Mixer Solutions for BMC and SMC Materials Production

Mixer Solutions for BMC and SMC Material Production Before addressing mixers related to SMC and BMC production, it would be helpful to briefly discuss SMC and BMC materials and their manufacturing processes.

SMC Material

SMC (Sheet Moulding Compound) is a material produced by combining chopped glass fibre with resin containing filler material in an SMC machine, resulting in a material with a sheet form. It would not be incorrect to compare the production of sheet-form SMC material to a two-stage process. In the first stage of this process, the main raw material—resin—and other auxiliary materials are mixed with the aid of a high-speed mixer, producing the SMC dough. In the second stage of the process, glass fibre is combined with this dough using an SMC machine.

BMC Material

BMC (Bulk Moulding Compound) is a material produced by combining chopped glass fibre with resin containing filler material in a sigma mixer machine, resulting in a dough-form material. As with SMC material, BMC dough production consists of a two-stage process, and different mixer machines are used in each stage. In the first stage of the process, a high-speed mixer is used, and in the second stage, a sigma mixer (sigma mixer) is used. While company internal procedures may vary, the general practice is as follows. In the first stage, resin and other raw materials are homogeneously mixed in a high-speed mixer to produce BMC pre-dough. The BMC pre-dough is then taken to the second stage of the process. In the second stage of the process, horizontal mixers known in our country as "Sigma Mixer" and referred to in the literature as "Sigma Mixer" are used. In the second stage of the process, BMC pre-dough and other raw materials (glass fibre, calcite, and depending on the product, other raw materials) are successively charged into the mixer bowl. Mixing is continued until a homogeneous BMC dough is obtained. The following mixers are the principal mixers used in producing these materials. These two mixers are the most commonly used or most preferred mixers in the composites sector.

Mixers Used in the Production of SMC and BMC Materials

  1. High-Speed Mixer 2. Sigma Mixer

1. High-Speed Mixer

Also known as a dissolver, high-speed mixers with sawtooth-blade propellers on the mixing shaft are the most widely used mixers in dispersion applications. The dispersion mixer to be used for SMC and BMC pre-dough production should have certain optional features different from standard high-speed mixers. Due to the viscosity of SMC and BMC pre-doughs, it would be beneficial to have a mixer with the following options: • A more powerful main electric motor, • A scraper blade option. Although SMC and BMC pre-doughs do not have extremely high viscosity values, the motor must be more powerful to mix the product without strain. The scraper blade provides additional mixing, assisting product movement from different areas of the bowl. Since the high-speed mixer continuously feeds the product, compared to a mixer without this feature, the scraper blade helps achieve a homogeneous mixture in a shorter time. The scraper blade ensures a more homogeneous product is obtained and better heat transfer is achieved.

2. Sigma Mixers

Sigma mixers are part of the horizontal mixer group and are used to produce liquid-liquid or liquid-solid mixtures, similar to fast-running vertical mixers. Due to their high torque production capability, sigma mixers sometimes appear as the only option in the production of products with very high viscosity. All twin-blade sigma mixer machines have 2 "Z"-shaped kneading blades in a "U"-shaped horizontal bowl. The mixing principle of sigma mixers is based on the crushing of material between the inner walls of the bowl and the blades and the mixing of the product by tearing or breaking it. The sigma mixer machine has 2 blades rotating towards each other at different speeds. Each blade moves the material in the bowl in different directions, thus ensuring perfect cross-mixing of all material. Sigma mixer blades pass as close as possible to both each other and the bowl wall. This closely occurring movement creates both breaking and tearing action, thus also reducing the particle size of solids. For a sigma mixer to demonstrate good mixing performance, it should have at least the following characteristics.   1. The mixing blades must pass as close as possible to both each other and the bowl wall. Thus, high shearing can be created, reducing particle size. Depending on mixer capacity, the clearance from the bowl wall should be between 2-5 mm. 2. Two types of blade movement can be observed in sigma mixers: tangential or overlapping movements. In tangential rotational movement, the sigma mixer blades rotate towards each other at different speeds. One blade rotates faster than the other. In overlapping rotational movement, the relative positions of the blades remain constant. This means these blades rotate at the same speeds. The key point here is that if a sigma mixer is to be used for a product with high viscosity such as adhesive or rubber, a sigma mixer with tangential blade movement characteristics should be selected. This is because the production of high-viscosity products requires tangential movement.

Alternative Mixers to Sigma Mixers: Combined Planetary Maltreater Machines

The design and characteristics of vertical mixers change depending on viscosity. When viscosity increases significantly during the production of a product, high-speed mixers with a fixed shaft cannot adequately mix the product within the mixing bowl, and this causes the product to have poor homogeneity. Combined planetary mixers differ from other vertical mixers due to two characteristics. The first and most important characteristic is that they have two planetary-type shafts and angled blades on these shafts. These planetary shafts rotate both around their own axes and in a certain orbit within the bowl. Due to the orbital motion of the planetary shafts within the bowl, the planetary blades are constantly moving within the production charge. The second characteristic is that it has a high-speed shaft, and this feature is a mandatory characteristic for producing certain formulations. The high-speed shaft with a dispersion blade is very effective in the processes of wetting and dispersing powders. In addition to these shafts, the combined planetary mixer machine also has another shaft called a scraper blade, which aids the mixing process by transporting the product from the bowl wall towards the center. Only the combined planetary mixer machine can be considered an alternative to the sigma mixer machine. It should be remembered that for the production of products with very high viscosity, sometimes the only option is the sigma mixer machine. This is because after a certain very high viscosity value, the combined planetary mixer machine will also fail to create adequate mixing within the mixing bowl. The combined planetary mixer machine has the following four mixing shafts. 1. Two high-speed dispersion propellers in the center, 2. A total of two planetary mixer agitators located to the right and left of the high-speed shaft, rotating both on their own axes and orbiting within the bowl, 3. Optional scraper blades—two in total—that scrape the inner wall of the bowl and create vortex in the mixture, enabling homogenization in shorter periods.     T. Emrah Sözer Industrial Engineer Administrative Affairs Director Sözer Makina