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标题:如何消除螺杆挤出中的凝胶?

1楼
上海北京顺德 发表于:2010/6/2 11:50:00

 

 

Stop Those Gels!消除凝胶!

By Paul Waller President
Plastics Touchpoint Group Inc.

 

Gels are an unsightly but common problem in single-screw extrusion. They can be large or small, hollow or solid, clear or discolored. They appear in sudden “showers,” discrete stripes, or randomly dispersed. By identifying the specific type of gel and circumstances of its appearance, you stand a much better chance of tracking down and fixing the cause—whether it stems from problems in raw material, processing, or equipment.

 

凝胶面目可憎,但是在单螺杆挤出机中却是常见问题。它们或大或小,或空心或实心,或透明或变色。它们出现时如同骤然而至的“阵雨”,或是离散的条块,或无规分散的形态。通过甄别特定的凝胶类型和其出现的情形,我们可以追溯和确定其原因--不论它是由于原料、加工问题还是设备问题。

 

Know your gel types弄清凝胶的类型

Platelets, fisheyes, or disc-shaped gels are round and thickest at the center. Cut them open, and you won’t find any discoloration. These types of gels come from two sources: crosslinking and unmelts. Both crosslinked and unmelted gels can get through screen packs and disrupt polymer flow farther downstream.

 

片状、鱼眼状或盘状凝胶呈现为圆形,中间部分厚。把凝胶切开,不会发现任何变色。这些凝胶有两个来源:交联和不熔。交联和不熔凝胶都能通过过滤网并在下游扰乱聚合物的流动。

 

Crosslinked gels are formed by overheating the resin. They melt at a higher temperature than the surrounding resin. Crosslinked gels can come from raw-material contamination or use of too much reclaim. Unmelted gels have the same melting point as the surrounding resin, but remained colder than the average melt temperature. Causes include heating too quickly in the feed zone, melting too slowly in the transition zone, insufficient mixing, and excessive heat loss in the adapters or die. The preferred solution is to increase mixing time (recirculation flow in the metering section of the screw) by adding screens to raise backpressure.

 

交联凝胶是由过分加热树脂形成的。它们的熔融温度比周围的树脂高。交联凝胶可能来源于原材料污染或使用了太多的回料。不熔凝胶与周围树脂有同样的熔点,但是受热温度比平均温度低。其原因包括在喂料区加热时间太短和在过渡区熔融太慢、混合不良、在机筒和口模中存在过多的热量损失。较好的方案是通过添加滤网提高背压而增加混合时间(在计量区进行料流的再次循环加工)。

[此贴子已经被作者于2010-6-2 12:37:48编辑过]
2楼
上海北京顺德 发表于:2010/6/2 11:51:00
图片点击可在新窗口打开查看
Large, hollow gels, or lensing, are caused by moisture trapped in the melt. The answer is to pay more attention to resin storage and drying.

Another strategy for eliminating unmelts is to use what is commonly referred to as a “humped” temperature profile. First, lower the feed-zone temperature, which will result in more intensive friction in the transition zone. Screw motor amperage will usually increase. Average melt temperature may increase noticeably, so watch out for crosslinked, discolored, or burnt gels if friction is too severe. You may also have to lower the last barrel zone to cool the melt.

 

消除不熔凝胶的另一个方案是采用“驼峰”型温度组合。首先,降低喂料区温度,这将在输送区导致更强的摩擦。螺杆电机的电流将突出增加。这样,平均熔融温度可能显著增加,所以这时要当心由于过分摩擦导致的交联、变色和烧焦的凝胶。你也许需要降低最后一节机筒的温度来冷却熔体。

 

Pinpoint gels, referred to as “applesauce,” look like dust particles entrapped in resin. Causes include material contamination or too much reclaim. Another possible cause is incomplete melting. The preferred remedy is to heat the material more quickly by increasing frictional shear. A second option is to increase heating time by reducing screw speed, but this also reduces output.

 

微细凝胶,也成为“苹果酱”,看起来象混进树脂中的灰尘。原因包括材料污染或者使用太多的回料。另一个可能的原因是不完全熔融。较好的纠正措施是增加摩擦性剪切而更快地加热材料。第二个备用方案通过减少螺杆转速来延长加热时间。但是,这样也减少了产量。

 

 

Other possible culprits are insufficient backpressure or contamination from a dirty screw. If you need to raise backpressure, the preferred solution is to add more screens and increase screw speed. Expect the melt temperature to rise somewhat.

 

其它可能的错误是背压不够或者不干净螺杆的污染。如果你需要升高背压,较好的方案是添加更多的滤网并增加螺杆速度。希望把熔体温度升高一些。

 

Hollow gels, referred to as “lensing,” occur when vapor is trapped in the melt and carried forward to the die. Causes include wet resin or gases produced as the resin heats up. Pellets can get wet from condensation when resin is sucked from a cold silo into the hopper during winter. A solution is to allow resin to warm up inside the factory before using it. For hygroscopic materials like nylon and PET, make sure the resin is properly dried.

 

空心凝胶,也称作“透镜”凝胶,当熔体携裹蒸汽并带到口模时,会出现这种凝胶。原因包括树脂发湿或者过热分解气体。在冬季,当树脂从冷料仓吸入到料斗时,塑料粒子由于冷凝而变湿。一个可行的方案是在使用前加热。对于象尼龙和PET这样的吸湿性材料,要确保树脂能得到正确的干燥。

[此贴子已经被作者于2010-6-2 14:59:19编辑过]
3楼
上海北京顺德 发表于:2010/6/2 11:52:00

Other causes are insufficient heating in the feed section or turning the screw too fast. Air is normally forced backwards into the feed throat as the solid granules are compressed prior to melting. If only the surface of the granules melts before the pellets are compressed, air can get trapped.

 

其它原因包括喂料段加热不够或者螺杆转得太快。随着固体粒子在熔融前辈压缩,空气通常是向后排出,进入落料管。假如粒子在被压缩前只有表面熔融了,那么空气就会被物料裹挟。

hollow gels
A Rogues’ Gallery of Gels
These photos show lensing (top), arrowhead or chev­ron-shaped gels (middle), and applesauce (bottom). To prevent them, you need to know what caused them in the first place.
arrowhead gels
applesauce gels

The solution is raising the temperature in the first and second extruder zones so that the granules soften, compress, and squeeze the air out before melting begins. Another solution is to reduce your screw speed, but this also reduces output. Other causes to look for are insufficient drying or funnel flow inside the drying hopper—when material at the center flows quickly while material near the wall stagnates.

 

解决方法是升高挤出机第一段和第二段的温度,以便粒子软化、压缩并在熔融之前将空气挤出。另一个办法是减少你的螺杆转速,但是这样也减少了产量。其它原因就是看看是否干燥不够或者在干燥料斗中存在漏斗流动---中心材料流动得快,而料斗壁附近的料静止不动。

 

Gels can also appear as arrowheads, chevrons, and V or J shapes. Causes again include resin contamination or use of too much reclaim. If pro­­cessing conditions are at fault, the problem is likely to be too low a temperature of the melt or die or too little backpressure, all of which result in insufficient mixing. The preferred solutions are increasing frictional shear by lowering extruder feed and transition zone temperatures. Raising adapter and die temperatures may solve the problem if heat is being lost too rapidly after the extruder.

 

凝胶也可以以箭头形、V形和J形出现。原因也包括树脂污染或使用太多的回料。假如加工条件没有问题的话,问题可能是熔体或口模的温度太低,或者背压太低。这些原因都导致混合不够。较好的方案是通过减少挤出机的喂料段和输送段的温度而增加摩擦剪切。当热量损失太快时,升高机筒和口模的温度,也可以解决问题。

 

Discolored gels are caused by overheating or burning. Material causes include contamination, too much reclaim, or insufficient antioxidant. Faulty processing conditions to look for are excessive frictional shear inside the extruder, too hot a die, or too much recirculation flow in the metering section.

 

变色凝胶是由过热和烧焦引起的。材料方面的原因包括污染、太多的回料或者抗氧剂使用不足。要检查的错误的加工条件包括挤出机内部的过分的摩擦剪切、口模太热,或者计量段存在过多的返混。

 

Possible equipment problems are a clogged screen pack or dirty screw or die. Or, you could be using the wrong screw and/or barrel, a screw that’s too short, adapters and flow channels that are too wide, inappropriate die design, or incorrect heater or thermocouple locations. One solution is to maintain sufficient shear to wipe the metal surfaces constantly, since most polymer degradation occurs when polymer contacts metal for too long.

 

可能的设备原因包括阻塞的滤网、不干净的螺杆或口模。否则,你可能使用了错误的螺杆和/或机筒、太短的螺杆、太宽的连接通道、不恰当的口模设计、或者不正确的加热圈或热电偶位置。一个办法是保持足够的剪切以快速清洁机筒内表面。因为当聚合物与金属接触太长时间后会发生聚合物降解。

 

It’s also possible that you are overheating the resin locally in certain parts of the extruder screw. Variable-depth melt thermocouples in the adapter can often detect this problem even when barrel temperature controllers do not detect overheating.

[此贴子已经被作者于2010-6-2 18:17:08编辑过]
4楼
上海北京顺德 发表于:2010/6/2 11:52:00

Patterns of occurrence

If gels are randomly dispersed, they usually come from the raw material or the extruder. Raw-material problems that produce gels include contamination, too much recycled or reclaimed material, and insufficient antioxidant. Always check for contamination first because it’s easy to detect and simple to fix. If gels form in lanes in the machine direction, alternating with lanes free of gels, the problem is usually downstream from the extruder in the die or die block.

When large quantities of gels repeatedly appear and disappear, the phenomenon is called gel showers. They may be caused by improper processing conditions and are common when materials with very different melting rates are blended together. Pigment masterbatches, for example, are designed to melt quickly to maximize mixing time in the extruder. But if the feed zone is too hot, pigment granules melt too quickly and become encrusted on the screw. Encrusted granules then break off occasionally, resulting in a sudden cascade of gels the same color as the pigment masterbatch. They aren’t burnt gels, just heavily pigmented unmelts. Extruder motor amps will rise because the sudden increase in pigment concentration adds more friction to the transition zone. The preferred solution is to lower the temperature slightly in both the feed and transition zones to delay melting of the masterbatch.

5楼
忍者深闺 发表于:2010/6/3 14:40:00
好经验,收藏收藏
6楼
上海北京顺德 发表于:2012/9/13 20:18:00
 

It’s also possible that you are overheating the resin locally in certain parts of the extruder screw. Variable-depth melt thermocouples in the adapter can often detect this problem even when barrel temperature controllers do not detect overheating.


也可能在挤出机螺杆的局部过分加热了树脂。在适配器中的可变深度的熔体热电偶常常能探测到这个问题,尽管筒体温度控制器没有探测到过分加热。


Patterns of occurrence凝胶发生的模式

If gels are randomly dispersed, they usually come from the raw material or the extruder. Raw-material problems that produce gels include contamination, too much recycled or reclaimed material, and insufficient antioxidant. Always check for contamination first because it’s easy to detect and simple to fix. If gels form in lanes in the machine direction, alternating with lanes free of gels, the problem is usually downstream from the extruder in the die or die block.


如果凝胶是无规分散的,那么它们通常来自于原材料或者挤出机。产生凝胶的原材料问题包括物料污染、太多的回料以及抗氧剂添加不够。总是要首先检查污染物,因为污染物容易检测和确定。如果凝胶以条线形式(in lanes)出现在机器方向(也就是料流方向),而且条线之间没有凝胶,问题通常来源于挤出机下游的口模或口模模组。


When large quantities of gels repeatedly appear and disappear, the phenomenon is called gel showers. They may be caused by improper processing conditions and are common when materials with very different melting rates are blended together. Pigment masterbatches, for example, are designed to melt quickly to maximize mixing time in the extruder. But if the feed zone is too hot, pigment granules melt too quickly and become encrusted on the screw. Encrusted granules then break off occasionally, resulting in a sudden cascade of gels the same color as the pigment masterbatch. They aren’t burnt gels, just heavily pigmented unmelts. Extruder motor amps will rise because the sudden increase in pigment concentration adds more friction to the transition zone. The preferred solution is to lower the temperature slightly in both the feed and transition zones to delay melting of the masterbatch.


当大量的凝胶重复性地出现和消失时,这个现象叫做凝胶阵雨。这些凝胶可能是不正确的加工条件引起的,更经常是由于不同熔融速率(有的熔得快,有的慢些)的物料掺混在一起造成的。例如,色母粒会很快熔融从而使挤出机中的混炼时间达到最大。如果喂料段太热,色母粒就会快速熔融,并在螺杆上形成包覆层。包覆粒子不时破掉,导致一阵一阵的凝胶,其颜色和色母粒的颜色一样。它们不是烧焦的凝胶,而是被染色很浓的未熔物。色粉浓度的突然增加会增加输送段的摩擦力,从而使挤出机马达电流升高。最好的办法是略略降低喂料段和输送段的温度,以延长色母粒的熔融时间。


上海北京顺德评述


这篇英语文章详尽讨论了凝胶的这种起源、形态和解决方案。做挤出改性和做挤出成型的人一定会很喜欢这篇文章的。我尝试把文章中提到的凝胶来源归纳到下面的表中,这样更清晰一些。


表1    凝胶形成的原因分类

一级分类

二级分类

三级分类

四级分类

不熔凝胶

原料带来的凝胶

交联凝胶

暴聚造成的鱼眼

微交联凝胶

污染凝胶

有机碳化物

回料中的高熔点聚合物

机筒内形成的凝胶

中段降解凝胶

剪切降解凝胶

过分加热导致的降解凝胶

玻纤口、真空口停留降解凝胶

返混导致停留过久降解凝胶

口模降解凝胶


机筒内污染凝胶

有机降解碳化物


不干净的滤网


未熔凝胶

(可熔凝胶)

困气未熔凝胶

困空气未熔凝胶


困湿气未熔凝胶


分解困气未熔凝胶


塑化不良未熔凝胶

温度过低未熔凝胶

输送段维度过低

机头、口模温度过低

混合不良未熔凝胶

剪切不足

使用过多的回料

停留时间过短

螺杆转速太快

返混比例太低

7楼
jason 发表于:2012/9/14 10:22:00
  廖上海别老顾着翻译,原文资料给来一份啊。
8楼
上海北京顺德 发表于:2012/9/14 11:48:00
以下是引用jason在2012-9-14 10:22:17的发言:
  廖上海别老顾着翻译,原文资料给来一份啊。

上面不是有原文吗?我给另发原文到你email?

9楼
jason 发表于:2012/9/14 13:05:00
  是的啊,你都给翻译了看起来没啥悬念
10楼
laoying1122 发表于:2012/9/14 15:07:00

好资料!!谢谢老廖!!!

共14 条记录, 每页显示 10 条, 页签: [1] [2]

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