFFECTS OF RESIN CONTENT, RESIN PROPERTIES AND SHELL THICKNESS ON THE PROPERTIES OF SHELL MOLDS SUMMARY Shell moulding is a mass production, precisioncasting met hod which casting parts can be produce with better surface quality, low dimensional tolerances, high reproducibility, low scrap, low fettling and machining work, comparing with the conventional sand casting. Unfortunately the resin binder, used in shell molds, is an expensive material and this increases the cost of the shell molds. On the other hand savings due to the low scrap and low fettling and machining, decrease the total cost of the casting. The increase in quality of casting, because of high surface quality, low dimensional tolerances and high reproducibility together with the savings in post operations pay off the relatively high cost of shell molding. So this process is feasible only, if it can be applied in a correct way that you can obtain all the benefits, the method offers to you. In the present work it was aimed to find optimum combina tions of resin content and mold thickness to reach up the optimum mold and casting properties and optimum mold cost. It was also aimed to make a good correlation between the resin properties and the mold properties. Two different resins were used as mold binder for this reason. The resins were actually supposed to be the same type, one was produced locally, the other was imported. The difference was first observed by a foundry which was trying to subsitute the local resin for the imported one. The main difficulty was the high peel-back rate which avoids the usage of the shell sand, coated with the local resin.The silica sand used as refractory material and hexamine used as catalyst were produced locally. Physical and chemical properties of two resins were deter mined first and differences in these properties is recorded to be used to make correlations between the resin properties and shell mold properties. Optimum coating parameters were determined later to obtain optimum sand properties and to avoid any deviation in the sand properties due to the improper coating conditions. It was found that locally produced resin had lower visco sity and stick point, but higher flow distance and B-time than the imported one. Free phenol and formaldehyde ratios, Carbon, Hydrogen and Nitrogen contents were higher in locally produced resin while PH value and water content were lower than the imported one. As a result it has been concluded that imported resin had higher condensation value (either due to higher condensation time or higher condensation temperature or both). In spite of these differences in physical and chemical properties there was no big differences between the optimum coating parameters of the resins. 5-10 C lower coating temperature and 0.1-0.2 % lower cooling water for the locally produced resin were the main differences. Resins were solid novolac resins and hot coating were applied. Effects of resin content and mold thickness were investi gated seperately first. Either resin content or mold thickness were kept constant while the other was changing. With this the effect of these parameters has been determined when only one of them were changing. The main idea was changing these two main parameter simul taneously. According to the formula GR K2 M = `. ` 6 IXShell mold bending moment (M) changes with the bending strength (G`) and the square of mold thickness. It has been proved that bending strength changes with the resin content. So with many different combinations of resin contents and mold thicknesses the same bending moment can be obtained. That means by using different resin contents and mold thicknesses, shell molds which have the same load carrying capacity can be produced. Which combinations is beneficial from the mold properties, casting properties and mold cost point of views have been sought. The effect of increasing resin with the constant mold thick ness was as follows. Room temperature strengths, mold making tempe rature strengths, room temperature gas permeability the amount of gas evolved at the casting temperature, thermal shock resistance, total breakdown time, deformation with the load and secondary exponsion were increased. Deformation against load (thermal expan sion) was decreased first, after reaching a minimum showed a small increase again with the increasing resin content. Investment rate and grain size of the coated sand were increased, stick point was decreased, peel back rate was increased first, after reaching a maximum was decreased. The main difference between the performance of the two resin coated sand was the peel-back rate. Although they have showed the similar tendency with the increasing resin content, domestic resin has showed quite a higher peel-back rate than the imported one, up to the 4.5 % resin content. Between 2% and 3% resin content, domestic resin coated sand showed 10% to 20% peel back rate. With this peel back rate shell mold making is impossible. Peel back rate has dropped from 10% (at 3% resin content) to 2% (at 4.5% resin content). 2% is a reasonable peel back rate and does not create a serious problem during mold making. Peel back rate of imported resin coated sand has never exceeded 5.5%. Beyond 4.5% of resin content peel-back of the two resins has been eliminated. Imported resin has been superior in mold making temperature strengths while the domestic one has been superior in room tempe rature strengths. No big difference has been observed on the other properties. An increase in thermal shock resistance and total breakdown time of the mold, and decreases in deformation against load, defor mation with the load and secondary expansion at casting temperature were observed, when the shell thickness was increased while the resin content of the sand remained constant. The room temperature gas permeability was decreased as well.Three main properties have been chosen and observed while the resin content and mold thickness have been changed simulta neously in such a way that the bending moment has remained unchanged. These three properties are thermal shock resistance, hot distorsion behaviour, and gas permeability. Thermal shock resistance has been chosen because of its importance in shell mold practice. If thermal shock resistance is not adequate for the purpose, the mold cannot be used due to the premature cracking. It has been observed thermal shock resistance has showed a small increase than a decrease and an increase while the resin content was decreasing and mold thickness was increasing. The minimum thermal shock resistance has been observed at 3.0% resin content. Hot distortion behaviour determines the dimensional proper ties of the casting produced with shell mold. The deviation from the original state of the mold either upwards by expansion or downwards by deformation is important. The greater the deviation from the original state at the point of skin forming freezing of metal, the greater the dimensional deviation of casting. Deforma tion agains load (thermal expansion) was decreased first, but it has increased again as the resin content was decreased and mold thickness was increased. Deformation with the load (downward distortion) and the secondary expansion were increased, total breakdown time were decreased in the same range. Gas permeability was observed to determine the degree of permeability drop, because the lack of adequate permeability may cause improper filled molds especially in thin sections. Gas permeability was dropped while resin content was decreasing and mold thickness was increasing at the same time. The amount of materials used in shell molds, and their ratios to each other were changed as the resin content was increased and shell thickness was decreased while the bending moment remained constant. Since the cost of the materials are different, that results, changes in shell mold material cost. Material cost was decreased first and after reaching a minimum value at 3,0% resin content increased again. Material cost was increased, as the bending moment was increased. The miminum material cost was obtained at different shell thicknesses for different bending moment values, but resin content remained constant at 3,0% for minimum material cost. xiThe contribution of the labor cost to the shell mold cost was also investigated. It was found that labor cost for per piece was decreased, as resin content was increased and shell thickness was decreased, due to an increase in the number of the molds produced in a unit time. But it was also found that the material cost was increased at the same time and the influence of the material cost was determining factor for total shell mold cost. The miminum cost point was remained same, until the ratio of labor cost to material cost was reached to 0,8. After this point, the minimum cost point was changed from 3,0% to 3,4% resin content. XII