Kapileri emme mekanizması ile enjeksiyonda kalıplanmış parçadan bağlayıcının alınması
Binder removal from injection molded part by capillary wicking mechanism
- Tez No: 39495
- Danışmanlar: PROF. DR. ADNAN TEKİN
- Tez Türü: Yüksek Lisans
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 69
Özet
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Özet (Çeviri)
Powder injection molding (PIM) is one of the forming techniques capable of mass producing small ceramic components with complicated shapes in an economical manner since the early 1930s. In recent years, injection molding technique has been attracting considerable attention in manufacturing industries due to its advantages such as near-net shape material production, elimination of grinding and finishing steps, its speed, reproducibility and efficient material utilization during the process. The main principle of PIM is to inject a powder-organic binder plastic mass into a die under pressure, and then debind and sinter the molded part. Accordingly, the main steps can be classified as follows; 1) powder preparation and binder formulation; 2) powder and binder mixing; 3) injection of the plastic powder-binder mixture into the mold; 4) removal of the binder from the molded part; 5) sintering at high temperature; and 6) finishing operations. The final component will ideally replicate the shape and surface texture of the mold cavity, but will be isotopically reduced in size due to debinding and sintering shrinkage.PIM is commonly divided into two distinct categories based on the injection pressure: high pressure injection molding (HPIM) ( pressure > 0.5 MPa) and low pressure injection molding (LPIM) ( pressure < 0.5 MPa ). High pressure injection molding which is a technique basically borrowed from the plastic industry, is the more common of the two techniques, but recently low pressure injection molding has been receiving attention as an alternative technique. The primary advantages of low pressure injection molding relative to high pressure PIM are : 1) It is a simple and low cost process with smaller dimensions, 2) with LPIM there is less wear on the machine parts in contact with the powder batch resulting in less contamination of ceramic mix by metallic tooling , 3) it is easier to control flow behavior of the batch in LPIM, and 4) the use of LPIM eliminates the separate mixing , pelletizdng and granulation steps necessary in HPIM.Binder formulation that is employed in the injection molding process is a very critical part of the processing steps. Binders act as a vehicle to carry the powder into the mold in a uniform fashion, and ideally do not affect the composition of the finished product. Binders must have the right flow behavior during molding, must wet but not chemically degrade the powders, must lead to easy removal from the mold, and must also have physical or chemical properties which enable them to be easily removed during the debinding process. Paraffin is typically the major binder addition for the A1203 based ceramic materials in the low pressure injection molding process. However, there are problems due to poor wetting and compatibility. A1203 powder is wetted by paraffin with a contact angle of 15 degrees at 160°C. Therefore, empirical binder formulations have been developed to improve the molding conditions at temperatures lower than 160°C. Oleic acid is known to improve the molding conditions by enhancing the wetting behavior of A1203 powder when paraffin is used as the major binder. Two component binders usually result in a very narrow working range as far as molding conditions and debinding steps are concerned. In order to enlarge this narrow working range, multicomponent binder formulation systems are preferredAfter molding, the next stage is the removal of the binders from the green powder compacts. This is a critical step, and improper debinding can lead to distortion and the formation of defects inside the samples. There are two main debinding techniques, solvent and thermal processes. Solvent extraction involves immersing the compact in a fluid that dissolves some of the binder system, thereby leaving an open pore structure for subsequent further debinding by evaporation One form of solvent debinding technique is condensed solvent extraction in which liquid and vapor extraction debinding techniques are used together. A solvent with a wide evaporation temperature range within which lies the melting temperatures of the soluble binder components, a \ porous substrate or wicking powder that can adsorb the outflowing binder-solvent solution, and an optimum temperature in extraction debinding are the required variables. Another one is supercritical solvent extraction in which solvent is pressurized and/or heated until it exceeds the critical condition where the liquid and vapor are indistinguishable. Another variant of solvent extraction is fast catalytic debinding in which the polyacetal-based binder is depolymerised in the presence of an acidic gas and volatilizes as formaldehyde starting from compact surface to the interior.The alternative to solvent debinding is to remove binder by thermal mechanisms where weight loss can be identified when a ceramic-polymer suspension is heated In the first place, thermal degradation of the polymer produces low molecular weight products of chain scission which, for an irreversible reaction, are generated uniformly throughout the organic phase. Some of these evaporate from the surface and the resulting concentration profile stimulates the outward diffusion of products. Secondly, in the presence of a reactive gas (e.g. oxygen) oxidative degradation occurs near the surface of the body and the reaction rate is limited by the diffusion of oxygen into the polymer melt. This result in shrinking unreacted organic core. Thirdly, if the organic vehicle incorporates a very low molecular weight thermoplastic suchas wax, evaporation from the surface of the body may occur without degradation process controlling the rate. In these three techniques binder will be removed in a gaseous form either by diffusion or permission. Lastly, liquid extraction using a contacting wicking substrate. This method is carried out at a temperature which is high enough so that the binder has sufficiently low viscosity to flow out of the compact into the pores of a wick substrate. Wicking is the general binder removal technique when the parts are produced via low pressure injection molding method by utilizing paraffin wax based binder formulation. Wicking binder from compact through the porous wick substrate is effected from those parameters; a) temperature which determines fluidity of the binder, b) powders particle size (in compact and in powder bed) which determines capillary attraction force magnitude, c) time, d) compact minimum thickness, e) fraction of the powder in compact, f) environmental conditions, and g) physical and chemical properties of the powders and binder system.In this work, batch preparation, mixing and, molding operations were carried out using a Peltsmarm MIGL-33 Model low pressure injection molding unit. This machine was especially designed for ceramic injection molding studies. In contrast to high pressure injection molding machine, it does not require granulation or a feeder. It consists of a tank assembled with a feeder pipe with independent temperature control. The batch is prepared within the tank reservoir under vacuum using a planetary double arm blade mixer that continuously mixes the batch. The batch, pipe and orifice temperatures can be monitored and controlled automatically by a set point adjustment on the instrumentation panel. The die is installed to the opening of the feeder pipe with an air cylinder. Molding is performed by applying air pressure to the surface of the ceramic mix in the tank under constant pressure.In this study, melt wicking of the binder from part through powder bed by capillary action was investigated in detail. Those parameters, time, temperature, particle size of samples used in part, particle size of powder bed and part thickness were studied in order to understand effect of those variables for debinding time and debinding mechanism. Binder removal is typically carried out in two successive stages. The first stage is the removal of the lower melting binder components at temperatures just below their boiling temperature. This can be a lengthy process, but is significantly speeded up if it is placed in a bed of loosely packed powder with smaller average size. The finer size powders act as a wicking agent, drawing the binders out of the green component by action of capillary forces. Ideally, after completion of this stage the porosity should be interconnected and the compact is relatively permeable. For this purpose, samples were embedded in fine grade alumina powders in this study. The second step is the thermal degradation of the remaining binder. Here, the residual binder components are evaporated and depolymerized by action of the heat and oxidizing atmosphere. The gases generated during depolymerization and oxidation process of carboneous residue readily diffuse through the open, porous structure. In developing an appropriate debinding cycle, the minimum compact thickness, the packing traction, binder composition, and pore size are main variables which need to be considered. Because thermal degradation and evaporation produce gases, conventional thermal methods require heating rates of approximately 5-10°C/hour so that bubbles and defects do not form in the compact. However, in the wicking method, the most of the binders are removed in the liquid state, so by using this method fewer defects are introduced and remaining binders may be removed in a shorter time.Starting powders were Alcoa A-16 and Bayer HVA A1203 to prepare the batches and also to use as a wick powder bed. Particle size distribution of the each powder used is measured by the X-ray sedimentation technique(Micromeritics 5000 D Sedigraph) and specific surface area of each powder were measured by BET method(Micromeritics). Alcoa A-16 SG A1203 powders have a very fine average particle size of 0.4um and with a specific surface area of 9 m2/g. Bayer HVA A1203 powders have an average particle size of 4um and with a specific surface area of 0.85 m2/g. In order to investigate three different powders instead of two powders, another powder with an average particle size of lum was produced by attrition milling of the Bayer HVA powders for half hour. And powder specific surface area was increased to 4 m2/g.A commercial grade of low density polyethylene was used to modify binder properties in this study. The thermal degradation behavior of the binder components and batches were studied by TGA and DSC methods. These investigations showed that while paraffin has a thermal decomposition temperature around 200°C, polyethylene shows the same behavior around 400°C. The presence of small amounts of polyethylene expands the degradation range of paraffin and oleic acid binders to higher temperatures.Powders were compounded with binder at 110°C using planetary double blade mixer under vacuum until it became fluid and homogeneous. The solid loading 55% by volume, and volume fractions of the binders were 40% paraffin, 3% oleic acid, and 2% polyethylene. The weight percentages of the different size powders in the batch were 86% A1203.The prepared batch was molded using the following machine settings. The die temperature was 50°C, and the tank, pipe and orifice temperatures were 140, 130 and 120°C, respectively. The molding pressure was 5 atm, and the injection hold time was 5 seconds. These machine settings were kept the same during the molding trials.The thermal degradation of the binder commences at approximately 225 °C and is completed at 450°C. Therefore, the critical temperature range of debinding by thermal decomposition process for the batch is between 250 and 350 °C range where majority of the binder can be removed. On the other hand, when the samples were immersed in powder bed, wicking process for the binder removal becomes an important mechanism for easy debinding.Different debinding programs were employed in experiments during which four different set-point temperatures, 80°C, 120°C, 160°C and, 200°C were applied to parts that were immersed in three different powder. For each program, the heating was continued until the preset temperatures were reached and then temperatures were kept constant during wicking. When the furnace reached the preset temperature , first samples were taken in order to weigh for weight losses then the rest of the parts were weighted in 20 minutes intervals.After sintering at 1550°C for one hour, the relative density was found to be 93% of the theoretical density (3.94 gr/cc) and the linear shrinkage was 18%. There was not any significant density change in samples that were exposed to different binder removal processes. 5°C/min heating rate during sintering was sufficient to allow enough time to oxidize and remove the remaining binder. For example after 200°C of the debinding process only 5.8 vol % of the bulk body is covered by the organic binder, which can easily be removed during slow heating rate of sintering process.In this study, aim was to investigate wicking mechanism in detail in order to decrease debinding time and reduce energy usage during debinding for the parts produced via low pressure injection molding method. For this reason, powder size( 0.4, 1, 4um), debinding temperature(80, 120, 160, 200°C) and part thickness(2, 4, 8, 12mm) were used as main parameters of wicking of binder from compact through porous powder bed. During study, binder system, volume fraction of the powders in the compact and environmental conditions were kept constant.Three batches were prepared from A1203 powders with a mean particle size of 0.4, 1 and 4um. These batches were injected into bar shape via low pressure injection molding method as described before. Parts from each batch were debinded with wicking mechanism in three different powder bed in order to find out effect of particle size to debinding. In other word, each part from three different batch was embedded into three different powder bed and they were heated to preset temperature and temperature was kept constant during this debinding operation. Those studies were repeated for 80, 120, 160 and 200°C temperatures in order to find out that what is effect of temperature to the amount of the binder removal.In all of the experiments 9 parts were debinded same way to measure weight loss at 9 different time interval(0, 15, 30, 45, 60, 75, 135, 195, 255 minutes) and rest of the parameters were kept constant. Effect of minimum thickness of the parts, that were injected into the cylinder shape from batches of 0.4 and 4um powders , to the amount of binder removal by wicking mechanism was examined by embedding parts into the 0.4um powder bed at 180°C temperature. As a final study, critical amount of binder that can be safely left in the compact for normal sintering program not to have any defect after sintering was investigated for three batches. For this study, parts that consisting different amount of residual binder were sintered in air furnace and after sintering visual inspection and also density measurements were made. From these examinations defect free parts and their binder contents were found.During wicking studies, depending on differences between size of the powder particle used in compact and in wick powder bed and also depending on the temperature, two mechanisms were observed At higher temperature and bigger size differences, capillary suction was the dominant mechanism. When the size of the powder particle used in porous wick powder bed is equal and/or greater than particles used in compact, then viscous flow was observed as a dominant binder removal mechanism. When the particle size is larger it is easy for viscous flow to occur because of reduced side wall effect. On the other hand, capillary wicking process was the main mechanism at 160 and 200°C temperatures when parts that were produced from 4 and lum powder, were embedded into 0.4um powder bed during debinding. 90% binder was removed in 30 minutes while capillary mechanism is in progress. Unlike capillary suction, viscous flow continuously removed binder as time pass.As a conclusion, binder system that developed for this low pressure injection molding studies showed a bingham type of fluid behavior. At high temperature, viscosity of the batches increased when the number of the particle was increased in the unit batch volume. In other word, when the particle size is reduced while amount of the solid fraction in the batch is the same, then viscosity will increase. When the parts, that were produced via low pressure injection molding, were debinded, binder will be removed from compact through wick powder bed either with capillary wicking or with viscous flow mechanism. Capillary wicking is a main mechanism when wick powder particles are much smaller than compact particles. If wick powder particles are equal or bigger than compact particles then viscous flow will be main mechanism. Capillary wicking is much faster mechanism compare to viscous flow, since 90% binder can be removed from compact in 30 minutes via capillary wicking if the right environmental conditions are chosen. Temperature increase will decrease viscosity of the binder and this will actually lead to decrease in debinding time and increase in amount of the removed binder. When the parts, which have various amount of the binder, were sintered in a air furnace with a normal sintering regime, some of the part contained defects. If 35-40% of the binder is removed before sintering then they didn't have any defect but if it is less then they had defects. Neither capillary wicking nor viscous flow show any dependence on to the part thickness. While the amount of the removed binder is increased, produced pore size and amount increased in the compact.
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