A  thermosetting resin is used as a main component, and a plastic which  forms a product is formed by a crosslinking curing process in  combination with various necessary additives. It  is liquid in the early stage of the manufacturing or molding process,  and it is insoluble and infusible after curing, and it cannot be melted  or softened again. Common  thermosetting plastics are phenolic plastics, epoxy plastics,  aminoplasts, unsaturated polyesters, alkyd plastics, and the like. Thermoset plastics and thermoplastics together constitute the two major components of synthetic plastics. Thermosetting plastics are divided into two types: formaldehyde cross-linking type and other cross-linking type.
When  the thermosetting plastic is heated for the first time, it can soften  the flow, heat to a certain temperature, and generate a chemical  reaction-crosslinking reaction to solidify and harden. This change is  irreversible, and thereafter, when heated again, it can no longer  soften. It  is by this characteristic that the molding process is carried out by  using the plasticizing flow at the time of the first heating, filling  the cavity under pressure, and solidifying into a product having a  determined shape and size.
Thermosetting  plastics are characterized by a chemical reaction and hardening after  heating, pressurizing or adding a hardener at a certain temperature for a  certain period of time. The  chemical structure of the hardened plastic changes, the texture is  hard, it is insoluble in the solvent, and the heating does not soften  any more. If the temperature is too high, it will decompose. In  the thermoplastic, the molecular chains of the resin are linear or  branched, and there is no chemical bond between the molecular chains.  When heated, the flow is softened. The process of cooling and hardening  is a physical change.
Formaldehyde  cross-linking plastics include phenolic plastics, aminoplasts (such as  urea-formaldehyde-melamine-formaldehyde, etc.). Other crosslinked plastics include unsaturated polyesters, epoxy resins, phthalyl acrylate resins, and the like.
Commonly  used thermosetting plastics are phenolic resin, urea-formaldehyde  resin, melamine resin, unsaturated polyester resin, epoxy resin, and  silicone resin.
Phenolic resin (PF)
Phenolic  resin is one of the longest plastic varieties in history. It is  commonly known as bakelite or bakelite. Its appearance is yellowish  brown or black. It is a typical representative of thermosetting  plastics. Phenolic  resin is often used in various filling materials. The properties of the  finished product vary depending on the filler used. Phenolic resin is  used as a molding material in the field where heat resistance is  required, but also as a bonding agent for plywood and grinding wheels. And brake pads.
2. Urea-formaldehyde resin (UF)
Urea-formaldehyde  resin is a colorless plastic which can be used as a molding material, a  binder, etc., and is prepared from urea and formaldehyde. The urea-formaldehyde resin molding compound is filled with cellulose. Moreover, the hardness and mechanical strength are excellent. On  the other hand, there are disadvantages such as brittleness, water  absorption, and poor dimensional stability, and cracks often occur even  when standing. Urea-formaldehyde  resin can be used to manufacture daily necessities and mechanical parts  such as tableware and bottle caps, and can also be used as a binder.
3. Melamine-formaldehyde resin (MF)
Melamine-formaldehyde resins are also known as melamine, melamine, and melamine. This  plastic makes up for the shortcomings of urea-formaldehyde resins that  are not water-resistant, but at a higher price than urea-formaldehyde  resins. Since  melamine-formaldehyde resin is as colorless and transparent as  urea-formaldehyde resin, it has bright color and is available because of  its heat resistance, surface hardness, mechanical properties,  electrical properties, water resistance, solvent resistance and chemical  resistance. In the field of tableware, various daily necessities (including furniture), industrial supplies.
4. Unsaturated polyester resin (UF)
The unsaturated polyester resin is a pale yellow or amber transparent liquid having different viscosities. Because  unsaturated polyester resin is not high in strength, it is often added  to reinforcing materials such as glass fiber. The product is commonly  called glass fiber reinforced plastic. The  unsaturated polyester resin is liquid before curing, and can be molded  without being pressurized, and can be cured even at normal temperature,  and thus can be processed into articles by various processing methods.
5. Epoxy resin (EP)
Epoxy resin is a thermosetting plastic that is 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-corrosive coating and  adhesive, which is commonly used for the sealing of printed circuit  boards and electronic components.
6. Silicone Resin (SI)
Unlike the above-mentioned respective resins, the main component is not carbon but silicon, so the price is high. However,  the silicone resin is heat-resistant at 180 ° C, can withstand 500 ° C  after special treatment, has good cold resistance, and does not change  with physical properties. It is a thermosetting plastic with excellent  chemical resistance, water resistance and weather resistance, and its  heat-resistant product. It is a material for the production of components for the electronics industry.
7. Polyurethane
There are many varieties of polyurethanes, ranging from lightweight thermoplastic elastomers to rigid foams. Polyurethane  flexible foam has a density of 0.015 ~ 0.15g / cm, and the flexible  foam is formed into a block shape, which is easy to cut into furniture  and packaging materials. Rigid foam can be made in various types.
Application
The main markets for thermoset materials produced by injection molding include:
Automotive industry: engine components, headlight reflectors and brakes.
Electrical industry: circuit breakers, switch housings and coil formers.
Household appliances: bread oven board, coffee maker base, motor commutator, motor housing and waste handler housing.
Other: power tool housing, lamp housing, gas flow meter and tableware.