Decision on the production status of the meshing contract

Let's study these parameters together.

The main sources of heat for the increase in material temperature during twin-screw extrusion process are:

1) The heat conduction of the cylinder wall to the material;

2) The viscous dissipation heat generated by the shear action of materials in a twin-screw.

Temperature directly affects the viscosity of materials. As the temperature increases, the activity of molecules increases, and intermolecular interactions such as molecular diffusion, chain arrangement, entanglement, and internal friction of molecules decrease, resulting in a decrease in material viscosity. For temperature sensitive materials, when the temperature exceeds a certain limit value, accidents such as combustion and explosion may occur. Therefore, in actual operation, temperature should be strictly controlled and operated within a safe temperature range.

In the twin-screw extruder, the lead of the forward threaded element remains unchanged, and compared with the absence of the reverse threaded element, the temperature peak value of adding the reverse threaded element in the twin-screw extruder increases significantly; In a twin-screw extruder, the lead of the reverse threaded element remains unchanged, while the lead of the forward threaded element decreases and the temperature peak increases. It is because in the process of twin-screw mixing of materials, both the reverse threaded element and the small lead forward threaded element help to establish pressure during the mixing process, improve the filling rate of materials in the screw, increase friction between materials and the cylinder wall, and raise the temperature of materials.

Pressure has a certain impact on the fluidity of materials. Due to the effect of pressure, the free volume inside the material molecules will decrease, leading to a decrease in the activity of molecular chains, an increase in viscosity, an increase in pressure, a decrease in material fluidity, and an increase in viscosity. During the process of twin-screw mixing materials, the material will be subjected to a certain external pressure. Under the pressure, the distance between molecules will be reduced, the range of molecular chain movement will be reduced, the intermolecular interaction will be enhanced, and the viscosity will increase. In the actual production process, if the pressure is too high, it will cause the material viscosity to be too high, making the processing difficult. For pressure sensitive substances, when the pressure exceeds the limit value, it can cause accidents such as combustion or explosion.

Compared to reverse kneading blocks, reverse threaded components can establish greater pressure. In practical engineering, reverse threaded components or reverse kneading blocks are generally placed in front of the screw exhaust section to establish a certain degree of vacuum. Although the reverse threaded component can establish a higher vacuum degree than the reverse kneading block, which helps with gas discharge, it can lead to increased head resistance, increased screw torque, and reduced production. Therefore, the structure of the twin-screw extruder needs to be selected based on the actual processing object and operating process.

Viscous dissipation is the work done during the mixing process of materials to overcome the resistance caused by their own viscosity, which is further converted into the internal energy of the material itself.

During the mixing process of a twin-screw extruder, the shear rate of the polymer fluid in the flow channel directly affects the viscosity of the material, thereby affecting the viscosity dissipation value of the material mixing process. If the shear rate is too high, the viscosity dissipation will be greater, the polymer melt will rupture, and even mechanical degradation will occur. If it is too small, it will not meet the needs of polymer processing and affect the final quality of the product

Mixing is a process of reducing non-uniformity between materials. According to the Brodkey mixing mechanism, the mixing process involves three forms of molecular motion: molecular diffusion, volume diffusion, and vortex diffusion. In a twin-screw extruder, the material is stretched or sheared during screw rotation, resulting in tensile or rotational flow. In order to quantitatively characterize the degree of material mixing, the concept of mixing index is introduced, which is the ratio of the stretching rate tensor to the sum of the stretching rate tensor and the rotational tensor.

During the mixing process of materials in a twin-screw extruder, the mixing index reflects the interrelationship between shear and stretching effects. The higher the mixing index, the more significant the influence of stretching during the material mixing process.

An increase in the lead of the forward kneading block and the lead of the reverse threaded element will both enhance the stretching effect of the material in the twin-screw extruder, promoting better mixing and uniformity of the material in the extruder. However, in actual production, the mixing index is difficult to directly explain the mixing effect of materials in the twin-screw extrusion process, so this can only be used as a reference and should still be based on reality.

The residence time distribution curve is an important means to describe axial mixing in a twin-screw extruder, and the average residence time of materials in the twin-screw extruder directly affects the quality of the final product. The longer the residence time of the material in a twin-screw extruder, the more continuous mixing effect the material has in the barrel. After a sufficient residence time, the mixing state of the material will reach its optimal state. On the contrary, the shorter the residence time of the material in the twin-screw extruder, the stronger the self-cleaning ability of the screw.

Reverse threaded components and kneading blocks can prolong the residence time of materials in the twin-screw and enhance mixing.

For twin-screw extruders, torque is an important parameter that can characterize the load-bearing capacity of the twin-screw extruder and protect its safe operation. In actual processing, it is necessary to study the viscoelasticity of polymers based on the torque limit of the twin-screw extruder to ensure that the extrusion process is within a safe and controllable range.

1. The temperature peak increases with the increase of the lead of the reverse threaded element and decreases with the increase of the lead of the forward threaded element.

2. Compared to reverse kneading blocks, reverse threaded components can establish greater pressure. In practical engineering, reverse threaded components or reverse kneading blocks are generally placed in front of the screw exhaust section to establish a certain degree of vacuum.

3. Adding reverse threaded elements and reverse kneading block elements to the screw components will lead to an increase in screw filling and viscosity dissipation.

4. The mixing index increases as the lead of the forward kneading block and the lead of the reverse threaded element increase.

5. Reverse threaded components and kneading blocks can extend the residence time of materials in twin screws.

6. The highly significant influencing factors of torque are the lead of forward threaded components and the lead of reverse threaded components.

PC is a common engineering plastic, but ordinary PC materials have poor impact resistance and chemical resistance, and cannot be applied in many fields. The usual approach is to modify ordinary PC, such as PC/ABS and PC/PBT alloys, but sometimes transparency or rigidity may be lost. And through copolymerization, higher performance products can be obtained.

There are various types of PC copolymerization, and today we will use a few questions to mainly introduce the commonly used commercially available - silicon copolymerization modified PC.

 What level of modification does silicon copolymerization modification of PC belong to?

The three levels of polymer material modification:

The first method involves adding small molecule additives and short aspect ratio fillers to achieve lubrication, plasticization, filling, and reinforcement effects, due to the limited entanglement force between the filling material and the polymer molecular chains.

2. The second method involves blending and modifying two types of polymers, which have more entangled molecular chains and complementary properties, resulting in a more comprehensive performance.

3. The third method involves copolymerization modification, which essentially combines the two polymers together. The performance is more stable than that of blending, and the comprehensive performance better reflects the characteristics of the two different polymers.

By copolymerizing and modifying PC with organic silicon components, the flowability, hydrolysis resistance, high and low temperature resistance, low temperature impact resistance, and flame retardancy of PC materials have been improved. Greatly improving the comprehensive cost-effectiveness of PC materials.

What are the characteristics of silicon co polymer PC?

Introducing flexible organosilicon segments increases the length of the soft segments, reduces the rigidity of the benzene ring, and increases the flexibility of the molecular chains, thereby improving the fluidity of PC materials.

The introduced organosiloxane "- Si-O-Si -" is a hydrophobic group that can effectively change the surface properties of materials, making them hydrophobic. Therefore, the hydrolysis resistance of polycarbonate is greatly improved.

Due to the unique structure of organic silicon, it combines the properties of both inorganic and organic materials. It has basic properties such as low surface tension, high compressibility, high gas permeability, and excellent characteristics such as high and low temperature resistance, electrical insulation, oxidation stability, weather resistance, flame retardancy, hydrophobicity, corrosion resistance, non-toxic and odorless, and physiological inertness.

Therefore, by incorporating organic silicon into the PC, the high and low temperature resistance of the PC is improved, resulting in a notch impact strength of 40KJ/m between -30 ℃ and -40 ℃ ²， Still able to maintain mechanical properties at room temperature; And the oxidation stability and weather resistance characteristics of organic silicon improve the oxidation resistance performance of PC; Enhanced resistance to yellowing.

Is there no drawback to silicon co polymerization modified PC?

The preparation process of silicon copolymer PC is long and complex, and its polymerization requires the use of interface phosgene method; In addition, at the consumer level, the downstream market of silicon copolymer PC has relatively limited consumption scenarios, with long certification cycles in some fields, small product batches, high costs, and obvious disadvantages in cost-effectiveness.

Benefiting from factors such as good performance characteristics and production costs (including monomer PDMS prices, depreciation costs, etc.) support, the price of silicon copolymer PCs has always been relatively strong. The market price of typical brands such as SABIC EXL9330 is about 2-3 times that of ordinary bisphenol A type PCs. It is precisely due to price reasons that the current consumption of silicon copolymer PCs is mainly concentrated in the fields of 5G base stations and photovoltaics.

Processing ability still slightly lacking：

Because the substrate is PC, although the fluidity of silicon copolymer PC products is slightly improved, there is still a certain distance compared to other categories. For large-sized parts, the yield rate is relatively average.

In the copolymerization system of polysiloxane and PC, the length of siloxane segments and the content of siloxane in the copolymer have a significant impact on the aggregated structure and properties of the final polymer. Research has shown that low temperature impact resistance and other properties are directly related to the content of siloxane. When the content of siloxane exceeds 20%, it is easy to phase out. The smaller the chain segment of siloxane, the easier it is to obtain a homogeneous copolymer. If the chain segment of siloxane is too long or the dosage is too high, it will have an impact on the appearance of the product.

Therefore, in downstream practical applications, it is often necessary to modify high silicon content silicon copolymer PC to reduce silicon content and meet usage requirements.

In addition, with the rapid development of the domestic new energy vehicle industry, as one of the ideal shell materials for products such as charging piles and charging guns, silicon copolymer PC often requires partial flame retardant modification.

Since the 1960s, companies such as General Electric Plastics (GE) and Bayer (now Covestro) have been developing silicon copolymer PCs, with GE being the most successful. After 2000, based on the promising market prospects of silicon copolymer PC, engineering plastic giants such as Chuguang Xingchan and Teijin in Japan, as well as Sanyo and LG in South Korea, have successively launched their own silicon copolymer PC products. After acquiring GE Plastics in 2007, SABIC continued to invest in research and development in the field of silicon copolymer PCs. After years of development, it has become the leader in the silicon copolymer PC market.