Oct. 07, 2024
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.
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Development History and Application of BMC&#; BMC material in the early s, 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 s, 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-s, 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.
&#;1&#;Electrical components
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.
&#;2&#;Auto parts
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.
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.
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.
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.
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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.
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.
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