Thermoset BMC injection molding
(1) Features and applications of BMC
Brief introduction of BMC: (the full name is Bulk Molding Compound)
BMC (DMC) is the abbreviation of Bulk (Dough) Molding Compound.
At domestic, it is usually called unsaturated polyester bulk molding plastic.
Its main materials are bulk prepreg made through the intensive mixing of GF (chopped glass fiber), UP (unsaturated pitch), MD (material padding) and various additives.
DMC materials were applied firstly in former West Germany and Britain in the 1960s, and then greatly developed in America and Japan respectively in the 1970s and 1980s.
As BMC bulk molding plastics possess excellent electric performance,
mechanical performance, heat-resistance, and chemical resistance are also adaptable for the various molding processes,
which meets the requirements of a variety of products for performance,
therefore, have also been welcomed by a majority of users increasingly.
I. Low voltage class of electrical parts: RT series, disconnecting switch, air switch, switchboard, electric meter shell, etc.
High voltage class: Insulator, insulation cover, arc-suppression cover, closed lead-wire plate, ZW and ZN vacuum series.
II. Reflector lights of auto parts, almost all of the Japanese reflector lights are made form DMC; Auto igniter, separating disc and decorative plate, car horn box and so on;
III. Motor parts, air-conditioner motor, motor shaft, coil former, power-driven, and steam-driven parts.
IV. Daily necessities, microwave oven tableware, the shell of electric iron, etc.
Packing, transportation, storage BMC series molding compound should be packed and sealed in the composite film bags, 25kgs per bag, put into carton after bandage;
Beware of heavy load, being heated or damaged package in transit;
The storage period will be three months since the date of production at the temperature lower than 25℃ in the dry warehouse of avoiding sunlight,
if, beyond the storage life, it cannot be used until being qualified through re-inspection.
BMC series molding compound forming process; compression molding temperature: 150±5℃; Injection molding temperature:125±5 ℃, pressure holding time: 60-70s/mm; pressure of mold pressing: 10-20Mpa.
The specific process depends on product and mold structure,
if the thickness of the product is relatively thick, then the temperature should be relatively low, generally, the molding temperature is 135-145℃,
the more complicated shapes, the higher the molding temperature.
（2）BMC Material performance
It mainly possesses the following superior performance:
The appearance is beautiful and bright, and the dimensions of the finished product are very stable,
with good coloring, low shrinking rate, no easy to deform,
high strength, excellent electric insulative,
excellent flame resistance, resistance to corrosion, no easy to weathering.
For the injection molding equipment of BMC, in BMC, as the glass fiber is added,
the used equipment is somewhat different from the normal thermosetting plastic injection molding equipment,
the injection molding machine used is OK whether it is plunger type or screw type;
1) Feeding system:
No matter screw type or plunger type, an extrusion-type feeding device must be added to force materials into the barrel;
this feeding device mainly adopts the plunger-type forcing feedstock.
2) Injection molding system:
As the injection amount of the plunger-type injection molding machine is accurate and constant, which makes the glass fiber scatter into the melting materials with less damage,
therefore, the plunger-type injection molding machine is mostly used, with the inconvenient exhaust.
3) Heating system:
In the injection molding of BMC, it is very important to control the barrel temperature,
there must be a set of the control system to control temperature and endure the optimum temperature between the feeding segments and nozzle.
At present, the thermostatted water or thermostat oil is mainly used for heating. Electrical heating can also be used.
4) Mold closing mechanism:
The mold closing mechanisms, such as the mechanical, hydraulic and fully hydraulic types are mainly used.
BMC injection mold is somewhat different from regular thermoplastic and thermoset injection molding, mainly because the shrinking percentage of BMC is relatively low,
therefore, variation should be made when the dimensions of mold cavity is designed.
（3）BMC injection molding process
1) Barrel temperature and mold temperature.
During the injection molding process, BMC is required to maintain the flow state of low viscosity for a long time under the barrel temperature;
Generally, the barrel temperature should be able to meet the lower limit value of BMC.
The barrel temperature is generally divided into two or three segments to control,
the temperature at the end near the hopper is relatively low, and the temperature close to the nozzle is relatively high.
Generally, the difference is 20℃-60℃; Mold temperature is often controlled between 135℃and 185℃.
2) Injection pressure:
As the bad mobility, fast curing speed and complicated mold structure,
the injection pressure is better to select higher pressure, generally being 80-160MPa.
3) Injection speed:
The increase in injection speed is helpful for improving the surface quality of plastic parts,
shorten the curing time, not conducive to exhaust, and also increases the orientation degree of fiber.
Therefore, under the premise of ensuring the surface quality of plastic parts, the lower injection speed should be adopted, usually being 1.8 – 3.5m/min.
4) Screw rotational speed and backpressure:
If screw-type injection molding machine is adopted,
when BMC is injected, the damage on the glass fiber by the screw is greater, in order to reduce the damage of glass fiber as much as possible,
the rotating speed of the screw is better to choose the low value, generally 20-50r/min.
While according to the viscosity of BMC, it is better to adopt low back pressure, generally being 1.4-2.0MPa.
5) Molding cycle:
As the sizes and complexity of plastic parts are different,
the processing time of each segment is also different; generally, the injection time is 2-20s, the temperature holding time is 10-20s/mm (thickness).