BMC is the acronym of Bulk Molding Compound in English and its Chinese name is ball molded plastic or polyester ball. BMC is a semi-dry molding / injection intermediate material for manufacturing glass fiber reinforced thermosetting products, which is pre-mixed into paste by unsaturated polyester resin, low shrinkage / low profile additive, initiator, internal release agent, mineral filler, etc., and then mixed evenly by adding thickener, colorant, etc., and put into a special kneading kettle, the short-cut glass fiber is fully kneaded / impregnated, and the special materials need to add thickening process. Finally, mass / bulk materials are formed.
A brief introduction of BMC
Development History and Application of BMC： BMC material in the early 1960s, the low-shrinkage, non-shrinkage BMC, developed by BIP company in the United Kingdom and R&H company in the United States has greatly improved the dimensional stability, smoothness and appearance of polyester molded plastics. In the 1970s, in applications requiring dimensional accuracy, high rigidity and heat resistance, such as office machine brackets, the main tone was still black. Because of the single color, the application range is narrow. In the mid-1980s, BMC, which can be freely colored, was developed to expand its scope of application. Because BMC has excellent electrical properties, mechanical properties, heat resistance, chemical corrosion resistance, and adapt to various molding processes, it can not only meet the performance requirements of various products, but also meet the requirements of appearance, so it is more and more recognized by the majority of users.
Low voltage category: isolation switch, air switch, switchboard, watt-hour meter shell, etc.; high voltage category: insulator, insulator cover, arc suppression cover, closed lead plate, ZW, ZN vacuum series.
Headlight transmitter. Almost all of the lamp reflectors are made of BMC; car igniter, separation plate and decorative board, horn box, etc.; motor parts, air conditioning motor, motor shaft, coil frame, electric and steam parts. Household items such as microwave oven tableware, electric iron shell and other high voltage insulation.
The basic composition of BMC
1. Three Parts
From the perspective of proportion, BMC material is mainly composed of three parts. It is composed of matrix resin-unsaturated polyester resin and low shrinkage system, reinforced materials-mainly glass fiber, fillers-calcium carbonate, talc powder, aluminum hydroxide, porcelain clay and other components. Although the amount of these components accounts for a small proportion in the total composition, they play a very special role in the formulation system of BMC, which will have an important impact on the preparation process, product molding and final properties of BMC.
2. Low shrinkage and low profile additives (LSA, LPA)
Low shrinkage mechanism:Incompatible types-polyethylene, Polyvinyl chloride, etc., to improve the surface roughness of products; to pre-curing compatible classes-thermoplastic polyester, Polyvinyl acetate, Polymethyl methacrylate, etc., can achieve real zero shrinkage or even negative shrinkage.
3. BMC special glass fiber
The reinforced material of BMC is mainly short-cut glass fiber, which is an important guarantee of the mechanical properties of BMC. The length of short-cut fiber used in BMC is generally 6.4mm and 12.7mm. The content of BMC glass fiber depends on the strength requirements of the products, but too high or too low glass fiber content will increase the difficulty of production and molding, which is generally controlled between 50% and 55%.
The initiators used in BMC are mainly high temperature initiators which have a long storage life and function at high temperature, and some peroxides are commonly used, such as: BPO (Dibenzoyl Peroxide, TBPB (Tert-butyl Benzoate Peroxide) and so on. In order to improve the production efficiency and reduce the molding temperature of BMC, two different initiators can be used together, which is a new curing system. Although it may shorten the storage life of BMC, it can improve the curing speed and reduce the molding temperature to meet some special requirements of the products, such as too high temperature will damage the products.
When the resin is mixed with other raw materials to form a resin paste, it is necessary to maintain a low viscosity in order to infiltrate well with glass fiber. After the BMC production is completed, the viscosity of the sheet needs to increase rapidly to reach a very high viscosity. In other words, the viscosity should be “small first, then big, slow first and then fast”. Only in this way can the molding process of BMC be carried out, which requires the addition of thickeners.
Internal release agent
We know that in order to release the BMC molded products smoothly, a release agent must be added, the one coated on the surface of the mold is called the external release agent, and the one added to the resin paste is called the internal release agent. The mold internal release agents are some long-chain fatty acid salts, whose melting point is lower than the molding temperature, and will melt and migrate to the product surface before the material is heated and solidified, isolating the mold and the products to achieve the demoulding effect. The commonly used internal release agents are zinc stearate (ZnSt), calcium stearate (CaSt) and so on.
Coloring agent refers to the colorant, which makes BMC have a variety of required colors, so that SMC products are more full of vitality, the pigments used by BMC are not ordinary colorants, because BMC is molded at high temperature, the choice of colorant is very important, ordinary colorants in high temperature, high pressure flow process will color separation, fading and other phenomena.
In addition to the above main components in the BMC component, it is sometimes necessary to add some trace auxiliaries according to the special requirements of the product and process, such as anti-phase separation and viscosity reducer and so on.
Advantages and disadvantages of BMC process.
Qualified BMC composites not only have water and oil resistance, but also have good corrosion resistance. In addition, it also has a good service life, so we can use BMC composite materials in the manufacturing process of some electric motors or mechanical equipment. As can be seen from the above, common BMC composites provide a wide range of product sizes and specifications. We need to understand its advantages or disadvantages before buying, and compare and draw reasonable and reliable suggestions and solutions to achieve the maximum use value of the product.
The difference between BMC Composite and SMC
BMC material is the abbreviation of Bulk (Dough) molded plastics, that is, bulk molded plastics. It is called unsaturated polyester block molding plastics in China. The main raw material is dough prepreg, which is fully mixed with GF (chopped glass fiber), UP (unsaturated resin), MD (filler) and various additives. DMC materials were first used in the former West Germany and the United Kingdom in the 1960s, and then in the United States and Japan in the 1970s and 1980s. Because BMC block molding plastics have excellent electrical properties, mechanical properties, heat resistance, chemical resistance, adapt to a variety of molding processes, can meet the performance requirements of various products, and are widely used in automobile production and railway transportation, building accessories, mechanical and electrical products and other fields.
SMC composite material is the abbreviation of Sheet molding compound, and Sheet molding compound is sheet molding plastic. The main raw materials are composed of GF (special yarn), UP (unsaturated resin), low shrinkage additive, MD (filler and various additives. It first appeared in Europe in the early 1960s, and the process was developed by the United States and Japan around 1965. At the end of 1980s, China introduced advanced SMC production line and production technology. SMC composites and their SMC molded products have excellent electrical insulation, mechanical properties, thermal stability and chemical resistance. Therefore, SMC products are widely used. The current trend is that SMC composites will eventually replace BMC materials.
Advantages of BMC composite materials
1) BMC composites have a short molding cycle, can be molded or injected, and are suitable for mass production.
2) A large number of fillers can be added to the molding of BMC composites to meet the requirements of flame retardancy and dimensional stability, and the cost is low.
3) Some complex products can be formed as a whole. Plug-ins, holes, tables, ribs, grooves, etc., can be formed at the same time.
4) Compared with ordinary thermoplastics, bmc composite products have higher heat resistance, insulation and elastic modulus.
5) Due to the low skills of workers, bmc composites are easier to automate and save labor.
6) The size of the product using bmc composite material is accurate.
7) The working environment is good.
Shortcomings of BMC composite materials
1) BMC composites are only suitable for manufacturing products with small size and low strength requirements (usually the strength of BMC is about 30% lower than that of SMC).
2) The price of injection molding machine is higher.
The above examples are about all aspects of BMC composites, including its advantages and disadvantages. Therefore, it can be known that BMC composites have many properties. For example, water resistance, oil resistance and excellent corrosion resistance. The advantages of good heat resistance and electrical properties make it possible to stand out in the highly competitive material market. But in addition, it can be further found that BMC composites have higher requirements on the post-use environment than other materials, so you should refer to the shortcomings of this part to make the most reasonable purchase plan. You should also learn the above and understand the differences between various products to determine a satisfactory purchase price.