Generally, the screw is divided into three sections, namely the feeding section, the compression section, and the homogenization section
Feeding section: the bottom warp is small, the main role is to transport raw materials to the rear section, so it is mainly a problem of conveying capacity, parameters (L1, h1), h1 = (0.12-0. 14) D.
Compression section: The bottom warp changes, and the main role is to compact, melt the material, and establish pressure. Parameter compression ratios s=h1/h3 and L2. The accuracy should be graded A=(h1-h3)/L2.
Homogenization section (metering section): squeeze the melted material in the compression section to the front end of the screw at a quantitative and constant temperature, and the parameters (L3, h3), h3=(0.05-0.07)D.
For the entire screw, the parameter L/D - L/D ratio
L/D pros and cons: L/D and speed n, is an important factor in the plasticizing capacity and effect of the screw, L/D large material in the barrel for a long stay, conducive to plasticization, while the pressure flow, leakage is reduced, improve the plasticizing capacity, at the same time the temperature distribution requirements are higher for the material is beneficial, but after the large, there is a negative impact on the use of manufacturing assembly, generally L/D is (18~20), but there is a trend of increasing.
For other pitch S, the spiral angle φ=πDtgφ, and generally D=S, then φ=17°40'.
φ has an impact on the plasticizing capacity, and generally speaking, the larger the φ, the faster the conveying speed, so the φ of the material varies depending on the shape. Powder can be taken φ = about 25 °, cylindrical material φ = 17 °, square material φ = about 15 °, but the difference in φ is also more difficult for processing, so generally φ take 17 ° 40'.
Ridge width e, for materials with small viscosity, e should be larger as much as possible, too small and easy to leak, but too large to 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 test means, there is no complete design means for the design of the screw. Most of them have to formulate parameters according to different material properties to meet different needs based on experience, and each plant is roughly the same.
The following is a brief introduction to the design of several special screws combined with their material characteristics:
PC material (polycarbonate) features:
(1) Amorphous 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, the thermal stability is good in the normal processing temperature range, the long-term stay at 300 °C basically does not decompose, and the viscosity begins to decompose when it exceeds 340 °C, and the viscosity is less affected by the shear rate.
(3) Strong water absorption
Parameter selection:
a.L/D in view of its good thermal stability, viscosity characteristics, in order to improve the plasticizing effect as far as possible to choose a large aspect ratio, the factory takes 26. Due to its wide melting temperature range and long compression, a gradient screw is used. L1 = 30% of the full length, L2 = 46% of the full length.
b. Compression ratio: by the degree of gradation A needs to adapt to the melting rate, but the current melting rate can not be calculated, according to the characteristics of PC melting from 225 °C to 320 °C can be processed, the value of the gradation degree A can be relatively taken to the upper middle value, in the case of L2 is larger, the ordinary gradient screw s=2~3, the factory takes 2.6.
c. Because of its high viscosity and strong water absorption, a mixing structure is added to the screw before the homogenization section and after the compression section to strengthen the disintegration of the solid bed, and at the same time, the entrained water can be turned into a gas to escape.
d. Other parameters such as E, S, O and the gap with the barrel can be the same as other ordinary screws.
2. PMMA (Plexiglass) Features:
(1) The glass transition temperature is 105 °C, the melting temperature is greater than 160 °C, the decomposition temperature is 270 °C, 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 a gradient screw with a length-diameter ratio of 20~22, and the accuracy requirements of its product molding are generally L1=40% and L2=40%. b. The compression ratio is ε, generally 2.3~2. 6.
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 should not be too small.
3. PA (Nylon) Characteristics:
(1) Crystalline plastics, there are many types, different types, their melting points are not the same, and the melting point range is narrow, the melting point of PA66 generally used is 260 °C ~ 265 °C.
(2) Low viscosity, good fluidity, obvious melting point, poor thermal stability.
(3) General water absorption.
Parameter selection
a.L/D selects a mutant screw with an aspect ratio of 18~20.
b. Compression ratio, generally selected 3~3.5, in which to prevent overheating decomposition h3=0.07~0. 08D.
c. Because of its low viscosity, the gap between the check ring and the barrel should be as small as possible, about 0.05, and the gap between the screw and the barrel should be about 0.08.
d. Other parameters can be designed according to the general screw.
4. PET (Polyester) Characteristics:
(1) The melting point is 250 °C ~ 260 °C, and the molding temperature of blow molding grade PET is a little wider, about 255 °C ~ 290 °C.
(2) The viscosity of blow molding grade PET is high, and the temperature has a great influence on the viscosity, and the thermal stability is poor.
Parameter selection
(2) L/D is generally taken as 20, and the three-stage distribution is L1=50%-55%, and L2=20%.
(2) The screw with low shear and low compression ratio is adopted, and the compression ratio e is generally 1.8~2, and the 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 more sensitive to temperature, and the general manufacturer uses more 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 purpose. At the same time, in terms of the use of recycled materials (most of them are flakes), 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, and has achieved relatively good results.
PVC (Polyvinyl Chloride)
Heat-sensitive materials are generally divided into hard and soft, the difference lies in the amount of plasticizer added to the raw material, less than 10% is hard, and more than 30% is soft.
Peculiarity:
(1) There is no obvious melting point, it becomes soft at 60 °C, viscoelastic at 100 °C ~ 150 °C, melts at 140 °C, and decomposes at the same time, decomposes rapidly at 170 °C, the softening point is close to the decomposition point, and the decomposition is released in HC1 gas.
(2) Poor thermal stability, temperature and time will lead to decomposition and poor fluidity.
Design principles
a. The temperature is strictly controlled, and the screw design should be as low as possible to prevent overheating.
b. The screw and barrel should be corrosion-proof.
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 check ring, the head taper is 20 ° ~ 30 °, which is more suitable for soft rubber, such as the product requirements are higher, no metering section can be used, separated screw, this screw is more suitable for hard PVC, and in order to cooperate with temperature control, add cooling water or oil hole inside the screw of the feeding section, add cold water or oil groove outside the barrel, and the temperature control accuracy is about ± 2 °C.
Injection molding process of PC material and selection of screw
PC has excellent performance, high transparency, good impact toughness, creep resistance, wide temperature range, and the process characteristics of PC are: the sensitivity of melt viscosity to shear rate is small, and the sensitivity to temperature is large, there is no obvious melting point, the melt viscosity is high, the resin is easy to hydrolyze at high temperature, and the product is easy to crack. In view of these characteristics, we should pay special attention to the difference: to increase the fluidity of the melt, it is not by increasing the injection pressure but by increasing the injection temperature. The flow channel and gate of the mold are required to be short and thick to reduce the pressure loss of the fluid, and at the same time to have a high injection pressure. The resin needs to be fully dried before molding to keep its moisture content below 0.02%, and the resin should be kept warm during processing to prevent re-moisture absorption. Not only reasonable product design is required, but also the molding process should be correctly mastered, such as increasing the mold temperature, post-processing the product, etc., which can reduce or eliminate internal stress. Adjust the process parameters in time according to the different conditions of the product.
Let's talk about the molding process
1. The injection temperature must be based on the shape, size and mold structure of the product. The product performance, requirements and other aspects can be considered before it can be made. Generally, the temperature selected in the molding is between 270 ~ 320 °C, if the material temperature is too high to exceed 340 °C, the PC will decompose, the color of the product will become darker, and defects such as silver wires, dark bars, black spots, bubbles and other defects appear on the surface, and the physical and mechanical properties will also be significantly reduced.
2. Injection pressure has a certain impact on the physical and mechanical properties of PC products, internal stress, molding shrinkage, etc., which has a greater impact on the appearance and release of the product, too low or too high injection pressure will make the product have some defects, the general injection pressure is controlled between 80-120MPa, for thin-walled, long-process, complex shape, small gate products, in order to overcome the resistance of melt flow, in order to fill the mold cavity in time, a higher injection pressure (120--145MPa) is selected. The result is a complete product with a smooth surface.
3. Holding pressure and holding time The size of the holding pressure and the length of the holding time have a greater impact on the internal stress of PC products, the holding pressure is too small, the shrinkage effect is small, and it is easy to appear vacuum bubbles or concave on the surface, the holding pressure is too large, and the internal stress around the gate is easy to occur. The choice of holding time should depend on the thickness of the product, the size of the gate, the mold temperature, etc., generally small and thin products do not need a long holding time, on the contrary, the large and thick products should have a longer holding time. The length of the holding time can be determined by the test of the gate sealing time.
4. The injection speed has no obvious impact on the performance of PC products, except for thin-walled, small-gate, deep-hole, and long-process products, medium or slow processing is generally used, and it is best to use multi-stage injection, and generally use slow-fast-slow multi-stage injection method.
5. The mold temperature can be generally controlled at 80-100 °C, and it can also be increased to 100-120 °C for products with complex shapes, thinner and higher requirements, but it can not exceed the mold heat deflection temperature.
6. Screw speed and back pressure Due to the large viscosity of PC melt, it is beneficial to plasticization, exhaust, and maintenance of the plastic machine, and to prevent excessive screw load, and the speed requirements of the screw should not be too high, which is generally controlled at 30-60r/min, and the back pressure is controlled between 10-15% of the injection pressure.
7. The use of release agent should be strictly controlled in the injection molding process of PC, and the use of recycled material should not exceed three times, and the amount used should be about 20%.
Requirements for the production of PC products: the maximum injection amount of the product (including runner, gate, etc.) should not be greater than 70-80% of the nominal injection amount, the screw is selected with a single thread and other pitches, a gradient compression screw with a check loop, the length and diameter ratio of the screw is 15--20, and the geometric compression ratio is C/R
