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防止挤出型材的翘曲  发帖心情 Post By:2010/3/17 19:15:00 [只看该作者]


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Stop Profile Warpage<!-- InstanceEndEditable -->
<!-- InstanceBeginEditable name="Synopsis" --><!-- InstanceEndEditable -->
<!-- InstanceBeginEditable name="Author" -->

By Bob Bessemer, Conair

Processors today want to run faster, minimize scrap, and do short production runs that may only last a few hours. However, running plastic profiles faster often causes warping, especially with complex hollow shapes having varying wall thicknesses, as well as wood/plastic composites or foamed profiles. So what do you do when your profile warps?

The simple answer is: Cool the profile more evenly. That means control heat transfer so thin sections don’t freeze off before thick sections dissipate heat. Resin and fillers also play a role. The thermal conductivity of a resin is its density times its ability to conduct heat. The thermal conductivity of PVC is 0.051, PP is 0.075, and HDPE is 0.16, where the higher number means slower heat transfer. Thus PE profiles require 20% to 30% longer tanks than PVC because of PE’s slower cooling.

An insulating=
An insulating layer of warm water surrounding the profile inhibits cooling.

VARIOUS WAYS TO COOL
How heat dissipates through a material is a matter of the materials’ thermal conductivity and thickness. As the outer skin of a profile solidifies, it insulates the inner material from the cooling medium. If a wall is thick, it becomes a more severe cooling issue.

Depending on your cooling equipment, there are different ways to fix uneven cooling. Cooling methods include air racks, water tanks, and spray-cooling tanks. Air cooling is the simplest and slowest method of heat transfer. Air has only 1% to 10% of the cooling capacity of water, so it’s less likely to cool unevenly and cause profiles to warp, twist, and bow. Typical throughput rates with air cooling are below 250 lb/hr for materials like polycarbonate, rigid PVC, and PS.

An air rack is typically 10 to 20 ft long with templates or form guides to support the profile while it cools. Clamps, bent wires, vise grips, and other devices can also help to support and guide the profile. In the first 10 ft, nozzles are typically used to blow air at specific sections of the profile, while the last 10 feet use more generalized air cooling with blowers above and below the profile.

In hollow profiles for window and door lineals, with complex shapes and wall thicknesses that can range from 0.085 in. to 0.200 in., even a slight bow can put them out of spec. Cooling with air racks can easily be modified by directing air nozzles at the heavier wall to cool it faster. Cardboard or some other shield can be used to block air from cooling the thin wall too quickly.

The solution=
The solution is turbulence to break up the laminar flow of water, which can be aided by baffles in the tank.

Cooling the profile by immersing and pulling it through a water tank is faster but creates a zone of heated water around the profile, which inhibits heat transfer. The solution is to break up this insulating layer of hot water by agitation. In most cases, water-distribution bars can be installed in the corners of the tank. If this causes a warpage problem, it sometimes helps to raise the overall water temperature and slow the rate of heat transfer. This requires a water temperature-control system, usually consisting of a water pump, filter, heat exchanger, and valving to control chilled water input.

Current profile extrusion practice typically involves dry calibration tables, which can be 30 ft or more long, in front of a water immersion tank with form guides under low vacuum. In some cases, these tanks can be more than 100 ft long. Longer tanks minimize the potential for distortion. However, equipment and floorspace costs are high, and even long tanks may not speed production enough.

As extrusion rates increase and processors run out of floorspace, they have been forced to look at more efficient heat transfer with spray-cooling tanks. The beauty of spray cooling is its heat-transfer efficiency. Nearly microscopic water droplets from spray nozzles come in contact with an extruded profile that is at 250 F or hotter. The droplets immediately flash off to steam, carrying away tremendous amounts of Btus.

Evaporative spray cooling works best when the spray is directed onto the extrudate as soon as it enters the tank, when the plastic is hottest, within the first inch or so after the material leaves the die. If dry calibration tooling is installed, there is essentially no evaporative effect. Once the surface temperature of the extrudate drops below 250 F, spray droplets no longer flash off to steam. But even at lower temperature, spray cooling still has 10% to 20% better cooling potential than immersion cooling because of the large surface area of droplets and the turbulence of spray.

FASTER MAY NOT BE BETTER
Immersion cooling removes heat faster than air, and spray cooling removes heat even faster, which makes it the most likely cooling method to cause complex profiles to warp. For some solid or irregular profiles, the answer may actually be to slow the heat transfer.

Water spray=
Water spray cools intensively by flash evaporation of small droplets as they hit the hot profile.

For instance, when rigid PVC profiles are cooled with high-turbulence spray tanks and properly sized water-circulation systems, chilled water in the range of 45 to 55 F has historically been used throughout. However, it may be better to temper the water in the initial tank to 80 to 130 F. This pulls heat out more slowly so the profile skin doesn’t freeze off immediately. A form-guide system can also be mounted in the spray tank to direct spray onto a thicker side to optimize cooling. Then, as the profile proceeds downstream, you can progressively drop the water temperature to complete the cooling.

If tempering the water alone doesn’t fix the warping, try controlling the temperature in the dry or wet-over-dry calibrators too. Calibration tooling can be designed with independent water-temperature control. A process that typically requires three 8- to 12-in.-long dry-calibration tools can easily optimize temperature in each section, as is typical in foamed profile extrusion. This level of temperature control also allows the process to be highly repeatable the next time the run is set up.

It is also possible to create independently controlled temperature zones within a calibration tool—such as top, bottom, and even side zones—to enhance heat transfer from the most difficult profile shapes.

An extrusion line for 1500 lb/hr of foamed 50/50 HDPE/wood composite might use a series of dry-calibration tools mounted on a manifold-cart in front of two 16-ft, high-intensity spray tanks, typically chilled to 40 to 55 F. But foam insulates and thus inhibits cooling, so these shapes are prone to retain heat and curl, especially if they have ribs or tongue-and-groove shapes.

In such cases, start by tempering the water in the primary sizing spray tank to 100 to 130 F to control the rate at which the skin forms while pulling maximum heat from the wall. To control water temperature within 1?F, a temperature-control unit can be used between the chiller and process-water loops, along with remote thermocouples in the tank. Depending on profile size, use 24-, 36-, or even 48-kW heaters with 1.5- to 7.5-hp pumps and modulating valves.

 

[此贴子已经被作者于2010-3-17 19:17:07编辑过]


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  发帖心情 Post By:2010/3/17 20:16:00 [只看该作者]

Processors today want to run faster, minimize scrap, and do short production runs that may only last a few hours. However, running plastic profiles faster often causes warping, especially with complex hollow shapes having varying wall thicknesses, as well as wood/plastic composites or foamed profiles. So what do you do when your profile warps?

 

当今的加工者希望机器运转快速,加少回料,把生产循环周期控制在几个小时。但是,生产塑料型材太快会导致翘曲,具有复杂空洞形状的产品由于壁厚不匀更是如此,还有木塑复合材料或者发泡材料也是这样。那么,型材翘曲时,我们该怎样做呢?

 

The simple answer is: Cool the profile more evenly. That means control heat transfer so thin sections don’t freeze off before thick sections dissipate heat. Resin and fillers also play a role. The thermal conductivity of a resin is its density times its ability to conduct heat. The thermal conductivity of PVC is 0.051, PP is 0.075, and HDPE is 0.16, where the higher number means slower heat transfer. Thus PE profiles require 20% to 30% longer tanks than PVC because of PE’s slower cooling.

 

简单的答案是:均匀地冷却型材。这就意味着在厚的部分散热之前别让薄的部分冷却掉了。树脂和填料也有影响。PVC的热导率是0.051,PP是0.075,HDPE是0.16,其中,较高的数值意味着较慢的热交换(好像是错误的)。这样PE型材的温度控制通道比PVC长20%-30%,这是因为PE冷却较慢。


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  发帖心情 Post By:2010/3/17 20:24:00 [只看该作者]

 
An insulating=
An insulating layer of warm water surrounding the profile inhibits cooling.

VARIOUS WAYS TO COOL各种冷却方法
How heat dissipates through a material is a matter of the materials’ thermal conductivity and thickness. As the outer skin of a profile solidifies, it insulates the inner material from the cooling medium. If a wall is thick, it becomes a more severe cooling issue.

 

某种材料的散热能力与材料的热导率和厚度有关。随着型材外层的固化,它会将内层材料与冷却介质隔绝开来。壁太厚会导致更严重的冷却问题。

 

Depending on your cooling equipment, there are different ways to fix uneven cooling. Cooling methods include air racks, water tanks, and spray-cooling tanks. Air cooling is the simplest and slowest method of heat transfer. Air has only 1% to 10% of the cooling capacity of water, so it’s less likely to cool unevenly and cause profiles to warp, twist, and bow. Typical throughput rates with air cooling are below 250 lb/hr for materials like polycarbonate, rigid PVC, and PS.

 

取决于冷却设备,有不同的方法来安排不均匀冷却。冷却方法包括空气架、水箱和喷水冷却箱。空气被冷最简单,但是热交换也最慢。空气只有水的冷却能力的1-10%,所以在不均匀冷却时引起翘曲、扭曲和弯曲的可能性较小。空气冷却的典型生产效率,对于象PC、硬PVC和PS等材料,低于250lb/hr。

 

An air rack is typically 10 to 20 ft long with templates or form guides to support the profile while it cools. Clamps, bent wires, vise grips, and other devices can also help to support and guide the profile. In the first 10 ft, nozzles are typically used to blow air at specific sections of the profile, while the last 10 feet use more generalized air cooling with blowers above and below the profile.

 

空气架一般10-20英尺长,带有模板或结构导轨,以支持型材的冷却。钳子、弯线、大力钳和其它一些设备也有助于对型材起支撑和导送作用。在头10英尺中,喷嘴用于在特定的部位吹空气;在后10英尺中,用风箱使用更一般的空气在型材上下进行风冷。

 

In hollow profiles for window and door lineals, with complex shapes and wall thicknesses that can range from 0.085 in. to 0.200 in., even a slight bow can put them out of spec. Cooling with air racks can easily be modified by directing air nozzles at the heavier wall to cool it faster. Cardboard or some other shield can be used to block air from cooling the thin wall too quickly.

 

在门窗等空心型材中,形状复杂,壁厚从0.085英寸到0.2英寸,即使轻轻一吹也可能使型材超出规格。用空气架冷却可以很容易地用直吹空气喷嘴来校准壁厚大处,以使冷却快速。硬纸板或者别的屏蔽物可用于阻止空气,防止薄壁冷却太快。

 

The solution=
The solution is turbulence to break up the laminar flow of water, which can be aided by baffles in the tank.

Cooling the profile by immersing and pulling it through a water tank is faster but creates a zone of heated water around the profile, which inhibits heat transfer. The solution is to break up this insulating layer of hot water by agitation. In most cases, water-distribution bars can be installed in the corners of the tank. If this causes a warpage problem, it sometimes helps to raise the overall water temperature and slow the rate of heat transfer. This requires a water temperature-control system, usually consisting of a water pump, filter, heat exchanger, and valving to control chilled water input.

Current profile extrusion practice typically involves dry calibration tables, which can be 30 ft or more long, in front of a water immersion tank with form guides under low vacuum. In some cases, these tanks can be more than 100 ft long. Longer tanks minimize the potential for distortion. However, equipment and floorspace costs are high, and even long tanks may not speed production enough.

 

把型材浸没于水中通过一个水箱拉动的冷却方法,会在型材周围建立一个热水区,这个热水区会阻碍热交换。方案是用搅拌的方法打破热水构成的隔热层。在很多情况下,水搅拌浆安装在水箱一角。如果这样还有翘曲问题,那么升高总的水温并降低热交换速度是有益的。这就要求一个水温控制系统,通常包括水泵、过滤装置、热交换器、阀门以控制冷却水流量。

 

目前,型材挤出实践包括干标定台,它有30多英尺长,位于浸渍水箱的前部,结构导轨处于负压。在某些情况下,这些箱子的长度超过100英尺长,因为较长的箱子减小了翘曲的倾向。但是这样会导致设备和地面空间成本上升,更长的箱子妨碍了高速生产。

 

As extrusion rates increase and processors run out of floorspace, they have been forced to look at more efficient heat transfer with spray-cooling tanks. The beauty of spray cooling is its heat-transfer efficiency. Nearly microscopic water droplets from spray nozzles come in contact with an extruded profile that is at 250 F or hotter. The droplets immediately flash off to steam, carrying away tremendous amounts of Btus.

Evaporative spray cooling works best when the spray is directed onto the extrudate as soon as it enters the tank, when the plastic is hottest, within the first inch or so after the material leaves the die. If dry calibration tooling is installed, there is essentially no evaporative effect. Once the surface temperature of the extrudate drops below 250 F, spray droplets no longer flash off to steam. But even at lower temperature, spray cooling still has 10% to 20% better cooling potential than immersion cooling because of the large surface area of droplets and the turbulence of spray.

 

随着挤出速率增加和加工者地面空间不足,他们被迫着眼寻求更高效的带喷溅冷却的交换器。喷雾冷却的好处在于它的热交换效率。从喷雾喷嘴中出来的微水滴与处于250华氏度的挤出型材密切挤出。水滴立即闪蒸成蒸汽,带走大量的热量。

当喷雾直接作用于型材时,蒸发喷雾冷却作用最好,其最佳时机是型材刚刚进入箱子,此时型材最热,也就是在型材离开口模约一英寸时最好。如果安装了干标定处理装置,那么基本没有蒸发效应。

 

 

[此贴子已经被作者于2010-3-19 19:53:50编辑过]

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  发帖心情 Post By:2010/3/18 9:27:00 [只看该作者]

The simple answer is: Cool the profile more evenly. That means control heat transfer so thin sections don’t freeze off before thick sections dissipate heat. Resin and fillers also play a role. The thermal conductivity of a resin is its density times its ability to conduct heat. The thermal conductivity of PVC is 0.051, PP is 0.075, and HDPE is 0.16, where the higher number means slower heat transfer. Thus PE profiles require 20% to 30% longer tanks than PVC because of PE’s slower cooling.

 

简单的答案是:均匀地冷却型材。这就意味着在厚的部分散热之前别让薄的部分冷却掉了。树脂和填料也有影响。PVC的热导率是0.051,PP是0.075,HDPE是0.16,其中,较高的数值意味着较慢的热交换(好像是错误的)。这样PE型材的温度控制通道比PVC长20%-30%,这是因为PE冷却较慢。


是不是能把where the higher number means slower heat transfer理解成高的数值意味着较低的热交换(要求)?

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  发帖心情 Post By:2010/3/18 16:04:00 [只看该作者]

以下是引用jason在2010-3-18 9:26:48的发言:

The simple answer is: Cool the profile more evenly. That means control heat transfer so thin sections don’t freeze off before thick sections dissipate heat. Resin and fillers also play a role. The thermal conductivity of a resin is its density times its ability to conduct heat. The thermal conductivity of PVC is 0.051, PP is 0.075, and HDPE is 0.16, where the higher number means slower heat transfer. Thus PE profiles require 20% to 30% longer tanks than PVC because of PE’s slower cooling.

 

简单的答案是:均匀地冷却型材。这就意味着在厚的部分散热之前别让薄的部分冷却掉了。树脂和填料也有影响。PVC的热导率是0.051,PP是0.075,HDPE是0.16,其中,较高的数值意味着较慢的热交换(好像是错误的)。这样PE型材的温度控制通道比PVC长20%-30%,这是因为PE冷却较慢。


是不是能把where the higher number means slower heat transfer理解成高的数值意味着较低的热交换(要求)?

“慢”和“低”没有本质区别吧?文中的几个热导率也不对啊。我翻译漏了一句:The thermal conductivity of a resin is its density times its ability to conduct heat. 可翻译为:树脂的热导率等于它的密度乘以其导热能力。


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  发帖心情 Post By:2010/3/18 20:40:00 [只看该作者]

以下是引用上海北京顺德在2010-3-17 20:23:35的发言:

 

[此贴子已经被作者于2010-3-18 20:42:34编辑过]

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