Pre-pressing is the operation of pre-pressing powder or fibrous molding compound into a certain shape to improve product quality and molding efficiency.
Preheating In order to improve the processability of the molding compound and shorten the molding cycle, the molding compound is heated before molding.
Molding: add the required amount of plastic into the mold, close the mold, exhaust, keep it at the molding temperature and pressure for a period of time, then demould and clean the mold.
The main equipment for molding is press and plastic mould. The most widely used hydraulic press is self-contained hydraulic press, with tonnage ranging from tens to hundreds of tons. There are downward pressure and upward pressure. 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 advantage of compression molding is that it can form large flat products and mass production, but the disadvantage is 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 a laminator, which is equipped with a plurality of 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 and asbestos cloth. These resins are phenolic resin, epoxy resin, unsaturated polyester and some thermoplastic resins.
3. Cold pressing forming. Cold pressing forming, also known as cold pressing sintering forming, is different from ordinary compression forming in that the material is pressed 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 a 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 reaching the curing temperature, the plastic should have greater fluidity, and after reaching the curing temperature, it should have a faster curing speed. Phenolic aldehyde, melamine formaldehyde and epoxy resin can meet this requirement.
Transfer molding has the following advantages: (1) less waste edges of products, which can reduce the post-processing capacity; (2) products with fine or fragile inserts and perforations can be molded, and the correct positions of the inserts and perforations can be maintained; (3) uniform product performance, accurate size and high quality; ④ The die wear is small. The disadvantages are: ⑤ the manufacturing cost of the mould is higher than that of the press mould; ⑥ Large plastic loss; ⑦ Fiber reinforced plastics are anisotropic due to fiber orientation; Nowadays, the plastic around the plug-in sometimes reduces the strength of the product because it is not firmly fused.
5. Low pressure molding. The molding pressure is equal to or lower than 1.4 MPa.
Low pressure molding is used to manufacture reinforced plastic products. Reinforcing materials, such as glass fiber, textiles, asbestos, paper, carbon fiber, etc. The most commonly used resins are thermosetting resins, 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 compressed evenly, which becomes a method of making parts. According to the different methods of causing fluid pressure, it can be generally divided into pressure bag forming, vacuum bag forming and autoclave forming.
(2) spray molding. When molding reinforced plastic products, chopped fibers and resin are simultaneously sprayed on the mold with a spray gun and solidified into products.
6. Extrusion molding. Extrusion molding, also known as extrusion molding or extrusion, is a method of continuous molding of materials through dies 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 components. There are two types of extruders: screw (single screw and multi-screw) and plunger. The extrusion process of the former is continuous, while 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 molding. Extrusion molding is one of the molding methods of thermosetting fiber reinforced plastics. It is used to produce profiles with fixed cross-sectional shape and infinite length. The molding process is to stretch the continuous fiber impregnated with resin glue through a heating die, and then further cure the resin through a heating chamber to make a unidirectional high-strength continuous reinforced plastic profile.
There are three kinds of resins commonly used in pultrusion: unsaturated polyester, epoxy resin and silicone resin. Among them, unsaturated polyester resin is the most widely used.
Extrusion molding machine usually consists of fiber arrangement device, resin tank, preforming device, mold and heating device, traction device and cutting equipment.
8. Injection molding. Injection molding is a method in which a thermoplastic or thermosetting molding compound is uniformly plasticized 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 (several seconds to several minutes), and the quality of molded products can range from a few grams to dozens of kilograms. Molded products with complex shapes and precise dimensions and metallic or nonmetallic inserts can be molded at one time. Therefore, the method has strong adaptability and high production efficiency.
Injection molding machines are divided into plunger injection molding machines and screw injection molding machines, which are composed of injection system, mold locking system and plastic mold. Its forming method can be divided into:
(1) Exhaust injection molding. The exhaust injection molding machine for exhaust injection molding has an exhaust port in the middle of the barrel, and the exhaust port 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, which improves 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 to say, plastic is continuously plasticized and extruded into the mold cavity at a certain temperature. After the plastic fills the mold cavity, the screw stops rotating, and the material in the mold is kept under pressure for a proper time under the thrust of the screw, and then cooled and molded. Flow injection molding overcomes the limitation of equipment for producing large-scale products, and the product quality can exceed the maximum injection volume of injection machine. Its characteristic is that the plasticized object is not stored in the barrel, but is constantly extruded 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 a 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. The invention relates to a molding method, which directly injects molten material into each mold cavity through 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 state and will not fall off with the product when demoulding, so there is no runner residue in the product. This molding method not only saves raw materials and reduces the cost, but also reduces the working procedures and can realize 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. 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 blocky thermosetting plastics are plasticized into a viscoplastic state under the action of screws in a machine barrel with strictly controlled temperature. Under high injection pressure, the material enters 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. Phenolic plastic is the most widely used plastic in thermosetting plastic injection molding.
9. Blow molding. A method for expanding a hot parison enclosed in a mold into a hollow product or a tubular parison into a tubular film by gas pressure without a mold. The method is mainly used for manufacturing various packaging containers and tubular films. All materials with melt index of 0.04~ 1. 12 are excellent hollow blow molding materials, such as polyethylene, polyvinyl chloride, polypropylene, polystyrene, thermoplastic polyester, polycarbonate, polyamide, cellulose acetate, polyacetal resin, etc., among which polyethylene is the most widely used.
(1) injection blow molding. Plastic is made into a bottomed parison by injection molding, and then the parison is moved into a blow mold to blow-mold 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-mold into a hollow product.
The difference between injection blow molding and extrusion blow molding lies in the different methods of manufacturing parisons, and the blow molding process is basically the same.
In addition to injection molding machines and extruders, blow molding equipment is mainly used for blow molding. The blow mold usually consists of two valves, in which there are coolant channels, and the small holes on the parting surface can be inserted with pressurized gas blowpipes.
(3) stretch blow molding. Stretch blow molding is a bi-directional stretch blow molding. The method is to stretch the die longitudinally first, and then inflate it with compressed air to realize 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) film blowing method. A method for molding a thermoplastic film. The plastic is extruded into a cylinder by extrusion, and then it is continuously expanded into a cylinder film with a certain size by blowing air into the cylinder, and then folded and wound into a double-layer flat film after cooling.
Plastic films can be made by many methods, such as blow molding, extrusion, casting, calendering, casting and so on. But blow molding is the most widely used.
This method is suitable for making polyethylene, polyvinyl chloride, polyamide and other films.
10. casting. A method of injecting liquid monomer, resin or their mixture into a mold to make it into a solid product without pressure or with a little pressure. Casting methods are divided into static casting, embedded casting, centrifugal casting, plastic casting, rotary casting, rotary casting and belt casting.
(1) static casting. Static casting is a simple and widely used casting method. 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 melt casting thermoplastic resin into a mold cavity for molding.
(2) Embedded pouring. Embedded casting, also known as packaging molding, is a molding technology that packages various samples and parts into plastic. That is, put the insert into the mold, inject liquid such as monomer, prepolymer or polymer, and then polymerize or cure (or harden) and demould. This technology has been widely used in the electronics industry. The plastics used in this molding process include formaldehyde, unsaturated polyester, plexiglass and epoxy resin.
(3) Centrifugal casting. Centrifugal casting is a method of forming tubular or hollow cylindrical products by centrifugal force. A certain amount of liquid resin or resin dispersion is injected into a rotationally heated container (that is, a mold) through an extruder or a special funnel to make it rotate around a single shaft at a high speed (tens to two thousand revolutions per minute). At this time, the material is forced to distribute near the mold wall under the action of centrifugal force. While rotating, the put materials are solidified, and then the products are obtained by cooling or post-treatment as required. When molding reinforced plastic products, reinforcing fillers can also be added at the same time.
Centrifugal casting usually uses thermoplastics with low melt viscosity and good thermal stability, such as polyamide and polyethylene.
(4) enamel. Slurry molding is a method of molding hollow products. During molding, the plastic paste is poured into an 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. This method is to put the liquid material into a closed mold, make it rotate around a single axis or multiple axes at a low speed (from several revolutions to dozens of revolutions per minute), make the material distribute on the inner wall of the mold under the action of gravity, and then cure or harden it by heating or cooling, so as to get the product from the mold. Rotating around a single axis is used to produce cylindrical products, moving around two axes or vibrating to produce closed products.
(6) Rotational molding (rotational molding). A molding method similar to rotary casting, except that the material used is not liquid but sintered dry powder. Its process is to put the powder into the mold, make it rotate around two mutually perpendicular axes, heat and fuse it evenly 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 for rotational molding.
(7) Tape casting. A method for preparing a thin film. In the manufacturing process, liquid resin or resin dispersion is first spread on a moving carrier (usually a metal belt), then it is cured (or hardened) by a suitable method, and finally the film can be peeled off from the carrier.
Plastics used to produce cast films include cellulose triacetate, polyvinyl alcohol, vinyl chloride and vinyl acetate copolymer. In addition, some engineering plastics such as polycarbonate can also be used to produce cast films.
1 1. Hand paste molding. Hand paste molding, also known as hand paste molding and contact molding, is one of the methods to manufacture reinforced plastic products. In this method, the reinforcing material is laid by hand on the mold coated with release agent, and the resin is painted to the required thickness, and then the product is cured and demoulded. Synthetic resins used for hand paste molding are mainly epoxy resin and unsaturated polyester resin. Reinforcing materials include glass cloth, untwisted plaid cloth, glass felt, etc.
12. Fiber winding forming. Under the condition of controlling tension and predetermined line shape, the continuous wire impregnated with resin glue is wound on the mandrel or die to form the reinforced plastic product. This method is only suitable for manufacturing cylindrical and spherical rotating bodies. Commonly used resins are phenolic resin, epoxy resin and unsaturated polyester resin. Glass fiber is a commonly used reinforcing material for winding forming. There are two kinds of glass fiber: twisted fiber and untwisted fiber.
13. Scroll. A method of forming a film or sheet by passing thermoplastic through a series of heated rollers and connecting them under the action of extrusion and stretching. Yating products include films, sheets, artificial leather and other coating products. The raw materials used in calendering are mainly PVC, cellulose, modified polystyrene and so on.
Calendering equipment includes calenders and other auxiliary machines. Calender is usually classified according to the number and arrangement of rollers. According to the number of rolls, there are two rolls, three rolls, four rolls, five rolls or even six rolls, and the three-roll or four-roll calender is most used.
14. coating. A method of coating a thin plastic layer (such as less than 0.3 mm) on the surface of fabric, paper, metal foil or plate in the form of liquid or powder, which is used for anticorrosion, insulation and decoration.
The most commonly used plastics for coating method are generally thermoplastics, such as polyethylene, polyvinyl chloride, polyamide, polyvinyl alcohol, polytetrafluoroethylene and so on.
The coating process includes hot melting, fluidized spraying, flame spraying, electrostatic spraying and plasma spraying.
(1) thermal welding. The plastic powder is sprayed on the surface of the preheated workpiece by compressed air through a spray gun, and the plastic melts and cools to form a covering layer.
(2) Fluidized spraying. Immerse the preheated workpiece in a container suspended with resin powder, and the resin powder melts and adheres to the surface.
(3) Flame spraying. The invention relates to a method for spraying by melting fluidized resin in a conical flame zone of a spray gun nozzle.
(4) Electrostatic spraying. Electrostatic field is caused by high-voltage static electricity, that is, the workpiece is grounded to a positive level, and the plastic powder is negatively charged when sprayed, so the plastic is sprayed onto the workpiece by static electricity.
(5) plasma spraying. Inert gas (such as mixed gas of argon, nitrogen and helium) flowing through the plasma generation zone is made into high-speed high-energy plasma flow at 5500~6300℃ by a plasma spray gun, and powder resin is entrained and sprayed on the surface of the workpiece at high speed to form a coating.
15. Foaming molding. Foaming molding is a process that makes plastic produce microporous structure. Almost all thermosetting and thermoplastic plastics can be made into foam plastics, and the commonly used resins are polystyrene, polyurethane, polyvinyl chloride, polyethylene, urea-formaldehyde and phenolic resin.
According to the cell structure, foams can be divided into two categories. If most of the pores are interconnected, they are called open-cell foams. If most of the pores are separated from each other, it is called closed-cell foam. The open-cell or closed-cell foam structure is determined by the manufacturing method.
(1) chemical foaming. The plastic melt is full of gas produced by specially added chemical foaming agent, thermal decomposition or chemical reaction between raw material components. Gases released by chemical foaming agents during heating include carbon dioxide, nitrogen and ammonia. Chemical foaming is often used in the production of polyurethane foam.
(2) Physical foaming. Physical foaming is a method of dissolving gas or liquid in plastic and then expanding or evaporating it. Physical foaming is suitable for a variety of plastics.
(3) mechanical foaming. Gas is mixed into the liquid mixture by mechanical stirring, and then foamed plastics with cells are formed by molding process. This method is often used for urea-formaldehyde resin, and other resins such as polyvinyl formal, polyvinyl acetate and polyvinyl chloride sol are also suitable.
16. Secondary molding. Secondary molding is one of the methods of plastic molding. A method for making plastic profiles or parisons into products with required shapes by heating and external force.
(1) thermoforming. Thermoforming is a molding method in which a thermoplastic sheet is heated until it softens, and a product is made with a suitable mold or fixture under air pressure, hydraulic pressure or mechanical pressure. There are many methods of plastic thermoforming, which can be generally divided into:
Molding adopts single mold (male mold or female die) or opposite mold, and the sheet is made into various products by external mechanical pressure or self-weight, which is different from one-time molding. This method is suitable for all thermoplastics.
Differential pressure forming adopts single mode (punch or die) or counter-die, or no die is needed. Under the action of gas pressure difference, the heated plastic sheet is made close to the surface of the mold and cooled to make various products. Differential pressure forming can be divided into vacuum forming and air pressure forming.
Thermoforming is especially suitable for manufacturing thin-walled and large-surface products. Commonly used plastics include various kinds of polystyrene, plexiglass, polyvinyl chloride, ABS, polyethylene, polypropylene, polyamide, polycarbonate and polyethylene terephthalate.
Thermoforming equipment includes clamping system, heating system, vacuum and compressed air system and forming die.
(2) biaxial stretching. So as to reorient the molecules of thermoplastic films or plates, especially the biaxial stretching process above the glass transition temperature. Tensile orientation should be carried out between the glass transition temperature and melting point of the polymer. After directional stretching and rapid cooling to room temperature, the mechanical properties of the film or monofilament in the stretching direction are greatly improved.
Polymers suitable for directional stretching include: polyvinyl chloride, polyethylene terephthalate, polyvinylidene chloride, polymethylmethacrylate, polyethylene, polypropylene, polystyrene and some styrene polymers.
(3) solid phase molding. Solid-phase molding is a method of molding thermoplastic profiles or blanks into products with dies under pressure. The molding process is lower than the melting (softening) temperature of plastic (at least lower than the melting point 10-20℃). All belong to solid phase molding. Among them, the processing of amorphous plastics in the high elastic region above the glass transition temperature and below the melting point is often called hot forming, while the processing below the glass transition temperature is called cold forming or room temperature forming, and is also often called cold working method or room temperature plastic processing of plastics.
This method has the following advantages: short production cycle; Improve the toughness and strength of products; The equipment is simple and can produce large and super-large products; Reduce costs. Disadvantages are: it is difficult to produce complex and precise products; The production process is difficult to control, and the product is easy to deform and crack.
Solid-state forming includes: sheet rolling, deep drawing or sheet stamping, hydroforming, extrusion, cold stamping, rolling forming, etc.