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Basic knowledge Of plastics

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Origin of plastic

Plastics in English is "Plastics" or "Plastic", which comes from the Greek "plastics", which means ""molding" and "development".

Plastic is taken from the characteristics of plastic itself, which means "free to form".


Classification of plastics

1. Classification by application fields

(1) General plastics: can only be used as non-structural materials, with large output, relatively low price, average performance, and many manufacturers.

(2) Engineering plastics: can be used as structural materials with excellent mechanical properties, electrical properties, heat resistance, chemical resistance, dimensional stability and processability, etc.

(3) It can be divided into general engineering plastics and special engineering plastics. The latter has higher performance, smaller output, few manufacturers, and higher sales price.


2. Classification according to the physical and chemical properties of plastics:

(1) Thermoplastics: During the processing process, generally only the physical volume changes. That is, plastics that can be repeatedly heated, softened and cooled to harden within a specific temperature range. (Common: polyvinyl chloride, polyethylene, polypropylene, polystyrene, ABS, SAN, nylon, polyformaldehyde, polycarbonate, polyphenylene ether, polymethacrylate, polyurethane, polyethylene terephthalate ester)

(2) Thermosetting plastics: chemical changes occur during processing. When heated, they begin to soften and have a certain degree of plasticity. However, with further heating, the resin hardens and shapes. If reheated, it generally cannot be remolded and can only be crushed for use. filler.


Thermoplastics are divided into crystalline plastics and amorphous plastics

Applications of plastics: Crystalline plastics: films, washing machines, gears, pipes, safety helmets, fiber products, ski shoe bodies

Amorphous plastics: TV casings, office supplies, pipes, plates, profiles, fan blades and mobile phones

Thermosetting plastics: packaging, adhesives, trays, billiard balls, bathtubs, etc.

Elastomer plastic: foam body, belt, rope, film, bowling ball, etc. (Elastomer is also a thermoplastic plastic)


3. Crystalline and amorphous materials are separated due to different molecular structures.

(1) Crystalline plastics: PA, PP, PE, POM, PBT, PET, PPO;

(2) Amorphous plastics: PC, ABS, PS, PVC, PMMA.

Crystalline plastics have a distinct melting point, and the molecules are regularly arranged when solid. The regularly arranged area is called the crystalline area, the disorderly arranged area is called the amorphous area, and the percentage of the crystalline area is called crystallinity. Generally, polymers with a crystallinity of more than 80% are called crystalline plastics.


Factors affecting crystallization

(1) Polymer chain structure, polymers with good symmetry, no or few branches, small side groups, and large intermolecular forces are easy to be close to each other and prone to crystallization.

(2) Temperature, the polymer moves from the disordered coils to the surface of the growing crystal. When the mold temperature is higher, the mobility of the polymer is increased and crystallization is accelerated.

(3) Pressure. If there is external force during the cooling process, it can also promote the crystallization of the polymer. Therefore, the injection pressure and holding pressure can be increased during production to control the crystallinity of crystalline plastics.

(4) Nucleation agent, because low temperature is conducive to rapid nucleation, but slows down the growth of grains. Therefore, in order to eliminate this contradiction, nucleation agents are added to the molding material, so that the plastic can be processed at high mold temperatures. Rapid crystallization.


Effect of crystallization on plastic properties

(1) Mechanical properties: Crystallization makes plastics brittle (impact strength reduced), strong toughness, and poor ductility.

(2) Optical properties. Crystallization makes the plastic opaque because light scattering occurs at the interface between the crystalline region and the amorphous region. Reducing the size of the spherulites to a certain degree not only increases the strength of the plastic (reducing intergranular defects) but also improves the transparency (no scattering occurs when the size of the spherulites is smaller than the wavelength of light).

(3) Thermal properties. Crystalline plastics do not appear in a highly elastic state when the temperature rises. When the temperature rises to the melting temperature TM, they exhibit a viscous flow state. Therefore, the use temperature of crystalline plastics is increased from Tg (glass transition temperature) to TM (melting temperature).

(4) Solvent resistance, permeability, etc. are improved because the crystal components are arranged more closely.

Crystalline Plastics for Injection Molding Machines and Molds

(5) Crystalline plastics require more energy to destroy the crystal lattice when melting, so more heat needs to be input when converting from solid to molten melt. Therefore, the plasticizing capacity of the injection molding machine must be large, and the maximum injection volume must also be large. Increase accordingly.

(6) The melting point range of crystalline plastics is narrow. In order to prevent the rubber crystal from clogging the nozzle when the temperature of the nozzle decreases, the aperture of the nozzle should be appropriately enlarged, and a heating ring that can independently control the temperature of the nozzle should be installed.

(7) Since the mold temperature has an important impact on the crystallinity, there should be as many water channels in the mold as possible to ensure uniform mold temperature during molding.

(8) Crystallinity undergoes a large volume shrinkage during the crystallization process, causing a large molding shrinkage. Therefore, the molding shrinkage must be carefully considered in mold design.

(9) Due to the significant anisotropy and large internal stress, attention should be paid to the location and size of the gate, the location and size of the reinforcement ribs in the mold design, otherwise warping deformation will easily occur, and it will be quite difficult to improve it through the molding process. of.

(10) The degree of crystallinity is related to the wall thickness of the plastic part. The wall thickness cools slowly, the crystallinity is high, the shrinkage is large, and shrinkage holes and pores are prone to occur. Therefore, attention should be paid to controlling the wall thickness of the plastic part during mold design.


Molding process of crystalline plastics

(1) The heat released during cooling is large, so it must be fully cooled. Pay attention to the control of cooling time when molding at high mold temperatures.

(2) The difference in specific gravity between the molten state and the solid state is large, resulting in large molding shrinkage and prone to shrinkage and pores. Pay attention to the setting of the holding pressure.

(3) When the mold temperature is low, cooling is fast, crystallinity is low, shrinkage is small, and transparency is high. The degree of crystallinity is related to the wall thickness of the plastic part. When the wall thickness of the plastic part is large, the cooling is slow, the crystallinity is high, the shrinkage is large, and the physical properties are good. Therefore, the mold temperature of crystalline plastics must be controlled as required.

(4) The anisotropy is significant and the internal stress is large. The uncrystallized molecules tend to continue to crystallize after demoulding. They are in a state of energy imbalance and are prone to deformation and warping. The material temperature should be appropriately increased and the mold temperature should be increased to medium. Injection pressure and injection speed.


Post-processing differences

(1) The surface of crystalline plastic is slippery and cannot be painted or chromed. It is difficult to decorate the surface. The colors currently painted are not long-lasting and fall off easily. For example, when spraying, printing, or coloring packaging bags, high-current electronic cone hair is generally used to print colors, but it is not long-lasting and the adhesive is difficult to find.

(2) The surface of amorphous plastic can absorb other molecules, such as ink, chrome plating, spray coating, etc. Therefore, general product casings, watch cases, TV cases, etc. are made of amorphous plastic, which is easy to decorate and not easy to fall off.


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