The molding process of plastics refers to the process by which polymers made by a synthetic resin manufacturer are made into a final plastic product. Processing methods (often referred to as primary processing of plastics) include compression molding (molding), extrusion molding (extrusion molding), injection molding (injection molding), blow molding (blow molding), and calendering.
Compression molding
Compression molding, also known as molding or compression molding, compression molding is mainly used for phenolic resins, urea-formaldehyde resins, unsaturated polyester resins and other thermosetting plastics molding.
Extrusion
Extrusion, also known as extrusion molding, is the use of extrusion machine (extruder) will be heated resin continuously through the mold, extrusion of the desired shape of the product method. Extrusion is sometimes used for thermosetting plastics and can be used for foam molding. The advantage of extrusion is that various shapes of products can be extruded, high production efficiency, automation, continuous production; disadvantage is that thermosetting plastics can not be widely used in this method of processing, product size is prone to deviation.
Injection molding
Injection molding is also known as injection molding. Injection molding is the use of injection molding machine (or injection machine) will be thermoplastic melt under high pressure into the mold by cooling, curing to obtain the product method. Injection molding can also be used for thermosetting plastics and foam molding. The advantages of injection molding are fast production speed, high efficiency, operation can be automated, can mold complex shaped parts, especially suitable for mass production. Disadvantages are high cost of equipment and molds, injection molding machine cleaning is more difficult.
Blow molding
Blow molding is also known as hollow blow molding or hollow molding. Blow molding is with the help of compressed air pressure to make the closed in the mold of the hot resin billet blowing for a hollow product method, blow molding, including blow molding film and blow molding hollow products two methods. Blow molding method can produce film products, a variety of bottles, barrels, pots and containers and children's toys.
Calendering
Calendering is the resin and a variety of additives by the expected treatment (kneading, filtration, etc.) through the calender of two or more calendering rollers turning the opposite direction of the gap processed into a film or sheet, and then peeled off from the calender rollers, and then cooled by a molding method. Calendering is a molding method mainly used for polyvinyl chloride resins, which can manufacture films, sheets, plates, artificial leather, floor tiles and other products.
Classification, composition and properties of plastics
Plastic is a synthetic polymer material with a wide range of uses, and plastic products abound in our daily lives. From the toiletries we use when we get up, the tableware we use at breakfast, to the stationery we use when we work and study, the seat cushions and mattresses we use when we rest, as well as the shells of televisions, washing machines, computers, and the various shapes of lamps and lanterns that give us light at night ......
Plastics have gradually replaced many Has been used for decades and centuries of materials and utensils, become an indispensable assistant in people's lives. Plastic set of metal hardness, lightweight wood, glass transparency, ceramic corrosion resistance, rubber elasticity and toughness in one, so in addition to daily necessities, plastic is more widely used in aerospace, medical equipment, petrochemicals, machinery manufacturing, national defense, construction and other industries.
One, the classification of plastics
There are many types of plastics, so far the world into the production of plastics about three hundred kinds. Plastic classification methods, commonly used in two ways:
1, according to the nature of the plastic heat is divided into thermoplastic and thermosetting plastics
Thermoplastic molecular structure is a linear structure, when heated, softening or melting, can be molded into a certain shape, and then harden after cooling. In the heat to a certain extent and re-soften, after cooling and harden, this process can be repeated many times. Such as polyvinyl chloride, polyethylene, polystyrene and so on. Thermoplastic molding process is relatively simple, can be continuous production, and has a fairly high mechanical strength, so the development is very fast.
Thermoset plastics molecular structure is the body structure, in the heat also occurs when the softening, can be molded into a certain shape, but heat to a certain extent or add a small amount of curing agent, the hardening of the stereotypes, and then will not soften the heat and change the shape. Thermosetting plastics processed after molding, heat is no longer softened, and therefore can not be recycled, such as phenolic plastics, aminoplastics, epoxy resins and so on belong to this type of plastic. Thermosetting plastic molding process is more complex, so continuous production has some difficulties, but its heat resistance is good, not easy to deformation, and the price is relatively inexpensive.
2, according to the different uses of plastics are divided into general-purpose plastics and engineering plastics
General-purpose plastics refers to the production of large, low-priced plastics with a wide range of applications, including polyolefins, polyvinyl chloride, polystyrene, phenolic plastics and aminated plastics five varieties. Many products used in people's daily life are made of these general-purpose plastics.
Engineering plastics are plastics that can be used as engineering structural materials and replace metal to make machine parts. For example, polyamide, polycarbonate, polyformaldehyde, ABS resin, PTFE, polyester, polysulfone, polyimide and so on. Engineering plastics with low density, high chemical stability, good mechanical properties, superior electrical insulation, easy processing and molding characteristics, widely used in automotive, electrical appliances, chemical, mechanical, instrumentation, instrumentation and other industries, but also in cosmic navigation, rockets, missiles and other aspects.
Second, the composition of plastic
We usually use plastic is not a pure substance, it is formulated by many materials. One of the polymer (or synthetic resin) is the main component of plastic, in addition, in order to improve the performance of plastic, but also in the polymer to add a variety of auxiliary materials, such as fillers, plasticizers, lubricants, stabilizers, coloring agents, etc., in order to become a good performance of plastic.
1, synthetic resins
Synthetic resins are the most important component of plastics, its content in plastics is generally 40% to 100%. Because of the large content, and the nature of the resin often determines the nature of the plastic, so people often see the resin as a synonym for plastic. For example, polyvinyl chloride resin and polyvinyl chloride plastics, phenolic resins and phenolic plastics mixed together. In fact, resin and plastic are two different concepts. Resin is a raw unprocessed polymer, it is not only used in the manufacture of plastics, but also coatings, adhesives and synthetic fibers raw materials. In addition to a very small number of plastics containing 100% resin, the vast majority of plastics, in addition to the main component of resin, but also need to add other substances.
2, filler
Filler, also known as filler, it can improve the strength and heat resistance of plastic, and reduce costs. For example, phenolic resin can greatly reduce the cost of adding wood flour, so that phenolic plastics become one of the cheapest plastics, but also significantly improve mechanical strength. Fillers can be divided into two categories of organic fillers and inorganic fillers, the former such as wood flour, rags, paper and a variety of fabric fibers, etc., the latter, such as glass fibers, diatomaceous earth, asbestos, carbon black, etc..
3, plasticizers
Plasticizers can increase the plasticity and flexibility of plastics, reduce the brittleness of plastics easy to process molding. Plasticizers are generally miscible with the resin, non-toxic, odorless, light, heat-stable high-boiling organic compounds, the most commonly used is phthalate esters. For example, the production of polyvinyl chloride plastic, if you add more plasticizers can be obtained soft polyvinyl chloride plastic, if you do not add or add less plasticizer (dosage < 10%), the hard polyvinyl chloride plastic.
4, stabilizers
In order to prevent synthetic resins in the processing and use of the process of light and heat decomposition and destruction, to extend the service life, to add stabilizers in plastics. Commonly used stearate, epoxy resin and so on.
5, colorants
Colorants can make plastics have a variety of bright, beautiful colors. Commonly used organic dyes and inorganic pigments as colorants.
6, lubricants
The role of the lubricant is to prevent the plastic in the molding does not stick to the metal mold, at the same time can make the surface of the plastic smooth and beautiful. Commonly used lubricants are stearic acid and its calcium and magnesium salts.
In addition to the above additives, plastics can also be added to the flame retardant, foaming agent, antistatic agent, etc., to meet the different requirements of use.
Three, the characteristics of plastic
1, plastic has plasticity
As the name suggests, plastic is a material that can be molded. The so-called plasticity of plastic is a solid plastic can be softened by heating methods, and then the softened plastic in the mold, so that it is cooled and re-solidified into a certain shape of the solid. This property of plastic also has certain defects, that is, easy to soften and deform when exposed to heat, and some plastics will be deformed even with higher temperature water, so plastic products should not be in contact with boiling water in general.
2, plastic with elasticity
Some plastics are also like synthetic fibers, with a certain degree of elasticity. When it is stretched by external forces, the curled molecules are straightened by the flexibility, but once the tension is canceled, it will return to the original curled state, which makes the plastic has elasticity, such as polyethylene and polyvinyl chloride film products. But some plastics are not elastic.
3, plastic has high strength
Plastic is not as hard as metal, but compared with glass, ceramics, wood, etc., or has a relatively high strength and wear resistance. Plastic can be made into strong gears and bearings on the machine.
4, plastic has corrosion resistance
Plastic is not like metal in the moist air will rust, not like wood in the wet environment will rot or be microbial erosion, in addition to plastic corrosion resistant to acids and alkalis. Therefore, plastics are often used as water and liquid pipelines in chemical plants, doors and windows in buildings, and so on.
5, plastic has insulating
Plastic molecular chain is atoms to *** valence bonding combined, the molecules can not be ionized, but also in the structure of the transfer of electrons, so the plastic has insulating properties. Plastics can be used to make wrappings for electrical wires, electrical sockets, and housings for electrical appliances.
Attachment: Commonly used types of plastics are:
①Polyvinyl Chloride (PVC)
It is one of the most used plastics in construction. Rigid polyvinyl chloride density of 1.38 ~ 1.43g/cm3, high mechanical strength, good chemical stability, the use of temperature range is generally between -15 ~ +55 ℃, suitable for the manufacture of plastic windows and doors, downpipes, trunking and so on.
② polyethylene (PE)
Polyethylene plastic in the building is mainly used for water supply and drainage pipes, sanitary ware.
③ Polypropylene (PP)
Polypropylene density in all plastics is the smallest, about 0.90 or so. Polypropylene is commonly used to produce pipes, sanitary ware and other building products.
④Polystyrene (PS)
Polystyrene is a colorless and transparent glass-like plastic. Polystyrene is mainly used in construction to produce foam insulation materials, translucent materials and other products.
5 ABS plastic
ABS plastic is a modified polystyrene plastic, acrylene eye (A), butadiene (B) and styrene (S) based on the composition of the three components.ABS plastic can be made to embossed patterns of plastic decorative panels and so on.