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别把挤出机零区不当回事儿  发帖心情 Post By:2010/12/13 11:49:00 [只看该作者]


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Know-How: Extrusion

<!-- InstanceEndEditable --><!-- InstanceBeginEditable name="ArticleTitle" -->Don’t Forget Zone ‘Zero’<!-- InstanceEndEditable -->
<!-- InstanceBeginEditable name="Synopsis" --><!-- InstanceEndEditable -->

By Jim Frankland

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Have you ever heard of zone zero? Do you know where it is? Well, it’s the feed throat. It’s the zone most extruder manufacturers do not treat as a barrel zone. But nothing goes through the extruder that doesn’t go through the feed throat first. Feed throat design and temperature control can have a lot to do with extruder output and energy efficiency.

In an earlier column I discussed feed throat design and how it can affect extruder performance. Now, let’s focus on the thermal aspects. The same solids-feeding principles that occur in the barrel immediately past the feed throat apply to the feed throat itself. That is, the polymer must stick to the barrel more than it does to the screw to move forward. The most effective thing for improving polymer-to-barrel friction is to get heat into the barrel wall. As I said, everything that goes through the extruder goes through the feed throat first, so this can be a controlling aspect of the process.

Over-cooling the feed throat can extract as much as 20% of the total drive power by pulling heat constantly from zone 1, where it’s critical to develop solids feeding, solids bed compaction, and initial melting.

As a processor, the only way you can maximize feed-throat performance is to adjust its temperature. That is not as easy as it sounds, because the only control typically supplied by the machine builder is a coolant flow valve, and often that valve is not designed for accurate flow control. Moreover, the coolant temperature and flow rate often change during the day as the demand or temperature changes inside or even outside the plant. Consequently the feed throat can change temperature with no adjustment by the operator.

You can solve this problem by adding a modulating valve as a flow control, which can be a controlled by a thermocouple attached to the feed throat. This does not optimize feed throat performance but does keep it at a fixed point.

Have your operators ever had a case where some polymer had melted in the feed throat during a prolonged screw stoppage while the barrel heaters were left at operating temperatures? Not wanting that to ever happen again, most operators run full coolant flow to the feed throat. They are not considering the feed throat as part of the process, but rather as a disconnected apparatus that has nothing to do with the process.

Nothing could be further from the truth. Not only is the feed rate established in the feed throat, but over-cooling the feed throat can extract as much as 20% of the total drive power by pulling heat constantly from zone 1. As you can see from Fig.1, the feed throat has a metal-to-metal connection that is very effective at transferring heat. Since the first zone thermocouple is some distance down the barrel from the feed throat, a gradient is developed between the zone 1 setting and the feed throat. That gradient further affects the feed rate by cooling the barrel in the location where it’s critical to develop solids feeding, solids-bed compaction, and heat to initiate melting.

Here, the barrel extends through a feed port jacket. The temperature at the barrel inner wall is maintained at a much higher level, as thermal conductivity through the barrel is much higher than from the barrel to the jacket and then to the coolant. Thus, the inner barrel wall is much hotter at the feed opening. This can add as much as 23% higher output with the same screw.

The point is further illustrated by the performance of extruders that do not use a separate feed throat—i.e., the barrel extends through a feed-port jacket. Such a design is shown in Fig. 2. The temperature at the barrel inner wall is maintained at a much higher level, as the thermal conductivity through the barrel is much higher than the conductivity from the barrel to the jacket and then to the coolant. As a result, the inner barrel wall is much hotter at the feed opening. In tests, this has accounted for as much as 23% increased output with the same screw. The main disadvantage to the jacket design is that the polymer can melt in the feed opening and on the screw much quicker, requiring more operator attention when the screw is stopped. However it shows the positive effect on output of a hotter feed throat. I have also found that extruders with this design generally have more stable output.

For most polymers, I like to start with a feed throat that is hot to the touch (110 to 120 F), and gradually reduce the coolant flow, thereby increasing the feed throat temperature until I see no increase in output. This takes a while, as the feed throat is slow to change temperature because of its mass and the transfer of heat to the gearbox, hopper, and resin. Each polymer, extruder, and operating condition will require a slightly different throat temperature to optimize output—but hotter is usually better.

About the Author
Jim Frankland is a mechanical engineer who has been involved in all types of extrusion processing for more than 40 years. He is now president of Frankland Plastics Consulting, LLC. Contact jim.frankland@comcast.net or (724)651-9196.

<!-- InstanceEndEditable -->


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  发帖心情 Post By:2010/12/13 12:08:00 [只看该作者]

Have you ever heard of zone zero? Do you know where it is? Well, it’s the feed throat. It’s the zone most extruder manufacturers do not treat as a barrel zone. But nothing goes through the extruder that doesn’t go through the feed throat first. Feed throat design and temperature control can have a lot to do with extruder output and energy efficiency.

 

你听说过“零区”吗?你知道它在哪里吗?它就是喂料口,许多挤出机制造商不把它当做机筒区。但是,不先通过喂料口,就没有什么东西通过挤出机。喂料口(管)设计和温度控制与挤出机产率和能量效率有很大关系。

In an earlier column I discussed feed throat design and how it can affect extruder performance. Now, let’s focus on the thermal aspects. The same solids-feeding principles that occur in the barrel immediately past the feed throat apply to the feed throat itself. That is, the polymer must stick to the barrel more than it does to the screw to move forward. The most effective thing for improving polymer-to-barrel friction is to get heat into the barrel wall. As I said, everything that goes through the extruder goes through the feed throat first, so this can be a controlling aspect of the process.

 

在早期的专栏里,我讨论过喂料口(管)设计,以及它是如何影响挤出机的性能的。现在,让我们把注意力集中到热量方面。喂料口之后出现的固体喂料原理同样也适用于喂料口(管)本身。也就是说,聚合物必需粘着于机筒而不是螺杆才能向前移动。改进聚合物对筒壁摩擦的最有效方法将筒壁加热。如我所说,任何通过挤出机的东西都要先通过喂料口(管),所以这是工艺控制的一个方面。


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  发帖心情 Post By:2010/12/13 12:22:00 [只看该作者]

As a processor, the only way you can maximize feed-throat performance is to adjust its temperature. That is not as easy as it sounds, because the only control typically supplied by the machine builder is a coolant flow valve, and often that valve is not designed for accurate flow control. Moreover, the coolant temperature and flow rate often change during the day as the demand or temperature changes inside or even outside the plant. Consequently the feed throat can change temperature with no adjustment by the operator.

 

作为一个加工者,优化喂料口的唯一方法就是调整它的温度。这可不像听起来那么容易,因为设备制造者在这方面的唯一控制就是冷却水阀,而且这种阀控制常常涉及得不够精确。还有,冷却水的温度和流动速率常常随着工厂内部乃至外部的温度变化而几天一变。所以,喂料口(管)的温度改变常常不是来源于操作者自己。

 

You can solve this problem by adding a modulating valve as a flow control, which can be a controlled by a thermocouple attached to the feed throat. This does not optimize feed throat performance but does keep it at a fixed point.

 

你可以加一个调节阀作为流体控制的手段来解决这个问题,你在喂料口上加一个热电偶就可以了。这样做不会优化喂料口性能,但是的确可以把它确定到某个状态。


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  发帖心情 Post By:2010/12/13 12:49:00 [只看该作者]

Have your operators ever had a case where some polymer had melted in the feed throat during a prolonged screw stoppage while the barrel heaters were left at operating temperatures? Not wanting that to ever happen again, most operators run full coolant flow to the feed throat. They are not considering the feed throat as part of the process, but rather as a disconnected apparatus that has nothing to do with the process.

 

你的操作人员遇到过这种情况吧:某个聚合物由于螺杆停机太久而在喂料口熔融了,这时机筒加热圈仍然处于加工温度。不希望再次出现这种情况,许多操作者就把喂料口的冷却水打满。他们没有把喂料口当做工艺的一部分,而是作为一个与工艺无关的孤立部件。

 

Nothing could be further from the truth. Not only is the feed rate established in the feed throat, but over-cooling the feed throat can extract as much as 20% of the total drive power by pulling heat constantly from zone 1. As you can see from Fig.1, the feed throat has a metal-to-metal connection that is very effective at transferring heat. Since the first zone thermocouple is some distance down the barrel from the feed throat, a gradient is developed between the zone 1 setting and the feed throat. That gradient further affects the feed rate by cooling the barrel in the location where it’s critical to develop solids feeding, solids-bed compaction, and heat to initiate melting.

 

没有什么比这更不靠谱的了。不仅是在喂料口建立了喂料速率,使喂料口处于过冷状态还多消耗了总驱动能耗的20%,这是通过在一区恒定吸热实现的。就象你在图1所见的那样,喂料口有一个金属对金属的连接,它可以有效转移热量。由于一区的热电偶在喂料口下游的某个距离,在一区温度设定和喂料口温度之间就建立了一个温度梯度。通过在某个地方冷却筒体,而这个地方是建立固体喂料、固体床,进而开始熔融的地方,这个梯度就这样进一步影响喂料速率。

 


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  发帖心情 Post By:2010/12/15 11:36:00 [只看该作者]

资料和翻译都很棒,赞一个,辛苦了!


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  发帖心情 Post By:2010/12/16 16:02:00 [只看该作者]

学了不少知识,好!

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  发帖心情 Post By:2010/12/17 1:30:00 [只看该作者]

喂料区到底是高温好,还是低温好?

 

上文作者说喂料区热一些好,可以提高产量,因为树脂的前进动力还是粘着于机筒。另外,喂料区温度低了,会导致从一区吸热,损失热量。

 

但是,我们经常担心,喂料区的温度高了,会导致螺杆上部堆积的料液熔融了,这些熔融的料粘成一团而无法象固体粒子那样自由落下到螺杆的槽里,这样就会在螺杆表面形成一个熔体包覆层,使进料无法实现。我观察到的事实怎么与作者矛盾呢?

 

 


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  发帖心情 Post By:2010/12/20 19:00:00 [只看该作者]

用户已被锁定

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  发帖心情 Post By:2012/6/1 15:49:00 [只看该作者]

人家说进料区要热点,也没有说要热到多少度。还是要看加工材料。据说加工聚丁烯材料,进料口就需要热点,使得螺杆更容易被抱紧。当然也不可能太热。要不然,螺杆都转不动。听说而已,没有亲见。请指正。

 

廖总和denny观察也是没有错的。 要不然我们就不用在进料口通冷却水了。


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  发帖心情 Post By:2012/6/1 19:19:00 [只看该作者]

尤其是加TPP的时候,一二区温度都关闭的

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