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
文章来自于塑料产业论坛（www.plas2006.com）
英文原载plasticstechnology
廖家志翻译和整理
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