Classification and selection of screw in single screw extruder
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Date：2016-12-30
单螺杆挤出机系统：挤出机挤压系统包括螺杆、机筒、料斗、机头、和模具，塑料通过挤压系统而塑化成均匀的熔体，并在这一过程中所建立压力下，被螺杆连续的挤出机头。
Single screw extruder system: the extruder extrusion system includes screw, barrel, hopper, head and die. The plastic is plasticized into uniform melt through the extrusion system, and is continuously extruded by the screw under the pressure established in this process.
螺杆：是挤塑机的最主要部件，它直接关系到挤塑机的应用范围和生产率，由高强度耐磨耐腐蚀的合金钢制成。
Screw: it is the most important part of the extruder. It is directly related to the application scope and productivity of the extruder. It is made of high-strength wear-resistant and corrosion-resistant alloy steel.
螺杆的基本型式及主要参数
Basic type and main parameters of screw
一般螺杆分为三段即加料段，压缩段，均化段。
加料段——底经较小，主要作用是输送原料给后段，因此主要是输送能力问题，参数（L1，h1），h1=（0．12－0．14）D。
压缩段——底经变化，主要作用是压实、熔融物料，建立压力。参数压缩比ε=h1／h3及L2。准确应以渐变度A=（h1－h3）／L2。
均化段（计量段）——将压缩段已熔物料定量定温地挤到螺杆最前端、参数（L3，h3），h3=（0．05－0．07）D。
L／D长径比：L／D与转速n，是螺杆塑化能力及效果的重要因素，L／D大则物料在机筒里停留时间长，有利于塑化，同时压力流、漏流减少，提高了塑化能力，同时对温度分布要求较高的物料有利，但大之后，对制造装配使用上又有负面影响，一般L／D为（18～42），但目前有加大的趋势。
螺距S，螺旋升角φ=πDtgφ，一般D=S，则φ=17°40′。 φ对塑化能力有影响，一般来说φ大一些则输送速度快一些，因此，物料形状不同，其φ也有变化。粉料可取φ=25°左右，圆柱料φ=17°左右，方块料φ=15°左右，但φ的不同，对加工而言，也比较困难，所以一般φ取17°40′。
棱宽e，对粘度小的物料而言，e尽量取大一些，太小易漏流，但太大会增加动力消耗，易过热，e=（0.08～0.12）D。
Generally, the screw is divided into three sections, namely feeding section, compression section and homogenization section.
Feeding section - the bottom diameter is small, which is mainly used to transport raw materials to the rear section, so it is mainly about the conveying capacity. Parameters (L1, H1), H1 = (0.12-0.14) d.
Compression section - the change of bottom longitude, which is mainly used to compact and melt materials and establish pressure. Parameter compression ratio ε= H1 / H3 and L2. The accuracy shall be based on the gradient a = (h1-h3) / L2.
Homogenization section (metering section) - extrude the melted material in the compression section to the front end of the screw quantitatively and at constant temperature. Parameters (L3, H3), H3 = (0.05-0.07) d.
L / D length diameter ratio: L / D and rotating speed n are important factors for screw plasticizing capacity and effect. If l / D is large, the materials will stay in the barrel for a long time, which is conducive to plasticization. At the same time, the pressure flow and leakage flow will be reduced, which improves the plasticizing capacity. At the same time, it is beneficial to the materials with high temperature distribution requirements, but if l / D is large, it will have a negative impact on manufacturing, assembly and use, The general L / D is (18 ~ 42), but it tends to increase at present.
Pitch s, spiral rise angle φ= πDtg φ， Generally d = s, then φ= 17°40′。 φ It has an impact on the plasticizing ability. Generally speaking φ Larger ones have faster conveying speed. Therefore, the shape of materials is different φ There are also changes. Powder is desirable φ= About 25 °, cylindrical material φ= About 17 °, block material φ= About 15 °, but φ It is also difficult for processing, so it is generally φ Take 17 ° 40 '.
Edge width e: for materials with low viscosity, e shall be larger as much as possible. If it is too small, it is easy to leak, but it will increase power consumption and overheat, e = (0.08 ~ 0.12) d.
下面就几种专用螺杆的设计结合其物料特性作简单介绍：
The following is a brief introduction to the design of several special screws combined with their material characteristics:
一．PC料（聚碳酸酯）
①非结晶性塑料，无明显熔点，玻璃化温度140°～150℃，熔融温度215℃～225℃，成型温度250℃～320℃。
②粘度大，对温度较敏感，在正常加工温度范围内热稳定性较好，300℃长时停留基本不分解，超过340℃开始分解，粘度受剪切速率影响较小。
③吸水性强
参数选定：
a．L／D针对其热稳定性好，粘度大的特性，为提高塑化效果尽量选取大的长径比，金韦尔取38。
由于其融熔温度范围较宽，压缩可较长，故采用渐变型螺杆。L1=30％全长，L2=46％全长。
b．压缩比ε 由渐变度A需与熔融速率相适应，但目前融熔速率还无法计算得出，根据PC从225℃融化至320℃之间可加工的特性，其渐变度A值可相对取中等偏上的值,在L2较大的情况下，普通渐变型螺杆ε=2～3，金韦尔取2．6。
c．因其粘度高，吸水性强，故在均化段之前，压缩段之后于螺杆上加混炼结构，以加强固体床解体，同时，可使其中夹带的水份变成气体逸出。
d．其它参数如e，s，φ以及与机筒的间隙都可与其它普通螺杆相同。
1、 PC material (polycarbonate)
① Amorphous plastic, no obvious melting point, glass transition temperature 140 ° ~ 150 ℃, melting temperature 215 ℃ ~ 225 ℃, molding temperature 250 ℃ ~ 320 ℃.
② It has high viscosity and is sensitive to temperature. It has good thermal stability within the normal processing temperature range. It basically does not decompose when it stays at 300 ℃ for a long time. It begins to decompose when it exceeds 340 ℃, and the viscosity is less affected by the shear rate.
③ Strong water absorption
Parameter selection:
a. L / D has good thermal stability and high viscosity. In order to improve the plasticizing effect, a large aspect ratio is selected as far as possible, and Kingwell is 38.
Because the melting temperature range is wide and the compression can be long, the gradual change screw is adopted. L1 = 30% of the total length, L2 = 46% of the total length.
b. Compression ratio ε The gradient a needs to adapt to the melting rate, but the melting rate cannot be calculated at present. According to the machinability of PC from 225 ℃ to 320 ℃, the gradient a value can be relatively higher than the middle value. In the case of large L2, the ordinary gradient screw ε= 2 ~ 3, Kingwell takes 2.6.
c. Because of its high viscosity and strong water absorption, a mixing structure is added on the screw before the homogenization section and after the compression section to strengthen the disintegration of the solid bed. At the same time, the water entrained in it can become gas and escape.
d. Other parameters such as e, s, φ And the clearance with the barrel can be the same as other ordinary screws.
二．PMMA（有机玻璃）
①玻璃化温度105℃，熔融温度大于160℃，分解温度270℃，成型温度范围很宽。
②粘度大，流动性差，热稳定性较好。
③吸水性较强。
参数选择
a．L／D选取长径比为35～42的渐变型螺杆，视其制品成型的精度要求一般L1=40％， L2=40％。
b．压缩比ε ，一般选取2．3～2．6。
c．针对其有一定亲水性，故在螺杆的前端采用混炼环结构。
d．其它参数一般可按通用螺杆设计，与机筒间隙不可太小。
2、 PMMA (plexiglass)
① The glass transition temperature is 105 ℃, the melting temperature is more than 160 ℃, the decomposition temperature is 270 ℃, and the molding temperature range is very wide.
② High viscosity, poor fluidity and good thermal stability.
③ Strong water absorption.
Parameter selection
a. L / D selects the gradual change screw with the length diameter ratio of 35 ~ 42. Generally, L1 = 40% and L2 = 40% depending on the accuracy requirements of product forming.
b. Compression ratio ε ， Generally, 2.3 ~ 2.6 are selected.
c. In view of its certain hydrophilicity, the mixing ring structure is adopted at the front end of the screw.
d. Other parameters can generally be designed according to the general screw, and the gap with the barrel cannot be too small.
三．PA（尼龙）
①结晶性塑料，种类较多，种类不一样，其熔点也不一样，且熔点范围窄，一般所用PA66其熔点为260℃～265℃。
②粘度低，流动性好，有比较明显的熔点，热稳定性差。
③吸水性一般。
参数选择
a．L／D选取长径比18～20的突变型螺杆。
b．压缩比，一般选取3～3．5，其中防止过热分解h3=0．07～0．08D。
c．因其粘度低，故止逆环处与机筒间隙应尽量小，约0．05mm，螺杆与机筒间隙约0．08mm
d．其它参数、可按通用螺杆设计。
3、 PA (nylon)
① There are many kinds of crystalline plastics with different melting points, and the melting point range is narrow. Generally, PA66 has a melting point of 260 ℃ ~ 265 ℃.
② Low viscosity, good fluidity, obvious melting point and poor thermal stability.
③ Average water absorption.
Parameter selection
a. L / D selects the mutant screw with length diameter ratio of 18 ~ 20.
b. The compression ratio is generally 3 ~ 3.5, in which H3 = 0.07 ~ 0.08d to prevent overheating decomposition.
c. Due to its low viscosity, the clearance between the check ring and the barrel should be as small as possible, about 0.05mm, and the clearance between the screw and the barrel should be about 0.08mm
d. Other parameters can be designed according to the general screw.
四．PET（聚酯）
①熔点250℃～260℃，吹塑级PET则成型温度较广一点，大约255℃～290℃。
②吹塑级PET粘度较高，温度对粘度影响大，热稳定性差。
参数选择
①L／D一般取32，三段分布L1=50％－55％，L2=20％。
②采用低剪切、低压缩比的螺杆，压缩比ε ，一般取1．8～2，同时剪切过热导致变色或不透明h3=0．09D。
③螺杆前端不设混炼环，以防过热，藏料。
④因这种材料对温度较敏感，而一般厂家多用回收料，为提高产量，金韦尔采用的是低剪切螺杆，所以可适当提高马达转速，以达到目的。同时在使用回收料方面（大部分为片料），本厂根据实际情况，为加大加料段的输送能力，也采取了加大落料口径在机筒里开槽等方式，取得了比较好的效果。
4、 Pet (polyester)
① The melting point is 250 ℃ ~ 260 ℃, while the molding temperature of blow molding grade PET is a little wider, about 255 ℃ ~ 290 ℃.
② Blow molding grade PET has high viscosity, great influence of temperature on viscosity and poor thermal stability.
Parameter selection
① L / D is generally 32, and the three-stage distribution L1 = 50% - 55%, L2 = 20%.
② The screw with low shear and low compression ratio is adopted to reduce the compression ratio ε ， Generally, it is 1.8 ~ 2. At the same time, shear overheating causes discoloration or opacity, H3 = 0.09d.
③ There is no mixing ring at the front end of the screw to prevent overheating and material storage.
④ Because this material is sensitive to temperature, most manufacturers use recycled materials. In order to improve the output, Jinwei adopts low shear screw, so the motor speed can be appropriately increased to achieve the purpose. At the same time, in the use of recycled materials (most of them are sheet materials), according to the actual situation, in order to increase the conveying capacity of the feeding section, the factory has also adopted the methods of increasing the blanking diameter and slotting in the barrel, which has achieved good results.
五．PVC（聚氯乙烯）
热敏性物料，一般分为硬质和软质，其区别在于原料中加入增塑剂的多少，少于10％的为硬质，多于30％为软质。
①无明显熔点，60℃变软，100℃～150℃粘弹态，140℃时熔融，同时分解，170℃分解迅速，软化点接近于分解点，分解释放于HC1气体。
②热稳定性差，温度、时间都会导致分解，流动性差。
设计原则a．温度控制严格，螺杆设计尽量要低剪切，防止过热。
b．螺杆、机筒要防腐蚀。
c．挤出工艺温度需严格控制。
5、 PVC (polyvinyl chloride)
Heat sensitive materials are generally divided into hard and soft materials. The difference lies in the amount of plasticizer added to the raw materials. Less than 10% is hard and more than 30% is soft.
① No obvious melting point, softened at 60 ℃, viscoelastic at 100 ℃ ~ 150 ℃, melted at 140 ℃ and decomposed at the same time. It decomposed rapidly at 170 ℃, the softening point was close to the decomposition point, and the decomposition released HC1 gas.
② Poor thermal stability, temperature and time will lead to decomposition and poor fluidity.
Design principle a. the temperature shall be strictly controlled, and the screw shall be designed with low shear as far as possible to prevent overheating.
b. The screw and barrel shall be anti-corrosion.
c. The extrusion process temperature needs to be strictly controlled
一般讲，螺杆参数为L／D=20～42，h3=0．07D，ε =1．6～2 ，L1=40％，L2=40％。 分离型螺杆，此种螺杆对硬质PVC较适合，而且为配合温控，加料段螺杆内部加冷却水或油孔，机筒外加冷水或油槽，温度控制精度±1℃左右。
Generally speaking, the screw parameters are L / D = 20 ~ 42, H3 = 0.07d, ε = 1．6～2 ，L1=40％，L2=40％。 Separate screw, which is more suitable for hard PVC. In order to cooperate with temperature control, cooling water or oil hole is added inside the screw in the feeding section, and cold water or oil tank is added to the barrel. The temperature control accuracy is about ± 1 ℃.
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