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The basic type and main parameters of injection molding machine screw

 

Generally, the screw is divided into three sections, namely the feeding section, the compression section, and the homogenization section.

 

Feeding section - the bottom diameter is small, the main function is to transport raw materials to the latter section, so it is mainly a problem of conveying capacity, parameters (L1, h1), h1=(0.12-0.14)D.

 

Compression section - the basal length changes, the main function is to compact and melt the material and build up the pressure. Parameter compression ratio ε=h1/h3 and L2. Accurately, the gradient should be A=(h1-h3)/L2.

 

Homogenization section (metering section) - extrude the molten material in the compression section to the front end of the screw quantitatively and at a constant temperature, parameters (L3, h3), h3=(0.05-0.07)D.

 

For the whole screw, the parameter L/D-length-diameter ratio

 

L/D Advantages and Disadvantages: L/D and speed n are important factors for the plasticizing capacity and effect of the screw. If L/D is large, the material will stay in the barrel for a long time, which is conducive to plasticizing. At the same time, the pressure flow and leakage flow are reduced. It improves the plasticizing ability, and is beneficial to materials with high temperature distribution requirements, but when it is large, it has a negative impact on the use of manufacturing and assembly. Generally, the L/D is (18 to 20), but there is a tendency to increase at present. 

 

Other pitch S, helix angle φ=πDtgφ, generally D=S, then φ=17°40′. φ has an impact 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 material is preferably about φ=25°, the cylindrical material is about φ=17°, and the block material is about φ=15°, but the difference in φ is also difficult for processing, so generally φ is 17°40′.

 

Width of edge e, for materials with low viscosity, e should be as large as possible, too small is easy to leak, but too large will increase power consumption and easy to overheat, e=(0.08～0.12)D.

 

All in all, under the current circumstances, due to the lack of necessary test methods, there is no complete design method for screw design. Most of them need to formulate parameters according to different material properties and experience to meet different needs, and each factory is roughly the same. 

 

The following SUDA briefly introduces the design of several special screws combined with their material characteristics:

 

1. PC material (polycarbonate)

 

Features: ①Amorphous plastic, no obvious melting point, glass transition temperature 140°～150°C, melting temperature 215°C～225°C, molding temperature 250°C～320°C.

 

②The viscosity is high, and it is sensitive to temperature. It has good thermal stability in the normal processing temperature range. It basically does not decompose after staying at 300℃ for a long time. When it exceeds 340℃, it starts to decompose. The viscosity is less affected by the shear rate.

 

③Strong water absorption

 

Parameter selection:

 

a. In view of its good thermal stability and high viscosity, L/D should choose a large aspect ratio as much as possible to improve the plasticizing effect, which is 26 in our factory.

 

Due to its wide melting temperature range and longer compression, a gradient screw is used. L1=30% full length, L2=46% full length.

 

b. The compression ratio ε needs to be adapted to the melting rate from the gradient A, but the melting rate cannot be calculated at present. According to the processability of PC from 225 ° C to 320 ° C, the value of the gradient A can be relatively moderate. The upper value, in the case of larger L2, the ordinary gradient screw ε=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 gas to escape.

 

d. Other parameters such as e, s, φ and the clearance with the barrel can be the same as other ordinary screws. 

 

two. PMMA (plexiglass)

 

Features: ①The glass transition temperature is 105℃, the melting temperature is greater than 160℃, the decomposition temperature is 270℃, and the molding temperature range is very wide.

 

②High viscosity, poor fluidity and good thermal stability.

 

③ Strong water absorption.

 

     Preferences 

 

a. L/D selects the graded screw with the length-diameter ratio of 20 to 22, and generally L1=40%, L2=40%, depending on the precision requirements of its product molding.

 

b. The compression ratio ε is generally selected from 2.3 to 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. 

 

three. PA (Nylon)

 

Features: ①Crystalline plastics have many types, different types, different melting points, and narrow melting point range. Generally, the melting point of PA66 is 260℃～265℃.

 

②Low viscosity, good fluidity, obvious melting point and poor thermal stability.

 

③The water absorption is average.

 

     Preferences 

 

a. L/D selects a mutant screw with a length-diameter ratio of 18-20.

 

b. The compression ratio is generally selected from 3 to 3.5, and h3 = 0.07 to 0.08D to prevent overheating decomposition. 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 is about 0.08. If necessary, depending on the material, the front end can be equipped with a check ring and a nozzle. should be self-locking.

 

d. Other parameters can be designed according to general screw.

 

Four. PET (polyester)

 

Features: ①The melting point is 250℃～260℃, and the blow molding grade PET has a wider molding temperature, about 255℃～290℃.

 

②Blow molding grade PET has high viscosity, temperature has a great influence on viscosity, and thermal stability is poor.

 

     Preferences 

 

①L/D is generally taken as 20, three-stage distribution L1=50%-55%, L2=20%.

 

②Use a screw with low shear and low compression ratio, and the compression ratio ε is generally 1.8 to 2. At the same time, the shear overheating leads to discoloration or opacity h3=0.09D.

 

③There is no mixing ring at the front end of the screw to prevent overheating and material storage.

 

④Because this material is more sensitive to temperature, and most 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 purpose. At the same time, in terms of using 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 also adopted methods such as increasing the blanking diameter and grooving in the barrel, and achieved relatively good results. Effect. 

 

five. 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.

 

Features: ①No obvious melting point, it becomes soft at 60℃, viscoelastic at 100℃～150℃, melts at 140℃, decomposes at the same time, decomposes rapidly at 170℃, the softening point is close to the decomposition point, and the decomposition is released in HC1 gas.

 

②Poor thermal stability, temperature and time will cause 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 anti-corrosion.

 

c. The injection molding machine 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 material accumulation, there is no non-reverse ring, and the taper of the head is 20°~30°, which is more suitable for soft rubber. If the product requirements are higher, a separate screw without metering section can be used. This screw is more suitable for rigid PVC. , And in order to cooperate with temperature control, cooling water or oil holes are added inside the screw in the feeding section, and cold water or oil grooves are added to the barrel, and the temperature control accuracy is about ±2 °C.

 

Injection molding machine process of PC material and selection of screw

 

PC has excellent performance, high transparency, good impact toughness, creep resistance, and wide temperature range. 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 different treatment: to increase the fluidity of the melt, it should be achieved by increasing the injection temperature instead of increasing the injection pressure. The runner 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, a higher injection pressure is required. The resin needs to be fully dried before molding, so that its moisture content is controlled below 0.02%. In addition, insulation measures should be taken for the resin during processing to prevent re-absorption of moisture. Not only a reasonable product design is required, but also a correct grasp of the molding process, such as increasing the mold temperature and post-processing the product, can reduce or eliminate internal stress. Adjust the process parameters in time depending on the different conditions of the product. 

 

Let's talk about the molding process

 

1. The injection temperature must be integrated with the shape, size and mold structure of the product. Product performance, requirements and other aspects of the situation can be considered before making. Generally, the temperature is selected between 270 and 320 °C in the molding process. When the material temperature is too high, such as over 340 °C, the PC will decompose, the color of the product will become darker, and defects such as silver wires, dark stripes, black spots, and bubbles will appear on the surface. , while the physical and mechanical properties are also significantly reduced.

 

2. The injection pressure has a certain influence on the physical and mechanical properties, internal stress, and molding shrinkage rate of the PC product, which has a great influence on the appearance and mold release of the product. Too low or too high injection pressure will cause the product to appear certain. For some defects, the injection pressure is generally controlled between 80-120MPa. For products with thin walls, long processes, complex shapes and small gates, in order to overcome the resistance of the melt flow and fill the mold cavity in time, a higher value is selected. The injection pressure (120-145MPa). Thereby a complete and smooth surface product is obtained.

 

3. Holding pressure and holding time The size of the holding pressure and the length of the holding time have a great influence on the internal stress of PC products. If the holding pressure is too small, the feeding effect is small, and it is easy to cause vacuum bubbles or surface shrinkage. Concave, the holding pressure is too large, and large internal stress is easily generated around the gate. In actual processing, it is often solved by means of high material temperature and low holding pressure. The selection of holding time should depend on the thickness of the product, gate size, mold temperature, etc. Generally, small and thin products do not need a long holding time. On the contrary, 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 effect on the performance of PC products. Except for thin-walled, small gate, deep-hole, and long-process products, medium-speed or slow-speed processing is generally used, preferably multi-stage injection, generally slow- - Fast - slow multi-stage injection.

 

5. The mold temperature can generally be controlled at 80-100 °C. For products with complex shapes, thinner products and higher requirements, it can also be increased to 100-120 °C, but it cannot exceed the mold thermal deformation temperature.

 

6. Due to the high viscosity of PC melt, the screw speed and back pressure are favorable for plasticization, exhaust, and maintenance of the plastic machine, preventing the screw from being overloaded. The screw speed requirements should not be too high, generally controlled at 30 ——60r/min is appropriate, and the back pressure should be controlled between 10-15% of the injection pressure.

 

7. PC should strictly control the use of mold release agent in the process of injection molding machine. At the same time, 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 large injection volume of the product (including runners, gates, etc.) should not be greater than 70-80% of the nominal injection volume. Gradient compression type screw, the length-diameter ratio L/D of the screw is 15-20.

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