Generally, the screw is divided into three sections, […]
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.
One. 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 parameter selection:
a. L/D is characterized by its good thermal stability and high viscosity. In order to improve the plasticizing effect, the long aspect ratio is selected as much as possible.
Due to its wide melting temperature range and long compression, a tapered screw is used. L1 = 30% full length, L2 = 46% full length.
b. The compression ratio ε is adapted to the melting rate by the gradient A. However, the current melting rate cannot be calculated. According to the processing characteristics of PC from 225 ° C to 320 ° C, the gradient A value can be relatively medium. The value of the upper side, in the case of a large L2, the ordinary gradient type screw ε = 2 ~ 3, the factory takes 2.6.
c. Because of its high viscosity and high water absorption, before the homogenization section, the mixing section is added to the screw after the compression section to strengthen the solid bed disintegration, and at the same time, the entrained water can be turned into gas.
d. Other parameters such as e, s, φ and the gap with the barrel can be the same as other common screws.
two. PMMA (Plexiglas)
Features: 1 glass transition temperature 105 ° C, melting temperature greater than 160 ° C, decomposition temperature 270 ° C, molding temperature range is very wide.
2 The viscosity is large, the fluidity is poor, and the thermal stability is good.
3 strong water absorption.
a. L/D selects a progressive type screw with a length to diameter ratio of 20 to 22, and the precision of the product forming is generally L1=40% and L2=40%.
b. The compression ratio ε is generally selected from 2.3 to 2.6.
c. For its hydrophilicity, a mixing ring structure is used at the front end of the screw.
d. Other parameters can generally be designed according to the general screw, and the clearance between the barrel and the barrel should not be too small.
three. PA (nylon)
Characteristics: 1 crystalline plastics, many types, different types, different melting points, and a narrow melting point range, generally used PA66 melting point of 260 ° C ~ 265 ° C.
2 low viscosity, good fluidity, relatively obvious melting point, poor thermal stability.
3 water absorption is general.
a. L/D selects a mutant screw with a length to diameter ratio of 18-20.
b. The compression ratio is generally selected from 3 to 3.5, in which the overheat decomposition is prevented from h3=0.07 to 0.08D.
c. Because of its low viscosity, the gap between the reverse ring and the barrel should be as small as possible, about 0.05, and the gap between the screw and the barrel is about 0.08. If necessary, depending on the material, the front end can be matched with the reverse ring, the nozzle The place should be self-locking.
d. Other parameters can be designed according to the universal screw.
four. PET (polyester)
Characteristics: 1 melting point 250 ° C ~ 260 ° C, blow molding PET is a much broader molding temperature, about 255 ° C ~ 290 ° C.
2Blow molding grade PET has high viscosity, temperature has great influence on viscosity and poor thermal stability.
1L/D generally takes 20, and the three-segment distribution L1=50%-55%, L2=20%.
2 using a low shear, low compression ratio of the screw, compression ratio ε, generally take 1.8 ~ 2, while shearing overheating leads to discoloration or opacity h3 = 0.009D.
3 There is no mixing ring at the front end of the screw to prevent overheating and storage.
4 Because this material is sensitive to temperature, and the general manufacturers use recycled materials. In order to increase the output, our factory uses a low shear screw, so the motor speed can be appropriately increased to achieve the goal. At the same time, in the use of recycled materials (mostly flakes), according to the actual situation, the factory has increased the feeding capacity of the feeding section, and also adopted the method of increasing the blanking caliber in the barrel and so on. Effect.
Fives. PVC (polyvinyl chloride)
Heat sensitive materials are generally classified into hard and soft, the difference being the amount of plasticizer added to the raw material, less than 10% being hard, and more than 30% being soft.
Features: 1 no obvious melting point, softening at 60 °C, viscoelastic state at 100 °C ~ 150 °C, melting at 140 °C, simultaneous decomposition, rapid decomposition at 170 °C, softening point close to decomposition point, decomposition released in HC1 gas.
2 The thermal stability is poor, and the temperature and time will cause decomposition and poor fluidity.
Design principle a. The temperature control is strict, and the screw design should be as low as possible to prevent overheating.
b. The screw and barrel should be protected against corrosion.
c. The injection molding process needs to be strictly controlled.
Generally speaking, the screw parameters are L/D=16-20, h3=0.07D, ε=1.6~2, L1=40%, L2=40%.
In order to prevent the storage of materials, there is no end reverse ring, the head taper is 20°~30°, which is suitable for soft rubber. If the product requirements are high, it can be used without measuring section and separating screw. This kind of screw is suitable for hard PVC. And in order to cooperate with the temperature control, the cooling screw or oil hole is added inside the screw of the feeding section, and the cold water or oil groove is added to the barrel, and the temperature control precision is about ±2 °C.