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With China Plastic Machinery
With China Plastic Machinery
The property of a substance that can flow and deform when heated, and can maintain a certain shape after cooling. Linear or branched polymers have the property of being able to soften by heating and harden by cooling repeatedly within a certain temperature range.
thermoplasticity
Most linear polymers exhibit thermoplasticity and are easily processed by extrusion, injection or blow molding.
In daily life, things like plastic bags and plastic clothes hangers are thermoplastic. Therefore, they can be melted by heating for sealing, bonding and other operations.
General polyethylene plastic and polyvinyl chloride plastic are thermoplastic.
Thermoplastic materials melt when exposed to fire and produce droplets, which will burn again when exposed to fire.
Raincoat, food bag or packaging bag
The relative concept of thermoplasticity is thermosetting, which refers to the property that it cannot soften and be repeatedly molded when heated, nor can it be dissolved in solvents. Bulk polymers have this property. Plastic with thermosetting properties are called thermosetting plastic.
Thermosetting plastic can soften and flow when heated for the first time. When heated to a certain temperature, a chemical reaction occurs, cross-linking solidification occurs, and the plastic becomes hard. This change is irreversible. After that, it can no longer soften and flow when heated again. It is precisely with the help of this characteristic that molding is carried out, using the plasticizing flow during the first heating to fill the mold cavity under pressure, and then solidify into a product of a certain shape and size. The resin of thermosetting plastic is linear or branched before solidification. After solidification, chemical bonds are formed between the molecular chains to form a three-dimensional network structure. Not only can it no longer melt, but it can also not be dissolved in solvents. Plastic used in harsh environments such as heat insulation, wear resistance, insulation, and high voltage resistance are mostly thermosetting plastic. The most commonly used ones should be frying pan handles and high and low voltage electrical appliances.
Common thermosetting plastic include phenolic resin, urea-formaldehyde resin, melamine resin, unsaturated polyester resin, epoxy resin, silicone resin, polyurethane, etc.
Thermoplastic refer to plastic that have the characteristics of softening when heated and hardening when cooled. Most of the plastic we use in our daily lives belong to this category. When heated, they become soft and even flow, and when cooled, they become hard. This process is reversible and can be repeated. Polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyoxymethylene, polycarbonate, polyamide, acrylic plastic, other polyolefins and their copolymers, polysulfone, polyphenylene ether, chlorinated polyether, etc. are all thermoplastic. The resin molecular chains in thermoplastic are all linear or branched structures, and there are no chemical bonds between the molecular chains. When heated, they soften and flow, and when cooled and hardened, they are physical changes.
Thermoplastic can be divided into general-purpose plastic, engineering plastic, special plastic, etc. according to their performance characteristics, wide range of uses, and versatility of molding technology.
The main characteristics of general-purpose plastic: wide range of uses, easy processing, and good comprehensive performance. For example, polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), polystyrene (PS), and acrylonitrile-butadiene-styrene (ABS) are also commonly known as the "five major general-purpose plastic." The characteristics of engineering plastic and special plastic are: some structures and properties of polymers are particularly prominent, or the molding and processing technology is difficult, etc., and they are often used in professional engineering or special fields and occasions. The main engineering plastic are: nylon (Nylon), polycarbonate (PC), polyurethane (PU), polytetrafluoroethylene (Teflon, PTFE), polyethylene terephthalate (PET), etc., special plastic such as "synthetic heart valves" and "artificial joints" of the "medical polymer" category.
According to the aggregate structure and performance characteristics of copolymers, they can be divided into two categories: crystalline plastic and non-crystalline plastic. Non-crystalline plastic are also called amorphous plastic. Different classification angles or methods can also be divided into different results.
The molecular weight of polymers in general thermoplastic can reach hundreds of thousands to millions, and the length of macromolecular chains can reach 10mm. These macromolecules can be linear, such as LLDPE and HDPE; they can also be branched, such as LDPE. The macromolecules are entangled with each other, arranged in a disordered or relatively orderly manner, forming an "aggregate structure".
When the macromolecules are completely disordered, we call it amorphous thermoplastic. Such as PVC, PC, PMMA, etc. Its performance characteristics are: good transparency, low mechanical strength, good flexibility. Those with some macromolecules or partially uniformly arranged structures are called crystalline thermoplastic. Such as LLDPE, POM, nylon, etc., their performance characteristics are: poor transparency, high mechanical strength, low flexibility.
The factors that affect the shrinkage of thermoplastic molding are:
1. Plastic variety During the molding process of thermoplastic, due to the volume change caused by crystallization, strong internal stress, large residual stress frozen in the plastic part, strong molecular orientation and other factors, the shrinkage rate is larger than that of thermosetting plastic, the shrinkage rate range is wide, and the directionality is obvious. In addition, the shrinkage after molding, annealing or humidity treatment is generally larger than that of thermosetting plastic.
2. Characteristics of plastic parts During molding, the molten material contacts the surface of the cavity and the outer layer is immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the inner layer of the plastic part cools slowly and forms a high-density solid layer with large shrinkage. Therefore, the thicker the wall, the slower the cooling, and the thicker the high-density layer, the larger the shrinkage. In addition, the presence or absence of inserts and the layout and number of inserts directly affect the direction of material flow, density distribution, shrinkage resistance, etc., so the characteristics of the plastic part have a greater impact on the shrinkage size and directionality.
3. The form, size, and distribution of the feed port directly affect the direction of material flow, density distribution, pressure holding and shrinkage compensation, and molding time. Direct feed ports and feed ports with large cross-sections (especially thicker cross-sections) have small shrinkage but large directionality, and feed ports with short widths and lengths have small directionality. Those that are close to the feed port or parallel to the material flow direction have large shrinkage.
4. Molding conditions The mold temperature is high, the molten material cools slowly, has high density, and shrinks greatly. Especially for crystallized materials, the shrinkage is greater due to the high degree of crystallinity and large volume changes. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic part, which directly affects the shrinkage size and directionality of each part. In addition, the pressure and time of holding also have a great influence on the shrinkage. The higher the pressure and the longer the time, the smaller the shrinkage but the greater the directionality. The higher the injection pressure, the smaller the viscosity difference of the molten material, the smaller the interlayer shear stress, and the greater the elastic rebound after demolding, so the shrinkage can also be reduced appropriately. The higher the material temperature, the greater the shrinkage, but the less directionality. Therefore, adjusting the mold temperature, pressure, injection speed, cooling time and other factors during molding can also appropriately change the shrinkage of the plastic part.
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