挤出口模流涎积垢问题

挤出口模流涎积垢问题 解决流涎口模问题 Die drool, or plateout, can cause a host of problems for extrusion processors. Here is how to stop the problem before it starts. 口模流涎或者叫模垢，可能对挤出加工者引起很多问题。 本文就是解决这些问题的方法。 Click Image to Enlarge One extrusion processor was emphatic (强调的)to me about it: “If you extrude olefinic or thermoplastic elastomeric compounds,” he told me, “you will experience the die drool problem.” 一个挤出加工者对我强调说:“如果你挤出聚烯烃或者热塑性弹性体共混物，你可能会遇到口模流涎问题。” He’s right. It doesn’t matter if you are extruding sheet, film, pipe, tubes, or profiles—you can come face-to-face with drooling dies in any of these processes. 他是对的。但是，不要紧。如果你挤出片材、膜、管、软管或者型材---你可得在任何这些工艺中面对口模流涎问 题。 Die drool shows up as a buildup on the die lips. This buildup can either be the same color as the material being extruded, or in some cases appears yellow or red. Often this buildup will create unacceptable machine-direction lines in the extrudate. And after a period of time this buildup will ultimately break-off and attach itself to the extrudate, resulting in additional aesthetic flaws. 口模流涎显著现为模唇堆料(堆积)。这种堆积物可能与挤出的颜色一样，在某些情况下也可能是黄色或 红色的。这些堆积物常常沿着流动方向在挤出物上划出不可接受的纹道。过了一段时间后，这些堆积物会最终破 碎并附着到挤出物上，最终引起额外的美学缺陷。 Die drool is also commonly referred to as plateout. To find the root cause of the problem, you’ll need to analyze the “big three” areas to identify what’s to blame(归咎于): 口模流涎常常也叫做积垢。为了找到问题的根本原因，你需要分析三大板块，以确定要归咎于什么原因: •Material: Each thermoplastic is unique, with compounded alloys being the most problematic. Compounded thermoplastics (those that have additives combined with the base resin) have the highest probability of additives exuding, also known as migration. For example, in rigid PVC compounds, there are internal and external lubricants, typically low-melting waxes, which can migrate out of the material matrix. Along with these constituents, plasticizers are added to make PVC flexible; these are also prone to migration when critical shear rates are achieved. 材料:每种热塑性塑料都是独特的，共混合金是最蛋疼的品种。共混热塑性塑料(挤出树脂，助剂)具有助剂析 出的最大可能性，助剂析出也称为迁移。例如，在硬PVC共混物中，添加有内外润滑剂，一般是低熔点蜡，它 们会迁移到基体材料的表面。在这些组分中，还有增塑剂也添加到PVC中以增加PVC柔性。当达到临界剪切 速率时，这些助剂都有析出倾向。 Reactor grades of olefinic polymers such as polypropylene and high-, medium-, or low-density polyethylene are less problematic; however, they still can be the bane (灾星，祸害)of the processor. Many olefinic thermoplastics are a combination of different molecular-weight grades to achieve certain physical characteristics. The resin supplier can blend high-molecular-weight resin with a low molecular weight to increase the modulus (stiffness), and at high shear rates there can be migration, leading to die buildup. However, on polyolefins and non-compounded thermoplastics, the root cause of die buildup typically is not material-related, so further investigation will likely be necessary: 反应器牌号的聚烯烃，如聚丙烯和高、中、低密度聚乙烯较少有问题。但是，它们仍然可能成为加工者的灾星。 许多聚烯烃热塑性塑料是不同分子量牌号的并用，以求达到某种特性。树脂供应商可能用低分子量树脂掺混高分 子量树脂，以提高模量(刚性)。在高剪切速率下，低分子量树脂可能迁移，导致口模积垢。但是，对于聚烯烃 和非共混热塑性塑料，口模积垢的根本原因一般不是与材料相关的，所以进一步的研究是必要的。 •Machine: The next area to analyze is the extruder. It is important to know the physical condition of your extruder, especially the barrel and screw. Excessive screw wear or even moderate barrel wear can cause problems. Barrel and screw wear are problematic because a single-screw extruder is not a positive pumping device, and any wear reduces output, requiring a higher screw speed to maintain the desired rate. Higher screw speeds, in turn, lead to additional frictional heat imparted to the thermoplastic, causing a melt separation and die drool/buildup. 机器:接着要分析的一个版块是挤出机。最重要的是要知道你的挤出的实际条件，特别是机筒和螺杆。过分的螺 杆磨损甚至是中等的机筒磨损都能引起问题。机筒和螺杆之所以引起问题，是因为单螺杆挤出机不是一个正向泵 送设备，任何磨损都减少产量，从而要求更高的螺杆转速以维持所要求的产率。反过来，较高的螺杆转速又导致 额外对热塑性塑料有害的摩擦热，最终引起熔体分离和口模流涎/积垢。 •Tooling: Tooling design plays an important role in the die-drool problem. The two key design criteria(指标) are drawdown(牵引)and land length(通道长度，指螺杆末端到口模外表面的长度). But before even thinking about die design you should inspect the condition of the die lip. Die lips should be sharp and “dressed” (deburred) to a 0.25-mm (0.010-in.) edge. If the edge has a larger radius, say 0.75 mm (0.030 in.), material can build up inside the radius and create an area that will lead to continued die drool. The aesthetic flaw will exhibit itself in the form of “smear marks” on the extrudate. On the matter of design, drawdown and land length are inseparable parameters. They are a main root cause for die drool, but probably the least understood. To best understand their inter-relationship, let’s review them in detail: 成型加工设备:加工设备在口模流涎问题上起重要作用。两个关键设计指标是牵引装置和通道长度。但是，在考 虑口模设计之前，你应该检查模唇的状况。模唇应该是平直的，倒角到0.25mm。如果倒角半径较大，比如说 0.75mm，材料可能在半径内积垢，出现一个连续流涎的区域。外观缺陷表现为挤出物上出现污痕。 Drawdown: The size of the designed die dimensions in deference to(根据，依据)the final part dimensions. The dimensional ratio of the die to final part used to offset the “swell” of the thermoplastic as it exits the die. 牵引装置:根据最终部件的尺寸来确定的所设计口模的尺寸。口模尺寸对最终部件尺寸的比例用于抵消热塑性塑 料挤出口模时的“膨胀”。 Land length: The length of the steel that runs parallel to the polymer flow. The land-length ratio is the ratio of the land length to the die gap (wall thickness). 通道长度:平行于聚合物料流方向的钢的长度。通道长度比是通道长度与口模间隙(壁厚)的比例。 Also be mindful of four equally important facts: 还要注意以下四个同样重要的事实: 1. Short-land dies exhibit greater thermoplastic swell. 短的通道口模表现出较大的热塑性塑料膨胀。 2. Long-land dies exhibit less thermoplastic swell. 长的通道口模表现出较小的热塑性塑料膨胀。 3. Higher outputs exhibit viscous heat generation (frictional heat). 较高的产量表现出更多的粘性热生成(摩擦热)。 4. Higher outputs exhibit greater thermoplastic swell. 较高的产量表现出较大的热塑性塑料膨胀。 With that said, the higher the expected output, the greater the die swell, which must be accommodated with either a longer land, an undercut of the die opening, or a greater drawdown. Remember that excessive drawdown can lead to residual stress imparted to the thermoplastic. Once stretched beyond its normal boundaries, the plastic is under severe stress and wants to return to a more “comfortable” (relaxed) position and thus is prone to fatigue if subjected to impact or pressure. 由此看来，所希望的产量越大，出口膨胀就越大，这就必须匹配一个较长的通道，一个口模开口的翻边结构，或 者一个较大的牵引。要记住，过大的牵引会导致危害热塑性塑料的残留应力。一旦拉伸超出限度，塑料就处 于严重的应力状态，这时塑料想回到“舒服的”(松弛的)位置，这样在受到冲击或压力时就会疲劳。 Figure 1 shows the effect of a long-land die that controls the thermoplastic swell to a normal range, drawing down into a sizer. Figure 2 shows where the swell of the thermoplastic is excessive and impinges on the outer die lip. This will result in die drool. If you try to correct the problem only by drawing the extrudate down, the processing window will be narrowed and there will be issues with part quality as well. 图1显示了长通道口模的效果，它将热塑性塑料膨胀控制到一个标准范围，牵引进入定径套。图2显示热塑性 塑料膨胀过度，在外模唇处造成危害。这将导致口模流涎。如果你只想通过牵引解决问题，加工窗口将很窄，部 件质量也会存在问题。 Paying attention to detail is critical to the tooling aspects of land length and drawdown parameters to maintain a process that is in control.关注细节对于通道长度的加工和牵引参数是关键的，这样可以将加工维持在可控状 态。 About the Author Daniel Cykana has devoted his 48-year career to plastic extrusion. He is the director of technology for Extrusion Solutions, LLC in Sheboygan Falls, Wis., where he focuses on extrusion training, die design, and processing problems. Contact: (920) 918-3250; email: danielcykana@aol.com; website: extrusionprocessing.com.You might also like: