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如何解决挤出机口模积料?  发帖心情 Post By:2010/3/13 14:54:00 [只看该作者]


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Troubleshooting: Extrusion

<!-- InstanceEndEditable --><!-- InstanceBeginEditable name="ArticleTitle" -->Stop Die Buildup<!-- InstanceEndEditable -->
<!-- InstanceBeginEditable name="Synopsis" --><!-- InstanceEndEditable -->

<!-- InstanceBeginEditable name="Author" -->By Thomas Cunningham, 

  Extrusion Technological Services &
John Perdikoulias, Compuplast International

Edited by  Jan Schut

 

 

Die buildup, also called die drool, die bleed, or plate-out, can plague any extrusion process. It causes a variety of problems, from visible surface imperfections on extrudates to process disruption and outright product failure. Processors typically expend time and labor to remove die buildup because they don’t know how to eliminate it any other way.

Simple die shapes allow for cleaning while the line is running. Changing the angle at which the extrudate is drawn away from the die can change the placement of buildup on the die and make it easier to clean. Die buildup can also be intentionally pushed onto the product and removed further down the line. For sheet, mechanical automatic scrapers are available to remove buildup.

But processors often have to schedule costly downtime to remove die buildup. Applying mold release or silicone to the die exit area after cleaning can reduce the rate of buildup and extend the time between cleanings. Slowing down the process also reduces die buildup, but this hurts output. All these approaches treat the symptom, not the problem. A better solution may be to address what causes buildup.

die buildup
Thick, fluffy die buildup may be caused by partial foaming in the melt, resin compatibility issues, or high die stress. (Photo: Extrusion Technical Services)

Die buildup is related to stress at the die exit. Resin flowing along the die inner surface moves relatively slowly and then is suddenly accelerated as it exits the die. This sudden acceleration causes stress within the melt. Under stress, low-molecular-weight polymer fractions and other components in the melt can separate and be deposited at the die exit. Solutions are either to reduce die-exit stress or to reduce the tendency for components to separate from the melt. These may involve changes to the process, the material, or the die.

CHANGE THE PROCESS
Higher die and melt temperatures are one way to reduce die-exit stress. But gains may be offset by additional degradation, causing an increase in low-MW fractions. Changes in melt and die temperatures should be explored independently. Sometimes lower die temperatures can create a cool layer of resin on the inner surface of the die that creeps slowly to the die exit and then separates from the bulk flow, causing buildup.

Start by determining the true melt temperature and set the die body to that temperature. Standard melt thermocouples are often wrong, so check the melt temperatures by hand. It’s difficult to do, but worth the effort when troubleshooting. Also, the die exit surface can be significantly cooler than the die itself. Try checking the die exit temperature with a surface thermocouple.

Air sweeps pointed at the die exit can help reduce and control buildup. The air sweep is a wand contoured to the extrusion profile shape, with holes drilled in it to distribute compressed air. The air sweep can direct any smoke and condensables away from the die. Air sweeps also have the benefit that they can cool die buildup so it doesn’t oxidize and turn dark. Nitrogen is also used to prevent oxidation, but take care not to blow too hard or cool the die so much that it distorts the extrudate.

CHANGE THE MATERIAL
Different materials produce different sorts of die buildup, from thin and runny to thick and fluffy. Thin, runny buildup is often caused by low-MW fractions in the formulation, which volatilize at the die exit and condense on the die face. Thick, fluffy, buildup is often caused by partial foaming within the melt, compatibility issues, or high stress inside the die. Check for excessive moisture in the resin and for resin degradation, possible melt fracture, or inadequate venting.

Resins from some suppliers will generate more die buildup than others, even when the resins have exactly the same specifications. If die buildup occurs, try a similar grade from another supplier. If you see an improvement, your original resin supplier may be interested to hear about the performance issue with its material and may be able to help resolve the issue.

Die flow
Die flow can be modeled to study stresses on a resin at the die exit, which can lead to die buildup. (Illustration: Compuplast)

Resins from different suppliers can have similar shear viscosity but very different elongational viscosities. If no other differences can be seen between resins, try testing the elongational viscosity. The resin with the higher elongational viscosity will likely have more problems with buildup due to higher stresses at the die exit.

Resins with high die swell sometimes have high rates of die buildup. Resins with narrow molecular-weight distribution can have lower die swell, but this doesn’t necessarily mean less buildup. Resins with narrow MWD are often hard to process, which can generate low-MW components, which in turn can cause die buildup.

You may want to test the die buildup chemically to see what’s in it. Reclaimed scrap usually contains low-MW fractions produced by thermal degradation during prior processing. Additives that reduce degradation or chain extenders that repolymerize degradation components may help. If that doesn’t work, sell your reclaim to someone else.

Sometimes adding lubricants helps reduce buildup. But too much lubricant can actually increase die buildup. Chemical compatibility of different components in a formulation also plays a role. For example, extreme die buildup often occurs when highly incompatible polymers are melt blended. In such cases, use of compatibilizers can help. Fluoropolymer processing aids can also be added in small amounts to reduce stress at the die exit.

Modified die-exit geometries=
Modified die-exit geometries (some patented) reportedly reduce die buildup with certain materials. (Illustration: Extrusion Technical Services)

Some types of die buildup oxidize quickly and turn brown or black. Adding antioxidants to the formulation can help. It won’t solve the die buildup, but it can make any buildup that attaches to the product less visible

CHANGE THE DIE
Coating die surfaces with fluoropolymers like PTFE has had only limited success in preventing buildup. More effective is application of metallic coatings that incorporate fluoropolymers to the inside of the die to reduce stress at the die exit.

Die-flow modeling has also been used successfully to study the stresses occurring at the die exit and to evaluate changes in die-exit geometry. Modifying the exit geometry to reduce stress is an area of ongoing development.

Some exit geometries have been patented by processors and resin producers. Modified die exits that claim to decrease die buildup include a sharp square exit, sharp pointed exit, radiused exit, outward-stepped exit, inward-stepped exit, and outward-flared exit (see diagram).

Increasing die land length can also help because it reduces swell. Or you can use a larger die opening to reduce die-exit stress, and then draw the extrudate down to the desired size.



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  发帖心情 Post By:2010/3/13 14:56:00 [只看该作者]

Die buildup, also called die drool, die bleed, or plate-out, can plague any extrusion process. It causes a variety of problems, from visible surface imperfections on extrudates to process disruption and outright product failure. Processors typically expend time and labor to remove die buildup because they don’t know how to eliminate it any other way.

Simple die shapes allow for cleaning while the line is running. Changing the angle at which the extrudate is drawn away from the die can change the placement of buildup on the die and make it easier to clean. Die buildup can also be intentionally pushed onto the product and removed further down the line. For sheet, mechanical automatic scrapers are available to remove buildup.

But processors often have to schedule costly downtime to remove die buildup. Applying mold release or silicone to the die exit area after cleaning can reduce the rate of buildup and extend the time between cleanings. Slowing down the process also reduces die buildup, but this hurts output. All these approaches treat the symptom, not the problem. A better solution may be to address what causes buildup.

die buildup
Thick, fluffy die buildup may be caused by partial foaming in the melt, resin compatibility issues, or high die stress. (Photo: Extrusion Technical Services)

Die buildup is related to stress at the die exit. Resin flowing along the die inner surface moves relatively slowly and then is suddenly accelerated as it exits the die. This sudden acceleration causes stress within the melt. Under stress, low-molecular-weight polymer fractions and other components in the melt can separate and be deposited at the die exit. Solutions are either to reduce die-exit stress or to reduce the tendency for components to separate from the melt. These may involve changes to the process, the material, or the die.

CHANGE THE PROCESS
Higher die and melt temperatures are one way to reduce die-exit stress. But gains may be offset by additional degradation, causing an increase in low-MW fractions. Changes in melt and die temperatures should be explored independently. Sometimes lower die temperatures can create a cool layer of resin on the inner surface of the die that creeps slowly to the die exit and then separates from the bulk flow, causing buildup.

Start by determining the true melt temperature and set the die body to that temperature. Standard melt thermocouples are often wrong, so check the melt temperatures by hand. It’s difficult to do, but worth the effort when troubleshooting. Also, the die exit surface can be significantly cooler than the die itself. Try checking the die exit temperature with a surface thermocouple.

Air sweeps pointed at the die exit can help reduce and control buildup. The air sweep is a wand contoured to the extrusion profile shape, with holes drilled in it to distribute compressed air. The air sweep can direct any smoke and condensables away from the die. Air sweeps also have the benefit that they can cool die buildup so it doesn’t oxidize and turn dark. Nitrogen is also used to prevent oxidation, but take care not to blow too hard or cool the die so much that it distorts the extrudate.

CHANGE THE MATERIAL
Different materials produce different sorts of die buildup, from thin and runny to thick and fluffy. Thin, runny buildup is often caused by low-MW fractions in the formulation, which volatilize at the die exit and condense on the die face. Thick, fluffy, buildup is often caused by partial foaming within the melt, compatibility issues, or high stress inside the die. Check for excessive moisture in the resin and for resin degradation, possible melt fracture, or inadequate venting.

Resins from some suppliers will generate more die buildup than others, even when the resins have exactly the same specifications. If die buildup occurs, try a similar grade from another supplier. If you see an improvement, your original resin supplier may be interested to hear about the performance issue with its material and may be able to help resolve the issue.

Die flow
Die flow can be modeled to study stresses on a resin at the die exit, which can lead to die buildup. (Illustration: Compuplast)

Resins from different suppliers can have similar shear viscosity but very different elongational viscosities. If no other differences can be seen between resins, try testing the elongational viscosity. The resin with the higher elongational viscosity will likely have more problems with buildup due to higher stresses at the die exit.

Resins with high die swell sometimes have high rates of die buildup. Resins with narrow molecular-weight distribution can have lower die swell, but this doesn’t necessarily mean less buildup. Resins with narrow MWD are often hard to process, which can generate low-MW components, which in turn can cause die buildup.

You may want to test the die buildup chemically to see what’s in it. Reclaimed scrap usually contains low-MW fractions produced by thermal degradation during prior processing. Additives that reduce degradation or chain extenders that repolymerize degradation components may help. If that doesn’t work, sell your reclaim to someone else.

Sometimes adding lubricants helps reduce buildup. But too much lubricant can actually increase die buildup. Chemical compatibility of different components in a formulation also plays a role. For example, extreme die buildup often occurs when highly incompatible polymers are melt blended. In such cases, use of compatibilizers can help. Fluoropolymer processing aids can also be added in small amounts to reduce stress at the die exit.

Modified die-exit geometries=
Modified die-exit geometries (some patented) reportedly reduce die buildup with certain materials. (Illustration: Extrusion Technical Services)

Some types of die buildup oxidize quickly and turn brown or black. Adding antioxidants to the formulation can help. It won’t solve the die buildup, but it can make any buildup that attaches to the product less visible

CHANGE THE DIE
Coating die surfaces with fluoropolymers like PTFE has had only limited success in preventing buildup. More effective is application of metallic coatings that incorporate fluoropolymers to the inside of the die to reduce stress at the die exit.

Die-flow modeling has also been used successfully to study the stresses occurring at the die exit and to evaluate changes in die-exit geometry. Modifying the exit geometry to reduce stress is an area of ongoing development.

Some exit geometries have been patented by processors and resin producers. Modified die exits that claim to decrease die buildup include a sharp square exit, sharp pointed exit, radiused exit, outward-stepped exit, inward-stepped exit, and outward-flared exit (see diagram).

Increasing die land length can also help because it reduces swell. Or you can use a larger die opening to reduce die-exit stress, and then draw the extrudate down to the desired size.


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  发帖心情 Post By:2010/3/13 15:04:00 [只看该作者]

Die buildup, also called die drool, die bleed, or plate-out, can plague any extrusion process. It causes a variety of problems, from visible surface imperfections on extrudates to process disruption and outright product failure. Processors typically expend time and labor to remove die buildup because they don’t know how to eliminate it any other way.

 

翻译:口模积料,也叫口模流涎或叫口模积垢,它可以危害任何挤出过程。它引起许多问题,从挤出物上可见的表面缺陷到工艺紊乱,乃至产品报废。加工者往往采取延长时间和投入劳动力来去除口模积料,因为他们不懂得采用任何别的方法来消除口模积料。


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  发帖心情 Post By:2010/3/13 15:35:00 [只看该作者]

Simple die shapes allow for cleaning while the line is running. Changing the angle at which the extrudate is drawn away from the die can change the placement of buildup on the die and make it easier to clean. Die buildup can also be intentionally pushed onto the product and removed further down the line. For sheet, mechanical automatic scrapers are available to remove buildup.

翻译:对于简单的口模形状,可以在线运行时进行积料清除。改变挤出物的牵引角度,就可以改变挤出物在口模上的堆积位置,这样就容易清除积料了。有些积料可以故意让其附着于产品上,然后从生产线上将其除掉。对于片材,可以用自动除榍装置除掉积料。


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  发帖心情 Post By:2010/3/13 15:44:00 [只看该作者]

But processors often have to schedule costly downtime to remove die buildup. Applying mold release or silicone to the die exit area after cleaning can reduce the rate of buildup and extend the time between cleanings. Slowing down the process also reduces die buildup, but this hurts output. All these approaches treat the symptom, not the problem. A better solution may be to address what causes buildup.

 

翻译:但是加工者常常得拿出昂贵的停机时间来清除积料。清除积料后在口模出料口使用脱模剂或者硅酮可以减轻积料程度并延长两次清除之间的时间。放慢加工速度也可以减少积料,但这牺牲了产率。所有这些方法都解决的是现象而不是根本问题。因此,需要一个好的方案来解决是什么引起了积料的问题。


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  发帖心情 Post By:2010/3/13 15:58:00 [只看该作者]

Die buildup is related to stress at the die exit. Resin flowing along the die inner surface moves relatively slowly and then is suddenly accelerated as it exits the die. This sudden acceleration causes stress within the melt. Under stress, low-molecular-weight polymer fractions and other components in the melt can separate and be deposited at the die exit. Solutions are either to reduce die-exit stress or to reduce the tendency for components to separate from the melt. These may involve changes to the process, the material, or the die.

 

口模积料与口模的出口应力有关。沿口模内表面流动的树脂移动相对慢,随后在流出口模时突然被加速。这种突然加速在熔体中引起应力。在应力作用下,低分子量聚合物部分和熔体中的其它组分就会(与聚合物熔体)分离,并沉积在口模出口上。解决方法要么是减少口模出口应力,要么是减少组分从熔体中分离的倾向。这些解决方法要涉及到工艺、材料或者口模。

[此贴子已经被作者于2010-3-13 15:59:19编辑过]

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  发帖心情 Post By:2010/3/13 17:03:00 [只看该作者]

CHANGE THE PROCESS改变工艺
Higher die and melt temperatures are one way to reduce die-exit stress. But gains may be offset by additional degradation, causing an increase in low-MW fractions. Changes in melt and die temperatures should be explored independently. Sometimes lower die temperatures can create a cool layer of resin on the inner surface of the die that creeps slowly to the die exit and then separates from the bulk flow, causing buildup.

 

较高的口模和熔体温度是减少口模出口应力的一种方法。但是利润可能被额外的降解抵消了,结果还会引起低分子量组分的增加。改变熔体温度和改变口模温度应该分开考虑。有时,较低的口模温度会在口模内表面上生成一个冷却的树脂层,这个树脂层慢慢地移出到口模出口,然后与本体料流分离,从而引起积料。

 

Start by determining the true melt temperature and set the die body to that temperature. Standard melt thermocouples are often wrong, so check the melt temperatures by hand. It’s difficult to do, but worth the effort when troubleshooting. Also, the die exit surface can be significantly cooler than the die itself. Try checking the die exit temperature with a surface thermocouple.

 

从确定真正的熔体温度开始,然后按照这个温度设定口模体的温度。标准的熔体温度常常是错误的,所以要用手工检查熔体温度。这样做很难,但是要解决问题就值得努力。此外,口模出口表面可能比口模本身要冷很多。要设法用表面热电偶检查口模出口温度。

 

Air sweeps pointed at the die exit can help reduce and control buildup. The air sweep is a wand contoured to the extrusion profile shape, with holes drilled in it to distribute compressed air. The air sweep can direct any smoke and condensables away from the die. Air sweeps also have the benefit that they can cool die buildup so it doesn’t oxidize and turn dark. Nitrogen is also used to prevent oxidation, but take care not to blow too hard or cool the die so much that it distorts the extrudate.

 

对着口模出口进行空气吹扫能帮助减少和控制积料。空气清扫装置是一个具有挤出物外形的套筒,里面钻了很多孔,可以使压缩空气呈现出一定的分布。空气清扫装置可以将任何烟雾物质和可压缩物吹离口模。空气清扫还有冷却积料的好处,以便积料不会氧化和变黑。也可以用氮气防止氧化,但是要记住,不要吹得太过火,否则会将口模过分冷却,这样会导致挤出物变形。


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  发帖心情 Post By:2010/3/13 17:39:00 [只看该作者]

CHANGE THE MATERIAL改变材料
Different materials produce different sorts of die buildup, from thin and runny to thick and fluffy. Thin, runny buildup is often caused by low-MW fractions in the formulation, which volatilize at the die exit and condense on the die face. Thick, fluffy, buildup is often caused by partial foaming within the melt, compatibility issues, or high stress inside the die. Check for excessive moisture in the resin and for resin degradation, possible melt fracture, or inadequate venting.

 

不同的材料产生不同种类的积料,从稀的流水状的积料到粘稠的绒毛状的积料。稀的流水状的积料常常是配方中低分子组分引起的,它口模出口处挥发,然后在口模表面冷凝。粘稠的绒毛状的积料常常是熔体内部分发泡、相容性问题或口模内部高应力引起的。检查物料的树脂是否过度潮湿或者树脂降解,检查是否存在熔体破裂或者排气不良。

 

Resins from some suppliers will generate more die buildup than others, even when the resins have exactly the same specifications. If die buildup occurs, try a similar grade from another supplier. If you see an improvement, your original resin supplier may be interested to hear about the performance issue with its material and may be able to help resolve the issue.

 

某些供应商的树脂会比别的供应商产生更多的积料,即使这种树脂具有同样的规格。如果发生口模积料,尝试使用别的供应商的类似牌号。如果你看到情况有改善,你原来的树脂供应商也许有兴趣了解跟材料有关的性能问题,也许能帮助解决这些问题。

图片点击可在新窗口打开查看
Die flow can be modeled to study stresses on a resin at the die exit, which can lead to die buildup. (Illustration: Compuplast)

Resins from different suppliers can have similar shear viscosity but very different elongational viscosities. If no other differences can be seen between resins, try testing the elongational viscosity. The resin with the higher elongational viscosity will likely have more problems with buildup due to higher stresses at the die exit.

Resins with high die swell sometimes have high rates of die buildup. Resins with narrow molecular-weight distribution can have lower die swell, but this doesn’t necessarily mean less buildup. Resins with narrow MWD are often hard to process, which can generate low-MW components, which in turn can cause die buildup.

You may want to test the die buildup chemically to see what’s in it. Reclaimed scrap usually contains low-MW fractions produced by thermal degradation during prior processing. Additives that reduce degradation or chain extenders that repolymerize degradation components may help. If that doesn’t work, sell your reclaim to someone else.

 

来自不同供应商的树脂可能有相似的剪切黏度,但是也许有很不相同的拉伸黏度。如果看不出两种树脂间存在别的差别,那就测试一下它们的拉伸黏度。具有更高拉伸黏度的树脂因为在口模出口处具有高的应力而将可能具有更多的积料问题。具有高的出口膨胀的树脂可能具有更高程度的积料。具有窄分子量分布的树脂会有低的出口膨胀,但这不意味着它一定有较少的口模积料。具有窄分子量分布的树脂往往难于加工,它还产生低分子组分,这反过来又引起积料。你可能想测试口模积料的化学成分以确定里面究竟有什么。回收料通常里面还低分子量组分,这些组分是在之前的加工中的热降解产生的。减少降解的助剂或者扩链剂会有助益。如果这些助剂不起作用,那么就把你的回料卖给别人。

 

Sometimes adding lubricants helps reduce buildup. But too much lubricant can actually increase die buildup. Chemical compatibility of different components in a formulation also plays a role. For example, extreme die buildup often occurs when highly incompatible polymers are melt blended. In such cases, use of compatibilizers can help. Fluoropolymer processing aids can also be added in small amounts to reduce stress at the die exit.

 

有时添加润滑剂有助于减少积料。但是,太多的润滑剂可能在实际上增加口模积料。配方中不同组分的化学相容性也有影响。例如,高度不相容的聚合物熔融摻混时,常常出现极度的口模积料。在此情况下,使用相容剂会有帮助。也可以添加少量的氟聚合物加工助剂来减轻口模出口的应力。

 

Some types of die buildup oxidize quickly and turn brown or black. Adding antioxidants to the formulation can help. It won’t solve the die buildup, but it can make any buildup that attaches to the product less visible

 

一些类型的口模积料会很快氧化并变成棕色或者黑色。在配方中添加抗氧剂会有帮助。这不会解决口模积料问题,但是它会使得附着于产品的任何积料不容易被看见。

[此贴子已经被作者于2010-3-13 19:35:13编辑过]

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hnix
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  发帖心情 Post By:2010/3/13 19:34:00 [只看该作者]

谢谢廖工,翻译辛苦啦,学习了图片点击可在新窗口打开查看

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  发帖心情 Post By:2010/3/13 19:59:00 [只看该作者]

     挤出机口模积料是最常有的问题,亦是最难解决的问题。分析得全面,准确啊,对照多年实践经验:一句话实用,经典,值得拜读!

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