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主题:在双螺杆共混中优化喂料赚更多的钱

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Conversely, materials with a high energy requirement to melt may be limited in rate by the amount of power that the motor and gearbox combination is able to supply. Limitations in the downstream equipment include high discharge pressure in the case of high-viscosity materials and restrictive dies, and high pellet temperature in the case of inadequately sized cooling equipment. Only once the limitation is understood can steps be taken to remove it. This process is one of continuous improvement, where if one limitation is removed, another will be uncovered at a higher throughput rate. 

 

相反,具有高能耗要求才能熔融的材料,可能受限于马达和齿轮箱组合所能提供的动力能力。下游设备的限制包括,在高粘材料和限制性口模条件下的高的加料压力,在设备冷却能力不足条件下的造粒温度。只有这些限制被理解了,才能采取措施先出它们。这个过程是连续改进的一个过程,其中,当一个限制被消除了,在更高产量下的另一个限制才能被发现。



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FEED SYSTEM

喂料系统


To achieve the highest throughput rates, feeding equipment must deliver an accurate and consistent feed to the extruder. To do this, the feed system must be designed according to the materials and throughput rates being fed (see Fig. 1). Easily fluidizing powders are handled well by a feeder with twin, narrow-pitch, concave augers(螺旋输送器,螺旋喂料器), as the smaller clearances and longer path to the discharge abate the fluidization behavior. On the other hand, solid polymer pellets may become impinged in the tight clearances between these screws, so pellets benefit from being fed with a coarse-pitch, single auger. Other materials, such as wet or cohesive aggregates, tend to be best suited for a belt feeder rather than a screw feeder. Consult your feeder OEMs for their specific recommendations.

 

为了达到最高的产率,喂料设备必须精准和持续地对挤出机进行喂料。为此,喂料系统根据材料和喂料产率(见图1)进行设计。易于流态化的粉末要处理好,就要使用双螺旋、窄螺距、凹式喂料器,因为较小的间隙和较长的卸料路径将会减轻流态化行为。另一方面,固体聚合物粒子会在螺纹之间的紧密间隙中会受到妨碍,所以,采用粗螺距、单螺旋喂料器,会有利于颗粒料的喂进。别的材料,诸如湿的或有粘着力的团聚体,适用于带式喂料器而不是螺旋喂料器。请咨询你的喂料器主机厂,寻求具体的推荐产品。


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For materials that tend to bridge, rat hole, or stick to the hopper, agitation is needed to keep the product flowing consistently. Classically this has been done by some type of a mechanical stirrer. One downside of this method is that it can compact sticky materials, and increase the cleaning time when changing over to different materials. An alternative to mechanical stirring is the ActiFlow system, which works through an intelligent vibratory drive attached to the outside of the feed hopper. It continuously optimizes frequency and amplitude based on changing material flow conditions and provides feedback to the feeder controller to remove the vibratory noise from the load-cell signal. 

 

对于易于架桥、形成鼠洞或粘附料斗的材料,需要搅拌装置使得产品持续流动。通常是用一些类型的机械搅拌器完成这个工作。这种方法的一个副作用是,它会把粘性材料压紧了,当切换不同不同材料时,增加了清扫时间。替代机械搅拌的一个设备是ActiFlow系统,它通过一个智能驱动装置工作,附着在喂料料斗的外侧。该装置基于变化的材料流动情况,连续优化频率和振幅,提供反馈给喂料控制器,以消除来自于喂料传感器信号的震动噪音。


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In addition to selecting feeding equipment to best suit the materials, take time to consider feeder control systems. One example is the automatic hopper-refill control. If the hopper is refilled too frequently, the controller spends less time measuring the loss in weight over time, which can reduce the accuracy of the feeder. On the other hand, if the level in the hopper is allowed to drop too low, the sudden pressure of material dropping into the hopper during a refill may cause a surge of material to slip through the screw, especially for low-bulk-density and easily fluidizing materials. When this surge of material falls into the extruder, it will cause a momentary increase in load on the motor, manifesting itself as an increase in torque on the extruder control screen. These torque spikes cannot exceed 100% of the extruder’s available torque or else safety interlocks will shut down the extruder to avoid an overload condition. As a result, the normal operating torque of the process must be kept lower to create room for these potential spikes.

 

除了选用最适于材料的喂料设备外,还要花时间考虑喂料控制系统。一个例子是自动料斗-补充控制。如果料斗补料过于频繁,那么,控制器就会花较少的时间测量单位时间内减少的重量,这将减少喂料器的精度。另一方面,假如料斗被允许下料太慢,在补料过程中落入料斗的材料的突然增压会引起材料波动导入螺旋,对于低的堆密度和易于流态化的材料,尤其是这样。当这种材料波动导入挤出机时,它会引起马达的负荷的瞬时增加,挤出机控制显示屏上的扭矩增加就是明证。这种扭矩陡升不可能超过100%的挤出机容许扭矩,否则互锁装置将会停掉挤出机以避免过载。因此,标准的加工运行扭矩必须保持较低,从而为这些潜在的陡升留有空间。


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The increase in profitability with feed-system optimization can be calculated by the increased productivity. An example of this calculation is shown in Table 1 for a 70-mm twin-screw extruder processing a polyolefin masterbatch with a profit margin of 10¢/kg. In this example, a feeder that was experiencing surging during refill was replaced with one better suited for the material, allowing the throughput to be increased from 2000 kg/hr to 2300 kg/hr (4409 to 5070 lb/hr). This results in a possible $180,000 in additional annual profit, more than four times the cost of the new feeder. Additionally, since the degree of fill in the twin-screw extruder increases, specific mechanical energy input to the material decreases. This results in an energy savings of 5% or 138 MWh/yr

 

因为喂料系统优化而增加的获利可以通过增加的产率加以计算。这种计算的一个例子示于表1,其中采用的是一个双螺杆挤出机加工一种聚烯烃母粒,利润幅度在10¢/kg。在这个例子中,一个在补料中存在波动的喂料机被替代以一种最适应材料的喂料机,允许产量从2000 kg/hr增加到 2300 kg/hr (4409 to 5070 lb/hr)。这产生了一个可能$180,000的额外年度利润,超过新喂料机成本的四倍。此外,由于挤出机充满度增加,材料的比机械能投入减少。这导致节能5%或138 MWh/年。


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FEED INTAKE

喂料进料


After the feed system delivers material to the extruder, it must be conveyed downstream in the feed intake zone. A limit to the throughput rate occurs when the intake zone has insufficient capacity to convey the materials. The conveying capacity of the feed intake zone is determined by the free area of the twin screw and the pitch of the screw elements in this zone, along with screw speed and a few product-related parameters.

 

在喂料系统将材料输送到挤出机后,材料必须输送到下游的喂料进料区。当喂料进料区没有足够的空间输送材料时,就会出现对产率的限制。决定喂料进料区的输送能力的因素包括,螺纹的自由体积,这个区域的螺纹元件的螺距,连同螺杆速度以及一些与产品相关的参数。


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The free cross-sectional area of the extruder is fixed in a standard twin-screw extruder. Extruders with a larger outer-diameter to inner-diameter ratio will have more free area. It is important to consider this when procuring a new extruder. The product parameters are tied to the selection of raw materials. Changes are often complicated for purchasing or product- quality reasons.

 

在一个标准双螺杆挤出机中,挤出机的自由截面面积时固定的。具有更大的外径/内径比的挤出机,将具有更多的自由体积。重要的是,当生产一个新的挤出机时,要考虑这一点。产品参数与原材料选择有关。因为采购或产品质量原因,变化常常变得复杂。


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Therefore, the parameters that can be optimized on an existing machine are the pitch of the screw elements and the screw speed. Increasing screw speed will typically result in a higher energy input to the material. This increase in energy can be partially offset by an increase in feed rate. However, this increase tends to trail off(变弱,变小) at increasing screw speeds. This results in a higher melt temperature, which among other downsides could cause issues in pelletizing or in product quality.

 

因此,在现有机器上可以优化的参数是螺纹元件的螺距和螺杆速度。增加螺杆速度一般会对材料产生一个较高的能量输入。这种能量增加会部分地被喂料速率增加而抵消。但是,在增加螺杆速度时,这种增加会趋于变小。这导致了一个较高的熔体温度,连同别的副作用,这可能会在切粒或产品质量上造成问题。


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Increasing the pitch of the screw elements in the feed intake zone is the most innocuous (无害的)change. As a rule of thumb, it is recommended to always start with the widest-pitch screw elements in the feed intake zone. Feed intake limitations can result when feeding powders, especially low-bulk-density or easily fluidizing ones, due to air becoming entrained as the powders drop from the feeder into the extruder.

 

增加喂料进料区的螺纹间距是最无害的改变。一般来说,我们推荐在喂料进料区的起始部分,总是使用最宽的螺距。当喂入粉末时,特别是低堆密度或易于流态化的粉末时,会产生喂料进料限制,因为当粉末从喂料机落入挤出机时,会存在困气。

 

In typical polymer processes, there is a melt zone comprised of a section of kneading blocks fully filled by polymer. Since this section of the extruder is fully filled, the entrained air cannot move downstream past the melt seal with the molten polymer. Instead, it is forced to escape through the nearest upstream opening, which is usually the feed opening (Fig. 2). As a result, there is a competing flow of the raw materials entering and the entrained air exiting.

 

在典型的聚合物加工中,有一个熔融区,包含一段充满聚合物的捏合块。由于这段挤出机被完全充满,困住的空气移动到下游通过具有熔融聚合物的熔体密封层。相反,它被迫通过最近的上游开口逸出(图2)。结果,在进入的原材料与存在的困气之间存在一个竞争性的流动。

 

One way to reduce the amount of entrained air is to place the powder feeder on the same vertical level as the extruder, as close to the feed hopper as possible. This minimizes the drop and reduces the amount of air that gets entrained.

 

减少困气量的一个方法是,将粉末喂料器置于与挤出机同样的竖直水平,尽可能地接近喂料料斗。这样将缩小落差,减少空气受困数量。


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Another strategy is to design vents into the system to give the air alternate escape routes. A vent can be installed in the feed hopper itself. To increase the effect, the hopper should be designed so that the feeder discharges material down the side of the hopper on the same side as the down-turning screw in the twin-screw pair, with the vent stack on the other side of the hopper. This allows the air to flow away from the polymer on its way down to the extruder (Fig. 3).

 

另一个策略是,设计排气口,通入体系中,给空气以替代性的逸出路径。一个排气口可以安装到喂料料斗本身上。为了增加效果,料斗应该这样设计,以便喂料器把物料卸入到料斗底部一侧,这一位于双螺杆对的下旋螺杆的同一侧,排气口架在料斗的另一侧。这允许空气与流到挤出机的的聚合物分开流动(图3)。

 

The other location where a vent can be installed is in the twin- screw, upstream of the feed barrel. The entrained air is easily able to flow backwards in the twin-screw to the upstream vent barrel, while the solid powder is conveyed by the screws downstream.

 

安装排气口的另一个位置是在双螺杆挤出机中,位于喂料机筒的上游。困住的空气可以轻易在螺杆中往后流回到机筒的上游排气口。而固体粉末被螺杆输送到下游。

These three strategies can be employed individually or in parallel with each other.

 

这三种策略可以单独使用或彼此平行使用。


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