吹膜模头设计的一些见解

1、模头设计的一些见解林楚漂2012年1 模头的设计趋向紧凑型设计，减小模头高度和直径，科学分布流道，从而缩短流道长度，减少熔体滞留时间；减少流道壁面面积，降低模内压力损失。紧凑型模头设计一方面减少钢材用料，降低生产成本；另一方面有效提高薄膜质量和产量。缩短流道减少滞留时间的作用：（1）使熔体不至于过烧形成晶点和流道线，保证薄膜质量；（2）降低熔体受流道壁加热而升高温度的影响。模头加热温度的设定应使流道温度略高于熔体温度，提高熔体流动性，以使熔温和流动速率在圆周上分布均匀。温度较高的流道壁将热量传递给熔体使之升温。熔体温度升高将增大冷却负荷，降低模头产量；影响膜泡泡形，降低薄膜力学性能。（3）缩短

2、换料换色时间，降低生产成本。降低模内压力损失的作用：（1） 模头压力过大将对挤出机造成负荷。高压下螺杆旋转挤出熔体会形成大量剪切热，提高熔体温度，也加大冷却风机负载。压力回流降低挤出机产量。（2） 模头压力损失是模内熔体间以及熔体与流道壁间剪切应力的宏观表现。熔体受剪切（摩擦）流动困难，延长滞留时间。流动克服摩擦动能转化为热能使温度升高。模头内压力损失并不是越小越好。压力损失不可小于某一最小值（各种材料最小值不同）以保证足够大的熔体剪切速率，使得熔体不会粘附在流道壁上。机器以较高产量正常开机时，各层压力约为30-35MPa为佳。2 模头各部分流道的设计模头内流道基本可以分为4个部分：分配系统、

3、螺旋部分、汇流部分、口模部分。流道设计应遵循压力损失小、滞留时间短的原则。（1）分配系统即是主流道至螺旋起始点之间的流道分配。功能是实现熔体在圆周上均匀等分分配。设计时应保证在1分2、2分4、4分8等各段流道内剪切速率一致，以使流动稳定，实现均匀等分。剪切速率越低，压力损失越小。剪切速率有最小值。不同原料，最小剪切速率的值不同。例如eva为3-5（s-1），LDPE8（s-1）。 根据流体力学理论，圆形截面流道压力损失与流道长度成正比，与流道直径成反比。压力损失对流道直径的变化更加敏感。所以，在满足输送能力和最小剪切速率的前提下，分配流道设计要尽量加大直径，缩短长度。对于特定的模头，主流道入口

4、与螺旋起始点的位置是相对固定的。所以分配系统的设计应遵循“大而长，小而短”的原则。即是说，大直径流道应尽可能拉长长度，小直径流道应尽可能缩短长度。从而最大限度降低熔体流经分配系统的压力损失。（2）螺旋部分流道设计目的是使熔体在圆周方向上得到均匀的温度和流动速度，从而得到高质量的薄膜。该部分要求当改变物料、产量和料温等条件时，熔体沿圆周的分布仍然均匀。熔体既在螺槽内沿螺槽流动，同时也在环形间隙（漏流间隙）内沿轴向流动。螺槽内熔体流动速度在螺槽入口处最大，沿着螺槽流动速度逐渐减小直至为0。环形间隙内的流速在入口处最小，沿着轴向逐渐变大。随着螺槽的变浅和间隙的增大，熔体流动由主要沿螺槽的方向过渡到沿

5、环形间隙的轴向流动。该设计促使熔体在螺槽与环形间隙间交互流动，最终达到均化的目的。流道总高度一般是螺距的6倍。该高度范围内螺旋均起到作用的话足以达到均化熔体流动的目标。为使流道在一开始就起作用，要求起始漏流间隙不得过小，最好在0.8以上。起始漏流位置与螺槽起始处重叠。此处设计最好经过模拟计算才能确定各项设计参数，得到一个较好的设计。螺旋结束后较大间隙的储料段流道起到承接的作用：消除熔体沿圆周切线方向流动的影响；释放压力；储料段内熔体由于汇流段的高压阻而沿周向流动，为汇流段的均匀挤出做好准备。（3） 汇流部分流道处各层熔体贴合挤出。该处的设计对薄膜质量有较大影响。此处流道间隙一般很小，剪切速率大

6、，长度不宜过长。流道宽度应通过模拟计算来保证汇流时各层剪切速率相近，从而防止鲨鱼皮的产生。各层熔体剪切不可能完全一致，汇流后需要一段直的成形段流道来得到稳定的层间边界。然后才能进行流道的压缩或扩张进入口模。（4） 口模部分是最接近薄膜成型的一部分流道。与薄膜的表面质量有直接关系。为保证熔体稳定挤出口模，口模成形段流道之前同样需要一段储料段流道。二者之间的过渡流道倾斜角度常取7.5。口模成型段应有足够长度以使流动取向稳定。口模间隙一般在0.9-3.2间取值。间隙小，模头产量低但是薄膜质量好，过小则薄膜表面会产生鲨鱼皮缺陷。间隙大，模头产量高，过大则影响薄膜质量。Opinions about di

7、e head design1 die head design tend to compact style，reduce height and diameter，distribution the channels scientifically and shorten the length of channels，reduce melt held up time；decrease channel walls area，reduce inner die pressure expense。One side compact die head style design can reduce steels，

8、decrease producing cost；the other side improve the film quality and output effectively。The effect of shorten channels and reduce residence time：（1）The melt will not be over heated that form crystal dots and channel lines，ensure the quality of film；（2）Decreasing the effect of channel walls are heated

9、 up and the melt increase temperature。Die head heat up temperature setting should make the channel temperature a little higher than melts，improve the flowability of melt，make the melting temperature and flow rate distribute on the circle uniformity。The higher temperature channel walls deliver the he

10、at to melt and make the temperature rise。Melt temperature rising up will increase cooling load and reduce the die head output；effect bubble shape，decrease mechanical property。（3）shorten the time of changing material and colours，reduce producing cost。Effects of reducing inner die pressure：（5） Too muc

11、h die head pressure will cause a loading to extruder。Under high pressure screw rotating extrude melt will form a mount of cutting heat，improve melting temperature and cooling fan loading 。pressure backflow reduce the output of extruder。（6） Macroscopic expression of die head pressure expense。Melt flo

12、wing difficultly because of being cut ，so extend residence time。Flowing put up with friction kinetic energy transform to heat and make the temperature rise。Inner die head pressure expense is not the smaller the better。Pressure expense cannot less than one of minimum（different material with different

13、 minimum）ensure the melt shearing rate is large enough，enable the melt will not adhere to the channel wall。When the machine normal runs with higher output，each layer pressure is better with about 30-35MPa。2 design of channelsDie head inner channel can be 4 parts：distribute system、spiral、conflux、die

14、lip。Channel design should follow the rule that little pressure expense and short resort time。（1）Distribution system is the channel distribution from main channel to the between of spiral initial point。The function is achieving melt uniformity distribution on the circle。When designing we should ensur

15、e the shearing rate in 1 divides into 2、2 divides into 4、4 divides into 8 and each part of channel is accordance，enable the flowing steady and achieve aliquot uniformity。The lower shearing rate，the less pressure expense。Cutting speed has least value。Different material，different min. shearing rate。Fo

16、r example，EVA is 3-5（s-1），LDPE8（s-1）。 According to hydrodynamics，rotundity section channel pressure expense is in direct ratio to channel length and in inverse ratio to channel diameter。Pressure expense is more sensitivity for the changing of channel diameter。So based on the precondition that satisf

17、y transportation ability and min. shearing rate，distribution channel design should increase the diameter and shorten length as possible。For special die head，main channel entrance and spiral starting spot is relatively stable。So distribution system design should follow the principle that“big and long

18、，small and short”。It means that，channel with big diameter should lengthen the length and channel with small diameter should shorten length if possible。So that maximum reduce the pressure expense that melt flow by distribution system。（2） The channel design of spiral is purpose to make the melt temper

19、ature and flow speed are uniformity on the circle direction，enable to get a high quality film。This part request that when change the conditions such as material、output and temperature，the distributing of melt on the circle still uniformity。Melt flow in the spiral groove along spiral groove，also flow

20、 in the annular gap（leak flow gap）along axes direction at the same time。In spiral groove， melt flow speed is highest at the spiral entrance and along the spiral groove decrease till 0 gradually。The flow speed in the annular gap is lowest at the entrance and increase gradually along the axes directio

21、n。Along with spiral groove is getting flat and the gap is bigger，melt flow from the direction mainly along the spiral groove to annular gap axes direction flow。This design make the melt flow in both of spiral groove and annular gap and achieve to the purpose of equal。Channel total height is commonly

22、 6 times as screw-pitch gauge。In this height range if spirals all effect is enough to reach the target that equal the melt flow。In order to make the channel works at the beginning，the request is starting leak gap cannot be too small and is better over 0.8。Starting leak flow place and the starting of

23、 spiral groove are superposition。This design is better to do a simulation calculate to confirm the design parameter and get a better design。The bigger gap storing material sect work a carry on action after spiral is over：eliminate the effect that melt flow along circle tangent direction；release pres

24、sure；the melt of storing material sect flow along the circle because of its high pressure，prepare for the equal extude of conflux sect。（3） Channel of conflux part each layer melt near close to extrude。This part of design is make a bigger effect to film quality。This part of channel gap is usually very small and big shearing rate，the length willnot be too long。Channel breadth should ensure each layer shearing rate is approach when conflux by simulation calculating and prevent the sharkskin。Each layer melt shearing