便攜式縫紉機(jī)
便攜式縫紉機(jī),便攜式,縫紉機(jī)
外文翻譯
專 業(yè) 過(guò)程裝備與控制工程
學(xué) 生 姓 名 楊麗麗
班 級(jí) B裝備031班
學(xué) 號(hào) 0310140118
指 導(dǎo) 教 師 咸 斌
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縫紉機(jī)設(shè)計(jì)——影響喂料系統(tǒng)的壓應(yīng)力
P. Guigon
楊麗麗 譯（有刪節(jié)）
摘要
在文章的第一部分，敘述了縫紉機(jī)的主要特點(diǎn)。 然后,講述了喂料和擠壓質(zhì)量之間的關(guān)系。 對(duì)于某個(gè)靜態(tài)差距(無(wú)負(fù)載) ，縫紉機(jī)的處理量是由螺旋喂料速度決定的,與滾筒的速度快慢和需要壓實(shí)的生產(chǎn)材料無(wú)關(guān)。當(dāng)處理量是多種多樣的時(shí)候，控制差距是一個(gè)獲得相同質(zhì)量壓坯的好方法。對(duì)強(qiáng)烈環(huán)節(jié)緊湊的應(yīng)力分布的解釋和說(shuō)明，這些應(yīng)力是分布在由一根周期旋轉(zhuǎn)的螺桿喂料的縫紉機(jī)上的。
關(guān)鍵詞：輥壓； 喂料裝置； 壓坯異質(zhì)； 差距控制
1．引言
由于縫紉機(jī)簡(jiǎn)單、低營(yíng)運(yùn)成本的理念，而且用材廣泛，所以被用在了許多不同的行業(yè)(化工、制藥、 食品加工、采礦、礦產(chǎn)、冶金)上。廣泛的垃圾回收或處理就是一個(gè)新興的應(yīng)用領(lǐng)域?？p紉機(jī)的擠壓要比第一眼看上去的復(fù)雜。 對(duì)很多參數(shù)和對(duì)縫紉機(jī)理的缺乏了解導(dǎo)致了縫紉機(jī)沒(méi)有產(chǎn)品的優(yōu)越性。 這篇文章將講述縫紉機(jī)的主要部分。 文中將注意力集中在了解喂料裝置是如何影響壓實(shí)質(zhì)量的。
2. 縫紉機(jī)的概說(shuō)〔1-5〕
縫紉機(jī)的滾壓是一個(gè)連續(xù)的過(guò)程。 功能原理很簡(jiǎn)單：料粉是通過(guò)重力方式或者通過(guò)一根連接兩個(gè)方向相反正在旋轉(zhuǎn)的輥?zhàn)拥穆輻U喂入。 由材料和滾筒表面產(chǎn)生的摩擦在輥?zhàn)又g的狹小空間里帶出料粉，在這些空隙里粉末產(chǎn)生的強(qiáng)大應(yīng)力導(dǎo)致了其結(jié)構(gòu)緊湊。 如果滾筒是平滑的或者是槽型的，物料被壓緊成致密片 而口袋卷筒將形成煤球型的(如圖1所示)。
圖1：縫紉機(jī)中的壓塊和壓坯
2.1. 壓實(shí)機(jī)制
輥?zhàn)又g的空間，一般分為三區(qū)，在這三個(gè)區(qū)由不同的機(jī)制作用。喂料區(qū)：在這個(gè)區(qū)顆粒的整理應(yīng)力很小而且致密性很純粹； 壓實(shí)區(qū)：在這個(gè)區(qū)擠壓力作用明顯；擠壓區(qū)：顆粒開始塑性變形和/或被壓碎。在喂料區(qū)和壓實(shí)區(qū)之間的角度是鈍角或者是銳角。
圖2：由壓電傳感器測(cè)量的應(yīng)力分布
2.2. 典型應(yīng)力
輥?zhàn)娱g隙間的壓應(yīng)力的正常分布如圖2所示。在喂料區(qū)當(dāng)滾筒作用在粉末上的壓力很小時(shí)(小于0.1兆帕)， 它不能用壓電傳感器測(cè)量。 只有壓實(shí)區(qū)的應(yīng)力才可以用它測(cè)量。 應(yīng)力擴(kuò)增在小于直角的情況下發(fā)生。 應(yīng)力增加至最大值，這個(gè)最大值相當(dāng)于到達(dá)中性角度。 在許多情況下，角度的改變不和輥?zhàn)娱g隙成比例，是因?yàn)椴牧细采w在滾筒的表面。 直角之后，壓坯被排出。 彈出物對(duì)應(yīng)的壓力急劇下降。
2.3.縫紉機(jī)所具有的優(yōu)缺點(diǎn)：
縫紉機(jī)滾壓物料有以下幾個(gè)優(yōu)點(diǎn)：
(1)允許連續(xù)運(yùn)行和有多功能的高生產(chǎn)能力:適合重工業(yè)每小時(shí)幾百噸的生產(chǎn) (礦產(chǎn)、肥料等)。
(2)壓實(shí)成本低。 帶動(dòng)滾筒和螺桿運(yùn)轉(zhuǎn)的能量是有限的。通常，干燥這一步是不需要的。
(3)需要壓實(shí)的熱材料的氣溫高達(dá)1000攝示度是可能的。
然而，這項(xiàng)技術(shù)目前還有一些弊端：
壓坯的外形和尺寸比沖模擠壓出來(lái)的不規(guī)則。料粉的泄漏也要重點(diǎn)解決。未壓碎的料粉也需要再擠壓。使用真空除塵系統(tǒng)可以大大減少(可降百分之幾)細(xì)粉的泄漏[2]。
圖3：縫紉機(jī)的結(jié)構(gòu)
2.4.技術(shù)
無(wú)論制造商是誰(shuí)，縫紉機(jī)的原理都是一樣的,而且縫紉機(jī)都有相似的結(jié)構(gòu)配置。 市場(chǎng)上賣的縫紉機(jī)的輥?zhàn)佑兴椒胖玫?，有垂直放置的，有傾斜放置的，(如圖3所示)。兩種不同的結(jié)構(gòu)設(shè)計(jì)要根據(jù)縫紉機(jī)放置位置的合理性來(lái)選擇最優(yōu)的設(shè)計(jì)方案。
在懸臂軸設(shè)計(jì)中，棍子是被置于框體外面的（如圖3所示）。 這種設(shè)計(jì)通常被用于小型機(jī)器；這樣的設(shè)計(jì)便于輥?zhàn)拥木S修。 比較大型的機(jī)器用中間軸的設(shè)計(jì)結(jié)構(gòu)，這就意味著，軸的兩端是由鉸鏈連接軸承旋轉(zhuǎn)的，而且輥?zhàn)邮俏挥诳蝮w里面的。制造商對(duì)A、B、C三種結(jié)構(gòu)的優(yōu)點(diǎn)持有不同的意見。一般來(lái)說(shuō)，一個(gè)輥?zhàn)拥妮S承在機(jī)體里的作用是固定不變的，然而其他可移動(dòng)的輥?zhàn)拥妮S承是靠水壓力調(diào)節(jié)的
2.5.滾動(dòng)和擠壓系統(tǒng)
輥?zhàn)舆x擇的方法一般有兩種:幾何特征(光滑、槽、 和容器設(shè)計(jì))和表面硬度。 對(duì)于壓塊，容器造型的優(yōu)先使用，這是為了減少排除物的問(wèn)題和擠壓造成的破壞：作用于壓坯上的最大允許壓力很大程度上取決于輥?zhàn)拥闹睆健?越大的壓力被用于越大的機(jī)器上。 輥?zhàn)拥尿?qū)動(dòng)組件必須保證兩根軸間有一個(gè)恒定的轉(zhuǎn)距和一個(gè)相等的速度，這是為了阻止輥?zhàn)虞^早的被磨損壞和破壞壓坯的剪應(yīng)力的形成。為了防止壓塊，兩個(gè)輥?zhàn)娱g的旋轉(zhuǎn)速度必須一樣。一般來(lái)說(shuō),液壓系統(tǒng)是用來(lái)維持滾動(dòng)軸承座的. 采用這種系統(tǒng)，應(yīng)用力的調(diào)整范圍可以更廣泛。
2.6.喂料系統(tǒng)及隔離
喂料系統(tǒng)是一個(gè)好的擠壓過(guò)程的關(guān)鍵。它必須完成一個(gè)統(tǒng)一的連續(xù)的物料流動(dòng)，這是為了恰當(dāng)而充分的填滿輥?zhàn)娱g的量從而使壓坯形成不均勻質(zhì)。 該喂料系統(tǒng)還用于密封和除塵裝置。 兩種不同類型的喂料系統(tǒng)主要是依靠流動(dòng)特性和粉末的密度來(lái)區(qū)分使用的。致密性需要制作壓坯有足夠的質(zhì)量保證：重力的自由向下喂料和強(qiáng)迫喂料(粉末是被一個(gè)或幾個(gè)螺桿推向輥?zhàn)拥?。
2.7.粉末的除塵
粉末中的空氣有兩種逃走的方法:通過(guò)料粉的軸中心，來(lái)到喂料裝置處；通過(guò)輥?zhàn)又g的空隙和面夾板。 一些空氣可以在棍子內(nèi)被壓縮， 這是一個(gè)限制生產(chǎn)量和壓實(shí)質(zhì)量的關(guān)鍵因素[2]。 在壓實(shí)區(qū)使用真空除塵可以有效的優(yōu)化輥壓質(zhì)量和減小未擠壓的粉末的泄漏。
3. 在實(shí)驗(yàn)室縫紉機(jī)中喂料和壓實(shí)相互關(guān)系的闡述
3.1.實(shí)驗(yàn)室縫紉機(jī)
實(shí)驗(yàn)室進(jìn)行實(shí)驗(yàn)的過(guò)程如圖示4所示。 縫紉機(jī)配備了垂直安裝的130毫米直徑50毫米寬的圓盤?？p紉機(jī)的詳細(xì)描述和須知將在3-6給出。
圖4：實(shí)驗(yàn)室的縫紉機(jī)：（1）輥?zhàn)樱?）軸承座（3）輥軸（4）水平支撐系統(tǒng)（5）螺旋喂料（6）攪拌器（7）喂料漏斗（8）金屬夾（a）壓電變換器（b）移動(dòng)變換器
3.2. 縫紉機(jī)的吞吐量
對(duì)于細(xì)粉而言，縫紉機(jī)的進(jìn)料量是由兩個(gè)因素限制的。一方面，進(jìn)料量是由細(xì)粉的除塵能力限制的。而另一方面，壓實(shí)速度又是由顆粒的彈性度限制的。一般來(lái)說(shuō)，當(dāng)達(dá)到臨界流量時(shí)壓實(shí)的質(zhì)量比較差。在這種情況下，要么是由壓實(shí)引起的風(fēng)流影響了喂料（除塵能力差），要么是壓實(shí)的速度太快。這項(xiàng)研究的所有實(shí)驗(yàn)都將在低于這個(gè)臨界流量時(shí)進(jìn)行。因此，當(dāng)出現(xiàn)細(xì)粉壓實(shí)沒(méi)有產(chǎn)生帶鋼或者帶鋼的質(zhì)量差的問(wèn)題時(shí)，不是由除塵能力差或滾壓速度過(guò)高（擠壓時(shí)間短）引起的。
3.3.擠壓率好的擠壓場(chǎng)合
擠壓速度和螺桿轉(zhuǎn)速的范圍大可在實(shí)驗(yàn)室縫紉機(jī)中得到解決。因此，我們研究了在擠壓帶鋼成形中滾壓速度和螺桿速度對(duì)它的影響。為了清楚地發(fā)覺高低滾壓速度的限制對(duì)擠壓成形的影響我們使螺桿速度固定選擇它的擠壓速度。在滾壓速度低時(shí)將發(fā)過(guò)度擠壓，而在滾壓速度高時(shí)將不形成帶鋼。
三個(gè)操作條件規(guī)定如下：
當(dāng)喂料不足時(shí)，由螺旋喂料提供的大量粉末的操作滾壓率會(huì)太小。在這種情況下，不能擠壓微粒物質(zhì)。
當(dāng)喂料過(guò)多時(shí)，由螺旋喂料提供的大量粉末的操作滾壓率會(huì)太大。滾子與滾子之間空隙的增大是很重要的。在喂料過(guò)多的情況下，擠壓出來(lái)的物質(zhì)質(zhì)量會(huì)差而且未擠壓的粉末的流失也很嚴(yán)重。
好的擠壓率是在處于喂料不足和喂料過(guò)多之間的擠壓率。當(dāng)擠壓材料時(shí)產(chǎn)生的帶鋼具有足夠的凝聚力和力學(xué)強(qiáng)度時(shí)，才會(huì)有好的擠壓率。
圖5：不同輥?zhàn)訉?duì)應(yīng)不同旋轉(zhuǎn)速度的壓坯的輸出
圖6：輥?zhàn)拥牟煌俣葘?duì)應(yīng)不同的旋轉(zhuǎn)速度而且物料的輸出依靠旋轉(zhuǎn)速度而不是和輥速成正比
當(dāng)螺桿轉(zhuǎn)速固定時(shí)，滾壓吞吐量是由多種能夠形成好的擠壓的滾壓速度衡量的（如圖5所示）。對(duì)于固定的旋轉(zhuǎn)速度，縫紉機(jī)的吞吐量也是個(gè)常數(shù)。在圖6中，吞吐量是由多種滾壓速度下的旋轉(zhuǎn)速度決定的。這個(gè)吞吐量要比螺桿單獨(dú)作用時(shí)的吞吐量小。由滾動(dòng)產(chǎn)生的壓力改變了粉末在螺桿內(nèi)的滑動(dòng)狀態(tài)。
3.4. 軋輥輥縫的變化
如果上布的軋輥能縱向移動(dòng)，當(dāng)傳動(dòng)力是恒定時(shí)軋輥的縫就能從初值增加到一個(gè)恒定值。恒定值是軋輥?zhàn)饔迷趬簩?shí)材料上的平均壓應(yīng)力的作用。它也是輥速度vr的作用，軋輥的生產(chǎn)量是QC,材料的壓實(shí)密度是Qs，軋輥寬是L，壓實(shí)材料的摩擦系數(shù)是f[3]：e=Qc／[LVrρs(1-ξ)] 輥縫測(cè)量有許多工作要點(diǎn)（輥速度和螺旋轉(zhuǎn)動(dòng)速度），國(guó)際質(zhì)量曲已給出(如圖7所示)。
圖7：輥?zhàn)雍吐輻U的速度之間的標(biāo)準(zhǔn)間隙差距，初次間隙是0.8mm
3.6. 應(yīng)力的波動(dòng)與鐳的不同成分的關(guān)系如表[6]
緊湊的密度分布的特點(diǎn)是通過(guò)衡量一個(gè)氯化鈉晶體的傳遞分布。 適當(dāng)?shù)膲毫δ苁孤然c晶體支離破碎。因此，同樣的氯化鈉晶體不是到處都能傳遞光的。 因?yàn)槁然c的透光性能是與局部緊湊地方的壓力有關(guān)的。 承受較少壓力的地方因此出現(xiàn)暗色(如圖11所示)。機(jī)械性能良好可以作為獲得緊湊性的特點(diǎn)，例如粉碎被使用過(guò)的氯化鈉(硼粉74時(shí))。 施加在物料上的壓力既不符合輥寬度也不符合時(shí)間常數(shù)。 期刊的分布不均。 周期現(xiàn)象就是螺旋反饋線的周期。 事實(shí)上，施加在滾軸間隙上的壓力分布與喂料系統(tǒng)壓力的分布有關(guān)。喂料系統(tǒng)的壓力有來(lái)自螺旋饋線的。喂料壓力的不均勻是由于最后螺旋的螺桿的傳動(dòng)力不均勻。
圖11：氯化鈉的透光性（氯化鈉d50，Am74），上圖
氯化鈉照片的標(biāo)準(zhǔn)灰色度，下圖
4. 結(jié)論
喂料和壓縮特性之間相互作用得到了證明。 因?yàn)槭褂寐菪o料器，大的壓應(yīng)力產(chǎn)生了，并被當(dāng)作滾動(dòng)和轉(zhuǎn)動(dòng)的作用力。壓力的大小僅由螺桿給料器決定，和生產(chǎn)材料以及棍子的轉(zhuǎn)速都沒(méi)有關(guān)系。結(jié)果表明差距曲線可以近似于國(guó)際質(zhì)量曲線。因此，當(dāng)壓力的量不同時(shí)，控制差距是一個(gè)好方法，可以用這種方法來(lái)獲得相同的壓應(yīng)力。對(duì)墻的觀測(cè)表明，顆粒運(yùn)動(dòng)喂料區(qū)不是連續(xù)的。螺桿自轉(zhuǎn)的應(yīng)力周期得到了證明。從單螺桿喂料的應(yīng)力分布看，如果它們有相同周期就可以被觀察到。
Roll press design—influence of force feed systems on compaction
P. Guigon *, O. Simon1
Universite′ de Technologie de Compie`gne, BP 20529, 60205 Compie`gne cedex, France
Abstract
In the first part of the article, the main features of roll compactor design are reviewed. Then, the interaction between feeder and compact quality is demonstrated. For a given static gap (no load), the throughput of the press is only a function of the screw feeder speed no matter of the roller speed as long as compacted material is produced. Control of the gap is a good way to obtain compacts of the same quality when throughput is varied. The strong link of the stress distribution of the compact issued from a roll press fed by a single screw with the periodicity of the screw was demonstrated and explained.
Keywords: Roll compactor; Feeding device; Heterogeneity of compact; Gap control
1. Introduction
Because of their conceptual simplicity and low operating cost, roll compactors are used in many different industries (chemical, pharmaceutical, food processing, mining, minerals, and metallurgical) for a wide variety of materials. A new emerging application is the vast field of waste recycling or disposal. Compaction in a roll press is more complicated than it looks at first sight. Many parameters are involved and a lack of understanding of compaction mechanisms results in products that do not possess the required characteristics. This article will review the main features of roll compactors. Then, attention will be focused on the understanding of how the feeding device influences the quality of compacts.
2. Generality about roll compaction
Compaction in a roll press is a continuous process. Functional principle is simple: powder is fed by gravity or by means of a screw through two counter currently rotating rollers. Friction between the material and roller surface brings the powder towards the narrow space between the roll (gap), where the powder is submitted to high stresses leading to the formation of compact. If the rolls are smooth or fluted, material is compacted into
dense sheets, whereas pocket rolls will form briquettes
(Fig.1).
P. Guigon, O. Simon / Powder Technology 130 (2003) 41–48 42
Fig. 1. Briquetting and compaction in a roll press
2.1. Compaction mechanisms
The space between the rolls is generally divided into three zones, where different mechanisms occur: the feeding zone, where the stresses are small and densification is solely due to rearrangement of particles; the compaction zone, where the pressing forces become effective and the particles deform plastically and/or break; and the extrusion zone. The limit between the feeding and the compaction zone is the gripping angle or nip angle
2.2. Stress profile
A typical distribution of the normal stress versus the position in the gap between the rolls (roller angle) is represented in Fig. 2.
Fig. 2. Stress profile measured by the piezoelectric transducers.
As the stress exerted by the rollers on the powder in the feeding area is very small (less than 0.1 MPa), it can not be measured by piezoelectric transducers. Only the stress exerted in the compaction area is observable.
The stress augmentation takes place below the nip angle. The stress increases until a maximum which corresponds to the neutral angle. In many cases, the neutral angle does not coincide with the roll gap because the material slips along the roller surface. After the neutral angle, the compact is ejected. The ejection corresponds to a rapid decrease of the stress profile.
2.3. Advantages and drawbacks of roll compaction
Agglomeration in roll presses has the following advantages:
– The process is continuous and allows with multiple units of high production capacities: several hundred tons perhour are suitable for heavy industry (mineral, fertilizers,……).
– The compaction costs are low. The energy consumption is limited to the power to drive the rolls and the screws. Normally, no drying step is necessary.
– Compaction of hot materials with temperatures up to 1000 ℃is possible.
However, this technique presents some drawbacks:
– Aspect and dimension of compacts made by briquetting are less regular than those produced by die pressing.
– Powder leakage can be important. It is usually necessary to recycle the uncompacted powder. Use of vacuum desecration systems can greatly reduce (down to few percent) the leakage for very fine powder [2].
2.4. Technology
Whatever manufacturer, the roll presses consist of the same elements and have similar configurations. Commercially available roller compactors have rolls mounted in a horizontal, vertical or even inclined position as shown in Fig. 3. Two different frame designs exist which are distinguished by the location of the press rollers with respect to the frame.
Fig. 3. Configuration of roll presses.
In cantilever-shaft designs, the rollers are located outside the frame (Fig. 3). This design is normally used for smaller machines; it allows easy access to the rolls for maintenance tasks. Most larger machines use the mill-shaft frame design. This means, both ends of the two shafts are pivoted by bearings and the rolls are located within the frame.
Manufacturers are not unanimous about the advantages of configurations A, B, and C.
Generally, bearings of one of the rollers are fixed in relation to the frame,while the bearings of the other movable (floating) roller are maintained by an adjustable hydraulic force.
2.5. Rolls and pressurization system
Roll choice is essential in two ways: geometrical characteristics
(smooth, fluted, and pocket design) and surface hardness. For briquetting, pocket shapes are optimized in order to diminish ejection problems and breakage of compacts: maximum applicable stress on the compact depends greatly on roll diameter. Higher stresses are used on larger machines.
Roll drive assembly must ensure a constant torque and an equal velocity of the two roll shafts in order to prevent early wear of the rolls and shearing forces which will fracture the compact. In the case of briquetting, both rolls must rotate with exactly the same speed.
Generally, a hydraulic system is used to maintain the bearing blocks of the movable roller. By using such a pressurizing system, the applied force can be adjusted within wide limits.
2.6. Feeding systems and confinement
The feeding system is the key to a good compaction process. It must achieve a uniform and continuous flow of material in order to fill the nip between the rollers correctly and sufficiently, so that the formed compacts are not heterogeneous. The feeding systems are also used as densification and desertion devices.
Two different types of feeding systems are used depending on the flow properties, the density of the powder, and the densification needed to produce compacts of sufficient quality:
（1）gravity feeder for free flowing particles and force feeder (powder is pushed towards the rolls by one or several screws).
2.7. Powder desecration
The air fed with the powder can only escape by two paths: axially through the powder, counter currently to the feed; and through the gap between rolls and cheek plate. Some air can be compressed inside the compact. This is a key factor limiting compaction production throughput and compact quality [2]. Use of vacuum desecration before the nip roll region is efficient in optimizing roller compaction and minimizing uncompacted powder leakage.
3. Demonstration of the interaction between feeding and
compaction in a laboratory roll press
3.1. Laboratory roll press
Experiments were carried out on a laboratory roll press (KomarekR B100QC) shown in Fig. 4. The roll press was equipped with 130-mm diameter and 50-mm wide smooth rolls, which were vertically arranged. Detail description of the roll press and instrumentation is given in Refs. [3–6].
Fig. 4. The laboratory roll press: (1) roll, (2) bearing block, (3) roll shaft, (4) supporting hydraulic system, (5) screw feeder, (6) paddle mixer, (7) feed hopper,
(8) cheek plate. (a) Piezoelectric transducers, (b) displacement transducer.
3.2. Roll press throughput
For fine powder, the roll press throughput is principally limited by two factors. On one hand, the throughput is limited by the powder deaeration ability, and on the other hand, the compaction speed is limited by the elasticity of the particles. Generally, a poor quality compaction takes place when a critical throughput is reached. In this case, either the airflow generated by compaction disturbs the feeding (bad deaeration) or the compaction is too fast [1]. All experiments in this study were conducted below this critical throughput. Therefore, when no strip of compacted powder was produced or when the strip was of poor quality, the problem was not due to poor deaeration or to a too high roller speed (too short compaction time).
3.3. Compaction rate, good compaction settings
A wide range of roller speeds and screw feeder speeds can be set on the laboratory roll press. Therefore, we investigated the influence of roller speed and screw speed on the formation of a compacted strip. The screw feeder speed was fixed and the roller speed was chosen in order to detect visually the higher and lower limits of roller speed that enabled the compaction. At low roller speeds, overcompaction occurred, and at high roller speeds, no strip was formed.
Three operating conditions were defined as follows.
The subfeeding, corresponding to the operating rate of the roll press when the amount of powder that is provided by the screw feeder is too small. In this case, the particulate material is not compacted.
The over-feeding, which corresponds to the operating rate of the roll press when the amount of powder provided by the screw feeder is too large. The compact is extruded between the rolls and the roll gap increase is important. In this case, the compacted material is of poor quality and the powder loss as noncompacted powder is very important.
The ‘‘good compaction rate’’ is an operating rate between
sub- and overfeeding. It corresponds to the production of a strip of compacted material that exhibits enough cohesion and mechanical strength.
For a fixed screw speed, the roll press throughput was measured for several roller speeds Vr, enabling production of a good compact (Fig. 5). For a constant screw speed Vs, the roll press throughput is constant. In Fig. 6, the throughput is measured as a function of Vs for various Vr. This throughput is smaller than the throughput of the screw alone. The counter pressure created by the rollers modifies the slip between the powder and the screw barrel.
Fig. 5. Compactor throughput versus roll speed for different screw speeds.
Fig. 6. Compactor throughput versus screw speed for different roll speeds
and comparison with throughput delivered by the screw when not coupled
with the roll.
3.4. Roll gap variation
If the upper roll can move vertically, the roll gap increases from its initial value to an equilibrium value when the powder is compacted. This equilibrium value is a function of the mean stress applied by the rolls on the compacted material. It is also a function of the rollers speed Vr, the roll press throughput Qc, the density of the compacted material qs, the rolls width L, and the slip of the compacted material on the roll surface f [3]:
e=Qc／[LVrρs(1-ξ)]
The roll gap was measured for many working points (sets of Vs and Vr), and iso-gap curves were computed (Fig. 7).
Fig. 7. Calculated iso-gap curves (mm) versus roll and screw speed. Initial gap is 0.8 mm.
3.6. Heterogeneity of compact in relation to the fluctuations
of stress [6]
Distribution of the compact density was characterized measuring the distribution of light transmitted through a sodium chloride compact. The fragmented sodium chloride crystals are oriented by the applied stress, and therefore, light is not diffused similarly in all directions. For sodium chloride, the light transmission property is linked with the stress that has been applied locally on the compact. The zones that have endured less stress transmit less light and therefore, appear darker (Fig. 11). To obtain a compact with good mechanical properties that can be characterized, a comminuted sodium chloride (d50=74 Am) was used.
Fig. 11. Light transmitted through a sodium chloride compact (sodium chloride d50: 74 Am) (top). Iso-grey-levels of the photography of the light transmitted
through a sodium chloride compact (bottom).
The stress exerted on the compact is neither homogeneous on the roll width nor constant versus time. The heterogeneity distribution is periodical. The period of the phenomenon is the period of the screw feeder. In fact, the distribution of the stress exerted on the compact in the roller gap is due to the distribution of the feed pressure that is induced by the screw feeder.
The heterogeneity of the feed pressure is due to the inhomogeneous compaction of the powder in the last spiral of the screw.
4. Conclusions
The interaction between feeding and compact quality was
demonstrated. For the use of a screw feeder, the domain where compacts of good quality are produced was drawn as a function of roll and screw speeds. The throughput of the press is driven only by the screw feeder, no matter of the roller speeds as long as compacted material is produced. It was shown that iso-gap curves can be assimilated to isoquality curves. Therefore, controlling the gap is a good way to obtain compact of the same quality when throughput is varied.
Observations at the wall in the feeding region showed that the particle motion is not continuous. A strong link to the periodicity of the screw rotation was demonstrated. The same periodicity can be observed in the stress distribution of the compact issued from a roll press fed by a single screw.
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