雙螺桿擠出機(jī)的技術(shù)難點(diǎn)

近幾年來(lái)，同向旋轉(zhuǎn)雙螺桿擠出機(jī)在國(guó)內(nèi)外得到了突飛猛進(jìn)的發(fā)展，其產(chǎn)量、扭矩和轉(zhuǎn)速大幅度地提高，應(yīng)用也日益廣泛。目前，我國(guó)生產(chǎn)的同向平行雙螺桿擠出機(jī)多為中小型機(jī)，螺桿直徑約在φ25～φ100之間，國(guó)內(nèi)所需的大型擠出機(jī)全部依賴于進(jìn)口。國(guó)內(nèi)大部分廠家生產(chǎn)的雙螺桿擠出機(jī)總體性能較低，應(yīng)用范圍較窄，在很多行業(yè)的應(yīng)用領(lǐng)域還尚未開拓；對(duì)雙螺桿擠出機(jī)理的研究才剛起步，在脫揮理論的研究方面還處于空白狀態(tài)；在雙螺桿的設(shè)計(jì)、計(jì)算以及制造技術(shù)方面仍有不少難點(diǎn)需要克服，這些都制約了我國(guó)雙螺桿擠出技術(shù)與裝備的發(fā)展。鑒此，我們結(jié)合國(guó)家“九五”重大技術(shù)裝備攻關(guān)項(xiàng)目——“高效多功能雙螺桿擠出造粒機(jī)組”的研究，對(duì)開發(fā)研制中的技術(shù)難點(diǎn)和脫揮理論進(jìn)行了重點(diǎn)研究。
本課題以國(guó)外第六代雙螺桿擠出機(jī)的水平為趕超目標(biāo)，圍繞雙螺桿擠出機(jī)高的生產(chǎn)效率、強(qiáng)的混煉塑化能力、良好的操作穩(wěn)定性、低的能耗以及多功能、多用途等六項(xiàng)主要技術(shù)性能指標(biāo)，開發(fā)研制成功了國(guó)內(nèi)第一臺(tái)φ30高效多功能雙螺桿擠出造粒機(jī)組。在高扭矩、高轉(zhuǎn)速雙螺桿專用傳動(dòng)裝置、高粘度齒輪計(jì)量泵、組合式推力軸承裝置的設(shè)計(jì)與制造以及激光測(cè)試技術(shù)等方面取得了關(guān)鍵性的突破；完善了雙螺桿擠出機(jī)的核心部件——螺紋元件的幾何造型程序，實(shí)現(xiàn)了三維模型的參數(shù)化設(shè)計(jì)，圖形精度和計(jì)算速度有顯著的提高，程序的適用性和可操作性大為增強(qiáng)。這些都可為大型同向旋轉(zhuǎn)雙螺桿擠出機(jī)的國(guó)產(chǎn)化以及多功能雙螺桿擠出機(jī)的系列化開發(fā)提供經(jīng)驗(yàn)。
在廣泛查閱國(guó)內(nèi)外有關(guān)聚合物的脫揮機(jī)理以及各種螺桿擠出脫揮理論和實(shí)驗(yàn)研究的文獻(xiàn)基礎(chǔ)上，對(duì)同向旋轉(zhuǎn)雙螺桿擠出機(jī)的脫揮機(jī)理進(jìn)行了深入研究。通過分析物料在擠出過程中低分子揮發(fā)分氣體形成和排出過程，研究了加入物料物性、揮發(fā)分含量、擠出工藝條件、螺紋元件幾何參數(shù)和組合形式對(duì)氣體揮發(fā)物脫揮效率的影響，分析了氣泡形成條件和加速氣泡破裂的各種因素。
借鑒靜止液體中氣泡均勻成核和非均勻成核速率表達(dá)式，提出了在一定的溫度和螺桿轉(zhuǎn)速下同向旋轉(zhuǎn)雙螺桿擠出機(jī)中的氣泡成核速率表達(dá)式，并由實(shí)驗(yàn)得出模型中含有的相關(guān)參數(shù)。在此基礎(chǔ)上，結(jié)合物料在同向旋轉(zhuǎn)雙螺桿擠出機(jī)中的流動(dòng)特性，利用結(jié)構(gòu)微發(fā)泡中氣泡長(zhǎng)大模型，建立了同向旋轉(zhuǎn)雙螺桿擠出機(jī)的起泡控制脫揮和擴(kuò)散控制脫揮的有限膜物理和數(shù)學(xué)模型。采用國(guó)際上先進(jìn)的大型商業(yè)有限元軟件ANSYS對(duì)非牛頓流體三維流動(dòng)進(jìn)行了數(shù)值計(jì)算，得到了物料在雙螺桿擠出機(jī)內(nèi)的壓力、速度和粘度分布以及螺桿特性曲線，求得了物料在雙螺桿擠出機(jī)內(nèi)的有效充滿長(zhǎng)度、用于脫揮的未充滿段長(zhǎng)度和脫揮傳質(zhì)面積。這些數(shù)據(jù)的獲得對(duì)脫揮效率的計(jì)算是必不可少的。
為了驗(yàn)證本研究所建立的同向旋轉(zhuǎn)雙螺桿擠出機(jī)脫揮問題的物理、數(shù)學(xué)模型和理論計(jì)算值的正確性，我們進(jìn)行了一系列實(shí)驗(yàn)。在此基礎(chǔ)上用回歸分析法求得了起泡脫揮時(shí)氣泡群密度計(jì)算公式中的若干經(jīng)驗(yàn)參數(shù)。通過對(duì)實(shí)驗(yàn)數(shù)據(jù)的整理分析，得到如下結(jié)論：
1、主螺桿轉(zhuǎn)速、加料量以及機(jī)筒設(shè)定溫度是雙螺桿擠出機(jī)中脫揮過程的主要影響因素。這些因素的變化又會(huì)影響物料溫度、螺槽充滿度、停留時(shí)間以及有效充滿長(zhǎng)度，從而多方面地影響脫揮。對(duì)于特定的工藝，有一個(gè)最佳工作點(diǎn)，在穩(wěn)定工作的情況下，可以得到最高的脫揮效率。
2、提高螺桿轉(zhuǎn)速，有利于氣泡形成、長(zhǎng)大和破裂，有利于降低物料在螺槽中的充滿長(zhǎng)度、增強(qiáng)物料質(zhì)量傳遞表面的更新作用，可以提高脫揮效率；但過高的轉(zhuǎn)速，使物料在脫揮段的停留時(shí)間急驟減少，脫揮效率反而下降。
3、適當(dāng)降低喂料量可以減少排氣段的充滿率，使脫揮效率提高；但過低的喂料量不僅使擠出量減少并產(chǎn)生波動(dòng)，而且由于充滿率太低，不足以形成熔池，使起泡脫揮效率下降，因此喂料量必須適中。
4、提高熔體溫度，使擴(kuò)散系數(shù)和Henry常數(shù)增大，熔體粘度下降，有利于脫揮過程的進(jìn)行。
5、增加物料在脫揮段的停留時(shí)間、增加脫揮段長(zhǎng)度可以提高脫揮效率，為此在螺桿結(jié)構(gòu)設(shè)計(jì)中可以考慮增加排氣段長(zhǎng)度和采用多階排氣。
綜上所述，φ30新型高效多功能雙螺桿擠出造粒機(jī)組的開發(fā)研究成功，既可為我國(guó)聚合物混合、改性造粒工藝提供最新的裝備，又可為后續(xù)大型雙螺桿造粒機(jī)組的開發(fā)研制在技術(shù)和理論兩方面積累經(jīng)驗(yàn)。實(shí)驗(yàn)結(jié)果還表明，本研究建立的同向旋轉(zhuǎn)雙螺桿擠出機(jī)起泡控制和擴(kuò)散控制脫揮模型應(yīng)用于生產(chǎn)實(shí)際是可行的。它對(duì)聚合物生產(chǎn)過程的下游工序如清洗、凝固、脫揮、擠壓脫水、干燥和造粒以及反應(yīng)擠出等工藝參數(shù)選擇、大型多階排氣式雙螺桿擠出機(jī)的開發(fā)研究有重要參考價(jià)值。
During the past few years, the co-rotating twin-screw extruder has achieved great development. Its throughput, torque and screw speed has been increased to a higher lever, and its application has been extended. The co-rotating twin-screw extruders made in our country are middle or small size machines, whose diameters are from 25mm to 100mm. The large-scale twin-screw extruders are all imported. The performance of most homemade extruders is low, and applications are narrow. The study of twin-screw extruding mechanism in China has just started, and the research on devolatilization theory is still blank. There are a lot of technical difficulties in twin-screw design, calculation and manufacture to be overcome. All these problems hamper the development of twin-screw extruding technology and devices of our country. So based on"National Nine-Five Plan"subject—the development of high performance, multifunction twin-screw extruder, we studied some technical difficulties and emphasized on research of devolatilization theory for co-rotating twin-screw extruder.
Aimed at the level of the international 6〓 generation co-rotating twin-screw extruder, featured in high efficiency, strong mixing & plasticating capacity, fine stability, multifunction and low consumption, we successfully developed the first homemade high performance, multifunction φ 30 co-rotating twin-screw pelletizing line. In many areas, such as high torque, high screw speed twin-screw gearbox, high viscosity gear metering pump, series thrust bearings design, manufacture and laser testing technology, we have gained great breakthrough. We consummated the geometrymodeling program of heart part of twin screw extruder—the screw element, and realized its 3D parametric design. The graphic precision and compute speed has been remarkably improved. The applicability and operability of the program have been also enhanced. All these establish valuable foundation for localization of large-scale co-rotating twinscrew extruder and development of a series of multifunction co-rotating twin-screw extruders.
Based on the analysis of references about polymer devolatilization mechanism and all kinds of screw extruders'devolatilization theory and experiments, the devolatilization mechanism of co-rotating twin-screw extruder was deeply studied. By analyzing the forming and discharging of volatile gas during extrusion, the effects of material properties, volatile content, operation parameters, screw element's geometry parameters and combination on devolatilization efficiency were discussed. The conditions for bubble nucleation and the causes for accelerating bubble rupture were qualitatively analyzed.
Referring to the expressions of bubble homogeneous and heterogeneous nucleation rate within a quiescent liquid, we propose an expression of bubble nucleation rate of corotating twin-screw extruder at limited temperature and screw speed, and get relative parameters in the expression via experiment. Integrating with the flow characteristics of co-rotating twin-screw extruding, referring to bubble growth model in microstructure foam, we built the physical and mathematical models of foam-controlled devolatilization and diffusion-controlled devolatilization. We have done extensive 3Dflow analysis of non-Newtonian fluid in screw element by using commercial finited element analysis software—ANSYS. Then we got the distribution of pressure, velocity, viscosity of the filled material in co-rotating twin-screw channels and the characteristic curve of the screw element. Finally we calculated the filled length, unfilled length for devolatilization and area for mass transfer in extruder. All these data are necessary for calculation of devolatilization efficiency.
In order to test and verify the correctness and the reasonableness of the physical model and the mathematics model, a series of experiments have been done. Based on the experiment data collection, we regress the parameters in the bubble density expression. By the experiment data arranging and analyzing, the following conclusions can be obtained:
1. The screw speed, throughput and temperature settings of barrel are the main factors that affect the devolatilization efficiency. They affect the devolatilization efficiency through changing the temperature of material, the degree of filling, residence time and filled length of screw. These affections are various and interactive. To a specified process condition, there is an optimal working point, which can reach maximum devolatilization efficiency at stable working condition.
2. Screw speed increase makes for bubble nucleation, growth and rupture, and reduces the degree of filling, increases polymer surface renewal, thus enhances devolatilization efficiency. But overhigh screw speed sharply reduces the residence time, which decreases the devolatilization efficiency.
3. Adequacy feeding rate decrease enhances devolatilization efficiency by reducing the degree of filling in venting zone. But overlow feeding rate will reduce throughput and cause fluctuation. Because of overlow fillage, the rolling pool of polymer cannot be formed, thus decreases devolatilization efficiency. So the feeding rate must be moderate.
4. The increase of material temperature gives rise to a decrease in melt viscosity, an increase in diffusion coefficient and Henry's law constant, thus enhances devolatilization efficiency.
5. As residence time and unfilled length for devolatilization increases, the devolatilization efficiency increases. Thus we can consider increasing devolatilization length and adopting multi-vent at screw structure design.
To summarize, the successful development of the high performance, multi-function φ 30 co-rotating twin-screw extruding line not only supplied a new generation device for polymer mixing and pelletizing process, but also founded the technical and theoretical basis for developing large-scale co-rotating twin-screw extruder. From the results of experiment, it can be seen that the foam-controlled and diffusion-controlled models we built are feasible. They can be used to help selecting of process parameters of flushing, degassing, dewatering etc. , and to developing large-scale twin-screw extruder with multi-stage vents.