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湖南工業(yè)大學(xué)本科畢業(yè)設(shè)計(jì)（論文）過(guò)程管理資料 本科畢業(yè)設(shè)計(jì)（論文）中期報(bào)告 填表日期： 2010 年 5月10 日 學(xué)院(部) 機(jī)械工程學(xué)院 班 級(jí) 機(jī)械工程及自動(dòng)化062班 學(xué)生 姓名 蘇志遠(yuǎn) 課題名稱：雙螺桿擠出機(jī) 課題主要任務(wù)： （1）設(shè)計(jì)生產(chǎn)PVC的雙螺桿擠出機(jī)； （2）寫(xiě)出設(shè)計(jì)過(guò)程的說(shuō)明書(shū)； （3）畫(huà)出總裝圖和主要零部件的CAD圖紙； 1、簡(jiǎn)述開(kāi)題以來(lái)所做的具體工作和取得的進(jìn)展或成果 查閱相關(guān)資料，分析所給題目的要求，了解雙螺桿擠出機(jī)工作原理，選擇合適的設(shè)計(jì)型號(hào)并寫(xiě)出設(shè)計(jì)過(guò)程說(shuō)明書(shū)，并初步修改。 2、下一步的主要研究任務(wù)，具體設(shè)想與安排 完成設(shè)計(jì)圖紙，總裝圖1張，主要零、部件4-5張，繼續(xù)完善設(shè)計(jì)說(shuō)明書(shū)。交指導(dǎo)老師評(píng)閱，再修改。 3、存在的具體問(wèn)題 設(shè)計(jì)說(shuō)明書(shū)完成過(guò)程有大量計(jì)算，出現(xiàn)了很大的困難。 圖形的繪制比較困難。 很多細(xì)節(jié)上的東西考慮不到位。 4、指導(dǎo)教師對(duì)該生前期研究工作的評(píng)價(jià) 指導(dǎo)教師簽名： 日 期： 11 湖南工業(yè)大學(xué)本科畢業(yè)設(shè)計(jì)（論文）過(guò)程管理資料 畢業(yè)設(shè)計(jì)（論文）指導(dǎo)教師評(píng)閱表 學(xué)院（部）：機(jī)械工程學(xué)院 學(xué)生姓名 蘇志遠(yuǎn) 學(xué) 號(hào) 06405600206 班 級(jí) 機(jī)工062班 專 業(yè) 機(jī)械工程及自動(dòng)化 指導(dǎo)教師 姓 名 林益平 課題名稱 雙螺桿擠出機(jī) 評(píng)語(yǔ)：（包括以下方面，①學(xué)習(xí)態(tài)度、工作量完成情況；②檢索和利用文獻(xiàn)能力、計(jì)算機(jī)應(yīng)用能力；③學(xué)術(shù)水平或設(shè)計(jì)水平、綜合運(yùn)用知識(shí)能力和創(chuàng)新能力；） 是否同意參加答辯： 是□ 否□ 指導(dǎo)教師評(píng)定成績(jī) 分值： 指導(dǎo)教師簽字： 年 月 日 畢業(yè)設(shè)計(jì)（論文）評(píng)閱教師評(píng)閱表 學(xué)院（部）： 機(jī)械工程學(xué)院 學(xué)生姓名 蘇志遠(yuǎn) 學(xué) 號(hào) 06405600206 班 級(jí) 機(jī)工062班 專 業(yè) 機(jī)械工程及自動(dòng)化 課題名稱 雙螺桿擠出機(jī) 評(píng)語(yǔ)：（對(duì)論文學(xué)術(shù)評(píng)語(yǔ)，包括選題意義；文獻(xiàn)利用能力；所用資料可靠性；創(chuàng)新成果及寫(xiě)作規(guī)范化和邏輯性） 針對(duì)課題內(nèi)容給設(shè)計(jì)者（作者）提出3個(gè)問(wèn)題，作為答辯時(shí)參考。 1. 2. 3. 評(píng) 分： 是否同意參加答辯 是□ 否□ 評(píng)閱人簽名： 年 月 日 畢業(yè)設(shè)計(jì)（論文）答辯及最終成績(jī)?cè)u(píng)定表 學(xué)院（部）（公章）：機(jī)械工程學(xué)院 學(xué)生姓名 蘇志遠(yuǎn) 學(xué)號(hào) 06405600206 班級(jí) 機(jī)工062班 答辯 日期 2010.6.12 課題名稱 雙螺桿擠出機(jī) 指導(dǎo) 教師 林益平 成 績(jī) 評(píng) 定 分值 評(píng) 定 教師 1 教師 2 教師 3 教師 4 教師 5 小計(jì) 課題介紹 思路清晰，語(yǔ)言表達(dá)準(zhǔn)確，概念清楚，論點(diǎn)正確，實(shí)驗(yàn)方法科學(xué)，分析歸納合理，結(jié)論嚴(yán)謹(jǐn)，設(shè)計(jì)（論文）有應(yīng)用價(jià)值。 30 答辯 表現(xiàn) 思維敏捷,回答問(wèn)題有理論根據(jù)，基本概念清楚，主要問(wèn)題回答準(zhǔn)確、深入,知識(shí)面寬。 70 合 計(jì) 100 答 辯 評(píng) 分 分值： 答辯小組長(zhǎng)簽名： 答辯成績(jī)a： × ％＝ 指導(dǎo)教師評(píng)分 分值： 指導(dǎo)教師評(píng)定成績(jī)b： × ％＝ 評(píng)閱教師評(píng)分 分值： 評(píng)閱教師評(píng)定成績(jī)c： × ％＝ 最終評(píng)定成績(jī)： 分?jǐn)?shù)： 等級(jí)： 答辯委員會(huì)主任簽名： 年 月 日 說(shuō)明：最終評(píng)定成績(jī)＝a+b+c，三個(gè)成績(jī)的百分比由各學(xué)院（部）自己確定。 14 編號(hào) 無(wú)錫太湖學(xué)院 畢業(yè)設(shè)計(jì)（論文） 相關(guān)資料 題目： 反向旋轉(zhuǎn)型雙螺桿擠壓機(jī) 及擠壓部件設(shè)計(jì) 機(jī)電 系 機(jī)械工程及自動(dòng)化專業(yè) 學(xué) 號(hào)： 　　　 0923168　　　 學(xué)生姓名： 沈杰 指導(dǎo)教師： 　戴寧 （職稱：副教授 ） （職稱： ） 2013年5月25 目 錄 一、畢業(yè)設(shè)計(jì)（論文）開(kāi)題報(bào)告 二、畢業(yè)設(shè)計(jì)（論文）外文資料翻譯及原文 三、學(xué)生“畢業(yè)論文（論文）計(jì)劃、進(jìn)度、檢查及落實(shí)表” 四、實(shí)習(xí)鑒定表 無(wú)錫太湖學(xué)院 畢業(yè)設(shè)計(jì)（論文） 開(kāi)題報(bào)告 題目： 反向旋轉(zhuǎn)型雙螺桿擠壓機(jī) 及擠壓部件設(shè)計(jì) 機(jī)電 系 機(jī)械工程及自動(dòng)化 專業(yè) 學(xué) 號(hào)： 0923168 學(xué)生姓名： 沈杰 指導(dǎo)教師： 戴寧 （職稱：副教授 ） （職稱： ） 2012年11月25日 課題來(lái)源 用于食品生產(chǎn)的工程實(shí)踐性自擬課題。 科學(xué)依據(jù)（包括課題的科學(xué)意義；國(guó)內(nèi)外研究概況、水平和發(fā)展趨勢(shì)；應(yīng)用前景等） （1）課題科學(xué)意義 擠壓機(jī)是擠壓加工技術(shù)的關(guān)鍵。 擠壓加工技術(shù)作為一種經(jīng)濟(jì)實(shí)用的新型加工方法在食品生產(chǎn)中得到迅速發(fā)展。 擠壓加工主要由一臺(tái)擠壓機(jī)一步完成原料的混煉、熟化、粉碎、殺菌、預(yù)干燥、成型等工藝, 制成膨化、組織化產(chǎn)品或制成不膨化的產(chǎn)品。 只要簡(jiǎn)單地更換擠壓模具, 便可以很方便地改變產(chǎn)品的造型。反向型螺桿擠壓機(jī)用于食品生產(chǎn)具有工藝簡(jiǎn)單、一機(jī)多能、生產(chǎn)連續(xù)化、效率高、能耗低、投資少、收效快的特點(diǎn)。生產(chǎn)出來(lái)的食品口感細(xì)膩、易消化吸收、營(yíng)養(yǎng)成份損失少、貯藏時(shí)間長(zhǎng)、不易產(chǎn)生“回生”現(xiàn)象、食用方便. 目前，擠壓技術(shù)已經(jīng)發(fā)展成為最常用的膨化食品生產(chǎn)技術(shù)之一。反向型雙螺桿擠壓機(jī)的研究與開(kāi)發(fā)也勢(shì)在必行。 （2）反向雙螺桿擠壓機(jī)的研究狀況及其發(fā)展前景 上世紀(jì)60 年代, 開(kāi)始出現(xiàn)了雙螺桿擠壓機(jī), 并用于食品加工領(lǐng)域. 我國(guó)從70 年代開(kāi)始研究食品擠壓技術(shù)和擠壓加工機(jī)械. 1980 年3 月, 北京食品研究所仿制出第一臺(tái)自熱式PJ ) 1 型谷物膨化擠壓機(jī). 1982 年無(wú)錫輕工業(yè)大學(xué)從法國(guó)Clext ral 公司引進(jìn)一臺(tái)BC ) 45 型雙螺桿擠壓機(jī), 開(kāi)始了對(duì)擠壓加工技術(shù)的研究。 與此同時(shí), 國(guó)內(nèi)許多生產(chǎn)廠家也先后從世界各大公司引進(jìn)了先進(jìn)的擠壓設(shè)備。國(guó)際上有代表性的擠壓機(jī)生產(chǎn)企業(yè)除法國(guó)Clext ral 公司外，還有美國(guó)Wenger 公司，德國(guó)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。在引進(jìn)國(guó)外設(shè)備的同時(shí)，國(guó)內(nèi)的許多廠家也先后生產(chǎn)了不同類型的擠壓設(shè)備。 反向型雙螺桿擠壓技術(shù)在近幾年得到了迅速地發(fā)展。研究表明,反向型雙螺桿擠出技術(shù)具有無(wú)法比擬的優(yōu)越性能, 如物料能充分、徹底混合揉捏, 并且在反向型雙螺桿擠出機(jī)運(yùn)轉(zhuǎn)時(shí), 由于反向雙螺桿互相嚙合而具有自行擦凈的功能, 避免了螺桿堵塞的物料在套筒內(nèi)產(chǎn)生表面結(jié)焦的現(xiàn)象。同時(shí)反向型雙螺桿擠壓機(jī)還具有廣泛的原料適應(yīng)性的優(yōu)點(diǎn)。反向型雙螺桿擠出機(jī)因其具有突出的高效工作性能, 受到了食品行業(yè)的廣泛重視。根據(jù)收集的相關(guān)文獻(xiàn), 對(duì)反向雙螺桿擠壓機(jī)在食品工業(yè)中的應(yīng)用、發(fā)展前景、主要組成部分，以及擠壓機(jī)的各項(xiàng)參數(shù)等進(jìn)行綜合的分析和論述,希望對(duì)我國(guó)反向型雙螺桿食品擠壓的研究與發(fā)展有益。 研究?jī)?nèi)容 ① 螺桿擠壓機(jī)的擠壓膨化原理和結(jié)構(gòu)特點(diǎn) ② 反向旋轉(zhuǎn)型雙螺桿擠壓機(jī)的工作原理 ③ 反向旋轉(zhuǎn)型雙螺桿擠壓機(jī)主要參數(shù)計(jì)算 ④ 反向旋轉(zhuǎn)型雙螺桿擠壓機(jī)的總體結(jié)構(gòu)設(shè)計(jì) ⑤ 反向旋轉(zhuǎn)型雙螺桿擠壓機(jī)擠壓部件的設(shè)計(jì) 擬采取的研究方法、技術(shù)路線、實(shí)驗(yàn)方案及可行性分析 （1）實(shí)驗(yàn)方案 掌握反向旋轉(zhuǎn)型雙螺桿擠壓機(jī)的工作原理，通過(guò)對(duì)其結(jié)構(gòu)及特點(diǎn)的研究了解擠壓機(jī)的內(nèi)部結(jié)構(gòu)，從而進(jìn)行對(duì)擠壓部件的研究和設(shè)計(jì)。 （2）研究方法 通過(guò)實(shí)驗(yàn)了解擠壓機(jī)的結(jié)構(gòu)參數(shù)，對(duì)擠壓部件的參數(shù)進(jìn)行計(jì)算及確定，按照擠壓機(jī)的結(jié)構(gòu)進(jìn)行裝配圖及擠壓部件零件圖的繪制。 研究計(jì)劃及預(yù)期成果 研究計(jì)劃： 2012年10月12日-2012年12月31日：按照任務(wù)書(shū)要求查閱論文相關(guān)參考資料，完成畢業(yè)設(shè)計(jì)開(kāi)題報(bào)告書(shū)。 2013年1月1日-2013年1月27日：學(xué)習(xí)并翻譯一篇與畢業(yè)設(shè)計(jì)相關(guān)的英文材料。 2013年1月28日-2013年3月3日：畢業(yè)實(shí)習(xí)。 2013年3月4日-2013年3月17日：反向旋轉(zhuǎn)型雙螺桿擠壓機(jī)螺桿、機(jī)筒的主要參數(shù)計(jì)算與確定。 2013年3月18日-2013年4月14日：反向旋轉(zhuǎn)型雙螺桿擠壓機(jī)總體結(jié)構(gòu)設(shè)計(jì)。 2013年4月15日-2013年4月28日：部件及零件圖設(shè)計(jì)。 2013年4月29日-2013年5月21日：畢業(yè)論文撰寫(xiě)和修改工作。 預(yù)期成果： 了解擠壓機(jī)的工作原理、內(nèi)部結(jié)構(gòu)以及反向旋轉(zhuǎn)型雙螺桿擠壓機(jī)的優(yōu)缺點(diǎn)，熟練繪制擠壓機(jī)的裝配圖，擠壓部件的零件圖。 特色或創(chuàng)新之處 ① 雙螺桿擠壓機(jī)在食品工業(yè)中應(yīng)用更廣泛。 ② 反向旋轉(zhuǎn)型雙螺桿擠壓機(jī)結(jié)構(gòu)簡(jiǎn)單，更適合對(duì)高粘度食品的輸送加工。 已具備的條件和尚需解決的問(wèn)題 ① 設(shè)計(jì)方案思路已經(jīng)非常明確，已經(jīng)具備機(jī)械設(shè)計(jì)的能力和圖紙?zhí)幚矸矫娴闹R(shí)。 ② 結(jié)構(gòu)設(shè)計(jì)的能力尚需加強(qiáng)。 指導(dǎo)教師意見(jiàn) 指導(dǎo)教師簽名： 2012年 12 月 15 日 教研室（學(xué)科組、研究所）意見(jiàn) 教研室主任簽名： 年 月 日 系意見(jiàn) 主管領(lǐng)導(dǎo)簽名： 年 月 日 A simplified twin screw co-rotating food extruder: design, fabrication and testing Abstract 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 speci?c 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 120°C, screw speed 125 rpm, feed moisture 15% and with a die ori?ce size of 3 mm. When the alternative compression screw was tested very similar results were achieved with no signi?cant improvement in product expansion. ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Twin screw extruder; Design; Low cost; Snack food; Continuous cooker; Local construction; Cereal mixtures Nomenclature A Die diameter (mm) B Channel width (mm) C Screw circumference (mm) d Screw core diameter D Outer diameter of screws (mm) H Flight depth (mm) M Moisture content (% wet basis) n Number of fight turns N 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, adequateˉow of feed into the screws and, particularly in this case, ˉow 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 ˉow characteristics pushing the material positively forward towards the die but reducing the degree of mixing 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 de?ned 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 a clearance 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