Plastics that are made of thermosetting resin as the main component and are combined with various necessary additives through a cross-linking curing process to form finished products. They are liquid in the early stages of the manufacturing or molding process, and after curing they are insoluble and infusible, and cannot be melted or softened again. Common thermosetting plastics include phenolic plastics, epoxy plastics, amino plastics, unsaturated polyesters, alkyd plastics, etc. Thermosetting plastics and thermoplastics together constitute the two major component systems of synthetic plastics. Thermosetting plastics are divided into two types: formaldehyde cross-linked type and other cross-linked types.
Definition
Thermosetting plastics can soften and flow when heated for the first time. When heated to a certain temperature, a chemical reaction occurs, i.e. a cross-linking reaction, and the plastics solidify and harden. This change is irreversible. After that, they can no longer soften and flow when heated again. It is precisely with the help of this characteristic that the molding process is carried out. The plasticized flow during the first heating is utilized to fill the mold cavity under pressure, and then solidify into a product of a certain shape and size.
The characteristic of thermosetting plastics is that they harden due to chemical reactions after being heated, pressurized or added with a hardener at a certain temperature for a certain period of time. The chemical structure of the hardened plastic changes, the texture becomes hard, it is insoluble in solvents, and it no longer softens when heated. If the temperature is too high, it will decompose. The resin molecular chains in thermoplastics are linear or branched structures. There are no chemical bonds between the molecular chains. They soften and flow when heated. The process of cooling and hardening is a physical change.
Formaldehyde cross-linked plastics include phenolic plastics and amino plastics (such as urea-formaldehyde-melamine-formaldehyde, etc.). Other cross-linked plastics include unsaturated polyesters, epoxy resins, diallyl phthalate resins, etc.
Commonly used thermosetting plastics include phenolic resin, urea-formaldehyde resin, melamine resin, unsaturated polyester resin, epoxy resin, silicone resin, polyurethane, etc.
Types
1. Phenolic resin (PF)
Phenolic resin is one of the longest-standing plastic varieties in history. It is commonly known as bakelite or bakelite. It is yellow-brown or black in appearance and is a typical representative of thermosetting plastics. Various fillers are often used in phenolic resin molding. Depending on the different fillers used, the performance of the finished product is also different. As a molding material, phenolic resin is mainly used in fields that require heat resistance, but it is also used as an adhesive for plywood, grinding wheels and brake pads.
2. Urea-formaldehyde resin (UF)
Urea-formaldehyde resin is a colorless plastic that can be used as molding material, adhesive, etc. It is made from urea and formaldehyde. Urea-formaldehyde resin molding material is filled with cellulose. It has excellent hardness and mechanical strength. On the other hand, it has the disadvantages of being brittle, having water absorption, and poor dimensional stability, and even cracks often occur when left still. Urea-formaldehyde resin can be used to manufacture tableware, bottle caps and other daily necessities and mechanical parts, and can also be used as an adhesive.
3. Melamine-formaldehyde resin (MF)
Melamine-formaldehyde resin is also called melamine, melamine-metic, and melamine. This plastic makes up for the shortcoming of urea-formaldehyde resin that it is not water-resistant, but its price is higher than that of urea-formaldehyde resin. Because melamine-formaldehyde resin is colorless and transparent like urea-formaldehyde resin, and its molding color is bright, and because it has heat resistance, high surface hardness, mechanical properties, good electrical properties, water resistance, solvent resistance and chemical resistance, it can be used in the fields of tableware, various daily necessities (including furniture), and industrial products.
4. Unsaturated polyester resin (UF)
Unsaturated polyester resin is a light yellow or amber transparent liquid with different viscosities. Because unsaturated polyester resin is not strong, it is often added with reinforcing materials such as glass fiber, and the product is commonly known as fiberglass. Unsaturated polyester resin is liquid before solidification, and can be formed without pressure, and can even be solidified at room temperature, so it can be processed into products by various processing methods.
5. Epoxy resin (EP)
Epoxy resin is a thermosetting plastic cured with a curing agent. It has excellent adhesion, excellent electrical properties, and good mechanical properties. The main use of epoxy resin is as a metal anti-corrosion coating and adhesive, and is often used in the encapsulation of printed circuit boards and electronic components.
6. Silicone resin (SI)
Unlike the aforementioned resins, the main component is not carbon, but silicon, so the price is high. However, silicone resin is heat-resistant to 180°C, and can withstand 500°C after special treatment. It has good cold resistance, and its physical properties do not change with temperature. It is a thermosetting plastic with excellent chemical resistance, water resistance and weather resistance. Its heat-resistant products are materials for the production of electronic industrial components.
7. Polyurethane
There are many varieties of polyurethane, which can be made into everything from lightweight thermoplastic elastomers to rigid foams. The density of polyurethane soft foam is 0.015 to 0.15g/cm³. Soft foams are formed into blocks, which are easy to cut into furniture and packaging materials. Rigid foams can be made into a variety of shapes.
Processing characteristics
Commonly used thermosetting plastics include phenolic, amino (melamine, urea-formaldehyde) polyester, poly(dipropylene phthalate), etc. They are mainly used for compression molding, extrusion molding, and injection molding. Plastics such as silicone and epoxy resin are mainly used for low-pressure extrusion packaging of electronic components and casting molding.
Shrinkage
Forming shrinkage is mainly manifested in the following aspects:
1. The linear size shrinkage of plastic parts is due to thermal expansion and contraction, elastic recovery during demolding, plastic deformation, etc., which causes the size of plastic parts to shrink after they are demolded and cooled to room temperature. Therefore, compensation must be considered when designing the cavity.
2. Directionality of shrinkage During molding, molecules are arranged in a certain direction, making the plastic part anisotropic. The shrinkage is large and the strength is high along the material flow direction (i.e. parallel direction), while the shrinkage is small and the strength is low in the direction perpendicular to the material flow (i.e. vertical direction). In addition, during molding, the density and filler distribution of each part of the plastic part are uneven, so the shrinkage is also uneven. The shrinkage difference makes the plastic part prone to warping, deformation, and cracking, especially during extrusion and injection molding. Therefore, the shrinkage direction should be considered when designing the mold, and the shrinkage rate should be selected according to the shape of the plastic part and the direction of the material flow.
3. Post-shrinkage When plastic parts are formed, a series of stresses are caused by factors such as forming pressure, shear stress, anisotropy, uneven density, uneven filler distribution, uneven mold temperature, uneven hardening, and plastic deformation. These factors cannot completely disappear in the viscous flow state, so residual stress exists when the plastic parts are formed under stress. After demolding, the residual stress changes due to the balance of stress and the influence of storage conditions, which causes the plastic parts to shrink again, which is called post-shrinkage. Generally, plastic parts change the most within 10 hours after demolding, and are basically fixed after 24 hours, but the final stability takes 30 to 60 days. Generally, the post-shrinkage of thermoplastics is greater than that of thermosetting plastics, and that of extrusion and injection molding is greater than that of compression molding.
4. Post-processing shrinkage Sometimes plastic parts need to be heat treated after forming according to performance and process requirements, which will also cause changes in the size of the plastic parts. Therefore, when designing molds for high-precision plastic parts, the errors of post-shrinkage and post-processing shrinkage should be considered and compensated.
Liquidity
The ability of plastic to fill the cavity under a certain temperature and pressure is called fluidity. This is an important process parameter that must be considered when designing a mold. High fluidity can easily cause excessive overflow, loose filling of the cavity, loose plastic part structure, accumulation of resin and filler, easy mold sticking, difficulty in demolding and cleaning, and premature hardening. However, low fluidity will result in insufficient filling, difficulty in forming, and high forming pressure. Therefore, the fluidity of the selected plastic must be compatible with the requirements of the plastic part, the forming process and the forming conditions. When designing a mold, the pouring system, parting surface and feeding direction should be considered based on the fluidity performance. The fluidity of thermosetting plastics is usually expressed in Raschig fluidity (in millimeters). The larger the value, the better the fluidity. Each type of plastic is usually divided into three different levels of fluidity for selection of different plastic parts and forming processes. Generally, when the plastic part has a large area, many inserts, the core and insert are thin and weak, and there are narrow deep grooves and thin walls with complex shapes that are not conducive to filling, plastics with better fluidity should be used. Plastics with a Lasig fluidity of more than 150 mm should be used for extrusion molding, and plastics with a Lasig fluidity of more than 200 mm should be used for injection molding. In order to ensure that each batch of plastics has the same fluidity, the batch method is often used in practice to adjust, that is, plastics of the same type but with different fluidities are used together to make the fluidities of each batch of plastics compensate for each other to ensure the quality of plastic parts. It must be pointed out that the injection properties of plastics are not only determined by the type of plastics, but are also often affected by various factors when filling the cavity, which changes the actual ability of plastics to fill the cavity. For example, fine and uniform particle size (especially round particles), high humidity, high moisture and volatiles, appropriate preheating and forming conditions, good mold surface finish, and appropriate mold structure are all conducive to improving fluidity. On the contrary, poor preheating or forming conditions, poor mold structure, large flow resistance, or too long storage period, overdue storage, high storage temperature (especially for amino plastics) will lead to a decrease in the actual flow performance of plastics when filling the cavity, resulting in poor filling.
Specific volume and compression ratio
Specific volume is the volume occupied by one gram of plastic (in cm³/g). Compression rate is the ratio of the volume or specific volume of plastic powder and plastic part (its value is always greater than 1). They can be used to determine the size of the mold loading chamber. A large value requires a large loading chamber volume, and at the same time indicates that there is a lot of gas in the plastic powder, exhaust is difficult, the molding cycle is long, and the productivity is low. A small specific volume is the opposite, and it is conducive to tablet pressing and pressing. However, the specific volume value often has errors due to the particle size and particle unevenness of the plastic.
Hardening properties
During the molding process, thermosetting plastics are transformed into a plastic viscous state under heating and pressure, and the fluidity increases to fill the cavity. At the same time, a condensation reaction occurs, the cross-linking density continues to increase, the fluidity decreases rapidly, and the molten material gradually solidifies. When designing the mold, attention should be paid to facilitating loading, loading and unloading of inserts, and selecting reasonable molding conditions and operations for materials with fast hardening speed and short flow state, so as to avoid premature hardening or insufficient hardening, resulting in poor molding of plastic parts.
The hardening speed is generally related to the type of plastic, wall thickness, shape of the plastic part, and mold temperature. However, it is also affected by other factors, especially the preheating state. Appropriate preheating should be maintained under the condition that the plastic can exert maximum fluidity and maximize its hardening speed. Generally, the higher the preheating temperature and the longer the time (within the allowable range), the faster the hardening speed. In particular, the hardening speed of the pre-pressed billet is significantly accelerated by high-frequency preheating. In addition, the hardening speed increases with high forming temperature and long pressurization time. Therefore, the hardening speed can also be properly controlled by adjusting the preheating or forming conditions. The hardening speed should also be suitable for the requirements of the forming method. For example, injection and extrusion molding should require slow chemical reactions and slow hardening during plasticization and filling, and should maintain a long-term fluid state, but when the cavity is filled, it should harden quickly under high temperature and high pressure.
Moisture and volatile matter content
Various plastics contain different levels of moisture and volatiles. When there is too much moisture and volatiles, the fluidity increases, the material is easy to overflow, the retention time is long, the shrinkage increases, and the defects such as ripples and warping are easy to occur, which affects the mechanical and electrical properties of plastic parts. However, when the plastic is too dry, it will also lead to poor fluidity and difficulty in forming. Therefore, different plastics should be preheated and dried as required. For materials with strong hygroscopicity, especially in humid seasons, even the preheated materials should be prevented from absorbing moisture again.
Since various plastics contain different amounts of water and volatiles, and condensation occurs during the condensation reaction, these components must be converted into gases and discharged from the mold during molding. Some gases are corrosive to the mold and irritating to the human body. Therefore, when designing the mold, we should understand the characteristics of various plastics and take corresponding measures, such as preheating, chrome plating the mold, opening exhaust grooves, or setting up exhaust processes during molding.
Injection molding process
The injection molding process of thermosetting plastics is the same as that of thermoplastics, but the process parameters are different. Common injection molding machines can be plunger injection molding machines or screw injection molding machines. The injection molding method (taking a screw injection molding machine as an example) is as follows. Add the thermosetting plastic into the plasticizing barrel, and the heated plasticizing barrel and the rotating screw melt and plasticize the raw materials into a molten state. At this time, a physical reaction occurs in the raw materials, which are then pushed forward to the screw head by the rotating screw. When the molten material reaches the injection volume, the screw moves forward to inject the molten material into the injection molding mold at a higher injection pressure and injection speed. At this time, the molten material in the injection molding mold reacts with the curing agent added at the same time under high pressure and high temperature conditions to produce a cross-linking reaction. This chemical reaction also releases low molecular weight substances such as water and ammonia. After the molten material cools down and hardens, it can be taken out of the injection molding mold to become an injection molded product of thermosetting plastics.
Related Cases
Generally speaking, plastic recycling refers to thermoplastics. Since thermosetting plastics form a cross-linked structure after solidification, they cannot be melted and reshaped again, so recycling is more difficult and there are few actual recycling applications. Only polyurethane and other plastics are recycled on a small scale commercially. The amount of thermosetting plastics accounts for about 15% of all plastics, which is a large absolute number, so its recycling is becoming more and more important and urgent.
There are not many types of commonly used thermosetting plastics, mainly polyurethane (PU), epoxy resin (EP), phenolic resin (PF), unsaturated polyester (UP), melamine resin (UF), etc. Among them, PU and PF are the most used, each accounting for about 1/3 of the total amount of thermosetting plastics. Post-consumer thermosetting plastic waste is very small in urban solid waste, mainly in industry and commerce.
Application Areas
More than half of PU production is used for soft foam, which is used more than furniture, mattresses, and automotive interior parts. Hard foam is the second largest use of PU, mainly used in construction, industrial insulation materials, packaging, and transportation. Reaction injection molding and casting PU are mainly used in automotive interior parts. In addition, it can also be used in agriculture, mining, sports and other equipment.
The main use of PF is in the manufacture of plywood, adhesives, glues, coatings, etc., while molding resins only account for a very small part.
UP is mainly used for large accessories, such as greenhouses, storage tanks, and car bodies. EP is mainly used to make adhesives, coatings, etc. It can also be used for molding, castings, printed circuit boards, etc. UF molded parts are mainly used for electrical equipment, tableware, and buttons.
There are three recycling methods for thermosetting plastics: physical recycling, chemical recycling, and energy recycling.
Precautions
1. Thermosetting plastics for injection molded products should be linear structural granules or powders with small relative molecular mass.
2. The melt of thermosetting plastics after plasticization should have good thermal stability and fluidity, and should have good fluidity when it stays in the barrel for a long time (within 10 minutes); the melt is stable at low temperatures and the cross-linking reaction is rapid at high temperatures.
3. The heating medium of the barrel is water, and the heating medium of the injection molding mold is oil. Use constant temperature control and the temperature fluctuation difference should be as small as possible.
4. The melt should be filled into the mold with a higher injection pressure and a faster injection speed. When adjusting, the lowest value should be taken to ensure the quality of the plastic product filling and molding.
5. Pay attention to the structural design of the screw head and nozzle, and no residual material is allowed to remain after injection. The nozzle is open, with an aperture of 2~2.5mm, and the melt channel is smooth and clean.
6. Pay attention to the selection of the cross-sectional dimensions of the exhaust channel in the injection molding mold. Cross-sectional dimensions that are too large or too small will have a certain impact on the molding quality of the plastic product.
