In the three processes, the blow-molded products are hollow; Because there is an injection port in injection molding, there will be a little more unwanted part in the molded product, and there will be a process to cut it off, but it can still be found if you observe it carefully; The biggest feature of rotational molding products is that there is no seam and no injection port. Plastic molding and processing methods Plastic molding and processing is an engineering technology, which involves various processes of transforming plastics into plastic products. In the process of transformation, one or more of the following situations often occur, such as the rheology of polymers and the changes of physical and chemical properties. Plastic molding method 1. Compression molding. Compression molding, also known as compression molding, is a molding method in which molding compound is pressed (usually heated) in a closed mold cavity. Generally, compression molding is suitable for thermosetting plastics, such as phenolic plastics, amino plastics, unsaturated polyester plastics and so on. Compression molding consists of three processes: preloading, preheating and molding: preloading is the operation of pre-pressing powder or fibrous molding compound into a certain shape to improve product quality and molding efficiency. Preheating To improve the processability of molding compound and shorten the molding cycle, the molding compound is heated before molding. Molding: adding the required amount of plastic into the mold, closing the mold, exhausting, keeping it at molding temperature and pressure for a period of time, and then demoulding and cleaning the mold. The main equipment for compression molding is press and plastic mold. The most widely used hydraulic press is self-contained hydraulic press, with tonnage ranging from tens to hundreds of tons. There are down press and up press. The molds used for compression molding are called compression molds, which are divided into three categories; Overflow mold, semi-overflow mold and non-overflow mold. The main advantages of compression molding are that it can mold large flat products and mass production, but its disadvantages are long production cycle and low efficiency. 2. Laminated molding. A method of combining two or more layers of the same or different materials into a whole by heating and pressing with or without adhesive. Lamination molding is usually operated by laminating machine, which is equipped with multiple floating hot pressing plates between the moving platen and the fixed platen. The commonly used reinforcing materials for lamination are cotton cloth, glass cloth, paper, asbestos cloth, etc. The resins are phenolic resin, epoxy resin, unsaturated polyester and some thermoplastic resins. 3. Cold pressing molding. Cold-press molding, also called cold-press sintering molding, is different from ordinary compression molding in that materials are pressed and molded at room temperature. The molded product after demoulding can be heated again or cured by chemical action. This method is mostly used for the molding of PTFE, and also for some high temperature resistant plastics (such as polyimide). The general technological process includes three steps: blank making, sintering and cooling. 4. Transfer molding. Transfer molding is a molding method of thermosetting plastics. When molding, the molding compound is first heated and softened in a heating chamber, and then pressed into the heated mold cavity for curing and molding. Transfer molding has different types of work according to different equipment: ① movable plate; 2 tank type; ③ plunger type. The requirements of transfer molding for plastics are: before the curing temperature is reached, the plastic should have greater fluidity, and after the curing temperature is reached, it must have a faster curing rate. Phenolic aldehyde, melamine formaldehyde and epoxy resin can meet this requirement. Transfer molding has the following advantages: (1) less waste edges of products can reduce the amount of post-processing; (2) can mold products with fine or fragile inserts and perforations, and can keep the correct position of the inserts and perforations; ③ The product has uniform performance, accurate size and high quality; ④ The wear of the mould is small. The disadvantages are: ⑤ the manufacturing cost of the die is higher than that of the compression die; ⑥ Plastic loss is large; ⑦ Fiber reinforced plastics are anisotropic due to fiber orientation; Today, the plastic around the insert will sometimes reduce the strength of the product because it is not firmly fused. 5. Low pressure molding. A touch pressing or laminating method with a molding pressure equal to or lower than 1.4 MPa is used. Low pressure molding method is used to manufacture reinforced plastic products. Reinforcing materials such as glass fiber, textile, asbestos, paper, carbon fiber, etc. Most commonly used resins are thermosetting, such as phenolic resin, epoxy resin, amino resin, unsaturated polyester resin and silicone resin. Low-pressure forming includes bag pressing method and injection method. (1) bag compression molding. With the help of elastic bag (or other elastic diaphragm) to receive fluid pressure, the reinforced plastic between rigid mold and elastic bag is uniformly compressed, which becomes a method of making parts. According to the different methods of causing fluid pressure, it can be generally divided into pressure bag molding, vacuum bag molding and autoclave molding. (2) spray molding. When molding reinforced plastic products, the method of spraying chopped fibers and resin on the mold at the same time with a spray gun and curing them into products. 6. Extrusion molding. Extrusion molding, also known as extrusion molding or extrusion, is a method in which materials are continuously formed through a die in a flowing state by heating and pressurizing in an extruder. Extrusion method is mainly used for the molding of thermoplastics, and can also be used for some thermosetting plastics. The extruded products are all continuous profiles, such as pipes, rods, wires, plates, films, wire and cable coatings, etc. In addition, it can also be used for mixing, plasticizing granulation, coloring and blending of plastics. Extrusion molding machine consists of extrusion device, transmission mechanism, heating and cooling system and other main parts. There are two types of extruders: screw (single screw and multi-screw) and plunger. The extrusion process of the former is continuous and the latter is intermittent. The basic structure of single screw extruder mainly includes transmission device, feeding device, barrel, screw, head and die. The auxiliary equipment of extruder can be divided into three categories: material pretreatment equipment (such as material conveying and drying), extrudate treatment equipment (setting, cooling, traction, cutting or rolling) and production condition control equipment. 7. Extrusion forming. Extrusion molding is one of the molding methods of thermosetting fiber reinforced plastics. It is used to produce profiles with fixed section shape and unlimited length. The molding process is to draw the continuous fiber impregnated with resin glue through a heating die, and then further cure the resin through a heating chamber to prepare a unidirectional high-strength continuous reinforced plastic profile. There are three resins commonly used in pultrusion: unsaturated polyester, epoxy and silicone. Among them, unsaturated polyester resin is the most used. Extrusion molding machine usually consists of fiber arrangement device, resin tank, preforming device, die and heating device, traction device and cutting equipment. 8. Injection molding. Injection molding (injection molding) is a method in which thermoplastic or thermosetting molding compound is first plasticized uniformly in a heating barrel, and then pushed into the cavity of a closed mold by a plunger or a moving screw. Injection molding is suitable for almost all thermoplastics. In recent years, injection molding has also been successfully used to mold some thermosetting plastics. The molding cycle of injection molding is short (a few seconds to several minutes), and the quality of molded products can range from a few grams to tens of kilograms. It is possible to mold molded products with complex shapes and accurate dimensions and metal or nonmetal inserts at one time. Therefore, the method has strong adaptability and high production efficiency. Injection molding machines are divided into plunger injection machines and screw injection machines, which are composed of injection system, mold locking system and plastic mold. Its molding methods can be divided into: (1) Exhaust injection molding. The exhaust injection molding machine used in exhaust injection molding has an exhaust port in the middle of the barrel, which is also connected with the vacuum system. When the plastic is plasticized, the vacuum pump can pump away the water vapor, monomers, volatile substances and air contained in the plastic through the exhaust port. Raw materials do not need to be pre-dried, thus improving production efficiency and product quality. It is especially suitable for molding easily hygroscopic materials such as polycarbonate, nylon, plexiglass and cellulose. (2) Flow injection molding. Flow injection molding can use ordinary mobile screw injection machine. That is, the plastic is continuously plasticized and extruded into the mold cavity with a certain temperature. After the plastic fills the mold cavity, the screw stops rotating, and the materials in the mold are kept under pressure for a proper time by the thrust of the screw, and then cooled and shaped. Flow injection molding overcomes the equipment limitation of producing large-scale products, and the quality of the products can exceed the maximum injection volume of the injection machine. Its characteristic is that the plasticized objects are not stored in the barrel, but are constantly squeezed into the mold, so it is a method combining extrusion and injection. (3) Injection molding. * * * Injection molding is a method of injecting different kinds or colors of plastics into the mold simultaneously or successively by using an injection machine with two or more injection units. Composite products of various colors and/or plastics can be produced by this method, and the representative injection molding is two-color injection and multi-color injection. (4) Runner-free injection molding. A molding method in which the molten material is directly injected into each mold cavity by an extended nozzle of an injection machine without setting a shunt channel in the mold. During the injection process, the plastic in the runner keeps in a molten flow state, and does not come off together with the product when demoulding, so there is no runner residue in the product. This molding method not only saves raw materials and reduces costs, but also reduces working procedures, and can achieve full-automatic production. (5) Reaction injection molding. The principle of reactive injection molding is that the raw materials are metered by a metering device, pumped into a mixing head, collided and mixed in the mixing head, and then injected into a closed mold at high speed for rapid curing, demoulding and taking out the product. It is suitable for processing some thermosetting plastics and elastomers such as polyurethane, epoxy resin, unsaturated polyester resin, silicone resin and alkyd resin. At present, it is mainly used in the processing of polyurethane. (6) Injection molding of thermosetting plastics. Granular or lumpy thermosetting plastics are plasticized into a viscoplastic state under the action of screws in a barrel with strictly controlled temperature. Under a high injection pressure, the materials enter a mold with a certain temperature range for cross-linking and curing. Thermosetting plastics injection molding has not only physical changes, but also chemical changes. Therefore, compared with thermoplastic injection molding, there are great differences in molding equipment and processing technology. The following table compares the differences between thermosetting and thermoplastic injection molding. Comparison of injection molding conditions between thermosetting plastics and thermoplastics Process conditions Thermosetting plastics Thermoplastics Thermoplastics barrel temperature plasticizing temperature is low, barrel temperature is below 95℃, temperature control requirements are strict, plasticizing temperature is high, barrel temperature is above 15℃, The temperature control is not strict. The time in the barrel is short, and the heating mode of the barrel is long. The liquid medium (water, oil) is electrically heated. The mold temperature is below 15-2℃ and the injection pressure is below 1-2 MPa. The injection quantity is small, and the residue in the front of the barrel is small. The injection quantity is large, and the residue in the front of the barrel is more. Phenolic plastics are the most widely used in injection molding of thermosetting plastics. 9. Blow molding. A method of inflating a hot parison closed in a mold into a hollow product or a tube parison into a tube film without die by gas pressure. The method is mainly used for manufacturing various packaging containers and tubular films. All materials with melt index of .4 ~ 1.12 are excellent hollow blow molding materials, such as polyethylene, polyvinyl chloride, polypropylene, polystyrene, thermoplastic polyester, polycarbonate, polyamide, cellulose acetate and polyacetal resin, among which polyethylene is the most widely used. (1) Injection blow molding. The plastic is made into a bottomed parison by injection molding, and then the parison is moved to a blow mold to blow into a hollow product. (2) Extrusion blow molding. The plastic is first made into a bottomed parison by extrusion, and then the parison is moved to a blow mold to blow into a hollow product. The difference between injection blow molding and extrusion blow molding is that the method of manufacturing parison is different, and the blow molding process is basically the same. In addition to the injection machine and extruder, the blow molding equipment is mainly the mold for blow molding. The blow mold is usually composed of two valves, in which there is a coolant channel, and the small hole on the parting surface can be inserted into the pressurized gas blowing pipe. (3) stretch blow molding. Stretch blow molding is a kind of blow molding with biaxial directional stretching. The method is to stretch the mold longitudinally first, and then inflate it with compressed air to achieve transverse stretching. Stretch blow molding can greatly improve the transparency, impact strength, surface hardness and rigidity of products, and is suitable for blow molding of polypropylene and polyethylene terephthalate (PETP). Stretch blow molding includes injection parison directional stretch blow molding, extrusion parison directional stretch blow molding, multi-layer directional stretch blow molding, compression molding directional stretch blow molding and so on. (4) Blow film method. A method of molding thermoplastic film. The plastic is extruded into a tube by extrusion method, and then it is continuously expanded into a tube film with a certain size by blowing air into the tube, and then it is folded and wound into a double-layer flat film after cooling. Plastic films can be made by many methods, such as blow molding, extrusion, tape casting, calendering, casting, etc., but blow molding is the most widely used. This method is suitable for the manufacture of polyethylene, polyvinyl chloride, polyamide and other films. 1. casting. A method of injecting liquid monomer, resin or their mixture into a mold and making it into a solid product without or with a little pressure. Casting methods are divided into static casting, embedded casting, centrifugal casting, slush casting, rotary casting, rotary casting and tape casting. (1) Static casting. Static casting is a simple and widely used method in casting. This method usually uses liquid monomer, partially polymerized or condensed slurry, solution of polymer and monomer, additives (such as initiator, curing agent, accelerator, etc.), or thermoplastic resin melt cast into the mold cavity for molding. (2) embedded casting. Embedded casting, also known as encapsulation molding, is a molding technology that encapsulates various samples and parts into plastic. That is, the embedded object is placed in the mold, and liquid such as monomer, prepolymer or polymer is injected, and then it is polymerized or solidified (or hardened) and demoulded. This technology has been widely used in the electronic industry. The plastics used in this kind of molding process include formaldehyde, unsaturated polyester, plexiglass and epoxy resin. (3) centrifugal casting. Centrifugal casting is a method of molding tubular or hollow cylindrical products by centrifugal force. A certain amount of liquid resin or resin dispersion is injected into a rotating and heated container (that is, a mold) through an extruder or a special funnel to make it rotate around a single axis at a high speed (tens to two thousand revolutions per minute). At this time, the materials put in are forced to be distributed near the wall of the mold by centrifugal force. At the same time of rotation, the put materials are solidified, and then the products can be obtained by cooling or post-treatment as needed. Reinforcing fillers can also be added at the same time when molding reinforced plastic products. Centrifugal casting usually uses thermoplastics with low melt viscosity and good thermal stability, such as polyamide and polyethylene. (4) enameling. Slurry molding is a method of molding hollow products. When molding, the plastic paste is poured into the open hollow mold until the specified capacity is reached. The mold should be heated before or after loading, so that the material can become gel on the inner wall of the mold. When the gel reaches a predetermined thickness, the excess liquid material is poured out, heated and melted, and the product can be peeled off from the mold after cooling. The plastic used for plastic lining is mainly polyvinyl chloride. (5) Rotary casting. In this method, the liquid material is put into a closed mold to make it rotate around a single axis or multiple axes at a low speed (several revolutions per minute to dozens of revolutions), so that the material can be distributed on the inner wall of the mold by gravity, and then cured or hardened by heating or cooling, and the product can be obtained from the mold. Rotating around a single axis is used to produce cylindrical products, and moving around two axes or by vibration is used to produce closed products. (6) Rotational molding (rotational moulding). A molding method similar to rotary casting, except that the material used is not liquid, but sintered dry powder. The process is to put the powder into the mold, make it rotate around two perpendicular axes, be heated and evenly fuse on the inner wall of the mold, and then cool it to get the hollow product from the mold. Polyethylene, modified polystyrene, polyamide, polycarbonate and cellulose plastics are used in rotational molding. (7) casting and casting