双螺杆挤出机设计【φ72MM双螺杆塑料挤出机】
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编号无锡太湖学院毕业设计(论文)相关资料题目: 反向旋转型双螺杆挤压机 及挤压部件设计 机电 系 机械工程及自动化专业学 号: 0923168学生姓名: 沈杰 指导教师: 戴宁 (职称:副教授 ) (职称: )2013年5月25目 录一、毕业设计(论文)开题报告二、毕业设计(论文)外文资料翻译及原文三、学生“毕业论文(论文)计划、进度、检查及落实表”四、实习鉴定表无锡太湖学院毕业设计(论文)开题报告题目: 反向旋转型双螺杆挤压机 及挤压部件设计 机电 系 机械工程及自动化 专业学 号: 0923168 学生姓名: 沈杰 指导教师: 戴宁 (职称:副教授 ) (职称: )2012年11月25日 课题来源用于食品生产的工程实践性自拟课题。科学依据(包括课题的科学意义;国内外研究概况、水平和发展趋势;应用前景等)(1)课题科学意义挤压机是挤压加工技术的关键。 挤压加工技术作为一种经济实用的新型加工方法在食品生产中得到迅速发展。 挤压加工主要由一台挤压机一步完成原料的混炼、熟化、粉碎、杀菌、预干燥、成型等工艺, 制成膨化、组织化产品或制成不膨化的产品。 只要简单地更换挤压模具, 便可以很方便地改变产品的造型。反向型螺杆挤压机用于食品生产具有工艺简单、一机多能、生产连续化、效率高、能耗低、投资少、收效快的特点。生产出来的食品口感细腻、易消化吸收、营养成份损失少、贮藏时间长、不易产生“回生”现象、食用方便. 目前,挤压技术已经发展成为最常用的膨化食品生产技术之一。反向型双螺杆挤压机的研究与开发也势在必行。(2)反向双螺杆挤压机的研究状况及其发展前景上世纪60 年代, 开始出现了双螺杆挤压机, 并用于食品加工领域. 我国从70 年代开始研究食品挤压技术和挤压加工机械. 1980 年3 月, 北京食品研究所仿制出第一台自热式PJ ) 1 型谷物膨化挤压机. 1982 年无锡轻工业大学从法国Clext ral 公司引进一台BC ) 45 型双螺杆挤压机, 开始了对挤压加工技术的研究。与此同时, 国内许多生产厂家也先后从世界各大公司引进了先进的挤压设备。国际上有代表性的挤压机生产企业除法国Clext ral 公司外,还有美国Wenger 公司,德国的WP 公司, 意大利MAP 公司,日本的恩奴比食品有限公司,瑞士的Buchcler 公司等。在引进国外设备的同时,国内的许多厂家也先后生产了不同类型的挤压设备。反向型双螺杆挤压技术在近几年得到了迅速地发展。研究表明,反向型双螺杆挤出技术具有无法比拟的优越性能, 如物料能充分、彻底混合揉捏, 并且在反向型双螺杆挤出机运转时, 由于反向双螺杆互相啮合而具有自行擦净的功能, 避免了螺杆堵塞的物料在套筒内产生表面结焦的现象。同时反向型双螺杆挤压机还具有广泛的原料适应性的优点。反向型双螺杆挤出机因其具有突出的高效工作性能, 受到了食品行业的广泛重视。根据收集的相关文献, 对反向双螺杆挤压机在食品工业中的应用、发展前景、主要组成部分,以及挤压机的各项参数等进行综合的分析和论述,希望对我国反向型双螺杆食品挤压的研究与发展有益。 研究内容 螺杆挤压机的挤压膨化原理和结构特点 反向旋转型双螺杆挤压机的工作原理 反向旋转型双螺杆挤压机主要参数计算 反向旋转型双螺杆挤压机的总体结构设计 反向旋转型双螺杆挤压机挤压部件的设计拟采取的研究方法、技术路线、实验方案及可行性分析(1)实验方案 掌握反向旋转型双螺杆挤压机的工作原理,通过对其结构及特点的研究了解挤压机的内部结构,从而进行对挤压部件的研究和设计。(2)研究方法 通过实验了解挤压机的结构参数,对挤压部件的参数进行计算及确定,按照挤压机的结构进行装配图及挤压部件零件图的绘制。研究计划及预期成果研究计划:2012年10月12日-2012年12月31日:按照任务书要求查阅论文相关参考资料,完成毕业设计开题报告书。2013年1月1日-2013年1月27日:学习并翻译一篇与毕业设计相关的英文材料。2013年1月28日-2013年3月3日:毕业实习。2013年3月4日-2013年3月17日:反向旋转型双螺杆挤压机螺杆、机筒的主要参数计算与确定。2013年3月18日-2013年4月14日:反向旋转型双螺杆挤压机总体结构设计。2013年4月15日-2013年4月28日:部件及零件图设计。2013年4月29日-2013年5月21日:毕业论文撰写和修改工作。 预期成果:了解挤压机的工作原理、内部结构以及反向旋转型双螺杆挤压机的优缺点,熟练绘制挤压机的装配图,挤压部件的零件图。特色或创新之处 双螺杆挤压机在食品工业中应用更广泛。 反向旋转型双螺杆挤压机结构简单,更适合对高粘度食品的输送加工。已具备的条件和尚需解决的问题 设计方案思路已经非常明确,已经具备机械设计的能力和图纸处理方面的知识。 结构设计的能力尚需加强。指导教师意见 指导教师签名:2012年 12 月 15 日教研室(学科组、研究所)意见 教研室主任签名: 年 月 日系意见 主管领导签名: 年 月 日A simplified twin screw co-rotating food extruder: design, fabrication and testingAbstract A simplified co-rotating twin screw food extruder was designed, fabricated and tested in England, followed by extensive testing in Sri Lanka. It was built as a model to meet the specic product and nancial constraints of less developed countries and was expected to be used in those countries to widen the production capabilities of extruded foods. The machine had an estimated delivery of 10 kg/h and was made mainly with mild steel. Two types of screw were made, one with a constant pitch of 14 mm and the other with varying pitch in segments of 14, 12 and 10 mm. The machine was powered by a 2.2 kW electric motor with electronic speed controls. The machine also had electrical heating with a temperature controller and a pressure sensing device. The cost of fabrication of the machine was estimated at 2000 with most of the parts built in a fairly simple workshop. A mixture of rice and dried banana was successfully extruded as a potential snack food and on the basis of maximum expansion the best results was obtained from a barrel temperature of 120C, screw speed 125 rpm, feed moisture 15% and with a die orice size of 3 mm. When the alternative compression screw was tested very similar results were achieved with no signicant improvement in product expansion. 1999 Elsevier Science Ltd. All rights reserved.Keywords: Twin screw extruder; Design; Low cost; Snack food; Continuous cooker; Local construction; Cereal mixturesNomenclatureA Die diameter (mm)B Channel width (mm)C Screw circumference (mm)d Screw core diameterD Outer diameter of screws (mm)H Flight depth (mm)M Moisture content (% wet basis)n Number of fight turnsN Speed angular (rev/min)p Pitch (mm)Q Delivery rate (mm3/min)S Total helical length of screws (mm)t Temperature (C)T Residence time (min) Overlap angle of screw fights (degrees) Calender gap (mm) Side clearance (mm) Product density (g/mm3 ) Helix angle (degrees)1. Introduction Extrusion cooking is finding ever increasing applications in the food process industry. Apart from providing a means of manufacturing new products, it has successfully revolutionised many conventional manufacturing processes (Harlow, 1985, Frame, 1994). Today,extruders come in a wide variety of sizes, shapes and method of operation. There are three types of food extruder found in industry: hydraulic ram, roller and screw type extruders (Frame, 1994). The screw extruders are very different to the other two having special features such as continuous processing and mixing ability. Single and twin screw types are both widely used in the food process industry. Unfortunately, most of the food extruders available in the market are either so costly that less developed countries cannot a.ord to buy them except by some form of assistance or outside investment or else are not appropriate for the wide variety of materials that need to be processed. As a result the growth of extrusion technology of food into these countries has been hindered despite its many advantages. In particular there appears not to be a suitable twin screw extruder available at suffciently low cost for use in developing countries. The high cost of these machines is largely due to the sophisticated constructional features with many controls that are not essential for many applications.The initial purchase cost is therefore a major barrier to the wider introduction of extrusion to developing countries where many raw materials are available to be processed into nutritious and palatable food items. Attempts have been made to utilise low pressure and simple extruders for baby weaning foods some of which have been very successful with programs through organizations such as Feed the Millions (Santa Monica,California, 90406, USA) and Thriposha (USDA/USAID). The need has long been recognised for simple,yet versatile machines at affordable prices which are capable of exploiting the natural food resources (Sahagun,1977), Jansen and Harper (1980), Harper and Jansen(1985). These could incorporate all the usual cereals such as rice, millet, maize and sorghum with a rich variety of nutritional and tasty additives such as fruits,vegetables and spices. The objective of this study therefore was to eliminate or simplify many of the unnecessary features of the currently available machines so that fabrication could be done in an unsophisticated workshop, while ensuring the versatility to permit the production of acceptable extruded foods especially using blends of fruits and cereals. This latter technique was developed by Gamlath (1995) using a single screw extruder to produce a range of extrudates that she adjudged to show great promise as a snack food or breakfast cereal in Sri Lanka in particular but also in many developing countries and also as a high value export. These products were largely based on mixtures of fruit and cereals such as mango, banana and tomato with maize, rice and wheat. Attractively textured and flavoured items were produced which could easily be further flavoured with spices, sugar and salt. Many developing countries use home based businesses to produce snacks and ready-to-eat meals and these product well into this well established culture. Machines made in the developing country, however, need to be low cost and relatively simple, easy to maintain and operate. In this study the particular features required were good transport characteristics for sticky materials and good mixing ability which are needed for the blended crops selected. Short residence times and low to medium pressures were acceptable.2. Design approach The design of the extruder in this study was mostly based on the results of extrusion studies done using a single screw laboratory scale extruders (Brabender,Warrington, Cheshire, UK) as part of the same programme,and supported by engineering principles and constraints applicable to a developing country. In the design process, the level of pressure likely to be developed within the extruder barrel becomes the most important factor as it determines the sizes of all the important components of the extruder such as bearings,shaft diameters and barrel dimensions. As these machines often operate under elevated temperatures, pressure tends to vary unpredictably because of complex rheological properties of food dough as related to varying temperature along the barrel. Therefore, actual measurements of extruder process variables, were made on the chosen range of food materials on the laboratory single screw extruder. This approach was reckoned to be more appropriate rather than a theoretical one, that cannot be generalised especially for materials of unknown rheology. Hence, approximate values for some of the most important parameters such as die sizes, pressures,temperatures etc. were quantified. These values were then tested again in the experiments described later for the twin screw system.2.1. Single vs twin screws Many comparisons between twin and single screw extruders have been made (Harper, 1992) (Hauck, 1985 ) (Hauck & Ben Gera, 1987) (Van Zuilichem, Stolp & Janssen, 1984) and many of the features discussed are relevant to the current application. Single screw machines meet most of the criteria for developing countries but unfortunately certain of the technical requirements can only be met by twin screws such as versatility, adequateow of feed into the screws and, particularly in this case, ow of the material down the barrel without spinning within it. This is a common problem with the rather sticky or high moisture materials that were envisaged with the incorporation of fruits and vegetables and other tropical crops and flavours and experienced by Gamlath (1995). The forward motion of the food dough relies entirely on the friction between the material and the interior surfaces of both screw and barrel. This is particularly so in a single screw but less so in a twin screw. Single screw extruders are generally cheaper than the twin screw type owing to their simpler construction, but they are more likely to block than twin screw extruders. The dilemma facing the designer, therefore, is that although the operational problems can be avoided by using twin screws their sophisticated constructional features would have to be simplified significantly to achieve the low cost objective. In twin screw extruders the screws can be made as either co-rotating or counter rotating, with differing amounts of intermesh, pitch and clearances, all of which affect the processing characteristics of the machine (Martelli, 1983). Since the flight of one screw engages the channel of the other the twin screws prevent material from sticking to the screws and rotating with the screw, hence encouraging forward motion of dough with reduced slip (Jansen, 1978). This also enhances the mixing of materials in the channels of the screws. The counter rotating type of twin screw extruder can give better material ow characteristics pushing the material positively forward towards the die but reducing the degree ofmixing of materials within the extruder. Another unfortunate feature is that separating forces between the screws can cause severe wear of the side walls. On the other hand, co-rotating, twin screw extruders give additional advantages such as pressure balance in transverse planes of the screws and a wiping action of the fights due to the opposing linear velocities at the fights of intermeshing region. Therefore, it was decided that a twin screw co-rotating extruder would provide all the versatility and characteristics required for processing the food materials envisaged in a developing country. The only problem then was to design a satisfactory machine that would also meet the cost criteria.2.2. Screws The central part of an extruder is the screw. The screws rotate in a tightly fitting barrel and convey the material from feed end to die end, mixing and com-pressing the ingredients. The capacity of the extruder is mainly determined by the screw outer and root diameters, pitch and fight width (Fig. 1). Therefore, the screw overall dimensions have to be first determined according to the desired output to initiate the design process. One other important factor is the residence time, as it determines the degree of cooking taking place in the extruder. Exploratory trials and the work of Gamlath (1995) indicated that a residence time of about 16 s would be adequate at the mean speed. It is, however, influenced by many of the design parameters including the length, diameter and pitch of the screws although it can be controlled largely by the operational screw speed of the extruder and the selected die size. The rheology of the material also has a major influence on the throughput so residence time cannot be accurately predicted. In this study several dimensions of the screws were determined in order to suit the expected output and residence time. The geometry and equations relating many of these factors are given in Fig. 1 and Eqs. (1)(3) below. The maximum output was not the rst priority but similar sized extruders and single screw trials indicated a typical delivery rate of up to 10 kg/h. The residence time was designed to be short as the novel products planned were not expected to rely on long processing times. The short residence time was particularly sought in order to maintain the vitamin C content of perishable products which could be easily destroyed if cooked too long (Seiler, 1984). The influence of screw pitch is such that low values with small helix angles lead to longer residence times because of the increased number of cycles that the material has to go through. On the other hand, large pitches would reduce the degree of mixing and uniformity of heat distribution due to the increase in volume of material contained in the screw channels. Furthermore, if the fight width is also in-creased in proportion with the pitch to avoid large gaps between engaging fights, the power consumption is in-creased due to the increased surface area of shear be-tween the fight tip and the barrel. Therefore, in order to maintain the channel volume within reasonable limits and to have a fairly short residence time the pitch was selected as 14 mm. In view of the foregoing the outer diameter of the extruder was chosen at 30 mm with a helix angle at 8.5 which is in the range recommended by Martelli (1983) for similar plastic extrusion machines. The parameter most affected by the outer diameter is the extruder throughput, which was not a critical factor in this study apart from it being a low value to suit a large numbers of experiments. Another feature affected by the outer diameter is the screw center distance which in turn determines the fight height and the amount of intermesh. The screw center distance is vital as it establishes the space available for thrust bearings. Therefore, the screw center distance, the amount of intermesh and the root diameter all have to be carefully adjusted so that a maximum space is made available for the thrust bearings. The screw length, with pitch, governs the number of fight turns and has a major influence on the residence time which was planned to be short compared to most extruders. The length of the screws had to be relatively short therefore which was fortunate as this suited the manufacturing facilities available to construct a twin screw extruder barrel. It is not easy to machine two overlapping holes to accommodate the twin screws with a simple machine tool especially when the length is large as drills tend to wander. The length of barrel and screws to give the required residence time was 224 mm. The foregoing design choices permitted calculation of the number of turns as 16 (Eq. (1) with a preliminary value of fight height of 4 mm.The residence time is dened assuming no slip as,Referring to Fig. 1, the non-intermeshing part of the screws the length of circumference, C can be written as, ,=overlap angle= (1)The channel width is a major factor affecting the volume of screw channels and hence the throughput. Since the pitch is already xed, the value of the channel width depends on the selection of the values for the width of the fight and the clearance needed between engaging fight flanks. Considering the primary dimensions of the screw profile, the fight width at the root was calculated as 5 mm, leaving a reasonable side clearance of 2 mm. This established the channel width as 9 mm. The channel height was taken as 4 mm to have an output of 10kg/h at 200 rpm. A simple volumetric determination based on no-back flow was used to estimate the delivery rate (Q) as given by Eq. (2). (2)Referring to Fig. 1, the root diameter and the center distance between two screws were calculated as 22 and 27 mm, respectively, leaving a clearance (calender gap) of 1 mm between the tip of the screw fight and the bottom of the channel of the other screw.The fight flank angle of 7 was selected to avoid possible binding of the fights in operation. The minimum angle was given by the Eq. (3) (Jansen, 1978). (3)Compression of the product in the single screw barrels can be achieved by one of several methods as described by Harper (1979). These include varying pitch with constant root and outer diameters, use of tapered screws having constant pitch and tapered root diameter with constant outer diameter. Not all these possibilities can be applied to twin screws because of their geometrical interdependency. One of the simplest way of making screws achieve compression is to change the pitch along the screw and thus change the channel volume from feed end to die end. In this study it was decided to give the screws three different pitches in equal lengths along the screw using a lathe. The pitches selected were 14, 12 and 10 mm and in equal lengths along the screw with aclearance of 7 mm between each segment allowed for machining purposes. An alternative for simple compression screw design is to increase the root diameter of the screws from feed to delivery but this was considered to be somewhat harder to make on a simple lathe than the system chosen.2.3. Barrel The extruder barrel that accommodates the screws should be mechanically strong enough to withstand the pressures developed in the barrel and to resist the wear. The exact pressures inside the barrel are not known and, therefore to estimate the barrel wall thickness as well as to design the thrust bearings a level of pressure had to be assumed. It was measured during the preliminary work in the single screw extrud
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