BMC (bulk moulding compound) formula&production

What is BMC material?

Bulk Molding Compound (BMC) is a polyester resin molding compound made by premixing method. The premix consists of unsaturated polyester resin, inorganic mineral fillers, short-cut glass fibers, initiators, pigments and lubricants. The length of the glass fiber affects the processing performance of the molded material and the final performance of the product, the general length of the glass fiber is 1.3 ~ 1.6cm, the longest 3.0cm. too short product strength is low, and too long is not conducive to uniform dispersion, but also affect the processing fluidity.

Typical ratio of bulk molding compound

Compared with sheet molding compound, the reinforcing material used is more extensive, the fiber length used is shorter (6~25mm), the content is lower (15% -25%), the filler content is higher, the physical and mechanical properties are slightly lower, the molding process performance and product appearance quality is better, suitable for molding a variety of complex structure but the size of functional parts.

What are the components of the common formulations of bulk molding compounds?

The basic composition of bulk molding compounds contains resins, low shrinkage additives, reinforcements, fillers, initiators, chemical thickeners, internal mold release agents, colorants, etc. The following is a list of general-purpose DMCs, which can be used in the molding process. Common formulations for general purpose DMC, low shrinkage DMC and BMC are listed below. There are also many special types of formulations for different application conditions and requirements, such as arc resistant, fast curing, high impact, heat resistant, water resistant, corrosion resistant, hot mix, high fluidity and inexpensive, etc.

General purpose DMC formulations

Low Shrinkage DMC Typical Formulation

General purpose BMC formulation

Production method and process of bulk molding compound

The production method of BMC can be divided into two types of batch-mixing method and continuous method according to the characteristics of the production process. The batch mud method is more economical and easy to achieve the turnover of BMC varieties, so it is more widely used. According to the different materials and manufacturing methods used, the production method of BMC can be divided into solvent-free batch mixing method and solvent-free pre-impregnation method.

In the solvent-free batch mixing method, liquid pure resin is used and the mixing process of the components is carried out in a spiral type dyed leaf kneader. The resulting BMC requires no further treatment, but can usually be extruded into rods or pellets to facilitate the molding operation.

In the solvent-free prepreg method, the untwisted roving is pre-impregnated with a solvent-free liquid resin and then cut and pressed into a dense, soft compound by a cutter.

In solvent-containing batch blending and prepreg methods, the resins used are solid resins with volatile solvents, such as alkyd resins, DAP (prepolymer), phenolic resins or silicone resins. The role of the solvent is to enable the solid resin to impregnate the glass fiber and other components, but the solvent needs to be removed by heating after mixing or impregnation. BMC made by this method is relatively hard and not easy to operate in the molding machine.

Bulk Molding Compound Production

The mixing procedure for bulk molding compounds is to first mix the resin, pigments, initiator and possibly some fillers, and then add the remaining fillers and reinforcements. Before the reinforcing material is added, the mixing time should not be too long and the mixing intensity should not be too large. And when the glass fiber is added, the mixing time is limited to meet its uniform distribution in the system.

1. Production of standard general-purpose bulk molding compound

1. ① Add all the accurately weighed powder fillers (calcium carbonate, zinc stearate, flotation asbestos) into the same container and mix in the mixer for 5min.
2. ② Add the weighed initiator into another container with resin and mix for 20min, then add into the mixer and mix for 5min.
3. ③ Weigh the glass fiber into a separate container, add a small amount in batches into the mixer, and mix 1min, 2min or 5min according to the process (the time should be as short as possible, so as not to separate the glass fiber).
4. ④ After mixing, empty the molded plastic and clean the mixer.
5. ⑤ Extrude the mold according to the quality or shape as needed.

2. Production of thickened low-shrinkage bulk molding compound

1. ①In the same container, add initiator and magnesium oxide into the blended resin, mix for 20min, and let it stand to defoam.
2. ②Add various powder fillers into the mixer and mix for 5min.
3. ③Pour the resin into the mixer and mix for 5min.
4. ④Add glass fiber to the mixer in small batches and mix for several minutes according to the process requirements.
5. ⑤Keep loose or squeeze as needed.
6. ⑥The material should be left for 24h to give full play to the thickening effect.

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