提高双螺杆挤出机共混性能的十大绝招

There are many techniques known to operators and plantengineers for increasing the performance of a twin-screw compounding extruder. Because of their informal nature,however, most of these methods do not appear in any equipment manuals. Nor arethey generally mentioned in textbooks and technical papers. But sometimesattention to several small details  canadd up to make a big difference in extruder performance. The tips described inthis paper fall into three broad categories: processing techniques,machine modifications, and maintenance procedures。

有很多提高双螺杆共混挤出机性能的方法，为操作人员和工厂工程师所熟知。但是，由于这些信息的性质，其中的许多方法没有出现在设备手册里。这些方法一般也不在教科书和科技论文中被提到。但是，关注一些小的细节会对增加挤出机性能大有裨益。本文描述的技巧分成三个大的类别：加工工艺、设备改造和维护程序。

1. THERMAL INSULATOR GASKETS 热绝缘垫片

The feed barrel section is almost always water cooled. But because this barrel isbolted flange to flange against barrel section 2, which is heated,heat iscontinuously transferred from the hotter to the cooler barrel  section. As a result, barrel #2 is often notable to maintain a high enough temperature. In a typical case, the operator hasa setpoint of 180 C for barrel 2 but can never get above 135 C because of heatloss to the feed barrel. The easiest solution is to install an insulator gasketbetween the flanges of the feed barrel and barrel 2. These gaskets aretypically 1 mm thick, and we recommend stacking two together for betterinsulating performance. There will still be some heat conduction through thebolts and the screw shafts, but not nearly as much as without the insulation.New extruders are often supplied with the insulator in place, but manyoperators don't realize its importance and throw it away when disassembling thebarrels. After several years the insulator gasket should be replaced, as thematerial tends to degrade and crumble.

机筒的喂料段几乎总是水冷的。但是，由于这段筒体是用法兰对法兰紧挨第二段筒体(加热的）锁定的，热量可以连续从温度高的筒体传导到冷的筒体。结果2筒体不能维持足够的温度。通常，操作人员在2#筒体设定180℃的温度，但是由于喂料段的热量损失，2段从来没有达到超过135℃的温度。最简单的方案就是喂料段与2段筒体法兰之间加一个隔热垫片。这个垫片一般是1mm厚。我们建议叠加两个垫片，可以具有更好的绝热效果。还有一些热传导通过螺栓和螺杆轴心损失掉，但是有没有隔热没有打的影响。新的挤出机常常提供恰当的绝热措施。但是许多操作人员没有意识到隔热的重要性而在拆卸机筒时把它们扔掉了。过了几年后，隔热垫片应该更换，因为这些材料有老化降解和皱缩的倾向。

2. SIDE STUFFING OF POWDERS 粉体侧喂料

Side stuffing is widely used for feeding fillers into twin-screw extruders.Many processors desire very high loadings of fillers that are oftenlow-bulk-density (fluffy) materials. The ultimateloading attainable is usuallylimited by two parameters:

A) Volumetric capacities of the side-stuffer and mainextruder screws.  The volumetric capacityis based on the free volume of the side-stuffer's   twin screws and of the main extruder screws,as well as the rpm of both screw sets. If material is test run through the sidestuffer unbolted from the extruder and discharging into a drum, it probablywill easily feed a high rate of material. But when the stuffer is attached tothe extruder, capacity is often limited by the amount of material the mainextruder screws can accept. Consequently, it is best to for the main extruderscrews to have flighted elements with a long pitch (long flight advance) at thestuffer location, extending 2D to 4D downstream of the stuffer. This is to keepthe melt in the extruder moving rapidly forward to allow the maximum freevolume for the filler to enter. If the screw design causes any'dam-up' of material downstream of thestuffer, it will severely limitthe amount of filler that can be fed.
B) Venting capacity to allow air to escape the extruder. The object of ventingis to allow air to escape easily, while preventing large amounts of filler frombeing lost out the vent. The best configuration is a top vent in the barrelimmediately upstream of the side stuffer. Sometimes a small half-slot ventinsert can also be used in the top of the side stuffer.
Here are some other factors to consider:
■feederdrop height: Ideally, the feeder should be positioned as closeas possible above the side stuffer to minimize the drop height. If a fluffymaterial is allowed to drop through air, it becomes aerated to the point whereits bulk density is significantly reduced. This can limit the throughput rateof the entire line.
■feederagitation type: Make sure the agitator in the filler feeder isn'taerating the material and reducing the bulk density. Many feeder manufacturershave special agitator designs for powders.
■make surestuffer hopper is vented: Along with the filler, thestuffer also introducesa lot of air into the extruder. An open top on the side-stuffer chute will takecare of venting. If you have a solid cover on the chute with a round stubup andflex connector to tbe feeder, it's important to also have a vent opening.
■aroundall hoppers/chutes to drain static electricity: Somematerials generate static electricity from friction. Static charge can causepowder to cling to the inside surfaces of hoppers and chutes, leading toproblems such as caking. An easy fix is to run a ground wire (10 gauge isrecommended) from the chute to a known good ground on the machine frame.

■compressed-airblaster: If caking persists, sometimes a special solutionis needed. Hopper vibrators can be used, but are tricky to size and mount. Analternative is a 'blaster' witb air-jet nozzles strategically placedwithin the wall of the chute to break up any cakes before they get too large.The air jets are connected to a solenoid valve, which is actuated by a repeatcycle timer. This lets you set both the blast period and dwell time betweenblasts. It's best to have a small air accumulator tank just upstream of thesolenoid valve to provide a sharp pulse of air.

侧喂料通常用于把填料喂进双螺杆挤出机。许多加工者需要做很高填充量的填料，这些填料往往又具有很低的堆积密度。最终的填充量受两个因素制约：

A)侧喂料的体积喂料能力和挤出机主螺杆的体积吃料能力。体积喂料能力取决于侧喂料机和主螺杆螺杆的自由体积，以及这两个螺杆的转速。如果一种材料在测试中可以不受到主螺杆的阻力而流到储料鼓中，那么这种材料喂料速度可以达到很好。如果侧喂料机直接挨着主螺杆连接，那么助剂螺杆可以接受的材料量将受到限制。因此，主机螺杆最好有长螺距的螺纹元件，在侧喂料口下游延伸2D-4D长度。这将允许熔体快速通过侧喂料口，使填料最大限度地被主螺杆吃进。如果主机螺杆设计引起任何侧喂料喂进材料的堆积，那么将严重限制填料的喂料量。

B)允许气体从挤出机排出的排气能力。排气的目的就是使空气容易逸出，同时还要保护填料不从排气口大量逸失。最好的配置是在侧喂料口的上游开置向上排气的排气口。有时，在侧喂料机的上部开一个小的半狭缝排气。还有一些其它因素要考虑：

---喂料机的落料高度。理想情况下，喂料螺杆要尽可能近地安装于侧喂料口顶部，以减小落料高度。如果某种蓬松的材料从空气中流过，那么它可能变得充满空气，从而使它在落下后假密度大大减小。

---喂料机搅拌类型。要确保填料喂料机中的搅拌器不使填料流态化。许多搅拌器制造商设计了特殊的搅拌器用于填料喂料。储料器会连同填料一起引进空气进入挤出机。一个顶部敞开的斜槽料斗有助于排气。假如你的斜槽喂料料斗有一个实心盖子并带一个圆管，并且与喂料器是柔性连接，那么重要的要有一个排气口。

---料斗斜槽要接地防静电。一些材料由于摩擦产生静电。静电引起粉体对料斗挂壁，从而导致粉体饼状化。一个简易的方案就是接地。

---压缩空气爆破器。如果料饼持续存在，常常需要一些特殊办法。可以使用料斗振动器，但是它们占空间，还需要安装。一个替代方案是安装一个带空气喷嘴的爆破器，安装在斜槽的恰当位置上，在料饼长大之前将其吹破。

3. HIGH-PRESSURE WATER PUMP高压水泵

It's well known that turbulentflow m a pipe will induce far greater heat transfer from the pipe wall thanwill laminar flow. Laminar flow occurs with low fluid velocities caused bylowpressure delivery. In a laminar-flow situation, a stable boundary layerdevelops, which acts just like an insulator between the main fluid flow and thepipe wall. 'Layers' of fluid slide over adjacent layers, withoutmixing and without removing much heat. With turbulent flow induced byhigh-pressure delivery, there is a high degree of transverse momentum exchange,which breaks up the boundary layer. As a result, the violent fluid motioncausesmuch greater heat transfer from the pipe wall to the fluid. The easiestway to induce turbulent flow in tbe barrel cooling bores is to increase tbedelivery pressure. Extruder cooling recirculation systems typically have supplypressures from 20 to 60 psi. To achieve turbulent flow, a pressure of approximately120 psi is desired. This can be attained relatively easily by changing the pumpin the recirculation system to a high-pressure type. Almost all extrudercooling system components (hoses and valves) are rated for at least 150 psi, so120 psi still allows some safety factor. The advantage will be apparentimmediately in a process that is highly exothermic. Temperature zone overridescan often be greatly reduced or even eliminated. A side benefit of turbulentflow is that it inhibits fouling of the cooling bores from scale buildup.

大家知道，水泵中的紊流比层流可以导致更多的管壁热交换。层流出现在低流速低压输送条件下。在层流情况下，建立了一个稳定的边界层，就象在主料流和管壁之间建立了一个绝热层。料流层沿着临近的料流层滑过，没有混合，没有带走更多的热量。高压传输中的紊流有高度的横向动量交换，它击破了边界层。结果剧烈的流体运动引起了管壁与流体之间大得多的热交换。挤出机冷却循环系统一般提供20-60psi的压力。为了达到紊流，最好达到120PSI的压力。把循环系统的水泵改成高压泵就行了。挤出机几乎所有的冷却系统部件（软管和阀）都要升级到150PSI,才能满足120PSI的安全系数。优点是在加工时马上排出大量的热。

4. ACID-FLUSH COOLING BORES用酸冲洗冷却水道

Water-cooled extruder barrels are heat exchangers, and like all other heatexchangers the coolant bores are subject to fouling from scale buildup. Mostoperators notice that the cooling performance of a new extruder is much betterthan an extruder after three or four years of running. This is because the newextruder barrel has smooth, shiny, fresh-drilled cooling bores. The oldermachine has a layer of crusty mineral deposits lining the bores, which act likean insulator. If left unchecked, scale buildup can lead to a much more seriousproblem. Eventually the cooling bores can become completely blocked so there iszero water flow. If this happens, the only solution is to remove the barrelsfrom the extruder and drill out all the cooling bores—a time-consumingprocedure.

水冷的挤出机机筒就是一个热交换器，象所有的热交换器一样，水道易于结垢而变坏。许多操作人员注意到，一个新挤出机的冷却性能要比使用三、四年的挤出机好很多。这是由于新挤出机机筒具有光滑的新钻的冷却水道。老机器有一层垢附着在水道内表面，象一个隔热层。如果年久失修，垢层会导致严重的问题。最终冷却水道被完全堵死而滴水不流。到了这个时候，唯一的办法就是从挤出机上摘掉机筒，拿出所有的冷却水道--很费时间的一个手续。

5. SWITCHTO SYNTHETIC GEAR OIL 切换到合成齿轮油

The gearbox is the heart of a twin-screw extruder. If it fails, it's likely tobe expensive to repair, and it may take quite awhile. There is one thingeveryone can do, easily, to avoid gearboxproblems: switch to synthetic gearoil. Synthetic oil is a huge advance in lubrication technology:
■It's more'slippery,' resulting in less friction
■Gears,bearings, and seals last longer.
■Thegearbox runs cooler and quieter.
■Syntheticoil doesn't lose viscosity from mechanical shear.
■It maintains higher viscosity at high temperatures.

■It improves the overall efficiency of the gearbox.

齿轮箱式双螺杆挤出机的心脏。如果它完蛋了，那要花很多钱维修，而且颇为费时。这时，我们能做的一个事情就是避免齿轮箱问题：切换到合成齿轮油。合成油在润滑上是一个巨大进步：

---更加滑腻，导致较小的摩擦；

---齿轮、轴承和密封将更持久；

---齿轮运转温度更低，更安静；

---合成油不因为机器剪切而损失粘度；

---它在高温下维持较高的粘度；

---改进综合齿轮效率

6、USEANTI-SEIZE LUBRICANTS（翻译略）

Most people in plastics are aware of the need for anti-seize lubricants.There are several different types: copper-based, molybdenum, lithium, etc. For something like a bolt thread, probably any type will work. Butfor extruder spline shafts the choice of anti-seize is much more critical.Screw elements fit onto the spline shafts with a very small gap tolerance, butlow-viscosity polymers tend to creep down between adjacent elements and getinto this gap. With time and heat, degraded polymer forms a high-strengthadhesive just like an epoxy.
If screws are left together in an extruder without being taken apartperiodically, they can be extremely difficult to remove from the shafts. Thisis why the choice of anti-seize is so important. Many types of anti-seizelubricant are acceptable for a short period of time, but over long periods athigh temperature, they degrade into a fine powder. This makes them a poorchoice for spline shafts, where you need the anti-seize to perform when you'reready to disassemble the screws months or years later. After testing all theanti-seize brands on the market over a period of many years, Leistritz has foundone that is ideal for this application: Fuchs Gleitmo 820, manufactured byFuchs Lubricants inGermany.Gleitmo 820 is a white grease containing high solids. It is very slippery andis rated for up to 1150 C. What makes it perfect for spline shafts is that itdoesn't degrade into a powder. When the screw elements are removed aftermonthsof use, the Gleitmo 820 is still a slippery grease. Fuchs Lubricants hasU.S.distributors and a website (fuchs.com) with technical information.

7. PURGE OFTEN 常常清洗机器

Purging is widely used as an easy way to clean out materials from the extruderand die and to avoid a time-consuming stripdown cleaning There are many typesof materials used for purging, both commercial purge products and 'homeengineered' purges. These materials generally fall into two categories:chemical purges and mechanical abrasive purges. Many combine both chemical andabrasive actions.Chemical purges work by attacking the plastic residue like asolvent. Many of them also incorporate a foaming action and require a 5-min'soak' in tbe barrel to allow the chemical action to work.

Mechanicalabrasive purges work by scouring and scrubbing material off the metal surfaces,using friction and shear. Processors do a lot of trial-and-errorexperimentation to find the right purge material and technique. No'magic' purge material will work best for all materials. Commercialpurge products are generally more expensive than 'home engineered'purges. But if you can use less material with the commercial purge, or get tbejob done quicker, the total cost may actually be less.

Processorshave run all sorts of materials through extruders, in the quest for a cheap andeffective purge material. For instance,in the U.S. Midwest many operators use cattle-feedcorn, because it's readily available and does a fairly good job of scouring outthe machine. Here are two effective homemade purges:
Mix #/; S0% HDPE (or another htgh-viscosity resin) 50%diatomaceous earth (DE): DE is finely milled fossil shells of tiny seaorganisms. It is readily available in garden supply stores. DE looks like afine powder, but the particles have very sharp, angular edges, which produce ascouring action. Processors who run sheet and film use this purge, as it cansometimes get rid of die lines, saving a lengthy die teardown.
*Mix #2; HDPE or other viscous resin with kitchen cleanser andwater: Mix a slurry of water and three or four cans of Comet or Ajax kitchen cleanser in abucket. Feed HDPE at a normal purging rate and temperature, and slowly pour theslurry into the main feed port. The water in the slurry will flash to steam,which gives a very good cleaning effect combined with the abrasive in thekitchen cleanser. (Safety note: Warn any operators nearby that steam maydischarge from extruder openings. Do not look into the die or vents unlesswearing a full polycarbonate face sbield andlong-sleeved shirt.) Probably themost important thing is to purge often. The object is to prevent old materialsfrom being 'baked on' the metal surfaces.Once this happens, purgingwill generally not be able to remove thematerial, and a manual stripdown cleaningwill be the only remedy. One mistake many processors make is to run the purgeonly at a fixed screw rpm. It's much more effective to vary the speed, changingfrom low to medium to high rpm, running for perhaps30 sec at each speed. Thisinduces different shear rates against the metal surfaces, helping to dislodgeold material from the walls.

清机是把用于把挤出机和口模内材料清除出来的简易而广泛的方法，它避免了耗时的拆机清洗。有很多材料可用于清机。既有商用的清机产品，也有自个儿特制的清机产品。这些材料常分成两类：化学清洗剂和机械摩擦清洗剂。许多清洗剂同时具有化学和摩擦功能。化学清洗剂象一种溶剂一样进攻塑料残渣。许多化学清洗剂还加入一种发泡功能，需要在机筒内“浸泡”5分钟，从而发挥化学作用。

机械摩擦清洗的原理是从金属表面摩擦和冲刷掉残留材料，使用的是摩擦和剪切的方式。加工者做了很多试错试验来找到合适的清洗材料和方法。没有什么“神奇”的材料对所有材料都管用的材料。商用清洗材料一般比自己制造的清洗材料贵。但是，假如你使用较少的商用清洗剂材料能更快把活干好，那么实际成本可能更小。

加工者需要廉价而有效地清洗材料。比如，在美国中西部许多操作者使用喂牛的玉米，因为它容易得到，对机器有好的清洁作用。下面介绍两种自己制造的有效地清洗剂：

---1#混合物：50%HDPE＋50%硅藻土

---2#混合物：HDPE＋水＋去污粉。将HDPE以通常速度在熔融温度下加入螺杆。将去污粉用水在桶里搅拌成浆液，从主喂料口慢慢加入浆液。这些浆液因为发泡而冲击、摩擦、清洗螺杆。

8. EFFICIENT CHANGEOVERS 有效切换

Color or material changeovers tend to create problems. Extrusion people wouldlike to always have nice long runs. But in the real world, demands fromcustomers mean frequent changeovers to make 80 Ib of Product A, followed by 200Ib of Product B, etc. So it is important to approach changeovers in a waytominimize operator effort and problems:
■Know howcritical is cleanliness to the next product to be run. There are anecdotesaround any extrusion shop of somebody purging for 16 hr to avoid pulling thescrews to brush them off. The point is, if you know the next product requires athorough manual brush cleaning,don't waste time and material trying to get bywith purging.
■Ifpossible, plan runs in order of decreasing cleanliness requirements. Forexample, this means going from light to dark colors. That way, you should faceonly one difficult changeover,when going from the dark color back to a lightcolor.
■Havewritten purging or strip-down procedures to run each product. This helps operatorsplan their time, and eliminates discussions later on about how many pounds ofproduct have to bescrapped because the operator didn't clean the machineproperly.

颜色和材料切换常弄出问题。搞挤出的人总是想把机器开得长一些时间。但是在现实中，由于客户的需求，我们常常需要不断切换，生产80磅产品A，然后再生产200磅的产品B，等等。重要的是要找到一种方法，减少操作人员的工作量和出现的问题:

---知道下一个产品的清机是如何重要。假如你知道下一个产品要求彻底的手工清机，那你就不要浪费时间和材料来糊弄清机。

---假如可能，要按照减少清机的顺序排产。这就意味着要从浅色做到深色。反之，你只有拆机。

---写出生产每个产品的清机或拆机的程序，这有助于操作人员计划时间，消除他们关于会产生多少机头料的争论。

---产品的批量要与机器大小相对应。这似乎是不言而喻的。但是许多加工者排产无计划，用75机生产500磅的单子，结果产生了300磅的废料。

---帮助操作人员，把清洁的操作工具摆放在挤出机周围，如果频繁换料，那就在挤出机周围搞一个工作台，把需要的东西都准备好。

---研究原材料和成品流进流出操作现场的顺序。安排好以使操作人员花费较少的精力在移动运输工具上。

9.WARM-UP/COOL DOWN 加热和冷却

Extrusion people tend to keep the machinery hot all the time. Operatorstypically turn the heat zones on the minute they come in the plant at 7:00 am.If you're really going to start the extruder at 8:00 am, there's nothing wrongwith this. But often people get distracted with maintenance chores, and theextruder sitsidle, heated to full operating temperature for hours at a time.This is a bad practice because it allows polymer to bake onto the screws andbarrels. Once baked on, the material will continue to degrade and carbonize.Purging will not remove this material. Even worse, as the extruder is runningproduct, black specks will periodically flake off the screws and barrels,contaminating the product and causing customer complaints. The best way tohandle warm-up is to plan for when you really want to start running theextruder. Then start heating up only as soon as will allow for complete warm-upplus adequate soak time. There are also preferred shutdown/cooldown proceduresto avoid problems with black specks. Before shutting the extruder down, it'sgood practice to fill the extruder and die to the maximum degree with someinert polymer such as HDPE to 'seal' the machine. The HDPE displacesoxygen, and coats the metal surfaces to greatly slow down oxidation andformation of carbon. After the extruder has been 'sealed,' it isbetter to crash cool the machine, rather than let it cool just from exposure toambient air over the next 18 hr. Crash cooling involves turning all thetemperature setpoints down to zero to force the cooling solenoid valves toopen. With the coolant pump running, this will force cool water through allbarrel sections, bringing the temperature down quickly. Quick cooldown doesn'tgive the polymer a chance to degrade and carbonize

开挤出机的喜欢一直把挤出机加热着。操作人员习惯早晨七点一到工厂就把挤出机开着。如果你是真的要在8：00点开机，那也倒没有什么错误。但是，人们常常对维修杂事搞得抓狂。现在挤出机又闲置在那里加热到满量程，一放几个小时。这是一个坏习惯，因为这样做将使聚合物粘附到螺杆和机筒上。一旦粘附上，这些聚合物材料将持续降解和炭化。清机弄不掉这些材料。更糟糕的是，一旦机器生产产品，黑点将周期性地从螺杆和机筒里跑出来，污染产品，引起客户抱怨。处理加热的最好办法是计划好你什么时候真正要操作挤出机，然后开始加热，只需温度完全到位并可机筒充料即可。也有更好的停机冷却程序，可以避免黑点问题。好的习惯是，在停机之前，最大限度地在挤出机里填充惰性聚合物，象HDPE，来密封机器的缝隙。HDPE赶走氧气，涂覆在金属表面，极大地降低了碳的氧化和形成。在挤出机被“密封”后，最好急冷挤出机，而不是放在空气中冷18个小时。所谓急冷，就是把所有温度设定到零，让冷却水阀全部打开。开着冷却水泵，这将使冷却水通过所有的筒体部位，将温度快速降下。快速降温将不给聚合物降解和炭化的机会。

文章来自于塑料产业论坛（www.plas2006.com）

英文原载plasticstechnology

廖家志翻译 

(转自艾邦高分子）