Classification and selection of extruder screws


The extruder extrusion system includes a screw, a barre […]

The extruder extrusion system includes a screw, a barrel, a hopper, a machine head, and a mold. The plastic is plasticized into a uniform melt by an extrusion system, and continuously extruded by the screw under the pressure established in the process. The nose.
  Screw: It is the most important part of the extruder. It is directly related to the application range and productivity of the extruder. It is made of high strength and corrosion resistant alloy steel.

The basic type and main parameters of the screw
     Generally, the screw is divided into three sections, namely, a feeding section, a compression section, and a homogenization section.
     Feeding section - the bottom is small, the main function is to transport the raw material to the rear section, so it is mainly the conveying capacity problem, the parameter (L1, h1), h1 = (0.12-0.14) D.
     Compression section - the bottom is changed, the main function is to compact and melt the material and establish pressure. The parameter compression ratio is ε=h1/h3 and L2. Accuracy should be based on the gradient A = (h1-h3) / L2.
     Homogenization section (metering section)——Squeeze the molten material of the compression section to the foremost end of the screw at a fixed temperature, the parameter (L3, h3), h3=(0.05-0.07)D.
     For the entire screw, the parameter L/D-length to diameter ratio
     L/D pros and cons: L/D and speed n are important factors for screw plasticizing ability and effect. L/D is large, the material stays in the barrel for a long time, which is beneficial to plasticization, and the pressure flow and leakage are reduced. The plasticizing ability is improved, and at the same time, the material with higher temperature distribution is favorable, but after the big, it has a negative impact on the manufacturing and assembly. Generally, the L/D is (18-20), but there is an increasing trend.
     The other pitch S, the helix angle φ = πDtgφ, generally D = S, then φ = 17 ° 40 '.
     φ has an influence on the plasticizing ability. Generally speaking, the larger the φ, the faster the conveying speed. Therefore, the shape of the material is different, and the φ also changes. The powder can take φ=25°, the cylindrical material φ=17°, and the square material φ=15°, but the difference of φ is also difficult for processing, so the general φ is 17°40′.
     The rib width e, for materials with small viscosity, e is as large as possible, too small and easy to leak, but too large will increase power consumption, easy to overheat, e = (0.08 ~ 0.12) D.
     All in all, in the current situation, due to the lack of necessary testing methods, there is no complete design method for the design of the screw. Most of them have to formulate parameters according to different materials according to their experience to meet different needs. The plants are roughly the same.
     The following is a brief introduction of the design of several special screws combined with the material characteristics: PC material (polycarbonate)
     Features: 1 non-crystalline plastic, no obvious melting point, glass transition temperature 140 ° ~ 150 ° C, melting temperature 215 ° C ~ 225 ° C, molding temperature 250 ° C ~ 320 ° C.
     2 The viscosity is large, sensitive to temperature, and the thermal stability is better in the normal processing temperature range. The residence at 300 ° C is not decomposed for a long time, and the decomposition starts at 340 ° C, and the viscosity is less affected by the shear rate.
     3 strong water absorption