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What is BMC compression moulding?

Join Date: 2022-07-11

BMC(Bulk Molding Compound) is suitable for the production of complex products with high molding speed and low cost. The products are mainly electrical products. The method of forming BMC is the same as that of compression molding or transfer molding of thermoset plastics, which is a high-pressure compression molding.

BMC compression moulding processing

BMC-compression-moulding

Firstly, resin, filler, initiator and other components are pre-mixed in the mixing kettle to make resin paste, in order to solve the contradiction between the low viscosity of resin when impregnating glass fiber and the high viscosity of molded plastic when molding. Plastic viscosity of the pair of contradictions, often also add thickeners.

Glass fiber after heat treatment with a wire cutting machine into a certain length, short-cut glass fiber and resin paste mixing is generally completed by the kneader. In the kneading process mainly control the kneading time and resin system viscosity, mixing time is too long, fiber strength loss is too large, but also lead to thermal effects, affecting the infiltration; time is too short, will be mixed uneven spoon. After mixing, the resulting BMC must be sealed in polyethylene film bags and can generally be stored at room temperature for 3-4 weeks.

Bulk Molding Compound (BMC) Molding Process Parameters

Molding Pressure

Since BMC (such as unsaturated polyester resin) has good flowability and no volatiles are generated during the curing process, it is not necessary to use a high molding pressure to fill the cavity and complete the curing process. The curing process can be completed without using high molding pressure. However, higher molding pressures can be beneficial in speeding up the molding process by allowing excess material to overflow and producing precise, well-filled products, but high pressures can sometimes cause difficulties in release due to severe overflow.

For the same component of BMC, the molding pressure is mainly determined by the complexity of the product, the nature of the product and other molding process conditions. When pressing some simple shapes For simple products, a pressure of 0.7MPa is sufficient; for more complex products with tabs or blind holes, a higher pressure may be required. The type of mold also has an influence on the choice of pressure, the overflow mold is less than the practical pressure of the semi-overflow mold, while the pressure of the non-overflow mold (rarely used for BMC pressing) is larger, or even several times higher.

Forming Temperature

The molding temperature of BMC is generally 110~170°C. The temperature of molding is related to the curing speed, complexity of the product, shrinkage control and flow conditions. For fast molding, the molding temperature can be increased. If the shape of the product is complex, the molding temperature can be lower to ensure that the material flow has enough time to fill all corners of the mold cavity. Sometimes, in order to make the products stick to the mold selectively, the temperature difference between upper and lower mold can be adjusted appropriately, but generally not more than 10°C.

Curing time

The curing time of the product is related to the curing speed of the material, and also related to the thickness of the product and the molding temperature. Flat products, a general election in the mold to make it fully cured. The thicker the product, the longer the curing time, but in order to shorten the time of the product in the hydraulic press, mold combination, often use the post-treatment process.

Generally speaking, it is difficult to find the exact curing time for a BMC molded product because the material formulation, shape, and heating position of the actual product will affect the time required. The time required is influenced by the material formulation, shape and heating position of the actual product. However, this problem is not difficult to solve, because during compression molding, "over-curing" (i.e., exceeding the required time for the product) and "under-curing" (i.e., not reaching the required curing time for the product) are not uncommon. (i.e., the required curing time of the product) are undesirable and cannot be obtained to meet the quality requirements of the product. Unlike thermosets in general, the transition from the undercured state to the fully cured state is a very rapid process when BMC is pressed. In a short period of time, BMC may be cured, but the surface of the product may appear bubbles without any gloss. This phenomenon (defect) can be eliminated by properly adjusting the curing time, and when the product has met the quality requirements, the curing time can be determined, but the accuracy is relative.

  • General purpose and low shrinkage BMC molding process parameters: molding pressure of 3.5-7MPa, molding temperature of 132 - 149°C, holding time of 45s.
  • Low shrinkage thickening BMC molding process parameters: molding pressure of 3.5-7MPa. molding temperature of 149-177°C, holding time should be as short as possible.

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