Feeding section - the bottom diameter is small, its main function is to transport raw materials to the subsequent section, so the main problem is the transport capacity, parameters (L1, h1), h1 = (0.12-0.14) D.

Compression section - the bottom diameter changes, its main function is to compact and melt materials and build up pressure. Parameter compression ratio ε=h1/h3 and L2. The accuracy should be based on the gradient A=(h1-h3)/L2.

Homogenization section (metering section) - squeeze the melted material in the compression section to the front end of the screw in a quantitative and constant temperature, parameter (L3, h3), h3 = (0.05-0.07) D.

For the entire screw, the parameter L/D - aspect ratio

L/D pros and cons: L/D and rotation speed n are important factors in 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 beneficial to plasticization. At the same time, pressure flow and leakage flow will be reduced. It improves the plasticizing capacity and is beneficial to materials with higher temperature distribution requirements. However, when it is large, it will have a negative impact on manufacturing, assembly and use. Generally, L/D is (18~20), but there is a current trend of increasing it.

For 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 φ of the material will also change depending on the shape of the material. For powder materials, φ=25°, for cylindrical materials, φ=17°, and for square materials, φ=15°. However, the different φs make processing difficult, so generally φ=17°40′.

For the rib width e, for materials with low viscosity, e should be as large as possible. If it is too small, it will cause leakage, but if it is too large, it will increase power consumption and make it easy to overheat. e = (0.08 ~ 0.12) D.

The following is a brief introduction to the design of several special screws combined with their material characteristics:

one. 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 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 staying at 300°C for a long time. It starts to decompose when it exceeds 340°C. The viscosity is less affected by the shear rate.

③Strong water absorption

Parameter selection:

a. L/D In view of its good thermal stability and high viscosity characteristics, in order to improve the plasticizing effect, a large aspect ratio should be selected as much as possible. Our factory chooses 26.

Because of its wide melting temperature range and long compression, a progressive screw is used. L1=30% of the total length, L2=46% of the total 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 processing characteristics of PC from melting from 225℃ to 320℃, the gradient A value can be relatively medium. For the upper value, when L2 is larger, the ordinary gradual screw ε=2~3, our 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 enhance the disintegration of the solid bed and at the same time, allow the entrained water to turn into gas and escape.

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

two. PMMA (plexiglass)

Features: ① The 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 a gradual screw with an aspect ratio of 20 to 22. Depending on the accuracy requirements of the product molding, generally L1=40% and L2=40%.

b. Compression ratio ε is generally selected from 2.3 to 2.6.

c. In view of its certain hydrophilicity, a mixing ring structure is adopted at the front end of the screw.

d. Other parameters can generally be designed according to the universal screw, and the gap with the barrel should not be too small.

three. PA (nylon)

Characteristics: ① There are many types of crystalline plastics, different types have different melting points, and the melting point range is narrow. The melting point of commonly used PA66 is 260°C ~ 265°C.

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

③The water absorption is average.

Preferences:

a. L/D selects a mutant screw with an aspect ratio of 18 to 20.

b. The compression ratio is generally selected from 3 to 3.5, among which h3 = 0.07 to 0.08D to prevent overheating decomposition.

c. Because of its low viscosity, the gap between the non-return 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 non-return ring and a nozzle. The place should be self-locking.

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

Four. PET (polyester)

Characteristics: ①The melting point is 250℃～260℃. The molding temperature of blow molding grade PET is wider, about 255℃～290℃.

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

Preferences:

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

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

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

④ Because this material is more sensitive to temperature, and most manufacturers use recycled materials. In order to increase production, 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 (mostly flakes), according to the actual situation, in order to increase the conveying capacity of the feeding section, our 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. 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: ① No obvious melting point, softens at 60℃, viscoelastic state at 100℃~150℃, melts at 140℃ and decomposes at the same time, decomposes rapidly at 170℃, softening point is close to the decomposition point, decomposes and releases HC1 gas.

②Poor thermal stability, temperature and time will cause decomposition, and poor fluidity.

Design principles a. The temperature control is strict, and the screw design should be as low as possible to prevent overheating.

b. The screw and barrel must 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 accumulation of materials, there is no non-return ring, and the head taper is 20°~30°, which is more suitable for soft glue. If the product requirements are higher, a separated screw without metering section can be used. This kind of screw is more suitable for hard 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 tanks are added outside the barrel. The temperature control accuracy is about ±2°C.

PC material injection molding process and screw selection

PC has excellent performance, high transparency, good impact toughness, creep resistance, and wide operating temperature range. The process characteristics of PC are: the melt viscosity is less sensitive to shear rate, but more sensitive to temperature, and has no obvious melting point. The viscosity of the melt is high, the resin is easily hydrolyzed at high temperatures, and the product is easy to crack. For these characteristics, we must pay special attention to different treatment: to increase the fluidity of the melt, it should be achieved not by increasing the injection pressure but by increasing the injection temperature.

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 require a higher injection pressure. The resin needs to be fully dried before molding and processing to control the moisture content below 0.02%. In addition, insulation measures should be taken for the resin during processing to prevent re-absorption of moisture. Not only reasonable product design is required, but also the molding process should be correctly mastered. For example, increasing the mold temperature and post-processing the product can reduce or eliminate internal stress. Adjust process parameters in a timely manner according to different conditions of the product.

Let’s talk about the molding process:

1. Injection temperature must take into account the shape, size and mold structure of the product. This decision can only be made after considering various aspects such as product performance and requirements. Generally, the temperature selected during molding is between 270 and 320°C. If the material temperature is too high, such as exceeding 340°C, PC will decompose, the color of the product will become darker, and defects such as silver threads, dark stripes, black spots, and bubbles will appear on the surface. , while the physical and mechanical properties also decreased significantly.

2. Injection pressure has a certain impact on the physical and mechanical properties of PC products, internal stress, molding shrinkage, etc. It has a greater impact on the appearance and demoulding properties of the product. Injection pressure that is too low or too high will cause certain defects in the product. For these 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 injection pressure is selected. Injection pressure (120-145MPa). This results in a complete product with a smooth surface.

3. Holding pressure and holding time. The holding pressure and 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 vacuum bubbles or surface shrinkage may easily occur. Concave, the holding pressure is too large, and large internal stress is easily generated around the gate. In actual processing, high material temperature and low holding pressure are often used to solve the problem. 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 require a long holding time. On the contrary, large and thick products should have a longer holding time. The length of the pressure holding time can be determined by testing 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 speed is generally used for processing. Multi-stage injection is best, and slow injection is generally used. ——Fast-slow multi-stage injection method.

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

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

7. The use of PC release agent must be strictly controlled during the injection molding process. At the same time, the use of recycled materials cannot exceed three times, and the usage amount should be about 20%.

Requirements for plastic machines that produce PC products: It is required that the maximum injection volume of the product (including runners, gates, etc.) should not be greater than 70-80% of the nominal injection volume. The screw should be single-head thread and equal pitch, with a check ring. Gradually compressed screw, the length-to-diameter ratio L/D of the screw is 15-20, and the geometric compression ratio C/R