The following article comes from composite material application technology, the author composite material aapplication technology
Composite application technology
Composite application technology
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BMC is the abbreviation of glass fiber reinforced unsaturated polyester thermosetting plastic, which is currently the most used type of reinforced thermosetting plastic.
(1) Features and applications of BMC
BMC has good physical, electrical and mechanical properties, so it has a wide range of applications. For example, it can be used to make mechanical parts such as gearbox components, intake pipes, valve covers, bumpers, etc.; Durable aviation, construction, furniture, etc. are also widely used; in its traditional electrical appliances field, its use is also more and more extensive.
(2) BMC injection molding equipment
In BMC, because glass fiber is added, the equipment used is somewhat different from the usual thermosetting plastic injection molding equipment. The injection molding machine used is either plunger type or screw type.
1) Feeding system. Regardless of the screw type or the plunger type, an extrusion-type feeding device must be added to force the material into the barrel, and the feeding device mostly adopts the plunger-type pressurized feeding.
2) Injection molding system. Since the injection volume of the plunger injection molding machine is accurate and constant, the glass fiber is dispersed in the melt with less damage. Therefore, the plunger injection molding machine is used more, but the exhaust is inconvenient.
3) Heating system. In BMC injection molding, it is very important to control the temperature of the barrel. There must be a control system to control the temperature to ensure the best temperature from the feeding section to the nozzle. Currently, constant temperature water or constant temperature oil is used for heating. Electric heating can also be used.
4) Clamping device. Several clamping devices such as mechanical, hydraulic and full hydraulic are mostly used.
5) Mould. The BMC injection mold is somewhat different from the usual thermoplastic and thermosetting injection molds, mainly because the shrinkage rate of BMC is relatively low, so the cavity size should be changed when designing.
(3) BMC injection molding process
1) Barrel temperature and mold temperature. During processing, BMC is required to maintain a low-viscosity flow state for a long time at the barrel temperature. Generally, the barrel temperature should meet the low limit of BMC. The temperature of the barrel is generally divided into two or three stages. The temperature near the hopper is lower and the temperature near the nozzle is higher. Generally the difference is 20~60℃, mold
The temperature is generally controlled at 135~185℃.
2) Injection pressure. Because BMC has poor fluidity, fast curing, and complex mold structure, the injection pressure should be higher, generally 80~160MPa.
3) Injection speed. The increase in the injection speed helps to improve the surface quality of the plastic parts and shorten the curing time, but it is not conducive to exhaust gas and increases the degree of glass fiber orientation. Therefore, a lower injection speed should be used under the premise of ensuring the surface quality of the plastic parts, usually 1.8~3.5m/min.
4) Screw speed and back pressure. If a screw injection molding machine is used, the screw will damage the glass fiber when injecting BMC. In order to minimize the damage to the glass fiber, the screw speed should be low, generally 20~50r/min. According to the viscosity of BMC, it is better to use low back pressure, generally 1.4~2.0MPa.
5) Molding cycle. Due to the different size and complexity of the plastic parts, the process time of each section is also different. Generally, the injection time is 2-20s, and the holding time is 10-20 (thickness).
⒈BMC (DMC) processing characteristics
⑴ Fluidity: BMC (DMC) has very good fluidity and can maintain good fluidity under low pressure.
⑵ Curability: BMC (DMC) has a fast curing speed, and the curing time is 30-60 seconds/mm when the molding temperature is 135-145°C.
⑶Shrinkage rate: The shrinkage rate of BMC (DMC) is very low, between 0 and 0.5%. The shrinkage rate can also be adjusted by adding additives as needed; it can be divided into no shrinkage (shrinkage rate <0.05%), low shrinkage rate ( 0.05~0.3%), high shrinkage rate (0.3~0.5%) three levels.
⑷Coloring: BMC (DMC) has good coloring.
⑸ Disadvantages: longer molding time and larger product burrs.
⒉BMC (DMC) molding compound preparation
BMC (DMC) is a mixture of liquid UP resin, solid powder filler, chopped glass fiber and other ingredients. In order to facilitate the feeding and other operations during molding, the premix is usually extruded into strips or pellets. shape.
The preparation of BMC (DMC) is divided into two steps: ① first add resin, initiator, colorant, release agent, part of fillers, etc. into a high-shear mixer and mix evenly, the mixing temperature reaches about 50 ℃, and then slowly Add the remaining fillers and mix evenly to obtain a resin premixed slurry; ②Put the premixed slurry into a Z-type reamer mixer or planetary mixer, add the dried chopped glass fiber, and stir After 10-15 minutes, pour out the dough, squeeze it into strips and pellets with an extruder, seal it with polyester film after drying, and store it for later use.
⒊BMC (DMC) compression molding
BMC (DMC) compression molding is to add a certain amount of molding compound into a preheated mold, pressurize and heat, and then solidify and shape. The specific process is feeding (adding solid molding compound to the preheated steel mold) → combination Mold→filling (molding compound flows under pressure and fills the entire cavity)→curing (full curing after maintaining a certain time under the set pressure and temperature)→opening the mold and taking out the product.
BMC (DMC) compression molding process conditions are as follows:
⑴ Molding pressure: 3.5~7MPa for ordinary products, 14MPa for products with high surface requirements;
⑵Molding temperature: The mold temperature is 145±5℃, and the fixed mold temperature can be adjusted down by 5~15℃ for demoulding;
⑶ Mold clamping speed: mold clamping must be completed within 50 seconds;
⑷Curing time: The curing time for the product with a wall thickness of 3mm is 3 minutes, the curing time for a wall thickness of 6mm is 4-6 minutes, and the curing time for a wall thickness of 12mm is 6-10 minutes;
⒋BMC (DMC) injection molding
BMC (DMC) has good processing fluidity, so it is very suitable for injection molding. The injection pressure is not high during molding, the injection time is short, and the mold can be filled quickly and evenly.
The injection molding of BMC (DMC) is different from other thermosetting plastics in that it is difficult to feed putty-like agglomerates and requires a forced feeding device; BMC (DMC) can be regarded as a pre-mixed material that has been plasticized when feeding. , The screw does not have the effect of plasticizing, but only mixing, conveying and metering.
BMC (DMC) injection molding process conditions are as follows:
⑴ Barrel temperature: Generally, the temperature is controlled below 60℃ to avoid curing;
⑵Injection pressure: generally in the range of 20~70MPa, and in order to prevent damage to the package during packaging molding, the pressure should be controlled at about 10MPa;
⑶Injection time: very fast, the shortest can be as low as 1 second;
⑷ Mold temperature: usually controlled at 140~160℃ to ensure rapid curing after full mold;
⑸Curing time: generally 15~60 seconds.
⒌BMC (DMC) die casting molding
Die-casting molding is a molding method between compression and injection molding, also known as transfer molding, injection molding or injection molding. It is to put a certain amount of BMC (DMC) molding compound into the feeding chamber for proper preheating , And then rely on the plunger to quickly press the molding compound in the die-casting chamber into the closed hot mold cavity, hold the pressure and heat after the mold is full, and after the product is completely solidified and shaped, the mold is opened and the product is taken out.
Die-casting molding is not suitable for large-scale products. It is suitable for insulating parts, support parts, structural parts and other products with complex processing results, openings, multiple inserts, and variable shapes; in addition, it is used for plastic packaging parts to provide insulation, anti-corrosion and anti-vibration Effect.
BMC (DMC) die casting molding process conditions are as follows:
⑴Molding temperature: the mold temperature is 120~150℃, and the preheating temperature is 15~20℃ lower than the mold temperature to prevent the material from solidifying in advance in the die-casting chamber due to the high temperature;
⑵Molding pressure: 1.5~2.5 times higher than compression molding, 14~28MPa;
⑶ Preheating time: 40~60 seconds/mm;
⑷ Mold filling time: 10~30 seconds;
⑸Curing time: 10~30 seconds/mm.