There are 3 Categories of Plastics

There are many categories of plastics. At present, tens of thousands of machinable plastic raw materials (including modified plastics) have been produced in the world, and more than 300 kinds are commonly used. There are also many ways to classify plastics. The commonly used categories of plastics methods are as follows.

Categories of Plastics: According to the resin molecular structure and performance after heating

According to the molecular structure and performance of the resin after heating, it can be divided into two categories of plastics: thermoplastic materials and thermosetting plastics materials.

• Categories of Plastics: Thermoplastic Materials

The resin’s molecular structure in thermoplastics is linear or branched, often called a linear polymer. It is molded into a certain shape of plastic products when heated and maintains the set shape after cooling. If it is heated again, it can be softened and melted and can be made into a plastic product of a certain shape again, which can be repeated many times and is reversible.

There is generally no chemical change in the above molding process, only physical change. Because thermoplastics have the above-mentioned reversible characteristics, scraps are produced in plastic processing. Waste products can be recycled, crushed into granules, and mixed into raw materials for use.

Thermoplastics can be divided into two types: crystalline plastics and amorphous plastics. The molecular chains of crystalline plastics are neatly arranged, stable, and compact, while the molecular chains of amorphous plastics are arranged in disorder. Therefore, crystalline plastics are generally more heat-resistant, opaque, and have higher mechanical strength, while amorphous plastics are the opposite. Commonly used polyethylene, polypropylene, and polyamide (nylon) are crystalline plastics, commonly used polystyrene, polyvinyl chloride, and ABS are amorphous plastics.

From the perspective of appearance characteristics, generally crystalline plastics are opaque or translucent, and amorphous plastics are transparent. But there are exceptions, such as poly-4-methyl pentene-1 is a crystalline plastic, but has high transparency, while ABS is an amorphous plastic, but is opaque.

• Categories of Plastics: Thermosetting Plastic Materials

At the start of heating, thermosetting polymers also have a chain or dendritic structure. They can be molded into plastic objects of predefined shapes and are reliable and plastic. The process of creating chemical links between the primary chains of these chains or dendritic molecules as they continue to heat results in the progressive formation of a network structure known as a cross-linking reaction.

As the temperature reaches a specific point, the cross-linking reaction intensifies and the molecules eventually undergo structural change, solidifying into a substance that neither melts nor dissolves. The shape of the plastic product is permanent and won’t change when heated again as a result of the chemical reaction between the molecules’ chains. Plastic has lost its malleability. Unless it is irreversibly burned at extremely high temperatures, plastic is no longer malleable. Physical and chemical changes occur during the molding process. Plastics made of thermosets cannot be recycled.

Thermoplastics are less heat resistant than thermosets. Thermosetting plastics include materials like phenol, melamine-formaldehyde, unsaturated polyester, etc. that are often utilized.

Injection, extrusion, and blow molding are frequently used to produce thermoplastics. Thermosets can be utilized for injection molding as well as compression molding often.

Polymers are the primary constituent of plastics, hence plastics frequently bear polymer names. Because most plastics have names that are difficult to say and write, they are frequently represented by widely recognized English acronyms.

Categories of Plastics: According to the properties and uses of plastics

According to the performance and use of plastics, they can be divided into general-purpose plastics, engineering plastics, and special plastics. General-purpose plastics refer to a class of plastics with large output, wide application, low price, and common performance, usually used as non-structural materials.

The six general-purpose plastics recognized in the world are polyethylene, polypropylene, polyvinyl chloride, polystyrene, phenolic plastics, and aminoplast, and their output accounts for more than 75% of the world’s total plastic output, constitute the main body of the plastics industry.

• Categories of Plastics: Engineering Plastics

Engineering plastics generally refer to industrial-quality plastics that can be used to manufacture mechanical parts or engineering structural materials. In addition to having high mechanical strength, this kind of plastic has better wear resistance, corrosion resistance, heat resistance, self-lubrication, and dimensional stability than general-purpose plastics. They have certain metal properties, so they have been widely used in engineering and technology departments such as machinery manufacturing, light industry, electronics, daily use, aerospace, missiles, and atomic energy, and are increasingly replacing metals as some mechanical parts.

At present, plastics that are widely used in engineering include polyamide, polyoxymethylene, polycarbonate, ABS, polysulfone, polyphenylene ether, polytetrafluoroethylene, etc, among which are the first four to develop the fastest and are internationally recognized as the four major projects plastic.

• Categories of Plastics: Specialty Plastics (functional plastics)

Refers to those plastics that have special functions and are suitable for a special occasion, mainly including medical plastics, photosensitive plastics, magnetically conductive plastics, superconducting plastics, radiation-resistant plastics, high-temperature resistant plastics, etc.

Its main component is resin, some of which are specially synthesized resins, and some of which use the above-mentioned general-purpose plastics and engineering plastics resins to obtain special properties after special treatment or modification, this kind of plastic output is small, has excellent performance, expensive.

As the scope of plastic applications becomes wider and wider, the boundaries between engineering plastics and general-purpose plastics have become difficult to draw. For example, general-purpose plastic polyvinyl chloride has been widely used in chemical machinery as a corrosion-resistant material.

Categories of Plastics: According to the crystal form of plastic

Plastics are typically categorized into crystalline plastics and non-crystalline plastics based on their various crystalline shapes (amorphous plastics).

Crystalline plastics, the majority of which are somewhat crystalline, are defined as plastics whose molecules, given the right circumstances, can generate a certain geometric structure (such as PE, PP, PA, POM, PET, PBT, etc.). Amorphous plastics, such as ABS, PC, PVC, PS, PMMA, EVA, and AS, are defined as plastics whose molecular structure and organization are not in a crystal structure but rather in a disordered form. Amorphous plastics are another name for these substances. Amorphous polymers are isotropic, meaning that their mechanical properties are the same in all directions.

Requirements of crystalline plastics for injection molding machines and injection molds

• Crystalline plastics need more energy to destroy the crystal lattice when melting, so more heat needs to be input when converting from solid to molten melt, so the plasticizing capacity of the injection molding machine is relatively large, and the rated injection volume should be corresponding improve.

• The melting point range of crystalline plastics is narrow. To prevent the plastic material from crystallizing and blocking the nozzle when the nozzle temperature drops, the nozzle aperture should be appropriately enlarged, and a heating coil that can individually control the nozzle temperature should be installed.

• Since mold temperature has an important influence on crystallinity, there should be as many mold cooling channels as possible to ensure uniform mold temperature during molding.

• Because crystalline polymers experience significant volume loss during the crystallization process, the molding shrinkage rate needs to be carefully taken into account while designing the mold.

• The position and size of the gate and the position and size of the reinforcing rib in the mold design must be carefully considered because of the significant anisotropy and high internal stress of crystalline plastics. If not, the mold is likely to warp or deform, and this can be difficult to correct by modifying the molding process.

• The wall thickness of plastic products is proportional to the degree of crystallinity. The wall thickness cools slowly, has a high degree of crystallinity, experiences significant shrinkage, and is prone to developing pores and cavities during that shrinkage. Hence, when designing a mold, care should be taken to manage the wall thickness of plastic items.

The molding process characteristics of crystalline plastics

• The heat released during cooling is large, so it must be fully cooled, and attention should be paid to the control of cooling time during high-temperature molding.

• The density difference between the molten state and the solid state is large, the molding shrinkage is large, and shrinkage cavities and pores are prone to occur. Pay attention to the setting of the holding pressure.

• When the mold temperature is low, the cooling is fast, the crystallinity is low, the shrinkage is small, and the transparency is high. Crystallinity is related to the wall thickness of plastic products. When the wall thickness of plastic products 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 according to requirements.

• After the plastic product is molded because the uncrystallized molecules tend to continue to crystallize, they are in a state of energy imbalance and are prone to deformation and warping. The material temperature and mold temperature should be appropriately increased, and medium injection pressure and injection speed should be adopted.