BMC's Injection Molding

BMC is the abbreviation of glass fiber reinforced unsaturated polyester thermosetting plastics, which is the most widely used type of reinforced thermosetting plastics.

 

(1) Characteristics and applications of BMC

BMC has good physical, electrical and mechanical properties, so it is widely used, such as making mechanical parts such as gearbox components, intake pipes, valve covers, bumpers, etc.; It has also been widely used in aviation, architecture, furniture and other aspects that require earthquake-resistant, flame-retardant, beautiful and durable; In its traditional electrical field, its use is also becoming more and more extensive.

 

(2) BMC's injection molding equipment

In BMC, due to the addition of glass fiber, the equipment used is somewhat different from the usual thermoset injection molding equipment, and the injection molding machine used, both plunger type and screw type.

1) Feeding system. Whether it is a screw type or a plunger type, an extrusion feeding device must be attached to force the material into the barrel, and the feeding device mostly adopts plunger type pressurized feeding.

2) Injection molding system. Because the injection volume of the plunger injection molding machine is accurate and constant, so that 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's injection molding, it is important to control the barrel temperature and have a control system to control the temperature to ensure that the temperature from the charging section to the nozzle is optimal. At present, constant temperature water or constant temperature oil is mostly used for heating. Electric heating is also available.

4) Mold clamping device. Several clamping devices such as mechanical, hydraulic and full hydraulic are mostly used.

5) Molds. BMC injection molds are a little different from the usual thermoplastic, thermoset injection molds, mainly because BMC has a lower shrinkage, so there should be some changes when designing the mold cavity size.

 

(3) BMC's injection molding process

1) Barrel temperature and mold temperature. During processing, BMC is required to maintain a low-viscosity flow dynamic for a long time at the barrel temperature, and the general barrel temperature should be able to meet the low limit of BMC. The barrel temperature is generally divided into two or three stages of control, with a lower temperature near the hopper end and a higher temperature near the nozzle end. Generally the difference is 20~60 °C, mold

The temperature is generally controlled at 135~185°C.

2) Injection pressure. Due to the poor fluidity, fast curing and complex mold structure of BMC, it is advisable to choose a higher pressure for injection pressure, generally 80~160MPa.

3) Injection speed. The increase in injection speed helps to improve the surface quality of the plastic part and shorten the curing time, but it is not conducive to outgassing and increases the orientation of the glass fiber. Therefore, a lower injection speed should be used on the premise of ensuring the surface quality of plastic parts, usually 1.8~3.5m/min.

4) Screw speed and back pressure. If the screw injection molding machine is used, when injecting BMC, the damage of the screw to the glass fiber is greater, in order to minimize the damage of the glass fiber, the screw speed should be selected at a low value, generally 20~50r/min. According to the viscosity of BMC, it is advisable to use low back pressure, which is 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, the general injection time is 2~20s, and the holding time is 10~20 (thickness).