Mixer Solutions for BMC and SMC Materials Production

Sözer Makina, which understands how important accumulated sector knowledge is in meeting customer demands and shares this experience across its sales and technical teams, continuously contributes to the industry with new, high-technology products. We are not just a machinery supplier; with our 73 years of experience and accumulated knowledge, we can produce professional solutions tailored to your needs. Before entering the topic of mixers related to SMC and BMC production, it would be useful to briefly discuss SMC and BMC materials and their production 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 paste-like material. It would not be incorrect to compare the production of paste-form SMC material to a two-stage process. In the first stage of this process, resin as the main raw material and other auxiliary materials are mixed with the help of a high-speed mixer, producing the SMC paste. In the second stage of the process, glass fibre is combined with this paste using an SMC machine.

SMC Material Production Flow Chart

BMC Material

BMC (Bulk Moulding Compound) is a material produced by combining chopped glass fibre with resin containing filler material in a Z-blade mixer machine, resulting in a paste-form material. As with SMC material, BMC paste production consists of a two-stage process, with different mixer machines used in each stage. In the first stage of the process, a high-speed mixer is used, and in the second stage, a Z-blade mixer (sigma mixer) is used. As internal processes may differ between companies, the general application is carried out as follows. In the first stage, resin and other raw materials are homogeneously mixed in a high-speed mixer to produce BMC pre-paste. The BMC pre-paste is then advanced to the second stage of the process. In the second stage of the process, horizontal mixers known in our country as "Z-blade Mixers" and referred to in literature as "Sigma Mixers" are used. In the second stage of the process, BMC pre-paste and other raw materials (glass fibre, calcite and, depending on the product, other raw materials) are sequentially charged into the mixer vessel. Mixing continues until a homogeneous BMC paste is obtained.

BMC Compound Production Flow Chart

Mixers Used in the Production of SMC and BMC Materials

The following mixers are the primary mixers used to produce these materials. These two mixers are the most commonly used or preferred mixers in the composites sector. 1. High-Speed Mixer (Dissolver) 2. Z-blade Mixer (Sigma Mixer)

1. High-Speed Mixer

Also known as a dissolver, high-speed mixers with a saw-tooth impeller on the mixing shaft are the most commonly used mixers in dispersion applications. The dispersion mixer used for SMC and BMC pre-paste production should have some optional features different from standard high-speed mixers. Due to the viscosity of SMC and BMC material pre-pastes, it would be beneficial to have a mixer with the following options: • A more powerful main electric motor, • A scraper arm option. Although SMC and BMC pre-pastes do not have very high viscosity values, a more powerful motor is needed to allow the motor to mix the product without strain. The scraper arm provides additional mixing, assisting product movement from different areas of the vessel. Since the high-speed mixer continuously feeds the product, compared to a mixer without this feature, the scraper arm helps achieve a homogeneous mixture in a shorter time. The scraper arm enables better homogeneous product to be obtained and better heat transfer to occur.

2. Z-blade Mixers (Sigma Mixers)

Z-blade mixers belong to the group of horizontal mixers and are used to produce liquid-liquid or liquid-solid mixtures, like fast vertical mixers. Due to their ability to generate high torque, Z-blade mixers sometimes appear as the only option in the production of products with very high viscosity. All dual-blade Z-blade mixer machines have 2 "Z"-shaped kneading blades in a "U"-shaped horizontal vessel. The mixing principle of Z-blade mixers is based on the material being compressed between the vessel inner wall and the blades, with the product being sheared or torn to achieve mixing. The Z-blade mixer machine has 2 blades rotating toward each other at different speeds. Each blade moves the material in the vessel in different directions, thus ensuring perfect cross-mixing of all material. Z-blade mixer blades pass at the closest possible distance to both each other and the vessel wall. This closely-spaced movement creates both shearing and tearing action, thereby reducing particle size of solids.

For a Z-blade mixer to exhibit good mixing performance, it must have at least the following characteristics:

1. The mixing blades must pass at the closest possible distance to both each other and the vessel wall. Thus, high shearing can be created, reducing particle size. Depending on mixer capacity, the clearance to the vessel wall should be between 2-5 mm, 2. Two types of blade movement can be observed in Z-blade mixers; these are tangential or overlapping movements. In tangential rotational movement, Z-blade mixer blades rotate at different speeds toward each other. 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: if a Z-blade mixer is to be used for a product with high viscosity such as adhesive or rubber, a Z-blade mixer with tangential blade motion characteristics should be selected. Because tangential motion is required for the production of high-viscosity products.

3. Combined Planetary Kneader Machines

The design and characteristics of vertical mixers change depending on viscosity. When viscosity increases significantly during product production, high-speed mixers with fixed shafts cannot adequately mix the product in the mixing vessel, causing the product to have poor homogeneity. With increasing viscosity, mixer selection shifts from left to right as follows: Combined planetary kneaders 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 specific orbit within the vessel. Due to the orbital movement of the planetary shafts within the vessel, the planetary blades continuously move within the production charge. The second characteristic is that they have a high-speed shaft, a feature that is necessary to produce some formulations. The high-speed shaft with a dispersion blade is very effective in wetting and dispersing powders. In addition to these shafts, the combined planetary kneader machine also has another shaft called a scraper arm, which assists the mixing process by moving product from the vessel wall toward the center. Only the combined planetary kneader machine can be evaluated as an alternative to the Z-blade mixer machine. It should also be remembered that for the production of products with very high viscosity, sometimes the Z-blade mixer machine is the only option. Because beyond a certain very high viscosity value, the combined planetary kneader machine will also fail to create adequate mixing in the mixing vessel.

The combined planetary kneader machine has the following four mixing shafts:

1. A high-speed dispersion shaft at the center, 2. Shafts with a total of two planetary mixers located on the right and left sides of the high-speed shaft, which rotate both around themselves and orbit within the vessel, 3. A scraper shaft designed both to scrape the vessel wall and to bring unmixed product to the center. This shaft also rotates in the opposite direction compared to the mixing shaft, helping the product reach homogeneous state in much shorter time. T. Emrah Sözer Production Director Sözer Makina Sanayi ve Ticaret Ltd. Şti.