Plastic molding refers to the process of making final plastic products from polymers produced by synthetic resin manufacturers. Processing methods (usually referring to one-time processing of plastics) include compression molding (compression molding), extrusion molding (extrusion molding), injection molding (injection molding), blow molding (hollow molding), calendering and so on.
Compression molding
Compression molding is also called compression molding or compression molding. Compression molding is mainly used for molding thermosetting plastics such as phenolic resin, urea-formaldehyde resin and unsaturated polyester resin.
Extrusion molding
Extrusion, also known as extrusion molding, is a method of using an extruder (extruder) to continuously pass heated resin through a die to extrude products with required shapes. Extrusion is sometimes used for the molding of thermosetting plastics and also for the molding of foamed plastics. The advantages of extrusion are that products with various shapes can be extruded, with high production efficiency and automatic continuous production; The disadvantage is that thermosetting plastics cannot be widely processed by this method, and the product size is prone to deviation.
injection moulding
Injection molding is also called injection molding. Injection molding is a method of injecting thermoplastic melt into a mold under high pressure by an injection molding machine (or injection machine), and then cooling and curing to obtain the product. Injection molding can also be used for the molding of thermosetting plastics and foamed plastics. The advantages of injection molding are high production speed, high efficiency, automatic operation, and the ability to mold parts with complex shapes, which is especially suitable for mass production. Disadvantages are high cost of equipment and molds, and difficult cleaning of injection molding machines.
Blow molding
Blow molding is also called hollow blow molding or hollow molding. Blow molding is a method of blowing hot resin parison enclosed in a mold into hollow products by using the pressure of compressed air. Blow molding includes two methods: blow molding film and blow molding hollow products. Blow molding can produce film products, various bottles, barrels, kettle containers and children's toys.
calender
Calendering is a molding method that resin and various additives are processed into films or sheets through the gap between two or more calendering rollers with opposite directions in a calender after the expected treatment (kneading, filtering, etc.). ), then peeled off from the calendering roll, and then cooled and molded. Calendering is a molding method mainly used for PVC resin, which can be made into films, sheets, plates, artificial leather, floor tiles and other products.
Classification, composition and characteristics of plastics
Plastic is a widely used synthetic polymer material, and plastic products abound in our daily life. From the toiletries used after getting up, the tableware used for breakfast, to the stationery used for work and study, the cushion mattress used for rest, the casing of TV, washing machine and computer, and the lights of various shapes that bring us light at night. ...
With its excellent performance, plastics have gradually replaced many materials and utensils that have been used for decades and hundreds of years, and become an indispensable assistant in people's lives. Plastic combines the hardness of metal, the lightness of wood, the transparency of glass, the corrosion resistance of ceramics and the elasticity and toughness of rubber. Therefore, in addition to daily necessities, plastics are more widely used in aerospace, medical equipment, petrochemical, machinery manufacturing, national defense, construction and other industries.
I. Classification of plastics
There are many kinds of plastics, so far there are about 300 kinds of plastics put into production in the world. There are many ways to classify plastics, and there are two commonly used methods:
1, according to the different properties of plastics after heating, it can be divided into thermoplastic and thermosetting plastics.
The molecular structure of thermoplastics is linear, which softens or melts when heated, can be shaped into a certain shape, and then hardens after cooling. It becomes soft when heated to a certain extent, and becomes hard after cooling. This process can be repeated many times. Such as polyvinyl chloride, polyethylene, polystyrene, etc. Thermoplastic molding process is relatively simple, can be produced continuously, and has quite high mechanical strength, so it develops rapidly.
The molecular structure of thermosetting plastics is a kind of bulk structure, which can be softened by heating and shaped into a certain shape. However, after heating to a certain extent or adding a small amount of curing agent, it will harden and set, and will not soften and deform after reheating. Thermosetting plastics, such as phenolic plastics, amino plastics, epoxy resins, etc. Can't be recycled after processing and molding. Thermosetting plastics are complicated in molding process and difficult to produce continuously, but they are good in heat resistance, difficult to deform and relatively low in price.
2. According to the different uses of plastics, they can be divided into general plastics and engineering plastics.
General plastics refer to plastics with large output, low price and wide application range, mainly including polyolefin, polyvinyl chloride, polystyrene, phenolic plastics and aminoplastics. Many products used in people's daily life are made of these general plastics.
Engineering plastics can be used as engineering structural materials instead of metals to manufacture machine parts. Such as polyamide, polycarbonate, polyoxymethylene, ABS resin, polytetrafluoroethylene, polyester, polysulfone, polyimide and the like. Engineering plastics have the characteristics of low density, high chemical stability, good mechanical properties, excellent electrical insulation and easy processing and molding. , widely used in automobiles, electrical appliances, chemicals, machinery, instrumentation and other industries, as well as aerospace, rockets, missiles and so on.
Second, the composition of plastic
The plastic we usually use is not a pure substance, it is made of many materials. Polymer (or synthetic resin) is the main component of plastic. In addition, in order to improve the properties of plastics, various auxiliary materials, such as fillers, plasticizers, lubricants, stabilizers, colorants, etc. , must be added to the polymer to become a plastic with good performance.
1, synthetic resin
Synthetic resin is the most important component in plastics, and its content in plastics is generally 40% ~ 100%. Because of its large content and the nature of resin often determines the nature of plastic, people often regard resin as synonymous with plastic. For example, PVC resin and PVC plastic, phenolic resin and phenolic plastic are confused. In fact, resin and plastic are two different concepts. Resin is a kind of raw material polymer, which is not only used to make plastics, but also used as raw material for coatings, adhesives and synthetic fibers. Except a few plastics contain 100% resin, most plastics need to add other substances besides the main component resin.
Step 2 pack
Fillers, also called fillers, can improve the strength and heat resistance of plastics and reduce costs. For example, adding wood flour to phenolic resin can greatly reduce the cost, make phenolic plastic one of the cheapest plastics, and significantly improve the mechanical strength. Fillers can be divided into two categories: organic fillers, such as wood flour, rags, paper and various fabric fibers; Inorganic fillers, such as glass fiber, diatomite, asbestos and carbon black.
3. Plasticizer
Plasticizers can increase the plasticity and softness of plastics, reduce brittleness, and make plastics easy to process and shape. Plasticizer is generally an organic compound with high boiling point, which can be mixed with resin, non-toxic and tasteless, and stable to light and heat. Phthalates are the most commonly used. For example, in the production of PVC plastics, if more plasticizer is added, and if no plasticizer is added or less plasticizer is added (dosage
4. Stabilizer
In order to prevent synthetic resin from being damaged by light and thermal decomposition during processing and use, and to prolong its service life, stabilizers should be added to plastics. Commonly used are stearate and epoxy resin.
5. Colorants
Colorants can make plastics have various bright and beautiful colors. Organic dyes and inorganic pigments are usually used as colorants.
6. Lubricants
The function of lubricant is to prevent the plastic from sticking to the metal mold during molding, and at the same time, it can make the plastic surface smooth and beautiful. Commonly used lubricants include stearic acid and its calcium and magnesium salts.
In addition to the above additives, there are flame retardants, foaming agents, antistatic agents and so on. It can also be added to plastics to meet different use requirements.
Third, the characteristics of plastics.
1, plastic has plasticity.
As the name implies, plastic is a moldable material. The so-called plasticity of plastic means that solid plastic can be softened by heating, and then the softened plastic is put into a mold, cooled and solidified into a certain shape again. This property of plastics also has certain defects, that is, it is easy to soften and deform when heated, and some plastics will deform even if they are scalded by high-temperature water, so plastic products are generally not suitable for contact with boiling water.
2. Plastic is elastic.
Some plastics are as elastic as synthetic fibers. When stretched by external force, the curled molecules are straightened by flexibility, but once the tension is released, they will return to the original curled state, thus making plastics elastic, such as polyethylene and polyvinyl chloride film products. But some plastics are not elastic.
3. The strength of plastic is very high.
Although plastic is not as hard as metal, it still has higher strength and wear resistance compared with glass, ceramics and wood. Plastic can be made into strong gears and bearings on machines.
4. Plastic has corrosion resistance.
Plastics will not rust in humid air like metal, nor will they rot or be corroded by microorganisms in humid environment like wood. In addition, plastics are resistant to acid and alkali corrosion. Therefore, plastics are often used as water and infusion pipes in chemical plants, doors and windows in buildings, etc.
5. Plastic is insulating.
The molecular chains of plastics are bound together by atoms with valence bonds. Molecules can neither ionize nor transfer electrons in the structure, so plastics are insulated. Plastics can be used to make wire sheaths, power sockets, electrical housings, etc.