The characteristic of thermosetting plastics is that they undergo chemical reactions and harden after being heated, pressurized, or added with a hardening agent for a certain period of time at a certain temperature. After hardening, the chemical structure of plastic undergoes changes, its texture is hard, insoluble in solvents, and it no longer softens when heated. If the temperature is too high, it will decompose. In thermoplastics, the resin molecular chains are linear or with branched chains. There is no Chemical bond between the molecular chains, which softens and flows 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 polyester, epoxy resin, phthalate propyl ester resin, etc.
Commonly used thermosetting plastics include phenolic resin, urea formaldehyde resin, melamine resin, unsaturated polyester resin, epoxy resin, organic silicon resin, polyurethane, etc.
Thermosetting plastic products
Thermosetting plastic products
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1. Phenolic resin (PF)
Phenolic resin is one of the longest plastic varieties in history, commonly known as bakelite or bakelite, with a yellow brown or black appearance, and is a typical representative of thermosetting plastics. Phenolic resin molding often uses various filling materials, and the performance of the finished product varies depending on the filling materials used. As a molding material, phenolic resin is mainly used in areas 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 a molding material, adhesive, etc. It is prepared from urea and formaldehyde. The urea formaldehyde resin molding material is filled with cellulose. And the hardness and mechanical strength are excellent. On the other hand, there are drawbacks such as brittleness, water absorption, poor dimensional stability, and even static cracking. Urea formaldehyde resin can be used to manufacture daily necessities and mechanical components such as tableware and bottle caps, and can also be used as an adhesive.
3. Melamine resin (MF)
Melamine resin is also called melamine, melamine and melamine melamine. This type of plastic compensates for the water resistance of urea formaldehyde resin, but its price is higher than that of urea formaldehyde resin. Because Melamine resin is as colorless and transparent as urea formaldehyde resin, and its molding color is bright, and because of its heat resistance, high surface hardness, good mechanical properties, 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 supplies.
4. Unsaturated polyester resin (UF)
Unsaturated polyester resin is a light yellow or amber transparent liquid with different viscosities. Due to the low strength of unsaturated polyester resin, reinforcement materials such as glass fiber are often added for use, and the product is commonly known as fiberglass. Unsaturated polyester resin is in a liquid state before curing, and can be formed without pressure, and can even solidify at room temperature, so it can be processed into products using 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, commonly used for sealing and casting printed circuit boards and electronic components.
6. Organic silicone resin (SI)
Unlike the aforementioned resins, the main component is not carbon, but silicon, resulting in high prices. However, organic silicone resin is heat-resistant to 180 ℃ and can withstand 500 ℃ after special treatment. It has good cold resistance and 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 used in the production of electronic industrial components.
7. Polyurethane
There are many kinds of polyurethane, which can be made from lightweight thermoplastic elastomer to rigid foam. The density of polyurethane flexible foam plastic is 0.015~0.15g/cm. The flexible foam plastic is formed into blocks, which is convenient for cutting as furniture and packaging materials. Rigid foam plastic can be made into various types.
Folding and editing the processing technology of this section
Common thermosetting plastics include phenolic, amino (melamine, urea formaldehyde) polyester, and poly (propylene Phthalic acid), etc. Mainly used for compression molding, extrusion molding, and injection molding. Silicone, epoxy resin and other plastics are mainly used for low-pressure extrusion packaging of electronic components and casting forming.
Fold shrinkage rate
1. Forming shrinkage is mainly manifested in the following aspects:
(1) The linear size shrinkage of plastic parts due to Thermal expansion, elastic recovery and plastic deformation during demoulding lead to the size reduction of plastic parts after demoulding and cooling to room temperature. Therefore, compensation must be considered when designing the cavity.
(2) When forming with directional shrinkage, molecules are arranged in the same direction, resulting in anisotropy of the plastic part. Along the direction of the material flow (i.e. parallel direction), the shrinkage is large and the strength is high, while in the direction perpendicular to the material flow (i.e. vertical direction), the shrinkage is small and the strength is low. In addition, due to uneven density and filler distribution in various parts of the plastic part during forming, shrinkage is also uneven. The occurrence of shrinkage differences makes plastic parts prone to warping, deformation, and cracking, especially in extrusion and injection molding where directionality is more pronounced. Therefore, when designing the mold, it is advisable to consider the shrinkage direction and select the shrinkage rate according to the shape and flow direction of the plastic part.
(3) When forming post shrinkage plastic parts, due to the influence of factors such as forming pressure, shear stress, anisotropy, uneven density, uneven distribution of fillers, uneven mold temperature, uneven hardening, plastic deformation, etc., a series of stresses cannot disappear completely in the viscous flow state. Therefore, there is residual stress when forming plastic parts under stress state. When the residual stress changes due to stress balance and storage conditions after demolding, resulting in re shrinkage of the plastic part, it is called post shrinkage. Generally, plastic parts have the greatest change within 10 hours after demolding, and are basically shaped after 24 hours, but the final stability takes 30-60 days. Usually, the post shrinkage of thermoplastic materials is greater than that of thermosetting materials, while those formed by extrusion and injection molding are greater than those formed by compression molding.
(4) Post processing shrinkage sometimes requires heat treatment after forming plastic parts according to performance and process requirements, which can 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.
Folding liquidity
The ability of plastic to fill the mold cavity at a certain temperature and pressure is called flowability. This is an important process parameter that must be considered during mold design. High fluidity can easily cause excessive overflow, loose filling cavity, loose plastic structure, accumulation of resin and filler at different ends, easy adhesion, demolding and cleaning difficulties, and premature hardening. However, if the fluidity is low, the filling is insufficient, making it difficult to form, and the forming pressure is high. So the flowability of selecting plastic must be compatible with the requirements of the plastic part, forming process, and forming conditions. When designing the mold, the pouring system, parting surface, and feeding direction should be considered based on the flow performance. The flowability of thermosetting plastics is usually expressed in terms of Rasig flowability (in millimeters). A large numerical value indicates good fluidity, and each type of plastic is usually divided into three different levels of fluidity for different plastic parts and forming processes to choose from. When the complex shapes of large plastic parts, multiple inserts, thin cores and inserts, narrow deep grooves, and thin walls are unfavorable for filling, plastic with good fluidity should be used. During extrusion molding, plastic with a Rasig flowability of over 150mm should be used, while during injection molding, plastic with a Rasig flowability of over 200mm should be used. In order to ensure that each batch of plastic has the same fluidity, in practice, a batch merging method is commonly used to adjust, that is, to match plastics of the same variety with different fluidity, so that the fluidity of each batch of plastic compensates with each other to ensure the quality of the plastic parts. It must be pointed out that the injectability of plastic is not only determined by the type of plastic, but also influenced by various factors when filling the mold cavity, which changes the actual filling capacity of plastic. If the particle size is fine and uniform (especially circular particles), the humidity is high, the moisture content and volatile matter are high, the preheating and forming conditions are appropriate, the surface smoothness of the mold is good, and the mold structure is appropriate, all of which are conducive to improving the fluidity. On the contrary, poor preheating or forming conditions, poor mold structure, high flow resistance, or long or expired plastic storage period, high storage temperature (especially for amino plastics) can all lead to a decrease in the actual flow performance of plastic filling the cavity, resulting in poor filling.
Folding specific volume and compression rate
The specific volume is the volume occupied by each gram of plastic (in cm/g). The compression ratio is the ratio of the volume or specific volume of the plastic powder to the plastic part (its value is always greater than 1). They can all be used to determine the size of the mold loading chamber. The large numerical value requires a large volume of the loading chamber, while also indicating that the plastic powder is filled with too much air, difficult to exhaust, has a long forming cycle, and low productivity. If the specific volume is small, the opposite is true, and it is beneficial for pressing the ingot. However, the specific volume value often has errors due to the particle size and unevenness of the plastic.
Folding hardening characteristics
During the forming process, the thermosetting plastic changes into a plastic viscous flow state under heating and pressure, and then the fluidity increases to fill the cavity. At the same time, Condensation reaction occurs, the cross-linking density increases continuously, the fluidity decreases rapidly, and the melt gradually solidifies. When designing molds, attention should be paid to the convenience of loading, loading and unloading inserts, and selecting reasonable forming conditions and operations for materials with fast hardening speed and short flow state to avoid premature hardening or insufficient hardening, leading to poor plastic forming.
The hardening rate is generally related to the type of plastic, wall thickness, shape of the plastic part, and mold temperature. However, it is also influenced by other factors, especially related to the preheating state. Proper preheating should be maintained to maximize the fluidity of the plastic, and the hardening rate should be increased as much as possible. Generally, if the preheating temperature is high and the time is long (within the allowable range), the hardening rate will be accelerated, especially if the pre pressed ingot blank is preheated at high frequency, the hardening rate will be significantly accelerated. In addition, high forming temperature and long compression time also increase the hardening rate. Therefore, the hardening rate can also be appropriately controlled by adjusting the preheating or forming conditions. The hardening speed should also meet the requirements of the forming method. For example, during injection and Extrusion moulding, the chemical reaction should be slow and the hardening should be slow during plasticization and filling, and the flow state should be maintained for a long time. However, when the cavity is filled, it should be hardened rapidly under high temperature and pressure.
Folding moisture and volatile matter content
Various plastics contain varying degrees of moisture and volatile matter content. When excessive, their fluidity increases, they are prone to overflow, maintain for a long time, shrink, and are prone to defects such as ripples and warping, which affect the mechanical and electrical properties of plastic parts. But when plastic is too dry, it can also lead to poor fluidity and difficulty in forming. Therefore, different plastics should be preheated and dried according to requirements. For materials with strong moisture absorption, especially in humid seasons, even after preheating, it should be prevented from re absorbing moisture.
Since various plastics contain moisture and volatiles of different components, and condensation moisture occurs during Condensation reaction, these components need to be turned into gas and discharged out of the mold during forming. Some gases have corrosive effects on the mold and also have irritating effects on the human body. Therefore, in mold design, it is necessary to have an understanding of various plastic characteristics and take corresponding measures, such as preheating, mold chrome plating, opening exhaust slots, or setting up exhaust processes during forming.
Injection molding process
The injection molding process of thermosetting plastics is the same as that of thermoplastic injection molding, but the process parameters and conditions are different. Common Notes
Thermosetting plastic molding machine
Thermosetting plastic molding machine
Plastic injection molding machines can be either plunger type or screw type. The injection molding method (using a screw injection molding machine as an example) is as follows. The thermosetting plastics are added into the plasticizer barrel. The heated plasticizer barrel and the rotating screw make the raw materials melt and plasticize in a molten state. At this time, a physical reaction is generated in the raw materials, and then the rotating screw pushes the molten materials forward to the screw head. When the molten materials reach the injection volume, the screw moves forward to inject the molten materials into the injection molding mold at a higher injection pressure and injection speed. At this point, the molten material in the injection molding mold undergoes a cross-linking reaction with the curing agent added simultaneously under high pressure and temperature conditions, which releases low molecular substances such as water and ammonia. After the molten material is cooled and hardened, it can be taken out of the injection molding mold and become an injection molded product of thermosetting plastics.
Related Cases
Generally speaking, plastic recycling refers to thermoplastic plastics, while thermosetting plastics cannot be melted again due to the cross-linked structure formed after solidification, making it difficult to recycle and have few practical recycling applications. Only polyurethane and other materials are commercially recycled on a small scale. The usage of thermosetting plastics accounts for about 15% of all plastics, and the absolute quantity is very large. Therefore, the recycling and utilization of thermosetting plastics is becoming increasingly important and urgent.
There are not many commonly used thermosetting plastics, mainly including polyurethane (PU), epoxy resin (EP), phenolic resin (PF), unsaturated polyester (UP), melamine resin (UF), etc. Among them, PU and PF are the most commonly used, each accounting for about one-third of the total amount of thermosetting plastics. The amount of post consumer thermosetting plastic waste in urban solid waste is very small, mainly in industry and commerce.
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More than half of the PU output is used for soft foam, while the amount of soft foam is more than furniture, mattresses, automotive interior parts, etc; Hard foam is the second largest use of PU, which is mainly used for thermal insulation materials in construction and industry, as well as packaging and transportation companies; Reaction injection molding and casting PU are mainly used in automotive interior accessory manufacturers. In addition, it can also be used in agricultural, mining, sports and other equipment.
The main use of PF is to manufacture plywood, adhesives, adhesives, coatings, etc., while molding resin only accounts for a small portion.
UP is mainly used for large accessories, such as greenhouses, storage tanks, car bodies, etc. The main use of EP is to manufacture adhesives, coatings, etc. It can also be used for molding, casting parts, printed circuit boards, etc. UF's molded parts are mainly used for electrical equipment, tableware, and buttons.
There are three recycling methods for thermosetting plastics: logistics recycling, chemical recycling, and energy recycling.
matters needing attention
1. Thermosetting plastics for injection molded products should be linear structured particles or powders with relatively low molecular weight.
2. The melted material after plasticization of thermosetting plastics should have good thermal stability and flowability, and should have good flowability when it stays in the barrel for a long time (within 10 minutes); The melt is stable at low temperatures, and the crosslinking reaction is rapid at high temperatures.
3. The heating medium of the machine barrel is water, and the heating medium of the injection molding mold is oil. It is controlled at a constant temperature, and the temperature fluctuation difference should be minimized as much as possible.
4. The melt should be filled with higher injection pressure and faster injection speed. The adjustment should be based on ensuring the quality of plastic product filling and molding, and the lowest value should be taken.
5. Pay attention to the structural design of the screw head and nozzle, and do not leave any residue after injection. The nozzle is an open type with a diameter of 2-2.5mm, and the melt channel is smooth and clean.
6. Pay attention to the selection of cross-sectional dimensions for exhaust channels in injection molding molds. Excessive or excessive cross-sectional dimensions can have a certain impact on the quality of plastic product molding.