Views: 28 Author: Site Editor Publish Time: 2021-11-05 Origin: Site
Reaction injection molding service originated from polyurethane plastics. With the advancement of process technology, the process has also been extended to the processing of a variety of materials. At the same time, in order to broaden the application fields of RIM technology, especially in the automotive industry, the process also introduces fiber-reinforced technology.
Introduction to Reaction Injection Molding (RIM)
Reaction injection molding ("RIM" for short) refers to the mixing of two-component materials with high chemical activity and low relative molecular weight, and then injecting them into a closed mold at room temperature and low pressure to complete polymerization, crosslinking and curing. The process of reacting and forming a product. This new process combining polymerization reaction and injection molding has the characteristics of high material mixing efficiency, good fluidity, flexible raw material preparation, short production cycle and low cost. It is suitable for the production of large thick-walled products, so it is well received by the world. The attention of all countries.
RIM was originally only used for polyurethane materials. With the advancement of process technology, RIM can also be applied to the processing of a variety of materials (such as epoxy, nylon, polyurea, polycyclopentadiene, etc.). The RIM process for rubber and metal molding is a hot spot of current research.
In order to broaden the application fields of RIM, improve the rigidity and strength of RIM products, and make them into structural products, RIM technology has been further developed, and there have been enhanced reaction injection molding (RRIM) specially used for the molding of reinforced products and specialized Structural reaction injection molding (SRIM) technology for structural parts molding.
RRIM and SRIM molding process principles are the same as RIM, the difference is mainly in the preparation of fiber-reinforced composite products. At present, typical RIM products include large products such as car bumpers, fenders, body panels, truck boxes, truck middle doors and rear door components. Their product quality is better than SMC products, the production speed is faster, and the amount of secondary processing required is smaller.
RIM molding process
The RIM process is as follows: the monomer or prepolymer enters the mixing head in a certain proportion through a metering pump in a liquid state for mixing. After the mixture is injected into the mold, it reacts quickly in the mold, crosslinks and solidifies, and becomes a RIM product after demolding. This process can be simplified as: storage → metering → mixing → filling → curing → ejection → post-processing.
2. Process control
(1) Storage. The two-component stock solution used in the RIM process is usually stored in two reservoirs at a certain temperature, and the reservoirs are generally pressure vessels. When it is not forming, the stock solution usually circulates continuously in the reservoir, heat exchanger and mixing head under the low pressure of 0.2~0.3 MPa. For polyurethane, the temperature of the stock solution is generally 20-40°C, and the temperature control accuracy is ±1°C.
(2) Measurement. The metering of the two-component raw liquid is generally completed by the hydraulic system, which is composed of pumps, valves and accessories (the piping system that controls the liquid material and the oil circuit system that controls the work of the distribution cylinder). During injection, the pressure is converted into the pressure required for injection through a high-low pressure conversion device. The original liquid is measured and output by a hydraulic quantitative pump, and the measurement accuracy is required to be at least ±1.5%, and it is best to control it at ±1%.
(3) Mixing. In the molding of RIM products, the quality of the product largely depends on the mixing quality of the mixing head, and the production capacity completely depends on the mixing quality of the mixing head. The generally used pressure is 10.34~20.68MPa, and a better mixing effect can be obtained within this pressure range.
(4) Filling the mold. The characteristic of reaction injection material filling is that the material flow velocity is very high. For this reason, it is required that the viscosity of the stock solution should not be too high, for example, the viscosity of the polyurethane mixture when filling the mold is about 0.1 Pa.s.
When the material system and mold are determined. There are only two important process parameters, namely filling time and raw material temperature. The initial temperature of the polyurethane material should not exceed 90℃, and the average flow velocity in the cavity should generally not exceed 0.5m/s.
(5) Curing. The polyurethane two-component mixture has high reactivity after being injected into the mold cavity and can be cured and set in a short time. However, due to the poor thermal conductivity of the plastic, a large amount of reaction heat cannot be dissipated in time, so the internal temperature of the molded object is much higher than the surface temperature, causing the curing of the molded object to proceed from the inside to the outside. In order to prevent the temperature in the cavity from being too high (not higher than the thermal decomposition temperature of the resin), the heat exchange function of the mold should be fully utilized to dissipate heat.
The curing time in the reaction injection mold is mainly determined by the formula of the molding material and the size of the product. In addition, the reaction injection product needs to be heat treated after it is ejected from the mold. Heat treatment has two functions: one is to supplement curing, and the other is to bake after painting to form a strong protective film or decorative film on the surface of the product.
1. Polyurethane RIM
The raw materials used in polyurethane RIM are different from general-purpose polyurethane raw materials: the liquid raw materials are required to have low viscosity, good fluidity and high reactivity, and the raw materials should be formulated into two components, A (polyol) and B (diisocyanate).
The process includes: placing the A and B components of the raw materials in the raw material tank of the injection machine, and keeping them in a N2 atmosphere at a certain temperature at a suitable viscosity (below 1Pa·s) and reactivity; The quantitative pump presses the two-component raw materials into the mixer according to a certain ratio and injects them into the sealed mold; the mixture is rapidly polymerized in the mold, and solidified. In this process, it only takes 1~4s from the raw material to fill the cavity, and the complete production cycle is 30~120s.
2. Polyurethane RRIM
The two components used in the polyurethane RRIM process are polyol and isocyanate. The polyol is a polyether type, with a relative molecular mass of 1?800~2?400, and a functionality of 2~3; the isocyanate is generally diphenylmethane diisocyanate (MDI) or a mixture of polyisocyanate and its isomers. The degree is 2~7. There are two main types of RRIM reinforcing materials, namely chopped reinforcing fibers and milled reinforcing fibers. The length of the fiber is generally 1.5~3.0mm, this length can not only ensure the reinforcement effect, but also facilitate the passage through the injection system. The greater the dispersion of the fiber length, the worse the reinforcement effect. The content of reinforcing fibers (mass fraction) in RRIM products is generally below 20%. For high-strength products with special requirements, the content of reinforcing fibers can reach 50%.
3. Epoxy RIM
Epoxy RIM products have high tensile strength and flexural modulus, low linear expansion coefficient, and have excellent chemical resistance and high heat resistance (compared to polyurethane and nylon). In order to improve the impact strength of epoxy resins, polyethylene glycol prepolymers with isocyanate groups and a relative molecular mass of 4,000 can be added to the raw materials.
In addition, in order to further improve the mechanical properties, various reinforcing materials, such as various fibers, whisker powder, flake powder, microbeads and long fibers, can be added to make them RRIM products. They are extremely useful in the automotive industry. Competitive.
4. Nylon 6 RIM
The raw materials used in nylon 6 RIM include polyether polyol and prepolymer (component A) made of catalyst and caprolactam (component B). During processing, first add caprolactam to the raw material tank, control the temperature to 74~85℃, then add the catalyst, close the container, stir vigorously to dissolve the catalyst in the caprolactam, and degas the mixture for 15min under N2.
Then mix caprolactam and prepolymer at a mixing temperature of 74-85°C, stir well and degas. Then under the action of pressure, the two liquid components enter the mold through the mixer and solidify and shape. Because the prepolymer and caprolactam undergo a block copolymerization reaction, the resulting product has good flexibility and high impact strength.
Nylon 6 RRIM products with reinforced materials have higher rigidity and lower linear expansion coefficient. Nylon 6 RIM and RRIM products are widely used, mainly in the automotive industry, such as fenders, door panels, engine hoods and crash covers.
5. Dicyclopentadiene (DCPD) RIM
The raw materials of DCPD RIM mainly include DCPD, catalysts, activators, stabilizers, regulators, fillers, antioxidants, elastomers, foaming agents, flame retardants and nucleating agents.
In the DCPD RIM system, various raw materials are generally divided into two components, A and B according to the formula requirements. The A component includes DCPD, catalysts, stabilizers and other additives. The B component includes DCPD, activator, regulator and other auxiliary agents.
During processing, the accurately metered components A and B are uniformly mixed in the mixing head, and then injected into the sealed mold. Rapid polymerization reaction occurs in the mold, followed by solidification and molding. It is important to note that before the mold is full, the polymerization reaction time regulator controls the chemical reaction. After the mold is filled, the polymerization is completed and the molding is completed in about 10 seconds. Products generally do not need to go through a post-curing process.
6. Polyurea RIM
Polyurea RIM uses a self-release material system containing internal release agent, which is made by the reaction of amino-terminated polyether, amine chain extender and isocyanate-terminated prepolymer (MDI) during molding. Polyurea.
The process has many excellent characteristics: due to the high reactivity of amine groups and isocyanate groups, no catalyst is required; when the reaction material is injected into the mold cavity, the viscosity is large, and the eddy current is reduced when the mold is filled, so less air is introduced, and the product is waste. The rate is low; the material gels within 1~2s after entering the mold, and only needs to stay in the mold for 20s; the material does not adhere to the cavity during demolding, and the selection of internal mold release agent system is less restricted; adding reinforced glass fiber to prepare poly In the case of urea RRIM products, it has no effect on the reaction between amine and isocyanate.
The entire reaction process of polyurea formation does not require a catalyst, so that there is no residual catalyst in the product, so the polyurea RIM product does not degrade at high temperature, and the product has good stability.
7. Variable fiber reaction injection molding (VFRIM)
MM/RIM technology is to lay the fiber in the mold cavity first, and then inject the liquid resin. The disadvantage of this process is that it requires prefabricated fiber mats, which complicates the process and increases the cost. In addition, because the fiber felt needs to be laid manually, the labor intensity is greatly increased. Based on this, variable fiber reaction injection molding (Variable fiber ※※※※ction reaction ※※※※cction molding, VFRIM) was born.
This technology was developed by the German company KraussMaffei and Italy's Cannon-Technos company in the 1990s. Its important feature is that the fiber roving is first sent to the shredder to cut into dispersed short fibers, and then the short fibers are sent to the L-shaped mixing head to mix with the resin, and finally the mixture is injected into the mold for curing and forming.
At present, the products that have been produced using VFRIM technology include car door panels, cushion trays, bumpers, sun visors, luggage trays and light truck box panels. The performance of low-density products produced with VFRIM technology is equivalent to that of traditional RIM products. Compared with RIM products, high-density products produced using VFRIM technology show better performance.